IntroductionIn this paper, we give the design equations and a procedure for the complete specification of the design parameters for a wide bandwidth acousto -optic modulator (AOM). Although the modulating medium, transducer material, and bonding agents determine the modulator performance, we concentrate here on the question of how to design an AOM, given specifications on its minimum modulation bandwidth, diffraction efficiency, and the maximum ellipticity of the output beam. Brief TheoryWe give a brief description of the theory of the operation of acousto -optic modulators and the pertinent design equations. References 1 and 2 provide more details on the basic theory of operation of acousto -optic devices. The following list defines the symbols to be used in the discussion:is the acoustic center frequency of the AOM is the optical wavelength in vacuum is the acoustic wavelength at the center frequency is the acoustic velocity in the modulator material is the index of refraction of the modulator material is the length of the transducer is the height of the transducer is the modulation bandwidth of the AOM is the fall -off in the bandwidth expressed in dB is the focal length of the focusing optical lens is the 1 /e2 diameter of the laser beam is a constant equal to 1.357 is the optical wavevector is the acoustic wavevector is the acoustic beam spread angle due to diffraction is the optical convergence angle due to focusing is the 1 /e2 diameter of the focused beam waist is the Bragg angle is the acoustic power is the diffraction efficiency is the acousto -optic figure of merit A typical AOM configuration is shown in Figure 1. In the Bragg -regime, the acousto -optic interaction is maximized for the following condition: Xo sin ûB -2nA(1) 0 When the Bragg angle BB is correctly aligned, a diffracted output beam is produced at an angle 20B from the direct optical beam. The momentum conservation law for the acousto -optic interaction is given by: kout ° kin + a (2) and is illustrated in Faure 2. The incident light beam has a range of wavevectors kin of constant magnitude but with an angular range of Soo because, in order to achieve a fast rise time, IntroductionIn this paper, we give the design equations and a procedure for the complete specification of the design parameters for a wide bandwidth acousto-optic modulator (AOM). Although the modulating medium, transducer material, and bonding agents determine the modulator performance, we concentrate here on the question of how to design an AOM, given specifications on its minimum modulation bandwidth, diffraction efficiency, and the maximum ellipticity of the output beam.
The e x p e r i m e n t a l r e s u l t s on i n t e r f e r om e t r i c B r a g g C e l l Spectrum Analyzer a r e indeed encouraging. An experiment w a s p e r f o r m e d using a 5 mW HeNe l a s e r in Mach-Zehnder i n t e r f e r o m e t r i c Configuration. A l i n e a r c h i r p w a s u s e d to g e n e r a t e the r e f e r e n c e b e a m . W e w e r e a b l e to obserw58 dB Dynamic Range f o r CW and 54 d B DR f o r 5ps pulsed signals with 100% Probability of I n t e r c e p t and less than 1 p s T i m e of A r r i v a l a c c u r a c y . Pseudo Noise Sequence w a s a l s o t r i e d i n a n o t h e r e x p e r i m e n t with good r e s u l t s at reduced s y s t e m bandwidth.
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