Single and tandem Fabry-Perot etalons as solar background filters for lidar

Atmospheric lidar is difficult in daylight because of sunlight scattered in to the receiver field of view. In this research methods for the design and performance analysis of Fabry-Perot etalons as solar background filters are presented. The factor by which the signal to background ratio is enhanced is defined as a measure of the performance of the etalon as a filter. Equat ions for evaluating this parameter are presented for single-, double-, and triple-etalon filter systems. The role of reflective coupling between etalo ns is examined and shown to substantially reduce the contributions of the s econd and third etalons to the filter performance. Attenuators placed betwe en the etalons can improve the filter performance, at modest cost to the si gnal transmittance. The principal parameter governing the performance of th e etalon filters is the etalon defect finesse. Practical limitations on eta lon plate smoothness and parallelism cause the defect finesse to be relativ ely low, especially in the ultraviolet, and this sets upper limits to the c apability of tandem etalon filters to suppress the solar background at tole rable cost to the signal. (C) 1999 Optical Society of America OCIS codes: 2 30.3640, 120.2230, 120.4570, 350.2460.