Bubbly laser
Researchers in the UK have designed and constructed a new type of laser that can generate pulsed or continuous mid-infrared (IR) emission in the challenging 3.1 to 3.2 micrometre wavelength range. Writing in The Optical Society's journal Optica, William Wadsworth and colleagues at the University of Bath, explain how they have achieved a spectral range that has been an obstacle for laser developers for a long time. The new system could be useful in mid-infrared spectroscopy, environmental sensing and detection and even consumer devices.
The new laser combines aspects of stable, high-quality, high-power hollow-fibre lasers with an appropriate gas in the hollow optical fibre to create a fibre gas laser. "Beyond about 2.8 micrometres, 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 3.5 micrometres," says Wadsworth explain the difficult gap. He led the research team alongside Bath's Jonathan Knight.
Gas and fibres
Key to the new laser's success is the team's development of silica hollow-core fibres that perform exceptionally well in the mid-IR. Hollow-core fibres are a new class of fibres that use internal glass structures to confine light within, whereas conventional optical fibres confine light in a solid core of glass. "You can think of the structures in our fibres as very long and thin bubbles of glass," muses 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." The light travelling within the hollow fibre remains mostly in the empty core and so wavelengths beyond 2.8 micrometres are not lost. Silica is the preferred material for optical fibres because it is relatively inexpensive, easy to manufacture and extremely strong.
The team points out that lasers require an electric current or a pumping laser to excite a material's electrons, which then emit photons as they drop back to their unexcited state. The researchers used acetylene gas, which emits in the mid-IR and can be excited, or pumped, using laser technology adapted from the telecommunications industry. The research shows that the hollow-core fibres can hold gas and trap light in the same place so that they might interact over lengths of 10 or 11 metres in the team's experiments. Such coupling has been done before, but the novelty of this work lies in the addition of a feedback fibre. This last component was essential to building a true laser.
The feedback fibre 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; this reduces the pump power required to produce a laser beam. Critically for future applications is the use of practical, inexpensive diode lasers from the already mature telecommunications sector.
Wavelength extension
"We developed a way to use light to pump molecules and generate light that is not that common to see in a laser system," explains team member Fei Yu. "This new way to construct a gas laser could be expanded to make more and more laser types that would have been impossible without our hollow-core fibre." The same approach should work with other gases allowing emission up to 5 micrometres. "This laser is just one use of our hollow-core fibre," adds team member Muhammad Rosdi Abu Hassan. "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."
The next phase in the resarch will involve improvements to the laser output, Hassan told SpectroscopyNOW. "At the moment, the maximum laser output is 4 milliwatts with 8.8% laser efficiency for synchronous pumping and 2.5 milliwatts with 6.7% laser efficiency for Continuous Wave (CW) laser," he explained. "Mid-infrared fibre lasers when compared to fibre lasers that emit at 1 or 2 micrometres are seen to produce significantly lower output power. However, our system is extremely low threshold running stably around 3 micrometres and scalable through the use of existing 1.5 micrometre laser technology using an oscillator/amplifier configuration."
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