MOGlabs DLC202, DLC102, DLC252, DLC502 User Manual

External Cavity Diode Laser Controller

DLC102, DLC202, DLC252, DLC502

Revision 9.09

Limitation of Liability

MOG Laboratories Pty Ltd (MOGLabs) does not assume any liability arising out of the use of the information contained within this manual. This document may contain or reference information and products protected by copyrights or patents and does not convey any license under the patent rights of MOGLabs, nor the rights of others. MOGLabs will not be liable for any defect in hardware or software or loss or inadequacy of data of any kind, or for any direct, indirect, incidental, or consequential damages in connections with or arising out of the performance or use of any of its products. The foregoing limitation of liability shall be equally applicable to any service provided by MOGLabs.

Copyrightc MOG Laboratories Pty Ltd (MOGLabs) 2014. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying or otherwise, without the prior written permission of

MOGLabs.

Contact

For further information, please contact:

MOG Laboratories P/L 18 Boase St Brunswick VIC 3056 AUSTRALIA +61 3 9939 0677 info@moglabs.com www.moglabs.com

MOGLabs USA LLC 419 14th St Huntingdon PA 16652 USA +1 814 251 4363 info@moglabsusa.com www.moglabsusa.com

MOGLabs Europe Goethepark 9 10627 Berlin Germany +49 30 21 960 959 christoph.p@moglabs.com

Preface

Diode lasers can be wonderful things: they are eﬃcient, compact, low cost, high power, low noise, tunable, and cover a large range of wavelengths. They can also be obstreperous, sensitive, and temperamental, particularly external cavity diode lasers (ECDLs). The mechanics and optics needed to turn a simple $10 120 mW AlGaAs diode laser into a research-quality narrow-linewidth tunable laser are fairly straightforward [1, 2, 3, 4], but the electronics is demanding – and, until now, not available commercially from a single supplier, let alone in a single unit.

The MOGLabs range of ECDL controllers change that. With each DLC unit, we provide everything you need to run your ECDL, and lock it to an atomic transition. In addition to current and temperature controllers, we provide piezo drivers, sweep ramp generator, modulator for AC locking, lock-in ampliﬁer, feedback servo system, laser-head electronics protection board, even a high-speed low-noise balanced photodetector.

We would like to thank the many people that have contributed their hard work, ideas, and inspiration.

We hope that you enjoy using the DLC as much as we do. Please let us know if you have any suggestions for improvement in the DLC or in this document, so that we can make life in the laser lab easier for all, and check our website from time to time for updated information.

MOGLabs www.moglabs.com

Safety Precautions

Safe and eﬀective use of this product is very important. Please read the following safety information before attempting to operate your laser. Also please note several speciﬁc and unusual cautionary notes before using the MOGLabs DLC, in addition to the safety precautions that are standard for any electronic equipment or for laser-related instrumentation.

CAUTION – USE OF CONTROLS OR ADJUSTMENTS OR

PERFORMANCE OF PROCEDURES OTHER THAN THOSE

SPECIFIED HEREIN MAY RESULT IN HAZARDOUS

RADIATION EXPOSURE

Laser output can be dangerous. Please ensure that you implement the appropriate hazard minimisations for your environment, such as laser safety goggles, beam blocks, and door interlocks. MOGLabs takes no responsibility for safe conﬁguration and use of your laser. Please:

• Avoid direct exposure to the beam.

• Avoid looking directly into the beam.

• Note the safety labels and heed their warnings.

• When the laser is switched on, there will be a short delay of

two seconds before the emission of laser radiation, mandated by European laser safety regulations (IEC 60825-1).

• The STANDBY/RUN keyswitch must be turned to RUN before the laser can be switched on. The laser will not operate if the keyswitch is in the STANDBY position. The key cannot be

iii

removed from the controller when it is in the clockwise (RUN) position.

• To completely shut oﬀ power to the unit, turn the keyswitch anti-clockwise (STANDBY position), switch the mains power switch at rear of unit to OFF, and unplug the unit.

• When the STANDBY/RUN keyswitch is on STANDBY, there can- not be power to the laser diode, but power is still being supplied to the laser head for temperature control.

CAUTION Please ensure that the unit is conﬁgured for the correct voltage

for your AC mains supply before connecting. The supply must include a good ground connection.

CAUTION To ensure correct cooling airﬂow, the unit should not be oper-

ated with cover removed.

WARNING The internal circuit boards and many of the mounted compo-

nents are at high voltage, with exposed conductors, in particular the high-voltage piezo driver circuitry. The unit should not be operated with cover removed.

NOTE The MOGLabs DLC is designed for use in scientiﬁc research

laboratories. It should not be used for consumer or medical applications.

Protection Features

The MOGLabs DLC includes a number of features to protect you and your laser.

Softstart A time delay (3s) followed by linearly ramping the diode cur-

rent (3 s max).

Circuit shutdown Many areas of the circuitry are powered down when not in use.

The high voltage supply and piezo drivers, the diode current supplies, the coil driver, and others are without power when the unit is in standby mode, if an interlock is open, or a fault condition is detected.

Current limit Sets a maximum possible diode injection current, for all op-

erating modes. Note that current supplied through the RF connector on the laser headboard is not limited.

Cable continuity If the laser is disconnected, the system will switch to standby

and disable all laser and piezo power supplies. If the laser diode, TEC or temperature sensor fail and become open-circuit, they will be disabled accordingly.

Short circuit If the laser diode, TEC or temperature sensor fail and become

short-circuit, or if the TEC polarity is reversed, they will be disabled accordingly.

Temperature If the detected temperature is below −5◦C or above 35◦C, the

temperature controller is disabled.

Internal supplies If any of the internal DC power supplies (+5, ±10, ±12 V) is

1 V or more below its nominal value, the respective components (temperature controller, diode current supply) are disabled.

Protection relay When the power is oﬀ, or if the laser is oﬀ, the laser diode

is shorted via a normally-closed solid-state relay at the laser head board.

Emission indicator The MOGLabs controller will illuminate the emission warn-

ing indicator LED immediately when the laser is switched on. There will then be a delay of at least 2 seconds before actual laser emission.

Mains ﬁlter Protection against mains transients.

Key-operated The laser cannot be powered unless the key-operated STANDBY

switch is in the RUN position, to enable protection against unauthorised or accidental use. The key cannot be removed from the controller when it is in the clockwise (RUN) position.

Interlocks Both the main unit and the laser head board have interlocks,

to allow disabling of the laser via a remote switch, or a switch on the laser cover.

Preface i

Safety Precautions iii

Protection Features v

1 Introduction 1

1.1 Basic operation . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Passive frequency control . . . . . . . . . . . . . . . . 2

1.3 DC locking to an atomic transition . . . . . . . . . . . 4

1.4 AC locking to an atomic transition . . . . . . . . . . . 5

2 Connections and controls 7

2.1 Front panel controls . . . . . . . . . . . . . . . . . . . 7

2.2 Front panel display/monitor . . . . . . . . . . . . . . . 10

2.3 Rear panel controls and connections . . . . . . . . . . 12

2.4 Internal switches and adjustments . . . . . . . . . . . 15

2.5 Feedback conﬁgurations . . . . . . . . . . . . . . . . . 20

2.6 Digital control . . . . . . . . . . . . . . . . . . . . . . . 23

2.7 Internal trimpots . . . . . . . . . . . . . . . . . . . . . 24

3 Operation 25

3.1 Simplest conﬁguration . . . . . . . . . . . . . . . . . . 25

3.2 Laser frequency control . . . . . . . . . . . . . . . . . 26

3.3 External scan control . . . . . . . . . . . . . . . . . . . 27

3.4 Locking to an atomic transition: DC . . . . . . . . . . 28

3.5 Locking to an atomic transition: AC . . . . . . . . . . 31

3.6 External sweep . . . . . . . . . . . . . . . . . . . . . . 34

3.7 Locking using an external signal . . . . . . . . . . . . 34

vii

viii Contents

3.8 External control of lock frequency setpoint . . . . . . 36

4 Optimisation 37

4.1 Frequency reference . . . . . . . . . . . . . . . . . . . 37

4.2 Noise spectra . . . . . . . . . . . . . . . . . . . . . . . 39

A Speciﬁcations 41

A.1 RF response . . . . . . . . . . . . . . . . . . . . . . . . 45

A.2 Sweep saturation and trigger . . . . . . . . . . . . . . 45

B Troubleshooting 47

B.1 STANDBY/RUN indicator . . . . . . . . . . . . . . . . . 47

B.2 Diode OFF/ON indicator . . . . . . . . . . . . . . . . . 48

B.3 250 kHz modulation . . . . . . . . . . . . . . . . . . . 49

B.4 Locking . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

C Using DBR/DFB diodes 53

C.1 Fine current control . . . . . . . . . . . . . . . . . . . 53

C.2 DC current feedback . . . . . . . . . . . . . . . . . . . 53

C.3 Slow current feedback . . . . . . . . . . . . . . . . . . 54

C.4 Lock saturation . . . . . . . . . . . . . . . . . . . . . . 54

C.5 Special options . . . . . . . . . . . . . . . . . . . . . . 54

D Modulation coils 55

D.1 Field requirements . . . . . . . . . . . . . . . . . . . . 55

D.2 Coil impedance . . . . . . . . . . . . . . . . . . . . . . 56

D.3 Impedance matching . . . . . . . . . . . . . . . . . . . 57

D.4 Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

D.5 Shielding . . . . . . . . . . . . . . . . . . . . . . . . . 59

E External modulators and injection current modulation 61

E.1 Coupling circuit . . . . . . . . . . . . . . . . . . . . . . 61

E.2 Injection current modulation . . . . . . . . . . . . . . . 62

F Photodetector 65

F.1 Photodiodes . . . . . . . . . . . . . . . . . . . . . . . . 66

G Laser head board 67

G.1 Headboard connectors . . . . . . . . . . . . . . . . . . 67

G.2 Dual piezo operation . . . . . . . . . . . . . . . . . . . 68

Contents ix

G.3 RF coupling . . . . . . . . . . . . . . . . . . . . . . . . 69

H Feedback overview 71

I Connector pinouts 75

I.1 Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

I.2 Photodetector . . . . . . . . . . . . . . . . . . . . . . . 76

I.3 Interlock . . . . . . . . . . . . . . . . . . . . . . . . . . 76

I.4 Digital control . . . . . . . . . . . . . . . . . . . . . . . 77

J PCB layout 79

K 115/230 V conversion 81

K.1 Fuse . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

K.2 120/240 V conversion . . . . . . . . . . . . . . . . . . . 81

References 86

x Contents

1. Introduction

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The MOGLabs DLC can be used in various conﬁgurations, including simple current/temperature controller, passive frequency controller with internal or external sweep/scan, and as a complete system for active frequency stabilisation with AC, DC or external locking signal. Here is a quick outline of some modes of operation, so that you can connect and go as quickly as possible. Details are provided in chapter 3.

1.1 Basic operation

In the simplest conﬁguration, the MOGLabs DLC will be used to control the diode injection current, and temperature. All connections are via a single cable to the MOGLabs laser. If using with a non-

MOGLabs laser, please see appendix G for information on connect-

ing the diode, thermoelectric Peltier cooler (TEC), and temperature sensor via the laser head interface board which is provided. For operation with DBR/DFB diodes, please see appendix C.

The front-panel display and selector switch can be used to monitor the diode current, current limit, diode dropout voltage, temperature, temperature setpoint, and TEC current; see ﬁgure 1.1.

Figure 1.1: MOGLabs DLC front panel layout.

2 Chapter 1. Introduction

120V

time

TRIG

STACK

FREQUENCY

SPANSPAN

Figure 1.2: Stack (or current bias) output and trigger pulse, when scanning. Note that the ramp slope can be inverted. Details of the ramp behaviour are described in section A.2.

1.2 Passive frequency control

The MOGLabs DLC controls the laser frequency via the diode current, and piezo electric actuators to control the cavity length of an ECDL.

In normal (SCAN) mode, a sawtooth is supplied to the main (STACK) actuator to linearly sweep the laser frequency at a rate determined by the rear-panel trimpot, f see ﬁgure 1.2.

, from 4 to 70 sweeps per second;

sweep

Critical DLC signals can be monitored using the CHANNEL A and

CHANNEL B outputs on the rear panel, synchronised to the TRIG

trigger output, which should be connected to the equivalent inputs on a two-channel oscilloscope. The particular signals are selected from the front-panel CHAN A and CHAN B selector switches. The signals are described in detail in the following chapter.

Figure 1.3 is an example of what is seen on the oscilloscope in a simple scanning conﬁguration. The laser beam transmitted by an atomic vapour cell is detected on the photodetector provided with the controller, as the laser frequency sweeps through atomic resonances, thus showing the atomic absorption spectrum.

The FREQUENCY knob controls the oﬀset to the piezo-electric actu-

1.2 Passive frequency control 3

Ch1 100mV Ch2 100mV 5.0ms

Figure 1.3: A simple absorption spectrum of rubidium with the controller in simple frequency scanning mode.

ator (STACK) and thus the mid-point frequency of the sweep. As the external cavity frequency changes, the laser may “mode-hop” due to competition between the external cavity and the internal cavity deﬁned by the rear and front facets of the diode saemiconductor chip itself. The internal frequency of the diode can be adjusted by changing the diode current, either manually as the FREQUENCY oﬀset is adjusted when modehops are observed. The current can also be automatically biased during the frequency sweep, if BIAS is enabled via the internal DIP switch 4. Note that adjusting the frequency oﬀset (FREQUENCY knob) will aﬀect the diode current if BIAS is enabled, but it may still be necessary to adjust the diode current as FREQUENCY is adjusted, to avoid modehops.

The extent of the frequency sweep is controlled with the SPAN con- trol. The maximum range is typically 10 − 100 GHz. Depending on the oﬀset, the span may be limited by the minimum and maximum voltage that can be applied to the actuator, as described in detail in section A.2.

4 Chapter 1. Introduction

M M

ECDL

Servo

Vapour cell

Offsets

λ/4 λ/4

1.3 DC locking to an atomic transition

Figure 1.4 shows one possible conﬁguration in which a MOGLabs

DLC is used to lock an ECDL to an atomic transition. Locking is to

the side of an absorption peak in a vapour cell; see for example Demtr¨oder [5] for more information on spectroscopy. The passive conﬁguration of §1.2 is extended with the MOGLabs DLC photodetector (see appendix F), and an atomic vapour absorption cell. Alternately, a Fabry-Perot optical cavity or other reference could be used.

Figure 1.4: Schematic setup for DC locking to an atomic transition. PD is the DLC photodetector. BS beamsplitter, M mirror, λ/4 a quarter-wave retarder.

The schematic shows a saturated absorption spectroscopy arrangement, but often simply locking to the side of a Doppler-broadened absorption peak will be adequate. The photodetector can be used in single channel mode (default) or with balanced diﬀerential inputs, for example to subtract a Doppler background from a saturated absorption spectrum.

The lock frequency is determined by the zero-crossing point of the photosignal. The photosignal oﬀset is adjusted via the INPUT OFFSET and ERROR OFFSET controls. Feedback can be via one or both piezo actuators, or the diode injection current, or all three.

1.4 AC locking to an atomic transition 5

250kHz

M M

Lock-inECDL

Servo

Vapour cell + coil

AOM

λ/4 λ/4

f ~ 150mm

f ~ −25mm

1.4 AC locking to an atomic transition

With AC locking (FM demodulation or “lock-in ampliﬁer” detection), the laser frequency can be locked to a peak centre. The AC approach oﬀers the advantage of inherently lower detected noise and thus the potential for improved laser frequency stability. The setup is similar to that for DC locking, but modulation of the laser frequency, or the reference frequency, is required. The MOGLabs DLC provides an internal 250 kHz oscillator which can directly dither the diode current, or drive an external modulator. In particular, it is designed to drive a Zeeman-shift modulation coil surrounding the atomic reference vapour cell; see appendix D.

Figures 1.5, 3.5, 3.6 show examples of AC locking arrangements, using a coil to Zeeman-modulate the atomic reference, or an acoustooptic modulator (AOM) for modulating the frequency of the beam passing through the vapour cell. If preferred, the modulator oscillator can be set to dither the diode current (see §2.4). Feedback can again be via one or both piezo actuators, the diode current, or all three.

Figure 1.5: Setup for AC locking to an atomic transition. PD DLC pho- todetector, BS beamsplitter, M mirror, λ/4 quarter-wave retarder. See also Figs. 3.5, 3.6.

6 Chapter 1. Introduction

2. Connections and controls

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2.1 Front panel controls

STANDBY/RUN In STANDBY mode, the DLC maintains the laser temperature, but

powers down all other components including the high-voltage piezo power, and the main on-board low-voltage power.

In RUN mode, the DLC activates all circuits, including the laser current driver and piezo drivers. The diode current is disabled, and the

STACK is on but not scanning, until the laser enable switch is ON.

On ﬁrst power-up, the STANDBY indicator will be red; this is normal and indicates there has been a power failure since last switched to RUN. The unit should then be set to RUN to initiate temperature control, and back to STANDBY if further operation is not desired.

If the unit fails to switch to RUN mode (indicator does not show green), see appendix B.

OFF/ON Diode injection current enable. Also activates the STACK ramp and

current bias (if DIP switch 4 in ON). The STANDBY/RUN key switch must ﬁrst be on RUN and the associated indicator must be green.

If the unit fails to switch to RUN mode (indicator does not show green), see appendix B.

8 Chapter 2. Connections and controls

CURRENT Diode injection current, 0 to 100/200/250/500 mA (DLC102 to DLC502).

The response is not linear; that is, the change in current varies for a given rotation of the knob. The mid-range sensitivity is reduced to allow greater precision at normal operating currents.

FREQUENCY The laser frequency will normally be controlled via a multilayer

piezo-electric actuator (STACK). This knob controls the oﬀset voltage applied to that actuator, 0 to 120 V (or 150 V; see LK2, p. 15). For

DFB/DBR diodes, the frequency control feedback signal can control

the diode current rather than the stack; see §2.4, DIP switch 16.

Note The FREQUENCY control will also aﬀect the diode current, if BIAS

(DIP switch 4) is enabled.

SPAN Frequency scan range, from 0 to 120 V (or 150 V; see LK2, p. 15).

The span may be limited by the minimum and maximum voltage that can be applied to the actuator; see detailed description in section A.2.

PHASE When AC locking, the controller demodulates the error signal from

the detected light intensity. PHASE adjusts the relative phase between the internal reference modulator and the detected signal, from 0 to 360◦. When DC locking, the sign of the error signal can be ﬂipped by rotating the PHASE control.

GAIN Overall error signal gain, 0 to 40 dB.

SLOW Gain for feedback to the slow (piezo) actuator, 0 to 40 dB.

FAST Gain for fast feedback to the diode current, 0 to 40 dB.

set

BIAS Feed-forward bias current. If DIP switch 4 is ON, changes in laser fre-

Temperature set point, 0 – 30◦standard; extended range optional.

quency, usually via the STACK actuator, will simultaneously change the current. This trimpot controls the slope dI/df of current with frequency. It can be positive or negative, with a range of ±25 mA for the full frequency span.

2.1 Front panel controls 9

INPUT OFFSET Oﬀset of input light intensity signal, 0 to −10 V. This can be ad-

justed to bring the photodetector light signal close to zero on the oscilloscope, and to shift the zero frequency lockpoint for DC locking.

OFF/MOD Modulator enable, to switch on the coil driver, diode current dither,

or external modulator.

ERROR OFFSET Oﬀset of the frequency error lock signal. The DLC will lock such

that the error signal plus ERROR OFFSET is zero, allowing for small adjustment of the lock frequency.

SCAN/LOCK Switch between scanning mode and lock mode. When switching

from scan to lock, the controller will ﬁrst reset the scanning actuator (usually STACK) to the oﬀset voltage at the trigger point, and then lock to the nearest frequency at which the error signal is zero.

+/− Sign of fast (current) feedback. The sign of the slow feedback can

be changed with the PHASE control, for both AC and DC locking.

OFF/LOCK Enable fast (current) feedback. The laser can be locked with slow

(piezo) locking or fast (current) locking alone. Best performance is usually obtained with both channels of feedback; see chapter 4 for feedback optimisation.

10 Chapter 2. Connections and controls

2.2 Front panel display/monitor

Display selector

The MOGLabs DLC includes a high-precision 4.5 digit LED display with four unit annunciators and 8-channel selector switch.

Current Diode current (mA) * see note below Curr max Current limit (mA)

(−) sign indicates limit rather than actual current

Voltage Diode voltage (V) Temp set Temperature set point ( Temperature Actual temperature (

TEC current Current to thermoelectric (Peltier) cooler (A) TEC voltage Voltage on thermoelectric (Peltier) cooler (V)

Frequency Frequency actuator oﬀset, usually slow piezo

(normalised to a range of ±1)

Note The current display shows the current set point, not the actual diode

current. If BIAS is enabled, then during the scan the actual diode current will be higher or lower than that shown, depending on the adjusted value of the BIAS trimpot. The current limit circuit prevents the actual diode current from exceeding the limit set by I (see page 13), even if the current setting plus current modulation (internal, external, or BIAS) would exceed I

Use CHAN B Current to see the actual diode current, and the eﬀect of

BIAS and current limit when scanning.

◦

max

2.2 Front panel display/monitor 11

CHAN A

Several important signals can also be monitored externally with an oscilloscope via the rear connectors CHANNEL A, CHANNEL B and

TRIG. The outputs to these can be selected with the CHAN A and CHAN B selectors.

Input Photodetector [30 mV/µW] Filter Filtered photodetector, 40 kHz low pass Freq Frequency-scanning actuator (STACK) [1 V/48 V] Slow Slow feedback STACK [1 V/0.24 V] DISC [1 V/4.8 V] Fast Current feedback [1 V/100 µA]

CHAN B

Input Photodetector [30 mV/µW] Error Feedback error Current Diode current [10 V/A*] Mod Modulator output current [1 V/A] Temp Temperature error [10 V/

◦

* Note: 5, 10, 12.5, 25 V/A for DLC102/202/252/502.

12 Chapter 2. Connections and controls

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2.3 Rear panel controls and connections

IEC power in/out The unit should be preset for the appropriate voltage for your coun-

try. Please contact MOGLabs for instructions if you need to change the power supply voltage.

The output IEC connector is a direct connection to the input power, after the input mains ﬁlter. This outlet should be used only to power a monitoring oscilloscope. It is provided to minimise ground-loop noise problems.

Fan The fan speed is temperature-controlled.

Interlock The DLC will not power on the laser unless the pins on this connector

are shorted. A standard 2.1 mm DC plug is provided.

LASER Connection to laser head. This connector provides diode current,

two piezo drives, temperature sense, and TEC current. A DVI-D Dual cable is provided.

WARNING: The piezo drive signals can be lethal. The high-voltage outputs, diode current and TEC current will be disabled if the cable is disconnected, or if the main or head interlocks are open-circuit, but these protection features should not be assumed.

gain

CHANNEL A, B Monitor outputs; connect to oscilloscope, channels 1 and 2.

Temperature control feedback gain. Increase this if the response time is too great or if the temperature error is large. Reduce this if

the temperature oscillates.

2.3 Rear panel controls and connections 13

TRIG Oscilloscope trigger, TTL-level. Connect to external trigger input on

oscilloscope. Set oscilloscope triggering for external, rising edge.

max

Diode current limit. The current limit can be set with the display selector set to Curr max. See page 10 for further information.

ERROR/CURR MOD Input for externally derived feedback error signal (DIP switch 5) or for

current modulation (DIP switch 6). Impedance 5 kΩ. Signal normally < ±1 V; max ±8 V. If used for external error, DIP switch 5 ON, applies in lock mode only.

If used for current modulation, DIP switch 6 ON, the sensitivity is

2.5 mA per volt.

Signal paths can be found in appendix H.

SWEEP/PZT MOD Input for externally generated frequency control (STACK, DIP switch

9 and/or DIP switch 13) or for piezo DISC modulation (DIP switch 14).

This signal is added to the internal error signal if DIP switch 15 is on.

If DIP switch 9 is on, the internal sweep ramp is replaced with the external sweep input. In that case, the external sweep signal should be 0 to 2.5 V and must cross 1.25 V to generate triggering for the oscilloscope (TRIG) and locking. Impedance 5 kΩ. Sensitivity 48 V per volt (120 V max).

If the external sweep is less than 0 to 2.5 V then the current bias dI/df will be reduced in proportion.

It is possible to add the SWEEP signal to the internally generated

STACK signal in all circumstances, for example to test actuator re-

sponse while locked to a transition. To do this, add a resistor (approximately 5k0, size 0603) at R113.

Signal paths can be found in appendix H.

Photodetector Connection to photodetector unit. A standard 6-pin FireWire (IEEE-

1394) cable is provided.

sweep

Scan rate, 4 – 70Hz. Note that the rapid return of the STACK sweep drive can excite mechanical oscillations in the laser. Slower sweeps

14 Chapter 2. Connections and controls

are recommended; usually 10 or 20 Hz works well but if ringing is observed at the start of the sweep, reduce f

MOD OUT Connection to external modulator, output is 0 to ±500 mA, ±8 V.

sweep

Current sensing with 1 Ω sense resistor. It can be directly connected to a 50 Ω load, giving a voltage of ±5 V if I

is adjusted to ±100mA.

set

See appendices D, E.

mod

Modulation depth: the range of current modulation on MOD OUT and if DIP switch 3 is on, the diode current.

2.4 Internal switches and adjustments 15

2.4 Internal switches and adjustments

See appendix H for schematic overviews of the piezo and diode current control signals, and the eﬀect of the diﬀerent DIP switches. See appendix J for the location of relevant internal components.

CAUTION The cover of the controller should be left on, even loosely, to ensure

proper airﬂow and cooling.

Interlock Link LK1 (rear right of main board) can be shorted internally to

avoid the requirement for an external interlock, if permitted by local safety regulations.

120 V Link LK2 (near LK1 and 160 V test point) can be shorted to limit

the piezo stack voltage to 120 V, or removed to increase it to 150 V.

DIP switches

OFF ON

1 DISC ﬁxed DISC ON 2 STACK ﬁxed STACK ON 3 Current dither OFF Current dither ON 4 Current bias OFF Current bias ON 5 Internal error External error 6 External current mod OFF External current mod ON 7 AC lock DC lock 8 Single photodiode Dual photodiode 9 Internal sweep External sweep 10 STACK feedback – STACK feedback + 11 STACK sweep + STACK sweep – 12 AC current feedback DC current feedback 13 STACK internal STACK external 14 DISC internal DISC external 15 Default External slow error 16 Current mod by SLOW control signal (for DBR/DFB)

16 Chapter 2. Connections and controls

DIP 1, 2 Please refer to section 2.5 below for discussion of feedback conﬁg-

urations.

DIP 3 With DIP 3 ON, the 250 kHz modulator directly modulates the injec-

tion current to cause frequency modulation of the laser frequency. In conjunction with a frequency-dependent absorption on the photodetector signal, for example with an atomic vapour cell or etalon (see section 3.5). The modulation depth is adjusted via internal trimpot

RT6 and the I

rear-panel trimpot. The modulation can be switched

mod

on and oﬀ via the front panel toggle switch OFF/MOD.

Caution Current dither (DIP 3 ON) inherently increases the eﬀective linewidth

of the laser. The modulation depth should be adjusted to the minimum which still provides a useful locking signal.

DIP 4 Enables injection current bias, sometimes called “feed-forward”. If

this switch in ON, the injection current will be modulated in conjunction with changes to STACK, for example as the laser frequency is ramped, or due to frequency feedback locking. The depth of bias modulation is controlled with the BIAS front-panel trimpot. Appropriate adjustment can substantially extend the mode-hop-free scan range of the laser.

DIP 5 Externally derived locking signals can be used to control the laser

current and piezo actuators. If DIP 5 is ON, the internally generated error signal is replaced with the signal from the rear-panel ERROR input, and then drives all feedback channels. The master gain adjustment, and both slow and fast gain adjustments, can be used.

DIP 6 If this switch is ON, the rear-panel ERROR/CURR MOD signal is added

to other current feedback signals, and the gain of the combined signal is enhanced by a factor of 25. All internal servo shaping ﬁlters are bypassed by the external current modulation. The gain knobs aﬀect the internally-generated error signals as usual. The

FAST gain knob and +/− also aﬀect the external current modulation.

The state of DIP 5 does not aﬀect ERROR.

2.4 Internal switches and adjustments 17

DIP 6 and DIP 12 If both DIP 6,12 are on, internal slow feedback to STACK, and external

current modulation to the diode current, are enabled.

DIP 7 Switch 7 determines whether AC (centre or top of peak) or DC (side

of peak) locking is used. Generally AC is preferred because the noise at the modulation frequency of 250kHz is much lower than at

DC; thus AC locking is largely free of slow drifts. However, for many

applications a DC reference is perfectly adequate and allows locking with wider bandwidth.

DIP 8 It can be convenient to subtract a background from the input sig-

nal, for example to remove a Doppler background from a saturated absorption reference. Switch 8 switches the photodetector to diﬀerential mode. The diﬀerence between the two photodiode signals is generated in the photodetector itself.

DIP 9, 13, 14, 15 These switches determine the function of the SWEEP input, for ex-

ample to provide an external frequency ramp, or to use an external locking circuit (see section 3.7) or to allow measurement of the actuator response functions.

DIP 9 With DIP 9 ON, the laser frequency sweep will be driven from an

external ramp. Note that the sweep, supplied via SWEEP, should be 0 to 2.5 V and must cross 1.25 V to generate triggering for the oscilloscope and locking. The TRIG signal will output at 1.25 V. The front-panel SPAN knob controls the amplitude.

DIP 10, 11 The sign of the response of the two piezo actuators can be reversed

with switches 10, 11. For example, increasing the potential on STACK may increase or decrease the cavity length, while DISC may act in the same or the opposite sense. It is important for locking that both operate in the same sense. Also, it may be useful to reverse the scan for some applications. To reverse the sign of DISC, reverse the error signal ﬁrst, and then adjust the sign of the STACK and current feedback.

Note The feedback to the STACK actuator reverses with DIP 1 and so DIP 10

should also be ﬂipped when DIP1 is ﬂipped, or the PHASE adjusted to

18 Chapter 2. Connections and controls

reverse the error signal. See section 2.5 below for further discussion.

DIP 12 Current feedback is normally AC coupled because slow feedback to

STACK takes care of slow drifts. For lasers without piezo control,

such as DBR and DFB diodes, DC feedback to current can be enabled by switching DIP 12 on. With external current modulation (see

DIP 6 above), DIP 12 on enables slow piezo feedback and AC coupled

external current modulation.

DIP 13 If DIP 13 is on, the internally generated STACK signal is replaced with

the external SWEEP signal, independent of the state of SCAN/LOCK. The change occurs after the oﬀset (FREQUENCY) and STACK polarity (DIP switch 11), before the SLOW gain adjust.

It is possible to add the SWEEP signal to the internally generated

STACK signal in all circumstances, for example to test actuator re-

sponse while locked to a transition. To do this, add a resistor (approximately 5k0, size 0603) at R113.

DIP 14 If DIP 14 is on, the internally generated DISC signal is replaced with

the external SWEEP signal, independent of the state of SCAN/LOCK.

To measure an actuator response, connect an external variablefrequency oscillator to the SWEEP input, and sweep through the frequency range of interest. Measure the laser frequency modulation amplitude from the transmitted intensity at the side of a FabryPerot fringe or saturated absorption transmission peak (e.g. ﬁg. 1.4), preferably with a lockin ampliﬁer.

DIP 15 If DIP 15 is on, the external SWEEP input replaces the normal internally-

generated slow (piezo) feedback error signal. The change occurs before the SLOW gain adjust. The fast (current) feedback is unaltered, except for the signal activated by DIP 16; see section 2.5 below. Sweep, oﬀset (FREQUENCY) and stack polarity (DIP switch

11) are unaﬀected.

DIP 12, 16 Switches 4, 12, 16 allow operation of DFB/DBR lasers without external

cavity feedback and thus with only current as an actuator. Please refer to section 2.5 below for discussion of feedback conﬁgurations.

2.4 Internal switches and adjustments 19

Use switch DIP4 (current feed-forward bias) to drive the current with the scanning ramp. Switch DIP 16 adds the fast DISC signal to the current. DIP 16 and DIP 4 can be active simultaneously. Switch 12 enables DC coupling of the current feedback, rather than the default

AC coupling, to allow current-only feedback locking.

20 Chapter 2. Connections and controls

2.5 Feedback conﬁgurations

The DLC is designed to drive up to three feedback actuators with appropriate frequency bandwidths for each. The actuators are STACK,

DISC and CURRENT. Suitable lasers include the MOGLabs ECDL which

has CURRENT and STACK feedback but no DISC piezo; DFB/DBR lasers which only oﬀer CURRENT feedback; and lasers with all three.

The nominal feedback bandwidths described below are deﬁned by the unit gain bandwidth when all controls (MASTER, SLOW, FAST) are at their centre positions. The actual closed-loop unity gain frequencies will depend on the particular laser, diode, and piezos used and on the reference signal, so the frequencies are only a guide.

For CURRENT feedback, phase lead adjust can increase the bandwidth to 40 kHz.

Summary of conﬁgurations

DIP 16 10 2 1 Description

A OFF OFF ON ON STACK slow DISC fast B ON ON ON ON STACK slow DISC fast C OFF ON ON OFF STACK fast DISC ﬁxed D OFF OFF OFF ON STACK ﬁxed DISC fast E ON X OFF OFF STACK ﬁxed DISC fast

For the MOGLabs ECDL, use option C (default) or, to increase the range for slow drift, option B.

The conﬁgurations above assume that increasing the voltage on

STACK increases the laser frequency (by reducing the cavity length).

Reverse DIP 10 if the opposite is true.

2.5 Feedback conﬁgurations 21

A: STACK slow, DISC fast

STACK: −20dB/decade, BW 50 Hz DISC: −40 dB/decade, BW 1.5 kHz CURRENT: −20 dB/decade, BW 15 kHz

B: STACK slow, DISC fast, extra CURRENT

STACK: −20dB/decade, BW 50 Hz DISC: −40 dB/decade, BW 1.5 kHz CURRENT: −20 dB/decade BW 15 kHz + ﬂat response

Additional CURRENT feedback with ﬂat response (no integrator) to boost low-frequency feedback. The combined current feedback gain is reduced 25×. In this conﬁguration, the error signal must be reversed; that is, the error signal should have a negative slope at the lock point, the +/- current feedback polarity toggle switch should be down (−). Note DIP 10 is ON.

C: STACK fast, DISC ﬁxed

STACK: −40dB/decade, BW 750 Hz DISC: ﬁxed CURRENT: −20 dB/decade, BW 15 kHz

High gain (fast) output to STACK reduces range of STACK to ±1 GHz before internal signal saturates.

D: STACK ﬁxed, DISC fast

STACK: ﬁxed DISC: −40 dB/decade, BW 1.5 kHz CURRENT: −20 dB/decade, BW 15 kHz

22 Chapter 2. Connections and controls

E: CURRENT only

STACK: ﬁxed DISC: ﬁxed CURRENT: ﬂat, BW 15 kHz DIP 12 should be ON for DC CURRENT feedback. DIP 4 ON to drive the current with the scanning ramp.

For DBR and DFB lasers and ECDLs when it is desirable to operate without piezo actuators.

2.6 Digital control 23

2.6 Digital control

HD12 is a 10-pin header which provides access to several control signals for locking and for sample-and-hold of the lock-point. HD12 is located near the DIP switches, slightly towards the front and lefthand side of the unit (see appendix J). The pinout of the header is described in section I.4. The signals are standard TTL-compatible, > 2.4 V HIGH and < 0.8 V LOW. The inputs are ORed with the front toggle switches, such that the signal is activated if either the digital input is active (i.e. HIGH) or the toggle switch is on (down).

Laser ON HIGH to switch the laser diode current on, regardless of the state of

the front-panel switch.

LOCK HIGH to SLOW lock, regardless of the state of the front-panel switch.

LOW to sweep, if the front-panel switch is up.

FAST HIGH to FAST lock.

HOLD HIGH to freeze STACK. With HOLD active, the feedback to the slow

piezo will be ﬁxed by a sample-and-hold circuit. The diode current can then be modulated via the rear-panel CURR MOD input (with DIP switch 6 ON), to jump the laser frequency quickly, without the error feedback circuit competing with the external modulation. External current modulation is independent of the FAST lock status.

FAST lock is asynchronous with HOLD active; that is, the FAST lock

will activate immediately, rather than the normal delay until the scan ramp reaches the sweep centre.

To relock, restore the CURR MOD input voltage, and return the HOLD input LOW; the locking feedback will then be reactivated. FAST lock can then be reactivated.

This ability can be used for auto-locking under computer control, and also for atom trapping experiments involving sequences with diﬀerent detunings for polarisation gradient cooling and for compression.

24 Chapter 2. Connections and controls

2.7 Internal trimpots

RT6 Current dither amplitude limit RT12 Phase lead RT13 Ambient temp for active sensors (AD590, AD592) RT15 TEC current limit

RT6 For AC locking, either the laser frequency or the external reference

must be modulated at the DLC dither frequency, 250 kHz. An external modulator (see appendix E) is normally used, but the laser injection current can be modulated directly. The modulation depth is then controlled by the rear-panel I modulation is factory set via RT6.

RT12 A phase-lead circuit is included on the current feedback channel,

to boost the output at higher frequencies (tens of kHz). RT12 controls the phase lead and can be adjusted for diﬀerent diodes; see appendix 4.

trimpot. The limit to the current

mod

RT13 Oﬀset adjustment for active temperature sensors (AD590, AD592),

so that temperature reads in◦C.

RT15 Current limit for TEC output. To set, change the set temperature

suddenly, and adjust RT15 while reading the TEC current.

3. Operation

3.1 Simplest conﬁguration

In the simplest application, the MOGLabs DLC will be used to control just the diode injection current and temperature. All connections are via a single cable to the MOGLabs laser. If using with a non-

MOGLabs laser, please see appendix G for information on connecting

the diode, thermoelectric Peltier cooler (TEC), and temperature sensor via the laser head interface board. For operation with DBR/DFB diodes, please see appendix C.

To operate in passive conﬁguration:

1. Ensure the power is on, and the STANDBY/RUN switch is on

STANDBY. In this mode, most circuits will be switched oﬀ, in-

cluding much of the main internal board, low and high voltage

DC supplies, photodetector, piezo and diode outputs. On ﬁrst

power-up, the STANDBY indicator will be red; this is normal. The switch should be set to RUN to initiate temperature control, and then may be returned to STANDBY.

2. Switch from STANDBY to RUN. The indicator should change from red (if just powered up), or orange, to green. If the indicator is not green, the TEC or sensor is not correctly wired. In

RUN mode, all electronics will be powered up, except for the

diode injection current supply and piezo drivers.

3. If the controller is switched back to STANDBY, all electronics will be powered down, except for the temperature controller, which will continue to operate normally.

4. Adjust the temperature setpoint: ﬁrst select Temp set on the display selector, then adjust T

5. Temperature control can be optimised by adjustment of the integrator gain, rear-panel trimpot T

via the front-panel trimpot.

set

. Adjust to minimise

gain

26 Chapter 3. Operation

120V

time

TRIG

STACK

FREQUENCY

SPANSPAN

the time to equilibrate the temperature (CHANNEL B output, front panel CHAN B set to Temp) after a sudden change in T

set

6. Adjust the current control knob to minimum (fully anti-clockwise).

7. Set the diode maximum current: select Curr max on the display selector, then adjust the maximum allowed diode injection current via the rear panel I

trimpot. Note that with the display

gain

set to Curr max, a negative sign (−) provides a visual reminder that the limit is being displayed rather than the actual current.

8. Switch the laser on. The indicator on the laser head board should illuminate, and the front-panel indicator above the switch should turn green.

Note that the SCAN/LOCK and fast-channel OFF/LOCK switches must be set to SCAN and OFF respectively. Other protection features will prevent current to the diode, including main cable disconnect, and open circuit on the rear-panel or laser head interlocks.

3.2 Laser frequency control

In normal (SCAN) mode, a sawtooth ramp is supplied to the the stack, at frequency of f

sweep

= 4 to 70 Hz; see ﬁg. 3.1. Depending on the

frequency oﬀset (FREQUENCY) and the width of the scan (SPAN), the

STACK can saturate either at the low or high frequency end of the

sweep. The spectrum may then be constant, although if current bias is enabled the laser frequency may still scan in that range, but at a smaller slope (see section A.2 for details).

Figure 3.1: Stack output voltage and trigger signal, when scanning.

3.3 External scan control 27

Several adjustments of the frequency sweep are possible:

SCAN/LOCK The SCAN/LOCK switch should be on SCAN. FREQUENCY Oﬀset; i.e. mid-point voltage of the ramp. SPAN Sets the height of the ramp; see ﬁg. 3.1. BIAS The BIAS front-panel trimpot controls the feed-

forward bias injection current which follows the ramp, to enable wider mode-hop-free scans. The bias can be adjusted in a trial-and-error manner to achieve the widest possible scans. BIAS is disabled unless internal DIP switch 4 is ON.

sweep

The rear-panel f

trimpot adjusts the ramp

sweep

rate from 4 to 70 Hz.

Note The rapid return of the STACK sweep drive can excite mechanical

oscillations in the laser. Slower sweeps are recommended; usually 20 Hz works well but if ringing is observed at the start of the sweep, reduce f

sweep

Figure 3.2 is an example of an absorption spectrum acquired with the simple scanning conﬁguration, using a standard (uncoated) diode and BIAS current feed-forward. The transmission of the laser through a rubidium vapour cell was detected on the DLC photodetector, as the laser frequency was scanned through the 52S

3.3 External scan control

An external source can be used to control the laser frequency while in SCAN mode.

1. Connect the external frequency control (ramp, or DC) signal to the rear-panel SWEEP external input.

2. Select external signal by setting DIP switch 9 to ON.

3. Set DIP switch 4 on if current bias is required.

1/2

→ 52P

3/2

levels.

28 Chapter 3. Operation

Rb87 F=2

F=3

F=2

F=1

Frequency (GHz)

−

2 4

Intensity

−0.

0.4

0.8

Saturated absorption spectrum for natural Rb

Figure 3.2: A saturated absorption spectrum of rubidium using a standard uncoated laser diode and low diﬀraction eﬃciency grating in Littrow conﬁguration (upper trace). The lower trace shows the AC-modulation error signal (see §3.5).

4. Toggle DIP switch 11 (external sweep has reverse polarity to internal).

5. Set SCAN/LOCK to SCAN. The front-panel SPAN knob controls the amplitude.

Note The frequency control supplied to SWEEP should be between 0 and

2.5 V and must cross 1.25 V to generate essential internal triggering. The TRIG signal will output at 1.25 V.

3.4 Locking to an atomic transition: DC

Figure 3.3 shows how an ECDL can be locked to an atomic transition as determined from absorption in a vapour cell. The basic conﬁguration described in §3.2 is extended with the DLC photodetector, and an atomic vapour absorption cell. A Fabry-Perot optical cavity or other frequency reference could also be used.

The photodetector can be used in single channel mode (default) or with balanced diﬀerential inputs, for example to subtract a Doppler

3.4 Locking to an atomic transition: DC 29

M M

ECDL

Servo

Vapour cell

Offsets

λ/4 λ/4

Ch1 100mV Ch2 100mV 20.0ms

Figure 3.3: Schematic setup for DC locking to an atomic transition. PD is the DLC photodetector. BS beamsplitter, M mirror, λ/4 retarder.

background from a saturated absorption spectrum.

Sample oscilloscope traces obtained in DC locking (“side of fringe”) mode are shown below, for wide and narrow spans. These traces were obtained with an 8 cm long Rb vapour cell at room temperature.

Figure 3.4: Examples of spectra for DC locking, for wide and narrow spans (upper traces) and error signals (lower traces).

To operate in DC locking conﬁguration:

1. Select DC locking by setting internal DIP switch 7 to ON.

2. If using diﬀerential inputs, set internal DIP switch 8 to ON.

3. Using an optical beamsplitter, a stray reﬂection, or by other

30 Chapter 3. Operation

means, deﬂect a fraction of the laser output through the vapour cell. The MOGLabs DLC is designed to operate best with about 250 µW incident on each of the Si-PIN photodiodes. Lensed and ﬁltered photodiodes are standard, to minimise the inﬂuence of background light, but best results will be obtained if light from incandescent or ﬂuorescent lamps is eliminated.

4. If using balanced inputs, the second light beam should illuminate the second photodiode.

5. Find an appropriate spectral feature.

6. Adjust front-panel INPUT OFFSET and ERROR OFFSET to obtain a zero-crossing ERROR signal at the desired frequency. The slope should normally be negative (depending on DIP switches

10, 11). The ERROR signal can be inverted by coarsely adjust-

ing the PHASE control.

7. Set SLOW and FAST gains to minimum (fully anti-clockwise).

8. Switch SCAN/LOCK to LOCK.

9. Switch OFF/LOCK to LOCK. It may be necessary to invert the sign of the fast lock with the ± switch.

10. Increase SLOW and FAST gains to minimise the error signal, ideally using an external audio spectrum analyser. The gains should be increased until the onset of oscillation, and then reduced. See chapter 4 for additional discussion of feedback optimisation.

Note that it is not necessary to “zoom in” on the desired lock point. The controller will automatically lock to the zero-crossing closest to the trigger point, i.e. to the centre of the oscilloscope trace.

When the laser is locked (step 8 above), the photodetector (INPUT) signal should be ﬁxed at the value corresponding to the lock frequency – in this case zero since for DC locking, the controller locks to the zero-crossing.

3.5 Locking to an atomic transition: AC 31

250kHz

M M

Lock-inECDL

Servo

Vapour cell + coil

AOM

λ/4 λ/4

f ~ 150mm

f ~ −25mm

3.5 Locking to an atomic transition: AC

Figures 3.5 and 3.6 show two alternate saturated absorption spectroscopy arrangements, useful for AC (“top of fringe”) locking. The laser frequency can be directly modulated via the diode current (see

§2.4, DIP switch 3), or using an external modulator. The controller includes a modulator driver with suﬃcient power to drive a coil directly for Zeeman modulation, or an external modulator such as an acousto-optic modulator can be used; see appendix D.

Sample oscilloscope traces obtained in AC locking mode are shown below, for wide and narrow spans. These traces were obtained with an 8 cm long Rb vapour cell at room temperature, using a Zeeman modulation coil as described in appendix D.

To operate in AC locking conﬁguration:

1. Select AC locking by setting internal DIP switch 7 to OFF.

2. Connect the photodetector module and optimise the photosig-

Figure 3.5: Schematic setup for AC locking to an atomic transition. PD is the DLC photodetector. BS beamsplitter, M mirror, λ/4 retarder. Beam expanding lenses increase signal power without power broadening.

32 Chapter 3. Operation

250kHz

Lock-inServo

Vapour cell + coil

λ/4

f ~ 150mmf ~ −25mm

Optical isolator

λ/2

PBS

ECDL

nal on CHANNEL A. The MOGLabs DLC is designed to operate best with about 250 µW incident on the Si-PIN photodiode. Lensed and ﬁltered photodiodes are standard, to remove most background light, and when AC locking at 250 kHz modulation frequency, any remaining photocurrent from background lighting should not be a problem.

3. Adjust the INPUT OFFSET such that saturated absorption trace is near zero.

4. Switch the modulation on with OFF/MOD.

5. Find an appropriate spectral peak and observe the dispersive error signal with CHAN B set to ERROR.

6. Optimise the error signal (usually for maximum slope) by adjusting the front panel PHASE. The error signal slope should normally be negative (depending on DIP switches 10, 11) at the desired frequency.

7. Adjust the GAIN such that the error peaks are roughly 250 − 500 mV peak-to-peak. Note that larger signals are not recom-

Figure 3.6: Schematic setup for a more compact and more easily aligned saturated absorption arrangement. PD is the DLC photodetector. PBS polarising beamsplitter, M mirror, λ/4 and λ/2 retarders. Beam expanding lenses increase the signal power while minimising saturation broadening.

3.5 Locking to an atomic transition: AC 33

Ch1 100m V Ch2 100m V 20.0m s

Figure 3.7: Examples of spectra for AC locking, for wide and narrow spans (upper traces), with error signals (lower traces).

mended; although the signal-to-noise may look better on an oscilloscope, that is a reﬂection of the noise of the oscilloscope and is not the case inside the DLC controller.

8. Adjust front-panel ERROR OFFSET such that the error signal is crossing zero at the desired frequency.

9. Set SLOW and FAST gains to minimum (fully anti-clockwise).

10. Switch SCAN/LOCK to LOCK.

11. Switch OFF/LOCK to LOCK. It may be necessary to invert the sign of the fast lock with the ± switch.

12. Increase SLOW and FAST gains to minimise the error signal, ideally using an external audio spectrum analyser (see chapter

4). The gains should be increased until the onset of oscillation, and then reduced. See chapter 4 for additional discussion.

Note again that it is not necessary to “zoom in” on the desired lock point. The controller will automatically lock to the zero-crossing of the error signal (in this case the peak of a spectral feature) closest to the trigger point, at the centre of the oscilloscope trace.

When the laser is locked (step 10 above), the photodetector (INPUT) signal should be ﬁxed at the value corresponding to the lock frequency. In contrast to the DC locking case, this should be the INPUT signal at the peak of the spectral feature, not zero.

34 Chapter 3. Operation

3.6 External sweep

An external ramp can be used to control the frequency sweep, for example if very slow sweeps are required, or for computer-controlled sweeps.

To operate with external sweep:

1. The external sweep signal MUST have 1.25 V oﬀset. That is, it must transition through 1.25 V at some time during the sweep.

2. The external sweep signal should be within 0 to 2.5 V range.

3. Connect the external sweep signal to the rear-panel SWEEP external input.

4. Select the external sweep signal by setting internal DIP switch

9 to ON.

5. Normally DIP switch 4 should be on so that current bias (feedforward) is enabled.

6. The front panel knobs FREQUENCY and SPAN will still work. Set FREQUENCY to its midpoint (0 V on the front-panel display, with Frequency selected). Set SPAN to fully clockwise. You can then change your external sweep DC oﬀset and peak-to-peak amplitude, and/or use FREQUENCY and SPAN to control the oﬀset and sweep amplitude.

Note: if you have a Rev. 8 controller, you will probably need to remove resistor R113. Contact MOGLabs for assistance.

3.7 Locking using an external signal

The MOGLabs DLC can be used with a wide variety of externally generated dispersive signals; see appendix E for examples, and appendix H for block diagrams of the control circuitry.

Note that this section refers to error and control signals. An error signal is a dispersive signal with a potential that depends on laser frequency. A control signal is a feedback servo signal generated from

3.7 Locking using an external signal 35

an error signal, usually with PID (proportional-integral-diﬀerential) or PIID (PID with a double integrator) response.

When using an external error or control signal, it will normally be advisable to switch oﬀ the modulator (DIP switch 3).

3.7.1 External error signal

To operate with externally generated error signal, but using the internal DLC servo PIID feedback control:

1. Connect the external error signal to the rear-panel ERROR external input.

2. Select the external locking signal by setting internal DIP switch

5 to ON. DIP switch 6 should be OFF.

3. Follow the procedure above for DC locking.

The bandwidth limit will be the same as for a DLC-generated error signal; that is, about 25 kHz on the fast (current) channel.

3.7.2 External fast (current) control for higher bandwidth

For higher bandwidth feedback, a fast control signal can be input on ERROR and enabled via DIP switch 6. The fast signal will then control current directly, without DLC feedback control.

The external feedback circuit must include appropriate response. If using current-only control, without piezo control, then PID or PIID is probably appropriate.

If the DLC is still controlling the piezo (with SLOW lock turned on) then the current control should be AC coupled, and include gain reduction at high frequencies to avoid servo loop oscillation.

3.7.3 External fast and slow

To control both current (fast) and piezo (slow) with external signals:

36 Chapter 3. Operation

1. Connect fast control signal to ERROR.

2. Enable fast current control with DIP switch 6.

3. Connect slow error signal to SWEEP.

4. Enable slow piezo control with DIP switch 15.

The piezo will be controlled by the DLC if SCAN/LOCK is on SCAN, and by the external slow signal when switched to LOCK.

The slow signal should be a dispersive error signal without PID or other servo response function. The fast signal should be AC coupled, and include gain reduction at high frequencies to avoid servo loop oscillation.

3.8 External control of lock frequency setpoint

It is often useful to have external control of the lock frequency setpoint, for example to suddenly change the detuning of a laser. See section 2.6 for discussion of such external control.

4. Optimisation

Laser frequency stabilisation is a complex and ongoing research topic. A thorough treatment would require extensive discussion of control theory, actuator response, mechanical design, laser-atom interactions and electronics. Here we consider the problem from a pragmatic perspective.

The laser is assumed to be moderately stable, operating close to the desired frequency, with a linewidth of a few MHz averaged over a typical measurement time of about one second. The very short-term linewidth is determined by the Schawlow-Townes (S-T) limit, which is typically less than 100 kHz. The MOGLabs DLC will stabilise the laser frequency to an external reference, usually an atomic absorption feature, and reduce the eﬀective linewidth as close as possible to the S-T limit.

Achieving the best frequency locking stability requires careful optimisation of the signal-to-noise ratio (SNR) of the frequency discrimination signal obtained from the saturated absorption or other reference. Then the phase and gain settings must be optimised, preferably by measuring the feedback error signal spectrum.

4.1 Frequency reference

The frequency reference is critical to the performance of the MOGLabs

DLC: the controller cannot reduce the laser frequency noise without

an appropriate frequency-dependent reference signal.

The DLC has been designed to work with a saturated absorption reference, as shown in ﬁgures 3.5 and 3.7. Users should familiarise themselves with saturated absorption spectroscopy, for example as described in Demtr¨oder [5].

The frequency discriminator (“ERROR”) SNR should be optimised to

38 Chapter 4. Optimisation

produce clear (low-noise) dispersive error signals as shown in the upper trace of ﬁg. 3.7. Note that the error signal should be about

0.5 V p-p. While the signal looks cleaner at larger amplitude relative to background oscilloscope noise, in fact the overall performance will deteriorate. Other important factors to consider:

Probe power The probe power should be about 250 µW. Higher

power will increase the photosignal, but the detector saturates at about 500 µW.

Probe intensity The probe intensity should be low to reduce power-

broadening. Thus, the probe beam should be expanded to 5 or 10 mm diameter, to allow high power and low intensity, as discussed in section 3.5.

Polarisation The frequency discriminator (ERROR) signal is sensitive

to the pump and probe polarisations. Good polarisers and careful alignment can be very helpful.

Coil design See appendix D.

Shielding The Zeeman coil produces substantial magnetic ﬁelds,

oscillating at 250 kHz. These ﬁelds can readily induce problematic potentials and currents in the laser head and/or main circuit board. In particular, it is quite possible to produce a larger frequency modulation from induced currents in the laser diode than from the Zeeman modulation of the reference. It is vital that the coil be located far from the main unit and from the laser, or that it be shielded. A layer of high-permeability material (soft iron or mu-metal) is probably adequate. To test this, simply reverse the polarity of the coil connection. If the error signal is also reversed, but otherwise similar, then the shielding is probably adequate.

4.2 Noise spectra 39

MOGLabs DLC-202 + ECD-003 monoblock laser noise spectra

Frequency (Hz)

0.01

0.10

1.00

10.00

1000.00

]zHtr/zHk[ DSL esion ycneuqerF

Unlocked

Piezo

Piezo & current

Off resonance

Figure 4.1: Error signal spectra, with laser unlocked, locked with SLOW (piezo) feedback only, and with SLOW and FAST (piezo+current) feedback. The oﬀ-resonance spectrum provides information on the eﬀective noise ﬂoor.

4.2 Noise spectra

The master, slow and fast gains can be set as described in chapter 3, increasing them until the onset of oscillation, and then reducing slightly. If possible, an audio frequency spectrum analyser can be used to provide better guidance. A generic computer sound card with spectrum analysis software gives reasonable results up to 20 kHz. A good sound card (24-bit 200 kHz, e.g. Lynx L22 or E-Mu 1212m) provides noise analysis up to 100 kHz with 140 dB dynamic range, surpassing most standalone audio spectrum analysers, at very low cost. Connect the spectrum analyser to the CHANNEL B output, and set the CHAN B selector to ERROR.

You should see curves similar to those shown in ﬁg. 4.1. The noise spectrum with laser unlocked was obtained in scan mode, but with zero span, and the frequency carefully set to an atomic resonance (the highest saturated absorption dip in ﬁg. 3.7). Similarly for the Oﬀ resonance curve, but with the laser tuned far away from all res-

40 Chapter 4. Optimisation

onances, outside a Doppler absorption peak. The Oﬀ resonance spectrum gives the frequency discriminator noise ﬂoor: it is meaningless to try to reduce the laser frequency noise below this level.

With SLOW feedback enabled, the noise for low Fourier frequencies is drastically reduced. A double-integrator is used for slow feedback, such that the suppression is 40 dB/decade. The SLOW gain adjusts the 0 dB gain point; in the ﬁgure, this reaches approximately 5 kHz. Higher gains result in oscillation at a frequency corresponding to a pole in the piezo actuator response (i.e. a mechanical resonance).

If conﬁgured to work with the stack actuator only (see §2.4), then the SLOW feedback will suppress noise only to a few tens of Hz.

FAST feedback adds an additional 20 dB/decade suppression, with

0 dB gain beyond 20 kHz, even as high as 40 kHz, depending on the diode, optical feedback, the frequency discriminator noise ﬂoor and other details. Typically we ﬁnd that the laser diode itself has a 90 phase lag at 15 to 100 kHz. Some compensation for that phase lag is provided by a phase lead compensator (see RT12, page 24).

◦

Ideally, the SLOW and FAST gains should be adjusted to minimise the integrated noise (the area under the error spectrum). The data in ﬁg. 4.1 show a small “Bode bump” at around 30 kHz, indicating excessive current gain, leaving the laser marginally stable. For lower

FAST gain, the Bode bump will be reduced, at the expense of reduced

suppression of the mechanical resonance noise peaks around 2 kHz.

The frequency discriminator SNR – that is, the diﬀerence between the Unlocked and the Oﬀ resonance spectra (in the data shown above, about 10 dB for high frequencies) – is critical. Improvements to the reference, for example using a Fabry-Perot etalon rather than saturated absorption spectroscopy, can provide much greater SNR and correspondingly greater laser frequency noise suppression. See

§E.2, page 62, for one approach.

A. Speciﬁcations

Parameter

Current regulator

Output current

Max diode voltage

Display resolution

Noise

Stability

CURR MOD

RF modulation

BIAS

Temperature controller

TEC current max

TEC voltage max

TEC power max

Stability

Sensor

Range

Display resolution

Speciﬁcation

0 to 100/200/250/500 mA

3.2 V at full current; 6 V at half current /HC models up to 6.5 V at full current

±0.01 mA

< 10 nA rms (10 Hz – 1 MHz)

Warmup time: 15 minutes

5 kΩ, ±8 V max, sensitivity 100 µA/V,

1.5 MHz bandwidth

SMA 50 Ω, 160 kHz – 2.5 GHz, see below

±25 mA over full sweep

±2.5 A

±9 V

22 W

±5 mK/◦C

NTC 10 kΩ, AD590, AD592

0–30◦standard; extended range optional

◦

±0.01

Note The TEC is controlled with a linear regulator, which will overheat if

the current load is high and the TEC voltage is low. Choose a TEC with resistance of 4 to 5 ohms to optimise power to the device.

42 Appendix A. Speciﬁcations

Parameter

Speciﬁcation

Piezos

STACK

0 to 120 V for FREQUENCY (default) 0 to 150 V optional (LK2 removed)

DISC

Scan rate

100 ± 16.4 V feedback

4 to 70 Hz

Note The default maximum piezo voltage is 120 V but can be increased to

150 V by removing jumper LK2; see page 15.

Note The maximum piezo drive current is 10 mA, which limits the scan

rates for piezos with high capacitance. For exmaple, for a 250 nF piezo, the rate should not be greater than 25 Hz.

Photodetector

Photodiodes

Si-PIN, IR ﬁltered 740 nm – 1100 nm, 1 × 1 mm2sensor, ±10◦ﬁeld of view See appendix F for spectral response. Options:

• unﬁltered 400 nm – 1100 nm

◦

Coupling

Diode separation

Bandwidth

Dimensions

• ± 20◦, ±70

AC and DC, single or diﬀerential

10 mm

720 kHz

25 × 25 × 60 mm

Feedback system

MOD OUT

PHASE

INPUT OFFSET

ERROR OFFSET

GAIN

Bandwidth (gains at midpoint)

Protection and status

External interlock Laser head

enclosure interlock

Key switch interlock

Delayed soft-start Open circuit

detect Diode current

limit

250 kHz, ±8 V, ±500 mA Current output (1 Ω sense) Control via I

rear-panel trimpot

set

0 to 360◦(min)

−10 V to +10 V

±0.5 V

MASTER ±20 dB SLOW MASTER ±20 dB FAST MASTER ±20 dB SLOW 0 dB at 700 Hz FAST 0dB at 80 kHz

2.1 mm DC power plug (provided)

2-pin MOLEX connector (provided)

STANDBY/RUN

3 s delay + 3 s ramp

Laser cable, TEC, temperature sensor

Rear panel trimpot I

max

44 Appendix A. Speciﬁcations

DARK AC mains oﬀ, or fault condition

detected (TEC failure, polarity reversed, open-circuit, cable unplugged, missing sensor, temperature out of range)

STANDBY/RUN LED

RED AC mains power on

ORANGE Standby

(temperature controller on)

GREEN Fully operational

(piezo, current, ramp)

RED Start sequence error or fault

(Either LOCK switch ON, interlock open, head cable dis-

STATUS LED

connected, temperature controller fault detected)

ORANGE Ready

GREEN Diode running

Mechanical & power

Display

Fan

IEC input

IEC output

Dimensions

Weight

4.5 digit LED; standard colour red

12 V DC ball-bearing Temperature controlled

110 to 130 V 60Hz or 220 to 260 V 50Hz Fuse: 5x20mm, anti-surge (slo-blo) ceramic, 250V/2.5A

Common ground with power input Intended for oscilloscope; 1 A max

Standard 2U 19”, WxHxD = 422 × 84 × 200 mm

4.3 kg (excluding cables, laser head board, photodetector). 8 kg shipping

A.1 RF response 45

RBW

30 k Hz

VBW

10 M Hz

SWT

17 s

Att

50 d B

-30 d Bm

Ref

-20 d Bm

Cente r

1.5 G Hz

Span

3 GHz

300 M Hz/

-70

-65

-60

-55

-50

-45

-40

-35

-30

-25

-20

A.1 RF response

Figure A.1: RF response, SMA input on laser headboard to diode SMA

output.

A.2 Sweep saturation and trigger

In normal scanning mode, a sawtooth is supplied to the stack piezo (or other laser frequency actuator), at a frequency of 1 to 70 Hz; see ﬁg. A.2. At the nominal midpoint of the sweep, a trigger (low to high) signal is output via the rear panel TRIG connection, for synchronising to an oscilloscope or external experiment.

The span may be limited by the minimum and maximum voltage that can be applied to the actuator, 0 and 120 V [150 V optional]. That is, the ramp may “saturate”, as shown in ﬁg. A.2. The period remains ﬁxed, and the trigger remains at the centre of the period, but the laser frequency will not scan for the entire period. Thus the spectrum will appear to shift to the left or right of centre and will be “ﬂat” for part of the span. For situations where complete linear spectra are needed, the actual ramp output should be monitored using the Freq selection of the CHAN A output.

46 Appendix A. Speciﬁcations

120V

time

TRIG

STACK

FREQUENCY

SPAN

Figure A.2: STACK output voltage and trigger pulse, when FREQUENCY

is set near the midpoint (upper) or moved closer to 0 V (lower), where the output voltage exceeds the maximum range.

B. Troubleshooting

The MOGLabs DLC detects a wide range of fault conditions and deactivates related circuitry accordingly. The front-panel LEDs provide indication of the state of these functions.

B.1 STANDBY/RUN indicator

Colour

DARK

RED

ORANGE

GREEN

Status

Temperature controller oﬀ.

Reset via keyswitch, RUN → STANDBY → RUN Possible faults:

• AC mains oﬀ

• Interlock(s) disconnected

• TEC open or short-circuit

• TEC polarity reversed

• Cable disconnected

• Temperature sensor disconnected

• Active temperature sensor connected to

thermistor pins

• Thermistor connected to active sensor pins

• Temperature out of range (< −5◦C or

> 35◦C)

• External sweep selected (DIP switch 9) but no external sweep supplied

• Wrong AC mains voltage

AC mains power failure (temperature controller

oﬀ) Standby (temperature controller on)

Fully operational (piezo, current, ramp)

48 Appendix B. Troubleshooting

B.2 Diode OFF/ON indicator

Colour

RED

ORANGE

GREEN

Status

Fault

Reset via OFF/ON switch ON → OFF → ON

Possible faults:

• SCAN/LOCK switch not up (SCAN)

• OFF/LOCK switch not up (OFF)

• Rear interlock disconnected

• Laser head interlock disconnected

• Laser head cable disconnected

• TEC disabled (temperature out of range)

• Any one of +5, ±10, ±12 V internal

supplies below nominal by more than 1 V

• External sweep selected (DIP switch 9) but no external sweep supplied

Standby: above conditions satisﬁed, diode ready to start

Diode fully operational, piezos active

If the indicator remains ORANGE after switching the diode ON, check the possible faults listed above, in particular the lack of a clock sync provided from internal or external sweep (see 2.4).

B.3 250 kHz modulation 49

R101

R100

R102

R99 R111

RT6

U27

R552

R139

R109

R110

C45

R96

C39

R117

R98

R116

U34

R128

R136

R120

U35

C50

R129

R137

R130

R119

C56

C38

R97

R93

R118

RT8

C47

U28

C40 L9

C48

U36

C55

C53

C51

U15

U24

R601

R22

R28

U19

R42

R43

C27

R63 C32

R54

R66

C33

R75

R65

C37

R79 R87

R91

C16

R21

C24

K1C2U4

R23

R19

R20

C18

R68

R37

R59

RT3

R36

U10

C19

R35

C35

RT4

C30

C28

R67

C34

R71

R58

R77

R76

R92

R69 R88

U25

C214

R72

RT5

R81

R80

U26

R85

R90

RT2RT1

TO220_KIT

mod

P35

Amp

Dist

Freq

Amp-D

Amp-I

B.3 250 kHz modulation

The 250 kHz sine-wave oscillator relies on critical non-linear behaviour of an electronic component. Due to component drift, the oscillator may cease, and the AC error signal is then lost. A few small adjustments of trimpots will restore the oscillator.

1. Measure test point P35 (with a multimeter) and adjust RT5

2. Probe U25, Pin14 (top pin on right-hand side of U25), and

Figure B.1: 250 kHz oscillator trimpots and testpoints.

(labelled Amp) for −1.15 volts. P35 is near RT5 Amp trimpot.

adjust RT4 Dist and RT3 Freq to obtain a 2.6 V peak-peak, 250 kHz sine wave. RT4 is used to bring the oscillator to life, and adjust the voltage gain. RT3 is used to adjust the frequency, only. Adjust RT4 ﬁrst, and once the sine wave appears, adjust RT3 for 250 kHz, then ﬁnally adjust RT4 for

50 Appendix B. Troubleshooting

the 2.6 V p-p.

3. Probe test point P7 (near RT8 Amp-D and U28), and adjust RT8 to obtain a 1.0 V peak-peak sine wave.

4. With the rear-panel I

trimpot set to maximum (fully clock-

mod

wise), probe test point P36 (just to the right of U59) and adjust RT6 Amp-D to obtain a 1.0 V peak-peak sine wave.

5. Finally adjust I

to the required modulation depth, typically

mod

about half way (6 turns anti-clockwise).

B.4 Locking 51

B.4 Locking

The MOGLabs controller provides feedback via three channels each with a complex servo loop function. A few common problems are addressed here; for more diﬃcult problems, MOGLabs will be happy to work with you to ﬁnd the best possible solution.

B.4.1 SLOW does not lock

• Try locking with STACK only, DISC only, or both (see DIP switches 1,2).

It can be very useful to watch the SLOW output (via CHAN A) when locking.

• Try locking with FAST channel only. If FAST locking works but not SLOW, then there is a gain or polarity problem, or a disconnect on one of the slow actuators (STACK, DISC).

• STACK feedback has wrong polarity. See DIP switch 10.

• Lock signal zero-crossing too far from trigger point.

• Gain too high. Try smaller and smaller gain, but be careful to

ensure that the lock error signal is crossing zero.

• Loop response too fast for actuator. The controller is normally shipped with slow-channel response gain of 1 (0dB) around 700 Hz. Please contact the factory for instructions on changing this for slower actuators.

B.4.2 SLOW locks only brieﬂy

Usually this is because the STACK feedback has the wrong polarity. Again, it can be very useful to watch the SLOW output (via CHAN A) when locking. Try ﬂipping DIP switch 10. Ensure the laser frequency is scanning properly, i.e. that the STACK is properly connected and working.

52 Appendix B. Troubleshooting

B.4.3 FAST does not lock

• FAST feedback has wrong polarity. Try reversing the polarity

with the front-panel switch.

• If the laser frequency is close to a mode hop (i.e. intrinsic diode cavity resonance is half way between two external-cavity longitudnal modes), the current response can be opposite to normal. Try adjusting the diode current very slightly.

• Lock signal zero-crossing too far from trigger point.

• Gain too high. Try smaller and smaller gain, but be careful to

ensure that the lock error signal is crossing zero.

B.4.4 FAST locks only brieﬂy

The FAST channel is normally AC-coupled (see DIP switch 12), with a time constant of 0.1 s. Thus with FAST feedback only, the laser will drift oﬀ resonance. Normally the SLOW channel is used to compensate for very slow drift, but the laser can be locked by current feedback only with DIP switch 10 ON. With DC current feedback, the feedback saturates at ±10 mA.

C. Using DBR/DFB diodes

DBR (Distributed Bragg Reﬂector) and DFB (Distributed FeedBack) diodes oﬀer a compact and robust alternative to ECDLs. The linewidth of DBR and DFB diodes is typically 2 to 3MHz, and they are very susceptible to external optical feedback, necessitating two or even three stages of Faraday isolator to prevent frequency instability. Their frequency of operation is controlled by temperature and current only, and the DLC must be reconﬁgured for optimum use without the usual piezo actuator control. The issues are discussed below.

C.1 Fine current control

Without piezo control of frequency, very ﬁnd control of the current is required. The coarse CURRENT knob can be used to set the current to within a milliamp or two, and the FREQUENCY knob must then be used. The FREQUENCY knob is normally used to adjust the piezo actuator oﬀset, but it also couples to the current via the current feed-forward (bias). The BIAS trimpot can be adjusted such that the FREQUENCY knob varies the current by up to ±25 mA. For ﬁner control, the BIAS can be reduced arbitrarily, from fully anti-clockwise (−25 mA range) to fully clockwise (+25 mA range). Note that DIP switch 4 must be ON.

C.2 DC current feedback

For locking, the current feedback is normally AC coupled because slow drifts are compensated by the STACK actuator. Change to DC current feedback by turning DIP switch 12 ON.

54 Appendix C. Using DBR/DFB diodes

C.3 Slow current feedback

The feedback signal that normally drives the DISC actuator can be coupled to the current feedback, by turning DIP switch 16 ON.

C.4 Lock saturation

Slow drift is normally compensated by the STACK actuator, and hence the DISC and current feedback signals only have small range, and with DBR/DFB diodes this is easily saturated. Use feedback conﬁguration B (see section 2.5) to maximise the lock range. Dip switch

1 should be ON.

C.5 Special options

Modiﬁcations can be made to the controller, including:

1. External control of temperature set-point, for example to enable slow frequency scans via the diode temperature.

2. Very slow locking feedback to the diode current.

3. Very slow locking feedback to the temperature set-point.

Contact MOGLabs for details.

D. Modulation coils

The MOGLabs DLC is designed to lock to an atomic transition, particularly using AC locking. The frequency of the laser light can be modulated (e.g. using internal current modulation or an external modulator), or the reference can be modulated. In the latter case, an atomic reference can be modulated at low cost using a solenoid coil wrapped around an atomic vapour cell, as shown below.

Figure D.1: Vapour cell, Zeeman coil, and primary excitation coil, mounted on PCB (available from MOGLabs).

D.1 Field requirements

Ideally the Zeeman dither coil should produce a frequency shift of about half the peak width, typically a few MHz. Atomic “stretched” state transitions will be Zeeman shifted by

so we need ﬁelds of around one Gauss (10−4Tesla). The magnetic ﬁeld inside a long solenoid is

µB=

e¯h

= 1.4MHz/Gauss (D.1.1)

B = µ0ni (D.1.2)

56 Appendix D. Modulation coils

where n is the number of turns per unit length and i the current. For wire diameter 0.4 mm, n = 2500m−1, and the current requirement is only 22 mA/MHz.

D.2 Coil impedance

However, driving an oscillating current through a coil is problematic because the impedance grows with the frequency. The impedance is given by XL= ωL where ω is the radial frequency and L the inductance. The inductance for a long solenoid is

L = µ0n2Al (D.2.3)

where A is the cross-section area of the coil (πr2for a circular crosssection) and l is the coil length. In practice, the inductance will be less (e.g. see Wheeler [9]):

Wheeler

N2r

228r + 254l

(mH) (D.2.4)

where N is the total number of turns, r is the coil radius in metres, and l is the length in metres (l > 0.8r). We have found that for dimensions typical of coils wound around vapour cells, these two formulae agree within a factor of two.

Note that the inductance increases with n2whereas the magnetic ﬁeld and hence modulation depth grows with n; thus for our purposes, we generally prefer small n and large currents. On the other hand, the driving voltage requirement (the “back emf”) is given by

ε = −L

di dt

= Li0ω (D.2.5)

max

for a sinusoidal current of amplitude i0. The required output slew rate is

dV /dt = −L

d2i

Max ≡ Li0ω2. (D.2.6)

D.3 Impedance matching 57

The MOGLabs DLC operates at ω = 250 kHz. For a cell of length 8 cm, 0.4 mm wire, and 20 mA, we ﬁnd L

= 20V, and the maximum slew rate is 32 V/µs.

max

Wheeler

≈ 650 µH, and

The MOGLabs DLC does not have that direct output capability. Reducing n helps: inductance, and thus ε and dV /dt fall with n2while the frequency modulation depth falls with n. Thus a coil of about 40 turns (500 m−1) and current amplitude of 150 mA should result in a modulation depth of 1.3 MHz. However, we prefer to use a two-coil impedance matching arrangement to increase the modulation depth at smaller currents.

D.3 Impedance matching

The DLC can drive up to ±0.5 A and ±8 V, with a slew rate of 6 V/µs. This can be impedance-matched to a high current coil using a transformer, or quite eﬀectively by directly winding a primary on the main Zeeman coil, as shown in the photo above.

For the main Zeeman coil, 0.4 mm to 0.6 mm diameter wire wound around the vapour cell, about 120 to 200 turns, works well. The coil is “balanced” for the standard modulation frequency of ω = 250 kHz using a capacitor. The coil is excited inductively by a primary, about ﬁve to ten turns, connected directly to the DLC modulator output (see ﬁgure). The cell, coils, and balancing capacitor can be conveniently mounted on a PCB, as shown in the image above, available from

MOGLabs.

Figure D.2: Circuit diagram for Zeeman coil and excitation coil. Typically the primary is 5 to 10 turns, and the secondary 120 to 200 turns.

The capacitor should be chosen such that the capacitive impedance

58 Appendix D. Modulation coils

equals the inductive impedance. That is,

D.4 Tuning

ωL =

Using the long-solenoid equation for inductance,

although in practice we ﬁnd that the inductance is about half the long-solenoid prediction and hence the capacitance should be doubled, typically about 1 to 5 nF. With this arrangement, energy is stored in the inductor-capacitor “tank”, and the DLC need only drive a small current (e.g. 50 mA peak-to-peak) to compensate for losses.

ωC

C =

ω2µ0n2Al

. (D.3.7)

ω2L

(D.3.8)

WARNING! The potential across the secondary Zeeman coil can

easily be hundreds of volts! Please ensure that your coil and capacitor do not have exposed connections! Also be sure to use capacitors with adequate voltage rating.

To maximise the current in the secondary, the capacitor should be chosen to tune the circuit to the DLC modulation frequency. A spectrum analyser with tracking generator is particularly helpful: connect the coil to the TG output, and to the SA input, and sweep through the resonance (see ﬁgure). Alternately, drive the coil with a function generator and measure the magnetic ﬁeld with another independent coil (e.g. 20 turns of ﬁne wire on a 1 cm diameter former) connected to an oscilloscope. Adjust the capacitor by adding or removing small capacitors in parallel, until the detected ﬁeld is maximum at 250 kHz. Again, be sure to use capacitors with suﬃcient voltage rating.

In some cases the Q of the circuit may be too high, such that a series resistor of about 0.5 ohm can result in increased current at 250 kHz, and reduced sensitivity to frequency drifts.

D.5 Shielding 59

Ref

-26 d Bm

Att

5 dB

-30 d Bm

Cente r

250 k Hz

Span

500 k Hz

50 kH z/

RBW

1 kH z

VBW

30 k Hz

SWT

2.5 s

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

1 Marke r 1 [T1 ]

-41. 37 dB m

25 0.00 00000 00 kH z

Figure D.3: Coil response acquired using a spectrum analyser with tracking generator. The response shows a strong resonance near 250 kHz.

D.5 Shielding

Large magnetic ﬁelds oscillating at 250 kHz can readily cause problematic electromagnetic interference (EMI). Induction in the laser head or the cable to the laser head can easily produce substantial diode current modulation. The coil (and vapour cell) should be located far from the laser and from the controller, or shielded with soft iron or a high permeability alloy such as mu-metal or Conetic. We ﬁnd that a tube made from thin (0.25 mm) sheet mu-metal, about 50% longer than the cell and coil, is adequate.

60 Appendix D. Modulation coils

E. External modulators and injection

current modulation

The MOGLabs DLC is designed for AC locking a laser to an external reference such as an atomic resonance or an optical cavity. In many cases it is convenient to use the internal modulator driver, and Zeeman modulation of an atomic transition, as described in appendix D. Zeeman modulation is not always possible (e.g. if the reference is an optical cavity), or desirable (e.g. due to magnetic interference). The MOGLabs DLC can dither the laser diode injection current (DIP switch 3), or drive an external modulator, such as an electro-optic modulator (EOM) or acousto-optic modulator (AOM).

E.1 Coupling circuit

The DLC provides a current-controlled modulation output, with 1 Ω sense resistor. It can be directly connected to a 50 Ω load, producing a voltage of ±5 V with I

adjusted to ±100 mA.

set

Impedance-matching and a DC level shift may be needed to drive an external modulator, as in the schematic below, designed for the D323B RF ampliﬁer from ISOMET.

Figure E.1: Coupling from MOD OUT to an external modulator.

The ISOMET D323B RF driver has a frequency control input with 4 to 17 V range. We AC couple using a simple 10T:10T ferrite bead trans-

62 Appendix E. External modulators and injection current modulation

Photodiode

Phase shifter

Lowpass

2.5MHz

10MHz

Probe

laser

PBS

λ/2

Mixer

µ-metal

Rb vapor cell

Fiber

Amplifier

Error

signal

Coupling

laser

Microwave

beatnote

Saturated

absorption

spectroscopy

Phase

lead

Photodiode

former. Primary and secondary were wound with 10 turns of PVCinsulated hookup wire around a ferrite bead approximately 15 mm diameter. A 500 Ω potentiometer allows control of the modulation amplitude, and a 9 V battery and 100 kΩ potentiometer provide a

DC shift to set the centre modulator frequency. The latter allows

frequency oﬀset control of the modulated light beam.

E.2 Injection current modulation

The MOGLabs DLC can dither the laser diode injection current (set by DIP switch 3), at the standard 250 kHz, or with high frequency modulation (e.g. 10 MHz) via the SMA RF input on the laser headboard. Very narrow linewidths can be achieved with suitably high bandwidth frequency discrimination, for example by phase locking two lasers. The diagram below shows an arrangement to lock two lasers to an EIT (electromagnetically induced transparency) resonance, which obtained a beatnote linewidth below 1 kHz [10].

Figure E.2: High bandwidth locking based on FM sideband demodulation [11, 6]. The probe laser is locked with high bandwidth, relative to the coupling laser, using electromagnetically induced transparency as a dispersive reference.

E.2 Injection current modulation 63

Att

25 dB

RBW

300 Hz

AQT

200 ms

Ref

-53 dBm

Center

6.8346 GHz

Span

750 kHz

75 kHz/

-100

-95

-90

-85

-80

-75

-70

-65

-60

-55

The coupling laser was locked to the 52S

F = 2 → 52P

1/2

3/2

F = 2 transition of87Rb using the Zeeman modulation technique, as in section 3.5. The probe laser was tuned to the F = 1 → F = 2 transition and modulated at 10 MHz. The two lasers copropagated through a Rb vapour cell and onto a photodiode. An electromagnetically induced transparency provided a dispersive reference. A frequency error signal was obtained by FM demodulation [11, 6]. The error signal is returned to the external error input on the probe laser MOGLabs DLC, which locked the laser with bandwidth up to about 40 kHz. The error signal was also coupled through a single stage passive phase-lead (high-pass) ﬁlter, and then combined with the 10 MHz modulation using a passive bias tee, and injected into the SMA modulation input, to provide feedback bandwidth of about 600 kHz.

Figure E.3: RF beatnote from two MOGLabs DLC-locked lasers. The

−3 dB peak width was 750 Hz with a spectrum analyser RBW setting of 300 Hz. For a 20 s average, the width was about 4 kHz.

64 Appendix E. External modulators and injection current modulation

F. Photodetector

5.72

The MOGLabs photodetector, shown below, can be used as a single detector, or as a diﬀerential pair (internal DIP switch 8). The photodetector is connected via the rear socket and cable provided. A number of M4 and 8-32 threaded holes allow mounting in diﬀerent conﬁgurations to minimise the footprint on an optical bench (see ﬁgure F.2).

Figure F.1: MOGLabs DLC balanced diﬀerential photodetector.

Figure F.2: M4 mounting holes are marked with a dimple; others are 8-32.

Single channel photodiode 1, diﬀerential signal 1 − 2.

66 Appendix F. Photodetector

λ (nm)

Relative detection efficiency (%)

400

600 800 1000 1200

100

400 600 800 1000 1200

100

λ (nm)

Relative detection efficiency (%)

F.1 Photodiodes

The standard photodetector uses Si-PIN photodiodes encapsulated in a coloured plastic which transmits in the near-infrared and blocks most room light. The diodes include a lens to reduce the acceptance angle to ±10◦. Unﬁltered diodes, and wider acceptance angles, are also available.

Photodiode Speciﬁcations

Parameter Standard Options

Spectral range(10% of max) 750 – 1100 nm 400 – 1100 nm Peak sensitivity 900 nm 850 nm Half angle ±10

◦

±20◦; ±75

Sensitive area 1 × 1 mm Max incident power 500µW Apparent sensitivity (CHAN A) 30 mV/µW

◦

Figure F.3: Photodiode spectral response, standard ﬁltered and unﬁltered.

G. Laser head board

A laser head interface board is provided to allow convenient connection breakout to the laser diode, TEC, temperature sensor, piezo actuators, and laser head interlock. It also includes a solid-state protection relay and passive protection ﬁlters, a laser-on LED indicator, and an SMA connection for direct diode current modulation via a microwave bias-tee. The laser head board can be mounted to the supplied laser head panel. Two versions are available; please refer to the MOGLabs laser user manual for information on the board provided with MOGLabs lasers.

Figure G.1: MOGLabs DLC laser head board showing headers for con- nection of laser diode, piezo actuators, temperature sensor, TEC and head enclosure interlock.

G.1 Headboard connectors

P1 Microwave RF modulation input (SMA) P3 Diode (SMA, high bandwidth) HD1 Diode (MOLEX, low bandwidth) HD2 Active temperature sensor (AD590 or AD592) HD3 Peltier TEC HD4 Interlock; laser disabled unless short-circuited HD5 Thermistor temperature sensor, 10 kΩ HD6 Primary piezo STACK HD7 Piezo DISC HD8 Secondary piezo STACK

68 Appendix G. Laser head board

Note only one temperature sensor should be connected. For high bandwidth RF modulation (see below), the diode should be connected to the SMA connector (P3) rather than to the MOLEX HD1. Another very small circuit board, to connect directly to the diode, is also available from MOGLabs, with SMA and MOLEX connectors. The MOGLabs DLC does not provide a mechanism for optical power control or measurement for diodes with an internal photodiode.

G.2 Dual piezo operation

The DLC provides outputs to two piezo elements. They can be conﬁgured as:

Single Typically, only a single “stack” actuator, such as the Tokin

AE0203D04 (available from Thorlabs, www.thorlabs.com), will

be required. The single stack actuator allows frequency scanning and frequency oﬀset selection, and active slow feedback (up to ≈ 100 Hz). Normally STACK HD6 is used.

Alternate single channel If there is a failure of the STACK electronic

driver, it is possible to use the DISC driver. To change to the alternate high voltage driver:

• Connect the STACK piezo actuator to HD7

• Insert a 0R0 resistor, size 0603, for R602

• Remove R601 (nominally 10R0)

• Change R372 from 30k0 (size 1206) to 270k (STACK at

120 V) or 390k (STACK at 150 V) (see LK2, p.15).

• Adjust RT7 fully clockwise.

Two channel The DLC feedback servos include a second channel for

high-speed piezo feedback, typically to a disc actuator. This would be connected to HD7.

Parallel The DISC channel can instead be used to drive a second

STACK actuator, for example to allow simultaneous translation

G.3 RF coupling 69

and tilt of a diﬀraction grating, to increase the mode-hop free tuning range. This is the default conﬁguration for Rev. 9.01. For earlier revisions, to activate this mode:

• Connect the second STACK to HD7

• Insert a 0R0 resistor, size 0603, for R602

• Remove R601 (nominally 10R0)

• Change R372 from 30k0 (size 1206) to 270k (STACK at

120 V) or 390k (STACK at 150 V) (see LK2, p.15).

• Adjust RT7 to vary the relationship of the potential to the second piezo from 0.3 to 1.0 times the potential on the main STACK.

G.3 RF coupling

The SMA connector on the laser head board allows high-frequency current modulation. The RF input is AC coupled, with low- and highfrequency limits of about 30 kHz and 2.5 GHz (see ﬁg. A.1). Capacitor C4 is now (Rev. 9.01+) 47 nF; earlier revisions used 100 pF. C4 can be changed to adjust the low-frequency cutoﬀ. For higher bandwidths, use an external bias-tee such as the Mini-Circuits ZFBT-

4R2GW-FT between the head board and the diode.

From Rev. 9.01, the input sensitivity has been reduced by a factor of approximately 20 compared to earlier revisions. The input impedance is 1 k. The sensitivity depends on the diode impedance but is now typically around 1 mA/V.

WARNING: The RF input is a direct connection to the laser diode. Excessive power can destroy the diode. It is separated from the head board relay by an inductor, and thus the relay does not provide protection from high frequency signals.

70 Appendix G. Laser head board

r e s a L

+ r o t s i m r e h T

- r o t s i m r e h T

+ r e i t l e P

- r e i t l e P

+ o z e i P k c a t S

- o z e i P k c a t S

+ o z e i P c s i D

- o z e i P c s i D

- r e s a L

+ r e s a L

- y a l e R

+ y a l e R

D E L

5 R

R 9 9 4

t u p n I t n e r r u C r e s a L F R

A M S e l a m e F

2 r i a P

d l e i h S 4 / 2 P

4 r i a P

1 e l g n i S

2 e l g n i S

3 e l g n i S

1 r i a P

0 1

d l e i h S 3 / 1 P

1 1

3 r i a P

2 1

3 r i a P

3 1

5 e l g n i S

4 1

6 e l g n i S

5 1

4 e l g n i S

6 1

0 r i a P

7 1

0 r i a P

8 1

d l e i h S 5 / 0 P

9 1

5 r i a P

0 2

5 r i a P

1 2

d l e i h S 6 P

2 2

6 r i a P

3 2

6 r i a P

4 2

e l o H t n u o M

1 D

1 D H

50R

4 R

R 3 4

I N D

2 D

+ r o s n e s e v i t c A

- r o s n e s e v i t c A

g i S

d n G

1 P

P 5 - A M S

k c o l r e t n I r e s a L

k c o l r e t n i n o t i i s o p d i L

t c a t n o c e e r F e g a t l o V

x o b n e h w s e s o l c t a h t

n o t i i s o p n i

4 D H

s d a e l g n i y l F

2 R

9 9 k 4

4 C

47nFV 0 5 2

g i S

d n G

3 P

P 5 - A M S

b a n d w i d t h R F h i g h

c o n n e c t i o n t o d i o d e

1 L

3.3uH

1.9A

2 7

C N

A 1 U

4 5

C N

B 1 U

1 R

R 0 9 3

v 5 P

h t r a E s i s s a h C

d l e i h S

v 5 P

d n G

1k0

Figure G.2: MOGLabs DLC laser head board schematic. The RF mod-

impedance (∼ 50Ω).

ulation low-pass cutoﬀ frequency is determined by C4 and the diode

H. Feedback overview

Photodetector

Firewire 6 pin

Mod out

Phase

Inamp

Demodulator

+/-

Photodiode oset

Input

signal

Sine ref

Dither current range

Rear panel Imod

Gain

RT6 current dither gain

Current dither enable

Dip 3

& front panel MOD

DC lock enable

Dip 7

Dierential PD

Dip 8

Test H/W

Dither on/o

Front panel MOD

Front panel

Phase adjust 0 − 360°

Front panel

Diode current dither

Gain +/-

Error oset

Summer

+/-

External signal error/current

Error oset

Rear panel

External error enable

Dip 5

Front Panel

Error signal

250kHz

Phase midpoint polarity control

Front panel

Master gain

Figure H.1: Overview of error signal.

Rear panel BNC

Trigger out (TTL)

DVI-D DL

Laser

Sweep generator

Summer

Sweep frequency

Buer

External sweep enable

Gain +/-

Frequency

Summer

Integrator

di2/dt

Gain +/-

HV Driver

Stack sweep enable

Gain

Summer

HV Driver

Disc sweep enable

+/-

Piezo gain

External sweep

(stack oset)

Stack drive polarity

compensation

Disc piezo

Stack

enable

Stack polarity

Stack oset

Rear panel

Rear panel BNC

Disc oset

External sweep signal

Slow current signal

Dip 9

Dip 10

Dip 1 & Dip 2

Dip 13

Dip 14

Dip 11

control

External sweep signal

Dip 15

External piezo control

External sweep signal

Piezo fast control enable

Dip 1

Piezo slow control enable

Dip 2 & NOT Dip 1

Front panel

Sweep amplitude

Front Panel

Scan/Lock

Front panel or DI-2

Stack piezo signal

Track/Hold

DI-4 (Hold)

Sweep

level

Error signal

R113

72 Appendix H. Feedback overview

Figure H.2: Overview of slow feedback and piezo signals. Note that resistor R113 is not installed by default.

Buer

Integrator

Gain

Summer

Current Regulator

Current setpoint

Current limit

+/-

Gain

Phase midpoint

External Signal Error / Current

Phase gain

Current gain

Current polarity

Current

Sweep level

Bias enable

Current regulator enable

Display : Current

Actual & Limit (-)

Diode Voltage

polarity control

Rear panel

Rear panel - BNC

Bypass AC

Dip 12

Dip 4

Phase lead

control

100uA/V

2.5mA/V

current enable

Dip 6

External

Dip 16 Piezo

current enable

Bias gainFront panel

Front panel

Front Panel

RT12

Front panel

Current control enable ‘FAST’

Front panel or DI-3

Front Panel or DI-1

soft start

Slow current signal

Diode current dither

DVI-D DL cable

Laser

Error signal

Figure H.3: Overview of fast feedback and diode current signals.

74 Appendix H. Feedback overview

I. Connector pinouts

1 8

17 24

I.1 Laser

WARNING: The LASER connector is a standard DVI-D Dual Link socket as used for consumer digital display devices. It should only be connected to the corresponding MOGLabs laser head board. It supplies the high-voltage signals to drive the laser piezoelectric actuators. The piezo drivers will be disabled if the cable is disconnected, but nevertheless considerable care should be taken to ensure that non-MOGLabs devices are not connected via this connector.

The MOGLabs cable can be replaced with a standard digital DVI-D

Dual cable. There is a bewildering assortment of apparently similar

cables available; only high quality dual-link digital DVI-D cables should be used.

Pin Signal Pin Signal Pin Signal

1 TEC – 9 DIODE – 17 DISC + 2 TEC + 10 DIODE + 18 DISC – 3 Shield 11 Shield 19 Shield 4 TEC – 12 DIODE – 20 STACK + 5 TEC + 13 DIODE + 21 STACK – 6 AD590/592 – 14 Relay GND 22 7 AD590/592 + 15 Relay +5V 23 NTC – 8 16 Interlock +5V 24 NTC +

Figure I.1: LASER connector on rear panel.

76 Appendix I. Connector pinouts

135

246

LASER ENABLE ø2.1mm

+5V/5k ø6.5mm

I.2 Photodetector

The photodetector is connected via standard 6-pin IEEE-1394 (FireWire) connectors. Note that ﬁrewire cables swap pins 3,4 with pins 5,6 so the pinout on the photodetector connector is diﬀerent to that on the controller.

Pin Controller Detector

1 Ground 2 Diﬀerential if GND 3 +12 V Signal – 4 −12 V Signal + 5 Signal – +12 V 6 Signal + −12 V

Figure I.2: Photodetector connector on rear panel of DLC and corre- sponding connector on photodetector. Diﬀerential output is enabled if pin 2 is grounded (0 V). Single-ended is open-circuit or high (+12 V). Note that ﬁrewire cables swap pins 3,4 with 5,6.

I.3 Interlock

The rear-panel interlock socket is a standard 2.1mm cylindrical DC power jack. The outer conductor is supplied with 5 V via a 5 k resistor. The inner pin is connected to ground via a 10k resistor. The laser should be enabled by shorting the two contacts.

Figure I.3: INTERLOCK connector on rear panel.

I.4 Digital control 77

I.4 Digital control

HD12 is a 10-pin header which provides access to several important control signals for locking and for sample-and-hold of the lockpoint, as described in section 2.6. The signals are standard TTLcompatible, > 2.4 V HIGH and < 0.8 V LOW. The inputs are ORed with the front toggle-switches, such that the signal is activated if either the digital input is active (i.e. HIGH) or the toggle switch is on (down).

Pin Signal Pin

1 Laser ON/OFF 2 GND 3 Lock/Sweep 4 GND 5 Fast Lock 6 GND 7 Hold 8 GND 9 +5 V 10 GND

78 Appendix I. Connector pinouts

J. PCB layout

HD1

C41

R102

R263

R100

D14 D16

R99

R111

R101

U27

R84

C10

R15

U115

RV1

R44

TR2

C14

U118

R440

R218

U11

R25

R26

C20

R47

R73

R559

R60

R78

HD12

U21

R578

U119

C160

RV7

R376

R375

HD6

C169

R386

R166

U117

R385

HD8

HD9

C152

D13 D15

C104

R272

C116

R312

U78

RV4

R279

C125

R295

RV5

R112

U116

R340

RV6

C141

R339

C134

R313

C136

C139

TR8

R256

U72

R262

TR9

TR10

C111

C113

R276

R277

C126

R288

R296

U76

C121

R284

R297

C72

C71

R162

C63

RV2

U110

R163

N20

R212

R64 R70

RV3

R193

R200

R205

R206

C80

HD4

C84

R209

R228

ZD2

C99

U55

C93

TR6

C94

HD2

HD3

R152

R153

R154

R603

R121

R606

U29

R160

TR3

R156

R155

R164

R16

U44

TR4

R158

C67

U45

R157

C69

C151

R197

R552

R219

R226

R225

R139

R149

U54

R176

R187

C107

U62

U108

R336

U109

R234

C102

RS1

R356

R113

L10

L12

R173

R174

U58

C127

C75

R169

R172

C42

R104

SW2

R103

C46

R106

R108

R94

R105

R107

U30

R132

R123

R131

U46

R124

U47

R135

R255

R89

C64

R602

C70

R210

R179

R201

R202

C86

R180

U50

C82

R203

U56

TR7

R229

C100

ZD1

C95

C96

C105

L14

R181

R183

R194

RT12

R186

R185

U51

R214

C88

R207

U52

R215

C89

R196

R195

C81

C74

C76

U53

C77

R208

R505

U111

R242

R222

R241

R217

R216

R230

U59

R221

U60

R239

R247

C106

R249

C101

R245

R246

R590

C90

R224

R220

R238

R248

U61

R223

R233

R243

R184

C43

R454

R114

C239

R115

R592

R593

R595 R594

U31 U32 U33

R598

R596 C54

C44

R599

R109

C45

R96

C39

R110

R98

U34

R128

C49

R136

R117

R116 R120

U35

R129

C52

R134

R143

C59

R452 R597

C240

R126

R159

U39

R168

R175

R144

R127

U113

U37

C50

R137

R138

R148

R130

C57

R150

R145 R146

R170

U40

R557

R147

R555

U41

R171

C131

R310

R341

C147

R257

TR11

C114

C108

U65

C110

L15

U77

C122

R298

R280

U71

C123

C142

C145

U92

R314

HD5

U79

R321

C135

D10

L18

C140

R320

U91

R322

U80

R328

U81

C133

R330

R329

R335

U87

R342

R343

C146

R347

C148

R348

R351

R352

C150

U95

R350

R358

TR13

R349

R354

TR14

U93

R353

R359

U96

R361

R360

U97

U66

R264

U67

R250 R258

R266

C115

R252

C117

R251

R267

R271

R591

R269

U68

C119

U73

R31

C118

R278

C120

C124

R285

C431 R281

R286

R299

R309

L17

R289

R290

R300

R282

C129

R287

R304

R291

R301

R302

C128

R303

R305

RT14

R315

R308

C109

RS2

R577

U69

R572

R576

R571

R589

R259

U70

R579

R261

R270

R511

R260

R604

R607

RS3

R575

N23

N22

R283

R331

L16

P17

R273

U74

R275

U75

RS4

R292

R274

RT13

R306

U82

R332

C132

U83

R323

C137

R324

R344

R333

R311

U84

R318

R316

C138

R325

R326

R317

R319

C144

U88

C143

U86

R49

RT15

U94

R357

R362

R337

U89

R338

U90

R345

TR12

R346

R327

C149

P18

P15

C56

C65

R177

RT10

C38

R97

R93

R118

C47

C60

RT8

U28

C40

RT6

U36

C48

R122

C55

R140

R151

R141

U38

R182

U42

C66

U43

C58

C61

R142

R161

C62

C73

L11

R165

C68

C78

R188

R178

R125 C427

C85

R190

R574

R211

R568

R569

R199

R497

R192

U48

R198

TR20

C319

U57

U49

C91

R231

C321

R227

R236 R235

C98

R573

U63

TR5

R496

D31

L13

U64

R237

TR19

R495

R232

R254

C103

31P11TR

C53

R119

P11

C51

C153

TR16

K7 H7

HD10

HD11

U98

R364

R365

R366

R363

R367

R543

TR15

C165

U99

C157

R544

C166

R583

R587

U102

C171

C155

R368

R372

C158

C156

R373

R56

D12

C154

R369

R371

R370

R588

R374

U100

C162

R584

R582

R377

C161

N21

R381

R564

R387

HD7

R50

C168

R389

LK2

R378

U101

R380

R383

R390

R384

R379

R382

R541

N10

D17

R392

R581

R391

LK1

C159 C164

P20

C170

R388

U24

R16

R10

U114

R12

R13

R24

R408

R22

C36

R409

C12

C11

R32

C17

C15

R605

RT7

C21

C22 R27

U12

R30

C13 C25

R38

R39

R419

R45

C26

SW1

R48

R46

C29

TR1

R82

R86

R52

R51

R53 R55

U20

R83

C325

R580

R413

U13

R418

U16

R34

R41

R17

U14

R42

R601

R28

U19

U18

R43

C27

U15

R61

R57

U17

R62

R40

R33

U22

R63

U23

R74

R54

C32

R66

C33

R75

R65

C37

R79 R87

R91

R1L1R3R6R2

R410

U107

R14

R21

C16

R561

R562

R600

R23L7R19

R20

C18

R29

R37

R68

R59

R36

RT3

C35

U10

C19

C24

R35

C30

C28

RT4

C214

R67

C34

R58

R77

R76

R92

R69 R88

U25

R71

R72

RT5

R81

R80

U26

R85

R90

RT2RT1

B1010 rev 9+

Main Board

Dip Switch Positions

16 ----------------------------- 1

-5v

1 2

I/O

FAN_IO

+12v

2*14 VAC

+ - T

2014

TO220_KIT

-12v-+12v

-12v

+5v

JTag

TO220_KIT

Ph.

OFF

+5v

+12v

TO220_KIT

Naux Paux

- 0v +

0 ADJ

TO220_KIT

Amp-D

Amp-I

Gain

TO220_KIT

Earth 120 VAC

Imax

160v

B1010P139

TO220_KIT

Freq.

Dist

Amp

80 Appendix J. PCB layout

K. 115/230 V conversion

K.1 Fuse

The fuse is a ceramic antisurge, 2.5A, 5x20mm, for example Littlefuse

021502.5MXP. The fuse holder is a red cartridge just above the IEC power inlet and main switch on the rear of the unit (Fig. K.1).

Figure K.1: Fuse catridge, showing fuse placement for operation at 230 Vac.

K.2 120/240 V conversion

The controller can be powered from AC 50 to 60 Hz, 110 to 120 V (100 V in Japan), or 220 to 240 V. To convert between 115 V and 230 V, the fuse cartridge should be removed, and re-inserted such that the correct voltage shows through the cover window.

82 Appendix K. 115/230 V conversion

Figure K.2: To change fuse or voltage, open the fuse cartridge cover with a screwdriver inserted into a small slot at the top of the cover, just above the red voltage indicator.

When removing the fuse catridge, insert a screwdriver into the recess at the top of the cartridge; do not try to extract using a screwdriver at the sides of the fuseholder (see ﬁgures).

Figure K.3: To extract the fuse cartridge, insert a screwdriver into a recess at the top of the cartridge.

When changing the voltage, the fuse and a bridging clip must be swapped from one side to the other, so that the bridging clip is always on the left and the fuse always on the right; see ﬁgures below.

K.2 120/240 V conversion 83

Figure K.4: Bridge (left) and fuse (right) for 230 V. Swap the bridge and fuse when changing voltage, so that the fuse remains on the right-hand side (see below).

Figure K.5: Bridge (left) and fuse (right) for 115 V.

84 Appendix K. 115/230 V conversion

Bibliography

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MOG Laboratories Pty Ltd 18 Boase St, Brunswick VIC 3056, Australia Tel: +61 3 9939 0677 info@moglabs.com

 2007 – 2015 Product speciﬁcations and descriptions in this document are subject to change without notice.

MOGlabs DLC202, DLC102, DLC252, DLC502 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual

Safety Precautions

Protection Features

Contents

1.1 Basic operation

1.2 Passive frequency control

2.1 Front panel controls

2.2 Front panel display/monitor

2.3 Rear panel controls and connections

2.4 Internal switches and adjustments

2.6 Digital control

2.7 Internal trimpots

3.2 Laser frequency control

3.3 External scan control

3.6 External sweep

3.7 Locking using an external signal

3.8 External control of lock frequency setpoint

4.1 Frequency reference

4.2 Noise spectra

A.2 Sweep saturation and trigger

B.3 250 kHz modulation

B.4 Locking

C.1 Fine current control

C.3 Slow current feedback

C.4 Lock saturation

C.5 Special options

D.1 Field requirements

D.2 Coil impedance

D.3 Impedance matching

D.4 Tuning

D.5 Shielding

E.1 Coupling circuit

E.2 Injection current modulation

F.1 Photodiodes

G.1 Headboard connectors

G.2 Dual piezo operation

G.3 RF coupling

I.1 Laser

I.2 Photodetector

I.3 Interlock

I.4 Digital control

K.1 Fuse

K.2 120/240 V conversion