Overcoming obstacles to design and fabricate integrated resonant channel-dropping filters

The integrated resonant channel-dropping filter was first proposed by H.A. Haus in 1991 for use in wavelength-division multiplexed (WDM) fiber-optic communication systems. The function of the filter is to spatially separate, or "drop," just one channel from a WDM bit, stream without terminating or otherwise disturbing the remaining channels. The WDM bit stream is carried along the center rib waveguide. Only that channel, or wavelength band, that excites the two side-coupled quarter-wave shifted distributed Bragg reflector (QWS-DBR) resonators is removed from the bus to the left-most waveguide. The remaining channels do not excite the resonators and therefore travel through undisturbed. The action of dropping a single channel from the bus can be reversed to create a narrow-band channel-adding filter. The ability to add or drop a single channel from the entire WDM bit stream augments the repertoire of functions that are available to the WDM system architect. There are three critical optical parameters that must be properly interrelated during design and controlled during fabrication to build a channel-dropping filter having specific spectral characteristics. The three parameters are the grating strength, the evanescent waveguide-waveguide coupling strength, and the phase-velocity mismatch between the resonators and the bus. While e-beam and x-ray lithographies are used to control the length and width dimensions to the order of 10 nm, so too must the vertical dimensions be controlled to the order of 10 nm. This requires accurate control of film thicknesses and etch depths. To the extent that all of the dimensions and materials indices cannot be controlled to the requisite tolerance, a trimming technique is required.