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Gas-hollow fiber laser delivers 'difficult' mid-IR output

26 Feb 2016

Acetylene-filled fiber laser produces 3.1µm-3.2µm wavelengths, long-sought by photonics R&D groups.

Researchers at Bath University, UK, have created a new kind of laser capable of pulsed and continuous mid-infrared emission between 3.1µm and 3.2µm, a spectral range that has long presented a challenge for laser developers. They say the achievement could aid in the development of new applications for mid-IR lasers, which are currently used in spectroscopy, environmental sensing and detecting explosives.

The new laser, described in a recent article “Cavity-based mid-IR fiber gas laser pumped by a diode laser”, in Optica, combines aspects of both gas and fiber lasers. Placing a suitable gas inside of a hollow optical fiber allowed the researchers to create a fiber gas laser with mid-IR emission.

“Beyond about 2.8 microns, 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 microns,” said William Wadsworth, who co-led the research team with Jonathan Knight, also at the University of Bath. “This has left a gap that has presented a great deal of difficulty.”

Hollow-core fibers

Key to this laser’s success was the team’s development of silica hollow-core fibers that perform exceptionally well in the mid-IR. Wadsworth explained, “You can think of the structures in our fibers as very long and thin bubbles of glass. 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.”

Because light traveling inside a hollow-core fiber remains mostly in the empty core, these fibers overcome the tendency of silica-based glass to absorb light at wavelengths beyond 2.8 microns.

Feedback fiber

The researchers recognized that their new hollow core fibers could enable a new type of fiber laser. The researchers used acetylene gas because it emits in the mid-IR 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 long distances — 10m or 11m in this case.

The Bath researchers have previously shown that gas inside a fiber can interact with light to produce mid-IR emission. In their new work, the researchers added a feedback fiber, the last component needed to consider the device a true laser. The feedback fiber takes a small amount of light produced in the fiber containing the acetylene gas and uses that light to seed another cycle of light amplification, thus reducing the pump power required to produce a laser beam.

One important advantage of the new design is its use of mature telecommunications diode lasers, which are practical, inexpensive, and available in high powers. The researchers plan to use a higher power pump laser to increase the fiber gas laser’s power.

Future expansion

Fei Yu, a member of the Bath team, said, “We developed a way to use light to pump molecules and generate light that is not that common to see in a laser system. This new way to construct a gas laser could be expanded to make more and more laser types that would otherwise have been impossible without our hollow-core fiber.”

The researchers say that a number of other gases should work with their fiber gas laser, allowing emission up to 5µm. “This laser is just one use of our hollow-core fiber,” said Muhammad Rosdi Abu Hassan, a doctoral student and first author of the paper. “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.”

About the Author

Matthew Peach is a contributing editor to optics.org.

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