32-fs Kerr-lens mode-locked Yb:CaGdAlO 4 oscillator optically pumped by a bright fiber laser

P Sévillano (1) , Patrick Georges (2) , Frédéric Druon (2) , D Descamps (3) , E Cormier (3)

1 FEMTO-ST - Franche-Comté Électronique Mécanique, Thermique et Optique - Sciences et Technologies (UMR 6174)
2 Laboratoire Charles Fabry / Lasers
3 CELIA - Centre d'Etudes Lasers Intenses et Applications

In this study, we report on a pure Kerr-lens mode-locked Yb:CaGdAlO 4 oscillator optically pumped by a diffraction-limited fiber laser. At the repetition rate of 96 MHz, several configurations have been studied to achieve either pulse duration of 40 fs with average powers up to the watt level or shorter pulse duration down to 32 fs. To the best of our knowledge, this represents the shortest pulse duration ever achieved with an Yb-doped bulk material and the highest average power for sub-40-fs Kerr-lens mode-locked Yb-bulk oscillator. OCIS codes: (140.4050) Mode-locked lasers; (140.3615) Lasers, ytterbium; (140.7090) Ultrafast lasers. http://dx.doi.org/10.1364/OL.39.006001 During the past decade, Yb-doped crystals have raised a great interest for the generation of high average power ultra-short pulses due to their excellent spectroscopic and thermal properties [1]. Among them, Yb:CaGdAlO 4 (Yb:CALGO) crystal is one of the most promising candidates for sub-50-fs oscillator thanks to its broad and flat emission band [2]. Recent works reported pulse generation as short as 40 fs in this material [3], which is very close to the shortest (35 fs) pulse duration ever obtained from an Yb-doped bulk material-based oscillator [4,5]. However, these ultra-short pulse oscillators generate average powers never exceeding a few tens of milliwatts. This power limitation results from the unavailability of bright powerful (diffraction limited) pump sources as well as the typical response time of saturable absorbers. The thin-disk technology provides an efficient alternative to significantly increase the average power [6]. Although very impressive powers are demonstrated, the shortest pulse duration reported in Yb:CALGO thin-disk oscillator is limited to 62 fs [7]. Improving the average power while maintaining the pulse duration as short as possible is a challenging goal we aim at achieving making use of pure Kerr-lens mode-locking (KLM) technique combined with high-brightness pumping. By this way, recent works on Yb:CaF 2 reported significant improvements in average power of bulk oscillators with the generation of sub-70 fs with several watts [8]. In this Letter, we report on a pure Kerr-lens mode-locked Yb:CALGO oscillator delivering pulse duration of 40 fs at the watt level by means of a powerful single mode, polarized Yb-doped fiber pump emitting around 979.5 nm. After optimization of the output coupler transmission, the oscillator can provide the shortest pulse ever produced in Yb-doped bulk material with pulses as short as 32 fs. To the best of our knowledge, it is the first realization of a pure Kerr-lens mode-locking in Yb:CALGO crystal. For this, we use a high-brightness optical pumping with a single-mode fiber laser source [9]. After describing the experimental setup, we present the performance of the KLM Yb:CALGO os-cillator and discuss the optimization to provide the shortest pulse durations. The experimental setup of the laser cavity and pumping geometry is sketched in Fig. 1. We use a 5-mm-long, 2 mm × 4 mm Brewster-angle cut, Yb:CALGO crystal doped at 5 at. %. It is mounted in a water-cooled copper holder. We have chosen the c-cut orientation to avoid the anisotropy of temperature-dependent refractive index that could appear at high average pump power [10] and to eliminate the polarization state sensitivity in cross-section emission [2]. The Yb:CALGO crystal is positioned between two concave mirrors (M 1 , M 2 , 100-mm-radius-of-curvature) in a standard X-fold cavity configuration. To compensate astigmatism due to the Brewster-angle incidence, these mirrors are tilted with an angle of 8°. On one side, the cavity ends by a high-reflection (HR) mirror and closed on the other side by a 35% output-coupler (OC). A pair of SF10 prisms separated by a distance of 450 mm is used for fine intra-cavity dispersion control. The net intracavity group delay dispersion (GDD) is calculated to be close to −2200 fs

Domaines: Optique / photonique

P Sévillano, Patrick Georges, Frédéric Druon, D Descamps, E Cormier. 32-fs Kerr-lens mode-locked Yb:CaGdAlO 4 oscillator optically pumped by a bright fiber laser. Optics Letters, 2014, 39 (20), pp. 6001-6004. ⟨10.1364/OL.39.006001⟩. ⟨hal-01222075⟩