Design and application of high performance Acousto-Optic Tunable Filters

Valle, Stefano (2017) Design and application of high performance Acousto-Optic Tunable Filters. EngD thesis, University of Glasgow.

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Printed Thesis Information: https://eleanor.lib.gla.ac.uk/record=b3301654

Abstract

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 di-
mension 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 limi-
tation 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 acous-
tic resonant cavities. The theoretical development and subsequent design of the first
resonant AOTF developed inside Gooch and Housego is carried out, and the exper-
imental 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 trans-
ducer) 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.

Item Type: Thesis (EngD)
Qualification Level: Doctoral
Keywords: Acousto-optic, resonant cavity, AOTF, tunable filters, UV, MIR, Tellurium Dioxide, Calomel, Crystal Quartz.
Subjects: Q Science > QC Physics
Colleges/Schools: College of Science and Engineering > School of Engineering
Funder's Name: Engineering and Physical Sciences Research Council (EPSRC)
Supervisor's Name: Johnson, Dr. Nigel
Date of Award: 2017
Embargo Date: 19 September 2020
Depositing User: Mr. S Valle
Unique ID: glathesis:2017-8323
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Sep 2017 09:58
Last Modified: 21 Mar 2018 14:31
URI: http://theses.gla.ac.uk/id/eprint/8323

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