Characterization of optical strain sensors based on silicon waveguides

Summary form only given. Strain gauges are widely employed in microelectromechanical systems (MEMS) for sensing of, for example, deformation, acceleration, pressure, or sound [1]. Such gauges are typically based on electronic piezoresistivity. We propose integrated optical sensors which have particular benefits: insensitivity to electromagnetic interference, no danger of igniting gas explosions with electric sparks, small multiplexers (1 mm<;sup>2<;/sup>) and high-speed readout. We use photonic microring resonators in SOI technology as accurate sensors that can be integrated with MEMS. In this paper, we present a characterization of the relation between an applied strain and the shift in the optical resonance wavelengths of such resonators. This characterization includes the influence of the width of the waveguide and of the orientation of the silicon crystal.Two sets of racetrack-shaped ring resonators were fabricated by ePIXfab/IMEC (Leuven, Belgium), both in an (100) SOI wafer with a light-guiding layer of 220 nm high and an oxide top cladding (Fig 1a). Its resonance wavelengths λm around vacuum wavelength λ = 1.55 μm were measured. Strain not only causes elongation of the racetrack circumference l, but also changes the effective index ne of the waveguide. This is because the guide cross-section shrinks due to Poisson's effect, and its refractive index changes due to the photo-elastic effect. Moreover, the guide is highly dispersive as described by its effective group index ng ≡ ne - λ(∂ne/∂λ). Having a long straight waveguide allows neglecting the influence of the bends, tapers, and couplers. The measured relation between the applied strain and λm is linear, so that the resonance shift is described by the first-order derivative of the resonance equation m · λm = l(εz) · ne(λm,εz) to strain εz. This gives ∂λm/∂εz = (λne/ng) + (λ/ng)∂ne/∂εz [2]. The net shift, ∂λm/∂εz, is measured. The term (λne/ng) is due to the elongation of the track, including dispersion. This term is computed, where ne is obtained from a mode solver and ng is measured. The last term is due to the strain-induced change in the effective index and is extracted. We characterized the photonic chips in an automated mechanical setup in which they are bent such that the top layer with the photonic circuitry experiences a homogeneous strain (Fig 1b). Transmission spectra of the resonators were recorded for elongations varying from 0 to 250 microstrain. The resonance positions, and the group index ng, were extracted from fitting a relation for ring resonator transmission [3]. Results are shown in Fig 1c&d. Wider waveguides are slightly more sensitive to strain, which is mainly due to the modal dispersion ne/ng. With this paper the authors present an extensive proof of the principle of SOI microring resonators operating as strain sensors as well as a complete study of the influence of the design choices and physical effects.