A team of applied physicists at the California Institute of Technology 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 the journal Nature, Caltech applied physics professor 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 stretched 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. The Caltech team uses the physics of the sphere to reduce both power and size. Normal Raman lasers turn on "with a shout"—these new devices require "only a whisper."
Central to this breakthrough was the ability to couple directly to the ring orbits, or whispering gallery modes, of the sphere while preserving the exquisite perfection of the sphere in terms of its ability to store and concentrate light. The Caltech team uses stretched optical fiber in the form of a taper 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 this wavelength flexibility.
Also, through a process called cascading, it is possible to cover even greater wavelength bands by using one Raman laser as the pump for another. In this way, a whole series of wavelengths can be generated in a kind of domino effect. More generally, it can be used to extend the wavelength range of other laser sources into difficult-to-access wavelength bands for sensing or other purposes.
The article is titled "Ultralow-threshold Raman laser using a spherical dielectric microcavity," and is available at www.nature.com.
In the photo, the sphere has been doped, which enables observation of the ring orbit as green luminescence. The photo is by M. Cai of Vahala group.
Further discussion of this and related work can be found at the Vahala Caltech group website: www.its.caltech.edu/~vahalagr.
