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Acousto-optic tunable filters

This is a continuation from the previous tutorial - acousto-optic deflectors. An optical grating can be used for the separation or filtering of optical frequencies, as is seen in any grating spectrometer and in s distributed Bragg reflector as discussed in the grating waveguide couplers tutorial. It is also possible to use the index grating generated by an acoustic wave in a medium for such purposes. One advantage of such an acousto-optic filter, or acousto-optic spectrometer, is that it is electronically tunable because the period of the acousto-optic grating can be varied by altering the acoustic frequency. This electronic tunability...

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Acousto-Optic Deflectors

This is a continuation from the previous tutorial - acousto-optic modulators. The acoustic wave of an acousto-optic deflector is frequency modulated. Unlike an acousto-optic modulator, which is an amplitude modulator, an acousto-optic deflector is a frequency modulator, which allows its acoustic frequency to be varied electronically. Acousto-optic deflectors have many applications. A frequency shifter, which has the sole purpose of generating a diffracted optical beam at an optical frequency shifted by the amount of the acoustic frequency from the input optical frequency, can be considered as the simplest form of an acousto-optic deflector. Acousto-optic deflectors are also used in such...

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Acousto-Optic Modulators

This is a continuation from the previous tutorial - acousto-optic diffraction. The acoustic wave of an acousto-optic modulator is amplitude modulated. The operation of an acousto-optic modulator is based on the dependence of the acousto-optic diffraction efficiency on the intensity of the acoustic wave. The acoustic intensity can be controlled by an electrical signal that generates the acoustic wave in a modulator. An acousto-optic modulator is an electronically addressed amplitude modulator that accepts an electrical modulation signal to vary the intensity of an optical beam accordingly. Acousto-optic modulators have been put to many different applications. The straightforward application is amplitude...

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Acousto-optic diffraction

This is a continuation from the previous tutorial - photoelastic effect. We see from the preceding two tutorials that the space- and time-dependent periodic permittivity changes induced by a traveling plane acoustic wave of the form given in (8-1) [refer to the elastic waves tutorial] can be generally expressed as \[\tag{8-34}\Delta\boldsymbol{\epsilon}=\Delta\tilde{\boldsymbol{\epsilon}}\sin(\mathbf{K}\cdot\mathbf{r}-\Omega{t})\] where \(\mathbf{K}\) depends on both the polarization and the propagation direction of the acoustic wave. In general, \(\Delta\tilde{\boldsymbol{\epsilon}}\) is a function of the strain and the rotation generated by the acoustic wave in the medium, the elasto-optic coefficients of the medium, the mode and direction of the acoustic wave,...

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Photoelastic effect

This is a continuation from the previous tutorial - elastic waves. Mechanical strain in a medium causes changes in the optical property of the medium due to the photoelastic effect.  The basis of acousto-optic interaction is the dynamic photoelastic effect in which the periodic time-dependent mechanical strain caused by an acoustic wave induces periodic time-dependent variations in the optical properties of the medium. The photoelastic effect is traditionally defined in terms of changes in the elements of the relative impermeability tensor caused by strain: \[\tag{8-7}\eta_{ij}(\mathbf{S})=\eta_{ij}+\Delta\eta_{ij}(\mathbf{S})=\eta_{ij}+\sum_{k,l}p_{ijkl}S_{kl}\] where \(p_{ijkl}\) are dimensionless elasto-optic coefficients, also called strain-optic coefficients or photoelastic coefficients, and they...

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