Rejection of Coherent Interference in Optical Modulation-Demodulation Experiments

Systems are described which reject both coherent interference (modulator-leakage pickup) and noise in laboratory modulation-demodulation systems, such as those used in microwave laser-communications experiments. The approach involves the use of audio-rate chopping and phase sensitive detection along with zero-IF or homodyne detection. Using these techniques, it is possible to make quantitative measurements of desired signals which have amplitudes several orders of magnitude smaller than coherently related interference signals, while rejecting both chopped and unchopped noise. Calibration techniques are discussed, and experiments are described which verify the operations of these systems.