With the increasing use of ultrasonography, especially in medical imaging, novel fabrication techniques together with novel sensor designs are needed to meet the requirements for future applications like three-dimensional intercardiac and intravascular imaging. These applications require arrays of many small elements to selectively record the sound waves coming from a certain direction. Here we present proof of concept of an optical micro-machined ultrasound sensor (OMUS) fabricated with a semi-industrial CMOS fabrication line. The sensor is based on integrated photonics, which allows for elements with small spatial footprint. We demonstrate that the first prototype is already capable of detecting pressures of 0.4 Pa, which matches the performance of the state of the art piezo-electric transducers while having a 65 times smaller spatial footprint. The sensor is compatible with MRI due to the lack of electronical wiring. Another important benefit of the use of integrated photonics is the easy interrogation of an array of elements. Hence, in future designs only two optical fibers are needed to interrogate an entire array, which minimizes the amount of connections of smart catheters. The demonstrated OMUS has potential applications in medical ultrasound imaging, non destructive testing as well as in flow sensing.

Abstract-Marcatili's famous approximate analytical description of light propagating through rectangular dielectric waveguides, published in 1969, gives accurate results for low-indexcontrast waveguides. However, photonic integrated circuit technology has advanced to high-index-contrast (HIC) waveguides. In this paper, we improve Marcatili's model by adjusting the amplitudes of the components of the electromagnetic fields in his description. We find that Marcatili's eigenvalue equation for the propagation constant is also valid for HIC waveguides. Our improved method shows much better agreement with rigorous numerical simulations, in particular for the case of HIC waveguides. We also derive explicit expressions for the effective group index and the effects of external forces on the propagation constant. Furthermore, with our method the phenomenon of avoided crossing of modes is observed and studied.

Recently there has been growing interest in sensing by means of optical microring resonators in photonic integrated circuits that are fabricated in silicon-on-insulator (SOI) technology. Taillaert et al. [Proc. SPIE 6619, 661914 (2007)] proposed the use of a silicon-waveguide-based ring resonator as a strain gauge. However, the strong lateral confinement of the light in SOI waveguides and its corresponding modal dispersion where not taken into account. We present a theoretical understanding, as well as experimental results, of strain applied on waveguide-based microresonators, and find that the following effects play important roles: elongation of the racetrack length, modal dispersion of the waveguide, and the strain-induced change in effective refractive index. © 2012 Optical Society of America OCIS codes: 120.4880, 160.1050, 130.7408, 280.4788, 160.6000, 000.2190 Piezoresistive electronic strain gauges are frequently used in micromachined electromechanical systems (MEMS) [1]. Alternatively, all-optical systems can be used, and they have particular benefits, such as being insensitive to electromagnetic interference, not having the danger of initiating gas explosions with electric sparks, and allowing for high speed readout. Guo and co-workers employed an optical polymer microring resonator as an ultrasound sensor, in which the deformation of the resonator was measured by monitoring its shift in resonance frequencies [2,3]. Taillaert et al. [4] proposed the use of a silicon optical microring resonator as a strain gauge. Silicon-on-insulator (SOI) technology has emerged as a focus platform for integrated photonics, with complementary metal-oxide semiconductor (CMOS) production lines, opening the possibility of mass fabrication [5]. Silicon is the commonly used material in MEMS, and we have shown the possibility of micromachining postprocessing of SOI photonic integrated circuits [6]. Strong lateral confinement of the light due to the high refractive index contrast of SOI waveguides (Δn ≈ 2) allows for small device footprint, but also comes with high sensitivity to the exact behavior of the modes in the waveguide, e.g., strong modal dispersion [7,8]. Amemiya et al.[9] reported on the photoelastic effect in strained SOI ring resonators without, however, considering the modal effects, such as dispersion. In this Letter, we first derive a model explaining the effects that play a role when considering the influence of strain on photonic waveguides. Then, we characterize these effects with a novel mechanical setup providing a well-defined strain. We describe the important effects when a strain is applied to a ring resonator that has a racetracklike shape with circumference l, as depicted in Fig. 1. Light is coupled from a connecting waveguide to the racetrack waveguide by means of a multimode interference (MMI) coupler [10,11]. Having such a long racetrack allows us to neglect the effect of the bends and of the coupler. The transmitted spectrum at the output port of the connecting waveguide shows dips at the wavel...

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Microscale strain gauges are widely used in micro electro-mechanical systems (MEMS) to measure strains such as those induced by force, acceleration, pressure or sound. We propose all-optical strain sensors based on micro-ring resonators to be integrated with MEMS. We characterized the strain-induced shift of the resonances of such devices. Depending on the width of the waveguide and the orientation of the silicon crystal, the linear wavelength shift per applied strain varies between 0.5 and 0.75 pm/microstrain for infrared light around 1550 nm wavelength. The influence of the increasing ring circumference is about three times larger than the influence of the change in waveguide effective index, and the two effects oppose each other. The strong dispersion in 220 nm high silicon sub-wavelength waveguides accounts for a decrease in sensitivity of a factor 2.2 to 1.4 for waveguide widths of 310 nm to 860 nm. These figures and insights are necessary for the design of strain sensors based on silicon waveguides.