To provide an optical heterodyne detection system allowing high polarization independency (non-dependency) or intensity noise suppression.

One example of this optical spectrum analyzing system comprises a depolarization element 307 having an input for receiving a sweep partial oscillator signal and an output for outputting a depolarization sweep partial oscillator signal, an optical coupler 310 having a first input for receiving an input signal, a second input optically connected to the depolarization element to receive the depolarization sweep partial oscillator signal, and an output for outputting a coupling signal including the input signal and the depolarization sweep local oscillator signal, and a heterodyne receiver 312 having an input for receiving the coupled optical signal from the optical coupler and an output for outputting an electric signal showing the coupled light signal.