Eliminating the birefringence in silicon-on-insulator ridge waveguides by use of cladding stress

We propose and demonstrate the use of the cladding stress-induced photoelastic effect to eliminate modal birefringence in silicon-on-insulator (SOI) ridge waveguides. Birefringence-free operation was achieved for waveguides with otherwise large birefringence by use of properly chosen thickness and stress of the upper cladding layer. With the stress levels typically found in cladding materials such as ${\mathrm{SiO}}_2$, the birefringence modification range can be as large as ${10}^{-3}$. In arrayed waveguide grating demultiplexers that were fabricated in a SOI platform, we demonstrated the reduction of the birefringence from $1.2\times {10}^{-3}$ (without the upper cladding) to $4.5\times {10}^{-5}$ when a $0.8‐\mathit{\mu }\mathrm{m}$ oxide upper cladding with a stress of $-320 \mathrm{MPa}$ (compressive) was used. Because the index changes induced by the stress are orders of magnitude smaller than the waveguide core–cladding index contrast, the associated mode mismatch loss is negligible.