Undersea object detection and recognition: the use of spatially and temporally varying coherent illumination

Increased optical range of target detection and recognition is always a problem in the marine environment. Methods for the formation of images in scattering media generally rely upon temporal or spatial methodologies. Time gating is a temporal example of image formation whereby a light source is time 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, this method requires exacting 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. Generally, the demands of maintaining a spatially coherent beam at optical frequencies is difficult over a long range thereby limiting the usefulness of the technique for image formation in turbid media. A paper submitted by the authors at the OCEANS '98 describes 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 and may use a continuous illumination source. Coherency of the spatial beam allows detection of the direct return, while scattered light appears as a noncoherent noise term. The theoretical foundation of the method and the initial results for turbid media were developed. This paper presents the latest ongoing research results.