PDF《on the AMC7820》

Application Report SBAA072A C May

2002 C Revised March

2005 1 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 SBAA072A

2 An Optical Amplifier Pump Laser Reference Design Based on the AMC7820 Figures Figure 1. DWDM Multiplexes Many Signals Onto One Fiber

3 Figure 2. EDFA Power Monitoring and Control

4 Figure 3. DWDM Transmission System.5 Figure 4. Thermoelectric Cooler Block Diagram

6 Figure 5. Thermistor Response Curve

7 Figure 6. TEC Control Loop.9 Figure 7. Temperature Measurement with Ratiometric Reference

10 Figure 8. Class D Power Driver for TEC

11 Figure 9. TEC Response with no Compensation.12 Figure 10. TEC Response with Compensation.13 Figure 11. Deviation from Setpoint vs Actual Temperature.14 Figure 12. Laser Control Loop.14 Figure 13. Current Sense Circuits

15 Figure 14. Digitally-Controlled Current Limit

16 Figure 15. Back Facet Diode Monitor.17 Figure 16. Complete AMC7820-Based EDFA Pump Laser System

18 SBAA072A An Optical Amplifier Pump Laser Reference Design Based on the AMC7820

3 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 C 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. SBAA072A