Automatic optical alignment by numerical ellipse fitting

An automatic system for surface alignment, which can be employed for the study and optical characterization of samples in many devices, is presented. The alignment is accomplished through a mechanical device, consisting in three stepping motors driving micrometry screws, which in turn allow the tilting of the specimen. Simultaneously, a laser beam is reflected on the surface sample and the reflection is collected through a Position-Sensitive Detector (PDS), which provides four photocurrents outputs proportional to the two-dimensional center of mass of the light spot impinging the surface of the detector, allowing to locate the relative positioning of the laser. Any desalignment generates an ellipse on the detector position and will be monitored through this array, feedbacked electronically through a personal computer. The algorithm developed for finding all parameters of the ellipse are evaluated and discussed.