Adaptive active imaging system based on radiation focusing for extended targets

An adaptive system for active imaging and radiation focusing on extended rough-surfaced objects in the presence of phase distortions was experimentally demonstrated. This technique is based on the use of two laser beams having different wavelengths, here a He-Ne and Argon laser. The Argon laser beam was used for adaptive radiation focusing on the rough target surface. The second laser beam served for target active imaging. The imaging system included the same wavefront correctors used for the radiation focusing system. For adaptive control of the wavefront correctors, a gradient technique based on a radiation focusing quality criterion optimization was applied. Statistical properties of the speckle field scattered by the target surface were used as the radiation focusing quality metric. To measure this criterion a single photodiode with a simple signal analysis method was employed. Adaptive correction was implemented with a nine electrode bimorph mirror and a liquid crystal spatial phase modulator having 127 individually addressed hexagonal liquid crystal cells.