Diffraction of optical waves by an acoustic grating is a well-known phenomenon that enables the design of versatile devices useful in photonic systems. For example, acousto-optic tunable filters (AOTFs) can be dynamically tuned by radio-frequency signals. Among possible material choice, paratellurite is often used for practical applications due to its high efficiency. In such a birefringent material, the interacton is anisotropic. This configuration is sensitive to optical input polarisation: a selective coupling between polarization modes occurs. The incident beam must be polarised and the diffracted mode polarisation is orthogonal to the incident one. However, during the design process a very specific operation point can be found that ensures the simultaneous diffraction of both the ordinary and the extraordinary optical modes. In this presentation, we introduce the design of AOTF in birefringent crystals and present the main parameters that are subject to trade-off. Acousto-optic diffraction efficiency is sensitive to the so-called phase matching condition between optical wave and the ultrasonic wave. The offset from synchronicity is considered introducing a phase mismatch parameter. Diffraction efficiency evolution with respect to Bragg condition are illustrated. A custom device is finally presented that ensures simultaneous diffraction of both polarisation modes and compared to experimental results.