With the development of electronic analog-to-digital converter (ADC) and photo detectors (PD) in bandwidth and speed, coherent detection with digital signal processing (DSP) has been attracting a great deal of interest again in recent years [1-4]. Thanks to DSP technology, optical transmission impartments can be equalized in electrical domain [2]. Homodyne detection has been discussed and investigated a lot in recent coherent communication system due to DSP in electrical domain [1-4]. However, inphase and quadrature (I/Q) signals should be separated in optical domain with full information. In this way, 4 balanced detectors with double hybrid structures and 4 channels ADCs are required.
Heterodyne coherent detection can simplify the coherent receiver with only half of the PDs and ADCs [1, 5]. The optical hybrid can be also simplified. However, this technique is limited by the bandwidth of PDs or ADCs which should have wider bandwidth for intermediate frequency (IF) signals. Thanks to the development of the PDs and ADCs, it gives a possibility to achieve a simplified coherent receiver with heterodyne detection, which has been investigated in subcarrier multiplexing system [6, 7] and radio over fiber system [8]. The most recent report on heterodyne detection is a limited 5 Gb/s 4 QAM signal over 20 km in [6] and limited 20 Mbaud 64 and 128 QAM over 525 km in [7].