Coherent detection is a new attractive detection method in optical transmission, especially in high capacity long-haul
fiber communication systems. Most coherent detection systems use a local laser as the local oscillator rather than using
an optical phase-locked loop (OPLL). In this case, the signal and local laser's wavelength must be as close as possible,
and both should be stable. Wavelength stability of DFB laser is a critical factor for optical coherent detection and other
applications. We use fiber couplers and a fiber phase shifter to buildup an optical 90o optical hybrid, by which the
wavelength fluctuation is monitored with a very high sensitivity.