<P>PROBLEM TO BE SOLVED: To improve the S/N of a heterodyne beat signal and to prevent a photo-detector from being damaged. <P>SOLUTION: An optical heterodyne detection system includes an attenuator (124) for attenuating an input signal (102) before composing the input signal with a local oscillator signal (106). An optimal attenuation level with respect to the input signal for improving the S/N of the heterodyne beat signal is found by obtaining a fundamental measured value of an output signal in response to the local oscillator signal and in the state of no input signal (402), obtaining a sample measured value of the output signal in response to the input signal as a function of a different attenuation level (404) and finding the optimal attenuation level as a function of the fundamental measured value and the sample measured value (406). A minimum attenuation level with respect to the input signal is found and the photo-detector of a receiver is protected from being saturated or damaged by the minimum attenuation level. <P>COPYRIGHT: (C)2004,JPO 
HETERODYNE-BASED OPTICAL SPECTRUM ANALYSIS OF USING CONTROLLED OPTICAL ATTENUATION