A Digital Algorithm for Controlling Programmable Acousto-Optic Filters: Numerical Simulation of Contrast and Computation
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Radiophysics and Quantum Electronics, Vol. 62, No. 11, April, 2020 (Russian Original Vol. 62, No. 11, November, 2019)
A DIGITAL ALGORITHM FOR CONTROLLING PROGRAMMABLE ACOUSTO-OPTIC FILTERS: NUMERICAL SIMULATION OF CONTRAST AND COMPUTATION SPEED K. B. Yushkov ∗
UDC534.2+535.4+535.58+621.373
Formation of radio-frequency waveforms with given spectra is the basis for controlling acoustooptic filters with synthesized transmission functions. Digital methods for synthesizing such radio signals on the basis of the discrete Fourier transform are analyzed. We propose an improved method for forming control radio signals, which allows one to improve the spectral-modulation contrast. Numerical simulation of the complex-valued transmission functions of an acousto-optic filter in the spectral-modulation regime is performed and the speed of the digital algorithms for the control-signal formation is estimated.
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INTRODUCTION
Recent years have shown considerable development of the methods for controlling wideband optical signals using acousto-optic tunable filters (AOTFs) [1]. The transmission function, which is determined by both the size of the region of light and sound interaction in the filter and the ultrasonic-field amplitude and frequency distribution in this region, is the main characteristic of the AOFT as a spectral device. It is well known that the broadening of the AOTF pass band and the spectrum processing in the specifiedwidth window can be performed using the frequency modulation of the control high-frequency signals [2, 3]. The variable-period Bragg grating is created in the acousto-optic interaction region during the fast frequency modulation of the high-frequency signal, which is supplied to the AOTF piezoelectric converter. If the period variation during the ultrasound front travel across the interaction region is rather strong, then the width of the AOTF transmission function is actually determined by the spectrum width of the high-frequency control signal. The formation of the nonstationary high-frequency signals by the Fouriertransform method allows one to obtain arbitrarily specified complex-valued transmission functions of the AOTF [4]. The basic applications of the acousto-optic methods for controlling the light spectrum are related to the formation of femtosecond laser pulses [5–12] and adaptive spectroscopy [13–16]. Digital synthesis of complex-shaped radio-frequency waveforms also has important applications in the related research fields, e.g., radar of biological objects [17]. In some promising problems of laser engineering and applied spectroscopy, the speed of a programmable spectral filter is the decisive factor. Femtosecond laser systems of gigawatt peak power with a pulse repetition rate of up to 100 kHz have been developed in recent years [12]. Spectrometers with a speed of 100 kHz have also been created on the basis of the femtosecond lasers [18]. The problem of adaptive control of the form of the laser pulses with such a repetition frequency has not yet been solved. Another important application
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