We explain how to obtain the bandwidth and tunable range required for wavelength-division multiplexing communication systems and how to design tunable liquid-crystal Fabry—Perot interferometer filters. The main factors determing the performance are liquid-crystal loss, mirror loss, surface roughness, and parallelism, as well as mirror reflectivity and cavity gap. Experimental results closely agree with the designed performance. Temperature dependence, response time, acceptable input power, and reliability are investigated. Pigtailed polarization-independent filter modules with a Peltier controller are made and are shown to have low polarization dependence (< 0.3 dB), low driving voltage (< 15 V), and compact size, as well as a narrow bandwidth (0.3 nm), high finesse of 189, and large tunable range (50 nm).
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