Acoustic Field Models and Diffraction Efficiency in Acousto-Optic Tunable Filters Based on Mercurous Bromide Crystals
Objective An acousto-optic tunable filter(AOTF)possesses significant advantages,including rapid wavelength switching,continuous tuning of the central wavelength,and flexible operation,making them widely utilized in spectral spectroscopic detection.Current commercial AOTF products employ tellurium dioxide crystals as the acousto-optic medium,with an operational spectral range limited to 4.5 μm due to the crystal's transparency range.In contrast,mercurous bromide crystals exhibit a higher transmission rate across the visible to the far-infrared spectrum(0.5-30 μm)and possess excellent acousto-optic performance.Therefore,researching the acoustic and optical field modeling within mercurous bromide crystals is of significant importance.Both tellurium dioxide and mercurous bromide crystals exhibit strong acoustic and optical anisotropy.While theoretical analysis of the impact of optical anisotropy on diffraction efficiency in acousto-optic devices has been progressively refined,the influence of acoustic anisotropy has been largely confined to considerations of the deviation in sound energy direction.Changes in sound field intensity and phase distribution caused by acoustic anisotropy have a non-negligible effect on diffraction efficiency that cannot be ignored.This paper proposes a method for acoustic field modeling using the angular spectrum approach,followed by the calculation of optical diffraction efficiency under the sound field distribution with the coupled wave method.The results provide acoustic field data for the computation of acousto-optic interactions in mercurous bromide crystals across the mid-long wave spectral region.Methods We utilize the angular spectrum method to construct an analytical model of the sound field,building a calculation model applicable to mercurous bromide and tellurium dioxide crystals,considering their acoustic anisotropy.This method achieves the distribution of the sound field within the mercurous bromide crystal and numerically iterates the diffracted light energy at any position in the sound field.Ultimately,it obtains the distribution of diffraction efficiency at different positions on the AOTF passband.The simulation results are compared and verified with actual measurements,thereby validating the accuracy of the simulation model.Results and Discussions The acousto-optic interaction surface intensity distribution at a driving frequency of 10 MHz is compared between mercurous bromide and tellurium dioxide[Figs.5(a)and 5(c)].The length of the Fresnel region in mercurous bromide's intensity distribution is greater than that of tellurium dioxide.The phase distribution of the acousto-optic interaction surface is also compared between tellurium dioxide and mercurous bromide[Figs.5(b)and(d)].As the sound propagation distance increases,the curved shape of equal phase lines in mercurous bromide reduces the diffraction efficiency.In contrast,the phase distribution of tellurium dioxide is closer to that of a plane wave,but its phase divergence is more severe than that of mercurous bromide.The simulated values are consistent with the measured values(Table 1).The main reasons for the deviation are threefold:the angle of the incident light in the test optical path deviates from the theoretical angle;the acoustic field energy is enhanced by the reflected waves at the crystal boundary,causing a large difference in energy along the direction of sound propagation;the acoustic field model does not consider sound energy conversion,resulting in the actual measured values being slightly less than the simulated data values.Conclusions We introduce a method for acoustic field modeling that is applicable to simulating the acoustic field within mercurous bromide crystals.Its effectiveness has been proven through experimental verification on a tellurium dioxide acousto-optic device.The simulation results show that within anisotropic acousto-optic crystals,the intensity and phase distributions of the acoustic field are uneven.A comparison of the acoustic field distribution in the two crystals under the same conditions reveals that the phase non-uniformity of mercurous bromide crystals is more pronounced in the acousto-optic tunable filter(AOTF).This finding highlights the importance of considering the acoustic field distribution when analyzing the diffraction efficiency of devices.The effect of the acoustic field distribution on the diffraction efficiency of the incident light was determined through the analysis of the coupled-wave model and numerical calculation methods.The experimental data also shows a downward trend in diffraction efficiency with an increase in the propagation length of the acoustic field and a significant reduction in diffraction efficiency at the boundaries of the transducer.These findings indicate that selecting an appropriate AOTF optical window is necessary,and the simulation results have been validated through actual measurement data.Therefore,this acoustic field simulation model can provide a theoretical basis for the selection of optical windows and system calibration in designing new mid-wave AOTF devices for mercurous bromide crystals.
acousto-optic tunable filtermedium to long wave infraredmercurous bromidesound field distribution
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