Analysis of fiber based coherent detection employing optical preamplifier and time-domain filter

We report on a novel fiber based coherent detection system employing an optical preamplifier, a spectrum 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 spectrum bandpass filter alone. 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 input power, 2.78 dB and 1 dB signal-to-noise (SNR) improvement has been achieved, respectively, when 5% and 50% time gating duty cycle is used.