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Optical Control Elements
Published in Chunlei Guo, Subhash Chandra Singh, Handbook of Laser Technology and Applications, 2021
Acousto-optic diffraction occurs from the pressure-induced grating and is maximum when the diffracted beam emerges at exactly twice the Bragg angle relative to the undeviated position. This angle depends on the acoustic frequency and, hence, can be controlled electronically through the drive signal fed to the transducer. The Bragg angle is given by θB=λ2nΛ, where θB is the Bragg angle, λ is the optical wavelength, Λ is the acoustic wavelength and n is the optical refractive index (see, e.g. [5]). The principle is shown schematically in Figure 13.3.
Vibrational Spectroscopy
Published in Grinberg Nelu, Rodriguez Sonia, Ewing’s Analytical Instrumentation Handbook, Fourth Edition, 2019
Peter Fredericks, Llewellyn Rintoul, John Coates
ACOUSTO-OPTICAL TUNABLE FILTERS. An AOTF functions as an electronically tunable bandpass filter. It consists of a birefringent crystal material with a RF transducer bonded to one face. An acoustic wave is generated across the crystal, which in turn produces a modulation of the index of refraction. Under the right conditions, if light is passed though the crystal, a portion of the beam will be diffracted. For a fixed RF frequency, only a narrow band of optical radiation satisfies the condition, and only those frequencies are diffracted. If the RF frequency is changed, the centroid of the optical frequencies will change correspondingly. Hence, it is possible to scan a spectral region with a particular optical bandpass by scanning a range of RF frequencies (Wang, 1992).
Interrogation Techniques for Fiber Grating Sensors and the Theory of Fiber Gratings
Published in Shizhuo Yin, Paul B. Ruffin, Francis T. S. Yu, Fiber Optic Sensors, 2017
Strong acousto-optic interactions occur when the Bragg condition between a light wave and a sound wave is satisfied, which is briefly shown in Figure 7.15. The Bragg condition (i.e., the momentum conservation, or phase matching condition) can be satisfied for a fixed light wavelength with an appropriate grating vector, the magnitude of which is determined by the acoustic frequency. Therefore, by varying the acoustic frequency, we can select the wavelength of the diffracted light. In other words, the wavelength tuning of the AOTF is obtained by varying the frequency of the RF (radiofrequency) signal that generates the sound wave via the transducer. As a result, by changing the RF of the AOTF, it is possible to interrogate a sensor grating. It can also be extended to multipoint grating sensors. It is possible to change the acoustic wave frequency from tens of hertz to hundreds of megahertz, thereby extending spectral tuning range. Thus, it is suitable for use in both dynamic and quasi-static detection of wavelength change in a broadband spectral range that may include multiplexed sensors. Furthermore, the AOTF technique has several advantages. It can be accessed at multiple wavelengths simultaneously as well as at random wavelengths [35]. This is obtainable by applying multiple RF signals of different frequencies. Hence, the AOTF can offer a parallel interrogation and a reduction of interrogation time in a multiplexed sensor array system.
Broadband quantum memory using electromagnetically induced transparency in atomic medium
Published in Journal of Modern Optics, 2019
Sumit Bhushan, Vikas S. Chauhan, Raghavan K. Easwaran
It has been observed that it is advantageous to have a quantum memory in which the multiplexing is realized via individual memory units as they can be accessed independently of each other (6), making the parallel processing of data more flexible and efficient. This is because, if any fault arises in any unit, it does not affect the functioning of other units. In ref. (6), multiplexing with 225 individual memory units has been demonstrated in a magneto optical trap (MOT) of rubidium atoms with the help of electromagnetically induced transparency (EIT). Here, the multiplexing and demultiplexing was facilitated by acousto-optic deflectors (AOD). In this setup, a little amount of cross-talk error in the retrieved signal is introduced as a result of the spread of the control laser to neighbouring memory units. This effect is on account of the different memory units being the part of one large macroscopic ensemble of trapped atoms. Such a situation can be avoided if the neighbouring memory units are made of completely physically separated atomic ensembles.
Anisotropy of acousto–optic figure of merit in lithium tetraborate crystals
Published in Journal of Modern Optics, 2018
Oksana Mys, Oleg Krupych, Rostyslav Vlokh
Acousto–optic (AO) materials are used for operating short-wavelength laser radiation in many branches of science and technology, e.g. ultraviolet (UV) lithography (1), LIDAR systems for monitoring ozone-depletion layer and atmosphere pollution (2–6), or disinfection and decontamination (7, 8). Paratellurite and lithium niobate crystals are well-known AO materials. For example, TeO2 manifests very high acousto–optic figure of merit (AOFM), 1200 × 10−15 s3/kg (9, 10). The AOFM for LiNbO3 is notably smaller (12.9 × 10−15 s3/kg – see Refs. (11)–(13)). However, these materials cannot be utilized in the UV spectral range, since their transparency region is limited by the wavelengths 0.40 and 0.35 μm, respectively (9).
Acousto-optic tunable filter chromatic aberration analysis and reduction with auto-focus system
Published in Journal of Modern Optics, 2018
Acousto-optic tunable filters (AOTFs) are electronic devices that use the interaction of acoustic and optical waves in acousto-optic crystals. They are used for a number of single-point applications, such as spectroscopy (1–5), laser tuning (6,7), and optical multiplexing (8,9). In recent years, it also be used for measuring the dispersion characteristic of fibres (10), spectropolarimetric detection along with photoelastic modulators or Liquid Crystal Variable Retarder (LCVR) (11,12), atmospheric trace gases remote sensing (13). Because the acousto-optic merit of TeO2 is higher, transmission region is 0.35~5 μm, covering the whole visible and part of the infrared region, a typical application is making abnormal acousto-optic devices, hence commonly use it as the crystal of AOTF acousto-optic devices. The acousto-optic material TeO2 has generally been used in the visible, and Tl3AsSe3 in the infrared. They can produce high-speed filtering with access times of a few microseconds.