Analog Devices AN655-b Application Notes
AN-655
MICROCONVERTER
ADuC832
AUTO-ZERO
OP AMP
AD8628
AUTO-ZERO
OP AMP
AD8628
TEC
CONTROLLER
ADN8830
WAVELENGTH
MONITOR PD
TEC
THERMISTOR
8
TUNABLE LASER
CW LASER
AVERAGE POWER
CONTROLLER
ADN2830
POWER
MONITOR PD
ETALON
FILTER
2.5V
REFERENCE
ADR291
12-BIT
DAC
12-BIT
DAC
12-BIT
SER ADC
T/H
MUX
GPIO
i8051
CW
LASER
AD8565
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/326-8703 • www.analog.com
Tunable Laser Reference Design for Designers with
the ADuC832/ADN8830/ADN2830
by Nobuhiro Matsuzoe
FEATURES
Single Board Solution for Tunable Lasers
8-Channel Selectable Wavelength Control
Wavelength Locking at 25 GHz/50 GHz Spacing
AutoPower Control (APC)
AutoTemperature Control (ATC)
AutoFrequency Control (AFC)
Laser Bias, Temperature Monitoring
EEPROM-Based Autorestoring
Serial Interfaces (SPI®/I2C®/RS-232) to Host System
Wavelength Stability < 2 pm (typ)
LD Temperature Stability < 0.01°C
APPLICATIONS
DWDM Transmission System
Optical Instrumentation
GENERAL DESCRIPTION
This tunable laser reference design offers a single board
solution with complete control requirements for DFB tunable laser subsystems. Designed to work from +3 V/–5 V
power supplies, it provides a low cost, low power tunable
laser solution designed for ease of use with standard
serial control interfaces.
A mixed-signal monolithic microprocessor, the ADuC832based wave locker feedback loop is designed to meet
the requirements of ITU-T grid spacing in a 50 GHz/25 GHz
system. An integrated 12-bit ADC with an 8- channel
multiplexer allows users to monitor laser bias current
and laser temperature via SPI, I2C, or RS -232C serial
interfaces. The ADN2830 laser bias controller can sink
up to 200 mA (single)/400 mA (dual) and its integrated
feedback control loop can maintain output optical power
constant over temperature changes during wave locking. The ADN8830 TEC controller enables excellent laser
temperature stability and precise wavelength control
with 1 pm resolution. A patented PWM/linear based TEC
drive architecture enables high efciency and minimizes
external ltering components.
REV. B
FUNCTIONAL BLOCK DIAGRAM
AN-655
–3–
AN-655
∆f F B
where:
F is the frequency slot
B is the bit rate.
S
S
<
(
)
– . /2 0 4
THERMISTOR TEC
WAVE
LOCKER
POWER
MONITOR
LASER
DIODE
MODULATOR
TEC CONTROLLER
ADN8830
I/V
AD8628
LASER CONTROLLER
ADN2830
ANALOG I/O
MICROPROCESSOR
DIGITAL I/O
SERIAL PORT
MICROCONVERTER ADuC832
I2C LINK TO HOST
2
CONTINUOUS
OPTICAL OUT
MODULATED
OPTICAL OUT
TRANSMIT DATA
2
TURNABLE LASER MODULE
REV. B
Figure 1. Typical Block Diagram of Optical
Transmitter with DFB Laser Module
WAVELENGTH TUNING
Because an optical transmitter requires a small formfactor design, use of the refractive index of a semiconductor laser is commonly used to alter the wavelength. As the
refractive index can be changed by both the temperature
and the density of carriers, transmitter designers can choose
from two different types of the wavelength-tunable laser
modules—distributed feedback (DFB) lasers and distributed Bragg reector (DBR) lasers. In general, the DBR
laser is capable of a fast tuning speed and a wide tuning
range. However, it requires multiple programmable current sources for wavelength control, which results in a
complex system design. In contrast, the DFB lasers can be
controlled by temperature of the laser chip, which results
in a lower system cost and higher reliability since the DFB
lasers are widely used today. The wavelength of the DFB
laser is related to temperature with a typical temperature
coefcient of 0.1 nm/K. By using a thermoelectric cooler
(TEC) and a thermistor with a built-in module, the laser
chip temperature can be adjusted, thus wavelength is
controlled. Figure 1 shows a simplied block diagram of
an optical transmitter designed with the DFB laser. The
wavelength tuning range of the DFB laser is limited by an
allowable operating temperature range. In fact, DFB lasers
provide a tuning range of a couple of nanometers which
is equivalent to 8 to 16 ITU-T grid channels depending on
the channel-to-channel spacing of the wavelength.
WAVELENGTH LOCKING
The internal or external wavelength locker generates
two monitor signals corresponding to the wavelength
and optical output power respectively. The wavelength
locker consists of the wavelength selective element and
the photodetector diode as shown in Figure 2a. The builtin Fabry Perot etalon works as a wavelength lter which
has a periodic characteristic similar to a comb lter. The
peak-to-peak range of the etalon lter, referred to as the
Free Spectral Range (FSR) cycle FS, is precalibrated by the
manufacturer to align with the ITU grid spacing as shown
in Figure 2b. Because the monitor signal has periodic
cycles, the algorithm of wavelength locking requires two
different tuning methods, coarse tuning, and ne tuning.
During the rst phase, the laser temperature must settle
to the particular temperature corresponding to the target
wavelength, where the wavelength is assumed within
the capture range. In the case of Figure 2b, there are two
locking points on both slopes, positive slope and negative
slope, within a single FSR. Thus, the capture range must
be half of the SFR. Feedforward control with a look-up
table is used in this coarse tuning phase. After the wavelength settles within the capture range, the ne tuning
phase acquires the wave length errors between the target
wavelength and actual wavelength by monitoring the wave
locker signal. Then the wavelength errors will be fed back
to the temperature control circuit. The ne tuning phase
maintains the wavelength within an allowable wavelength
deviation range over ambient temperature changes. The
ITU-T recommendation species wavelength stability as
a frequency deviation in G.692. This deviation is dened
as the difference between the nominal central frequency
and the actual central frequency (Figure 2c). A maximum
frequency deviation is given by
In 10 Gbps transmit applications with 50 GHz Grid spacing, the frequency deviation f must be less than 7.5 GHz
which means wavelength deviation
67 pm. The test result of the wavelength deviation taken
must be less than
by the reference design is shown in Figure 2d.
–2–
REV. B
ETALON
FILTER
I/V
I/V
OPTICAL
INPUT
POWER
MPD
WA
VELENGTH MPD
WAVELENGTH LOCKER
OPTICAL
OUTPUT
TO
µC
TO
µC
Figure 2a. Wavelength Locker Block Diagram
WAVELENGTH
MONITOR PD
LOCK POINT
WAVELENGTH (nm
)
MONITOR CURRENT (A)
POWER
MONITOR PD
FSR
F
S
TUNABLE RANGE = 3nm
F
OPTICAL POWER (dB)
WAVELENGTH (THz)
NOTES
1. 1-HOUR WAVELENGTH STABILITY AT 25C, WAVE LOCKER ENABLED,
2 SEC INTERVAL.
2. F: LOCK POINT 1.5pm MAX TO MIN: 3pm.
3. LASER: FLD5F15CA, FUJITSU QUANTUM DEVICES LTD.
4. THE WAVELENGTH STABILITY IS MEASURED IN 3pm ACCURACY.
5. AFC STABILITY IS AFFECTED BY ACCUMULATIVE ERRORS OF APC AND
ATC CONTROL LOOP.
RS-232C
CABLE
FC-APC
(PANDA FIBER)
LASER
MODULE
COMPUTER
POWER SUPPLY
(+5V/–5V)
WAVELENGTH
METER
R
TH
PD1
PD2
7
6
5 4 3 2
1
8 9
10 11 12 13 14
TEC
AN-655
DEMONSTRATION BOARD
The tunable laser reference design board (demo board)
demonstrates autopower control (APC), autotemperature
control (ATC) and autofrequency control (AFC). Figure 3
shows simplied setup of the demo board. The demo board
has a mount space for a tunable DFB laser module in a 14-lead
buttery package. The demo board also provides a power
supply terminal, analog I/O ports, and a serial port. To mount
laser modules, a power photodetector must be oating within
the laser module package as seen in Figure 4.
Figure 2b. Wavelength Locker Characteristics
Figure 2c. Frequency Slots and Allowable
Frequency Deviation
Figure 3. Demo Board Setup
Figure 4. Laser Module Pin Assignment (reference: Fujitsu Quantum Devices, FLD5F6CA Data
Sheet)
REV. B
Figure 2d. AFC Typical Performance
–3–
AN-655
–5–
AN-655
V V R I V
where:
V V
R
LOCKPOINT REF m
REF
= ×
(
)
[ ]
=
[ ]
=
[ ]
–
.
46 2
46
2 50
2490 Ω
V G T V V
where:
T is target temperature of the thermistor in Celsius.
G =
V V
DAC LASER OFFSET
LASER
OFFSET
= ×
(
)
[ ]
=
[ ]
–
.
– .
0 0658
0 659
REV. B
GETTING STARTED
To ensure proper operation, follow the steps below.
1) Calculate the target voltages of the wavelength lock point according to the following equation:
The photo current Im2 needs to be solved at each lock point, because the Im2 may differ from lock point to lock point, laser
to laser. Table 1 is an example of the calculation result from the 8-channel wavelength tunable laser module.
Table 1.
Wavelength Im2 V
[nm] [THz] [mA] [V] [Dec] [Hex]
0 1582.439 189.4496 521.4 1.202 1969 07B1
1 1582.851 189.4003 573.4 1.072 1757 06DD
2 1583.265 189.3508 511.4 1.227 2010 07DA
3 1583.692 189.2997 563.4 1.097 1798 0706
4 1584.110 189.2498 527.4 1.187 1944 0798
5 1584.529 189.1997 556.8 1.114 1824 0720
6 1584.942 189.1504 524.3 1.194 1957 07A5
7 1585.365 189.1000 553.3 1.122 1839 072F
LOCKPOINT
Lock Point ADC Code
2) Calculate the voltage setpoint for ADN8830 according to the following equation:
Table 2 is an example of the calculation result from the 8-channel wavelength tunable laser module.
Table 2.
Curve
Wavelength RTH T
[nm] [THz] [] [C] [V] [Dec] [Hex]
0 1582.439 189.4496 16810 12.191 0.144 236 00EC Positive
1 1582.851 189.4003 14370 15.941 0.390 639 027F Negative
2 1583.265 189.3508 12350 19.658 0.634 1040 0410 Positive
3 1583.692 189.2997 10630 23.433 0.883 1447 05A7 Negative
4 1584.110 189.2498 9220 27.106 1.125 1843 0733 Positive
5 1584.529 189.1997 8040 30.728 1.363 2233 08B9 Negative
6 1584.942 189.1504 7060 34.247 1.594 2612 0A34 Positive
7 1585.365 189.1000 6210 37.802 1.828 2996 0BB4 Negative
LASER
V
DAC Code Slope
DAC
–4–
REV. B
AN-655
Maximum TEC voltage V VLIM V= −
( )
×
[ ]
1 5 4.
3) Compile and download the software to the ADuC832. To select the download/debug mode, position the switch (S5) to
DEBUG, and then press the reset button (S3).This sets the ADuC832 to download mode. To select the normal mode, position the switch (S5) to NORMAL and press the reset button (S3). ADuC832 executes downloaded program after reset.
4) Mount the laser module to the mount pads labeled U13. The ADN2830 is capable of sinking a current up to 200 mA. The
maximum TEC voltage limit is set at 3.5 V ±5% by default. To change the TEC voltage limiter, change the value of R24 and
R25 which is congured as a voltage divider to set the voltage to the VLIM pin of ADN8830. Maximum TEC voltage can
be given by:
5) Set JP1 and JP2. To protect the laser module from accidental damage, it is recommended to close JP1 and JP2 if the
program has been changed. Shorting JP1 enables the ADN8830 to shut down mode regardless of control signals from
the ADuC832. Shorting JP2 enables the ADN2830 to ALS (Automatic Laser Shutdown) mode regard less of control signals
from ADuC832.
6) Apply +3 V and –5 V to the power supply terminal block (J1) located at the top of the demo board.
7) Leave JP2 open. ADN2830 starts to drive the laser diode.
8) Calibrate the optical output power by adjusting the multiturn potentiometer (R48).
9) Leave JP1 open. ADN8830 starts to control the laser temperature to the initial temperature setpoint selected by switch (S4).
10) Press S1 or S2 buttons to change the wavelength lock point. S1 increments and S2 decrements the wavelength point by 1.
11) To change the target wavelength lock point directly, congure the 3-bit DIP switch (S4) according to Table 3. This change
is effective only when the ADuC832 is powered up or after reset.
Table 3.
Ch# S4(1) S4(2) S4(3)
0 OFF OFF OFF
1 ON OFF OFF
2 OFF ON OFF
3 ON ON OFF
4 OFF OFF ON
5 ON OFF ON
6 OFF ON ON
7
12) 7-segment LED (DS1) displays the selected wavelength and DS1 blinks until the laser temperature is set. TEMPLOCK LED
(D1) is lit when the laser temperature is settled within the capture range. WL_LOCK LED (D2) is lit when the wavelength
is locked within ITU grid ±12pm.
ON ON ON
REV. B
–5–
AN-655
–7–
AN-655
V I V
im m2 2
2 5 2490= ×
(
)
[ ]
. –
NOT POPULATED
DAC1
2.5V REF
R57
0
R58
R46
249
I
m2
V
m2
A
VDO
DAC0
R1
TEMPSET
THERMIN
ADN8830
2.5V
ADuC832
R
TH
R3
R2
LINEARIZATION
ERROR < 0.5%
IDEAL
ACTUAL
2.5
0
CONTROL VOLTAGE (V)
THERMISTOR TEMPERATURE (C)
10 50
R = R exp B
1
T
–
1
T
where:
R
is thermistor resistance at 25 C.
B is thermistor constant
T
is temperature in K.
Typically, B = 3450 and R 10K
TH 25
x
25
25
25
25
×
°
=
R
R R
R R
R
R R
R R
R
R R R R – R R
R R
R
where:
R R R
R R
R
low high
low high
low high
low high
mid high mid low high low
high low mid
high high
low low
1
2
2
2
3
2
2
3
3
=
′ ′
′ ′
=
′ ′
′ +
′
=
+
+
′ =
+
′ =
+
–
–
REV. B
INTERFACING WAVELENGTH MONITOR PD
Figure 5 shows the current-to-voltage conversion circuit
on the demo board. The conversion gain is set by R46.
The input range of the wavelength monitor current Im2 is
up to 1.0 mA by default. The wavelength monitor voltage,
V
, is calculated by:
im2
Figure 5. I/V Conversion Circuit
INTERFACING 12-BIT DAC AND ADN8830
By using the interface circuit shown in Figure 6, the laser
temperature is controlled by DAC output voltage. This
scales the DAC voltage range from 0 V to 2.5 V for temperature range from 10°C to 50°C. The interface circuit
linearizes the thermistor transfer function. The TEMPSET
pin of ADN8830 is xed at 1.25 V. The THERMIN pin is
connected to the resistor network which includes the
thermistor. The characteristic of voltage-to-temperature
is shown in Figure 7.
Figure 6. Application Circuit Using DAC Control Voltage
Figure 7. V-to-Temperature Characteristic
To maintain optimal linearity over the required temperature
range, the value of the thermistor resistance should be
calculated at the lowest and the highest operating temperature according to the following equation:
R1, R2, and R3 are given by:
IMPLEMENTING THE WAVELENGTH LOCK
As the rst phase, the program executes the coarse tuning
with the ADN8830 TEC controller. The program reads S4
switch position, then generates the xed control voltage to
let the ADN8830 settle the laser temperature corresponding to the selected wavelength set by S4. Because the
ADN8830 has a local control loop for the temperature
control, the program waits for the temperature locked
signal from the ADN8830. After the laser temperature is
settled within the capture range of the wavelength lock,
the program starts the ne tuning. This phase uses the
monitor signal from the wavelength locker. The program
reads the actual wavelength and compares with the target
wavelength being stored in the memory. Then the program
adjusts the temperature control voltage, which corresponds
to the error amount between the target wavelength and
the monitored wavelength. Figure 8 shows the overview
of the program ow including the course and ne tuning.
Details of the course and ne tuning are shown in Figure 9
and Figure 10 respectively.
–6–
REV. B
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