INDEX Raman-Nath diffraction

Consequently, we can understand that sound waves change the refractive index of a medium according to Eq. (4.31). In a piezoelectric medium sound waves can induce a new electric field. This phenomenon is very complex because the induced electric field relates to a change in the refractive index of the medium. In general, the acousto-optic effects are classified into two phenomena depending on the frequency of the sound waves, the Raman-Nath diffraction and the Bragg diffraction (see Table 4.10). 

The Raman-Nath output is usually taken to be the zeroth-order mode. The modulation index equals the fraction of the light diffracted out of the zeroth-order and is expressed as... 

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