Optical Design for Advanced Lidar Detectors
Document ID: 90
Master's Thesis
The Pennsylvania State University
The Graduate School
Department of Electrical Engineering
Abstract
A LIght Detection And Ranging system, commonly referred to as lidar, transmits a laser beam at a certain wavelength into the atmosphere, and a telescope collects the return signals, including direct backscatter, vibrational Raman, and rotational Raman signals at different wavelengths. The return signals are then detected to obtain measurements of atmospheric properties. The potential use of lidar instruments as a replacement for current remote sensing technologies have fueled efforts to develop accurate, high resolution and autonomous lidar instruments. The Lidar Atmospheric Profile Sensor (LAPS) unit is a unique optical remote sensing instrument which was developed at Penn State University for the US Navy. It is used in the study of atmospheric properties and constituents using the technique of molecular Raman scattering. Experience with operating the LAPS instrument over a number of campaigns has highlighted design and performance benefits that could be gained from revisiting the original LAPS detector design. ALAPS (Advanced Lidar Atmospheric Profile Sensor), which is an advanced version of the LAPS instrument being developed as a commercial prototype at Penn State University, will benefit from the new designs developed for the advanced detector reported here. The optical paths in the LAPS detector and the advanced detector were investigated using optical simulation software called ZEMAX. Using the simulated performance we can determine the beam spot size and the location where lens should be placed relative to the fiber to obtain the best results over the wide range of wavelengths. The effects of aberrations and alignment errors that cause vignetting can be minimized or eliminated to arrive at a more efficient optical design. The advanced detector design aims to eliminate the uncertainties in measurements which have been identified with the LAPS instrument and also provide a commercially viable design. The advanced design incorporates a self-calibration mechanism using a filter-switching slide, which will be used to calibrate the instrument and monitor performance by measuring the relative transmission and efficiency of the channels. The new detector system, incorporates higher speed photomultiplier tubes (PMTs) and faster electronics to increase the data resolution from 75 m (500 ns) to 3 m (20 ns). The optical design includes the analysis of the new detector and testing to improve performance of the present LAPS detector. The detector design has been prepared to provide capability for self-calibration and for monitoring of detector performance, and it has been verified using simulations and experimental results.
| Citation: | G. S. Chadha, "Optical Design for Advanced Lidar Detectors", The Pennsylvania State University, Master's Thesis, May 2001, 63 pages |