A tunable optical filter is a potentially useful system element for optical wavelength selection in high-density multichannel wavelength-division-multiplexing (WDM) optical fiber networks.1,2 To date, WDM technology commonly utilizes interference (dielectric coated) optical filters or diffraction gratings. Interference filters have limited spectral resolution (of the order of 10 nm), and they are not tunable. Diffraction gratings have better spectral resolution (several nanometers), but they are not typically used in in-line applications (without a change in optical path direction). Furthermore, they are relatively bulky, lossy, and expensive. Several new methods were proposed to improve WDM techniques. The integrated optical ring in-line resonator3 opens new opportunities for high spectral resolution (of the order of 1 A) WDM applications, but it is not tunable, and its free spectral range is limited to <1 nm. A Mach-Zehnder interferometer was proposed as a tunable optical filter with very fine spectral resolution (also of the order of 1 Å),4 but the finesse of this filter is small (of the order of 2), which limits the number of channels. Moreover, its configuration is rather complicated (two directional couplers, PZT phase shifter, polarization-holding fibers, etc.) and expensive.

© 1988 Optical Society of America

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