Research On Fiber Acousto-Electro-Optic Modulation Technology

A new kind of fiber acousto-electro-optic modulator by using lithium niobate crystal is invented. Based on Maxwell equations and acousto-optic parameters interaction basic equation, acousto-optic interaction coupled wave equation is founded. The momentum mismatch caused by ultrasonic is zero while the fiber acousto-electro-optic modulator work at intrinsic operating frequency, and the momentum mismatch is completely determinated by electrical field. In acousto-optic interaction coupled wave equation, the momentum mismatch should be replaced by the momentum mismatch caused by electrical field. Thus the fiber acousto-electro-optic anisotropic interaction coupled wave equation is founded, and the formula of diffraction efficiency is deduced. In order to improve diffraction efficiency, the acousto-optic merit should reach a highest possible value and the half-wave voltage should reach a lowest possible value. According to this principle, an optimum operation mode of acousto-optic interaction is discovered, which is on the xoz plane of lithium niobate crystal, light travels along z axis, ultrasonic travels along x axis, and direct current electric field is loaded along y axis. Then the abaxial angle regarded as an independent variable, the geometric relations of the anisotropic acousto-optic interaction is deduced. In order to design modulator conveniently, the acousto-optic anisotropic interaction geometric relations parameters changing with intrinsic operating frequency are calculated by numerical value analysis. According to the transmission line of Mason equivalent circuit theory, the metallic membrane plating structure of piezoelectric transducer is determinated. Then, the length, width and thickness of piezoelectric transducer and acousto-electro-optic crystal are calculated. And the fiber acousto-electro-optic modulator is devised, made and tested. Then, the frequency character of absorbed electric power by modulator and diffraction intensity are measured, and the curves of diffraction efficiency changing with input electric power and direct current voltages are given respectively. Finally, some important performance parameters of the modulator are given, which include intrinsic operating frequency, intrinsic frequency bandwidth, highest diffraction efficiency and the rate of operating frequency changing with direct current voltages.