Gas/Fiber Hybrid Laser Achieves Stable MIR Emission
BATH, England, March 4, 2016 — A gas/fiber hybrid laser is capable of pulsed and continuous mid-infrared (MIR) emission between 3.1 and 3.2 μm, a spectral range that has long presented a major challenge for laser developers.
The achievement could aid in the development of additional applications for MIR lasers, which are currently used in spectroscopy, environmental sensing and detecting explosives.
Experimental layout of the laser cavity. Courtesy of Hassan, et al./The Optical Society (OSA).
“Beyond about 2.8 μm, conventional fiber lasers start to fall off in terms of power, and the other main technology for the mid-IR, quantum cascade lasers, doesn’t pick up until beyond 3.5 μm,” said professor William Wadsworth of the University of Bath. “This has left a gap that has presented a great deal of difficulty.”
Key to the laser’s success was the team’s development of silica hollow-core fibers that perform exceptionally well in the MIR. Hollow-core fibers use internal glass structures to confine light inside hollow cores, whereas traditional optical fibers confine light in a solid core of glass.
“You can think of the structures in our fibers as very long and thin bubbles of glass,” Wadsworth said. “By surrounding the region of space in the middle of the fiber with the bubbles, light that is reflected by the bubbles will be trapped inside of the hollow core.”
The fiber's long and thin bubbles of glass reflect light into the fiber's core much in the same way that light reflects off the surface of the soap bubble in the foreground, making it appear iridescent. Courtesy of the University of Bath.
The researchers realized the hollow core fibers could enable a new type of fiber laser. They used acetylene gas because it emits in the MIR and can be pumped using lasers designed for the telecommunications industry. The hollow-core fibers provided a way to trap the light and the gas in the same place so that they can interact for a very long distance — 10 or 11 μm in this case.
Because light traveling inside a hollow-core fiber remains mostly in the empty core, the team said these fibers overcome the tendency of silica-based glass to absorb light at wavelengths past 2.8 μm. Silica is the preferred material for optical fibers because it inexpensive, easy to manufacture and extremely strong.
The Bath researchers and other research groups have previously shown that gas inside a fiber can interact with light to produce MIR emission. In the new work, the researchers added a feedback fiber, the last component needed to consider the device a true laser. The feedback fiber took a small amount of light produced in the fiber containing the acetylene gas and used that light to seed another cycle of light amplification, thus reducing the pump power required to produce a laser beam.
The team said one important advantage of the design is its use of mature telecommunications diode lasers, which are practical, inexpensive and available in high powers. They plan to use a higher power pump laser to increase the fiber gas laser’s power.
Scanning electron micrographs of the two different forms of hollow fiber used in the experiment. At left, a gain fiber with transmission at 1.53- and 3.1-μm wavelengths. At right, a feedback fiber with low loss at 3.1 μm. Courtesy of Hassan, et al./The Optical Society (OSA).
The researchers say that a number of other gases should work with their fiber gas laser, allowing emission up to 5 μm.
“This laser is just one use of our hollow-core fiber,” said doctoral student Muhammad Rosdi Abu Hassan. “We see it stimulating other applications of the hollow fiber and new ways of interacting different types of laser beams with gases at various wavelengths, including wavelengths that you wouldn’t expect to work.”
The research was published in Optica, a publication of The Optical Society (OSA) (doi: 10.1364/optica.3.000218).
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