The acousto-optic tunable AOTF analyser

The two physical models that can be used to describe an AOTF device depend as usual on optics, and on wave and particle descriptions of the interactions involved. The basic physical layout of the device will be described, and then its principle of operation will be discussed. 

The two physical models for the operation of an AOTF depend on how one views the interaction of an incident NIR light beam with this acoustically excited crystal lattice. 

On one level it is a quantum effect, and can be described in terms of photon—phonon scattering. The incident NIR beam is a source of photons, and the energy from the piezotransducer provides a source of lattice phonons that propagate through the crystal. As in all collision processes, the twin principles of conservation of momentum and conservation of energy apply. The momentum of a quantum particle is linked to its wavevector by hk. The energy is linked to its frequency by hjj. 

Consider first the simpler case of acousto-optic interaction in an isotropic medium (i.e. a standard acousto-optic modulator). If the wavevector of the incident light is ko, that of the scattered light k+ or k and that of the phonon K, we have for conservation of momentum 

Secondly, this seemingly innocent relationship between input and scattered wavevec-tors has a significant effect on the scattered (diffracted) beam. Because in all realistic cases the scalar value of k0 will greatly exceed that of K, if one plots the wavevector diagrams one will see that k+ and ko form the two sides of a very nearly flattened triangle, and for 

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