Simple stable laser resonators with a single Yb:YAG thin disk module have been designed and demonstrated to produce up to 5 kW CW laser output at 1030 nm with <i>M<sup>2</sup></i> factor of 7. Pumped with 940 nm diodes, the optical-to-optical efficiencies were >50 % at full power. Simple I and V-shaped resonators consisting of only two and three optical elements were implemented, including the 16 mm diameter Yb doped thin disk acting as an active mirror. No additional adaptive optics for aberration or mode control was used; instead the results were achieved with laser cavity designs that take into account the changing radius of curvature of the pumped thin disk. The designs ensured the laser always operated well within the stable cavity zone and with an optimised and relatively large fundamental laser mode size on the thin disk. The low optical aberrations and effective thermal management of the thin disk, mounted on a diamond cooled heat sink, together with the above cavity design approach, enabled the realization of such high power and good beam quality thin disk laser in a simple single disk laser oscillator.
Recently, we have obtained a 23.5 W of 2-micrometers intracavity OPO output which is, to the best of our knowledge, the highest power from an intracavity OPO reported in the literature. To achieve such high average power 2-micrometers OPO output in a simple and compact laser system, we have adopted the diffusion-bonded walk-off compensated (DBWOC) KTP OPO pumped by the anisotropy Nd:YALO laser. The walk-off compensated twin KTP crystals reduce the aperture effect due to Poynting's walkoff in the critically phase-matched parametric generation. At the same time, it increases the acceptance angle for the nonlinear interaction, resulting in more efficient OPO conversion. In addition, the diffusion-bonded configuration eliminates the optical losses at the in/out facets and the need for alignment of the crystals. In order to low down the OPO threshold and increase the effective gain of KTP OPO, we bonded two pairs of crystals together. In this paper, we will compare the recent results of the 2-micrometers KTP OPO results with different pairs of DBWOC KTP OPO. With two pairs of DBWOC KTP device, we observed 78% higher 2-micrometers average output power compared to one pair of KTP device.
We demonstrated a 120-W side-pumped Tm:YAG laser with compound parabolic concentrators (CPC's) to couple the pump light into the laser rod. The optical-to-optical efficiency of this laser is 25.2% and the slope efficiency is 31.2%. At such high average power operation, we encountered severe thermal lensing in our Tm:YAG laser rod which prevented us from increasing the diode pump power due to the thermal rollover as the laser cavity become unstable. In this paper, Temperature was measured using IR camera. Temperature and stress distributions are obtained using finite element method. Those data can be used to estimate the fracture limit, the thermal lensing, the thermal distortion of the Tm:YAG laser and subsequently correct the thermal distortion using diffractive optical devices etc.
In this paper, we report a compact mid-IR intracavity OPO, which has 4.1 W of 3.5-micron output from a non-critically phase-matched (NCPM), type II, KTiOAsO4 (KTA) optical parametric oscillator (OPO). This KTA OPO was pumped within the cavity of a Q-switched diode-pumped Nd:YALO laser operating at 10 kHz. We adopted the simplest configuration with a compact diode-pumped Nd:YALO module pumping the singly resonant KTA OPO. Besides 4.1 W of 3.5 um, 10.9 W of 1.5 micron and 11.3 W of 1-micron radiation were obtained simultaneously.
A comparative study on GaN/sapphire has been performed by transmission electron microscopy (TEM) and IR reflectance (IR). TEM observations reveal that both the undoped and Si doped GaN epilayers have large density of threading dislocations. Dislocations in the undoped GaN tend to from open core structure, while dislocation lines in the Si-doped GaN are very sharp and the strain contrast is much more confirmed. It is believed that Si-doping causes the increase in undoped GaN to much more confirmed dislocation lines. Frank dislocation loops are also found lined up at a depth of about 110 +/- 10 nm from the interface. High resolution TEM study also reveals that the GaN buffer layer grown at low temperature has transformed into its thermodynamically stable wurtzite structure during the high temperature post- buffer GaN epilayer growth process. The comparative IR reflectance hows the corresponding behavior. The interference fringes of the Si doped sample, compared with the undoped ones, shows a contrast damping and reflectance reduction behavior, suggesting the presence of a transition/defect layers near the interface.