Small-footprint Integrated Bragg Gratings in SOI spiral waveguides

Integrated optical filters with flexible and precisely tailored spectral responses are required for many applications in communication and sensing. Integrated Bragg gratings (IBG) in silicon-on-insulator (SOI) waveguides can provide these filtering characteristics on a low-cost platform but a major drawback is the need for long grating structures to obtain elaborate spectral responses, which can pose problem when small footprints are desired. A mean to overcome this problem is to fabricate IBGs in spiral-waveguides (Fig. 1-a)). As a result, the footprint of the device is drastically reduced. However, the curvature modifies the waveguide effective index and distorts the grating spectrum. In this work, we design and fabricate IBGs in spiral waveguides using 193 nm deep UV photolithography and successfully avoid this distortion by adding a phase term, Ω(z), to compensate the effective index perturbation, δn(R(z)), caused by the curvature profile, R(z).