As the energy available from femtosecond disk lasers creeps above a microjoule, the devices become interesting for use in several applications, not the least of which is three-color laser projection. The high peak power available from these lasers greatly simplifies the nonlinear conversion of their fundamental infrared wavelength to the visible region.But recent attempts to push much beyond a microjoule stalled because of an unexplained instability at higher energies. Now scientists at Swiss Federal Institute of Technology Zurich have discovered that the instability plaguing their earlier experiments was caused by the Kerr nonlinearity of air. By replacing the air in the laser’s resonator with helium — whose nonlinearity is negligible compared with that of air — they boosted the laser’s energy from less than 2 μJ to more than 5, and they believe that further increases in energy should be relatively straightforward.The researchers used an Yb:YAG laser in which a 200-μm-thick thin disk provided the gain and a semiconductor saturable absorber mirror (SESAM) provided the passive modulation to mode-lock the laser (Figure 1). They designed the resonator to compensate for the 22.8-m thermal focusing of the thin disk and stretched the cavity to 12.2 m — corresponding to a 12.3-MHz mode-locking frequency — with a series of reimaging-mirror pairs.Figure 1. The researchers found that the nonlinearity of 12.2 m of air in the resonator destabilized soliton mode-locking, which depends on a delicate balance between intracavity dispersion and nonlinearity. They stabilized the laser by flooding the resonator with helium. Although the SESAM mode-locked the laser, the resulting femtosecond pulse was stabilized because it propagated in the resonator as a soliton. A soliton is a solitary wave that is stretched by dispersion and compressed by nonlinearity: The two effects exactly balance each other so that the wave propagates indefinitely without distortion. In the researchers’ laser, the nonlinearity was provided by self-phase modulation in the Brewster plate, and the group-delay dispersion was provided by eight specially designed dispersive mirrors. Source of nonlinearityTheoretically, the nonlinearity varies with the Brewster plate’s position in the resonator. The scientists previously used this effect to tune the pulse duration of other soliton mode-locked lasers. But in initial experiments with this 12.2-m-long, high-pulse-energy laser, they observed a pulse duration relatively independent of the Brewster plate’s position. This observation provided the clue they needed to identify the unexplained instability. If the nonlinearity introduced by the Brewster plate had only a modest effect on pulse duration, they reasoned, there must be another nonlinearity source in the resonator. Air in the intracavity beam path was the most likely culprit. Thus, they placed a box over the laser, flooded it with helium and — as they had hoped — the instability disappeared.When operating the laser shown in Figure 1 in a helium environment, the scientists obtained up to 63 W of average power, which was limited by the available pump power. The laser produced an 800-fs pulse with a 1.59-nm spectral bandwidth centered at 1030 nm. Pulse energies reached 5.1 μJ, corresponding to a peak power of 5.6 MW, and the M2 parameter of the output beam was 1.1.Optics Letters, Sept. 15, 2006, pp. 2728-2730.