Download A Reference Design for DWDM Pump Lasers

Application Report
SBAA072A – May 2002 – Revised March 2005
An Optical Amplifier Pump Laser Reference Design Based
on the AMC7820
Rick Downs
Data Acquisition Products
ABSTRACT
The AMC7820 is an integrated circuit designed for analog monitoring and control. Its
features are put to use in this reference design for laser and thermoelectric cooler control
in EDFA and Raman optical amplifiers. The resulting circuit fits into a credit-card sized
space.
Contents
Introduction .............................................................................................................................................3
Erbium-Doped Fiber Amplifier Basics ..................................................................................................4
Pump Laser Module ................................................................................................................................5
Laser Diode ...........................................................................................................................................6
Thermoelectric Cooler (TEC).................................................................................................................6
Thermistor..............................................................................................................................................7
Back Facet Monitor................................................................................................................................8
AMC7820: An Ideal Device for Control Loop Solutions ......................................................................8
Thermoelectric Cooler Control ..............................................................................................................8
Thermistor............................................................................................................................................10
Driver ...................................................................................................................................................10
Stability ................................................................................................................................................12
Laser Control.........................................................................................................................................14
Current Sense......................................................................................................................................15
Laser Driver .........................................................................................................................................16
Optical Power Monitor .........................................................................................................................17
Conclusion ............................................................................................................................................17
Schematics ............................................................................................................................................19
1
SBAA072A
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
2
Figures
DWDM Multiplexes Many Signals Onto One Fiber ..........................................................3
EDFA Power Monitoring and Control ...............................................................................4
DWDM Transmission System............................................................................................5
Thermoelectric Cooler Block Diagram .............................................................................6
Thermistor Response Curve .............................................................................................7
TEC Control Loop...............................................................................................................9
Temperature Measurement with Ratiometric Reference ..............................................10
Class D Power Driver for TEC .........................................................................................11
TEC Response with no Compensation...........................................................................12
TEC Response with Compensation.............................................................................13
Deviation from Setpoint vs Actual Temperature........................................................14
Laser Control Loop .......................................................................................................14
Current Sense Circuits .................................................................................................15
Digitally-Controlled Current Limit ...............................................................................16
Back Facet Diode Monitor ............................................................................................17
Complete AMC7820-Based EDFA Pump Laser System ............................................18
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
SBAA072A
Introduction
Optical networking is becoming a more important networking option, and it presents some
interesting control system challenges. One of these challenges is controlling the laser diode in a
DWDM system.
DWDM stands for Dense Wavelength Division Multiplexing – this is the same concept as
frequency division multiplexing that is used to send many channels down your cable TV line. In
this case, the “cable” is actually an optical fiber, and the many different channels of data are
multiplexed onto different wavelengths. This concept is illustrated in Figure 1.
Figure 1.
DWDM Multiplexes Many Signals Onto One Fiber.
As the optical signals travel down the fiber, their optical power needs to be maintained. This is
done in a fashion similar to using repeaters in radio; periodically along the fiber, the signals are
re-amplified to maintain the optimum optical power. This amplification takes place in the optical
domain, using an Erbium-Doped Fiber Amplifier, or EDFA.
Erbium is a rare-earth element that, when excited, emits light around 1.54 micrometers—the
low-loss wavelength for optical fibers used in DWDM. A weak signal enters the erbium-doped
fiber, into which light at 980nm or 1480nm is injected using a pump laser. This injected light
stimulates the erbium atoms to release their stored energy as additional 1550nm light. As this
process continues down the fiber, the signal grows stronger. The spontaneous emissions in the
EDFA also add noise to the signal; this determines the noise figure of an EDFA.
The key performance parameters of optical amplifiers are gain, gain flatness, noise level, and
output power. EDFAs are typically capable of gains of 30dB or more and output power of +17dB
or more. The signal gain provided with an EDFA is inherently wavelength-dependent, but it can
be corrected with gain flattening filters, which are often built into modern EDFAs.
Low noise is a requirement because noise, along with signal, is amplified. Because this effect is
cumulative, and cannot be filtered out, the signal-to-noise ratio is an ultimate limiting factor in the
number of amplifiers that can be concatenated and, therefore, the length of a single fiber link. In
practice, signals can travel for up to 120km (74mi) between amplifiers.
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
3
SBAA072A
Erbium-Doped Fiber Amplifier Basics
Figure 2 shows a detailed view of an EDFA. The optical power monitors can be seen near the
top of the diagram. The pump lasers must have a constant current flow to them, in order to keep
the optical power output constant and to keep the laser on wavelength.
Figure 2.
INPUT
CONNECTOR
EDFA Power Monitoring and Control.
Erbium - Doped Fiber
ISOLATOR
1550nm
WDM COUPLER
WDM COUPLER
Photo Power
Monitor
Photo Power
Monitor
1550nm
980 nm
980 nm
Laser Current
Controller
Laser Current
Sensor
Laser Current
Sensor
TEC Current
Power Driver
Fixed Temperature to Make
Stable Wavelength
TEC A
TEC Current
Power Driver
TEC Current
Controller
TEC Current
Controller
TEC Temp.
Sensor
TEC Temp.
Sensor
TEC B
PUMP LASER A
Laser Current
Controller
PUMP LASER B
Laser Current
Power Driver
Laser Current
Power Driver
Constant Pump Laser Current to
Make Stable Output Photo Power
OUTPUT
CONNECTOR
Photodiode /
TZA
Photodiode /
TZA
Photo Power Monitor
ISOLATOR
Main System Computer
There are actually several control loops here. Inside the laser module, there are control loops for
the pump laser current and the TEC, to control optical power and temperature. These loops are
relatively slow, almost DC control problems.
The loop outside the laser module, however, is much faster and this is the loop that monitors the
input and output optical power. This loop must be fast because the optical power must be quickly
adjusted when adding or dropping channels, to control the transient response of the EDFA.
Dropping channels can give rise to surviving channel errors, since the power of these channels
may surpass the threshold for nonlinear effects such as Brillouin scattering. Adding channels
can cause errors by depressing the power of surviving channels below the receiver threshold.
Response times of this loop are required to be in the range of 0.85µs to 3.75µs.
Often, a fast Analog-to-Digital Converter (ADC) is used to get the initial fast response time, and
a slower, higher-resolution converter is used to stabilize the loop to its final value. The design
presented in this application note addresses the slower control loop problem, which is to control
current through a laser diode to provide optimal optical power output, while maintaining tight
control of the laser’s temperature so that the laser diode will stay on the desired wavelength.
4
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
SBAA072A
The temperature of the laser diode is critical in maintaining a constant wavelength, so it must be
controlled. This can be challenging, because as significant current is driven into the laser diode
to provide the power desired, the temperature cannot change. These systems address this
problem by using a Thermo-Electric Cooler (TEC) inside the laser diode module. The cooling or
heating of the laser diode is controlled by the amount of current through the TEC. This current,
as well as the pump laser diode current, must be precisely monitored and controlled.
This means controlling the temperature within ±0.1°C, while driving the laser diode with as much
current as it can handle, all the while monitoring the laser current, the laser temperature, and the
TEC voltage and current.
Since these optical networking components are part of a large system, a standard has been
established for monitoring and reporting the status of each network component. This is called
the Management Information Base, and for the EDFA, parameters such as optical SNR, pump
laser temperature, pump laser current, and others must be able to be reported back to a central
computer that monitors these parameters. This is done to insure quality of service (QoS) and to
detect faults in the system. For this reason, these parameters are converted to digital through an
ADC.
Pump Laser Module
The pump laser diode module can be seen in Figure 3. The module consists of a laser diode and
a thermoelectric cooler. The cooler acts to keep the laser diode at the same temperature,
regardless of how much power is being used in the diode. This is critical, as the laser’s
wavelength will change with changes in temperature. An additional diode, used to monitor the
optical power output from the laser diode, is included, and is often fabricated on the back facet of
the laser; hence it is sometimes referred to as the back facet diode.
The control system around this laser diode module is hinted at here: a means of controlling laser
diode current is needed, as well as a temperature control loop that controls the thermoelectric
cooler. Various critical parameters of the system are monitored and controlled by the ADC and
the digital-to-analog converter (DAC).
Figure 3.
DWDM Transmission System.
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
5
SBAA072A
Laser Diode
A typical 980nm pump laser may only have an initial wavelength accuracy of ±5nm; when in
operation, however, the pump laser must remain within ±0.5nm over time and temperature to
maintain acceptable noise levels in the EDFA.
The laser module for this design is a 975nm pump laser, with 250mW output power. Laser
modules require drive currents in the range from 300mA up to 3A. The particular laser used in
this design requires a minimum threshold current of 35mA to operate, and can be driven up to
500mA when achieving maximum power output.
Thermoelectric Cooler (TEC)
All laser modules’ output wavelengths are temperature dependent. Modern lasers, like the one
used in this design, achieve ±0.02nm/°C temperature dependence. This is a great improvement
over just a year ago, when most lasers had a 1nm/°C temperature coefficient.
The thermoelectric cooler consists of a Peltier element in close contact with the laser diode, as
shown in Figure 4. A thermistor is provided to monitor the temperature at the laser diode. The
Peltier element is then driven by some kind of power driver which monitors the thermistor and
causes the driver to source or sink current through the Peltier element to maintain a constant
temperature.
Figure 4.
Thermoelectric Cooler Block Diagram.
fiber
laser
heat transfer
Peltier element
OUT_L
thermistor
LC low-pass filter
OUT_R
Vsupply
Embedded
Controller/DSP
GND
H-Bridge Configuration
Power Amp or discretes
6
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
ADC
SCI
IN_R
PWM
IN_L
serial comms link
SBAA072A
For system stability, note that maintaining ±0.1°C temperature stability with the newer laser still
may result in a change of 0.002nm out of the 975nm center wavelength. This is a change of
±2ppm. Even with the great advances made in the lasers over the past year, temperature control
is still a very important part of the optical system, as overall stability of approximately ±4ppm is
needed in these systems.
The thermoelectric cooler in the laser module used can draw up to 1.8A when the difference
between one side of the cooler and the other side is 50°C. The TEC must not have more than
3.4V applied across it, as higher voltages will damage the element.
Note that there are no accuracy specifications for the TEC — this is a device that is intended to
be used in a closed-loop control system. The accuracy of the temperature control will depend
upon the temperature measurement through the thermistor, and the control of the current
through the TEC.
Thermistor
The thermistor used in the laser module has a negative temperature coefficient. The nominal
value of the thermistor is 10K ±500Ω, at 25°C. Like all thermistors, its response is nonlinear over
a wide range of temperature.
Figure 5.
Thermistor Response Curve.
Thermistor Response
35000
30000
Resistance (Ω)
25000
20000
15000
10000
5000
0
0
10
20
30
40
50
60
70
80
Temperature (°C)
Since the thermistor is the transducer for measuring temperature, and the system must measure
and control temperature within 0.01°C, this transducer must be linearized. Fortunately, the
characteristic of this thermistor is known, so the nonlinearity can be taken into account.
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
7
SBAA072A
Back Facet Monitor
The diode formed on the back facet of the laser diode can be used as a photodiode to monitor
the optical power output. Typically, this diode is used only as a coarse indicator of laser power,
as the back facet diode is not very accurate. In the laser module used in this design, the
responsivity of this diode is typicially 7µA/mW, but can be as low as 2µA/mW and as high as
30µA/mW.
Tracking ratio is a specification worthy of attention. It is the linearity of measured power versus
the current output from the diode. At first glance, for the laser module used in this design, this
looks terrible: the current output for a given optical power may vary by as much as 30%! In
reality, that’s true only at very low powers. At close to the rated power for the laser, the tracking
ratio actually is closer to something like 1%.
Note that the wide variability of responsivity means that the system must be designed to handle
a dynamic range that will accommodate the lowest as well as highest responsivity.
AMC7820: An Ideal Device for Control Loop Solutions
The EDFA pump laser design requires two control loops: a temperature control loop, and a
current control loop for the laser diode. Setpoints for each loop will be under computer control,
requiring DACs, and monitoring of critical parameters must be done, requiring ADCs. Most of
the actual control loop functions can be realized in the analog domain, so several op amps are
also needed. While this function could be accomplished by putting discrete devices down on a
printed circuit board, this would require significant space. EDFAs are generally put in places
where space it at a premium.
The AMC7820 is a device that contains all the functions needed for this design. The AMC7820 is
a complete analog monitoring and control circuit in a 48-pin TQFP package that includes an 8channel, 12-bit ADC, three 12-bit DACs, nine operational amplifiers, a thermistor current source,
an internal +2.5V reference, and an SPITM serial interface. It is ideal for multi-channel
applications where low power and small size are critical.
In the following sections, the features of the AMC7820 will be put to use in developing solutions
to the control problems presented.
Thermoelectric Cooler Control
The first problem is to control the temperature of the laser diode. In this pump laser example, the
wavelength needs to be controlled to within ±0.5nm of the initial wavelength. This means that
only drift is a problem, not the initial value. As long as the temperature is close to the 25°C that
the laser wants to operate at, the pump will still work, but its stability is most critical.
Since many lasers have a 1nm/°C tempco, this means that the temperature should be controlled
to within ±0.5°C maximum. However, since drift appears as noise in the amplifier, any change
will degrade the optical signal-to-noise ratio. So the design goal is to achieve ±0.1°C drift to
minimize noise. This will also give ample room for drift due to amplifier aging. (Note that the
actual laser used is much more forgiving of temperature change, with a much lower tempco than
1nm/°C. While this is nice, the system is designed to control within ±0.1°C, to achieve maximum
SNR.)
8
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
SBAA072A
The temperature control loop is shown in Figure 6. Black squares denote pins on the AMC7820.
The temperature setpoint is determined by the output of DAC0, which has a 0V to 5V range. The
thermistor is biased with a 100µA current, which is supplied by the AMC7820’s internal current
source. At 25°C, this will result in 1V dropped across the thermistor. This voltage is applied to
the integrator built around OPA7. As the difference between the thermistor voltage and the
setpoint increases, the integrator will ramp up or down.
A key feature of the AMC7820 is the inclusion of switches to disable the TEC drive. This switch
comes off the output of the integrator. When connected to the integrator, the error voltage is
passed along to a limiter circuit, and then to a DRV591 PWM power driver. This circuit will drive
current through the TEC in either direction, to either heat or cool. OPA5 and OPA6 are
configured as difference amplifiers to sense the TEC voltage and current, respectively.
Figure 6.
TEC Control Loop.
Current Mirror
10K
1:4
11
ADC
CH6
VREF
5.62K
12
10K
10K
10K
44
THERM_I_OUT
45
ISET_RESISTOR
100K
2
T_SENSOR_VOLTAGE
1M
VREF
1uF
C FF
T
10K
OPA5 13
46
VREF
1/2
OPA2342
47
1
SW_2_OUT
DIS
VREF
Temperature
Setpoint
TEC
48
768K
DRV591
10uF
VREF
VREF
+
10uF
10uH
EN
OPA7
Cooling
-
10uH
1/2
OPA2342
10uF
0.1
DAC0
6
DAC0_OUT
562K
40K
40K
3
40K
7
DAC0_OUT_SET
ADC
CH7
8K
OPA6 5
4
8K
40K
VREF
DAC0 determines the temperature setpoint, and affords temperature resolution of approximately
0.03°C per LSB. The DAC requires a reference voltage; the internal AMC7820 reference will
have an initial tolerance of 2%, and a drift of 10ppm/°C. How can this error be eliminated?
The answer is simple: use the same reference for both the DAC and the thermistor, as shown in
Figure 7. If the current flowing through the thermistor is proportional to the reference voltage,
and the temperature setpoint of our control system is also proportional to the reference voltage,
then, in essence, what the control loop must do is cause the voltage across the thermistor to
equal the voltage out of the DAC. This bridge arrangement, as shown here, works extremely well
as the control loop is geared toward applications where two voltages must balance.
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
9
SBAA072A
Figure 7.
Temperature Measurement with Ratiometric Reference.
By using the reference in this ratiometric mode, the absolute value of the reference doesn’t
matter; moreover, reference drifts over time and temperature won’t affect the temperature
control loop at all.
Thermistor
As noted earlier, the thermistor characteristic is not linear, and this nonlinearity must be taken
into account. By knowing the characteristic of the thermistor, and the excitation it is being
provided, the microprocessor driving the setpoint DAC can calculate the voltage that
corresponds to a certain temperature.
Actually, the calculation will find a code that is a ratio of the output to full-scale; this code is what
is sent to the DAC. Remember, since the converter and the thermistor are all using the same
reference, the absolute voltage doesn’t matter; only the ratio to full-scale matters.
Driver
In this design, a switching PWM driver is used to drive the TEC. This is one approach to the
output stage. A linear driver features very low noise, and can be made very efficient when it
swings close to the supply rails. Driving a 2Ω TEC to its maximum current of 1.8A, efficiencies of
close to 90% are possible with some linear driver circuits. (See “Optoelectronics Circuit
Collection”, http://www-s.ti.com/sc/psheets/sbea001/sbea001.pdf.) The key to achieving good
efficiency with a linear driver is to match the TEC driver amplifier characteristics with appropriate
power supplies for your TEC.
10
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
SBAA072A
Switching or Pulse-Width Modulated (PWM) types of drivers, as used here, can achieve very
high efficiencies, dissipating less heat in the driver. This can be attractive in these optical
networking systems as space is usually at a premium, so large heatsinks are not desirable. The
downside to this approach is that the switching noise may couple into the element being driven.
For a TEC, this is usually not a problem, as long as adequate filtering is supplied to keep the
ripple current within the specifications of the TEC.
Figure 8.
Class D Power Driver for the TEC.
½ OPA2134
½ OPA2134
The PWM driver circuit shown in Figure 8, built around a Texas Instruments DRV591, can
supply up to ±3A to a TEC. The DRV591 has a fixed gain of 2.34, resulting in a transfer function
of:
VO = VO+ − VO − = 2.34(VIN + − VIN − )
In this application, the TEC cannot have more than 2.72V across it. Since VIN- is tied to the 2.5V
reference, this means that the voltage on VIN+ must be limited to less than 3.66V and must be
more than 1.34V. The OPA2342 circuit shown on the left side of Figure 8 serves that purpose.
The DRV591 provides for fault monitoring; in this circuit LEDs light up should an over current or
over temperature condition occur, but these could just as easily be brought back to the
microprocessor for reporting to the central computer.
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
11
SBAA072A
Stability
As the temperature setpoint is changed, the loop will attempt to force the temperature to the
setpoint. If the error amplifier/integrator is not compensated properly, the loop will overshoot and
ring for quite some time. As seen in Figure 9, a 4°C change in setpoint results in the loop taking
over 30 seconds to stabilize.
Figure 9.
TEC Response with no Compensation.
TEC Response to +4°C Change in Setpoint
30
25
Temperature, °C
20
15
10
5
0
0
5
10
15
20
25
30
35
40
45
50
Time, sec
TEC Temperature
Temperature Setpoint
Adding a feedforward capacitor (CFF) to the integrator greatly reduces the overshoot and the
time needed for the loop to stabilize. See Figure 10 for a 1µF capacitor used as the feedforward
capacitor; the system is still a bit underdamped. Note that the response time has reduced to
about 5 seconds.
12
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
SBAA072A
Figure 10.
TEC Response with Compensation.
TEC Response to ±4°C Change in Setpoint
30
25
Temperature, °C
20
15
10
5
0
0
10
20
30
40
50
60
70
80
Time, sec
TEC Temperature
Temperature Setpoint
The resulting operation of the TEC control loop is shown in Figure 11. Temperature stability,
even under a 20°C change in ambient temperature, was approximately ±0.005°C at steady
state. Note that for the most part, the temperature stability is within ±0.002°C over a long period,
and is within ±0.001°C for short periods.
Figure 11.
Deviation from Setpoint vs Actual Temperature.
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
13
SBAA072A
Laser Control
The second control problem is that of controlling the current through the laser diode. In real
EDFAs, a separate very accurate optical power monitor is used. In this design, the back facet
diode is used as a rough estimate of the optical power because the initial accuracy of the back
facet monitor is only ±20%, and at any given power level, only stable within ±1%. The actual
control, however, is not dependent upon optical power but on a current level that the system will
specify and measure. The sensitivity of the laser diode to current is approximately 0.5mW/mA in
its linear region.
The laser control loop is shown in Figure 12. As before, the setpoint for the current will come
from a DAC, in this case the AMC7820’s DAC2. The current through the laser is sensed with a
sense resistor; the voltage across this resistor is amplified using a gain-of-10 instrumentation
amplifier. This signal is fed back to an integrator built around OPA1, which integrates the
difference between the DAC setpoint and the actual current. This signal drives the external linear
power amplifier, which is an OPA561.
Figure 12.
Laser Control Loop
25
ADC
CH0
23
OPA3 24
Laser Current
Setpoint
17
DAC2_OUT
DAC2
EN
29
DIS
28
SW_1_OUT
40K
31
+5V
OPA1
30
CL
0.001uF
40K
OPA561
-5V
20
DAC2_OUT_SET
6.2K
+5V
ADC
CH4
REF
INA143
G = 10
0.3 ohm
0.15V => ILD = 500mA
-5V
LASER
+2.5V
This approach works well, but care must be taken with optimizing the loop for the appropriate
transient response. As signals are added or dropped on the optical fiber, the optical power must
change rapidly to maintain a constant power through the fiber. The proportional-integral method
shown here may not be fast enough for some systems. In that case, the integrator/power
amplifier combination can be replaced with a Howland Current Pump circuit (see
“Optoelectronics Circuit Collection”, http://www-s.ti.com/sc/psheets/sbea001/sbea001.pdf).
14
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
SBAA072A
In this control loop the reference is important. The sensor for feedback is the current sense
resistor and it is not able to be ratioed to the reference. The reference’s inaccuracies and drift
will therefore be part of the setpoint.
Fortunately, the laser is concerned mainly with changes in power once the initial power level is
approximately correct. The laser in this system has a response of approximately 0.5mW per mA
of current flowing through it. The setpoint for the DAC will come from the microprocessor, which
will send the DAC a code representing a particular laser current, which in turn is proportional to
an optical output power.
Current Sense
The circuit shown in Figure 13 will sense the laser current. For a given laser module, current can
only flow one direction, but this design was to be a universal circuit, so the driver and current
sense amplifier are designed to be bipolar.
Figure 13.
Current Sense Circuits.
+5V
0.15V => ILD = 500mA
-
INA143
G = 10
0.3 ohm
2.5V to 4V
REF
+
-5V
+2.5V
LASER
GROUNDED ANODE
+5V
0.15V => ILD = 500mA
+
INA143
G = 10
-5V
2.5V to 1V
REF
0.3 ohm
+2.5V
LASER
GROUNDED CATHODE
Thus, with a grounded anode laser, current will flow as shown in Figure 13. The output voltage
from the instrumentation amplifier will range from 2.5V at zero current up to 4V at 500mA of
current. Likewise, when current flows in the opposite direction, as with a grounded cathode
laser, the output will range from 2.5V at zero current down to 1V at 500mA.
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
15
SBAA072A
If the laser polarity is known, this circuit could be modified to only allow unipolar operation, by
grounding the INA143’s reference pin instead of connecting it to 2.5V. The input polarity of the
instrumentation amplifier would also need to be reviewed to make sure that the sense of the
signal would be correct for feeding back to the control loop. Increasing the size of the sense
resistor, to take advantage of an increased signal swing, would be possible. This would allow the
ADC to realize increased current resolution for monitoring purposes.
Laser Driver
The power amplifier in this control loop is a linear power op amp. In the case of the laser drive,
the noise in the drive current is critical, as noise here may couple into the signal path. Therefore,
switching amplifiers are not widely used for laser drive applications.
Many other linear drive circuits are possible. See “Optoelectronics Circuit Collection”,
http://www-s.ti.com/sc/psheets/sbea001/sbea001.pdf.
Driving too much current through it can damage the laser. A current limit on the laser driver is
therefore a good idea. The OPA561 allows for a fixed current limit by tying the current limit pin to
the negative rail through a resistor. A DAC could also be used to provide an adjustable current
limit, if it can provide between 0V and 1.2V swing above the negative rail (in this case, a swing
from –5V to –3.8V). For example, in a circuit with a grounded cathode, the OPA561 could
operate off of a single supply (note that the OPA561 requires a minimum +7V supply in single
supply mode). In that case, DAC1 could be used as shown in Figure 14 to control the current
limit. Note using the internal resistors for the DAC as part of a voltage divider to cause the DAC
output swing to be between 0V and 1.25V.
Figure 14.
Digitally-Controlled Current Limit.
+8V
Laser Drive
0 - 3.3V
OPA561
0.3 ohm
LASER
Current
Limit
DAC1
22
DAC1_OUT
40K
40K
21
DAC1_OUT_SET
8
0 - 1.25V
10
OPA2
9
40K
16
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
SBAA072A
Optical Power Monitor
The optical power output of the laser diode is monitored by the back facet diode. The back facet
diode is used in photovoltaic mode, meaning that no bias is applied to the diode. This means
that all the current from the diode needs to be converted into a usable voltage. Using a
transimpedance amplifier, as shown in Figure 11, does this. The feedback capacitor is chosen to
minimize gain peaking.
Figure 15.
Back Facet Diode Monitor.
2pF
14
16
4K
OPA4
15
BACKFACET
MONITOR
The ADC monitors the output signal from this circuit, in hopes that the back facet monitor diode’s
measurement of optical power is approximately correct. This is probably true as long as the
output power is close to the rated power (remember the tracking ratio is better at higher powers!)
and the monitor diode is kept at a constant temperature—and that has been taken care of
already by the TEC.
Now we must consider the latency introduced by the ADC if the optical power is used as a
feedback mechanism. In the system just designed, the optical power would have to be converted
by the ADC, understood by the host processor, and adjustments made to the DAC setpoint as
needed. As channels are added or dropped from the fiber, changes in optical power must be
responded to very quickly—in less than a microsecond. If the ADC were to be used in this
manner, a much faster ADC would be required. Likewise, a faster DAC would be required,
processor overhead would have to be quite small, and the power drive circuit would have to be
the Howland Current Pump rather than the proportional-integral power amplifier used.
Conclusion
Figure 16 is a photo of the board built from the design presented. This credit-card sized circuit
realizes the complete laser and TEC control loops of an EDFA amplifier. The pump laser module
can be seen in the upper right-hand corner of the board.
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
17
SBAA072A
Figure 16.
Complete AMC7820-Based EDFA Pump Laser System.
References
18
1.
AMC7820 datasheet, Texas Instruments (SBAS231B)
2.
“Optoelectronics Circuit Collection”, Texas Instruments, http://wwws.ti.com/sc/psheets/sbea001/sbea001.pdf
3.
980nm Optilock Pump datasheet, Corning Lasertronics
4.
“Optical Fiber Amplifiers for WDM Optical Networks”, Y. Sun, A.K. Srivastava, J. Zhou,
and J.W. Sulhoff, Bell Labs Technical Journal, January-March 1999, pp. 187-206.
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
A
B
C
D
R1
4.7K
+5V
PC PARALLEL PORT
1
Y1
Y2
Y3
Y4
U1B
A1
A2
A3
A4
+5V
18
16
14
12
TP6
TP5
TP4
TP3
TP2
MISO
MOSI
SCLK
/SS
/RESET
11
13
15
17
19
SN74AHC244PW
A1
A2
A3
A4
G
SN74AHC244PW
Y1
Y2
Y3
Y4
2
D/A CHANNEL ASSIGNMENTS
------------------------------------------------------DAC0
TEC Control
DAC1
Laser Drive Current Limit 0 - 1.2V - not used at this time
DAC2
Laser Control
A/D CHANNEL ASSIGNEMENTS
------------------------------------------------------CH0
Laser Drive Control Voltage
CH1
Back Facet Monitor
CH2
TEC Drive Control Voltage
CH3
Laser Drive Current Limit
CH4
Laser Current
CH5
Thermistor Voltage
CH6
Voltage Across TEC
CH7
TEC Drive Current
9
7
5
3
2
4
6
8
U1A
1
G
1
14
2
15
3
16
4
17
5
18
6
19
7
20
8
21
9
22
10
23
11
24
12
25
13
J2
2
4
6
8
10
R2
100K
TP8
C1
1uF
R3 1M
C25
C2
1uF
+5V
1uF
C3
D2
RED
3
TP9
TP7
37
38
39
40
41
42
43
44
45
46
47
48
S1812-104K
L1
100uH
R5
390
+5V
-5V
RESET
SML-LX0603GW-TR
D1
GREEN
R4
390
+5V
SML-LX0603IW-TR
1uF
EXTERNAL SPI
1
3
5
7
9
R6
390
.001uF
C4
R7
6.2K
TP18
EXT REF
D3
GREEN
SML-LX0603GW-TR
SCLK
MOSI
MISO
SS
BVDD
DVDD
DGND
THERM_I_OUT
ISET_RESISTOR
OPA7_INOPA7_OUT
OPA7_IN+
R8
40.2K
U2
OPA3_OUT
OPA3_INDAC1_OUT
DAC1_OUT_SET
DAC2_OUT_SET
AVDD
AGND
DAC2_OUT
OPA4_IN+
OPA4_OUT
OPA4_INOPA5_IN+
1uF
C5
+5V
24
23
22
21
20
19
18
17
16
15
14
13
R11
10K
10K
TP14
ED555/3DS
1uF
C6
GROUND
TP19
-5V
10K
R14
4
8.06K
40.2K
R16
R15
R12
10K
2pF
C7
R13 4K
TP11 TP12 TP13
+5V
2
TP10
VREF
AMC7820PFB
R10
8.06K
R9
J3
+/- 5 V
3
4
1
3
VREF
R17
40.2K
-5V
1uF
C10
JMP1
MODE
TP17
5
100K
100K
+
-
10K
10K
5
3
2
J7
ED555/2DS
THERMISTOR INPUT
2
1
OF
2
FILE
2
J5
CP09750-01-250-G2-01
CORNING
M1
ED555/2DS
BACK FACET MONITOR
2
1
ED555/2DS
DATE 15-Mar-2005
REV A
A
B
C
D
6
G:\AMC7820REF\Schematic\AMC7820 REF.Ddb - Documents\AMC7820 REF
SIZE B
AMC7820 REFERENCE DESIGN
6730 SOUTH TUCSON BLVD., TUCSON, AZ 85706 USA
DATA ACQUISITION PRODUCTS
HIGH-PERFORMANCE ANALOG DIVISION
SEMICONDUCTOR GROUP
TEC DRIVER IN +
TEC DRIVER OUT +
TEC DRIVE
2
1
J6
66
REVISION HISTORY
ENGINEERING CHANGE NUMBER APPROVED
ED555/2DS
LASER DRIVE
2
1
J4
S5AC
D7
TITLE
TEC -
REV
1
TEC DRIVER OUT -
TEC +
DOCUMENT CONTROL NO. 6436035
DRAWN BY BOB BENJAMIN
SHEET 1
3
CATHODE
ANODE
LASER
GND
LASER DIODE CURRENT
SENSE RESISTOR
TP16
R22
0.3
TP15
7
ENGINEER RICK DOWNS
Maximum Current .5 A
INA143U
U4
U3
7
10 uF
C12
OPA561PWP
UNIPOLAR
5
7
6
4
1
10 uF -5V
C11
2
3
+5V
R18
0.1 TEC CURRENT SENSE RESISTOR
TP1
+5V
1uF
C9
BIPOLAR
10K
R19
R20
6.65K
R21
402K
+5V
8
4
J1
DB25
20
VCC
GND
10
6
5
2
36
35
34
33
32
31
30
29
28
27
26
25
RESET
CH2
CH3
CH4
CH5
OPA1_INOPA1_OUT
OPA1_IN+
SW1_OUT
EXT_REF_IN
REF_OUT_2.5V
OPA3_IN+
SW2_OUT
T_SENSOR_VOLTAGE
OPA6_INOPA6_OUT
OPA6_IN+
DAC0_OUT
DAC0_OUT_SET
OPA2_INOPA2_OUT
OPA2_IN+
OPA5_INOPA5_OUT
1
2
3
4
5
6
7
8
9
10
11
12
1
2
3
6
NC
8
NC
9
NC
NC
5
LASER ANODE THERMISTOR
10
4
LASER CATHODE BFM CATH
11
BFM ANODE
NC
12
THERMISTOR
GND
13
COOLER (+)
COOLER (-)
14
1
SBAA072A
Schematics
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
19
An Optical Amplifier Pump Laser Reference Design Based on the AMC7820
A
B
C
1
RESET
VREF
TEC DRIVER IN +
10K
R25
432K
R24
768K
2
5
6
3
2
3.24K
+5V
R26
8
4
R23
2
C13
1 uF
OPA2342EA
7
U5B
OPA2342EA
U5A
1
BAT54S
D4A
BAT54S
D4B
3
3
220pF
120K
C14
R27
FAULT 0
FAULT 1
+5V
C16
C15
VDD
GND
ROSC
COSC
AREF
IN +
IN SHUTDOWN
R29
1K
D6
RED SML-LX0603IW-TR
D5
R28
120K
1
2
3
4
5
6
7
8
+5PV
SML-LX0603YW-TR
YELLOW
1uF
1uF
+5V
100uF
C17
C19
10uF
C18 1uF
4
32
31
30
29
28
27
26
25
R30
1K
C20
4
+5V
+5PV
C21
10uF
+5PV
10uH
CDRH104R-100NC
L3
10uH
CDRH104R-100NC
L2
Thermal pad under
DRV591. Dimensions 5mm
X 5mm. Vias should be
equally spaced. Drill size
is 0.33mm (13 mils).
Thermal Pad is connected
to GND pin, not PGND.
NOTE: PGND and GND should be seperate
traces and connected only at power input.
Traces should be sized to handle desired
current levels.
DRV591VFP
24
23
22
21
20
19
18
17
U6
1uF
OUT +
PGND
PGND
PGND
PGND
PGND
PGND
OUT -
FREQ
INT/EXT
PVDD
PVDD
PVDD
OUT +
OUT +
OUT +
FAULT 1
FAULT 0
PVDD
PVDD
PVDD
OUT OUT OUT -
20
9
10
11
12
13
14
15
16
D
1
5
C22
10uF
5
BOB BENJAMIN
SHEET 2
OF
2
FILE
SIZE B
TITLE
6
DATE 15-Mar-2005
REV A
TEC POWER DRIVER
6730 SOUTH TUCSON BLVD., TUCSON, AZ 85706 USA
DATA ACQUISITION PRODUCTS
APPROVED
HIGH-PERFORMANCE ANALOG DIVISION
SEMICONDUCTOR GROUP
REVISION HISTORY
ENGINEERING CHANGE NUMBER
6
G:\AMC7820REF\Schematic\AMC7820 REF.Ddb - Documents\DRV591
DOCUMENT CONTROL NO. 6436035
RICK DOWNS
DRAWN BY
TEC DRIVER OUT -
TEC DRIVER OUT +
ENGINEER
10uF
C24
10uF
C23
REV
A
B
C
D
SBAA072A
IMPORTANT NOTICE
Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications,
enhancements, improvements, and other changes to its products and services at any time and to discontinue
any product or service without notice. Customers should obtain the latest relevant information before placing
orders and should verify that such information is current and complete. All products are sold subject to TI’s terms
and conditions of sale supplied at the time of order acknowledgment.
TI warrants performance of its hardware products to the specifications applicable at the time of sale in
accordance with TI’s standard warranty. Testing and other quality control techniques are used to the extent TI
deems necessary to support this warranty. Except where mandated by government requirements, testing of all
parameters of each product is not necessarily performed.
TI assumes no liability for applications assistance or customer product design. Customers are responsible for
their products and applications using TI components. To minimize the risks associated with customer products
and applications, customers should provide adequate design and operating safeguards.
TI does not warrant or represent that any license, either express or implied, is granted under any TI patent right,
copyright, mask work right, or other TI intellectual property right relating to any combination, machine, or process
in which TI products or services are used. Information published by TI regarding third-party products or services
does not constitute a license from TI to use such products or services or a warranty or endorsement thereof.
Use of such information may require a license from a third party under the patents or other intellectual property
of the third party, or a license from TI under the patents or other intellectual property of TI.
Reproduction of information in TI data books or data sheets is permissible only if reproduction is without
alteration and is accompanied by all associated warranties, conditions, limitations, and notices. Reproduction
of this information with alteration is an unfair and deceptive business practice. TI is not responsible or liable for
such altered documentation.
Resale of TI products or services with statements different from or beyond the parameters stated by TI for that
product or service voids all express and any implied warranties for the associated TI product or service and
is an unfair and deceptive business practice. TI is not responsible or liable for any such statements.
Following are URLs where you can obtain information on other Texas Instruments products and application
solutions:
Products
Applications
Amplifiers
amplifier.ti.com
Audio
www.ti.com/audio
Data Converters
dataconverter.ti.com
Automotive
www.ti.com/automotive
DSP
dsp.ti.com
Broadband
www.ti.com/broadband
Interface
interface.ti.com
Digital Control
www.ti.com/digitalcontrol
Logic
logic.ti.com
Military
www.ti.com/military
Power Mgmt
power.ti.com
Optical Networking
www.ti.com/opticalnetwork
Microcontrollers
microcontroller.ti.com
Security
www.ti.com/security
Telephony
www.ti.com/telephony
Video & Imaging
www.ti.com/video
Wireless
www.ti.com/wireless
Mailing Address:
Texas Instruments
Post Office Box 655303 Dallas, Texas 75265
Copyright  2005, Texas Instruments Incorporated