Spatially variant and coherent illumination method for undersea object detection and recognition

Increased optical range of target detection and recognition is always a problem in the marine environment. The inherent optical properties of light absorption and scattering in water, compounded by suspended particulate matter scattering, limit both radiative and information transfer for image formation. Methods for the formation of images in scattering media generally rely upon temporal or spatial methodologies. Some interesting designs have been developed in an attempt to circumvent or overcome the scattering problem. Time gating is a temporal example of image formation whereby a light source is pulse projected toward a target and the detector is time gated to accept image forming illumination from a specific range. To be successful at eliminating much of the backscatter problem, this method requires range information and loses the simplicity of a continuous light source. Holography is one example of an image formation method requiring specific spatial relationships, i.e. 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. Generally, the demands of maintaining a spatially coherent beam at optical frequencies is difficult over long range thereby limiting the usefulness of the technique for image formation in turbid media. This paper describes a variation of the spatial interferometric technique that relies upon projected spatial gratings with subsequent detection against a quasi-coherent return signal.