Fiber-optic couplers for spherical, toroidal, and other, similarly shaped microscopic optical resonators that operate in whispering-gallery (WG) modes would be fabricated with integral alignment features, according to a proposal. These alignment features would facilitate coupling adjustments, which, heretofore, have been difficult and complex, as explained below.
Microsphere and similar resonators have been described in several previous NASA Tech Briefsarticles. The couplers in question were described in "Simple Fiber-Optic Coupling for Microsphere Resonators" (NPO-20619), NASA Tech Briefs, Vol. 25, No. 5 (May 2001), page 70. To recapitulate: In the WG modes of a transparent microsphere, light orbits inside the sphere, where it is confined by total internal reflection. The high degree of confinement results in high Q (where Q is the resonance quality factor). Light is coupled into or out of the microsphere by exploiting the overlap of (1) the evanescent field of the WG modes with (2) the evanescent field just outside an angled end face of a single-mode optical fiber.
For efficient transfer of energy, it is necessary to align the microsphere with the fiber such that they are in the correct relative position and orientation for optimum overlap of their evanescent fields:
- The intersection of the core of the optical fiber with the angled end face of the fiber must lie at the point of closest approach of the fiber to the sphere;
- The axis of the fiber must lie in the plane of symmetry of the a circumferential "belt of light" into which the WG modes are concentrated; and
- The gap between the sphere and the angled end face of the fiber must be maintained stable within the range of the evanescent fields typically between 0.5 and 1.5μm.
It should be apparent that coupling adjustment between the microsphere and the angled end face of the optical fiber is critical and difficult. Heretofore, it has been necessary to rely on bulky external mounts and translation mechanisms to establish and maintain the correct alignment. The proposed alignment features would ensure the mechanical stability of the gap while reducing the number of adjustment degrees of freedom from five to two, thereby reducing the difficulty of the coupling adjustment and the amount of bulky equipment needed.
The simplest integral alignment feature according to the proposal would be a trench etched into the angled end face of the optical fiber, coincident with the major axis of the face and centered on the end face of the fiber core (see Figure 1). The sphere would be brought into mechanical contact with the long rims of the trench. In this configuration, the depth and width of the trench would determine the size of the evanescent-field coupling gap between the sphere and the end face of the fiber. The contact between the rims and the sphere would not appreciably affect the performance of the microsphere as a resonator because the trench would be made wide enough that the points of contact would lie outside the belt of light.
To eliminate the need for an adjustment along the trench, one could replace the trench with a circular depression centered on the fiber core. As in the case of a trench, the depth and diameter of this depression would be chosen to obtain the desired gap size. Alternatively, as depicted in Figure 2, such a circular depression could be superimposed on a narrower trench.
This work was done by Vladimir Iltchenko of Caltech for NASA's Jet Propulsion Laboratory.
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Refer to NPO-30254.