title: Design and application of high performance Acousto-Optic Tunable Filters
Creators: name: Valle, Stefano
Publisher: name: University of Glasgow
Year: 2017
Subjects: subject: Acousto-optic, resonant cavity, AOTF, tunable filters, UV, MIR, Tellurium Dioxide, Calomel, Crystal Quartz.

description: The design and application of high performance Acousto-Optic Tunable Filters (AOTFs) is the topic of this thesis. The most common material commercially used to build AOTFs is Tellurium Dioxide because of its extended wavelength range from 380 nm up 4.5 μm, a good acousto-optic figure of merit, and the availability in crystal of large dimension with good optical quality. The performance of an AOTF can be summarized by passband versus wavelength, RF power to achieve peak diffraction efficiency and tuning range. One of the most relevant parameters which limits AOTF performance is the RF power versus diffraction efficiency, which increases with 2. A practical limitation has been imposed to 50mW /mm2 , above these level thermal gradient affects the performance. This level is easy reached for conventional AOTF for wavelength above 2.5 μm. The reduction of RF power requirement is achieved by means of acoustic resonant cavities. The theoretical development and subsequent design of the first resonant AOTF developed inside Gooch and Housego is carried out, and the experimental results are in good agreement with the predicted performances. The results obtained from the experience with resonant AOTFs lead to extending the wavelength range from 180 nm up to 10 μm using different acousto-optic materials with limited performance with conventional configuration. Crystal quartz is investigated to extend the wavelength range down to 180 nm, this material is well known in acousto-optic applications. The resonant configuration is particularly suitable for this crystal since the advantage factor is particularly high thanks to the low acoustic attenuation. The experimental results show good agreement with the predicted performances, leading to a large aperture AOTF operating between 180 nm and 380 nm with RF power for peak diffraction efficiency below 8 W. This is the first time that a resonant imaging AOTF made of crystal quartz is reported in literature, to the best of the authors knowledge. Calomel single crystal is investigated to realise an AOTF operating between 4.5 μm and 10 μm. Due to its physical properties which are incompatible with conventional manufacturing processes, different challenges (such as bonding the ultrasonic transducer) are solved by the author in order to build a high performance AOTF. Despite the preliminary results showing promise, the acousto-optic properties of the material are limiting the achievable performance, at least for acousto-optic tunable filters, a fact that does not seem to have been widely appreciated , and which cannot solved by just optimising the design. From the application point of view an optical spectrum analyser based on AOTF technology is investigated, where a dual polarization AOTF is developed in order to realize a spectrometer with a competitive price operating in the wavelength range between 2.5 μm and 4.5 μm. The effects of the acoustic attenuation on a large aperture AOTF is investigated theoretically and experimentally since the performance is affected in the UV range leading to non-ideal operating condition, and a solution is proposed to overcome this issue The results obtained will allow the design of high performance AOTFs in the industrial environment leading to a new types of devices with enhanced performance.