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).
Optical absorption and emission bands of Tm3+ ions in calcium niobium gallium garnet crystal
Abstract Absorption spectra of Tm3+ ions in Ca3Nb1.6875Ga3.1875O12 (CNGG) crystal have been investigated at various temperatures between 15 and 296 K. Luminescence spectra in a spectral region of 400–1750 nm are investigated under excitation into various excited states of Tm3+ and the conduction band of CNGG at room temperature. The absorption and emission bands of Tm3+ in CNGG are observed to be broader than those observed in other Tm3+-doped crystals such as LiNbO3. This is due to the disordered structure of CNGG. From the temperature dependence of absorption spectra, five Stark levels are derived for the 3H6 ground state. The highest Stark level is found to be 351 cm−1 above the ground level. It is suggested that the low efficiency of the 2.02 μm lasing at room temperature is due to the narrow splitting of the Stark levels.
An analytical approach to the design of electrodes in high-power, fast-transverse-flow CO2 lasers
Abstract Usually, the design of the electrodes of fast-transverse-flow CO 2 lasers relies more on experimental data than on theoretical analysis. Traditional systems sustain a stable, high-power discharge but the current distribution generates a peaked, non-uniform small-signal-gain pattern. We present a theoretical model suitable for the design of electrodes that achieves a quasi uniform gain distribution. The analysis, based on a two-temperature model and the gas-transport equations, computes the electron density that supports a flat gain pattern. Combining the desired electron density with the electron-balance equations, the model determines the required electric field. The results were used to design a new set of electrodes for a home-made fast transverse flow CO 2 laser. A stable, large volume discharge ( 4 × 8 × 90 cm 3 ) with very good uniformity was obtained. The resulting gain distribution was registered in a bi-dimensional map. The peak gain rose from 1.2 m - 1 , attained with the old electrode set, to 1.6 m - 1 with the new one and the gain showed a homogeneous profile.