Theoretical and experimental investigations have demonstrated the feasibility of compact white-light sensor optics consisting of unitary combinations of (1) low-profile whisperinggallery- mode (WGM) resonators and (2) tapered rod optical waveguides. These sensors are highly wavelength-dispersive and are expected to be especially useful in biochemical applications for measuring absorption spectra of liquids.
These sensor optics exploit the properties of a special class of non-diffracting light beams that are denoted Bessel beams because their amplitudes are proportional to Bessel functions of the radii from their central axes. High-order Bessel beams can have large values of angular momentum. In a sensor optic of this type, a low-profile WGM resonator that supports modes having large angular momenta is used to generate highorder Bessel beams. As used here, "lowprofile" signifies that the WGM resonator is an integral part of the rod optical waveguide but has a radius slightly different from that of the adjacent part(s).
An important difference between such an optic and an ordinary WGM resonator is that its modes decay primarily into Bessel modes of the optical waveguide, rather than to the outside. By changing the dimensions and shape of the WGM resonator and/or the radius of the adjacent part(s) of waveguide, it is possible to change the resonator loading and thereby tailor the degree to which light propagates from the resonator along the waveguide.
The feasibility of applications that involve exploitation of optical waves that have angular momentum depends on the propagation distances of such waves in free space. A high-order Bessel beam that propagates from a WGM resonator along a cylindrical waveguide with evanescentfield coupling cannot leave the waveguide; it propagates to an end of the waveguide, where it is totally internally reflected back along the waveguide toward the other end. However, if the waveguide is tapered, as in an optic of the present type, then the optic acts as radiator horn that preserves the angular momentum of the axially propagating Bessel beams while changing their axial momentum. A notable result of propagation along the taper is that upon reaching the wide end, the Bessel beams can be released into the space outside the waveguide and their shapes are preserved.