Integrated Optical Linear Edge Filters Using Apodized Sub-Wavelength Grating Waveguides in SOI

Abstract

An integrated optical apodized sub-wavelength grating waveguide is designed and demonstrated exhibiting high-extinction edge-filter characteristics with a linear roll-off in the dB scale. For a given waveguide cross-sectional geometry with fixed grating duty cycle and period, it has been shown that the apodized device length $L_{g}$ and width $W_{m}$ are the two important parameters deciding the band-edge position, roll-off and broadband extinction around the Bragg wavelength. The experimental results from fabricated devices ( $L_{g} = 70\,\,\mu \text{m}$ and $W_{m} = 2.5\,\,\mu \text{m}$ ) in silicon-on-insulator substrates (device layer ~ 220 nm and buried oxide layer $\sim ~2~\mu \text{m}$ ) show a smooth band-edge roll-off of > 3.5 dB/nm ( $\lambda _{edge} \sim ~1550$ nm) with a stopband ( $\Delta \lambda _{sb}>50$ nm) extinction of > 40 dB and a nearly flat-top passband ( $\Delta \lambda _{pb}>100$ nm) with negligible insertion loss of ~ 0.5 dB.