Novel fabrication process for vertical resonant coupler with precise coupling efficiency control

Semiconductor μ-cavities are attractive elements to incorporate into WDM optical networks as wavelength selective elements: narrow band filters, splitters, switches and so on. To achieve narrow linewidths suitable for WDM applications, a high Q-cavity with weak and precisely controlled input and output coupling is required. To realize a free spectral range of 30-50 nm, the ring or disk cavity diameter must be decreased to the range 5-10 μm and the cavity lateral index step must be large enough to support strongly guided ring or whispering gallery modes. In previous work these constraints were met by using evanescent coupling between waveguides and 5-20 μm diameter resonators formed with an air/semiconductor interfaces. For single mode operation such structures require waveguides with widths on the order of 1 μm and coupling separations ~0.1 μm. It is difficult to control the coupling in such structures even with electron beam lithography. The approach presented in this paper exploits the ability to produce precisely controlled coupling separations through epitaxial growth rather than lithography by using a vertical coupling rather than lateral coupling structures. Our implementation of this approach enables a generic methodology for complex three dimensional waveguide structures