The invention relates generally to optics, and relates more particularly to optical interconnects.
FIG. 1A illustrates a top view of one example of a conventional wavelength selective filter 100 (e.g., such as those used for wavelength division multiplexing). FIG. 1B illustrates a cross sectional view of the filter 100 of FIG. 1A, taken along line A-A′. The filter 100 includes a ring resonator 102 side-coupled to an access straight waveguide (or waveguide bus) 104. The ring resonator 102 is tuned to a wavelength channel of interest, such that the ring resonator 102 filters this channel from the bus 104. For high refractive index contrast planar lightwave waveguides and circuits, the coupling gap 106 (i.e., the distance that separates the ring resonator 102 from the bus 104) is typically on the order of a micrometer and controlled within a few nanometers of precision. Such control, however, is difficult to achieve by typical lithography methods.
FIG. 2A illustrates a top view of an alternative example of a conventional wavelength selective filter 200. FIG. 2B illustrates a cross sectional view of the filter 200 of FIG. 2A, taken along line A-A′. Like the filter 100, the filter 200 includes a ring resonator 202 coupled to a waveguide bus 204. However, as illustrated in FIG. 2B, the ring resonator 202 is formed in a high refractive index waveguiding layer that is separate from the layer in which the bus 204 is formed. In this case, the coupling gap 206 is vertically disposed and can be precisely controlled by an amount of gap material grown, for example, by molecular-beam epitaxy (MBE).
For silicon on insulator (SOI)-based planar lightwave circuits based on strip silicon single-mode waveguides with sub-micron cross sections, the approach illustrated in FIGS. 1A and 1B results in a coupling gap on the order of 100 nanometers, which should be controlled with nanometer precision. This makes fabrication tolerances very difficult to maintain. Applying the alternative approach illustrated in FIGS. 2A and 2B would require the growth of an oxide or other low refractive index material on top of the SOI structure to form the coupling gap, followed by growth of an additional top silicon layer for the ring resonator. This is likely to result in a polycrystalline or amorphous silicon structure on top of the silicon layer, which can lead to significant propagation losses (e.g., approximately twenty dB/cm) due to scattering on the grain boundaries. Losses are increased proportionally to the photon lifetime (inverse of the ring resonator quality factor) if the ring resonator or other resonator structure is located on the top layer of the circuit.
Thus, there is a need for a method and an apparatus for minimizing propagation losses in wavelength selective filters.