Analysis of fiber based coherent detection employing optical preamplifier and time-domain filter Youming Chen *a , William Torruellas a , Brian L. Mark b , Ralph Burnham a , Horacio Verdun a a Fibertek, Inc., 510 Herndon Pkwy, Herndon, VA 20170 b ECE Dept. George Mason University, MS 1G5, Fairfax, VA 22030 ABSTRACT 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. Keywords: coherent detection, optical preamplifier, time-domain filter (TDF), sensitivity, detection. 1. INTRODUCTION The sensitivity of signal detection is of major interest for optical high speed communication systems and LIght Detection And Ranging (lidar) systems. Sensitive receivers in fiber-optical networks can reduce transmitter power or amplifier amplification requirements and extend link spans. High receiver sensitivity allows links to be established over long distances in deep space satellite communication systems and large atmospheric attenuation to be overcome in terrestrial free space communications. For lidar systems, the sensitivity of signal detection determines how far and how accurately the lidar can detect the remote objects. Optical receivers employ either coherent or direct detection. In addition to amplitude, coherent detection extracts frequency and phase information from received signals, whereas direct detection extracts the received pulse amplitude only. In theory and previous experiments, coherent detection should yield the higher receiver sensitivity 1,2 . Another possible technique to improve detection sensitivity is to employ a fiber preamplifier. This technique has been successfully demonstrated in direct detection systems 3,4,5 , but not in the coherent detection systems. Due to the existence of ASE inside the amplifier, the sensitivity of coherent detection is degraded and varies with the data rate or pulse rate 6,7 . For this reason, fiber based optically preamplified coherent detection is not used in applications as commonly as direct detection. In this paper, we report on a new architecture for a coherent detection system, which employs an optical preamplifier and a time-domain filter. We provide a characterization of the detection system and a comparison of different detection schemes. In the follow sections, we briefly describe the concepts of the architecture design and noise analysis of the system. Then, we present the experiment results, including results for direct detection, and coherent detection with and without the time-domain filter. Discussions and conclusions are given in the final section. 2. CONCEPT OF THE OPTICAL PREAMPLIFIED COHERENT DETECTION SYSTEM FILTERED BY TDF AND BANDPASS FILTER * ychen@fibertek.com; phone 1 703 471-7671; fax 1703 471-5806; www.fibertek.com