Integrated Optical Rectangular-Edge Filter Devices in SOI

Abstract

An integrated optical design of a rectangular-edge filter device in 250-nm silicon-on-insulator platform is proposed and demonstrated experimentally. The device is designed with a multimode waveguide (supporting at least two modes) with asymmetric side-wall grating, which is adiabatically interfaced with input/output single-mode waveguides. The input/output access waveguides are terminated with grating couplers for optical characterizations. Design parameters are optimized for a sharp-edge or nearly rectangular-edge filter response in optical C-band (1530 nm $\leq \lambda _{\text{edge}} \leq $ 1565 nm). The submicron features and the entire footprint of the devices were defined with a single-step e-beam lithography process by using negative-tone resist and subsequent dry etching of $\sim$ 100 nm by using an inductively coupled reactive ion etching system. All the fabricated devices exhibit a rectangular-edge filter response at $\lambda _{\text{edge}} \sim $ 1560 nm with an edge-extinction of $>$ 40 dB at the rate of 118 dB/nm. The rectangular-edge is followed by a broad pass-band of $\sim $ 40 nm till the first-order Bragg reflected wavelength of $\lambda _B^{00} \sim $ 1600 nm in the transmission characteristics obtained for 1520 nm $\leq \lambda \leq$ 1620 nm. Tunability of a rectangular-edge filter is verified with cladding refractive index change and the observed refractive index sensitivity of the edge is $\sim$ 18 nm/RIU. The limit of detection for 1-dB transmitted power extinction at $\lambda _{\text{edge}}$ of a typical fabricated device is estimated to be 5.3 $\times\, \text{10}^{-4}$ RIU.