Multi-electrode array for spectral bandwidth control
Résumé
Acousto-optical tunable filters (AOTFs) are well used for practical applications and are most often based on the classical anisotropic non-collinear configuration in tellurium dioxide crystals. Any product sheet of such a commercially available AOTF presents two main features: the tuning range and the spectral bandwidth Δλ. The spectral transmission bandwidth1 is proportional to the ratio λ2 /(bW) where λ is the optical wavelength, b the dispersive constant and W the length of the transducer. The dispersive constant b is a rapidly decreasing function of the wavelength in the blue visible spectrum. As for examples: for a one octave tuning range starting from 400 nm to 800 nm, the spectral bandwidth is multiplied by ~10. For a tuning range in the infrared band 1250-2500 nm, the spectral bandwidth is multiplied by ~4. We present the design of a filter operating in the visible region from 450 nm to 650 nm. A double anisotropic interaction2 is considered allowing the simultaneous diffraction of the two optical modes propagating into the crystal by a single ultrasonic wave. A multi-electrode array of three identical consecutive electrodes Wi:1-3 = 3 mm is deposited on the transducer leading to a more precise control of the spectral bandwidth. The acousto-optic device has been fabricated by the AA-Opto company. With this component the spectral bandwidth is only multiplied by ~2 on from 400 nm to 650 nm.