The amplifier input is a voltage proportional to the photocurrent signal produced by a photodiode, which is irradiated by a modulated light beam at the same chopper frequency.

There is a growing interest in the frequency modulated continuous wave (FM-CW) SAR for its advantages such as small cubage, light weight, cost-effective and high resolution.

After frequency selecting when white light walks through the F-P sensor which cavity modulated by temperature, wavelength demodulating is realized with a plane grating and CCD.