Stanford Team Develops Self-Cooling Silica Fiber Laser

A Stanford University research team developed a self-cooling laser based on a silica fiber architecture. By overcoming the need for external cooling, such as with a water-based cooling system, the design charts a course for future laser-based devices capable of delivering exceptional purity and frequency stability.

Applications for a self-cooling laser constructed from a silica laser fiber include the creation of advanced fiber amplifiers — useful in the all-optical transportation of information over long distances — and low-power, high-precision metrology.

After repeated tests with the goal of successfully cooling a silica fiber when pumping it with laser light, Stanford University electrical engineering graduate student Jennifer Knall decreased the energy level of the light with which she was conducting tests. The results showed the silica fiber becoming definitively colder, not hotter, upon light excitation.

Knall deployed anti-Stokes fluorescence to achieve cooling. That process involves the addition of a rare-earth ion to the fiber. The rare-earth fiber (commonly, ytterbium) absorbs low-energy light and then emits it at a slightly higher energy level. The result is an overall reduction to the temperature of the fiber.

Anti-Stokes fluorescence is challenging in silica, though, because the energy that an excited ytterbium ion possesses can latch onto an impurity in the fiber and release heat energy. That process is called “concentration quenching.” To overcome it, the researchers identified a distinct glass composition that allowed them to incorporate as much ytterbium as possible without demonstrating the quenching effect, and achieve a high concentration.

“When the concentration of ytterbium is too low, the cooling is too small,” said team member Michel Digonnet, a Stanford University research professor of applied physics. “When it is too high, the ions lose their cooling efficiency.”

In a laser design, the temperature of a self-cooling silica fiber laser did not fluctuate, making the frequency and power of the light that they emitted more stable over time than lasers requiring external cooling. Their emission, therefore, is more consistent in terms of wavelength.

Knall and team members, including those from Mid Sweden University, Clemson University, Université Laval, and the University of Illinois Urbana-Champaign, also demonstrated the ability to integrate silica fiber into a laser amplifier. Should the team be able to increase performance efficiency, the system would be support high-power and large-scale laser applications beyond low-power tests and measurements. The team has developed two additional silica fiber compositions that self-cool since Knall’s first identification. Using the best candidate, she has amplified laser light more than fortyfold while maintaining a negative average temperature change along the length of the present fiber.

The researchers said they are currently extracting approximately 4% of the energy they are injecting into the fibers. That percentage and a correspondingly low level of efficiency, they said, make the technology an unlikely candidate for immediate adoption for high-power applications.

Research on the advancement was published in two papers in Optics Letters (www.doi.org/10.1364/OL.384658 and www.doi.org/10.1364/OL.395513).