On the limitations of optical characterization of intense ultrasound   fields in the Raman-Nath regime
  Accurate interpretation of optical measurement of acoustic fields generally assumes two criteria are met regarding the acoustooptic interaction. First, the light-sound interaction must be weak enough to be considered in the Raman-Nath regime. That is, only diffracting orders up to the first order are considered. Second, the effects of optical refraction that take place over the width of the acoustic signal are generally treated as insignificant. The quantitative conditions dictating where each criterion is applicable can be expressed in terms of the Cook-Klein parameter and the Raman-Nath parameter. However, these conditions, which are the basis for most optical methods of acoustic field characterization, are incomplete. This technical report demonstrates a third requirement that is also necessary in order to assure the validity of the approach in high pressure characterization. 
Gregory T. Clement
physics.optics