Abstract: The technique for realizing spectrum analysis of slpeech signal using coherent optical system is presented. By sacrificing the multichannel capability of such an optical system, speech signal with large space bandwidth product can be analyzed. The speech signal is converted into a twodimensional spatial format utilizing density modulation technique first. The power spectrogram is then obtained at output plane in an one-dimensional coherent optical spectrum analyzer. The principle of the system is indicated and some experimental results are also provided.