Optical-fiber microcavities reach angstrom-scale precision

A long-standing goal of optical science is to create miniature buffers, optical switches, and filters and assemble them into an all-optical computer—no electrons required. The fundamental structure needed for all those circuit elements is the microresonator, whose highly reflecting walls can confine a light signal in a tiny mode volume for hundreds of microseconds. Coupling a signal efficiently through a series of such resonators, though, requires that their resonance frequencies be precisely matched—a daunting task. Misha Sumetsky and colleagues at OFS Laboratories (a former branch of Bell Labs) have now developed a simple method for doing just that: They take ordinary silica fiber and heat it locally with a laser for a few seconds to anneal away stresses frozen into the fiber during its manufacture. As the fiber warms, the concomitant relaxation increases its local diameter and refractive index. The result is a mode-confining bulge that for a given laser power is reproducible to within a couple of angstroms. As proof of principle, the researchers fabricated a series of five nearly identical resonators that are each separated, like beads on a string, by 100 μm of fiber. As shown here, light introduced into the fiber through a tapered microfiber circulates as whispering-gallery modes whose intensity maxima are shown in red. The modes are confined radially by internal reflection and axially by the curvature of each resonator. (M. Sumetsky et al., Opt. Lett. 36, 4824, 2011.)—R. Mark Wilson