New modulation method in acousto-optic spectrometers

A new method is proposed for modulating radiation in collinear acousto-optic (AO) spectrometers, which is based on the sequential diffraction of incident polarised optical radiation using two sound packets with the same frequencies and initial phases. A theory of diffraction of optical radiation on two successive packets of sound waves propagating in a crystal is constructed, and the instrumental function of the AO spectrometer is found. It is established that the instrumental function of such a spectrometer becomes time-dependent, and the modulation frequency of this dependence, in turn, depends on the intensity of the sound wave and on the detuning from the phase-matching conditions. The frequency dependence of the measured photocurrent for some finite number of sound packets is obtained explicitly, and it is shown that its measurement allows the spectral composition of the incident optical radiation to be determined with greater accuracy.