Integrated optical rectangular-edge filter devices in SOI

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 ≤ λedge ≤ 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 ∼ 100 nm by using an inductively coupled reactive ion etching system. All the fabricated devices exhibit a rectangular-edge filter response at λedge ∼ 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 ∼ 40 nm till the first-order Bragg reflected wavelength of λ B00 ∼ 1600 nm in the transmission characteristics obtained for 1520 nm ≤ λ ≤ 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 ∼ 18 nm/RIU. The limit of detection for 1-dB transmitted power extinction at λedge of a typical fabricated device is estimated to be 5.3 ×, 10-4 RIU.