An optic of the present type can be made by cutting and polishing a bump/dip toroidal pattern on the side of a rod of transparent material or partly melting the tip of the rod. For example, the figure depicts such an optic made from a fused-silica rod of 30-mm length that tapers from 0.45-mm diameter at the narrow end to 3 mm at the wide end. The WGM resonator is a 500-μm axisymmetric bulge at the narrow end, formed by using a hydrogen torch to partly melt the narrow end. In operation, light is coupled into the WGM resonator via the cleaved tip of an optical fiber.

In use of such an optic as a sensor, the rod is dipped into liquid, the absorption spectrum of which one seeks to measure. Interference among the Bessel beams in the far-field region of the waveguide forms a helix-shaped light field. A chargecoupled- device camera is installed at a distance between 2 and 30 mm from the wide end of the optical fiber to observe this field. The dependence of the brightness of this field on the azimuth angle contains information on absorption as a function of wavelength.

This work was done by Dmitry Strekalov, Lute Maleki, Andrey Matsko, Anatoliy Savchenkov, and Vladimir Iltchenko of Caltech for NASA's Jet Propulsion Laboratory.