An Automated Fiber Positioning Mechanism for Lidar Application
Document ID: 19
Master's Thesis
The Pennsylvania State University
The Graduate School
Department of Electrical Engineering
Abstract
An automatic beam alignment mechanism for a monostatic lidar system has been designed and tested. The raw lidar data consists of photon count of the laser return, which are detected after scattering by molecules and particles in the target volume. The signal received by the telescope is channeled to the detector by the optical fiber. Since maximum detected signal is desired, it is crucial therefore to feed all the collected light into the aperture of this optical fiber. In practice however, it was frequently observed that after the laser beam of the LAMP instrument was perfectly aligned in the field-of-view of the telescope, at a later time was found to have moved by a significant fraction of the beam spot diameter in the transverse plane with respect to the optic axis of the fiber. A direct way of sensing this motion and correcting it has been realized by using a quadrant detector as a sensor. The error in beam alignment is quantified by differential quadrant currents, which are converted to voltages and are used to set a timer, that controls the drive of a DC motor. DC motors, are mounted on the x and y-axes of the translation stage, providing the necessary movement to re-position the detector, and hence the fiber, relative to the beam. This active closed loop control system completes one cycle in about 10 seconds.
Keywords: lidar design, lidar atmospheric measurement program (LAMP)
| Citation: | A. S. Venkatarao, "An Automated Fiber Positioning Mechanism for Lidar Application", The Pennsylvania State University, Master's Thesis, August 1994, 47 pages |