2272 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 24, DECEMBER 15, 2012 Silicon-On-Insulator In-Plane Gires–Tournois Interferometers Raphael St-Gelais, Thomas Kerrien, Hubert Camirand, Alexandre Poulin, and Yves-Alain Peter, Senior Member, IEEE Abstract— Gires–Tournois interferometers (GTIs) based on deep-etched silicon-air Bragg mirrors and on optical quality dicing of silicon are reported. Broadband reflectivity of the deep-etched Bragg mirrors allows operation on a wavelength range exceeding the C Band window. The waveguided in-plane configuration of the devices allows interferometer lengths that are not typically achievable on chips using out-of-plane designs (e.g., L > 1 mm for a 25-GHz free spectral range). Optical characterization of GTIs having two different free spectral ranges (i.e., 25 and 100 GHz) yield off-resonance insertion losses below 2 dB and polarization-dependant losses (PDL) below 1 dB. Insertion losses and PDL are, however, more important near the resonance wavelengths, reaching, respectively, 15 and 5 dB. Calculations show that the reported devices could be useful for Michelson-GTI bandpass filters, such as optical interleavers. Index Terms— Integrated optics, interferometers, optical waveguides, silicon-on-insulator technology. I. I NTRODUCTION D EEP-ETCHED silicon-air Bragg reflectors were previously integrated with waveguides to form tunable in-plane Fabry–Perot cavities [1], [2]. In the current work, we investigate similar devices, in which the two silicon-air mirrors now have different reflectivities in order to form on-chip Gires–Tournois interferometers (GTIs). GTIs are mainly used for optical fiber dispersion compensation [3], [4] and in Michelson-GTI band-pass filters [5] such as optical interleavers [6]–[9]. Previous on-chip integrations of GTIs relied on out-of-plane configurations [10], [11] (optical axis perpendicular to the substrate surface). The devices presented in the current work are based on an in-plane configuration, which could be advantageous, in some contexts, compared to out-of-plane devices. For example, in this configuration, the length ( L ) of the interferometer is not limited, as in [10], by the thickness of the substrate. Its free spectral range (FSR) can therefore be designed with great flexibility, from a 25 GHz DWDM (dense wavelength-division multiplexing) channel spacing ( L > 1 mm), to FSR > 100 nm ( L < 5 μm). The waveguided in-plane configuration could also permit, Manuscript received June 13, 2012; revised August 21, 2012; accepted October 25, 2012. Date of publication November 16, 2012; date of current version November 28, 2012. This work was supported by the National Science and Engineering Research Council of Canada. The authors are with the Department of Engineering Physics, Polytechnique Montréal, Montreal, QC H3C 3A7, Canada (e-mail: raphael.st-gelais@ polymtl.ca; thomas.kerrien@enspg.inpg.fr; hubert.camirand@polymtl.ca; alexandre-2.poulin@polymtl.ca; yves-alain.peter@polymtl.ca). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2012.2227142 (a) (b) Fig. 1. (a) Scanning electron microscopy pictures and (b) schematic cross-sectional view of a silicon-on-insulator Gires–Tournois interferometer. through design and fabrication process modifications, monolithic integration with other on-chip components such as couplers, signal modulators, and electrical circuits for spectral tuning (e.g., thermo-optic effect, carrier injection). II. DESIGN AND FABRICATION An in-plane GTI is presented in Fig. 1. The high reflectivity mirror (at the back of the cavity) is formed by plasma etching of a 3 μm period silicon-air Bragg mirror down to the buried oxide layer of a silicon on insulator wafer (11.5 μm thick silicon device layer). The waveguide cladding is then fabricated through a second photolithography and plasma etching step. The low reflectivity mirror is finally formed by dicing the waveguide entrance facet using an ADT 7100 provectus dicing saw equipped with a small diamond grit, resinoid matrix blade (ADT Part number 00777-8003-006- QKP). A protective photoresist coating is applied prior to dicing to ensure sample cleanliness. The sawing parameters are optimized (36,500 RPM, 1 mm/s feed speed) to ensure optical surface quality and low edge chipping. The amplitude reflection (r ) and transmission (t ) coeffi- cients at each mirror are defined in Fig. 1(b). The parame- ter η designates the intensity coupling coefficient between the anti-reflection coated single-mode fiber (Oz optics) and the waveguide. Using this notation, the amplitude reflection coefficient of the interferometer is given by Eq. 1, where 1041–1135/$31.00 © 2012 IEEE