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.
Admixtures to helium of 100 and 5 ppm of nitrogen, and 100 and 10 ppm of carbon monoxide were identified and measured in the helium discharge afterglow using an electrical probe placed into the plasma. For nitrogen and carbon monoxide gases, the measured electron energy spectra display distinct characteristic peaks (fingerprints). Location of the peaks on the energy scale is determined by the ionization energies of the analyte molecules. Nitrogen and carbon monoxide fingerprints were also observed in a binary mixture of these gases in helium, and the relative concentration of analytes has been predicted. The technically simple and durable method is considered a good candidate for a number of analytical applications, and in particular, in GC and for analytical flight instrumentation.
Propagation of sound in a medium where the rate of local heat addition is a function of gas density is analysed theoretically and the results are applied for modelling the experimentally observed effect of amplification of acoustic waves by an extended glow discharge in air. The model adequately describes the experimental dependences of the gain on the wave frequency and discharge power density and predicts that the amplification of sound by an unconfined glow discharge in air increases with discharge current density but does not change noticeably with gas pressure when the current density is kept constant. Quantitative estimates indicate that a gain of as high as 1 m−1 (or 9 dB for a 60 dB wave passing through 1 m of plasma) could be realized using a discharge in air with a current density of 100 mA cm−2.
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