The amplifier test set consists of couplers for monitoring the input and output power of the EDFA and a polarization controller/filter. The switches are used to select between gain measurement on the straight path and ASE measurement on the polarization controller/filter path.
The optical design of the polarization controller/filter is based on two retardation plates - one quarter-wave and one half-wave plate - and a linear dichroic polarizer . These parts are mounted on rotatable hollow shafts so that the collimated light beam can pass through the shafts and through the center of the optical codewheel. The whole assembly-optical parts and encoder - is driven by a dc motor coupled with a belt gear drive. With this design any incoming state of polarization can be transformned into any other state by rotating the retardation plates to defined angular positions. A polarizer is added at the output so that linear states of polarization can be extinguished.
In the polarization extinction method the polarization controller/filter is used as a polarization analyzer to determine the input state of polarization. Based on this measurement, the retardation plates are rotated to calculated angular positions, which change the signal and the amplified SSE to a linear state perpendicular to the pass direction of the polarizer. This causes the signal and the amplified SSE to be extinguished.
The amplifier test set contributes to the overall system uncertainty. Therefore, the couplers, switches, and polarization controller/filter were designed or selected for lowest polarization dependent loss. In addition, the switches were selected for very good repeatability and high return loss. The polarization controller/filter was also designed for low rotation dependent loss. Finally, an intelligent algorithm and optimized speed of the polarization controller/filter reduce the total measurement time.
Before starting a measurement, it is necessary to run a calibration. The first calibration step is to measure the coupling ratio of the coupler before the DUT with the help of the two power meter heads. The second step is to calibrate the optical spectrum analyzer and the power meter in conjunction with the attenuators and the losses of the paths. A feed-through is used to connect the test system's input and output ports. The calibration is verified by measuring the feed-through. In this case the gain is well-known (0 dB) and the output power must be the same as the input power.