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Combined gas and fibre laser gives mid-infrared emission

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

The new laser, described in the journal Optica, combines aspects of both gas and fibre lasers. Placing acetylene gas – which emits in the near infrared – inside of a silica hollow-core optical fibre allowed the researchers to create a fibre gas laser with mid-IR emission.

‘Beyond about 2.8µm, conventional fibre 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 3.5µm,’ 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.’

The researchers developed silica hollow-core fibres for the laser. These are a new class of fibres that use internal glass structures to confine light inside hollow cores, whereas traditional optical fibres confine light in a solid core of glass. The fibres perform exceptionally well in the mid-IR.

‘You can think of the structures in our fibres as very long and thin bubbles of glass,’ explained Wadsworth. ‘By surrounding the region of space in the middle of the fibre 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 fibre remains mostly in the empty core, these fibres overcome the tendency of silica-based glass to absorb light at wavelengths past 2.8µm. The hollow-core fibres also provide 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 metres in this case.

The University of Bath researchers as well as other research groups have previously shown that gas inside a fibre can interact with light to produce mid-IR emission. In the new work, the researchers added a feedback fibre, a component that takes a small amount of light produced in the fibre 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 fibre gas laser’s power.

The researchers say that a number of other gases should work with their fibre gas laser, allowing emission up to 5µm. ‘This laser is just one use of our hollow-core fibre,’ said Muhammad Rosdi Abu Hassan, a doctoral student and first author of the paper. ‘We see it stimulating other applications of the hollow fibre and new ways of interacting different types of laser beams with gases at various wavelengths, including wavelengths that you wouldn’t expect to work.’ 

Further information 

Paper in Optica: Cavity-based mid-IR fiber gas laser pumped by a diode laser

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