Fabrication of out-of-plane micromirrors in silicon-on-insulator planar waveguides

Emerging optical waveguide spectrometers, sensors, and biochips require the simultaneous acquisition of numerous optical signals. Aligning many tens or hundreds of optical fibers to waveguides is a formidable and costly exercise in packaging. We propose a data acquisition scheme using arrays of micromirrors to redirect light propagating in each waveguide out of the device surface, where all the beams can be collected by a single imaging array. Micromirrors, which are the key elements in this data acquisition scheme, were fabricated in 2μm thick silicon-on-insulator waveguides. Chemically assisted ion beam etching (CAIBE) was used to obtain inclined mirror facets, benefiting from the unique capability of CAIBE to tilt the sample relative to the ion beam. Finite difference time domain (FDTD) simulations of an ideal micromirror oriented at 45° to the propagation direction predict a >96% out-coupling efficiency (loss of 0.14dB) and a negligible polarization dependent loss (PDL). The measured total insertion loss for the fabricated waveguide and mirror was −5dB, including fiber-to-waveguide coupling loss, waveguide propagation loss, and mirror loss due to fabrication errors and diffraction, with a PDL of 0.7dB. The dependence of micromirror performance on facet angle and etch depth was studied by FDTD simulations. The influence of CAIBE etching chemistry and selectivity in obtaining optimum mirror parameters is discussed.