COMPARISON OF VARIABLE BIREFRINGENCE DEVICES AND FABRY-PEROT ETALONS FOR USE AS TUNABLE FILTERS

A comparison has been made of three device configurations which may be used as tunable filters:—a Fabry-Perot etalon used in transmission (FPT), a Fabry-Perot etalon used in reflection (FPR), and a variable birefringence filter used in transmission (VBF). Initial conditions are defined for the achievement of a contrast ratio of 100:1 and the implications of these results on material choice discussed. The effect of nonuniform device thickness on contrast ratio is analyzed for all three cases. Analytic expressions are derived which predict the behavior of the contrast ratio under differing device conditions and which give rule-of-thumb values for the maximum acceptable thickness variation for a given contrast ratio. Using these expressions it is shown that the configuration least sensitive to variations in thickness is the VBF, with the FPT coming second and the FPR last. Detailed computer modelling studies were also undertaken and it was found that the general trends predicted by the analytic expression were confirmed by the modeling. Experimental studies were carried out on three devices—a piezoelectric tunable air-spaced Fabry-Perot etalon, an electrooptically tunable liquid crystal Fabry-Perot etalon, and an electrooptically tunable liquid crystal variable birefringence filter. Measurements were made of the contrast ratio in all three configurations. The experimental trends were found to be in agreement with the theory and the modeling. © 1990 IEEE