Speckle reduction in coherent information processing

Speckle reduction techniques have been developed as one of the most active research fields in coherent optical information processing, together with the investigations of speckle statistics and coherence theory. The principles of speckle reduction are classified to five categories: 1) control of spatial coherence, 2) control of temporal coherence, 3) spatial sampling, 4) spatial averaging, and 5) digital image processing. This paper surveys the research works in the field of speckle reduction techniques involving four categories 1)-4) which have been conducted in the past 30 years.