A study on an active square-law combining method for a polarization-diversity coherent optical receiver

A polarization-diversity coherent optical receiver which realizes ideal square-law combining with active gain control is described. This receiver can suppress the additional power penalty due to the demodulator dynamic range limitation by equivalently expanding the demodulator dynamic range. A low-power penalty less than 1 dB was attained with a 400-Mb/s FSK (frequency-shift keyed) single-filter detection system. Throughout a 25-h 183-km transmission experiment, the stable operation of this receiver was confirmed.<<ETX>>