To provide an optical heterodyine detection system for generating a heterodyne signal having an improved S/N ratio.

The optical heterodyne detection system comprises an optical preselector (214, 814) having an adjustable pass band so as to track the waveform of a sweep local oscillation signal. In an embodiment an input signal (202) is coupled with a sweep local oscillation signal (206), and the preselector (214) filters the coupled optical signal. In another embodiment, a preselector (814) filters an input signal (802) before being coupled with the sweep local oscillation signal. This filters the input signal or the coupled optical signal so as to pass a frequency band for tracking the waveform of the sweep local oscillation signal, thereby reducing a noise resulting from WDM signals to widen the dynamic range of the detection system. An embodiment of the detection system comprises an optical coupler unit (210), a preselector (214) for coupled optical signals, and an optical detector (212). Another embodiment comprises an optical preselector (814) for input signals, an optical coupler unit (810) and an optical detector (812).