Soliton lasers overcome energy limitations

Generating ultrashort light pulses requires careful control of the light’s dispersion: Phase velocity depends on frequency, and because a real pulse contains a spread in frequency, it will broaden as it travels through an optical medium. Simple, inexpensive sources of sub-picosecond pulses, soliton lasers consist primarily of a laser diode and an optical fiber. They mitigate spreading by balancing it against Kerr focusing—the narrowing of a pulse caused when light’s electric field alters the medium’s refractive index—so each pulse travels as a soliton, and its duration remains unchanged. (For more on Kerr focusing, see Physics Today, August 2001, page 17 .)

Soliton lasers are attractive because of their simple construction, but they can’t achieve the high energies of techniques such as chirped-pulse amplification (see Physics Today, December 2018, page 18 ). That’s because the energy E and duration τ vary inversely, so shortening a pulse only increases its energy so much.

Now Antoine Runge and coworkers at the University of Sydney, with collaborators at Macquarie University and Nokia Bell Labs, have overcome that limitation. Their new pure-quartic soliton laser uses a spatial light modulator (SLM) to manipulate the light’s dispersion relation to allow for higher energy pulses.

A dispersion relation k(ω) describes how a wave’s frequency relates to its wavelength. For light in a conventional soliton laser, the function is approximately quadratic, and its second derivative describes how a pulse would spread in the absence of Kerr focusing. Nonzero higher-order derivatives that can make the solitons unstable are experimentally minimized. In 2016, however, researchers at the University of Sydney (including Andrea Blanco-Redondo and Martijn de Sterke , both authors of the current work) showed that higher-order dispersion can actually be useful. They designed a photonic crystal waveguide in which the effects of second- and third-order dispersion were eliminated by the waveguide’s geometry. Balancing of fourth-order dispersion with Kerr focusing was responsible for soliton formation.

The pure-quartic soliton laser built by Runge and colleagues employs the same principle. However, instead of a specially engineered waveguide, the researchers used a programmable SLM, shown in the figure, to create the desired dispersion profile. The researchers confirmed that the energy of the quartic pulses was proportional to τ⁻³, as predicted for fourth-order dispersion solitons, rather than τ⁻¹, as in conventional solitons. That scaling explains why pulse energies in pure-quartic lasers have the potential to surpass those in existing devices by a few orders of magnitude. The next step for Runge and colleagues is realizing those higher energies.

At tens of watts, soliton pulses aren’t breaking any records. However, applications like micromachining and soft-tissue surgery, which require quick bursts of high energy, could benefit from the compact, low-cost devices. (A. F. J. Runge et al., Nat. Photon., 2020, doi:10.1038/s41566-020-0629-6 .)