76 fs SWCNT-SA mode-locked Tm:MgWO4 laser at 2 μm

Abstract

Femtosecond lasers in the spectral region of ∼2 μm are essential for development of pump and seed sources in diverse optical parametric frequency conversion schemes to cover the mid-IR part of the optical spectrum [1]. Passive mode-locking in such solid-state lasers is mainly achieved by saturable absorbers (SAs). To obtain durations corresponding to ten or fewer optical cycles, materials with broadband gain spectrum extending beyond 2 μm are needed so that the structured water vapor atmospheric absorption can be avoided. The newly developed monoclinic tungstate Tm3+:MgWO4 (Tm:MgW) crystal exhibits such a broad and smooth gain profile due to the low-symmetry structure and the distortion of the crystal field caused by the mismatch of the ionic radii of Mg2+ and Tm3+ [2]. Using a graphene SA, the first sub-100-fs pulses at 2 μιη were obtained [3] while preliminary results with single-walled carbon nanotube (SWCN)-SAs indicated a pulse duration of 168 fs [4]. We report now on further progress with a SWCNT-SA generating nearly bandwidth-limited pulses of 76 fs duration at 2037 nm.