Solitons from new fibre laser could improve eye surgery

A fibre laser that emits high-energy, dispersion-free light pulses has been produced by researchers in Australia and the US. These soliton pulses are held together by a high-order term in the optical dispersion equation that had previously been a nuisance to scientists producing solitons. The researchers hope that their work will encourage the study of higher-order terms in the dispersion equation. Practical applications of high-power solitons include laser eye surgery.

Optical dispersion occurs when light at different frequencies (colours) travel at different speeds in a medium – causing a light pulse containing different frequencies to spread out in space and time. Familiar examples of dispersion are light being broken into different colours by a prism and the formation of rainbows. The equations governing dispersion, however, are complex. Adding second-order effects leads to non-dispersive solutions called solitons. In this case first and second order effects balance each other out and the pulse does not spread out.

Solitons in optical fibres were first observed in 1973 and have since have found applications in areas such as laser spectroscopy. However, their power is limited. In the early 1990s, researchers attributed this to higher-order terms in the optical dispersion equations, which become increasingly important as pulse power increases. This makes the pulses increasingly unstable and eventually causing them to break apart. Today, lasers that are used to produce high-power ultrashort pulses are complex and expensive. They produce pulses that chirp (change in frequency as the pulse progresses) and use additional equipment to reshape these chirped pulses into stable pulses.

All the heavy lifting

In 2018, Martijn de Sterke of the University of Sydney and colleagues found that fourth-order (quartic) dispersion is not necessarily destructive when they discovered a new type of optical soliton in a silicon waveguide. By “serendipity” de Sterke and colleagues found that, rather than being a perturbation “[the quartic dispersion] really did all the heavy lifting”: “The waveguide was not designed that way,” he explains, “it just happened to be that way, and our colleague Andrea Blanco-Redondo was clever enough to recognize that.”

In this latest work, de Sterke, Blanco-Redondo (now at Nokia Bell Labs in the US), Antoine Runge and Kevin Tam demonstrate the value of that serendipitous 2018 observation using an ultrafast fibre-laser that emits “pure quartic solitons”. The laser cavity incorporates a spectral pulse shaper that engineers the second- and third-order dispersion to be zero, while imparting a strong quartic dispersion. The resulting pulses agree well with the researchers’ theoretical predictions.

Crucially, Runge notes that much higher-power soliton lasers than previously possible may be achievable. “In a normal, quadratic soliton, if you halve the pulse duration, the energy goes up by a factor of two,” explains Runge, “In these new pure quartic solitons, if you halve the pulse duration, the energy goes up by a factor of eight.” This could allow them to be used in new applications: “A 100 fs quartic soliton could contain nanojoule energies with peak powers in the tens or hundreds of kilowatts,” he says. “For laser surgery and non-linear imaging that’s probably enough.”

Simplification needed

He says, however, that their device needs simplification to eliminate the pulse shaper before it could be commercially viable as a soliton laser: “At present, we don’t have fibres with the right dispersion,” he says, “but in future, if you managed to fabricate fibre like that – and we already have designs – then you wouldn’t need to do phase modulation or anything like that, and you would have a laser that was much simpler. What we did here was a proof of principle that these pulses exist and have these properties, but we’re still far away from a commercial pure quartic soliton laser.”

Runge hopes the work will encourage researchers to explore the potential of higher-order optical dispersion terms: “Now we have potentially an infinite family of pulses we can generate because we can tailor the dispersion the way we want and see if there’s something interesting for such or such application.”

Ultrafast laser spectroscopist Georg Herink of the University of Bayreuth in Germany is cautiously optimistic: “You definitely would get more energy into a soliton, and it definitely makes it interesting to look at even higher order terms,” he says. He cautions, however, that current de-chirping schemes produce orders of magnitude more energetic and shorter pulses than does the Sydney technique.

Herink suspects, however, that tuning the dispersion inside an optical cavity and producing multiple stable solutions of the optical dispersion equation has significant research potential: “Can you do something that hasn’t been observed before? Now you can explore a lot of academic questions that are maybe uniquely accessible with this platform experimentally.”

The research is described in Nature Photonics.