Research On Integrated Acousto-optic Chip Technology

Main applications of Integrated Acousto-optic(AO) frequency shifter includes Fiber Gyroscope, AO frequency spectral analysis, coherent laser testing, et al. As optic devices are more and more miniaturized and complicated, higher technology parameters are required to AO frequency shifter. In this paper, AO frequency shifter based on Ti diffused LiNbO₃ is developed. two AO waveguide Bragg modulators to complete AO frequency shifting and two non-aberration and non-spherical waveguide geodesic lens(GL) as collimating and focus integrated optic cell are developed based on Ti diffused LiNbO₃ substrate.The following results have been finished in this work:1. The mode distribution of electromagnetic field in asymmetry waveguide is analyzed. Differential coefficient equation and generatrix expression of geodesic lens is deduced. In profile design of GL, a suitable mathematic solution of lens profile in transitional zone eliminates effectively the curvature singularity of round-edge of lenses. To maintain continuity of first derivative and second derivative of lens profile, selection of power exponent of rim function is discussed. Optimized parameters are gotten. And generatrix expression of geodesic lens is expressed based on these optimized parameters.2. Through analysis of Bragg diffractive condition, the optical transmitting direction of acoustic wave and photonic wave is demonstrated. Design parameters of waveguide AO Bragg modulator are achieved. Cascade surface acoustic wave (SAW) transducer is designed and fabricated considering selection of proper piezoelectric material, optimized reaction model, improvement of diffractive efficiency and clarify separated conditions of diffracted light and undiffracted guided light.3. The rule of Gauss beam transmitting in optical fiber is analyzed. The impact of coupling effect to AO chip design is discussed. The Formulas of waveguide GL cardinal points and principal planes are derived; the geometry graphic imaging formation principles of GL are developed to get the object-image-relation of geodesic lenses. Utilization of geometric optical characters of GL, the equivalent light trail and phase transfer function are given. Simultaneously the distributing model is obtained. The focal spot intensity distribution of theory corresponds with the experiments.4. Integrated AO chip process method, performance indexes, and test results are discussed. A summary of current integrated AO chip technology and trend of AO chip technology are discussed. The innovation points of this research including:1. The profile model of waveguide geodesic lens is optimized. To normalize concave radius, divided to transition area and lens area, radiated loss decreased more, eliminated spherical aberration and curve loss are gained from different auxiliary functions in different region and optimized maximum curvature radius of joint between rim and planar waveguide.2. Optimized SAW AO Bragg diffractive condition and efficiency of SAW transducer, modulated efficiency of 85% is achieved by 500mW input driving power on device.3. Geometric optics and matrix optics is introduced into the research of waveguide geodesic lens, related completed transmitting theory in geodesic lens is obtained from light beam tracing and equivalent photonic circuit method. Under condition of adaxial incident light in small angle, phase transfer function and focus distributing model is obtained. Utilized computer simulation, theoretical focus distribution of GL is described, Which corresponds with the experiments.4. It is the first time in mainland China to design and fabrication of LiNbO₃ integrated AO chip, which characterizations are measured and analyzed. Acousto-optic(AO) frequency shifter that consists of a pair of geodesic waveguide lenses to collimate and focus and a pair of cascaded guided-wave AO Bragg cells has been realized in a Ti diffused Y cut LiNbO₃ waveguide substrate 46 cm by 2.5cm by 30 cm in size. Optimized ideal waveguide GL profile and AO parameters are calculated and designed by analytic method. Transmission loss of 1.2dB and spot size (FWHM) of 4.5μm of GL are achieved. All chip has provided a 3dB tunable bandwidth of 10 MHz on driving baseband frequency of 110MHz, coupling insert loss of 20dB with single-mode fiber.