High performance fiber-based optical coherent detection

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, coherent detection should yield the highest receiver sensitivity. Another possible technique to improve detection sensitivity is to employ a fiber preamplifier. This technique has been successfully demonstrated in direct detection systems but not in the coherent detection systems. Due to the existence of amplified spontaneous emission (ASE) inside the amplifier, the sensitivity of coherent detection varies with the data rate or pulse rate. For this reason, optically preamplified coherent detection is not used in applications as commonly as optically preamplified direct detection.;We investigate the performance of coherent detection employing a fiber amplifier and time-domain-filter. The fiber amplifier is used as the optical preamplifier of the coherent detection system. To reduce the noise induced by the preamplifier to a maximum extent, we investigate the noise properties for both a single pass amplifier and a double pass amplifier. The relative intensity noise and linewidth broadening caused by ASE have been experimentally characterized. The results show that the double pass amplifier has less phase distortion. To further reduce the effects of ASE, filters in both time and frequency domain have been explored. A spectral bandpass filter can be used to filter out out-band unnecessary spectral components, which are mostly generated by ASE. The time-domain filter can reduce the in-band, but no in-pulse power, which is also contributed mostly by ASE.;A single photodetector and a balanced photodetector have been studied in this dissertation research. The performance of the optical coherent detection system has been studied both theoretically and experimentally. This doctoral research has demonstrated that the combination of a fiber preamplifier and time-domain filter can improve optical coherent detection performance. Improvements have been demonstrated in the experiment by using different detection designs such as single pass, double pass, single photodetector, and balanced photodetector. The double pass preamplified balanced coherent detection demonstrated highest sensitivity.