Application Note September 2000 Transitioning from the A2300 DFB to the A1611A/B DFB Laser Module Optical Isolation Reflections caused by a phenomenon such as Rayleigh scattering in long fiber-optic links is minimized with built-in optical isolation. Reflected light entering the laser module is attenuated by at least 40 dB. This important feature is incorporated in all versions of the A2300 as well as the A1611A/B lasers. Pinouts and Impedances Offering the flexibility to accurately and cost-effectively address the requirements of forward-path applications, the A1611A/B module is optimized to assist OEM customers with 1310 nm CATV transmitter design. Introduction The information offered here is intended to aid in the transition from the Agere Systems Inc. A2300 series to the Agere A1611A/B series laser. Although similar in function, size, and shape, the A2300 and A1611 family of CATV lasers exhibit very distinct differences. Moreover, an industry standard pinout, higher power, better performance, and ease of integration combine to make the A1611A/B the laser module of choice when transitioning from the dependable but aging A2300. This document examines the basic differences (and similarities), methods for testing and test-fixture requirements, and also illustrates the ease of integrating the A1611A/B into a complete transmitter. Laser Profiles Both the A2300 and A1611A/B modules are analog 1310 nm DFB lasers mounted in a 14-pin industry-standard, butterfly-type package. Each is equipped with an optical isolator, monitor photodiode, thermistor, and thermoelectric cooling. The A2300 module offers two different pinout types, each with a different RF drive impedance. One is Agere's standard analog isolated laser module (ILM) pinout, which has an input impedance of 75 . The 75 input impedance eliminates the need for external matching circuits and is achieved using an RF transformer internal to the package. The second pinout type is the industry standard OC-48 compatible package characterized by a 25 resistive-matched input impedance. Similarly, the A1611A/B device uses the OC-48 pinout with a characteristic input impedance of 25 , although only one pinout type and RF drive impedance is offered. The pin configuration for all module versions is presented in Figure 1 and side-by-side pinout comparison is shown in Table 1. Transitioning from the A2300 DFB to the A1611A/B DFB Laser Module Application Note September 2000 Laser Profiles (continued) Pinouts and Impedances (continued) 7 6 5 8 9 10 4 3 2 1 11 12 13 14 1-1159(F) Figure 1. A2300 and A1611A/B Pin Configuration, All Versions Table 1. Industry-Standard OC-48 Pinout with 25 Resistive Input Impedance vs. the ILM Pinout with 75 Input Impedance Models A1611 and A2300; 25 2 A2300; 75 Pin No. Description Pin No. Description 1 Thermistor 1 Thermoelectric Cooler (+) 2 Thermistor 2 Thermistor 3 Laser (dc Bias) Cathode (-) 3 MPD Anode (-) 4 MPD Anode (-) 4 MPD Cathode (+) 5 MPD Cathode (+) 5 Thermistor 6 Thermoelectric Cooler (+) 6 Case Ground 7 Thermoelectric Cooler(-) 7 Case Ground 8 Case Ground 8 Case Ground 9 Case Ground 9 Case Ground 10 Case Ground 10 Case Ground 11 Laser Anode (+) 11 Laser Bias and RF Input; 75 12 RF Input; 25 (-) 12 NC 13 Laser Anode (+) 13 Laser Anode (+), Case Ground 14 Case Ground 14 Thermoelectric Cooler Agere Systems Inc. Transitioning from the A2300 DFB to the A1611A/B DFB Laser Module Application Note September 2000 Laser Profiles (continued) RF Modulation and Biasing Both the A2300 42-channel CENELEC and A1611A/B lasers use separate pins for RF modulation (pin 12) and biasing (pin 3). Unlike the OC-48 packages, the A2300 ILM shares the same pin (11) for biasing and modulation. To keep RF from feeding into the laser bias power supply, a simple circuit called the bias-T is necessary. A coil prevents RF from feeding back into the bias supply. A capacitor couples the modulating signal to the laser cathode via a 75 matching transformer designed into the package. FROM CONSTANT BIAS CONTROL CIRCUIT A2300 NTSC ILM 75 COIL dc + RF PIN 11 RF INPUT 75 1-1159(F) Figure 2. A2300 NTSC ILM 75 RF Modulation and Biasing CONSTANT BIAS CONTROL CIRCUIT RF INPUT 75 75 TO 25 Z MATCHING 1611, A2300 25 dc PIN 12 PIN 3 1-1162(F) Figure 3. A2300 and A1611A/B 25 RF Modulation and Biasing Agere Systems Inc. 3 Transitioning from the A2300 DFB to the A1611A/B DFB Laser Module Laser Profiles (continued) External Circuitry Recent breakthroughs in coupling efficiency (light out of the laser verses light into the fiber) have allowed the A1611 to achieve higher optical output powers. This advancement allows for longer fiber runs, makes more optical splits possible, and thereby reduces the number of transmitters needed in the system. To maintain stable module operation over the entire operating temperature range of -20 C to + 65 C, however, external control circuitry is necessary. Laser bias determines the optical output power. A closed-loop feedback circuit, using a sample of the optical output power provided by the built-in monitor photodiode, maintains the bias current at the correct level. Application Note September 2000 Similarly, chip temperature is maintained via an external feedback circuit. The thermoelectric cooler (TEC) inside the laser package uses the Peltier effect to transfer heat from the chip to the package. Varying the polarity of the current allows laser chip heating and cooling. In all cases, the laser package must be attached to a heat sink to ensure proper laser cooling. Agere can provide a set of boards (dc set) designed specifically to control laser bias and temperature. The dc set allows for a quick and easy solution to laser control when used in conjunction with Agere's T3641 transmitter engine-compatible lasers (such as the A1611A/ B), as configured in Figure 4. For more detailed information on laser control circuitry and theory of operation, please refer to Laser Control Circuits for A1611 Laser Module and T3641-Type Laser Transmitter Subassembly application note. 9404-xxx CONNECTOR bd. T3641 PREDISTORTOR ENGINE VR1 BIAS ADJUST 9028-xxx TEC bd. TEC ADUJUST 9383-xxx BIAS bd. 1611 LASER 1-1160(F) Figure 4. A1611A/B Bias and Temperature Control Using Agere's dc Boards 4 Agere Systems Inc. Transitioning from the A2300 DFB to the A1611A/B DFB Laser Module Application Note September 2000 Link Testing Link Test Setup Example Link testing plays an important role in determining the quality of a fiber link under real-world conditions. The link test can be used to simulate actual customer conditions such as carrier-noise ratio (CNR), distortion, fiber length, and other added passive losses. The following procedure is preliminary and is intended to be used only as a guide for setting up an in-house test station, which is shown in Figure 5. Link testing also provides a reliable method for comparing the A2300 and A1611A/B devices. The modules were evaluated using a recently calibrated manufacturing distortion station, 25 km of fiber, and 2 dB of passive loss. CNR and bias of both type lasers (A2300 and A1611) were set to the values found on the A2300 test data sheet and distortion was measured. For 80-channel loading, the A2300 exhibited good CTB and acceptable CSO performance with the edge in both categories going to the A1611. The measured CSO results for 110-channel loading were very marginal for the A2300. CTB was good in most cases. The A1611 passed both CSO and CTB with good margins. MATRIX GENERATOR First, the best bias must be determined. The best bias is found primarily by varying the laser bias in 10 mA increments until the optimized power and distortion point is found. Next, the correct fiber length must be inserted. This length can be varied to accommodate customer requirements. Now the matrix generator can be set up for correct channel loading. Once that is complete, CNR can be measured and the RF drive level can be adjusted to meet the required CNR specifications. Lastly, distortion (CSO, CTB) is measured. For distortion testing, optical power to the receiver may have to be reduced to prevent errors in measurement caused by overdriving the test receiver. SPECTRUM ANALYZER BANDPASS FILTERS OPTICAL RECEIVER RF INPUT 1611 LASER OPTICAL ATTENUATOR TEST FIXTURE LASER CONTROL BOX 1-1164(F) Figure 5. Link Test Hardware Configuration Agere Systems Inc. 5 Transitioning from the A2300 DFB to the A1611A/B DFB Laser Module Application Note September 2000 Link Testing (continued) The A2300 CENELEC requires an impedance-matching pad or transformer when used with the 75 output matrix generator, as shown in Figure 7. Test Fixture Requirements Different test fixtures are needed to fire-up each of the three lasers. The A2300 (ILM NTSC) uses a 257-type test fixture. The A2300 (OC-48 CENELEC) uses the 247-type test fixture. The A1611A/B uses a fixture (9331-xxx) manufactured on site. The A1611A/B test fixture can be optioned for 75 or 50 . It connects directly to the test hardware; bias-Ts and external impedance matching are not needed. A limited number of all test fixtures can be made available by Agere for short-term loan. The 75 version, as mentioned previously, will require an external bias-T. Figure 6 shows how the fixture can be used with the link test hardware setup. A2300 RF IN 75 (FROM MATRIX) BIAS-T TO RECEIVER dc BIAS ILX LIGHTWAVE LDC 3900 OR EQUIVALENT 1-1164(F) Figure 6. A2300 Test Fixture With External Bias-T and 75 RF Input A2300 RF IN 75 (FROM MATRIX) Z-MATCHING PAD OR XFMR TO RECEIVER ILX LIGHTWAVE LDC 3900 OR EQUIVALENT 1-1165(F) Figure 7. A2300 CENELEC Test Fixture With Impedance-matching Pad or Transformer and 75 RF Input 6 Agere Systems Inc. Transitioning from the A2300 DFB to the A1611A/B DFB Laser Module Application Note September 2000 Matched Impedance The T3641 transmitter engine consists of the A1611 laser mounted on the 9349 board. The 9349 board includes a low-noise hybrid amplifier, 75 to 25 impedance-matching circuitry, interface for connecting laser control boards, and the latest in-line predistortion technology. Most applications for the CATV laser call for a 75 impedance-matched input. This problem is neatly solved in the A2300 ILM by an in-package matching transformer. Limiting the input impedance to 75 , although very useful, can sometime limit design flexibility. Due to the previously mentioned pinout and impedance matching incompatibilities, the A2300 ILM cannot be used with the T3641 engine. (The 75 input impedance of the A2300 ILM is a mismatch for the 75 --25 transformer used in the 9349 board). For these reasons, the A2300 virtually requires a from-theground-up design effort. A predistortion board alone, designed for use with the A2300 using the older splitoff predistortion technology, is still available for purchase. A separate board will have to be fabricated to provide RF inputs, laser mounting, and control lines. The A1611A/B can easily be matched to most of the commonly used impedance requirements. A conceptual circuit showing the most common of the 25 to 75 requirements is shown in Figure 8. The heart of the circuit is a simple, external matching transformer. Board Integration Board integration can greatly reduce the time and expense required when designing a complete transmitter. Many of the necessary components such as predistortion, amplification, and impedance matching can be incorporated into a small form factor transmitter engine. Summary In view of the convenient advantages of the featurepacked A1611A/B laser module, including an industry standard pinout, higher power, better performance, and ease of integration, transitioning from the maturing A2300 to the new-generation A1611A/B is the next logical step. When the engine is combined into a complete transmitter design, however, new active components (such as amplifiers) can add distortion. Features such as electronic fine-tuning of CSO and CTB ensure the highest levels of performance. C7 GND 1 1 COAX PAD T1 3/5 TURN 2 RF INPUT TO LASER 25 2 3 75 GND INPUT C10 R1 C10 GND GND C8 GND GND C9 GND 1-1166(F) Figure 8. Conceptual Circuit Showing Typical A1611A/B Impedance-matching Requirements Agere Systems Inc. 7 Transitioning from the A2300 DFB to the A1611A/B DFB Laser Module Application Note September 2000 Related Information Description Introduction to the 3641-Type Transmitter Subassembly application note Laser Control Circuits for A1611A/B Laser Module and the 3641-Type Laser Transmitter Subassembly application note Mechanical Installation Requirements for the 3641-Type Transmitter Subassembly application note Electronic Fine Tuning of the 3641-Type Transmitter Subassembly application note Document Number AP00-071OPTO AP00-067OPTO AP00-069OPTO AP00-070OPTO 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 September 2000 AP00-063OPTO