Demonstration of a polarization diversity based SH-QPSK system with CMA-DFE equalizer

An optimum solution for a low power and low cost coherent optical receiver design can be obtained by a self homodyne (SH) system that uses analog domain processing. An SH system eliminates the need of a local oscillator (LO) and a carrier phase recovery (CPR) module at the receiver. Due to the inherent line-width tolerance of SH systems, an expensive laser is not required at the transmitter. In digital processing based receivers, analog-to-digital converters (ADC) are the dominant source of power and cost; signal processing in the analog domain can get rid of the usage of ADCs. Combination of these two approaches can lead to a huge power and cost saving. An SH-QPSK system with a polarization multiplexed carrier is experimentally demonstrated with a 28km standard single mode fiber (SSMF) channel and off-line signal processing at the receiver side. The system does not use any optical amplification and dispersion compensating fiber. A novel analog domain constant modulus algorithm-decision feedback equalizer (CMA-DFE) is proposed which is a cascade combination of two different types of equalizers. This proposed scheme gives improvement in the performance over an optimum CMA equalizer, because of the difference of minimum mean square errors (MMSE) of two different cost functions.