Passively mode-locked Yb:CNGG laser
Recently we achieved continuous-wave (cw) and Q-switched laser operation of Yb<sup>3+</sup> in the disordered crystal Ca<inf>3</inf>(NbGa)<inf>2−x</inf>Ga<inf>3</inf>O<inf>12</inf> or shortly CNGG [1] previously studied only with Nd-doping [2] and Tm-doping [3]. It is an isotropic material similar to YAG and exhibits a rather high thermal conductivity of 4.7 Wm<sup>−1</sup>K<sup>−1</sup>, about one third of that of YAG which is much larger than for glasses. The disordered nature of the CNGG crystal provides several different lattice positions for the trivalent dopant ions, resulting in significant inhomogeneous broadening in the absorption and emission spectra. The maximum absorption cross section was measured to 3.3·10<sup>−20</sup> cm<sup>2</sup> at 973.5 nm. Apart from the zero-phonon line the maximum emission cross section is 2.5·10<sup>−20</sup> cm<sup>2</sup> at 1028 nm with a bandwidth of 21 nm (FWHM), roughly twice that of Yb:YAG. The measured fluorescence lifetime amounts to 791 µs. We demonstrated 2.0 W of cw output power with a slope efficiency of 40% for Yb:CNGG and the passively Q-switched operation delivered 1.35 W of average output power at a PRF of 16.7 kHz [1]. Here we demonstrate passive mode-locking of the disordered laser crystal Yb:CNGG using a semiconductor saturable absorber mirror (SESAM).
Polarized laser oscillation of isotropic disordered Yb:Ca3(NbGa)(2-x)Ga3O12 crystal.
Linearly polarized laser oscillation is observed with the isotropic disordered Yb:Ca3(NbGa)(2-x)Ga3O12 crystal, resulting from stress-induced birefringence. At high pump levels, two orthogonal polarization states exist simultaneously. A continuous-wave output power of 2.0 W is obtained with an optical-to-optical efficiency of 32%, the slope efficiency amounts to 40%. In the passively Q-switched regime, the average output power reaches 1.35 W at a pulse repetition frequency of 16.7 kHz, with a slope efficiency of 37%; the laser pulse energy, duration, and peak power amount to 81 microJ, 25 ns, and 3.24 kW, respectively.
Optical properties of Er3+ ions in calcium niobium gallium garnet (CNGG) crystals
Single crystals of Er 3+ doped calcium niobium gallium garnet (CNGG) were grown using the Czochralski technique, and the single crystal fibers with diameter of 0,81 mm were grown by the micro pulling-down method. The absorption and luminescence spectra by Er 3+ ions in these materials have been investigated together with the luminescence decay. The same results are obtained between the crystals and fibers. Broad absorption lines are obtained which are suggested to be due to the Er 3+ ions with various site symmetries. By the excitation with 266 nm radiation, the luminescence bands are observed at 490, 535, 545, 554, 656 and 669 nm. The 545 nm luminescence due to the 4 S 3/2 → 4 I 15/2 transition is found to have two decay components: the fast decay component of 15.1 μs and the slow component of 30.5 μs. A delay time of 3.7 μs was found between the excitation pulse and the 656 and 669 nm luminescence bands due to the 4 F 9/2 → 4 I 15/2 transition while no delay was found for the other luminescence bands.