Fibers Talk, Lock, Make High-Brightness Laser

WATERFORD, Conn. -- For at least 30 years, researchers have attempted to combine the light from several separate, low-power semiconductor laser beams to produce a single high-quality, high-brightness beam. But getting the beams to lock phases together nearly perfectly and produce one spot at TEM₀₀ can be like trying to herd cats -- each wants to do its own thing. Scientists have run into high loss levels and difficulties in transferring power from one beam to another.
But in a forthcoming article in the Institute of Electrical and Electronics Engineers' Photonics Technology Letters, Peter K. Cheo, president of PC Photonics Corp., describes how he successfully managed to combine the light from seven independent Yb-doped optical fiber cores embedded in a common hexagonal cladding to achieve a high-quality beam with a light output of 350 W.
The fibers are arranged side by side in an isometric pattern, or ring, in such a way that the cores are equally distant from each other, allowing for equal coupling strength (Figure 1). This arrangement enables the light in the fibers to form an evanescent wave interaction in which the waves from each fiber core bounce off the other cores and the cladding, forcing them to align in a narrow range of frequencies and to form a stable, fundamental in-phase supermode.
In lay terms, Cheo explained that "the fiber cores all talk to each other," which makes them couple and get in phase to form one larger optical fiber laser. And because the cladding acts as the pumping mechanism, there is minimal power loss.
Cheo demonstrated proof of the principle with a prototype that achieved 85 percent quantum efficiency before an audience of researchers at the Laser Integration Technology facility at the Air Force Research Laboratory in Albuquerque, N.M. Craig Denman, director of the lab, said this was the first time a stable supermode had been achieved. "We're very excited about it," he added.
The US Air Force and the US Department of Defense are interested in developing a larger version operating at the kilowatt level for laser weaponry and target illumination.
The technique could be used for peaceful purposes as well, such as precision manufacturing or the high-speed drilling, cutting and welding of machine parts.
Cheo, who received US patent No. 6,031,850 for the technology, said there is no theoretical limit to the number of optical fiber cores that could be combined to increase power.