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A comparison is made of three device configurations which can 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. 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. >

Previous experimental measurements of beam noise have shown that spectrum levels measured with towed line arrays have considerable temporal and spatial variability. In the ambient noise region dominated by shipping noise (a few Hz to a few hundred Hz), variations in spectral levels of over 30 dB can occur. Attempts to model the predicted beam noise cumulative distribution function (R.M. Heitmeyer, L.T. Davis and N. Yen, NRL Report 8863, February 1985) required approximations for both the beam pattern and transmission loss to achieve an analytic solution. The computed detection gain in regions of reduced noise resulting from resolution of individual noise-interferers, termed "ship resolution gain" (SRG), is dependent on source distribution, acoustic transmission loss and beam pattern approximation. The paper uses numerical computation of SRG to treat arbitrary hydrophone shading and realistic ocean environments. Results are compared to earlier analytic predictions to show dependence on system and environmental parameters. >

Artificial birefringence in a silicon slit structure has been demonstrated using 10·6 µm radiation. The 4 µm period grating of slits, 2 µm wide and up to 50 µm deep, was formed by an anistropic etching technique. The birefringence was investigated as a function of temperature and found to agree with theory. The possibility of electronic control of devices based on the birefringent properties, such as waveplates and shutters, is discussed.