A scheme is proposed for coherent detection and real time Fourier processing of optical signals. In this scheme an input signal is photomixed at one photodetector with a variable (chirped) frequency output of a tunable diode last (TDL). Part of the TDL radiation is photomixed at a second photodetector with a fixed frequency output of a stable laser. The electrical signals from the two detectors are convolved by a surface scoustic wave convolver. After proper filtering and envelope detection, the convolver output can be shown to give the power spectrum of the input signal. This scheme may be used for spectroscopy in the infrared.