Digital Optical Coherent Detection And Its Application

Digital optical coherent detection has been a hot topic in high-speed optical transmission. By using coherent detection, it has higher receiving sensitivity and more effective spectrum usage. With the help of digital signal processing, full of linear and part of nonlinear impairments in transmission can be compensated. Many universities and research institutes all over the world have started research in this field. Besides, the corporations like Nortel, U2T are going to release their commercial 40Gb/s coherent receivers. Digital optical coherent detection has been a key point in the next optical transmission system.In digital optical coherent detection, we have done the following work.Based on low-cost fiber passive devices, we made the key component in digital optical coherent detection—polarization diversity optical 90o hybrid, the 4 paths'insertion loss difference of which is less than 0.3dB, and the time delay difference less than 5 pico-seconds. Compared to commercial hybrids, it is cheap, adjustable and stable. Using our home-made 90o hybrid, we set up the coherent detection platform and finished several transmission experiments. The key digital signal processing algorithms are completed, including IQ phase orthogonalization, chromatic dispersion compensation, digital polarization demultiplexing, and carrier phase estimation.We carried out a 20Gb/s polarization multiplexed DPSK system with a 280km single-mode fiber transmission. Further more, we developed an optical loop transmission system controlled by a MSP430 micro-controller. Using this fiber loop, a 40 Gb/s polarization multiplexed QPSK optical signal was transmitted for 1200km, the bit-error-rate of which is less than 10^-3.Digital optical coherent detection can be used to monitor the signal distortion caused by modulators. We use it to monitor the IQ phase bias in the parallel MZM for optical QPSK signal generation, the monitoring error of which is less than 5^o.Using mode-locked laser as the local oscillator, we have done some research work in linear optical sampling. The NRZ, DPSK signal's eye-diagram are re-constructed by optical sampling. The time width of the mode-locked laser is less than 1 pico-second, leading to a more than 200GHz sampling width.