Abstract
Acousto-Optic Tunable Filters (AOTFs) are electronically-controlled bandpass optical filters. They are often preferred in applications in spectroscopy where their agility and rapid random-access tuning can be deployed to advantage. When used for spectral imaging a large aperture (typically 10mm or more) is desired in order to permit sufficient optical throughput. However, in the mid IR the λ2 dependence on RF drive power combined with the large aperture can prove to be a hurdle, often making them impractical for many applications beyond about 2μm. We describe and compare a series of specialised free-space configurations of AOTF made from single crystal tellurium dioxide, that require relatively low RF drive power. We report on AOTFs specifically optimised for operation with a new generation of Supercontinuum source operating in the 2-4μm window and show how these may be used in a spectral imaging system. Finally, we describe an AOTF with an (acoustic) Fabry-Perot cavity operating at acoustic resonance rather than the conventional travelling-wave mode; the acoustic power requirement therefore being reduced. We present an analysis of the predicted performance. In addition, we address the practical issues in deploying such a scheme and outline the design of a prototype “resonant AOTF” operating in the 1-2μm region.
Highlights
The principles of acousto-optics have been known for some time, but it is only since the advent of the laser that the science has developed and practical devices found application
Bulk AO devices are used in many forms but principally Q-Switches, Mode-Lockers, Modulators, Deflectors, Frequency-Shifters [refs] and Tunable Filters, together with variations such as Cavity Dumpers and Pulse-Pickers [1]
The acoustic beam creates a sinusoidal perturbation of the refractive index of the medium due to the photoelastic effect which acts as a volume diffraction grating
Summary
The principles of acousto-optics have been known for some time, but it is only since the advent of the laser that the science has developed and practical devices found application. An optical beam will interact with the induced grating when the angle of incidence and wavelength are matched to the grating periodicity, causing the optical beam to be diffracted at a given angle. Most AO devices rely upon the “isotropic” AO interaction where the refractive indices of the incident and diffracted beams are the same; the acoustic beam propagates as a longitudinal mode. These devices often referred to as “Bragg Cells” form the basis of modulators and Q-switches etc [1]. This interaction requires an optically anisotropic (birefringent) medium, and the incident and diffracted beams have different (ordinary/extraordinary) refractive indices. This becomes very important when designing AOTFs to operate in the infra-red, beyond about 2μm
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