Detection and tracking of the backreflection of potassium dihydrogen phosphate images in the presence or absence of a phase mask

The potassium dihydrogen phosphate (KDP) crystals present in the final optics assembly at the National Ignition Facility (NIF) are used for conversion of an infrared laser light beam into an ultraviolet beam. The conversion is highest for a certain incident angle, the alignment of which is determined from the position of the backreflection beam, which exhibits a distinct characteristic shape. When a phase-plate device is introduced before the final assembly to increase the uniformity of the beam, the backreflection pattern changes drastically. The algorithm that is best for tracking the special-shaped beam is no longer suitable for tracking the phasemodified beam. We discuss our detection schemes for both situations. In particular, we demonstrate how the algorithm senses the modified beam by using a newly proposed criterion of correlation peak pedestal area and executes an alternate algorithm in real time without operator intervention. This new algorithm continuously tracks the beam pattern to guarantee reliable and repeatable sensing. Results from simulation and real-world implementation of the algorithm at the NIF facility are presented.