Reducing ASE Effect in Coherent Detection by Employing Double-Pass Fiber Preamplifier and Time-Domain Filter

We report on a novel fiber based coherent detection system employing a double-pass fiber preamplifier, a spectral bandpass filter, and a time-domain filter. The time-domain filter, a synchronous time gate, reduces the in-band amplified spontaneous emission (ASE) beat noise, which cannot be achieved by the spectral bandpass filter alone. The double-pass fiber amplifier further reduces the out-band ASE by about 20 dB with a fiber Bragg grating (FBG) at the end of the fiber amplifier. In preliminary experiments with a 100 GHz bandpass filter, no degradation is observed from the optically preamplified coherent detection compared to pure coherent detection. With a 10 ns pulse width, 500 kHz repetition rate, and 10 pW average input power, about 2 dB and 1 dB signal-to-noise ratio (SNR) improvements are achieved when 5% and 50% time gating duty cycles are used, respectively.