Microlaser delivers macropower

Caltech applied physicists have demonstrated an ultrasmall Raman laser that is 1,000 times more efficient than previous devices. The device could have significant applications for telecommunications and other areas where compact, highly efficient, and tunable lasers are desirable.

Reporting in the February 7 issue of Nature, Professor of Applied Physics Kerry Vahala and graduate students Sean Spillane and Tobias Kippenberg describe their progress in making the tiny device, which incorporates a small spherical glass bead and a stretched fiber-optic wire. The laser is especially efficient because of the way it stores light inside the microsphere, or resonator, as well as the manner in which the optical wire permits efficient coupling of light into the sphere.

According to Vahala, the light wraps around the sphere in a ring orbit and subsequently intensifies over hundreds of thousands of orbits, resulting in extreme concentration of optical power within the sphere. In this way, very weak signals applied to the sphere from the fiber-optic wire can build to enormous intensities within the sphere itself.

At these higher power levels, the physics within the sphere enters a nonlinear regime wherein conventional rules for light propagation break down. In the Caltech work, the molecules of the glass bead itself are distorted, resulting in a process called Raman emission and lasing. Because Raman lasers require enormous intensities to function, they are usually power-hungry devices, but Vahala’s team uses the physics of the sphere to reduce both power and size.

Central to this breakthrough was the ability to couple directly to the sphere’s ring orbits while preserving the sphere’s exquisite ability to store and concentrate light. The Caltech team uses stretched optical fiber to achieve coupling efficiencies, in which loss is negligible, both to and from the sphere.

Because Raman lasers and amplifiers can operate over a very broad range of wavelengths, they are important devices that extend other lasers into new or previously inaccessible wavelength bands.

For example, Raman amplifiers are now used widely in commercial long-distance fiber communications systems because of their wavelength flexibility. Also, it is possible to cover even greater wavelength bands by using one Raman laser as the pump for another, called cascading. In this way, a whole series of wavelengths can be generated in a kind of domino effect.

The article, “Ultralow-threshold Raman laser using a spherical dielectric microcavity,” is available at www.nature.com. More information can also be found at www.its.caltech.edu/~vahalagr.
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