Soliton laser overcomes energy limitation

The generation of ultrashort optical pulses requires careful control of the dispersion of light: the phase velocity depends on the frequency, and because the actual pulse contains frequency expansion, it will become wider as it propagates through the optical medium. Soliton laser is a simple and cheap sub-picosecond pulse source, which is mainly composed of laser diode and optical fiber. They balance diffusion by balancing with Kerr focusing (the pulse narrows when the electric field of light changes the refractive index of the medium), thereby mitigating the diffusion, so each pulse propagates as an soliton and its duration remains the same .

Soliton lasers are attractive because of their simple structure, but they cannot obtain the high energy of chirped pulse amplification and other technologies. This is because the energy E is inversely proportional to the duration τ, so shortening the pulse will only increase its energy.

Now, Antoine Runge and colleagues at the University of Sydney and collaborators at Macquarie University and Nokia Bell Laboratories have overcome this limitation. Their new pure quaternary soliton laser uses a spatial light modulator (SLM) to control the dispersion relationship of light to allow higher energy pulses to be obtained.

The dispersion relation k(ω) describes how the frequency of a wave is related to its wavelength. For light in a conventional soliton laser, the function is approximately quadratic, and its second derivative describes how the pulse will expand without Kerr focusing. The non-zero higher-order derivatives that may make the soliton unstable can be minimized experimentally. However, in 2016, researchers at the University of Sydney (including the authors of the current study, Andrea Blanco-Redondo and Martin de Steck) showed that higher-order dispersion is actually useful. They designed a photonic crystal waveguide in which the influence of second- and third-order dispersion was eliminated by the geometry of the waveguide. The balance of fourth-order dispersion and Kerr focusing is the cause of soliton formation.

The pure fourth-order soliton laser manufactured by Runge and colleagues uses the same principle. However, the researchers used programmable SLM instead of specially designed waveguides to create the required dispersion profile. The researchers confirmed that the energy of the fourth pulse is proportional to τ-3, as the predicted fourth-order dispersion soliton, rather than τ-1, as in the conventional soliton. This scaling explains why the pulse energy in a pure fourth-order laser may exceed the pulse energy in existing equipment by orders of magnitude. The next step for Lange and his colleagues is to realize these higher energies.

At tens of watts, the soliton pulse did not break any records. However, applications like micromechanical and soft tissue surgery that require rapid bursts of high energy can benefit from compact, low-cost equipment.