New Horizons for High-Power Fiber Lasers

Optical fibers, which are widely used in telecommunications, medicine, lighting, and many other applications, also can be used as high-power laser sources for scientific, industrial, and military purposes. A fiber laser, for example, is the master oscillator, or seed source, of the laser pulse that ultimately is amplified to more than four million joules of infrared energy in the NIF laser system.

Fiber lasers have several advantages over traditional chemical, gas, and solid-state high-power lasers: unequalled beam quality, good heat dissipation, high efficiency, and robust reliability. So far, most fiber lasers operate at wavelengths longer than one micron, as dictated by the application in which they’re used and developments in materials.

In a Laboratory Directed Research and Development (LDRD) project, LLNL researchers are exploring the use of new materials, fabrication methods, and fiber designs with the goal of extending fiber laser technology to shorter wavelengths. Such sources would benefit a variety of applications, including spectroscopy, remote sensing (LIDAR, or light detection and ranging, with water lines), adaptive optics systems (laser guide stars) to correct for atmospheric distortions in ground-based telescopes, underwater communications, and beam delivery (machining and directed energy).

"Fiber laser sources are unmatched in terms of brightness and efficiency due to the combination of waveguiding, long interaction lengths, excellent thermal management and ultrapure materials," said NIF & Photon Science physicist Paul Pax, the lead researcher on the project. "New short-wavelength pump diodes in the 400- to 450-nanometer (violet light) range are becoming readily available, and their powers are increasing," he said. "This opens up new avenues for short-wavelength fiber lasers that were not available before."

A fiber laser typically consists of a dual-core optical fiber, with one core nested inside the other; the inner core is doped with a rare-earth element such as erbium, ytterbium, or neodymium. Light from a pump laser is fired into the end or side of the fiber and guided along the fiber by the undoped outer core. As the pump light passes through the inner core, the dopant is stimulated to emit radiation, or lase, at one of the rare-earth element’s characteristic wavelengths.

Pax said an early success in the program has been the development and fabrication of a neodymium-doped fiber laser with a novel filtering waveguide—the structure that supports well-defined modes, or transmission paths, in the laser—that enables it to operate at a wavelength of 925 nanometers (nm) instead of neodymium’s otherwise-strongest characteristic wavelength of about 1060 nm.

"The strong 1060-nm line has to be suppressed or it depletes all the gain," he said. "This waveguide allows us to do just that, which makes operation at 925 possible. This wavelength is useful for remote sensing and, with harmonic frequency conversion, for blue-green-light underwater communications."

The laser is operating with "very good efficiency and good beam quality," Pax said. "Power is already a record for this type of laser—27 watts at 925 nanometers—limited by available pump power. And the waveguide design allows for scaling the power by increasing the core size."

The researchers also will test other dopants, including samarium, which lases directly in the visible spectrum at 651 nm, and terbium, which emits at about 545 nm. "The two immediate applications (directed energy and submarine communications) aren’t the only reason to pursue visible fiber lasers," Pax noted. "The possibilities are opening up because of the new pump diodes, and we want to be in a position to make use of them with novel active species (dopants) for fiber lasers."