Spectrum analyser for area charts using electro-optical signal processing

A new method for performing the Fourier analysis of an area chart using an electro-optical technique with an ultrasonic light modulator is treated theoretically and examined experimentally. The quadratic characteristic of photodetection is investigated and its disadvantage is effectively removed by adopting a both-side-band modulation. Experimental results for simple area charts verify the usefulness of this method. Analyses for a sinusoidal area chart indicate that the bias inherent in the optical signal yields low frequency spectra and that the spectral width due to truncation of the signal by the finite aperture reduces the frequency resolution. It is shown, however, that the wavelength of this chart is accurately measurable.