We calculate the π-power and rise time for several 1×1 y-branch MZI SOI thermo-optic switches as a function of device
size. These switches consist of waveguide core thickness ranging from 10 to 0.22 μm. Upon scaling the core thickness,
the best power and speed performance occurs at 0.7 μm. Further miniaturization results in an increased power and
reduced speed, mainly due to the mode expansion from the core into the cladding. We show that varying the MZI arm
separation is an effective approach to improve the performance of miniaturized thermo-optic switches.
We show that integrated optical star couplers can be useful characterization devices to measure the sidewall roughness-induced
scattering losses of planar waveguides. We describe the detailed fabrication processes of these star couplers on
the silicon-on-insulator (SOI) platform and the process improvements implemented to reduce the waveguide sidewall
roughness and scattering loss. We report the main process challenges, particularly to assure a clear gap between any
adjacent waveguides of the dense and closely spaced output waveguide array. These challenges are addressed by
optimizing the exposure dose of the resist and adding an oxygen ashing treatment to eliminate waveguide footings. We
demonstrate further improvement on the waveguide profile and sidewall roughness through the use of a thin Cr
hardmask for the dry plasma etching. This optimized fabrication process is capable of producing approximately a 3 nm
root-mean-square sidewall roughness, measured using both scanning electron microscopy (SEM) and atomic force
microscopy (AFM). Using the fabricated star couplers, we manage to measure the relative scattering losses of various
waveguides with the width varying from 0.2 to 2.0 μm in a single measurement, and show that the measured losses agree
with the measured sidewall roughness.
We describe a novel non-destructive technique to measure the sidewall roughness induced scattering loss of SOI ridge
waveguides using an integrated 5x17 star coupler. The accuracy of our technique is independent of the coupling
efficiency. In our technique, we capture the near field images of the full output waveguides array with varying width
ranging from 0.2 to 2.0 micron and use the intensity maps of these images to produce normalized intensity profiles, from
which the relative scattering losses of output waveguides are extracted. Using our technique, we have studied and
compared the scattering and polarization dependent losses of three different sets of SOI waveguide samples fabricated
by different processes. We have determined the root-mean-square roughness and autocorrelation length of these samples
using scanning electron microscopy (SEM). Relating the loss and roughness analysis, we have showed that the process
utilizing negative e-beam photoresist and Cr-hardmask with inductively coupled plasma (ICP) etching produced the
smoothest waveguide sidewalls and lowest scattering losses. We have also successfully modeled the measured ridge
waveguide losses as a function of waveguide width and demonstrated that the theoretical sidewall roughness is in
reasonable agreement with the measured roughness from SEM. Our technique is capable of studying roughness induced
scattering loss and thus provides an efficient way of optimizing and monitoring process parameters that affect sidewall
A monolithically integrated asymmetric graded index (GRIN) waveguide structure for coupling light into high index contrast waveguides is described. When analyzed in terms of its waveguide modes, the GRIN coupler is shown to be a multimode interference (MMI) device. The design parameters and tolerances are calculated for quadratic index profile and uniform index amorphous silicon (a-Si) GRIN couplers optimized for coupling light into silicon-on-insulator waveguides. Calculations of coupling efficiencies into 0.5 μm SOI waveguides show that asymmetric GRIN couplers operate over a very wide wavelength range with low polarization dependence, and fabrication requires lithographic resolution of only ±1 μm. Experimental results are presented for a 3 μm thick single layer a-Si coupler integrated with a 0.8 μm SOI waveguide. The measured variation of coupling efficiency with coupler length is in agreement with theory, with an optimal coupling length of 15 μm.
To miniaturize optical passive components or to have optical interconnects replace the current copper/low k interconnects for clock distribution, super high index contrast optics are needed because they allow optical waveguides with small bending radius, ie. < 50um. Silicon nitride core on oxide cladding has loss of <0.1dB/180° for 20um bending radius. However, coupling loss from the fiber to SiN waveguides, with 0.7umx0.7um cross section for single mode, is very large, > 20dB. To reduce the coupling loss, our approach is to have a double-core architecture, where fiber is first coupled to fiber matched waveguide, and then coupling from fiber match waveguide to SiN waveguide through a spot size mode converter. We have found the mode converter loss is reduced by 8dB by reducing the tip of the taper from 0.35um to 0.15um. In this paper, we are reported results of tips with less than 0.1um. We also describe the fabrication technology that enables us to make such fine tip with smooth surfaces.