Spatial coherence methods in undersea image formation and detection

Methods for the formation of images in scattering media generally rely upon temporal or spatial methodologies. Holography is a common example of image formation exploiting the mutual coherence between a reference beam and a signal beam. The coherence allows the formation of an interference pattern that carries the signal information on a "spatial carrier". In order for the method to be of use, the medium in which the beams are carried must preserve the coherence or phase spatially across the beams and in relation to the reference beam. In water, the distance over which the phase may be preserved is dependent upon many factors, including turbulence induced refractive index variations, thermal gradient structure, and relative motion. If pathlength differences exceed the temporal coherence length of the beam, interference is not obtained and the method breaks down. Holographic imaging has been demonstrated underwater and coherence measurement techniques have been suggested. Generally, the demands of maintaining a spatially coherent bean at optical frequency is difficult over long range thereby limiting the usefulness of the technique for image formation in turbid media. The authors describe a variation of the spatial interferometric technique that relies upon projected spatial gratings with subsequent detection against a quasi-coherent return signal. The method is advantageous in not requiring temporal coherence between reference and signal beams. Coherency of the spatial beam allow detection of the direction return, while scattered light appears as a noncoherent noise term. The theoretical foundation of the method and initial results for turbid media are developed.