Application Note, Rev. 1 November 2000 Laser Control Circuits for A1611A/B Laser Modules and the 3641-Type Laser Transmitter Subassembly 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 interface circuits. Incorporating the A1611A/B laser module, the 3641-type transmitter subassembly (shown below) provides significant advantages for OEMs, system integrators, and end users by enabling the optimization of transmitter performance in various cable television environments. The information offered here provides an overview of the two dc circuits needed to successfully drive these two devices. These are the optical power control 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 current at the proper level. Laser Control Circuits for A1611A/B Laser Modules and the 3641-Type Laser Transmitter Subassembly Application Note, Rev. 1 November 2000 Optical Power Control Circuit (continued) specified output power. The monitor photodiode should be biased at 5 V. Pin 4 is connected to signal ground. 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 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. 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 coupled 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 monitor 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 difference 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 A and 1800 A when the laser is at its 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 A, 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 factory 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 A 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 2 Agere Systems Inc. Application Note, Rev. 1 November 2000 Laser Control Circuits for A1611A/B Laser Modules and the 3641-Type Laser Transmitter Subassembly Optical Power Control Circuit (continued) Verification Tests Optical Power Status Monitoring 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. If a circuit such as that shown in Figure 1 is used for 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 comparators for triggering alarm circuits. Since the laser bias current generates the voltage across R9, this voltage can be used as a laser bias current monitor. Like the optical power monitor voltage, it can be used for a variety of status monitoring functions. 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 A and 1800 A. Agere Systems Inc. 3 Laser Control Circuits for A1611A/B Laser Modules and the 3641-Type Laser Transmitter Subassembly 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 temperature, 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 temperature 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 feedback 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 temperature-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. Application Note, Rev. 1 November 2000 R1, 2, 6, 7, CR1, and U1C are used to generate reference 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 voltage 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 integrating amplifier U1B. The output of U1B drives a bipolar voltage-to-current converter U2A, which is a highpower op amp. The voltage at the output of U2B is controlled so that the full range of TEC current can be supplied 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 maximum 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 k, R5 can usually be a fixed 10 k resistor. 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. A schematic diagram of a conceptual temperature control circuit is shown below in Figure 2. Figure 2. Conceptual Temperature Control Circuit Diagram 4 Agere Systems Inc. Application Note, Rev. 1 November 2000 Laser Control Circuits for A1611A/B Laser Modules and the 3641-Type Laser Transmitter Subassembly Temperature Control Circuit (continued) Understanding the Interface Requirements Thermistor (Pins 1 and 2). In addition to resistance, there is one other important parameter: the temperature coefficient of resistance. The temperature coefficient 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 k to 10.44 k. 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 (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. 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. Agere Systems Inc. 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 calculation of the chip temperature. For example, in the Figure 2 circuit, a monitor voltage of 1.22 V implies thermistor resistance of 9.56 k. Since the temperature 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 circuits a variety of status-monitoring functions. Another possible parameter to monitor is the TE cooler current. Since the voltage across R14 is directly generated by the TE current, this voltage can be used as a current monitor. This can be particularly useful to indicate when the transmitter temperature is approaching its limit; a high TEC current (approaching the maximum) indicates that the laser temperature is close to its maximum, which could mean that the transmitter temperature 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 k). 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 thermistor voltage divided by the measured thermistor current. 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. 5 Laser Control Circuits for A1611A/B Laser Modules and the 3641-Type Laser Transmitter Subassembly Application Note, Rev. 1 November 2000 Pin Information PIN 1 LEADS .030 (.8 mm) WIDE X .008 (.2 mm) THICK .400 (10.2 mm) MINIMUM LENGTH 6 SPCS @.100 (2.5 mm) Table 1. Pin Descriptions Pin Function 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Thermistor Thermistor dc Laser Bias (-) MPD Anode, Case Ground MPD Cathode TEC (+) TEC (-) Case Ground Case Ground NC Laser Common (+) Laser Modulation (-) Laser Common (+) NC LOCATION FOR LABEL INCLUDING SERIAL NUMBER AND DATE CODE (LABEL NOT SHOWN) .213 (5.4 mm) .049 (1.2 mm) .250 (6.4 mm) .500 (12.7 mm) .350 (8.9 mm) .19 .03 (5 .8 mm) 4X O .105 (2.7 mm) .078 (2.0 mm) .820 (20.8 mm) DIMENSIONS ARE IN INCHES (parenthesis) MILLIMETERS 1.025 (26.0 mm) 1.180 (30.0 mm) .70 .03 (18 .8 mm) .365 (9.3 mm) .215 .010 (5.5 .25 mm) .248 (6.3 mm) .040 (1.0 mm) Figure 3. A1611A/B Laser Module Pinouts For additional information, contact your Agere Systems Account Manager or the following: INTERNET: http://www.agere.com E-MAIL: docmaster@agere.com N. AMERICA: Agere Systems Inc., 555 Union Boulevard, Room 30L-15P-BA, Allentown, PA 18109-3286 1-800-372-2447, FAX 610-712-4106 (In CANADA: 1-800-553-2448, FAX 610-712-4106) ASIA: Agere Systems Hong Kong Ltd., Suites 3201 & 3210-12, 32/F, Tower 2, The Gateway, Harbour City, Kowloon Tel. (852) 3129-2000, FAX (852) 3129-2020 CHINA: (86) 21-5047-1212 (Shanghai), (86) 10-6522-5566 (Beijing), (86) 755-695-7224 (Shenzhen) JAPAN: (81) 3-5421-1600 (Tokyo), KOREA: (82) 2-767-1850 (Seoul), SINGAPORE: (65) 778-8833, TAIWAN: (886) 2-2725-5858 (Taipei) EUROPE: Tel. (44) 7000 624624, FAX (44) 1344 488 045 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. Copyright (c) 2001 Agere Systems Inc. All Rights Reserved November 2000 AP00-067OPTO-1 (Replaces AP00-067OPTO)