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| | 34 year old Plastic and Reconstructive Surgeon Courtney from McBride, has hobbies and interests which include mountain biking, property developers in [http://www.alemim.de/forum/member.php?u=19977 singapore property index] and drawing. In the last year has made a journey to Würzburg Residence with the Court Gardens. |
| [[Image:Acousto-optic Modulator.png|thumb|360px|right|An acousto-optic modulator consists of a [[piezoelectric transducer]] which creates sound waves in a material like glass or quartz. A light beam is diffracted into several orders. By vibrating the material with a pure sinusoid and tilting the AOM so the light is reflected from the flat sound waves into the first diffraction order, up to 90% deflection efficiency can be achieved. ]]
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| An '''acousto-optic modulator (AOM)''', also called a '''Bragg cell''', uses the [[acousto-optic effect]] to [[diffraction|diffract]] and shift the frequency of light using [[sound wave]]s (usually at [[radio-frequency]]). They are used in [[laser]]s for [[Q-switching]], telecommunications for signal [[modulation]], and in [[spectroscopy]] for frequency control. A [[piezoelectric transducer]] is attached to a material such as glass. An oscillating electric signal drives the transducer to vibrate, which creates 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 (see [[Brillouin scattering]]) off the resulting periodic index modulation and interference occurs similar to [[Bragg diffraction]]. The interaction can be thought of as [[four-wave mixing]] between [[phonon]]s and [[photon]]s.
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| ==Principles of operation==
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| The properties of the light exiting the AOM can be controlled in five ways:
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| ===Deflection===
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| :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
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| :: <math>\sin\theta = \left (\frac{ m\lambda}{2\Lambda} \right)</math>
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| : in the Bragg regime and
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| :: <math>\sin\theta = \left (\frac{ m\lambda_0}{n\Lambda} \right)</math>
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| : with the incident light being normal to the sound waves, where ''m'' = ..., −2, −1, 0, 1, 2, ... is the order of diffraction. Diffraction from a sinusoidal modulation in a thin crystal solely results in the ''m'' = −1, 0, +1 diffraction orders. Cascaded diffraction in medium thickness crystals leads to higher orders of diffraction. In thick crystals with weak modulation, only [[Nonlinear optics#Phase matching|phasematched]] orders are diffracted; this is called [[Bragg diffraction]]. The angular deflection can range from 1 to 5000 beam widths (the number of resolvable spots). Consequently, the deflection is typically limited to tens of [[milliradian]]s.
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| ===Intensity===
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| :The amount of light diffracted by the sound wave depends on the intensity of the sound. Hence, the intensity of the sound can be used to modulate the intensity of the light in the diffracted beam. Typically, the intensity that is diffracted into ''m'' = 0 order can be varied between 15% to 99% of the input light intensity. Likewise, the intensity of the ''m'' = 1 order can be varied between 0% and 80%.}
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| ===Frequency===
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| :One difference from Bragg diffraction is that the light is scattering from moving planes. A consequence of this is the frequency of the diffracted beam ''f'' in order ''m'' will be [[Doppler effect|Doppler]]-shifted by an amount equal to the frequency of the sound wave ''F''.
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| ::<math>f \rightarrow f + mF</math>
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| This frequency shift is also required by the fact that [[Nonlinear optics#Phase matching|energy and momentum]] (of the [[photon]]s and [[phonon]]s) are conserved in the process. A typical frequency shift varies from 27 MHz, for a less-expensive AOM, to 1 GHz, for a state-of-the-art commercial device. In some AOMs, two acoustic waves travel in opposite directions in the material, creating a [[standing wave]]. Diffraction from the standing wave does not shift the frequency of the diffracted light.
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| ===Phase===
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| :In addition, the phase of the diffracted beam will also be shifted by the phase of the sound wave. The phase can be changed by an arbitrary amount.
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| ===Polarization===
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| : Collinear [[Transverse wave|transverse]] acoustic waves or perpendicular [[longitudinal wave]]s can change the [[Polarization (waves)|polarization]]. The acoustic waves induce a [[birefringence|birefringent]] phase-shift, much like in a [[Pockels cell]]. The acousto-optic tunable filter, especially the [[acousto-optic programmable dispersive filter|dazzler]], which can generate variable pulse shapes, is based on this principle.<ref>{{vcite journal |author=H. Eklund, A. Roos, S. T. Eng |title=Rotation of laser beam polarization in acousto-optic devices |journal=Optical and Quantum Electronics |date=1975 |volume=7 |issue=2 |pages=73–79 |doi=10.1007/BF00631587 }}</ref>
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| ==Modelocking==
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| Acousto-optic modulators are much faster than typical mechanical devices such as tiltable mirrors. The time it takes an AOM to shift the exiting beam in is roughly limited to the transit time of the sound wave across the beam (typically 5 to 100 nanoseconds). This is fast enough to create active [[modelocking]] in an [[Ti-sapphire laser|ultrafast laser]]. When faster control is necessary [[electro-optic modulator]]s are used. However, these require very high voltages (e.g. 1-10 kilovolts), whereas AOMs offer more deflection range, simple design, and low power consumption (less than 3 watts).<ref>http://www.ulp.ethz.ch/education/ultrafastlaserphysics/7_Active_modelocking.pdf</ref>
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| ==Applications==
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| * [[Q-switching]]
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| * [[Ti-sapphire laser#Chirped-pulse amplifiers, chirped pulse amplification|Regenerative amplifiers]]
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| * [[Optical cavity|Cavity dumping]]
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| * [[Modelocking]]
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| * [[Laser Doppler vibrometer]]
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| * [[RGB Laser Light Modulation for Digital Imaging of Photographic Film]]
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| * [[Confocal microscopy]]
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| ==See also==
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| * [[Electro-optic modulator]]
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| * [[Jeffree cell]]
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| * [[Liquid crystal tunable filter]]
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| * [[Photoelasticity]]
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| * [[Pockels effect]]
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| ==External links==
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| *http://www.olympusfluoview.com/theory/aotfintro.html
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| ==References==
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| {{reflist}}
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| <references/>
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| {{DEFAULTSORT:Acousto-Optic Modulator}}
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| [[Category:Optical devices]]
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34 year old Plastic and Reconstructive Surgeon Courtney from McBride, has hobbies and interests which include mountain biking, property developers in singapore property index and drawing. In the last year has made a journey to Würzburg Residence with the Court Gardens.