Single-mode transmission in commensurate width-varied line-defect SOI photonic crystal waveguides

We have proposed a width-varied photonic crystal (PhC: triangular air-hole lattice) waveguide to overcome the disadvantage of SOI (Silicon on Insulator)-type PhC waveguides, which have no useful propagation mode in the photonic band gap (PBG), and accomplished propagation with a very low loss of 6dB/mm. However, this waveguide can only be made straight because its effective core width is varied by shifting PhC domains. That is, the width is not commensurate with the whole PhC lattice. To solve this problem, we designed a commensurate width-varied PhC waveguide that can be bent. The effective core width is varied by deforming the shape of the holes on either side of a single line defect so that the holes become elongated and tapered toward the center of the core. As a result of the hole deformation, in SOI-type PhC waveguides, the condition for single-mode propagation is satisfied in the PBG below the cladding layer light line. We fabricated this waveguide using SOI substrates and measured the TE-polarized transmission spectrums. The measured spectrums have a 25-50 nm transmission band in the PBG (1280-1580 nm), and they agree well with 3D-FDTD calculations. These results show that the deformed holes are effective.