Self-phase modulation cancellation in a high-power ultrafast thin-disk laser oscillator

Ultrafast high-power lasers are employed in a wide variety of applications in science and industry. Thin-disk lasers can offer the required performance directly at the output of a simple oscillator. However, the high intracavity peak power in these oscillators leads to a large amount of self-phase-modulation (SPM) picked up in the intracavity air environment. Consequently, the highest performance oscillators have been operated in a vacuum environment, but that approach is prohibitively complex for industrial lasers. In this letter, we introduce a new concept to overcome this hurdle. We cancel the SPM picked up in air by introducing an intracavity phase-mismatched second-harmonic-generation crystal. The resulting cascaded χ(2) processes provide a large nonlinear phase shift with a sign opposite to the one originating from the air. This enables laser operation in air at 210 W output power with 780-fs, 19-μJ pulses, the highest output power of any SESAM-modelocked laser operated in air to date