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From interferometry to color holography
Published in Stefano Discetti, Andrea Ianiro, Experimental Aerodynamics, 2017
An acousto-optic modulator (AOM), also called “Bragg cell,” uses the acousto-optic effect to diffract and shift the frequency of light using sound waves (usually at a frequency of the order of some MHz). A piezoelectric transducer is attached to a material such as glass (e.g., bismuth telluride). An oscillating electric signal drives the transducer to vibrate, thus creating sound waves in the glass. These can be thought of as moving periodic planes of expansion and compression that change the index of refraction. Incoming light scatters off the resulting periodic index modulation and interference occurs. A diffracted beam emerges at an angle θ that depends on the wavelength of the light λ relative to the wavelength of the sound Λ in the following relation: sin(θ)=mλ2Λ where m = …−2, −1, 0, 1, 2, … is the order of diffraction. In thick crystals with weak modulation as shown in Figure 8.3, only phase-matched orders are diffracted; this is called Bragg diffraction (only the zero and +1 orders are diffracted).
The Basics of Lasers
Published in Helmut H. Telle, Ángel González Ureña, Laser Spectroscopy and Laser Imaging, 2018
Helmut H. Telle, Ángel González Ureña
An acousto-optic modulator (AOM) is a device based on the modification of the refractive index by the oscillating mechanical pressure of a sound wave. As for EOMs, it can be used for controlling the power, frequency, or spatial direction of a laser beam by using an electrical drive signal. The active element of an AOM is a transparent crystal to which a piezoelectric transducer is attached, which excites a sound wave within the crystal. The sound wave generates a traveling periodic refractive index grating at which the light experiences Bragg diffraction (therefore, AOMs are also known as Bragg cells). For sufficiently high acoustic power, more than 50% of the optical power can be diffracted.
Brillouin-Based Distributed Temperature and Strain Sensors
Published in Arthur H. Hartog, An Introduction to Distributed Optical Fibre Sensors, 2017
In contrast with Raman scattering, where the interaction is with molecular vibrations, i.e. optical phonons, here the interactions occur with acoustic phonons, i.e. vibrations of the lattice. The process has some similarity to the acoustic diffraction of light in an acousto-optic modulator (AOM) [11], wherein an incident optical beam interacts with an ultrasonic acoustic wave to diffract some of the light.
Ultrafast laser micromachining of angled surfaces in fused silica
Published in Journal of Modern Optics, 2023
The short pulse laser employed in this study is a ytterbium-doped photonic crystal fibre femtosecond laser (Amplitude Systems). The optical components needed for laser power control such as half-wave plate, and acousto-optic modulator are built-in in the laser head unit. The laser generates 300 fs to 4 ps pulses at 1030 nm with up to 2 MHz repetition rate and pulse energy of 20 µJ. The beam is focused by a 20X high power micro focusing objective (0.4 NA) after passing a 4X beam expander and being reflected from a couple of mirrors. The quarter-wave plate is used to turn linearly polarized beam into circularly polarized beam. The process is controlled by Aerotech ultra-precise nanopositioning XY and Z stages possessing ±175 nm accuracy, 75 nm repeatability, 1 nm resolution, and <1 nm in position stability. 1 mm thick Corning 7980 fused silica plate is placed on a predesigned vacuum holding chuck with precision tip and tilt stage. The laser setup is shown in Figure 1.
Nuclear spin symmetry conservation studied by cavity ring-down spectroscopy of ammonia in a seeded supersonic jet from a pulsed slit nozzle
Published in Molecular Physics, 2020
G. Wichmann, E. Miloglyadov, G. Seyfang, M. Quack
The basic scheme of the experiment follows our earlier work in the near-IR [31,32,36,55,56] extended here to the mid-IR. An overview of the experiment is given in Figure 1. A single mode laser system using a cw optical parametric oscillator (OPO), with output power of about and referenced to a frequency comb, provides mid-IR photons for a laser cavity with a high finesse in the wavenumber range from to . The incident laser beam can be shifted by an acousto-optic modulator (AOM, ), switching the laser on and off in less than . The transmission through the laser cavity is recorded by an infrared detector (a liquid nitrogen-cooled InSb photodiode J10D, Judson Infrared, Inc.). When the laser radiation is switched off, the decay of the signal intensity (ring-down) is recorded [56] with is the initial intensity proportional to the circulating power in the cavity, R is the reflectivity of the mirrors, L is the length of the laser cavity, c is the speed of light, τ is the lifetime of the exponential decay and is the absorption by the gas between the mirrors. For further background and various general aspects and applications of the cavity ring-down technique we refer to [29,31,32, 55,56,57–66].
Grating-assisted non-linear directional coupler based optical switch with reduced critical power
Published in Journal of Modern Optics, 2018
Asymmetric waveguide directional coupler with co-directional coupling between the two waveguides due to a periodic grating (called grating-assisted directional coupler (GADC)) is an important guided wave component (11). It has many applications such as, distributed Bragg reflector source (12), wavelength filter (13–15), wavelength-division multiplexer (WDM) (16,17), optical half adder (18) and contradirectional coupler (19). In this manuscript, we have investigated a grating-assisted non-linear directional coupler (GANDC) using the coupled mode theory, and optimized the design parameters to reduce the critical power. The optimal device could have potential application as an optical switch. The proposed device can be fabricated using ultrafast laser inscription (ULI) technique; both linear and non-linear directional couplers were designed and fabricated inside gallium lanthanum sulphide (GLS) using ULI (20,21). The periodic modulation of refractive index can be realized by externally modulating the laser power using an acousto-optic modulator (22). we have designed the GANDC-based optical switch using parameters corresponding to GLS, which has a large Kerr non-linear coefficient and large transmission range (~0.5 to 10 μm). We have compared the coupling characteristics of the GANDC and that of an identical waveguide NLDC, and find that the critical power of the proposed GANDC is lower than that for the NLDC; the difference is particularly significant in the case of low-power switching. We have also studied the spectral characteristics of the proposed structure, and the effect of the grating on the critical power. A wide range of critical power can be achieved, depending on the requirement of the application, by tuning the grating parameters.