Agilent Technologies 86120B User Manual

Agilent 86120B Multi-Wavelength Meter User’s Guide

Agilent Part No. 86120-90033 Printed in USA February 2000

Agilent Technologies Lightwave Division 1400 Fountaingrove Parkway Santa Rosa, CA 95403-1799, USA (707) 577-1400

Notice.

The information contained in this document is subject to change without notice. Companies, names, and data used in examples herein are fictitious unless otherwise noted. Agilent Technologies makes no warranty of any kind with regard to this material, including but not limited to, the implied warranties of merchantability and fitness for a particular purpose. Agilent Technologies shall not be liable for errors contained herein or for incidental or consequential damages in connection with the furnishing, performance, or use of this material.

Restricted Rights Legend.

Use, duplication, or disclosure by the U.S. Government is subject to restrictions as set forth in subparagraph (c) (1) (ii) of the Rights in Technical Data and Computer Software clause at DFARS 252.227-7013 for DOD agencies, and subparagraphs (c) (1) and (c) (2) of the Commercial Computer Software Restricted Rights clause at FAR 52.227-19 for other agencies.

Warranty.

This Agilent Technologies instrument product is warranted against defects in

material and workmanship for a period of one year from date of shipment. During the warranty period, Agilent Technologies will, at its option, either repair or replace products which prove to be defective. For warranty service or repair, this product must be returned to a service facility designated by Agilent Technologies. Buyer shall prepay shipping charges to Agilent Technologies and Agilent Technologies shall pay shipping charges to return the product to Buyer. However, Buyer shall pay all shipping charges, duties, and taxes for products returned to Agilent Technologies from another country.

Agilent Technologies warrants that its software and firmware designated by Agilent Technologies for use with an instrument will execute its programming instructions when properly installed on that instrument. Agilent Technologies does not warrant that the operation of the instrument, or software, or firmware will be uninterrupted or errorfree.

Limitation of Warranty.

The foregoing warranty shall not apply to defects resulting from improper or inadequate maintenance by Buyer, Buyersupplied software or interfacing, unauthorized modification or misuse, operation outside of the environmental specifications for the product, or improper site preparation or maintenance.

No other warranty is expressed or implied. Agilent Technologies specifically disclaims the implied warranties of merchantability and fitness for a particular purpose.

Exclusive Remedies.

The remedies provided herein are buyer's sole and exclusive remedies. Agilent Technolo-

gies shall not be liable for any direct, indirect, special, incidental, or consequential damages, whether based on contract, tort, or any other legal theory.

Safety Symbols.

CAUTION

The

caution

sign denotes a hazard. It calls attention to a procedure which, if not correctly performed or adhered to, could result in damage to or destruction of the product. Do not proceed beyond a caution sign until the indicated conditions are fully understood and met.

WAR NING

The

warning

sign denotes a hazard. It calls attention to a procedure which, if not correctly performed or adhered to, could result in injury or loss of life. Do not proceed beyond a warning sign until the indicated conditions are fully understood and met.

The instruction manual symbol. The product is marked with this warning symbol when it is necessary for the user to refer to the instructions in the manual.

The laser radiation symbol. This warning symbol is marked on products which have a laser output.

The AC symbol is used to indicate the required nature of the line module input power.

The ON symbols are

used to mark the positions of the instrument power line switch.

The OFF symbols

❍

are used to mark the positions of the instrument power line switch.

The CE mark is a registered trademark of the European Community.

The CSA mark is a registered trademark of the Canadian Standards Association.

The C-Tick mark is a registered trademark of the Australian Spectrum Management Agency.

This text denotes the

ISM1-A

instrument is an Industrial Scientific and Medical Group 1 Class A product.

Typographical Conventions.

The following conventions are used in this book:

Key type

for keys or text located on the keyboard or instrument.

Softkey type

for key names that are displayed on the instrument’s screen.

Display type

for words or characters displayed on the computer’s screen or instrument’s display.

User type

for words or charac-

ters that you type or enter.

Emphasis

type for words or characters that emphasize some point or that are used as place holders for text that you type.

The Agilent 86120B—At a Glance

The Agilent 86120B Multi-Wavelength Meter measures the wavelength and optical power of laser light in the 700-1650 nm wavelength range. Because the Agilent 86120B simultaneously measures multiple laser lines, you can characterize wavelength-division-multiplexed (WDM) systems and the multiple lines of Fabry-Perot lasers.

NOTE

The front-panel OPTICAL INPUT connector uses a single-mode input fiber.

What’s new with the Agilent 86120B

This book directly applies to Agilent 86120B instruments with firmware version number 2.0. When first turned on, the instrument briefly displays the firmware version. These instruments have the added capability of measuring broadband devices and chirped lasers. Refer to “Measuring broadband devices

and chirped lasers” on page 2-10.

Characterize laser lines easily

With the Agilent 86120B you can quickly and easily measure any of the following parameters:

• Wavelengths and powers

• Average wavelength

• Total optical power

• Laser line separation

• Laser drift (

• Signal-to-noise ratios

• Coherence length

wavelength and power

)

iii

The Agilent 86120B—At a Glance

In addition to these measurements, a “power bar” is displayed that shows power changes like a traditional analog meter. You can see the power bar shown in the following figure of the Agilent 86120B’s display.

CAUTION

The input circuitry of the Agilent 86120B can be damaged when

total

input power levels exceed +18 dBm. To prevent input damage, this specified level must not be exceeded.

Print measurement results

You can get hardcopy results of your measurements by connecting a printer to the rear-panel

PARALLEL PRINTER PORT

connector.

Program the instrument for automatic measurements

The Agilent 86120B offers an extensive set of GPIB programming commands. These commands allow you to perform automated measurements on manufacturing production lines and remote sites. Chapter 4, “Programming” and Chap-

ter 5, “Programming Commands” provide all the information you’ll need to

know in order to program the Agilent 86120B.

Display wavelengths as if measured in vacuum or standard air

Although all measurements are made in air, displayed results are corrected for air dispersion to accurately show wavelength values in vacuum or in “standard air.” To ensure accurate wavelength measurements, make sure that you enter the elevation from which you will be making measurements as described in

Chapter 1, “Getting Started”.

The Agilent 86120B—At a Glance

Measurement accuracy—it’s up to you!

Fiber-optic connectors are easily damaged when connected to dirty or damaged cables and accessories. The Agilent 86120B’s front-panel INPUT connector is no exception. When you use improper cleaning and handling techniques, you risk expensive instrument repairs, damaged cables, and compromised measurements.

Before you connect any fiber-optic cable to the Agilent 86120B, refer to “Cleaning Con-

nections for Accurate Measurements” on page 1-13.

General Safety Considerations

This product has been designed and tested in accordance with IEC Publication 1010, Safety Requirements for Electronic Measuring Apparatus, and has been supplied in a safe condition. The instruction documentation contains information and warnings which must be followed by the user to ensure safe operation and to maintain the product in a safe condition.

Laser Classification: This product is classified FDA Laser Class I (IEC Laser Class 1).

WARNING

If this instrument is not used as specified, the protection provided by the equipment could be impaired. This instrument must be used in a normal condition (in which all means for protection are intact) only.

No operator serviceable parts inside. Refer servicing to qualified personnel. To prevent electrical shock, do not remove covers.

There is no output laser aperture

The Agilent 86120B does not have an output laser aperture. However, light less than 1 nw escapes out of the front-panel OPTICAL INPUT connector. Operator maintenance or precautions are not necessary to maintain safety. No controls, adjustments, or performance of procedures result in hazardous radiation exposure.

General Safety Considerations

WARNING

CAUTION

To prevent electrical shock, disconnect the Agilent 86120B from mains before cleaning. Use a dry cloth or one slightly dampened with water to clean the external case parts. Do not attempt to clean internally.

This is a Safety Class 1 product (provided with a protective earthing ground incorporated in the power cord). The mains plug shall only be inserted in a socket outlet provided with a protective earth contact. Any interruption of the protective conductor inside or outside of the product is likely to make the product dangerous. Intentional interruption is prohibited.

For continued protection against fire hazard, replace line fuse only with same type and ratings, (type T 0.315A/250V for 100/120V operation and 0.16A/250V for 220/240V operation). The use of other fuses or materials is prohibited. Verify that the value of the linevoltage fuse is correct.

• For 100/120V operation, use an IEC 127 5×20 mm, 0.315 A, 250 V, Agilent part number 2110-0449.

• For 220/240V operation, use an IEC 127 5×20 mm, 0.16 A, 250 V, Agilent Technologies part number 2110-0448.

Before switching on this instrument, make sure that the line voltage selector switch is set to the line voltage of the power supply and the correct fuse is installed. Assure the supply voltage is in the specified range.

CAUTION

This product is designed for use in Installation Category II and Pollution Degree 2 per IEC 1010 and 664 respectively.

VENTILATION REQUIREMENTS: When installing the product in a cabinet, the convection into and out of the product must not be restricted. The ambient temperature (outside the cabinet) must be less than the maximum operating

vii

General Safety Considerations

temperature of the product by 4°C for every 100 watts dissipated in the cabinet. If the total power dissipated in the cabinet is greater than 800 watts, then forced convection must be used.

CAUTION

Always use the three-prong ac power cord supplied with this instrument. Failure to ensure adequate earth grounding by not using this cord may cause instrument damage.

connect ac power until you have verified the line voltage is correct as

Do not

described in “Line Power Requirements” on page 1-6. Damage to the equipment could result.

This instrument has autoranging line voltage input. Be sure the supply voltage is within the specified range.

viii

The Agilent 86120B—At a Glance iii

General Safety Considerations vi

1 Getting Started

Step 1. Inspect the Shipment 1-3 Step 2. Check the Fuse 1-5 Step 3. Connect the Line-Power Cable 1-6 Step 4. Connect a Printer 1-7 Step 5. Turn on the Agilent 86120B 1-8 Step 6. Enter Your Elevation 1-10 Step 7. Select Medium for Wavelength Values 1-11 Step 8. Turn Off Wavelength Limiting 1-12 Cleaning Connections for Accurate Measurements 1-13 Returning the Instrument for Service 1-23

2 Using the Multi-Wavelength Meter

Displaying Wavelength and Power 2-3 Changing the Units and Measurement Rate 2-13 Defining Laser-Line Peaks 2-16 Measuring Laser Separation 2-20 Measuring Modulated Lasers 2-23 Measuring Total Power Greater than 10 dBm 2-25 Calibrating Measurements 2-26 Printing Measurement Results 2-28

3 Measurements Applications

Measuring Signal-to-Noise Ratios 3-3 Measuring Signal-to-Noise Ratios with Averaging 3-7 Measuring Laser Drift 3-9 Measuring Coherence Length 3-12

4 Programming

Addressing and Initializing the Instrument 4-3 Making Measurements 4-5 Monitoring the Instrument 4-16 Reviewing SCPI Syntax Rules 4-23 Example Programs 4-28

Contents-1

Contents

Lists of Commands 4-43

5 Programming Commands

Common Commands 5-3 Measurement Instructions 5-15 CALCulate1 Subsystem 5-26 CALCulate2 Subsystem 5-31 CALCulate3 Subsystem 5-43 CONFigure Measurement Instruction 5-64 DISPlay Subsystem 5-64 FETCh Measurement Instruction 5-67 HCOPy Subsystem 5-68 MEASure Measurement Instruction 5-68 READ Measurement Instruction 5-69 SENSe Subsystem 5-69 STATus Subsystem 5-74 SYSTem Subsystem 5-79 TRIGger Subsystem 5-84 UNIT Subsystem 5-86

6 Performance Tests

Test 1. Absolute Wavelength Accuracy 6-3 Test 2. Sensitivity 6-4 Test 3. Polarization Dependence 6-5 Test 4. Optical Input Return Loss 6-6 Test 5. Amplitude Accuracy and Linearity 6-9

7 Specifications and Regulatory Information

Definition of Terms 7-3 Specifications 7-6 Regulatory Information 7-10

8 Reference

Instrument

Preset

Conditions 8-2 Menu Maps 8-4 Error Messages 8-9 Front-Panel Fiber-Optic Adapters 8-15

Contents-2

Power Cords 8-16 Agilent Technologies Service Offices 8-18

Contents

Contents-3

Getting Started

The instructions in this chapter show you how to install your Agilent 86120B. You should be able to finish these procedures in about ten to twenty minutes. After you’ve completed this chapter, continue with Chapter 2, “Using the

Multi-Wavelength Meter”. Refer to Chapter 7, “Specifications and Regulatory Information” for information on operating conditions such as temperature.

If you should ever need to clean the cabinet, use a damp cloth only.

WARNING

CAUTION

Ventilation Requirements. When installing the product in a cabinet, the convection into and out of the product must not be restricted. The ambient temperature (outside the cabinet) must be less than the maximum operating temperature of the product by 4°C for every 100 watts dissipated in the cabinet. If the total power dissipated in the cabinet is greater than 800 watts, then forced convection must be used.

This product is designed for use in INSTALLATION CATEGORY II and POLLUTION DEGREE 2, per IEC 1010 and 664 respectively.

Install the instrument so that the ON/OFF switch is readily identifiable and is easily reached by the operator. The ON/OFF switch or the detachable power cord is the instrument disconnecting device. It disconnects the mains circuits from the mains supply before other parts of the instrument. Alternately, an externally installed switch or circuit breaker (which is really identifiable and is easily reached by the operator) may be used as a disconnecting device.

Install the instrument according to the enclosure protection provided. This instrument does not protect against the ingress of water. This instrument protects against finger access to hazardous parts within the enclosure.

1-2

Getting Started

Step 1. Inspect the Shipment

Verify that all system components ordered have arrived by comparing the shipping forms to the original purchase order. Inspect all shipping containers.

If your shipment is damaged or incomplete, save the packing materials and notify both the shipping carrier and the nearest Agilent Technologies sales and service office. Agilent Technologies will arrange for repair or replacement of damaged or incomplete shipments without waiting for a settlement from the transportation company. Notify the Agilent Technologies customer engineer of any problems.

Make sure that the serial number and options listed on the instrument’s rearpanel label match the serial number and options listed on the shipping document. The following figure is an example of the rear-panel serial number label:

1-3

Getting Started

Step 1. Inspect the Shipment

Table 1-1. Options and Accessories Available for the Agilent 86120B

Item Quantity

Option 010 Delete FC/PC connector — —

Option 011 Diamond HMS-10 connector interface 1 08154-61701

Option 013 DIN 47256 connector interface 1 08154-61703

Option 014 ST connector interface 1 08154-61704

Option 017 SC connector interface 1 08154-61708

Option 022 Replace flat physical contact interface with angled physical contact interface

Option 900 Great Britain power cord 1 8120-1703

Option 901 Australia, New Zealand, China power cord 1 8120-0696

Option 902 European power cord 1 8120-1692

Option 906 Switzerland power cord 1 8120-2296

Option 912 Denmark power cord 1 8120-2957

Option 917 India, South Africa power cord 1 8120-4600

Option 918 Japanese power cord 1 8120-4754

——

Agilent Technologies Part Number

Option 919 Israel power cord 1 8120-5181

Option UK5 Protective soft carrying case 1 9211-7314

Option UK6 Commercial calibration certificate with calibration data

Option AXE Rack mount kit with handles 1 86120-60031

Option IX4 Rack mount kit without handles 1 86120-60030

Option OB2 Additional user’s manual 1 86120-90001

Option 412 Add 10 dB external attenuator (FC/PC interface connector)

1-4

1—

1 1005-0587

Step 2. Check the Fuse

Locate the line-input connector on the instrument’s rear panel.

Disconnect the line-power cable if it is connected.

Use a small flat-blade screwdriver to open the pull-out fuse drawer.

Getting Started

WARNING

Verify that the value of the line-voltage fuse in the pull-out drawer is correct. The recommended fuse is an IEC 127 5×20 mm, 6.3A, 250 V, Agilent Technologies part number 2110-0703.

Notice that an extra fuse is provided in a drawer located on the fuse holder.

For continued protection against fire hazard, replace line fuse only with same type and ratings, (type T 6.3A/250V for 100/240V operation). The use of other fuses or materials is prohibited.

1-5

Getting Started

Step 3. Connect the Line-Power Cable

WARNING

CAUTION

This is a Safety Class 1 Product (provided with a protective earthing ground incorporated in the power cord). The mains plug shall only be inserted in a socket outlet provided with a protective earth contact. Any interruption of the protective conductor inside or outside of the instrument is likely to make the instrument dangerous. Intentional interruption is prohibited.

Always use the three-prong AC power cord supplied with this instrument. Failure to ensure adequate earth grounding by not using this cord may cause instrument damage.

connect ac power until you have verified the line voltage is correct as

Do not

described in the following paragraphs. Damage to the equipment could result.

This instrument has autoranging line voltage input. Be sure the supply voltage is within the specified range.

Verify that the line power meets the requirements shown in the following table.

Line Power Requirements

Power: 115 VAC: 110 VA MAX. / 60 WATTS MAX. / 1.1 A MAX.

230 VAC: 150 VA MAX. / 70 WATTS MAX. / 0.6 A MAX.

Voltage nominal: 115 VAC / 230 VAC

range 115 VAC: 90-132 V range 230 VAC: 198-254 V

Frequency nominals: 50 Hz / 60 Hz

range: 47-63 Hz

Connect the line-power cord to the instrument’s rear-panel connector.

1-6

Getting Started

Step 4. Connect a Printer

Connect the other end of the line-power cord to the power receptacle.

Various power cables are available to connect the Agilent 86120B to ac power outlets unique to specific geographic areas. The cable appropriate for the area to which the Agilent 86120B is originally shipped is included with the unit. The cable shipped with the instrument also has a right-angle connector so that the Agilent 86120B can be used while sitting on its rear feet. You can order additional ac power cables for use in different geographic areas. Refer to

“Front-Panel Fiber-Optic Adapters” on page 8-15.

Step 4. Connect a Printer

The Agilent 86120B can print hardcopies of measurement results on a printer. The output is ASCII text. If you don’t have a printer, continue with “Step 5.

Turn on the Agilent 86120B” on page 1-8.

• Using a standard parallel printer cable, connect the printer to the Agilent 86120B’s rear-panel

PARALLEL PRINTER PORT

connector.

1-7

Getting Started

Step 5. Turn on the Agilent 86120B

CAUTION

The front panel LINE switch disconnects the mains circuits from the mains supply after the EMC filters and before other parts of the instrument.

Press the front-panel should look similar to the following figure:

The front-panel ply after the EMC filters and before other parts of the instrument.

If the Agilent 86120B fails to turn on properly, consider the following possibilities:

• Is the line fuse good?

• Does the line socket have power?

• Is it plugged into the proper ac power source?

If the instrument still fails, return it to Agilent Technologies for repair. Refer to

“Returning the Instrument for Service” on page 1-23.

LINE

key. After approximately 20 seconds, the display

LINE

switch disconnects the mains circuits from the mains sup-

1-8

Getting Started

Step 5. Turn on the Agilent 86120B

Instrument firmware version

When the instrument is first turned on, the display briefly shows the instrument’s firmware version number. In the unlikely event that you have a problem with the Agilent 86120B, you may need to indicate this number when communicating with Agilent Technologies.

There is no output laser aperture

Measurement accuracy—it’s up to you!

Before you connect any fiber-optic cable to the Agilent 86120B, refer to “Cleaning Con-

nections for Accurate Measurements” on page 1-13.

1-9

Getting Started

Step 6. Enter Your Elevation

In order for your Agilent 86120B to accurately measure wavelengths and meet its published specifications, you must enter the elevation where you will be performing your measurements.

Press the

Press

Use the and softkeys to enter the elevation in meters. Entries jump in 500 meter steps from 0 m to 5000 m.

The elevation value selected with the softkeys must be within 250 meters of the actual elevation.

Press

Converting feet to meters

If you know your elevation in feet, you can convert this value to meters by using the following equation:

Setup

CAL

ELEV

RETURN

key.

softkey.

to complete the entry.

---------------=

3.281

1-10

Getting Started

Step 7. Select Medium for Wavelength Values

Because wavelength varies with the material that the light passes through, the Agilent 86120B offers wavelength measurements in two mediums: vacuum and standard air.

Press the

Make the following selection:

•Press

Press

Definition of standard air

Standard air is defined to have the following characteristics:

Barometric pressure: 760 torr Temperature: 15°C Relative humidity: 0%

Setup

CAL

VACUUM

STD AIR

RETURN

key.

softkey.

for wavelength readings in a vacuum.

for wavelength readings in standard air.

to complete the entry.

1-11

Getting Started

Step 8. Turn Off Wavelength Limiting

After the suring lasers between 1200 nm and 1650 nm. You can easily expand the input range to the full 700 nm to 1650 nm range with the following steps:

Press the

Press

All responses in the full 700 nm to 1650 nm range are now displayed.

Preset

Setup

WL LIM

LIM OFF

key is pressed, the input wavelength range is limited to mea-

key.

softkey.

to remove the limits on wavelength range.

1-12

Getting Started

Cleaning Connections for Accurate Measurements

Today, advances in measurement capabilities make connectors and connection techniques more important than ever. Damage to the connectors on calibration and verification devices, test ports, cables, and other devices can degrade measurement accuracy and damage instruments. Replacing a damaged connector can cost thousands of dollars, not to mention lost time! This expense can be avoided by observing the simple precautions presented in this book. This book also contains a brief list of tips for caring for electrical connectors.

Choosing the Right Connector

A critical but often overlooked factor in making a good lightwave measurement is the selection of the fiber-optic connector. The differences in connector types are mainly in the mechanical assembly that holds the ferrule in position against another identical ferrule. Connectors also vary in the polish, curve, and concentricity of the core within the cladding. Mating one style of cable to another requires an adapter. Agilent Technologies offers adapters for most instruments to allow testing with many different cables. Figure 1-1 on

page 1-14 shows the basic components of a typical connectors.

The system tolerance for reflection and insertion loss must be known when selecting a connector from the wide variety of currently available connectors. Some items to consider when selecting a connector are:

• How much insertion loss can be allowed?

• Will the connector need to make multiple connections? Some connectors are better than others, and some are very poor for making repeated connections.

• What is the reflection tolerance? Can the system take reflection degradation?

• Is an instrument-grade connector with a precision core alignment required?

• Is repeatability tolerance for reflection and loss important? Do your specifica-

1-13

Getting Started

Cleaning Connections for Accurate Measurements

tions take repeatability uncertainty into account?

• Will a connector degrade the return loss too much, or will a fusion splice be required? For example, many DFB lasers cannot operate with reflections from connectors. Often as much as 90 dB isolation is needed.

Figure 1-1. Basic components of a connector.

Over the last few years, the FC/PC style connector has emerged as the most popular connector for fiber-optic applications. While not the highest performing connector, it represents a good compromise between performance, reliability, and cost. If properly maintained and cleaned, this connector can withstand many repeated connections.

However, many instrument specifications require tighter tolerances than most connectors, including the FC/PC style, can deliver. These instruments cannot tolerate connectors with the large non-concentricities of the fiber common with ceramic style ferrules. When tighter alignment is required, Agilent Technologies instruments typically use a connector such as the Diamond HMS-10, which has concentric tolerances within a few tenths of a micron. Agilent Technologies then uses a special universal adapter, which allows other cable types to mate with this precision connector. See Figure 1-2.

1-14

Getting Started

Cleaning Connections for Accurate Measurements

Figure 1-2. Universal adapters to Diamond HMS-10.

The HMS-10 encases the fiber within a soft nickel silver (Cu/Ni/Zn) center which is surrounded by a tough tungsten carbide casing, as shown in Figure

1-3.

Figure 1-3. Cross-section of the Diamond HMS-10 connector.

The nickel silver allows an active centering process that permits the glass fiber to be moved to the desired position. This process first stakes the soft nickel silver to fix the fiber in a near-center location, then uses a post-active staking to shift the fiber into the desired position within 0.2µm. This process, plus the keyed axis, allows very precise core-to-core alignments. This connector is found on most Agilent Technologies lightwave instruments.

1-15

Getting Started

Cleaning Connections for Accurate Measurements

The soft core, while allowing precise centering, is also the chief liability of the connector. The soft material is easily damaged. Care must be taken to minimize excessive scratching and wear. While minor wear is not a problem if the glass face is not affected, scratches or grit can cause the glass fiber to move out of alignment. Also, if unkeyed connectors are used, the nickel silver can be pushed onto the glass surface. Scratches, fiber movement, or glass contamination will cause loss of signal and increased reflections, resulting in poor return loss.

Inspecting Connectors

Because fiber-optic connectors are susceptible to damage that is not immediately obvious to the naked eye, poor measurements result without the user being aware. Microscopic examination and return loss measurements are the best way to ensure good measurements. Good cleaning practices can help ensure that optimum connector performance is maintained. With glass-toglass interfaces, any degradation of a ferrule or the end of the fiber, any stray particles, or finger oil can have a significant effect on connector performance. Where many repeat connections are required, use of a connector saver or patch cable is recommended.

Figure 1-4 shows the end of a clean fiber-optic cable. The dark circle in the

center of the micrograph is the fiber’s 125 µm core and cladding which carries the light. The surrounding area is the soft nickel-silver ferrule. Figure 1-5 shows a dirty fiber end from neglect or perhaps improper cleaning. Material is smeared and ground into the end of the fiber causing light scattering and poor reflection. Not only is the precision polish lost, but this action can grind off the glass face and destroy the connector.

Figure 1-6 shows physical damage to the glass fiber end caused by either

repeated connections made without removing loose particles or using improper cleaning tools. When severe, the damage of one connector end can be transferred to another good connector endface that comes in contact with the damaged one. Periodic checks of fiber ends, and replacing connecting cables after many connections is a wise practice.

The cure for these problems is disciplined connector care as described in the following list and in “Cleaning Connectors” on page 1-20.

1-16

Getting Started

Cleaning Connections for Accurate Measurements

Use the following guidelines to achieve the best possible performance when making measurements on a fiber-optic system:

• Never use metal or sharp objects to clean a connector and never scrape the connector.

• Avoid matching gel and oils.

Figure 1-4. Clean, problem-free fiber end and ferrule.

Figure 1-5. Dirty fiber end and ferrule from poor cleaning.

1-17

Getting Started

Cleaning Connections for Accurate Measurements

Figure 1-6. Damage from improper cleaning.

While these often work well on first insertion, they are great dirt magnets. The oil or gel grabs and holds grit that is then ground into the end of the fiber. Also, some early gels were designed for use with the FC, non-contacting connectors, using small glass spheres. When used with contacting connectors, these glass balls can scratch and pit the fiber. If an index matching gel or oil must be used, apply it to a freshly cleaned connector, make the measurement, and then immediately clean it off. Never use a gel for longer-term connections and never use it to improve a damaged connector. The gel can mask the extent of damage and continued use of a damaged fiber can transfer damage to the instrument.

• When inserting a fiber-optic cable into a connector, gently insert it in as straight a line as possible. Tipping and inserting at an angle can scrape material off the inside of the connector or even break the inside sleeve of connectors made with ceramic material.

• When inserting a fiber-optic connector into a connector, make sure that the fiber end does not touch the outside of the mating connector or adapter.

• Avoid over tightening connections.

Unlike common electrical connections, tighter is

better. The purpose of

not

the connector is to bring two fiber ends together. Once they touch, tightening only causes a greater force to be applied to the delicate fibers. With connectors that have a convex fiber end, the end can be pushed off-axis resulting in misalignment and excessive return loss. Many measurements are actually improved by backing off the connector pressure. Also, if a piece of grit does happen to get by the cleaning procedure, the tighter connection is more likely to damage the glass. Tighten the connectors just until the two fibers touch.

1-18

Getting Started

Cleaning Connections for Accurate Measurements

• Keep connectors covered when not in use.

• Use fusion splices on the more permanent critical nodes. Choose the best connector possible. Replace connecting cables regularly. Frequently measure the return loss of the connector to check for degradation, and clean every connector, every time.

All connectors should be treated like the high-quality lens of a good camera. The weak link in instrument and system reliability is often the inappropriate use and care of the connector. Because current connectors are so easy to use, there tends to be reduced vigilance in connector care and cleaning. It takes only one missed cleaning for a piece of grit to permanently damage the glass and ruin the connector.

Measuring insertion loss and return loss

Consistent measurements with your lightwave equipment are a good indication that you have good connections. Since return loss and insertion loss are key factors in determining optical connector performance they can be used to determine connector degradation. A smooth, polished fiber end should produce a good return-loss measurement. The quality of the polish establishes the difference between the “PC” (physical contact) and the “Super PC” connectors. Most connectors today are physical contact which make glass-to-glass connections, therefore it is critical that the area around the glass core be clean and free of scratches. Although the major area of a connector, excluding the glass, may show scratches and wear, if the glass has maintained its polished smoothness, the connector can still provide a good low level return loss connection.

If you test your cables and accessories for insertion loss and return loss upon receipt, and retain the measured data for comparison, you will be able to tell in the future if any degradation has occurred. Typical values are less than 0.5 dB of loss, and sometimes as little as 0.1 dB of loss with high performance connectors. Return loss is a measure of reflection: the less reflection the better (the larger the return loss, the smaller the reflection). The best physically contacting connectors have return losses better than 50 dB, although 30 to 40 dB is more common.

1-19

Getting Started

Cleaning Connections for Accurate Measurements

Visual inspection of fiber ends

Visual inspection of fiber ends can be helpful. Contamination or imperfections on the cable end face can be detected as well as cracks or chips in the fiber itself. Use a microscope (100X to 200X magnification) to inspect the entire end face for contamination, raised metal, or dents in the metal as well as any other imperfections. Inspect the fiber for cracks and chips. Visible imperfections not touching the fiber core may not affect performance (unless the imperfections keep the fibers from contacting).

WARNING

CAUTION

Always remove both ends of fiber-optic cables from any instrument, system, or device before visually inspecting the fiber ends. Disable all optical sources before disconnecting fiber-optic cables. Failure to do so may result in permanent injury to your eyes.

Cleaning Connectors

The procedures in this section provide the proper steps for cleaning fiberoptic cables and Agilent Technologies universal adapters. The initial cleaning, using the alcohol as a solvent, gently removes any grit and oil. If a caked-on layer of material is still present, (this can happen if the beryllium-copper sides of the ferrule retainer get scraped and deposited on the end of the fiber during insertion of the cable), a second cleaning should be performed. It is not uncommon for a cable or connector to require more than one cleaning.

Agilent Technologies strongly recommends that index matching compounds

be applied to their instruments and accessories. Some compounds, such as

not

gels, may be difficult to remove and can contain damaging particulates. If you think the use of such compounds is necessary, refer to the compound manufacturer for information on application and cleaning procedures.

Table 1-2. Cleaning Accessories

Item Agilent Technologies Part Number

Pure isopropyl alcohol —

Cotton swabs 8520-0023

Small foam swabs 9300-1223

Compressed dust remover (non-residue) 8500-5262

1-20

Getting Started

Cleaning Connections for Accurate Measurements

Table 1-3. Dust Caps Provided with Lightwave Instruments

Item Agilent Technologies Part Number

Laser shutter cap 08145-64521

FC/PC dust cap 08154-44102

Biconic dust cap 08154-44105

DIN dust cap 5040-9364

HMS10/dust cap 5040-9361

ST dust cap 5040-9366

To clean a non-lensed connector

CAUTION

Do not use any type of foam swab to clean optical fiber ends. Foam swabs can leave filmy deposits on fiber ends that can degrade performance.

Apply pure isopropyl alcohol to a clean lint-free cotton swab or lens paper.

Cotton swabs can be used as long as no cotton fibers remain on the fiber end after cleaning.

Clean the ferrules and other parts of the connector while avoiding the end of the fiber.

Apply isopropyl alcohol to a new clean lint-free cotton swab or lens paper.

Clean the fiber end with the swab or lens paper.

scrub during this initial cleaning because grit can be caught in the

not

swab and become a gouging element.

Immediately dry the fiber end with a clean, dry, lint-free cotton swab or lens paper.

Blow across the connector end face from a distance of 6 to 8 inches using filtered, dry, compressed air. Aim the compressed air at a shallow angle to the fiber end face.

Nitrogen gas or compressed dust remover can also be used.

1-21

Getting Started

Cleaning Connections for Accurate Measurements

CAUTION

Do not shake, tip, or invert compressed air canisters, because this releases particles in the can into the air. Refer to instructions provided on the compressed air canister.

As soon as the connector is dry, connect or cover it for later use.

If the performance, after the initial cleaning, seems poor try cleaning the connector again. Often a second cleaning will restore proper performance. The second cleaning should be more arduous with a scrubbing action.

To clean an adapter

The fiber-optic input and output connectors on many Agilent Technologies instruments employ a universal adapter such as those shown in the following picture. These adapters allow you to connect the instrument to different types of fiber-optic cables.

Figure 1-7. Universal adapters.

Apply isopropyl alcohol to a clean foam swab.

Cotton swabs can be used as long as no cotton fibers remain after cleaning. The foam swabs listed in this section’s introduction are small enough to fit into adapters.

Although foam swabs can leave filmy deposits, these deposits are very thin, and the risk of other contamination buildup on the inside of adapters greatly outweighs the risk of contamination by foam swabs.

Clean the adapter with the foam swab.

Dry the inside of the adapter with a clean, dry, foam swab.

Blow through the adapter using filtered, dry, compressed air.

Nitrogen gas or compressed dust remover can also be used. Do not shake, tip, or invert compressed air canisters, because this releases particles in the can into the air. Refer to instructions provided on the compressed air canister.

1-22

Getting Started

Returning the Instrument for Service

The instructions in this section show you how to properly return the instrument for repair or calibration. Always call the Agilent Technologies Instrument Support Center first to initiate service service office. This ensures that the repair (or calibration) can be properly tracked and that your instrument will be returned to you as quickly as possible. Call this number regardless of where you are located. Refer to “Agilent

Technologies Service Offices” on page 8-18 for a list of service offices.

Agilent Technologies Instrument Support Center. . . . . . . . . . . (800) 403-0801

If the instrument is still under warranty or is covered by an Agilent Technologies maintenance contract, it will be repaired under the terms of the warranty or contract (the warranty is at the front of this manual). If the instrument is no longer under warranty or is not covered by an Agilent Technologies maintenance plan, Agilent Technologies will notify you of the cost of the repair after examining the unit.

When an instrument is returned to a Agilent Technologies service office for servicing, it must be adequately packaged and have a complete description of the failure symptoms attached. When describing the failure, please be as specific as possible about the nature of the problem. Include copies of additional failure information (such as the instrument failure settings, data related to instrument failure, and error messages) along with the instrument being returned.

returning your instrument to a

before

Preparing the instrument for shipping

Write a complete description of the failure and attach it to the instrument. Include any specific performance details related to the problem. The following

1-23

Getting Started

Returning the Instrument for Service

information should be returned with the instrument.

• Type of service required.

• Date instrument was returned for repair.

• Description of the problem:

• Whether problem is constant or intermittent.

• Whether instrument is temperature-sensitive.

• Whether instrument is vibration-sensitive.

• Instrument settings required to reproduce the problem.

• Performance data.

• Company name and return address.

• Name and phone number of technical contact person.

• Model number of returned instrument.

• Full serial number of returned instrument.

• List of any accessories returned with instrument.

Cover all front or rear-panel connectors that were originally covered when you first received the instrument.

CAUTION

Cover electrical connectors to protect sensitive components from electrostatic damage. Cover optical connectors to protect them from damage due to physical contact or dust.

Instrument damage can result from using packaging materials other than the original materials. Never use styrene pellets as packaging material. They do not adequately cushion the instrument or prevent it from shifting in the carton. They may also cause instrument damage by generating static electricity.

Pack the instrument in the original shipping containers. Original materials are available through any Agilent Technologies office. Or, use the following guidelines:

• Wrap the instrument in antistatic plastic to reduce the possibility of damage caused by electrostatic discharge.

• For instruments weighing less than 54 kg (120 lb), use a double-walled, corrugated cardboard carton of 159 kg (350 lb) test strength.

• The carton must be large enough to allow approximately 7 cm (3 inches) on all sides of the instrument for packing material, and strong enough to accommodate the weight of the instrument.

• Surround the equipment with approximately 7 cm (3 inches) of packing material, to protect the instrument and prevent it from moving in the carton. If packing foam is not available, the best alternative is S.D-240 Air Cap™ from

1-24

Returning the Instrument for Service

Sealed Air Corporation (Commerce, California 90001). Air Cap looks like a plastic sheet filled with air bubbles. Use the pink (antistatic) Air Cap™ to reduce static electricity. Wrapping the instrument several times in this material will protect the instrument and prevent it from moving in the carton.

Seal the carton with strong nylon adhesive tape.

Mark the carton “FRAGILE, HANDLE WITH CARE”.

Retain copies of all shipping papers.

Getting Started

1-25

Using the Multi-Wavelength Meter

In this chapter, you’ll learn how to make a variety of fast, accurate measurements. As you perform these measurements, keep in mind the following points:

• 700 nm to 1650 nm maximum input wavelength range

The range is normally limited from 1200 nm to 1650 nm. To use the full range,

refer to “Measuring lasers between 700 nm and 1200 nm” on page 2-8 and to “To use the full wavelength range” on page 2-8.

• +10 dBm maximum total displayed input power

• Laser linewidths assumed to be less than 10 GHz

CAUTION

• If you change the elevation where you will be using your Agilent 86120B, refer to “Calibrating Measurements” on page 2-26.

• Press the green

Do not exceed +18 dBm source power. The Agilent 86120B’s input circuitry can be damaged when total input power exceeds 18 dBm. You can measure power levels that are greater by adding attenuation and entering a power offset as described in “To measure total power exceeding 10 dBm” on page 2-25.

2-2

Preset

key to return the Agilent 86120B to its default state.

Using the Multi-Wavelength Meter

Displaying Wavelength and Power

This section gives you step-by-step instructions for measuring peak wavelength, average wavelength, peak power, and total input power. There are three display modes:

• Peak wavelength

• List-by-wavelength or power

• Average wavelength and total power

If the measured amplitudes are low, clean the front-panel nector.

This section includes:

Peak WL mode 2-4 List by WL or power modes 2-6 Total power and average wavelength 2-7 Measuring lasers between 700 nm and 1200 nm 2-8 Limiting the wavelength range 2-9 Measuring broadband devices and chirped lasers 2-10 Graphical display of optical power spectrum 2-11 Instrument states 2-12 Power bar 2-12

OPTICAL INPUT

con-

2-3

Using the Multi-Wavelength Meter

Displaying Wavelength and Power

Peak WL mode

Peak WL

When spectrum. This is the peak wavelength mode. The word screen. If multiple laser lines are present at the input, the number of lines located will be shown along the right side of the screen.

is pressed, the display shows the largest amplitude line in the

PEAK

is shown on the

Display after “Peak WL” key pressed

In addition to the digital readouts, there is a power bar. It provides a convenient analog “meter movement” for tuning laser power.

Although the that allow you to scroll through and display all the measured laser lines. You can scroll through the list according to the wavelengths or powers measured. The signals are displayed in order from shortest to longest wavelengths. The Agilent 86120B can measure up to 100 laser lines simultaneously.

To display peak wavelength and power

Connect the fiber-optic cable to the front-panel

To display the peak wavelength and power, do one of the following:

• Press the green

•Press

2-4

Peak WL

mode shows one signal at a time, softkeys are provided

Preset

OPTICAL INPUT

key.

connector.

To move the cursor to view other signals, press:

PREV WL

•

NEXT WL

•

PEAK

•

PREV PK

•

NEXT PK

•

to select next (previous) shorter wavelength.

to select next longer wavelength.

to signal with greatest power.

to select next lower power signal.

to select next higher power signal.

Using the Multi-Wavelength Meter

Displaying Wavelength and Power

2-5

Using the Multi-Wavelength Meter

Displaying Wavelength and Power

List by WL or power modes

In the list-by-wavelength or list-by-power modes, the measurements of five laser lines can be displayed at any one time. In list by wavelength mode, the signals are displayed in order from shortest to longest wavelengths. The

Agilent 86120B can measure up to 100 laser lines simultaneously. Use the

and softkeys to move the cursor through the list of signals; the list can contain up to 100 entries. Press the peak wavelength mode with the signal at the cursor displayed.

Annotation in the upper right corner of the display indicates whether the signals are ordered according to wavelength ( cursor shows the currently selected laser line. As you scroll through the responses, the current position of the selection cursor is shown along the screen’s right side.

SELECT

key, and the display changes to

BY WL

) or power (

BY PWR

). The

Display after “List by WL” key pressed

Also notice that power bars graphically show the relative power levels between laser lines.

To display multiple laser lines

Connect the fiber-optic cable to the front-panel

Press the green

List by WL

Press longest wavelength.

2-6

Preset

key.

to display the laser lines from the shortest wavelength to the

OPTICAL INPUT

connector.

Using the Multi-Wavelength Meter

Displaying Wavelength and Power

Press

List by Power

to display the laser lines in order of decreasing amplitudes.

Total power and average wavelength

In the third available display mode, the Agilent 86120B displays the average wavelength as shown in the following figure. The displayed power level is the total input power to the instrument. It is the sum of the powers of each laser line; it is

The following equation shows how individual wavelengths of laser lines are summed together to obtain the average wavelength value:

a measure of the average power level of the laser lines.

not

iλi

∑

i 1

avg

--------------------=

∑

i 1

where,

is the number of laser lines included in the average.

is the peak power of an individual laser line. Power units are in Watts (lin-

ear).

The following equation shows how individual powers of laser lines are summed together to obtain the total power value:

total

∑

i 1

2-7

Using the Multi-Wavelength Meter

Displaying Wavelength and Power

where,

is the number of laser lines included in the measurement.

is the peak power of an individual laser line. Power units are in Watts (lin-

ear).

To display average wavelength and total power

•Press the

Avg WL

key.

Measuring lasers between 700 nm and 1200 nm

After the suring lasers between 1200 nm and 1650 nm. This prevents the accidental display of spurious signals that may not exist. You can easily expand the input range to the full 700 nm to 1650 nm range, however you should learn how to identify spurious signals.

Spurious signals below 1200 nm may be displayed whenever low-power laser lines (power levels near the Agilent 86120B’s specified sensitivity) are present at the input. For example, a low-power laser line at 1550 nm has a second harmonic line at 775 nm. If this second harmonic is above the peak threshold level relative to the fundamental line, it is considered a peak. Its displayed power level may be rection at 775 nm is much greater (by about 15 dB) than that at 1550 nm (the interferometer is less sensitive at 775 nm).

You can also avoid displaying this second harmonic line by reducing the peak threshold below its preset value. Because the peak threshold level is used to determine which signals are to be displayed applied, the harmonic will be eliminated. Refer to “Defining Laser-Line Peaks”

on page 2-16.

Preset

key is pressed, the input wavelength range is limited to mea-

greater

than that of the fundamental because the amplitude cor-

amplitude corrections are

before

To use the full wavelength range

Press the

Press

Setup

WL LIM

LIM OFF

key.

softkey.

to remove the limits on wavelength range. All responses in the full

700 nm to 1650 nm range are now displayed.

2-8

Using the Multi-Wavelength Meter

Displaying Wavelength and Power

Limiting the wavelength range

The wavelength range of measurement can be limited with the wavelength limit function. Both start and stop wavelengths can be chosen. The units of wavelength start and stop are the same as the currently selected wavelength units. If wavelength units are later changed, the start and stop wavelength units will change accordingly. Note that a start wavelength limit in nm will become a stop wavelength limit if THz or cm

units of measure” on page 2-13.

The wavelength limit can be useful when laser modulation causes spurious wavelengths to be displayed. Reducing the wavelength range to the region of interest minimizes the number of spurious wavelengths displayed. Also, the graphical display uses these start and stop wavelength values to plot the power spectrum, whether the wavelength limit function is on or off.

Preset will turn the wavelength limit on and will set the start wavelength to 1200 nm and the stop wavelength to 1650 nm.

To limit the wavelength range

-1

is chosen. See “To change the

Press the

Setup

key.

WL LIM

LIM ON

STARTWL

STOP WL

softkey.

softkey if it is not already highlighted.

softkey to adjust the start wavelength value.

softkey to adjust the stop wavelength value.

2-9

Using the Multi-Wavelength Meter

Displaying Wavelength and Power

Measuring broadband devices and chirped lasers

When first turned on (or the green Preset key is pressed), the Agilent 86120B is configured to measure narrowband devices such as DFB lasers and modes of FP lasers. If you plan to measure broadband devices such as LEDs, optical filters, and chirped lasers, use the ment. When broadband devices are selected, the display shows the annotation on the screen.

The measurement algorithm for broadband devices determines the wavelength based upon the center-of-mass of the power spectrum. The sion function is used to determine the value of the integration limits. Care must be taken to ensure that the integration limits are above any noise. This is especially true when measuring devices with sloping noise floors, like an EDFA amplifier. For more information on peak excursion, refer to “Defining

Laser-Line Peaks” on page 2-16.

Instrument specifications apply when the Agilent 86120B is configured to measure narrowband devices. Specifications do not apply when the instrument is configured to measure broadband devices.

Setup

menu first to reconfigure the instru-

BROAD

peak excur-

This feature applies to Agilent 86120B instruments with firmware version number 2.0. When first turned on, the instrument briefly displays the firmware version. Instruments with a firmware version number less than 2.0 do not have this feature.

To measure broadband devices

Press the

Press

Press the

To return to measuring narrowband devices, press

2-10

Setup

key.

twice, and then the

BROAD

softkey.

DEVICE

softkey.

NARROW

Using the Multi-Wavelength Meter

Displaying Wavelength and Power

Graphical display of optical power spectrum

A graphical display of optical power versus wavelength is shown from the start wavelength value to the stop wavelength value. The start wavelength value is shown in the upper left corner of the graphical display, and the stop wavelength value is shown in the upper right corner of the graphical display. The power scale is a fixed dB scale, with +10 dBm at the display top and –53 dBm at the display bottom. The power scale is not affected by the Power Offset value. In most cases, the noise floor will be visible if the total input power is greater than about –5 dBm.

The Agilent 86120B graphical display.

The Peak Threshold value is displayed as a dotted line. All peaks above this dotted line are displayed in the List by Wavelength and List by Power modes. All peaks below this line are not displayed. Adjust the Peak Threshold value with the

The wavelength limit start and stop wavelength values are used for the graphical display even if the wavelength limit function is off.

The graphical display cannot be printed.

To see the graphical display

Press the

To exit the graphical display, press any softkey.

Setup

key, and the

List by WL or List by Power

GRAPH s

oftkey.

THRSHLD

key.

softkey.

2-11

Using the Multi-Wavelength Meter

Displaying Wavelength and Power

Instrument states

Four different instrument states can be saved and recalled at a later time. The actual instrument conditions that are saved are identical to those saved from the previous state after power is turned on. These conditions are shown in

Table 8-1 on page 8-2. If drift measurements or an application (such as signal-

to-noise) is on when an instrument state is saved, it is off when that state is recalled.

To save an instrument state

Press the

Press one of the four

To recall a state

Press the

Press one of the four

Setup

key.

SAV/RCL

SAVE

softkey.

Setup

key.

SAV/RCL s

RECALL

softkey.

oftkey.

softkey.

Power bar

To control the power bar

Press the

Press

Press display.

Setup

key.

twice, and then

BAR ON

to display the power bar, and press

SAVE

softkeys to save the instrument state.

RCL

softkeys to recall an instrument state.

PWR BAR

BAR OFF

to hide the power bar

2-12

Using the Multi-Wavelength Meter

Changing the Units and Measurement Rate

This section includes step-by-step instructions for changing the units and measurement rate.

This section includes:

Displayed units 2-13 Measurement rate 2-14 Continuous or single measurements 2-15

Displayed units

As described below, it’s easy to change the wavelength and amplitude units. You can choose between the following units:

Table 2-1. Available Units

Wavelength Power

nm dBm

–1

THz

To change the units of measure

Setup

Press

Press the

UNITS

softkey.

2-13

Using the Multi-Wavelength Meter

Changing the Units and Measurement Rate

Press WL and select one of the following units. Then, press

RETURN

to complete

your selection:

•

for nanometers

THZ

•

for terahertz

–1

•

Press

•

for wave number

POWER

and select one of the following units:

DBM

for decibels relative to a milliwatt

for milliwatts

for microwatts

Measurement rate

Under normal operation, the Agilent 86120B makes a measurement and displays the results about once every second. It is in this normal update mode that maximum accuracy and wavelength resolution are achieved. However, should a faster update be desired, for example when real-time feedback is required to tune a laser to its designated channel, the Agilent 86120B can be set to update approximately three times per second. This reduces both wavelength resolution and accuracy but can be beneficial in some applications.

FAST

When multiple wavelengths are present, these individual responses, with the reduced resolution, may no longer be recognized.

update is selected, one less digit of resolution is displayed. Also, if

To change the measurement speed

Press the

Select either

2-14

Setup

key.

softkey.

UPDATE

NORMAL

softkey.

FAST

Using the Multi-Wavelength Meter

Changing the Units and Measurement Rate

Continuous or single measurements

The Agilent 86120B continuously measures the input spectrum at the front-

OPTICAL INPUT

panel an asterisk (*) is displayed in the display’s upper right corner. When you switch between normal and fast update modes the rate that the asterisk blinks changes.

You can specify that the instrument perform a measurement only when the front-panel mode, and it is useful for capturing and preserving data. After capturing the data, you can display it using many of the procedures included in this chapter. You can return to continuous measurement mode at any time by pressing the

Cont

key.

To select single measurement acquisition

Single

connector. Whenever measurements are being acquired,

key is pressed. This is the single-acquisition measurement

•Press the

Single

key.

2-15

Using the Multi-Wavelength Meter

Defining Laser-Line Peaks

The Agilent 86120B uses two rules to identify valid laser-line peaks. Understanding these rules is essential to getting the most from your measurements. For example, these rules allow you to “hide” AM modulation sidebands or locate laser lines with small amplitudes.

In order to identify a laser line, the laser-line must meet rules:

of the following

both

Peak threshold limit

Peak excursion

• Power must be greater than the power established by the

• Power must rise and then fall by at least the

In addition, the input wavelength range can be limited as described in this section.

The peak threshold limit is set by subtracting the peak threshold value from the power of the largest laser line. So, if the largest laser line is 2 dBm and the peak threshold value is 10 dB, the peak threshold limit is –8 dBm

()

8 dBm–2 dBm 10 dB

40 dB.

The peak threshold’s default value is 10 dB. This ensures that any modulated signals being measured are not confused with their AM sidebands. For unmodulated lasers, or Fabry-Perot lasers, it may be desirable to increase this threshold to look for responses that are more than 10 dB from the peak.

Peak threshold can be used to suppress spurious signals. For example, a laser that is amplitude modulated in the audio frequency range can cause spurious wavelengths to be displayed below and above the correct wavelength. The power of these spurious wavelengths is below that of the correct wavelength. These spurious signals can be eliminated by decreasing Peak threshold from its Preset value.

The peak excursion defines the rise and fall in amplitude that must take place in order for a laser line to be recognized. The rise and fall can be out of the noise, or in the case of two closely spaced signals, out of the filter skirts of the

–=

. You can set the peak threshold value between 0 to

peak excursion

peak threshold limit

value

2-16

Using the Multi-Wavelength Meter

Defining Laser-Line Peaks

adjacent signal. The peak excursion’s default value is 15 dB. Any laser line that rises by 15 dB and then falls by 15 dB passes the rule. You can set the peak excursion value from 1 to 30 dB.

Examples of valid and invalid signals

In the following figure, three laser lines are identified: responses ➀, ➂, and ➃. Response ➁ is not identified because it is below the peak threshold. The portion of each signal that is within the peak excursion limits is shown in bold lines.

Because of the peak excursion rule, responses ➃ and ➄ are identified as one laser line—the minimum point between ➃ and ➄ does not drop to the peak excursion limit. This response has the highest power shown which is peak ➃.

Whenever the peak threshold limit or peak excursion value is changed, the new limits are applied to the current displayed measurements even if the instrument is in the

Single

measurement mode.

The following figure shows the same laser lines as the previous figure, but the peak-excursion value has been changed from 15 to 3 dB. Four laser lines are now identified with responses ➃ and ➄ identified as two distinct laser lines.

2-17

Using the Multi-Wavelength Meter

Defining Laser-Line Peaks

Limiting the input wavelength range

The Agilent 86120B’s preset condition limits the wavelength measurement range from 1200 nm to 1650 nm. You can expand the wavelength range to cover the entire 700 nm to 1650 nm range. Although wavelength range limiting reduces the number of laser lines found, its main purpose is to eliminate the identification of second harmonic distortion products as described in the following sidebar. To set the wavelength range, refer to “To use the full wave-

length range” on page 2-8

Distortion caused by low-power laser lines

Low-power laser lines (power level near the Agilent 86120B’s specified sensitivity) may be accompanied by second harmonic (or other) distortion. For example, a low-power laser line at 1550 nm has a second harmonic line at 775 nm. If this second harmonic is above the peak threshold level relative to the fundamental line, it is considered a peak. Its displayed power level may be

than that of the fundamental because the

greater

amplitude correction at 775 nm is much greater (by about 15 dB) than that at 1550 nm (the interferometer is less sensitive at 775 nm).

To avoid displaying this second harmonic line, limit the input wavelength range from 1200 nm to 1650 nm as described in “To use the full wavelength range” on page 2-8.

Or, reduce the peak threshold below its preset value. Because the peak threshold level is used to determine which signals are to be displayed

amplitude corrections are

before

applied, the harmonic will be eliminated.

2-18

To define laser-line peaks

Using the Multi-Wavelength Meter

Defining Laser-Line Peaks

Press the

Press

Setup

key.

THRSHLD

PX EXC

softkey.

, and enter the peak excursion value. Use the softkey to select

the digit that requires editing. Use the and softkeys to change the value.

The peak excursion value can range from 1 to 30 dB. The default value is 15 dB.

Press

RETURN

PK THLD

and then enter the peak threshold value.

The peak threshold value can range from 0 to 40 dB. Setting this value to 0 dB ensures that only the peak wavelength is identified. The default value is 10 dB.

Pressing the green

PRESET

key changes the peak excursion and peak threshold values to their default settings. It also turns wavelength range limiting on. Turning the Agilent 86120B’s power off and then on does not change these settings.

If too many lines are identified

If the following message is displayed, too many laser lines have been identified:

E15 MAX NUMBER OF SIGNALS FOUND

The maximum number of laser lines that the instrument can measure is 100. If this message appears, decrease the peak threshold value, increase the peak excursion value, or decrease the wavelength range of operation with the

functions.

WL LIM

....

START WL

and

STOP

2-19

Using the Multi-Wavelength Meter

Measuring Laser Separation

It is often important to measure the wavelength and power separation between multiple laser lines. This is especially true in wavelength-divisionmultiplexed (WDM) systems where channel spacing must be adhered to. The Agilent 86120B can display the wavelength and amplitude of any laser line relative to another. In fact, the following types of relative measurements can be made compared to the reference:

• Relative wavelength, absolute power

• Relative power, absolute wavelength

• Relative wavelength and power

This section includes:

Channel separation 2-21 Measuring flatness 2-22

2-20

Using the Multi-Wavelength Meter

Measuring Laser Separation

Channel separation

Suppose that you want to measure separation on a system having the spectrum shown in the following figure.

The Agilent 86120B displays separation on this spectrum as shown in the following figure. Notice that the 1541.747 nm laser line is selected as the reference. It is shown in absolute units. The wavelengths and powers of the remaining responses are shown relative to this reference. For example, the first response is 2.596 nm below the reference.

To determine channel spacing, simply read the relative wavelength measurement of the laser lines immediately preceding and following the reference. Use

the , , and channel spacing between each channel.

SELECT

softkeys to change the reference laser line and read the

2-21

Using the Multi-Wavelength Meter

Measuring Laser Separation

To measure channel separation

Press the front-panel

List by WL

Press

Press the Delta

Use the

Select the type of separation to observe:

•∆ WL displays channel separation.

WL /

•∆

Use the and softkeys to select the reference laser line.

SELECT

Press

SELECT

Press turn off the delta calculation.

Off

key to turn off the measurement.

∆

PWR

at any time to select a new reference. Press

Preset

key.

displays both channel separation and differences in power.

RESET

at any time to

Measuring flatness

You can use relative power measurements to measure flatness (pre-emphasis) in a WDM system. Simply select one carrier as the reference and measure the remaining carriers relative to the reference level. The power differences represent the system flatness. Press that all responses are shown in absolute wavelength and powers.

RESET

to turn off the delta calculations so

To measure flatness

key.

Preset

key.

Press the front-panel

List by Power

Press

This lists the input signals by power with the largest response listed first.

Press the Delta

Select ∆

Use the and softkeys to select the first laser line.

Press

Since the largest power signal is the reference, the relative power measurements for the other responses shows system flatness.

2-22

PWR

SELECT

Measuring Modulated Lasers

Using the Multi-Wavelength Meter

Measuring Modulated Lasers

Lasers modulated at low frequencies

A laser that is amplitude modulated at low frequencies (for example, modulated in the audio frequency range) can cause spurious wavelengths to be displayed below and above the correct wavelength. The power of these spurious wavelengths is below that of the correct wavelength. These spurious signals can be eliminated by decreasing the peak threshold. Refer to “Defining Laser-

Line Peaks” on page 2-16. Even when the laser is amplitude modulated, the

correct wavelength and power is displayed.

The spurious wavelengths caused by low frequency amplitude modulation will be located above and below the correct wavelength by the following wavelength spacing:

–

spacing

where F is the modulation frequency in Hz, and λ is the correct wavelength in nm. For example, an amplitude modulation of 10 kHz on a 1550 nm laser will produce spurious wavelengths spaced by 15 nm from the correct wavelength, and the spurious wavelengths will be at 1535 and 1565 nm.

Low frequency (10 kHz) AM modulation graph showing rounded sideband spurs.

2-23

Using the Multi-Wavelength Meter

Measuring Modulated Lasers

The graphical display is useful for locating these spurious wavelengths. Their amplitude will be below that of the correct wavelength and they will be broad, rounded peaks compared to the sharp peak of the correct wavelength. Use the Peak Threshold function to place the dotted line above the spurious peaks so they will not be displayed in the List by WL or List by Power table.

Lasers modulated at high frequencies

A laser modulated at high frequency (in the RF or microwave range) can also cause spurious wavelengths to be displayed, especially when the modulation is of a repetitive nature such as that of PRBS or SONET digital formats. In general, no spurious wavelengths will be displayed using preset instrument conditions. The preset condition includes peak excursion, peak threshold, and wavelength range limiting. However, increasing peak threshold can cause spurious wavelengths to be displayed. To control the wavelength range, refer to

“To use the full wavelength range” on page 2-8.

Even when the laser being tested is modulated with repetitive formats, the carrier’s correct wavelength and power is displayed; the wavelength and power of the spurious sidebands are incorrect.

The graphical display is useful to see the effects of high frequency modulation. Without modulation, the noise floor is typically 45 dB below the laser power. In general, high frequency modulation will raise the noise floor to about 25 dB below the laser power. The noise floor is typically flat, or white. The actual level of the noise floor depends on the type of data format and the data rate.

Directly modulated lasers

PRBS modulation graph showing raised noise floor.

Directly modulated lasers exhibit a linewidth that is broadband. To measure directly modulated lasers, refer to “Measuring broadband devices and chirped

lasers” on page 2-10.

2-24

Using the Multi-Wavelength Meter

Measuring Total Power Greater than 10 dBm

The maximum total power that can be measured by the Agilent 86120B is 10 dBm. However, with the addition of an external attenuator, more power can be applied. This may be necessary at the transmit end of a wavelength-division-multiplexed system where large signal levels are present. By entering an amplitude offset equal to the amount of attenuation at the instrument’s input, accurate amplitude measurements are shown on the display. Additional amplification can also be accounted for.

To measure total power exceeding 10 dBm

CAUTION

The maximum total input power that can be applied to the Agilent 86120B before damage occurs is 18 dBm. The maximum total input power that can be measured is 10 dBm.

Connect an optical attenuator between the front-panel and the fiber-optic cable.

The attenuator must reduce the total input power to the Agilent 86120B so that it is below +10 dBm.

Setup, MORE, CAL

Press

Notice that the is applied.

Use the softkey to select the digit that requires editing.

Use the and softkeys to change the value.

Power offset values are added to the display power readings. For example, if you placed a 10 dB attenuator on the front-panel connector, enter a power offset value of +10 dB. Negative values can also be entered if you connect an amplifier instead of an attenuator.

, and then

PWR OFS

annotation appears on the screen to indicate an offset

PWR OFS

OPTICAL INPUT

connector

2-25

Using the Multi-Wavelength Meter

Calibrating Measurements

The wavelength of light changes depending on the material that the light is passing through. To display meaningful wavelength measurements, the Agilent 86120B performs two steps:

Measures the wavelength in air.

Converts the wavelength to show values in either a vacuum or “standard air”.

For example, a laser line with a wavelength of 1550.000 nm in a vacuum would have a wavelength in standard air of 1549.577 nm.

Because all measurements made inside the Agilent 86120B are performed in air, the density of air, due to elevation, affects the wavelength results. You must calibrate the Agilent 86120B by entering the elevation. Elevations from 0 to 5000 meters can be entered. The elevation correction is immediately applied to the current measurement even if the instrument is in the single measurement acquisition mode.

Annotation on the display shows the current calibration elevation in meters and whether the wavelength measurements are shown for a vacuum ( standard air (

If you select frequency instead of wavelength measurements, switching between vacuum and standard air will not affect the measurement results. This is because the frequency of an optical signal does not change in different mediums—only the wavelength changes.

STD AIR

VAC

) or

Definition of standard air

Standard air is defined to have the following characteristics:

Barometric pressure: 760 torr Temperature: 15°C Relative humidity: 0%

2-26

To enter the elevation

Using the Multi-Wavelength Meter

Calibrating Measurements

ELEV

Setup

CAL

softkey.

key.

softkey.

Press the

Press

Use the and softkeys to enter the elevation in meters. Entries jump in 500 meter steps from 0 m to 5000 m.

In order for the Agilent 86120B to meet its published specifications, the elevation value selected with the softkeys must be within 250 meters of the actual elevation.

Press

RETURN

to complete the entry.

Converting feet to meters

If you know your elevation in feet, you can convert this value to meters by using the following equation:

---------------=

3.281

To select the medium for light

Press the

•Press

Press

Setup

CAL

VACUUM

STD AIR

RETURN

key.

softkey.

softkey, and make the following selection:

for wavelengths in a vacuum.

for wavelengths in standard air.

to complete the entry.

2-27

Using the Multi-Wavelength Meter

Printing Measurement Results

Measurement results can be sent directly to a printer. Simply connect a compatible printer to the rear-panel is ASCII text. An example of a compatible printer is Hewlett-Packard

series printer. Be sure to use a parallel printer cable to connect the

Jet printer.

The printer output is not a copy of the display. Rather, it is a listing of all signals present at the input (up to 100). The measurement values printed depend on the settings of the instrument when the

The following is an example of a typical printout:

Agilent 86120B SER US36151025 Firmware Ver. 2.000 List By Wavelength 8 Lines Power Offset 0.0 dB Vacuum Elevation 0 Meters Update Normal Peak Excursion 15 dB Peak Threshold 10 dB Device Narrow

PARALLEL PRINTER PORT

connector. The output

’s L aser-

key is pressed.

Input Wavelength Power

----------- ----------

1280.384nm -16.97dBm

1281.473 -13.14

1282.569 -13.92

1283.651 -13.34

1284.752 -11.69

1285.840 -8.11

1286.944 -10.38

1288.034 -14.65

1. Hewlett-Packard and LaserJet are registered trademarks of Hewlett-Packard Company.

2-28

To create a hardcopy

Using the Multi-Wavelength Meter

Printing Measurement Results

Connect the printer to the Agilent 86120B’s rear-panel connector.

Press

You can use the

ABORT

and

CONT

softkey to stop and restart a print job that is

in progress.

PARALLEL PRINTER PORT

2-29

Measuring Signal-to-Noise Ratios 3-3 Measuring Signal-to-Noise Ratios with Averaging 3-7 Measuring Laser Drift 3-9 Measuring Coherence Length 3-12

Measurements Applications

In this chapter, you’ll learn how to make a variety of fast, accurate measurements using the measurement tools accessed by pressing the

Appl’s

key.

3-2

Measurements Applications

Measuring Signal-to-Noise Ratios

Signal-to-noise measurements provide a direct indication of system performance. Signal-to-noise measurements are especially important in WDM systems because there is a direct relation between signal-to-noise and bit error rate. The Agilent 86120B displays signal-to-noise measurements in the third column. For example, the selected signal in the following figure has a signalto-noise ratio of 30.0 dB.

Signal-to-noise display.

During a signal-to-noise measurement, the absolute power of the carrier, in dBm, is compared to the absolute power of the noise at the carrier wavelength. See the following figure. The noise power at the carrier must be determined by interpolation because the carrier, in most cases, can not or should not be turned off.

You can select one of two methods used to determine the wavelength where the noise is measured: automatic interpolation or a user-entered wavelength. In the figure above, notice that “ automatic interpolation is selected.

S/N AUTO”

is displayed to indicate that

3-3

Measurements Applications

Measuring Signal-to-Noise Ratios

Location of noise measurements

Automatic interpolation

When the signal-to-noise “auto” function is selected, the Agilent 86120B first determines the proximity of any adjacent signal. If the next closest signal is

≤

200 GHz (approximately 1.6 nm at 1550 nm) away from the signal of interest, then the noise power is measured half way between the two channels and an equal distance to the other side of the signal of interest. See points P

and Pn2

in the following figure.

If the closest signal is more than 200 GHz from the signal of interest, or if there is no other signals present, then the noise power is measured at 100 GHz on either side of the signal of interest. The two measured noise power levels are then averaged to estimate the noise power level at the signal wavelength. The noise power measurements use linear interpolation to estimate the noise power level at the signal of interest’s wavelength.

Automatic interpolation

3-4

Measurements Applications

Measuring Signal-to-Noise Ratios

User-entered wavelength

Noise bandwidth

When the signal-to-noise “user” function is selected, the Agilent 86120B uses only one wavelength to measure the noise power for all signals. This wavelength is set by the user and all signals are compared to the noise level at this wavelength to determine their corresponding signal-to-noise ratios.

When measuring noise power, the Agilent 86120B must account for the noise bandwidth used during the measurement. Because noise bandwidth varies with measurement bandwidth (a wide bandwidth allows more noise to the Agilent 86120B’s detector than a narrow bandwidth), the Agilent 86120B normalizes all noise power measurements to a bandwidth of 0.1 nm. The annota-

0.1 nm

tion

is displayed to show that the noise bandwidth is being

normalized to a 0.1 nm bandwidth.

Repetitive data formats

The Agilent 86120B signal-to-noise application works best when the laser being tested is not modulated, or modulated with non-repetitive data formats. With repetitive data formats, such as PRBS data and SONET formats, there is significant low-frequency amplitude modulation of the laser. This modulation raises the noise floor of the Agilent 86120B significantly. The signal-to-noise measured can be limited to about 15 dB while measuring lasers modulated by repetitive data formats. For improved performance when the laser is modulated with repetitive data formats, use the Signal-toNoise with Averaging application.

To measure signal-to-noise

Press the front-panel

List by WL or List by Power

Press

Appl’s

Press

and then

Preset

S/N

key.

3-5

Measurements Applications

Measuring Signal-to-Noise Ratios

To select the wavelength reference for measuring the noise, do the following steps:

WL REF

Press

•press

, and

AUTO

to let the instrument interpolate the wavelength,

•press

If you chose

USER

to select the last wavelength manually entered.

USER

, you can specify the wavelength by pressing the softkey to select the digit that requires editing. Use the and softkeys to change the value.

While the signal-to-noise measurements are displayed, you can press

Press

RETURN

anytime to select the signal with the highest power.

USER WL

PEAK

. Use

3-6

Measurements Applications

Measuring Signal-to-Noise Ratios with Averaging

When the lasers being measured are modulated, especially with repetitive data formats such as SONET or PRBS, the noise floor is raised. Averaging reduces the noise floor and allows an improvement of greater than 10 dB in a signal-to-noise measurement. In general, averaging will decrease the noise floor caused by modulation until the true optical noise level is reached. The displayed signal-to-noise will improve with each average until the true optical noise level is reached, and then the displayed signal-to-noise will remain approximately constant. If, however, the true signal-to-noise is below the instrument sensitivity of approximately 40 dB (in a 0.1 nm noise bandwidth), it will not be measured.

Averaging can also improve the accuracy of measuring signal-to-noise of unmodulated lasers.

Signal-to-noise with averaging display.

Averaging is performed on the noise, not on the wavelength or power of the laser signals.

The signal-to-noise with averaging measurement uses the automatic interpolation method to determine the wavelengths where the noise is measured. Refer

to “Measuring Signal-to-Noise Ratios” on page 3-3 for a description of auto-

matic interpolation. There is no user-entered wavelength selection in signalto-noise with averaging.

3-7

Measurements Applications

Measuring Signal-to-Noise Ratios with Averaging

During a signal-to-noise with averaging measurement, the display indicates

S/N A xx,

where A indicates averaging and xx is the number of averages taken so far. The maximum number of averages is 900, the minimum number of averages is 10, and the default (Preset) value is 100 averages. A measurement with 100 averages takes about 2 minutes to complete. When the measurement is complete, the instrument switches to single measurement mode. Then, pressing the

Cont

key will start a completely new measurement. During a measurement and before the number of averages has been reached, pressing the

Single

key will stop the measurement. Then, pressing the

Cont

key will con-

tinue with the current measurement.

While making a signal-to-noise with averaging measurement, the number of averages can be changed. As long as the new number of averages is greater than the number of averages taken so far, the measurement continues. If the new number of averages selected is less than the number of averages taken so far, the measurement stops and the instrument switches to single measurement mode. Then, pressing the

Cont

key will start a completely new measure-

ment.

Noise bandwidth affects measurement

0.1 nm

tion

is displayed to show that the noise bandwidth is being

normalized to a 0.1 nm bandwidth.

To measure signal-to-noise with averaging

List by Power

Preset

S/N AVG

key.

NUM AVG

. The default (Preset) value

Press the front panel

List by WL

Press

Appl's

Press

and then

To change the number of averages, press is 100.

To stop the measurement at the current number of averages shown, press the

Single

key. Then press the

When the measurement is complete, the instrument will switch to the single

Cont

key to continue the present measurement.

measurement mode and stop.

To make a new measurement, press the

To exit, press the

EXIT

softkey, then press the

Cont

key.

Cont

key for continuous

measurement.

3-8

Measurements Applications

Measuring Laser Drift

In this section, you’ll learn how the Agilent 86120B can be used to monitor drift (changes to a laser’s wavelength and amplitude over time). Drift is measured simultaneously for every laser line that is identified at the input. The Agilent 86120B keeps track of each laser line’s initial, current, minimum, and maximum values and displays their differences relative to itself. This allows the Agilent 86120B to be used for laser transmitter evaluation, burn-in, or development. In addition, you can monitor system performance over time, temperature, or other condition.

The following display shows power and wavelength drift measured on five laser lines. The are being performed. The current relative drift values for wavelength and power are shown in items ➁ and ➂ respectively. Item ➃ indicates the absolute

reference values for the laser line indicated by the cursor . The reference values are measured before the measurement starts.

DRIFT

annotation, item ➀, tells you that drift measurements

You can restart the drift measurement at any time by pressing the key. All minimum and maximum values are reset to the reference values, and the Agilent 86120B begins to monitor drift from the current laser line values. Move the cursor up and down the listing to see the reference wavelength and power of each laser line.

RESET

soft-

3-9

Measurements Applications

Measuring Laser Drift

If measurement updating stops or the values become blanked

If, in the middle of a measurement, the number of laser lines present changes, the measurement stops until the original number of lines returns. You’ll notice that a

CLEAR

key appears and one of the following message is displayed:

E46 NUM LINES < NUM REFS E47 NUM LINES > NUM REFS

To view the data measured before the conditions changed, press CLEAR and then MAXMIN. Notice that the measurement acquisition is changed from continuous to single.

To restart testing, press CLEAR, the CONT key, and then RESET to use the new number of lines as the reference. Pressing CONT restarts continuous measurement acquisition. Or, you can restore the original number of lines on the input so that the drift measurement can continue.

To measure drift

soft-

Press the front-panel

Peak WL, List by WL

Press drift.

Appl’s

Press

Pressing

and then

DRIFT

sets the current laser-line values as the reference from which to

compare all drift.

Press

MAX-MIN

following paragraphs:

Display shows the current values of laser lines relative to

Display shows absolute maximum values since the drift

Display shows absolute minimum values since the drift

Preset

key.

List by Power

, or

DRIFT

to select the display style for observing

for the desired type of drift measurement as described in the

the wavelength and power values measured when the test was begun or the

RESET

softkey was pressed.

measurement was started. This measurement gives the

longest

wavelength and

greatest

power measured. The laser line of interest may have since drifted to a lesser value. Note that the maximum wavelength and maximum power may not have occurred simultaneously.

measurement was started. This measurement gives the

shortest

wavelength and

smallest

power measured. The

3-10

Measurements Applications

Measuring Laser Drift

laser line of interest may have since drifted to a greater value. Note that the minimum wavelength and minimum power may not have occurred simultaneously.

Display shows the

drift from the reference since the

total

drift measurement was started. Values represent the minimum wavelength and power drift values subtracted from the maximum drift values.

In the

List by WL

and

List by Power

displays, use the and softkeys to view the reference values (wavelength and power values of each laser line before the test was started).

During the measurement, you can change the display mode to

WL, List by Power

, or

Avg WL

. When

List by WL or List by Power

is selected, the signal

Peak WL, List by

list is sorted by reference values and not by the current, maximum, or minimum values.

To restart the drift measurements, press

RESET

. This resets the reference val-

ues.

3-11

Measurements Applications

Measuring Coherence Length

Coherence length is a measure of the distance over which a laser’s light retains the phase relationships of its spectrum. The Agilent 86120B measures coherence length of Fabry-Perot semiconductor diode lasers. The Agilent 86120B cannot measure coherence length of light emitting diodes (LEDs) or distributed feedback (DFB) lasers.

When you select coherence length measurements, the Agilent 86120B displays the following four values:

• Coherence length (Lc)

• Round trip optical length of diode laser cavity (2nLd)

•Alpha factor

• Beta factor

Coherence length in the region of 1 mm to 200 mm can be measured. The following figure shows a coherence length measurement.

To measure coherence length

Press the front-panel

Press

3-12

Appl’s

and then

Preset

key.

COH LEN

Measurements Applications

Measuring Coherence Length

Coherence length (L

)

The interferogram of the laser being tested is sampled and the envelope of the interferogram is found. This envelope has peaks (regions of high fringe visibility) at zero optical path delay and at delays equal to multiples of the laser cavity round-trip optical length. This is shown in the following figure of the interferogram envelope:

The amplitudes of the peaks decreases exponentially from the largest peak at zero path delay. The exponential decay constant is defined as the coherence length, Lc. The curve that connects the tops of the envelope peaks is given by the following equation:

Round trip optical length of diode laser cavity (2nLd)

OPD

------------–

decay curve

OPD is the optical path delay and Lc is the coherence length. Thus, at an optical path delay equal to the coherence length, the envelope peaks are down to

⁄

of their value at zero path delay peak. All envelope peaks found are used to determine the exponential decay constant (coherence length) using a least squares fit.

The average optical path delay spacing of the envelope peaks is measured. This is equal to the diode laser cavity round trip optical length, 2nLd.

3-13

Measurements Applications

Measuring Coherence Length

Alpha factor

Beta factor

The alpha factor is defined as the height of the first envelope peak away from zero path delay relative to the height of the envelope peak at zero path delay. The alpha factor is always between 0 and 1.

The

smaller

the alpha factor, the shorter the coherence length.

Alpha factor

------=

The beta factor is defined as the height of the fringe visibility envelope midway between the zero optical path delay peak and the next peak relative to the height of the envelope peak at zero path delay. The beta factor is always between 0 and 1.

The

smaller

the beta factor, the more longitudinal modes (wavelengths) the

laser has.

Beta factor

------=

3-14

Addressing and Initializing the Instrument 4-3

To change the GPIB address 4-3

Making Measurements 4-5

Commands are grouped in subsystems 4-7 Measurement instructions give quick results 4-9 The format of returned data 4-15

Monitoring the Instrument 4-16

Status registers 4-16

Queues 4-21 Reviewing SCPI Syntax Rules 4-23 Example Programs 4-29

Example 1. Measure a DFB laser 4-31

Example 2. Measure WDM channels 4-33

Example 3. Measure WDM channel drift 4-35

Example 4. Measure WDM channel separation 4-38

Example 5. Measure SN ratio of WDM channels 4-40

Example 6. Increase a source’s wavelength accuracy 4-42 Lists of Commands 4-44

Programming

This chapter explains how to program the Agilent 86120B. The programming syntax conforms to the IEEE 488.2 Standard Digital Interface for Programmable Instrumentation and to the Standard Commands for Programmable Instruments (SCPI).

Where to begin…

The programming examples for individual commands in this manual are written in HP

For more detailed information regarding the GPIB, the IEEE 488.2 standard, or the SCPI standard, refer to the following books:

®1

BASIC 6.0 for an HP 9000 Series 200/300 Controller.

Hewlett-Packard Company.

face Bus,

1987.

Hewlett-Packard Company.

Instruments,

1995.

International Institute of Electrical and Electronics Engineers.

Tutorial Description of Hewlett-Packard Inter-

SCPI—Standard Commands for Programmable

IEEE Standard

488.1-1987, IEEE Standard Digital Interface for Programmable Instrumentation.

International Institute of Electrical and Electronics Engineers.

New York, NY, 1987.

IEEE Standard

488.2-1987, IEEE Standard Codes, Formats, Protocols and Common commands For Use with ANSI/IEEE Std 488.1-1987.

New York, NY, 1987.

Types of commands

The Agilent 86120B responds to three types of commands:

• Common commands

• Measurement instructions

• Subsystem commands

All of these commands are documented in Chapter 5, “Programming Com-

mands”.

1. HP is a registered trademark of Hewlett-Packard Company.

4-2

Programming

Addressing and Initializing the Instrument

The Agilent 86120B’s GPIB address is configured at the factory to a value of

20. You must set the output and input functions of your programming language to send the commands to this address.

To change the GPIB address

Press the

Press

Use the and softkeys to change the GPIB address.

Press

Remote mode and front-panel lockout

Whenever the instrument is controlled by a computer, the displayed on the instrument’s screen and the softkey menu is blanked except for the panel control of the instrument.

You can specify a local lockout mode that prevents the being displayed. If the instrument is in local lockout mode, all the softkeys may be blanked. For example, if the instrument is first placed in local lockout mode and then placed in remote mode, no softkeys are displayed.

Consult the documentation for your programming environment to determine which commands are used to put an instrument in the remote and local lockout modes. These are not Agilent 86120B commands; they control GPIB control lines and do not send any characters to the Agilent 86120B.

Initialize the instrument at start of every program

It is good practice to initialize the instrument at the start of every program. This ensures that the bus and all appropriate interfaces are in a known state. HP BASIC provides a CLEAR command which clears the interface buffer and also resets the instrument’s parser. (The parser is the program that reads the instructions that you send.) Whenever the instrument is under remote pro-

Setup

key.

twice, then

RETURN

LOCAL

softkey. This softkey can be pressed by the user to restore front

GPIB

Remote

LOCAL

softkey from

message is

4-3

Programming

Addressing and Initializing the Instrument

gramming control, it should be in the single measurement acquisition mode. This is automatically accomplished when the *RST common command is used. The *RST command initializes the instrument to a preset state:

CLEAR 720 OUTPUT 720;”*RST”

Notice in the example above, that the commands are sent to an instrument address of 720. This indicates address 20 on an interface with select code 7. Pressing the green

Preset

key does not change the GPIB address.

Set single acquisition mode

An advantage of using the *RST command is that it sets the Agilent 86120B into the single measurement acquisition mode. Because the READ and MEASure data queries expect this mode, their proper operation is ensured.

Establish the wavelength range

At the start of each program, be sure to establish the input wavelength range using the Agilent 86120B’s :CALCulate2:WLIMit command. Setting this command to off enables the full wavelength range of the instrument. If you are measuring signals over a narrow wavelength range, use this command to ensure that spurious second harmonic peaks are not identified. Refer to

“WLIMit[:STATe]” on page 5-36, “WLIMit:STARt[:WAVelength]” on page 5-38,

and “WLIMit:STOP[:WAVelength]” on page 5-41. Refer also to “To limit the

wavelength range” on page 2-9.

4-4

Programming

Making Measurements

Making measurements remotely involves changing the Agilent 86120B’s settings, performing a measurement, and then returning the data to the computer. The simplified block diagram of the Agilent 86120B shown here lists some of the available programming commands. Each command is placed next to the instrument section it configures or queries data from.

Notice that there are two buffers from which data can be queried: an uncorrected data buffer and a corrected data buffer. With each scan of the input wavelength range, the analog-to-digital converter loads 65,536 data values into the uncorrected data buffer. This is considered to be one “measurement”. A fast-update measurement mode is available for quicker measurement acquisition. But, because only 8,192 data values are collected in fast-update measurement mode, the ability to resolve closely spaced signals is reduced.

After collecting the uncorrected data, the Agilent 86120B searches the data for the first 100 peak responses. (Searching starts at 1700 nm and progresses towards 700 nm for WLIMit OFF. If WLIMit is on, searching starts at

4-5

Programming

Making Measurements

WLIMit:STARt to WLIMit:STOP). These peak values are then placed into the corrected data buffer. Each peak value consists of an amplitude and wavelength measurement. Amplitude and wavelength correction factors are applied to this data.

For a listing of the programming commands (including a cross reference to front-panel keys), refer to the following tables:

Table 4-7, “Programming Commands,” on page 4-44 Table 4-8, “Keys Versus Commands,” on page 4-48

4-6

Programming

Making Measurements

Commands are grouped in subsystems

The Agilent 86120B commands are grouped in the following subsystems. You’ll find a description of each command in Chapter 5, “Programming Commands”.

Subsystem

Measurement Instructions Perform frequency, wavelength, wavenumber, and

CALCulate1 Queries

CALCulate2 Queries

CALCulate3 Performs delta, drift, and signal-to-noise measurements.

DISPlay Applies markers and displays power bars.

HCOPy Prints measurement results.

SENSe Sets elevation-correction values, selects readings for air or

STATus Queries instrument status registers.

SYSTem Presets Agilent 86120B and queries error messages.

TRIGger Stops current measurement. Acquires new measurement

UNIT Sets the amplitude units to watts or dBm.

Purpose of Commands

coherence length measurements.

uncorrected

corrected

vacuum, and enters amplitude offsets. Configures instrument for measuring broadband devices and chirped lasers. Queries time-domain values of the input data.

data. Also used to select single or continuous acquisition of measurement data.

frequency-spectrum data.

peak data and sets wavelength limits.

Table 4-1 on page 4-8 shows the kinds of measurements that the

Agilent 86120B can perform and the associated programming commands used to return that data. In some cases, there is more than one method that can be used to obtain the desired data. Refer to Chapter 5, “Programming Com-

mands” for the correct syntax for these commands.

4-7

Programming

Making Measurements

Table 4-1. Commands for Capturing Data

Desired Measurement

Command to Configure Measurement (partial listing)

Command to Query Data

Wavelength (nm) CONFigure, FETCh, READ, and MEASure MEASure:ARRay:POWer:WAVelength? Frequency (THz) CONFigure, FETCh, READ, and MEASure MEASure:ARRay:POWer:FREQuency? Wavenumber (m

–1

)

CONFigure, FETCh, READ, and MEASure MEASure:ARRay:POWer:WNUMber? Coherence Length (m) CONFigure, FETCh, READ, and MEASure FETCh, READ, or MEASure Power (W, dBm) CONFigure, FETCh, READ, and MEASure MEASure:ARRay:POWer? Average Wavelength,

CALCulate2:PWAVerage:STATe CALCulate2:DATA? Wavenumber, or Frequency

Total Power (W, dBm) CALCulate2:PWAVerage:STATe CALCulate2:DATA? Laser-Line Separation CALCulate3:DELTa:REFerence CALCulate3:DATA? Laser-Line Drift CALCulate3:DRIFt:STATe CALCulate3:DATA? Signal-to-Noise Ratio CALCulate3:SNR:STATe CALCulate3:DATA? Signal-to-Noise Ratio Average CALCulate3:ASNR:STATe CALCulate3:DATA? Time-Domain Data CALCulate1:TRANsform:FREQuency:POINts SENSe:DATA? Corrected Frequency Domain Data CALCulate1:TRANsform:FREQuency:POINts CALCulate2:DATA? Uncorrected Frequency Domain

CALCulate1:TRANsform:FREQuency:POINts CALCulate1:DATA? Data

4-8

Measurement instructions give quick results

The easiest way to measure wavelength, frequency, power, or coherence length is to use the MEASure command. The MEASure command is one of four measurement instructions: MEASure, READ, FETCh, and CONFigure. The syntax for measurement instructions

Instructions” on page 5-15.

Each measurement instruction has an argument that controls the measurement update rate. This is equivalent to using the

:MEASure command

MEASure configures the Agilent 86120B, captures new data, and queries the data all in one step. For example, to measure the longest wavelength, send the following command:

:MEASure:SCALar:POWer:WAVelength? MAX

Table 4-2. The Different Forms of MEASure

Making Measurements

documented in “Measurement

NORMAL

and

FAST

softkeys.

Programming

Desired Measurement Data

Power (W, dBm) :MEASure:ARRay:POWer? List by Power

Frequency (Hz) :MEASure:ARRay:POWer:FREQuency? List by WL

Wavelength (m) MEASure:ARRay:POWer:WAVelength? List by WL

–1

Wavenumber (m

Coherence Length (m) :MEASure:LENGth:COHerence? coherence length

)

Use this MEASure Query

:MEASure:SCALar:POWer? single wavelength mode

:MEASure:SCALar:POWer:FREQuency? single wavelength mode

MEASure:SCALar:POWer:WAVelength? single wavelength mode

:MEASure:ARRay:POWer:WNUMber? List by WL

:MEASure:SCALar:POWer:WNUMber? single wavelength mode

Display Format

(frequency)

Specifying SCALar places the display in the single wavelength format and returns a single value to the computer. Specifying ARRay places the display in

List by Power

the

List by WL

modes; an array of data is returned to the com-

puter.

4-9

Programming

Making Measurements

A common programming error is to send the :MEASure command when the instrument is in the continuous measurement acquisition mode. Because :MEASure contains an :INIT:IMM command, which expects the single measurement acquisition mode, an error is generated, and the INIT command is ignored.

:READ command

The READ command works like the MEASure command except that it does not configure the instrument’s settings. You can use the CONFigure command to configure the instrument for a particular measurement without returning any data.

The MEASure and READ commands are identical to combining the following commands:

Command

Equivalent Commands

:MEASure :ABORt;:CONFigure;:READ

:READ :ABORt;:INITiate:IMMediate;:FETCh

A common programming error is to send the :READ command when the instrument is in the continuous measurement acquisition mode. Because :READ contains an :INIT:IMM command, which expects the single measurement acquisition mode, an error is generated, and the INIT command is ignored.

:FETCh command

The FETCh command returns data from previously performed measurements; it does not initiate the collection of new data. Because FETCh does not configure the instrument or acquire new input data, you can use FETCh repeatedly on the same set of acquired data. For example, use two FETCh commands to return wavelength and then power values for the same measurement. This is shown in the following program fragment:

OUTPUT 720;”:INIT:CONT OFF;” OUTPUT 720;”:CONF:ARR:POW MAX” OUTPUT 720;”:INIT:IMM” OUTPUT 720;”:FETC:ARR:POW?” ENTER 720:powers$ OUTPUT 720;”:FETC:ARR:POW:WAV?” ENTER 720:wavelengths$

In the example above, the data in the power and wavelength arrays are returned in the same order so that powers can be matched to wavelengths.

Also, because new data is not collected, FETCh is especially useful when characterizing transient data.

4-10

Programming

Making Measurements

FETCh does not reconfigure the display. For example, if the display is in the

Peak WL List by WL

mode, sending :FETCh:ARRay does not configure the display to the

even though an array of data is returned to the computer.

A common programming error occurs when the :FETCh command is used after an *RST command. This generates error number –230,

. In this instance, you must send :INIT:IMM after the *RST command and

stale”

“Data corrupt or

before :FETCh command to capture a new array of measurement data.

:CONFigure command

The CONFigure command changes measurement settings without taking a measurement. The instrument is placed in the

List by WL, List by Ampl, Peak WL

display, or in the coherence length application.

CONFigure can be queried. The query returns the last configuration setup by the CONFigure command. The instrument returns a string which is the last instrument function sent by a CONFigure command or MEASure query. The returned string is in the short command form. Use caution when using this query, because if any instrument settings were changed since the last CONFigure command or MEASure query these changes may not be included in the returned string.

For example, if the last CONFigure command was

:CONFigure:SCALar:POWer:WAVelength 1300NM, MAX

a CONFigure? query would return a string that is similar to the following line:

“POW:WAV 1.300000e-6,0.01”

The

1300NM

and

resolution

values track the actual instrument settings and

input signals. Notice that the quotation marks are part of the returned string.

Return single or multiple measurement values

You can specify whether

FETCh, READ, or MEASure

returns a single value (SCALar) or multiple values (ARRay). The following example specifies SCALar data which returns a single value.

:MEASure:SCALar:POWer:WAVelength? MAX

4-11

Programming

Making Measurements

ARRay and the SCPI standard

According to the SCPI command reference, ARRay command causes an instrument to take multiple measurements. (A <size> parameter indicates the number of measurements to take.) However, the Agilent 86120B’s ARRay command refers to the measurements performed for one measurement sweep; this results in an array of measured signals. Because the <size> parameter does not apply, any <size> parameter sent will be ignored by the instrument. No syntax error will be generated if a <size> parameter is sent.

Always force the Agilent 86120B to wait for non-sequential commands

The Agilent 86120B normally processes its remote programming commands sequentially. The instrument waits until the actions specified by a particular command are completely finished before reading and executing the next command. However, there are a few non-sequential commands where this is not true. Non-sequential commands do

finish executing before the next com-

not

mand is interpreted.

The following is a list of the Agilent 86120B’s non-sequential commands:

:CALCulate1:TRANsform:FREQuency:POINTs :CALCulate2:PEXCursion :CALCulate2:PTHReshold :CALCulate2:WLIMit:STATe :CALCulate2:WLIMit:STARt:FREQuency :CALCulate2:WLIMit:STARt:WAVelength :CALCulate2:WLIMit:STARt:WNUMber :CALCulate2:WLIMit:STOP:FREQuency :CALCulate2:WLIMit:STOP:WAVelength :CALCulate2:WLIMit:STOP:WNUMber :CALCulate3:SNR:AUTO :SENSe:CORRection:ELEVation :INITiate:CONTinuous :INITiate[:IMMediate]

The following additional commands are also non-sequential commands if CALCulate3:SNR:AUTO is set to OFF:

:CALCulate3:REFerence:FREQuency :CALCulate3:REFerence:WAVelength :CALCulate3:REFerence:WNUMber

The benefit of non-sequential commands is that, in some situations, they can reduce the overall execution times of programs. For example, you can set the peak excursion, peak threshold, and elevation and use a *WAI command at the

4-12

Programming

Making Measurements

end to save time. However, non-sequential commands can also be a source of annoying errors. Always use the *OPC query or *WAI command with the nonsequential commands to ensure that your programs execute properly.

For example, suppose that you wanted to set the elevation correction value and then send an :INIT:IMM command. The following programming fragment results in an error

–213 “Init ignored”

. This occurs because the :ELEVation command causes the recalculation of the data which is like sending the :INIT:IMM command. When the actual :INIT:IMM is sent, the error occurs because the command is already in progress.

OUTPUT 720;”:INIT:IMM” OUTPUT 720;”:SENSe:CORRection:ELEVation 1000” OUTPUT 720;”:INIT:IMM”

Use an *OPC? query to ensure that the : as shown in the following lines:

OUTPUT 720;”:INIT:IMM” OUTPUT 720;”:SENSe:CORRection:ELEVation 1000” OUTPUT 720;”*OPC?” ENTER 720;Response$ OUTPUT 720;”:INIT:IMM”

Or, the *WAI command could be used:

OUTPUT 720;”:INIT:IMM” OUTPUT 720;”:SENSe:CORRection:ELEVation 1000” OUTPUT 720;”*WAI?” OUTPUT 720;”:INIT:IMM”

ELEVation

command has completed

4-13

Programming

Making Measurements

Measure delta, drift, and signal-to-noise

To select a measurement, use one of the following STATe commands:

CALC3:DELT:POW:STAT CALC3:DELT:WAV:STAT CALC3:DELT:WPOW:STAT CALC3:DRIF:STAT CALC3:SNR:STAT CALC3:ASNR:STAT

If you select a drift measurement, you can additionally select one of the following additional states:

CALC3:DRIF:DIFF:STAT CALC3:DRIF:MAX:STAT CALC3:DRIF:MIN:STAT CALC3:DRIF:REF:STAT

The :CALCulate3:DRIFt:PRESet command turns off the minimum, maximum, difference, and reference states but leaves the drift state on.

Attempting to turn more than one state on at a time results in a

tings Conflict”

The *RST and SYSTem:PRESet commands turn all calculations off.

CALCulate3:PRESet turns off any CALCulate3 calculations.

error.

(delta power) (delta wavelength)

(delta power and wavelength)

(drift)

(signal-to-noise ratios)

(signal-to-noise ratio averaging)

(difference)

(maximum drift)

(minimum drift)

(drift reference values)

“–221 Set-

4-14

Programming

Making Measurements

The format of returned data

Measurements are returned as strings

All measurement values are returned from the Agilent 86120B as ASCII strings. When an array is returned, the individual values are separated by the comma character.

Determine the number of data points

When a FETCh, READ, or MEASure command is used (with ARRay specified), the first returned value indicates the total number of measurement values returned in the query.

If you use the:CALCulate1:DATA?, :CALCulate2:DATA?, or :CALCulate3:DATA? queries to query data, send the :POINts? query first to determine the number of values returned in the string. The string does not contain a first value which specifies the string length. This is shown in the following example:

OUTPUT 720;”:CALCulate1:POINts?” ENTER 720;Length OUTPUT 720;”:CALCulate1:DATA?” ENTER 720;Result$

Data can be corrected for elevation and vacuum

Normally, the Agilent 86120B provides measurement values calculated for conditions in air at sea level. Use the :SENSe:CORRection:ELEVation command to compensate for air dispersion. Altitudes up to 5000 meters can be entered. Use the :SENSe:CORRection:MEDium command to switch to readings in a vacuum.

Amplitude units

The default amplitude units are dBm. If you need measurements in watts, use the :UNIT:POWer command. When the Agilent 86120B is turned on, the amplitude units are automatically set to the units used before the instrument was last turned off.

4-15

Programming

Monitoring the Instrument

Almost every program that you write will need to monitor the Agilent 86120B for its operating status. This includes querying execution or command errors and determining whether or not measurements have been completed. Several status registers and queues are provided to accomplish these tasks.

In this section, you’ll learn how to enable and read these registers. In addition to the information in this section, you should review the commands documented in “Common Commands” on page 5-3 and “STATus Subsystem” on

page 5-74.

Status registers

The Agilent 86120B provides four registers which you can query to monitor the instrument’s condition. These registers allow you to determine the following items:

• Status of an operation

• Availability of the measured data

• Reliability of the measured data

All three registers are shown in the figure on the following page and have the following uses:

Status Byte Monitors the status of the other three registers.

Standard Event Status This is the standard IEEE 488.2 register. Con-

OPERation Status Contains bits that report on the instrument’s

QUEStionable Status Contains bits that report on the condition of the

4-16

Definition

tains bits which indicate the status of the other two registers.

normal operation.

signal.

Programming

Monitoring the Instrument

Status Byte register

The Status Byte Register contains summary bits that monitor activity in the other status registers and queues. The Status Byte Register’s bits are set and cleared by the presence and absence of a summary bit from other registers or queues. Notice in the following figure that the bits in the Standard Event Status, OPERation status, and QUEStionable status registers are “or’d” to control a bit in the Status Byte Register.

If a bit in the Status Byte Register goes high, you can query the value of the source register to determine the cause.

The Status Byte Register can be read using either the *STB? common command or the GPIB serial poll command. Both commands return the decimalweighted sum of all set bits in the register. The difference between the two methods is that the serial poll command reads bit 6 as the Request Service (RQS) bit and clears the bit which clears the SRQ interrupt. The *STB? command reads bit 6 as the Master Summary Status (MSS) and does not clear the bit or have any effect on the SRQ interrupt. The value returned is the total bit weights of all of the bits that are set at the present time.

OPERation Status and QUEStionable Status registers

You can query the value of the OPERation Status and QUEStionable Status registers using commands in the STATus subsystem.

The STATus subsystem also has transition filter software which give you the ability to select the logic transitions which set bits in the OPERation Status and QUEStionable Status registers. For example, you can define the POWer bit of the QUEStionable Status register to report an event when the condition transitions from false to true. This is a positive transition. You can also specify a negative transition where the bit is set when the condition transitions from true to false.

4-17

Programming

Monitoring the Instrument

4-18

Agilent Technologies 86120B User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual

General Safety Considerations

Contents

Getting Started

Step 1. Inspect the Shipment

Step 2. Check the Fuse

Step 3. Connect the Line-Power Cable

Step 4. Connect a Printer

Step 6. Enter Your Elevation

Step 7. Select Medium for Wavelength Values

Step 8. Turn Off Wavelength Limiting

Cleaning Connections for Accurate Measurements

Choosing the Right Connector

Inspecting Connectors

Cleaning Connectors

Returning the Instrument for Service

Preparing the instrument for shipping

Using the Multi-Wavelength Meter

Displaying Wavelength and Power

Peak WL mode

List by WL or power modes

Total power and average wavelength

Measuring lasers between 700 nm and 1200 nm

Limiting the wavelength range

Measuring broadband devices and chirped lasers

Graphical display of optical power spectrum

Instrument states

Power bar

Changing the Units and Measurement Rate

Displayed units

Measurement rate

Continuous or single measurements

Defining Laser-Line Peaks

Measuring Laser Separation

Channel separation

Measuring flatness

Measuring Modulated Lasers

Measuring Total Power Greater than 10 dBm

Calibrating Measurements

Printing Measurement Results

Measurements Applications

Measuring Signal-to-Noise Ratios

Measuring Signal-to-Noise Ratios with Averaging

Measuring Laser Drift

Measuring Coherence Length

Programming

Addressing and Initializing the Instrument

Making Measurements

Commands are grouped in subsystems

Measurement instructions give quick results

Measure delta, drift, and signal-to-noise

The format of returned data

Monitoring the Instrument

Status registers