Multimode laser-diode pumped continuous-wave stoichiometric Yb<inf>3</inf>Al<inf>5</inf>O<inf>12</inf> laser

Abstract

Summary form only given. In recent years, ytterbium-ion (Yb)-doped materials have been recognized having attractive potential for laserdiode (LD) pumped solid state lasers. One of the features of the Yb-doped materials is that it can incorporate very high concentrations of the Yb-ions with low concentration quenching. Highly laser ion-doped materials with short absorption length permit the use of thin gain media with high pump absorption efficiency. The thin media are useful for high efficiency oscillation by high intensity pumping, i.e., LDs with low beam quality can be used as pump sources with high mode-matching efficiency between the laser beam and the pump beam. For example at room temperature, the highest efficiencies, which close to the laser quantum limit of 91% and are the highest in the lasers to our knowledge, are obtained by continuous-wave (CW) microchip Yb:YAG lasers with a high ion concentration of 20 at.% by high intensity pumping [1]. Although the pump source was a Ti:sapphire laser, these results suggest that these efficiencies can be also obtained from LD pumped CW lasers with higher Yb-ion concentration materials, such as the stoichiometric Yb-doped materials, in which Yb-ion concentration is the highest in the Yb-doped materials.Until now, laser oscillations of only two stoichiometric Yb materials have been reported [2-5]. On YbAG, which is one of the stoichiometric material, it has several advantages. The thermal conductivity of the YbAG is two times larger than that of the other. The fractional thermal population at the lower laser level, which is corresponding to the reabsorption loss, of the YbAG is much smaller than that of the other because of higher energy separation of the lower laser level and the ground state. The YAG is optically isotropic, stable, and robust compared to the other Yb-doped host materials. In previous studies, we have achieved continuous-wave YbAG laser oscillations by using a microchip gain material to minimize the reabsorption loss by optimum thickness [4]. Although the pump source was a Ti:sapphire laser, the pump wavelength of 937 nm is suitable for LD pumping. Here, we present a continuos-wave microchip stoichiometric YbAG laser at room temperature by multimode LD pumping [5]. It is the first report for the continuous-wave oscillation of stoichiometric Yb laser by LD pumping, in our knowledge. We used a microchip YbAG laser gain material which was also used in the previous experiment [4]. The pump source was a low-brightness, multi-mode single emitter LD (OSRAM) with an emitter area of 1 μm × 200 μm. When the output power of the LD was 720 mW, the wavelength was 940 nm. The LD beam was focused in the laser crystal with a spot area of 30 μm × 150 μm. The pump absorption efficiency was 94%. In this experiment, the maximum pump intensity in the laser crystal was 19 kW/cm2 for the absorbed pump power. The peak wavelength of the lasing was 1079 nm which is a similar result in the previous experiment by Ti:sapphire laser pumping [4]. Figure 1 shows the output power of the YbAG laser by multi-mode LD pumping and Ti:sapphire laser pumping as a function of the incident pump power. The lasing threshold intensity for the absorbed pump power was around 4 kW/cm2. The maximum slope efficiency and the maximum optical-to-optical efficiency were 13% and 10%, respectively for the absorbed pump power. These efficiencies are low compared to those of the Ti:sapphire-laser pumped laser due to its low mode-matching efficiency andhigh lasing threshold. The optical-to-optical and slope efficiencies will be increased by multi path pumping schemes, which are usually used in thin-disk lasers, and precise compensation of thermo-optic effects.