https://biblio.ugent.be/publication/818298/file/818329.pdf

Hydrogen-fueled internal combustion engines

We present a comprehensive overview of sensor technology exploiting optical whispering gallery mode (WGM) resonances. After a short introduction we begin by detailing the fundamental principles and theory of WGMs in optical microcavities and the transduction mechanisms frequently employed for sensing purposes. Key recent theoretical contributions to the modeling and analysis of WGM systems are highlighted. Subsequently we review the state of the art of WGM sensors by outlining efforts made to date to improve current detection limits. Proposals in this vein are numerous and range, for example, from plasmonic enhancements and active cavities to hybrid optomechanical sensors, which are already working in the shot noise limited regime. In parallel to furthering WGM sensitivity, efforts to improve the time resolution are beginning to emerge. We therefore summarize the techniques being pursued in this vein. Ultimately WGM sensors aim for real-world applications, such as measurements of force and temperature, or alternatively gas and biosensing. Each such application is thus reviewed in turn, and important achievements are discussed. Finally, we adopt a more forward-looking perspective and discuss the outlook of WGM sensors within both a physical and biological context and consider how they may yet push the detection envelope further.

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Whispering gallery mode sensors

Four types of hydrogen detectors are used by researchers, engineers, and manufacturers 
today, and if hydrogen continues to play a role in emerging alternative energy sources, 
there will be exponential growth in the use and need for more advanced and more robust 
devices in the future. The types of sensors reviewed in this chapter are (1) room-temperature hydrogen leak sensors; (2) thermometers, particularly useful at low temperature; 
(3) liquid hydrogen volume and mass gauges; and (4) para/ortho hydrogen ratiometers.

Whispering-gallery modes (WGM) on a spherical surface were first described by Lord Rayleigh at the beginning of the last century, but only after the invention of laser did they start to have some scientific relevance and only during the last two decades there has been a substantial move towards real devices and practical applications. WGM resonators have peculiar properties, the most notable being the potential of having an ultrahigh quality factor Q, which makes them very appealing both as laser cavities and as extremely sensitive sensors. Among the different types of WGM resonators, the microspherical ones represent a very important category, due to their simplicity, easy fabrication, and very high quality. In this review we provide a description of their fundamental properties and we summarize recent works on their application as filters, sensors and lasers.

Spherical whispering‐gallery‐mode microresonators

Confinement of light into small volumes has become an essential requirement for photonic devices; examples of this trend are provided by optical fibers, integrated optical circuits, semiconductor lasers, and photonic crystals. Optical dielectric resonators supporting Whispering Gallery Modes (WGMs) represent another class of cavity devices with exceptional properties, like extremely small mode volume, very high power density, and very narrow spectral linewidth. WGMs are now known since more than 100 years, after the papers published by John William Strutt (Lord Rayleigh), but their importance as unique tools to study nonlinear optical phenomena or quantum electrodynamics, and for application to very low-threshold microlasers as well as very sensitive microsensors, has been recognized only in recent years. This paper presents a review of the field of WGM resonators, which exist in several geometrical structures like cylindrical optical fibers, microspheres, microfiber coils, microdisks, microtoroids, photonic crystal cavities, etc. up to the most exotic structures, such as bottle and bubble microresonators. For the sake of simplicity, the fundaments of WGM propagation and most of the applications will be described only with reference to the most common structure, i.e. microspherical resonators.

/pdf/whispering-gallery-mode-microresonators-fundamentals-and-6yxtews7tz.pdf

Whispering gallery mode microresonators: Fundamentals and applications

A micro-optical force sensor concept based on the morphology-dependent shifts of optical modes of dielectric microspheres is investigated. The optical resonances, commonly referred to as the whispering gallery modes (WGM), were excited by evanescently coupling light from a tunable diode laser using a tapered single-mode fiber. A compressive force applied to the sphere induces a change in both the shape and the index of refraction of the sphere leading to a shift in WGM. By tracking the shifts, the force magnitude is determined using solid silica as well as solid and hollow Polymethyl-methacrylate (PMMA) microsphere resonators. A measurement sensitivity as high as dlambda/dF=7.664 nm/N was demonstrated with a 960 mum hollow PMMA sphere.

Micro-optical force sensor concept based on whispering gallery mode resonators.

An analysis and experiments were carried out to study the spreading and centerline property decay rates of underexpanded supersonic jets. The main purpose was to determine a suitable set of normalization parameters that would account for the initial expansion process and allow for a comparison of the asymptotic mixing rates of jets within a large range of exit-to-ambient pressure ratios. A set of expressions were developed for after-expansion equivalent jet exit diameter, velocity, temperature, and density to allow for a better collapse of jet spreading and centerline decay rates. Measurements were made for five different underexpanded sonic jets with jet exit-to-ambient pressure ratios of p e /p a =1, 2.5, 7.5, 15.5, and 20.3, corresponding to (isentropically) fully expanded jet Mach numbers of M j =1, 1.68, 2.38, 2.85, and 3.03, respectively. For each jet, both centerline and profile measurements were made using special probes that simultaneously measured local total pressure, static pressure, total temperature in the jet, as well as ambient conditions. These measurements were made in the subsonic flow regime, in some cases, extending as far as 270 nozzle diameters from the exit plane. The experimental results were analyzed and the asymptotic jet properties were determined using the new renormalization parameters.

Scaling parameters for underexpanded supersonic jets

The effect of hydrostatic pressure on solid and hollow microsphere optical resonators was investigated. The primary goal was to explore the feasibility of a micro-optical pressure sensor based on whispering gallery modes (WGMs) and to quantify the deleterious effect of environmental pressure changes on other WGM-based sensors. Expressions were developed for WGM shifts due to changes in hydrostatic pressure of the environment surrounding the spherical resonators. These expressions were validated through experiments in which the pressure-induced WGM shifts of hollow polymethyl methacrylate microspheres were monitored. The effect of atmospheric pressure variations on silica resonators is negligible, but hydrostatic pressure may be effective in the optical tuning of hollow polymer spheres.

Pressure tuning of whispering gallery mode resonators

In this paper we investigate the electrostriction effect on the whispering gallery modes (WGM) of polymeric microspheres and the feasibility of a WGM-based microsensor for electric field measurement. The electrostriction is the elastic deformation (strain) of a dielectric material under the force exerted by an electrostatic field. The deformation is accompanied by mechanical stress which perturbs the refractive index distribution in the sphere. Both strain and stress induce a shift in the WGM of the microsphere. In the present, we develop analytical expressions for the WGM shift due to electrostriction for solid and thin-walled hollow microspheres. Our analysis indicates that detection of electric fields as small as ~500V/m may be possible using water filled, hollow solid polydimethylsiloxane (PDMS) microspheres. The electric field sensitivities for solid spheres, on the other hand, are significantly smaller. Results of experiments carried out using solid PDMS spheres agree well with the analytical prediction.

Tuning of whispering gallery modes of spherical resonators using an external electric field

A simple plane wave, ray-tracing approach was used to derive approximate equations for the dielectric microsphere whispering gallery mode (WGM) resonant wavenumber and quality factor, as dependent on the surrounding medium’s refractive index. These equations are then used to determine the feasibility of a micro-optical sensor for species concentration. Results indicate that the WGMs are not sensitive enough to refractive index changes in the case of gas media. However, they can be sufficiently sensitive for measurements in liquids. Experiments were carried out to validate the analysis and to provide an assessment of this sensor concept.

M. Volkan Ötügen

Papers

Micro-optical force sensor concept based on whispering gallery mode resonators.

Scaling parameters for underexpanded supersonic jets

Pressure tuning of whispering gallery mode resonators

Tuning of whispering gallery modes of spherical resonators using an external electric field

Whispering gallery modes of microspheres in the presence of a changing surrounding medium: A new ray-tracing analysis and sensor experiment