PDF《on the AMC7820》

4 An Optical Amplifier Pump Laser Reference Design Based on the AMC7820 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. EDFA Power Monitoring and Control. 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 0.85?s to 3.75?s. 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. Main System Computer INPUT CONNECTOR ISOLATOR WDM COUPLER ISOLATOR Erbium - Doped Fiber OUTPUT CONNECTOR PUMP LASER A PUMP LASER B TEC A TEC B Photo Power Monitor Laser Current Power Driver Laser Current Controller Laser Current Sensor TEC Current Power Driver TEC Temp. Sensor TEC Current Controller Photodiode / TZA Laser Current Controller Laser Current Power Driver TEC Temp. Sensor TEC Current Power Driver Laser Current Sensor TEC Current Controller WDM COUPLER Photo Power Monitor Photodiode / TZA

980 nm 1550nm

980 nm 1550nm Photo Power Monitor Constant Pump Laser Current to Make Stable Output Photo Power Fixed Temperature to Make Stable Wavelength Main System Computer INPUT CONNECTOR ISOLATOR WDM COUPLER ISOLATOR Erbium - Doped Fiber OUTPUT CONNECTOR PUMP LASER A PUMP LASER B TEC A TEC B Photo Power Monitor Laser Current Power Driver Laser Current Controller Laser Current Sensor TEC Current Power Driver TEC Temp. Sensor TEC Current Controller Photodiode / TZA Laser Current Controller Laser Current Power Driver TEC Temp. Sensor TEC Current Power Driver Laser Current Sensor TEC Current Controller WDM COUPLER Photo Power Monitor Photodiode / TZA

980 nm 1550nm

980 nm 1550nm Photo Power Monitor Constant Pump Laser Current to Make Stable Output Photo Power Fixed Temperature to Make Stable Wavelength SBAA072A An Optical Amplifier Pump Laser Reference Design Based on the AMC7820

5 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 bein........