Small suspended SU-8 optical waveguides with a high refractive index contrast are designed and optimized. The singlemode
condition and the bending characteristics of the present waveguide are numerically calculated by using a fullvectorial
finite-difference method. Pure bending losses and transition losses of bending waveguides with different
bending radii and different core widths are studied. The simulation results show that a wider core is helpful to reduce the
pure bending loss. For example, when the core width is chosen as 2 m, a very small bending radius (~5 m) can be
achieved. However the transition loss is high. When a small total bending loss (<0.1dB/90º bend) is required, the
minimal bending radius is about 7 m when an optimal offset is introduced. A crossing structure with two arms to
support the suspended waveguides is designed and optimized by using a two-dimensional finite-difference time-domain
method. The structure parameters (e.g., the expanding width, the taper length and the length of the insert straight section)
of the crossing structure are optimized to minimize the excess loss. The optimal crossing structures exhibits a low excess
loss (<0.1 dB) and performs well in a wide wavelength range (1500 nm~1600 nm).
We review our recent work on chirped waveguide gratings for efficient coupling between standard single mode optical
fibers and silicon photonic wire waveguides. The use of a linear chirp in grating period reduces the second order Bragg
reflection from the waveguide gratings and increases the coupling efficiency for perfectly vertical optical fibers.
Measurement results obtained from devices fabricated using deep UV lithography yielded coupling efficiencies of over
34%. Techniques to further improve the coupling efficiency will be discussed. The use of chirped waveguide gratings
for low cost photonic packaging and the application of waveguide gratings for splitting/combining light will also be
presented.
By using small SU-8 polymer ridge waveguides, compact micro-racetrack resonator (MRR) add-drop filters are
presented. This used small ridge waveguide has an air-cladded SU-8 polymer core (n~1.573) on a SiO2 buffer (n~1.445)
so that the bending radius could be as small as ~102μm. Considering the resolution limitation of a regular UV-lithography
process, the gap in the coupling region of the present MRR in our design is no less than 1.0 μm. In order to
enhance the coupling between the access optical waveguide and the micro-racetrack, narrow optical waveguides are used
in the coupling regions. Meanwhile, wide optical waveguides are used in the bending sections to have a small bending
radius. And a single-side taper is used to connect these two sections with different widths so that the gap width is kept as
small as 1.0 μm in the whole region. With a regular UV lithography process, the designed micro-racetrack resonator
(MRR) add-drop filters are fabricated. The measured propagation loss for straight waveguides is about 0.1dB/mm and
the measured spectral responses of the through/drop ports show a Q-factor of 8000.
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