ON USING PERIODIC REFERENCE SIGNAL IN HOMODYNE ACOUSTO-OPTIC SPECTRUM ANALYZER

Abstract

Introduction. For a homodyne acousto-optic spectrum analyzer functioning a reference optical channel must be organized. The signal in this channel should provide uniform reference illumination throughout the spatial frequency range. In the general case, the spectrum analyzer functioning can be considered with a continuous photosensor and photosensor with charge accumulation. With the last one, the signal in the reference channel is proposed to be a wideband pulses periodic sequence. Objective. Analyze the spectrum analyzer functioning with a periodic reference signal. Materials and methods. We derive the mathematical expression to describe the influence of the reference signal structure on the analyzer’s output signal for the cases of continuous photosensor and photosensor with charge accumulation. Results. It is shown that in the case of continuous photosensor, the reference signal periodicity does not lead to characteristics degradation. However, in the case of many frequency resolution points it is impractical, since each photodetector signal is parallel, processing is required: filtering, amplification and digitization. In the case of using of the charge accumulation sensor, the discrete frequency grid appears, which means signals omissions in frequency. This can be avoided by choosing the accumulation time equal to the minimum among the values of the acousto-optic modulator time aperture and the reference signal period, which is hard to implement, or still leads to the signal omissions in frequency or time. Conclusion. To perform a real-time mode in the homodyne acousto-optic spectrum analyzer, the reference signal must be either non-periodic, which raises the question of its synthesis, or a continuous photodiode array should be used.