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Study on the Characteristics of the Acousto-optic Devices in Laser Coherent Detection

Author: HuoLei
Tutor: ZengXiaoDong
School: Xi'an University of Electronic Science and Technology
Course: Optical Engineering
Keywords: Acousto-optic devices Laser heterodyne interferometer Braggdiffraction Sound field distribution Diffraction efficiency
CLC: TN65
Type: PhD thesis
Year: 2012
Downloads: 214
Quote: 1
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Abstract

This work focuses on the influence of acousto-optic devices on heterodyneefficiency in the laser heterodyne interference detection. The features and applicationsof acousto-optic devices and encountered problems are discussed. The sound fielddistribution can influence the diffraction efficiency of the acousto-optic modulator andthe beam quality of the diffracted beam. In this research, the ultrasonic sound fielddistribution model generated from ultrasonic transducer is investigated, the factors ofnon-uniform sound field that affect the diffraction efficiency are analyzed, thenon-reciprocal effects of acousto-optic devices and its influence on the laser heterodyneinterference effects are also discussed. A summary of the development and applicationof acousto-optic devices and its operating characteristics is given.The first part of the paper, the general form of the sound and light coupled waveequations are derived from the parametric interactive point of view, and the interactionof the sound and light is analyzed by using the momentum matching and momentummismatch. The properties of the tellurium oxide acousto-optic devices are studiedthrough simulation and experiment, and then the acousto-optic diffraction efficiencycurves are obtained, the experimental results agree well with the theory. Based on thekind of non-uniform sound field theoretical analysis is made, and the result that thedecline in beam quality of the diffraction beam caused by the uneven sound field isexamined. The main reasons of the decline of diffraction beam quality led byacousto-optic crystal are studied. The experimental test measurement of theacousto-optic diffraction efficiency is made, and the measured data are processed andanalyzed. Through the comparison with the theoretical analysis, we can see that thediffraction beam quality deterioration and reduced efficiency was caused by the unevendistribution of spatial sound field.The second part of the paper, because of the fact that sound reflection leads touneven distribution of sound field in the crystal, a light beam diaphragm method isproposed. First of all, the non-uniform grating contrast model is proposed from theinfluence on volume grating contrast and diffraction efficiency. By use of the theoreticalsimulation and experimental measurements, proof of the transmission performance ofthe aperture method is examined. In addition, through the beam diaphragm method thecenter dark lines in spot of the diffracted beam is eliminated in the heterodyneinterferometer experiments, and the beam spot shows much more uniform. This methodcan improve the diffraction beam quality and improve heterodyne interferometer performance.The third part of the paper, based on the Rayleigh integral, the axisymmetricultrasonic transducer of sound field distribution characteristics is analysed. By use of anelliptical Gaussian modeling the piezoelectric transducer sound field distribution isdescribed, the radiation distribution for spatial sound field is analyzed, and numericalcomputation is also made. Through analysis and simulation modeling, the sound fielddistributions are discussed for different crystal location and attenuation. Then therelationship between the sound field distribution and the diffraction efficiencydistribution is derived, and the optical field distributions are computed for severaldiffraction conditions. At the end of this part, the condition under which the sound fieldin the crystal is uniform, and the diffraction efficiency on the acoustic axis are given.The fourth part of the paper, according to the beam forward and back transmissionin the acousto-optic devices, there exist different diffraction efficiency. From this kindof non-reciprocal phenomenon in the acousto-optic device, the consideration of thefactors to influence the diffraction efficiency and the laser heterodyne interferometerefficiency owing to the diffracted light attenuation are discussed. By use of thenon-reciprocal theory the acousto-optic device operating characteristics which havedriven by100MHz are analysed. The frequency shift resulted from non-reciprocal effectcan be determined by help of comparison of the transmission bandwidth of theacousto-optic device. Experimental results indicate that in tellurium oxide acoustic-opticdevices there exist non-reciprocal effect, it is shown that the direction of acoustic wavepropagation effects not only the light diffraction’s direction also the change oftransmission bandwidth. In tellurium oxide crystal such change can be as high as50~60%of the total bandwidth.In the research of laser heterodyne interferometer for coherent detection technology,the non-reciprocal effect in acousto-optic device can be used to determine the impactson heterodyne IF signal. And by use of the non-reciprocal effect the gain of laserheterodyne IF signal could be improved. The experiment shows: when the radiofrequency is tuned to a working range from100to105MHz for the acoustic opticmodulator (AOM), the diffraction efficiency reaches the maximum value of98%. Byuse of the non-reciprocal effect, a7.7dB voltage gain of diffraction efficiency can beobtained and heterodyne signal power increases by+13dBm to+21dBm.

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