Gaëlle Lucas-Leclin

Thermal lensing in diode-pumped ytterbium Lasers-Part I: theoretical analysis and wavefront measurements

A theoretical and experimental study of thermal lensing in Yb-doped crystals is presented. In this first part, we focus on theoretical considerations and we describe an original technique suitable for thermal lensing measurements in end-pumped materials. We first derive an expression of the temperature distribution with account of absorption saturation and pump beam divergence inside the crystal, and we address a more general discussion on the particularities of quasi-three-level lasers, as far as thermal effects and fracture issues are concerned. The thermal lens was then measured using a simple technique based on a Shack-Hartmann wavefront analyzer, under lasing and nonlasing conditions. We demonstrate that the technique allows precise wavefront measurements even on small spots. Thermal lensing measurements are finally presented in Yb-doped YAG, GGG, YCOB, GdCOB, KGW, and YSO crystals.

Thermal lensing in diode-pumped ytterbium Lasers-Part II: evaluation of quantum efficiencies and thermo-optic coefficients

A theoretical and experimental study of thermal lensing in Yb-doped crystals is presented. This papers follows the presentation of theoretical considerations and experimental wavefront measurements, which have been the subject of Part I. In this paper, we study the evolution of thermal lensing versus absorbed pump power, and we derive two parameters valuable for laser design and power scaling. The quantum efficiency and the thermo-optic coefficient, in Yb-doped YAG, GGG, GdCOB, YCOB, KGW and YSO. The clear difference between thermal lensing under lasing and nonlasing conditions is the proof that nonradiative effects occur in all the crystals under investigation. An analytical model which takes into account the laser extraction efficiency enables to explain all the experimental features and allows to infer the fluorescence quantum efficiency of the samples (in the range 0.7-0.96). Under nonlasing conditions, the thermal lens dioptric power experiences a roll-off, for which we propose an explanation based on the theory presented in Part I. These results are then used to yield the thermo-optic coefficient of the crystals. At last, we propose a simple analytical formulation useful for a rough estimation of the focal length.

Frequency doubling of an efficient continuous wave single-mode Yb-doped fiber laser at 978 nm in a periodically-poled MgO:LiNbO3 waveguide.

A single-mode Yb-doped fiber laser producing 2 W CW at 978 nm is demonstrated with a high slope efficiency of 72%. Thanks to its narrow bandwidth, lower than 0.02 nm, and its tunability of 6 nm, it has been efficiently frequency doubled in a periodically poled MgO:LiNbO3 waveguide, leading to a power of 83 mW at 489 nm and an internal conversion efficiency of 26 %.

Design and Simulation of Next-Generation High-Power, High-Brightness Laser Diodes

High-brightness laser diode technology is progressing rapidly in response to competitive and evolving markets. The large volume resonators required for high-power, high-brightness operation makes their beam parameters and brightness sensitive to thermal- and carrier-induced lensing and also to multimode operation. Power and beam quality are no longer the only concerns for the design of high-brightness lasers. The increased demand for these technologies is accompanied by new performance requirements, including a wider range of wavelengths, direct electrical modulation, spectral purity and stability, and phase-locking techniques for coherent beam combining. This paper explores some of the next-generation technologies being pursued, while illustrating the growing importance of simulation and design tools. The paper begins by investigating the brightness limitations of broad-area laser diodes, including the use of asymmetric feedback to improve the modal discrimination. Next, tapered lasers are considered, with an emphasis on emerging device technologies for applications requiring electrical modulation and high spectral brightness.

Narrow-line coherently combined tapered laser diodes in a Talbot external cavity with a volume Bragg grating

We present the phase locking of an array of index-guided tapered laser diodes. An external cavity based on the self-imaging Talbot effect has been built. A volume Bragg grating is used as the output coupler to stabilize and narrow the spectrum at 976nm. A power of 1.7W is achieved in the in-phase single main lobe mode with a high visibility. We have checked that each emitter is locked to the Bragg wavelength with a 100pm spectrum linewidth. The experimental results compare well with numerical simulations performed with two-dimensional wide-angle finite difference beam propagation method.

Nd :GdCOB: overview of its infrared, green and blue laser performances

Abstract GdCOB (Ca4GdB3O10) is a new non-linear optical material which can be grown in large sized crystals by the Czochralski method. In this material, part of the gadolinium ions can be replace by Nd3+ without significant decrease of the crystal quality. Nd:GdCOB single crystals exhibit laser action in the infrared at 1060 and 1091 nm simultaneously. The two transitions involved originating from the two Kramers doublets of the 4F3/2 state, their respective contributions depend upon the temperature of the Nd:GdCOB crystal. By combining the non-linear properties of the GdCOB matrix and the laser emission due to ND3+ ions, it is possible to generate directly by self-frequency doubling 115 mW of green laser light at 530.5 nm under diode pumping at 810 nm. It is also possible to self-double the 936 nm emission of Nd:GdCOB, leading to blue laser emission, but only under pulsed titanium-sapphire laser pumping. However, a CW blue laser beam is obtained in Nd:GdCOB by self sum-frequency mixing of the 1090 nm laser emission and the residual pump beam at ∼812 nm, yielding 1.2 mW of 465 nm laser light, with an optical to optical efficiency of 0.3%. This paper ends with a survey of the Nd:YCOB laser characteristics which are compared to those of Nd:GdCOB.

Diode-pumped self-frequency-doubling Nd:GdCa 4 O(BO 3 ) 3 lasers: toward green microchip lasers

An output power of 115 mW at 545 nm has been obtained from a diode-pumped self-frequency doubling Nd:GdCa4O(BO3)3 laser in a stable cavity. The infrared emission of the laser was found to be 1091 nm, not the 1060 nm that was expected when the highest line of the fluorescence spectrum was considered. We have demonstrated that the emission at 1091 nm was caused by the temperature increase at the focus of the pump beam. We demonstrated also, for what we believe was the first time, lasing operation of Nd:GdCOB in a plano–plano cavity and obtained an output power of 22 mW at 545 nm. To our knowledge, this is the highest output power ever reported with a self-frequency-doubling crystal in a plano–plano cavity.

Thermal lensing measurements in diode-pumped Yb-doped GdCOB, YCOB, YSO, YAG and KGW

A Shack–Hartmann wavefront sensor was used to measure thermal lensing in diode-end-pumped Yb-doped GdCOB, YCOB, YSO, YAG, and KGW crystals, under lasing or nonlasing conditions. Measured thermal lenses are aberration-free, and their focal lengths under lasing action range from 40 to 140 mm for 5 W of absorbed power. When laser action was inhibited the thermal lens dioptric power was increased significantly in most crystals, supplying evidence that nonradiative mechanisms exist. Reduction of thermal effects in a composite YCOB crystal is also investigated, as well as the dependence of thermal lensing on the emission wavelength in YSO. 2002 Elsevier Science B.V. All rights reserved.

Diffraction-limited polarized emission from a multimode ytterbium fiber amplifier after a nonlinear beam converter

The multimode and depolarized output beam of a highly multimode diode-pumped Yb-doped fiber amplifier is converted to a diffraction-limited, linearly polarized beam by a self-referencing two-wave-mixing process in an infrared-sensitive photorefractive crystal (Rh:BaTiO3). As much as 11.6 W of single-mode output is achieved with a 78% multimode-to-single-mode photorefractive conversion efficiency.

Coherent Dual-Frequency Emission of a Vertical External-Cavity Semiconductor Laser at the Cesium

We describe the dual-frequency and dual-polarization emission of a diode-pumped vertical external-cavity semiconductor laser at 852 nm dedicated to the coherent population trapping of cesium atoms. The output power reaches ~20 mW on each frequency, with a frequency difference in the gigahertz range.