Yb:CALGO thin-disk femtosecond oscillator

Abstract

Summary form only given. Thin-disk laser (TDL) technology has a crucial interest for high-power ultrafast laser oscillators. The thermal control of the gain medium allows the generation of high average power and high energy fs pulses directly out of an oscillator. From the past few years, some research efforts have focused on reducing the pulse duration using this technology [1]. In this point of view, we are presenting here the first femtosecond thin-disk oscillator using a very promising crystal, Yb:CALGO. This material combines atypical advantageous spectroscopic and thermal properties for an Yb-doped one. Moreover it has already demonstrated interesting results in bulk configuration with sub-50-fs pulses at low power and sub-100 fs with 12 W. These results motivated its use in the TDL configuration and the demonstration of high-power fs TDL using Yb:CALGO is shown with the production 28 W, 1.3 μJ, 300 fs pulses and 20 W, 0.9 μJ, 197 fs pulses.The femtosecond experiment was performed with a 2%-doped, 350-μm-thick, Yb:CALGO crystal. The setup is presented in fig. 1. The crystal diameter is 6 mm, and it is cut for the laser beam to propagate along the σ-axis, with access to both σ- and π-polarization. The pump diode delivers up to 230 W. The pump spot diameter on the disk is 2.9 mm and the laser beam waist diameter is 2.1mm. When the crystal is glued on a copper heat sink, one can observe a deleterious thermal astigmatism occurring for pump power around 130 W. In order to overcome this problem the crystal was glued on a diamond mount (fig. 1). In this case, in a multimode configuration we obtained up to 86 W in cw regime, corresponding to an optical-to-optical efficiency of 37.5 %. The cavity is optimized to operate in spatially single mode operation (fig. 1), with M2<;1.1. The average power in cw regime is then 50 W with a 6 % output coupler. Inserting in a cavity a 0.5 %-modulation-depth SESAM (from BATOP GmbH) with a saturation fluence of 90 μJ/cm2 and -2000 fs2 GTI mirrors, up to 28 W of average power has been obtained in femtosecond regime. The pulse duration is then 300 fs corresponding to a pulse energy of 1.3 μJ for a repetition rate of 23 MHz. In order to reduce the pulse duration a fused silica plate (3-mm-thick) is inserted in the cavity (fig. 1) to add Kerr self-phase modulation. In this configuration, 20 W has been obtained with a pulse duration of 197 fs corresponding to an energy of 0.9 μJ. The autocorrelation and spectral traces are plotted in fig. 1.In conclusion, we demonstrated the first femtosecond Yb:CaGdAlO4 thin-disk oscillator with the production of 28 W for 300-fs pulses and 20 W for sub-200-fs pulses. The efficiency of respectively 13% and 9% could be increase in the future and the results obtained so far represent the only first step with Yb:CALGO in TDL. Moreover, the production of 135 fs pulses has also been performed[2] but at low power (1.3 W) due to a lack of gain in the material and high unsaturable losses of the SESAM, and a tunability from 1032 nm to 1046 nm for sub-200 fs pulses has also been demonstrated at low power.