A1611BCB2000 - Agere Systems, Inc.

Application Note, Rev. 1

November 2000

Laser Control Circuits for A1611A/B Laser Modules and

the 3641-Type Laser Transmitter Subassembly

Incorporating the A1611A/B laser module, the 3641-type trans-

mitter subassembly (shown below) provides significant advan-

tages for OEMs, system integrators, and end users by enabling

the optimization of transmitter performance in various cable

television environments.

Introduction

The 3641-type laser transmitter engines and

A1611A/B laser modules are the RF/optical hearts

of broadband AM fiber-optic transmitter systems.

Successfully integrating these models into a full

transmitter requires the appropriate set of dc inter-

face circuits.

The information offered here provides an overview of

the two dc circuits needed to successfully drive

these two devices. These are the optical power con-

trol circuit and the temperature control circuit.

Conceptual schematic diagrams of a laser control

circuit, temperature control, and laser module pinout

are shown in Figures 1, 2, and 3, respectively, in the

following pages.

Optical Power Control Circuit

The following paragraphs refer to Figure 1. In order

for a transmitter to perform correctly, it is necessary

to precisely control the optical output power of the

laser. This section describes how this can be done.

In theory, the output of a laser is determined by the

amount of bias current. The goal of an optical power

control circuit, therefore, is to maintain the bias cur-

rent at the proper level.

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Application Note, Rev. 1

November 2000

the 3641-Type Laser Transmitter Subassembly

Laser Control Circuits for A1611A/B Laser Modules and

Optical Power Control Circuit (continued)

One way to do this is to sample the output power of the

laser and use a closed-loop feedback circuit to control

the bias current, and, as a result, the output power.

The monitor photodiode inside the laser package is

used to sample the output power. The back-facet light

from the laser, which is proportional to the power cou-

pled into the fiber, illuminates the monitor photodiode

and generates a current. This current is used as the

sampling element for the feedback control loop, which

keeps the laser power output constant.

The transimpedance amplifier (U1A) converts the mon-

itor photodiode current to a voltage. R3, CR1, and VR1

are used to generate an adjustable reference voltage.

The integrated amplifier (U1B) compares the reference

voltage with the monitor voltage, and outputs the differ-

ence voltage. A voltage-to-current converter, formed by

U1C, Q1, and resistors R5—R10, supplies the laser

with a negative bias. The integrating amplifier adjusts

the negative bias current so that the monitor voltage

equals the reference voltage.

Understanding the Interface Requirements

Photodiode Monitor (Pins 4 and 5). Pin 5 connects to

the cathode of the monitor photodiode in the laser

package. The current drawn by this pin is typically

between 100 µA and 1800 µA when the laser is at its

specified output power. The monitor photodiode should

be biased at 5 V. Pin 4 is connected to signal ground.

The correct operating photodiode current (PDI) is

recorded on the test data sheet provided with the laser

module or the transmitter board. Assuming the above

circuit is used, the monitor photocurrent generates a

voltage between 0.1 V and 1.8 V. Knowing the proper

PDI, the user would adjust the potentiometer VR1 to

obtain the current listed on the factory data sheet.

Laser Bias Current (Pin 3). The maximum current

required by the laser is 120 mA. At this current, the

voltage required (called the compliance voltage) is less

than or equal to –5 V. Therefore, the circuit driving this

pin must be capable of providing 120 mA at –5 V. An

analysis of the circuit shows that at 120 µA, there is a

–1.2 V drop across R9, leaving –1.8 V across transistor

Q1, which is sufficient to drive the laser current.

The operating laser bias current is recorded at the fac-

tory on the test data sheet. If the bias control circuit is

set up properly, the laser bias current and the PDI will

match the values recorded on the test data sheet.

Since the laser bias signal is connected to the laser,

noise or ripple will affect the laser optical output.

Bypass capacitors and series inductors installed on the

laser board attenuate high-frequency noise and ripple,

but low-frequency disturbances can be an issue. The

noise and ripple below 20 MHz should be kept below

50 µA peak-to-peak. This should be taken into account

in the design of the optical power control circuit.

Figure 1. Conceptual Laser Control Circuit Diagram

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Application Note, Rev. 1

November 2000 the 3641-Type Laser Transmitter Subassembly

Laser Control Circuits for A1611A/B Laser Modules and

Optical Power Control Circuit (continued)

Optical Power Status Monitoring

IfacircuitsuchasthatshowninFigure1isusedfor

optical power control, there are two voltages that can

be used to monitor the status of the transmitter.

The voltage at the output of U1A is proportional to the

output power of the laser. If a noninverting amplifier is

connected to this voltage, it can be used to provide a

scaled reading of the optical output power. The scaled

monitor can be used to drive analog test points, as well

as A/D converters for digital status monitoring, or com-

parators for triggering alarm circuits.

Since the laser bias current generates the voltage

across R9, this voltage can be used as a laser bias cur-

rent monitor. Like the optical power monitor voltage, it

can be used for a variety of status monitoring functions.

Verification Tests

Once a power control circuit is designed, it should be

tested to verify that it is working properly. The optical

power control circuit can be assumed to be working

when the optical output power is stable, and the bias

current is the same as the value recorded on the test

data sheet supplied with the laser board. If there is no

resistor in series between the bias control circuit and

the laser board, the bias current can be measured by

either inserting an ammeter between the bias control

circuit and the laser board bias pin, or by installing

a small sense resistor and measuring the voltage

across it.

As a further test, the reference voltage can be varied.

As it is varied, both the optical output from the laser

and the voltage at pin 6 (LSR-1) of the laser board

should be monitored. Any discontinuities in either the

voltage or the output power indicate problems with

the control circuitry.

The power control circuit should be tested over the full

possible range; that is, at PDI values between 100 µA

and 1800 µA.

44 Agere Systems Inc.

Application Note, Rev. 1

November 2000

the 3641-Type Laser Transmitter Subassembly

Laser Control Circuits for A1611A/B Laser Modules and

Temperature Control Circuit

The following paragraphs refer to Figure 2. The chip

temperature of the laser diode is a critical factor. It is

necessary to keep the chip operating at a fixed temper-

ature, usually 25 °C. The laser package is equipped

with devices that make temperature control possible.

The circuits required to do this are described here.

In theory, similar to the optical power control loop, a

feedback circuit is used to keep the laser chip tempera-

ture constant. The chip temperature is controlled with a

thermoelectric cooler (TEC) inside the laser package.

Positive current through the TEC pumps heat from the

laser chip to the laser package. The laser package

must be attached to a heat sink to further reduce the

laser temperature. Conversely, negative current

through the TEC moves heat from the laser package to

the laser chip, resulting in laser heating.

If a bidirectional current source is used, the laser chip

can be heated or cooled, which thereby enables laser

operation in various ambient environments.

To maintain constant laser chip temperature, a feed-

back element is necessary so that the control circuit

can sense the laser chip temperature. For this purpose,

a thermistor is mounted inside the laser package, very

close to the laser chip. Since a thermistor is a tempera-

ture-dependent resistor, a circuit that measures its

resistance is used to sense the chip temperature. The

control circuit then adjusts the TEC current to maintain

the proper laser chip temperature.

A schematic diagram of a conceptual temperature con-

trol circuit is shown below in Figure 2.

R1, 2, 6, 7, CR1, and U1C are used to generate refer-

ence voltages used in other parts of the circuit. R2 and

the laser thermistor form a voltage divider. This voltage

is buffered by U1A and compared to the reference volt-

age at the + terminal of U1B. The reference voltage is

generated by the voltage divider consisting of R3 and

R5. The difference between the reference voltage and

the buffered thermistor voltage is used to drive the inte-

grating amplifier U1B. The output of U1B drives a bipo-

lar voltage-to-current converter U2A, which is a high-

power op amp. The voltage at the output of U2B is con-

trolled so that the full range of TEC current can be sup-

plied without running into the 5 V or ground rails of the

op amp.

Since the integrating amplifier acts to drive the TEC

voltage (between pins 6 and 7), this dictates the maxi-

mum voltage (compliance voltage) between the TEC+

and TEC– pins that can be expected during operation.

It is specified at ±1.9 V. Therefore, the designer must

ensure that the TEC drive circuit can supply the

required current at up to ±1.9 V between its inputs to

zero. The overall circuit drives the thermistor resistance

to equal the resistance of R5. Since the specified chip

temperature is usually 10 kΩ,R5canusuallybeafixed

10 kΩresistor. However, if the user wants to be able to

change the operating temperature of the laser, then an

adjustable resistor could be used in place of R5.

Figure 2. Conceptual Temperature Control Circuit Diagram

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Application Note, Rev. 1

November 2000 the 3641-Type Laser Transmitter Subassembly

Laser Control Circuits for A1611A/B Laser Modules and

Temperature Control Circuit (continued)

Understanding the Interface Requirements

Thermistor (Pins 1 and 2). In addition to resistance,

there is one other important parameter: the tempera-

ture coefficient of resistance. The temperature coeffi-

cient dictates the temperature dependence of the

thermistor. It is typically –4.4% per °C. This means that

if the chip temperature drops from 25 °C to 24 °C, the

nominal thermistor resistance will increase from 10 kΩ

to 10.44 kΩ. Pin 1 is connected to temperature circuit

and Pin 2 is connected to signal ground.

Themoelectric Cooler (Pins 6 and 7). The TEC in the

laser package is connected to pins 6 (TEC +) and 7

(TEC–). When current flows from pin 6, through the

laser package, and out pin 7, heat is pumped from the

laser chip to the laser package, cooling the laser chip. If

current flows in the opposite direction, heat is pumped

from the laser package to the laser chip, heating the

laser chip.

Themoelectric Cooler Current (Between Pins 6

and 7). This specifies the maximum current required to

maintain the laser chip temperature. It is specified at

1.0 A at the minimum laser case temperature of –20 °C

and at 1.5 A at the maximum laser case temperature of

+65 °C. Therefore, the design of the TEC driver must

ensure that driver is capable of supplying this amount

of current.

Themoelectric Cooler Voltage (Between Pins 6

and 7). This indicates the maximum voltage (compli-

ance voltage) between the TEC+ and TEC– pins that

can be expected during operation. It is specified at

±1.9 V. Therefore, the designer must ensure that the

TEC drive circuit can supply the required current at up

to ±1.9 V. The polarity of the voltage depends on

whether the laser is being heated or cooled.

Temperature Status Monitoring

If a circuit such as that shown in Figure 2 is used for

temperature control, there are two voltages that can be

used to monitor the status of the transmitter.

The voltage at the output of U1A is related to the actual

temperature of the laser chip. Since the temperature

coefficient of the thermistor is known, this voltage can

be compared to the nominal voltage, allowing a calcu-

lation of the chip temperature. For example, in the Fig-

ure 2 circuit, a monitor voltage of 1.22 V implies

thermistor resistance of 9.56 kΩ. Since the tempera-

ture coefficient is –4.4% per °C, the laser temperature

in this case would be 26 °C. Similar to the optical power

monitor voltage discussed above, the chip temperature

monitor voltage can be connected to a variety of cir-

cuits a variety of status-monitoring functions.

Another possible parameter to monitor is the TE cooler

current. Since the voltage across R14 is directly gener-

ated by the TE current, this voltage can be used as a

current monitor. This can be particularly useful to indi-

cate when the transmitter temperature is approaching

its limit; a high TEC current (approaching the maxi-

mum) indicates that the laser temperature is close to its

maximum, which could mean that the transmitter tem-

perature is nearly out of range.

Verification Tests

This section describes a simple test that can be used to

make sure a temperature control circuit is operating

properly. The temperature control circuitry can be

assumed to be working when the thermistor resistance

is correct (generally 10 kΩ). Therefore, a method that

directly measures the thermistor resistance, without

disrupting circuit operation, would suffice. This can be

accomplished by inserting an ammeter between the

TEC control circuit and the thermistor pin on the laser

board, and a voltmeter from the thermistor pin to

ground.

The thermistor resistance is simply the measured ther-

mistor voltage divided by the measured thermistor cur-

rent. This setup can be used as the laser package

temperature is changed over its range. A properly

working temperature controller maintains a constant

thermistor resistance as the laser package temperature

is changed over its full temperature range.

Agere Systems Inc. reserves the right to make changes to the product(s) or information contained herein without notice. No liability is assumed as a result of their use or application.

November 2000

AP00-067OPTO-1 (Replaces AP00-067OPTO)

For additional information, contact your Agere Systems Account Manager or the following:

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Application Note, Rev. 1

November 2000

the 3641-Type Laser Transmitter Subassembly

Laser Control Circuits for A1611A/B Laser Modules and

Pin Information

Figure 3. A1611A/B Laser Module Pinouts

Table 1. Pin Descriptions

Pin Function

1 Thermistor

2 Thermistor

3 dc Laser Bias (–)

4 MPD Anode, Case Ground

5 MPD Cathode

6TEC(+)

7TEC(–)

8 Case Ground

9 Case Ground

10 NC

11 Laser Common (+)

12 Laser Modulation (–)

13 Laser Common (+)

14 NC

(5.4 mm)

.213

(LABEL NOT SHOWN)

AND DATE CODE

INCLUDING SERIAL NUMBER

LOCATION FOR LABEL

(1.2 mm)

.049

(12.7 mm)

.500

(parenthesis) MILLIMETERS

DIMENSIONS ARE IN INCHES

(30.0 mm)

1.180

(26.0 mm)

1.025

(20.8 mm)

.820

(8.9 mm)

.350

(2.0 mm)

.078

(2.7 mm)

4X Ø .105

(6.4 mm)

.250

PIN 1

6 SPCS @.100 (2.5 mm)

.400 (10.2 mm) MINIMUM LENGTH

.030 (.8 mm) WIDE X .008 (.2 mm) THICK

LEADS

(6.3 mm)

.248

(1.0 mm)

.040

(5.5 ± .25 mm)

.215 ± .010

(9.3 mm)

.365

(18 ± .8 mm)

.70 ± .03

(5 ± .8 mm)

.19 ± .03