Abstract
We describe optical-sensing measurements that utilize natural cavity resonance modes associated with glass spheres and cylinders with diameters between 10 and 100 μm. Radiation may be trapped by total internal reflection in the circular cross-section of such structures, and a resonant cavity mode occurs when a ray reflects back on itself in phase. Such modes have been referred to as morphology-dependent resonances (MDRs)1 since resonant frequencies and widths depend on the cavity size, shape, and refractive index. MDR positions and widths for spheres or circular cylinders can be calculated in terms of the size parameter (ratio of circumference to wavelength) for a given refractive index. These calculations predict a dense spectrum of optical modes with a broad range of possible widths or quality factors (Q's). MDR structure has previously been observed in a variety of different optical measurements from both fibers and. spheres2-4 Q's as high as 108 have been determined in fluorescence from dye-doped droplets.5 Recent observations of sharp MDR features in elastic scattering from undoped glass fibers and in emission from Nd3+ doped glass fibers suggest a new approach for optical sensors since these resonance positions are very sensitive to the cavity environment (temperature, surface quality, physical shape, etc.). By monitoring shifts in the resonances, one can infer changes in these environmental factors.
© 1993 Optical Society of America
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