Abstract

WDM systems are creating demand for wavelength demultiplexers to extract a single wavelength channel from a stream of multi-wavelength data. A popular filter design employs an asymmetric directional coupler, either with or without a grating inserted between two coupled waveguides (fig. 1). [1-5]. This device selects a single wavelength channel, determined by the phase matching condition for the two coupled waveguides. Several such filters have been demonstrated using vertical couplers on III-V semiconductors, because this approach enables precise control of waveguide thicknesses and indices as well as monolithic integration.[2-5] Recently, integration of these filters with photodetectors and/or optical amplifiers [6-8] was demonstrated for receiver applications. A major obstacle to using such chips is the polarization sensitivity of the asymmetric coupler. Polarization-dependent shifts in filter wavelength Δλο exceed the filter bandwidth,[2-6] resulting in extreme polarization sensitivity Δλο ≈ 30nm incompatible with conventional single-mode fiber communications. The only reported polarization-insensitive device of this type[9] employs a doubly-periodic grating, and cannot be used in couplers without a grating. We previously proposed a birefringence compensation technique to eliminate polarization dependence in devices either with or without grating-assisted coupling, and used simulations to show that the approach involves fabrication tolerances.[10] Here we report the first experimental demonstration of these devices, validating the design concept proposed in ref. [10].

© 1995 Optical Society of America