IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 1, NO. 1, OCTOBER 2010 1 Uniform and Sampled Bragg Gratings in SOI Strip Waveguides with Sidewall Corrugations Xu Wang, Student Member, IEEE, Wei Shi, Student Member, IEEE, Raha Vafaei, Nicolas A. F. Jaeger, Member, IEEE, and Lukas Chrostowski, Member, IEEE Abstract—We have demonstrated uniform and sampled Bragg gratings in silicon-on-insulator strip waveguides with symmetric sidewall corrugations. The fabrication is based on 193 nm deep ultra-violet lithography using a single mask. The measured reflection spectra of sampled gratings exhibit 10 usable peaks spaced by 4.2 nm, and show good agreement with theoretical predictions. Index Terms—Bragg gratings, sampled gratings, silicon-on- insulator, strip waveguides. I. I NTRODUCTION B RAGG grating structures are widely used in optical communication and sensing systems, such as in semi- conductor lasers and fibers. Recently, the integration of Bragg gratings on the silicon-on-insulator (SOI) platform has been attracting much interest [1]. Several approaches have been re- ported to achieve periodic modulation of the effective index of refraction in SOI waveguides: (1) using photorefractive effects via selective ion implantation [2]; (2) physically corrugating the waveguide, either on the top surface [1] or on the sidewalls [3], [4]; and (3) placing periodic corrugations next to the waveguide [5], [6]. Although the first approach can retain a planar surface that may be useful for subsequent processing, the ion implantation makes the fabrication more expensive. Typically, electron-beam lithography (EBL) was the workhorse for the fabrication of SOI Bragg gratings, but is unsuitable for commercial applications [7]. An alternative is to use deep ultra-violet (DUV) lithography, with high throughput and low cost, which has been recently demonstrated in [8] and [9]: the gratings in [8] were top-surface corrugated, hence requiring double-exposure lithography and precise alignment, while the sidewall-corrugated configuration in [9] can be used to define the waveguide and gratings in a single lithography step, which is much simpler and less expensive. Sampled gratings, an important derivative of Bragg gratings, are constructed by multiplying a sampling function and a conventional grating. Since the sampling function leads to a reflection spectrum with periodic maxima, sampled gratings are deployed in tunable semiconductor lasers to achieve wide tuning range through the Vernier effect [10]. Manuscript received October 22, 2010; revised December 14, 2010. This work was supported in part by the Natural Sciences and Engineering Research Council of Canada. The authors are with the Department of Electrical and Computer Engi- neering, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada (e-mail: xuw@ece.ubc.ca, lukasc@ece.ubc.ca) Digital Object Identifier 00.0000/LPT.2010.0000000 Fig. 1. Top view SEM image of a fabricated Bragg gratings. The designed square corrugation width is 25 nm with 50% duty cycle. The fabricated grating shape is rounded and the amplitude is reduced to ∼20 nm, due to the 193 nm lithography limitations. In this letter, we present sidewall-corrugated Bragg gratings in compact SOI strip waveguides fabricated by 193 nm DUV lithography with a single mask. The fabricated corrugations are much smaller than in [9] allowing a narrow bandwidth of 0.8 nm to be achieved. Based on the sidewall-corrugated grating structure, we present the first demonstration of sampled gratings in silicon waveguides. II. UNIFORM GRATINGS IN SOI WAVEGUIDES Fig. 1 shows the scanning electron micrograph (SEM) of a fabricated Bragg grating, fabricated at IMEC ePIXfab [7] using 193 nm DUV lithography in a single step. The SOI strip waveguide consists of a thin silicon layer (220 nm thick) on top of a buried oxide layer (2 μm thick) on a silicon wafer. The strip width w is 500 nm, and the corrugations are recessed on both sidewalls. The corrugation width on each side Δw c is approximately 20 nm in Fig. 1, which is much smaller than previous reports of similar gratings [3], [4], [9]. For all the devices reported here, the grating period Λ is 310 nm, the grating duty cycle is 50%, and the grating length L is 620 μm. The chip layout schematic is shown in Fig. 2, where high-efficiency integrated grating couplers [7] were used as input and output ports and AR coated lensed polarization- maintaining fibers were used for efficient vertical fiber cou- pling. The grating couplers were designed for transverse electric (TE) polarization only. A Y-branch splitter was used to collect the reflected light. The grating transmission was normalized using the transmission of a simple waveguide. The grating reflection was similarly normalized with the additional 3 dB Y-branch splitter loss. Therefore, in both cases, the losses due to propagation and fiber coupling have been de-embedded.