Nonlinear Methods In Image Formation

Nonlinear methods have produced significant improvements in images formed under adverse conditions. In particular, phase conjugation, both double pass (object-bearing beam passing twice through the inhomogeneity) and single pass (object beam passing only once) have led to a capability of image formation through inhomogeneities. This process can be significantly enhanced by a combination of aperture and coherence reduction, leading to resolution beyond the classical limit, or superresolution. The basis for this· second method is similar to phase conjugation in that an object bearing wave is mixed with a second beam at a detector. There are various differences, e.g., incoherent light is required, and a conjugation of the phase is not required. In the superresolution process, the object bearing wave behaves as if it had traversed the reference beam path, which is taken to be both aberration free and of significant size.