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.

An experimental investigation of the moderate Reynolds number plane air jets was undertaken and the effect of the jet Reynolds number on the turbulent flow structure was determined. The Reynolds number, which was defined by the jet exit conditions, was varied between 1000 and 7000. Other initial conditions, such as the initial turbulence intensity, were kept constant throughout the experiments. Both hot-wire and laser Doppler anemometry were used for the velocity measurements. In the moderate Reynolds number regime, the turbulent flow structure is in transition. The average size and the number of the large scale of turbulence (per unit length of jet) was unaffected by the Reynolds number. A broadening of the turbulent spectra with increasing Reynolds number was observed, This indicated that there is a decrease in the strength of the large eddies resulting from a reduction of the relative energy available to them. This diminished the jet mixing with the ambient as the Reynolds number increased. Higher Reynolds numbers led to lower jet dilution and spread rates. On the other hand, at higher Reynolds numbers the dependence of jet mixing on Reynolds number became less significant as the turbulent flow structure developed into a self-preserving state.

Performance characteristics of a force sensor concept based on the morphology dependent resonance ͑MDR͒ shifts of micro-optical resonators have been investigated. Previous experimental studies have indicated that microsphere sensors with diameters ranging between 30 and 950 m may have force resolutions reaching 10 −5 N ͓T. Ioppolo et al., Appl. Opt. 47, 3009 ͑2008͔͒. In the present, we carry out a systematic analysis and experiments to investigate the sensitivity, resolution, and bandwidth limits of MDR-based force sensors. Expressions for MDR shifts due to applied force in the polar direction are obtained for microspheres of various dielectric materials in the diameter range of 300-950 m. The analyses are compared with experimental results for polymethylmethacrylate and polydimethylsyloxane ͑PDMS͒ microsphere sensors. The results show that the strain effect on MDR shifts is dominant over that of mechanical stress. It also indicates that force sensitivities of the order of a 1 pN are feasible using hollow PDMS spheres. The sensor bandwidths range between 1 kHz and 1 MHz, depending on the sphere material.

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.

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.

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 pe/pa=1, 2.5, 7.5, 15.5, and 20.3, corresponding to (isentropically) fully expanded jet Mach numbers of Mj=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.

Temperature measurements were made using a novel microoptical sensor based on dielectric microspheres that are excited by coupling light from optical fibers. The technique exploits the morphology-dependent shifts in resonant frequencies that are commonly referred to as the whispering gallery modes. A change in the temperature of the microsphere leads to a change in both the size and the index of refraction of the sphere which results in a shift of the resonant frequency. By monitoring this shift, the temperature of the environment surrounding the sphere can be determined. The whispering gallery mode shifts are observed by scanning a tunable diode laser that is coupled into the optical fiber on one end and monitoring the transmission spectrum by a photodiode on the other. When the microsphere is in contact with a bare section of the fiber, the optical modes are observed as dips in the intensity of the light transmitted through the fiber. Temperature measurements were made in both air and water using this novel technique. Measurements by the microoptical sensor were compared to those by thermocouples with good agreement between the two sets of results

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