Many authors have investigated spherical resonators with WGMs. Relatively early investigations from theoretical and experimental views were published in the literatures. Furthermore, transmission characteristics of optical fibers including taper fibers have also investigated theoretically. Hollow glass microspheres were successfully pumped by hollow-glass fiber tapers and showed high-Q factors (quality factors) >109, where the matching between WGMs of hollow glass microspheres and transmission modes of fiber-tapers was inevitable for laser performance.
Nowadays, [fiber tapers]/[hollow-glass spheres] system was well established experimentally in a laboratory scale and theoretical background was understood satisfactory. As pointed out in the literature, however, from the theoretical point of views, hollow glass microspheres and optical coupling devices such as a fiber taper should have nearly equal refractive index for satisfying the mode matching. Moreover, fiber tapers, unclad and unsupported waveguides, are very fragile.
Therefore, if we have plans to open up a way to use the wide possibilities of hollow glass microspheres of various compositions, development of the pumping techniques that is conventional and applicable for commercial uses is an urgent issue. Similarly, tailoring the glass composition is requested for obtaining desirable physical and chemical properties from their wide-spreading properties. Our works on the high-index glass spheres aim at giving one of the answers to the issues: two pumping techniques are applicable to the practical uses and the multicomponent high-index glass is chosen to explore the possibilities. In terrace hollow glass microspheres, for obtaining high resonant emission, refractive index difference between the sphere and the terrace portion (nsphere/nterrace = nr > 1.3) is inevitable.
The pumping experiment of high-index spheres using a half-polisher fiber coupler showed the improvement of optical coupling by high-refractive-index film (nD = 1.73) coating on the polished core. These results were not explained by the above-mentioned theoretical consideration of the [fiber tapers]/[hollow-glass spheres] system. Theoretical support for our new experimental techniques is the next important issue to make advance in practical applications.
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