Study of the mechanical properties of Nd:YVO4 crystal by use of laser interferometry and finite-element analysis

Improved phase-locking of laser arrays by pump shaping

We introduce a method to enhance the phase-locking quality and duration of an end-pumped laser array by precisely shaping its pump beam to overlap with the array. Shaping the pump beam results in a significant improvement in lasing efficiency and reduces the pump power required to reach the lasing threshold compared to a typical uniform pumping configuration. Our approach involves shaping a highly incoherent laser beam by addressing smaller segments of the beam with higher local spatial coherence. We demonstrate a remarkable increase in the laser array output brightness by up to a factor of 10, accompanied by a substantial extension in the phase-locking duration.

11-watt single-frequency 1342-nm laser based on multi-segmented Nd:YVO4 crystal

High power continuous-wave (CW) single-frequency 1342 nm lasers are of interest for fundamental research, particularly, for laser cooling of lithium atoms. Using the popular Nd:YVO₄ laser crystal requires careful heat management, because strong thermal effects in the gain medium are the most severe limitations of output power. Here, we present a multi-segmented Nd:YVO₄ crystal design that consists of three segments with successive doping concentrations, optimized using a theoretical model. In order to quantify the optimization, we measured the thermal lens power of conventional crystal designs and compare them to our multi-segmented design. The optimized design displays a two times lower thermal lens dioptric power for the same amount of absorbed pump power in the non-lasing case. Using the optimized design, we demonstrate a high power all-solid-state laser emitting 10.0 W single-frequency radiation at 1342 nm when operating the laser crystal at room temperature. Further integration of the laser allows us to operate the laser crystal below room temperature for improving output power up to 11.4 W at 8°C. This is explained by the reduction of energy-transfer upconversion and excited-state absorption effects. Stable free-running operation at the low temperature of 8 °C is achieved with the power stability of ± 0.42 % by peak-to-peak fluctuation and frequency peak-to-peak fluctuation of ± 72 MHz in three hours.

Thermal stress and end-bulging in monoclinic crystals: the case study of double tungstates

An analytical description of the thermal stress and end-bulging component of thermal lensing is presented for monoclinic laser crystals, taking into account the anisotropy of their optical, thermal, and elastic properties for the first time, to the best of our knowledge. The geometry of longitudinal diode-pumping (plane stress approximation) is considered. The developed approach is applied to the monoclinic double tungstates (MDTs), Yb:KGW, Yb:KYW, and Yb:KLuW, yielding values of normal and shear stresses, tensile stress, and the end-bulging term. We show that for low-symmetry crystals, end-bulging is responsible for the astigmatism of the thermal lens. The geometry of this astigmatism is explained in terms of the principal meridional planes. We show that the crystal cut of the MDTs along the optical indicatrix axis N_g (propagation direction) provides low thermal stress and a purely positive and weakly astigmatic thermal lens, which is a key condition for power scaling and diffraction-limited laser output.

Highly efficient 12 W diode-pumped actively Q-switched Yb:KGd(WO4)2 laser

A compact diode-pumped actively Q-switched Yb:KGW laser is demonstrated with an optical-to-optical efficiency of 50%. In a 3-mirror laser cavity configuration output power of 12.2 W with repetition rate up to 50 kHz and pulse duration of 10-24 ns was obtained. The maximum pulse peak power of 70 kW was achieved. The laser output beam profile was Gaussian up to maximum pump powers with M2 factor less than 1.2.

Analytical solutions for anisotropic time-dependent heat equations with Robin boundary condition for cubic-shaped solid-state laser crystals

The problem of finding analytical solutions for time-dependent or time-independent heat equations, especially for solid-state laser media, has required a lot of work in the field of thermal effects. However, to calculate the temperature distributions analytically, researchers often have to make some approximations or employ complex methods. In this work, we present full analytical solutions for anisotropic time-dependent heat equations in the Cartesian coordinates with various source terms corresponding to various pumping schemes. Moreover, the most general boundary condition of Robin (or impedance boundary condition), corresponding to the convection cooling mechanism, was applied. This general condition can be easily switched to constant temperature and thermal insulation as special cases. To this end, we first proposed a general approach to solving time-dependent heat equations with an arbitrary heat source. We then applied our approach to explore the temperature distribution for three cases: steady-state pumping or long transient, single-shot pumping or short transient, and repetitively pulsed pumping. Our results show the possibility of an easier and more accurate approach to analytical calculations of the thermal dispersion, thermal stresses (strains), thermal bending, thermal phase shift, and other thermal effects.

Measurement of thermal lensing in a CW BaWO4 intracavity Raman laser

The thermal lens induced in an a-cut BaWO(4) crystal by stimulated Raman scattering is measured using lateral shearing interferometry. The strength of the lens is proportional to the Stokes output power. For light polarized parallel to the a-axis, and a Stokes mode radius of 120 μm, the lens is negative and highly astigmatic: -0.8 D W(-1) in the plane parallel to the a-axis and -7.7 D W(-1) in the plane parallel to the c-axis. The implications of this thermal lens for Raman laser design are discussed.

Comparative investigation on performance of acousto-optically Q-switched dual-rod Nd:YAG-Nd:YVO(4) laser and dual-rod Nd:YVO(4)-Nd:YVO(4) laser

We demonstrated an acousto-optically (AO) Q-switched dual-rod laser with different gain crystals in the cavity, the combination of Nd:YAG crystal and Nd:YVO(4) crystal. An AO Q-switched dual-rod Nd:YVO(4)-Nd:YVO(4) laser was also configured for comparative investigation. A 69 and 75?W near diffraction-limited laser in CW operation was obtained for the dual-rod Nd:YAG-Nd:YVO(4) laser and dual-rod Nd:YVO(4)-Nd:YVO(4) laser, respectively. In the Q-switched operation, 500?kHz output with 64.2?W average power and 650?kHz laser with 73.2?W average power was achieved for the Nd:YAG-Nd:YVO(4) laser and Nd:YVO(4)-Nd:YVO(4) laser, respectively. The detailed comparison on the performance of pulse duration, pulse repetition rate, output power, and optical spectrum was also investigated and discussed.

High repetition rate dual-rod acousto-optics Q-switched composite Nd:YVO4 laser

We demonstrate the high-repetition rate acousto-optic Q-switching of a dual-rod Nd:YVO4 laser. Two thermally bonded composite YVO4-Nd:YVO4-YVO4 rod crystals were used as the gain medium to reduce the serious thermal effect. The pump mode and its influence on the laser mode were analyzed, and the effective pump radius was controlled by tuning the spectrum of laser diodes with adjustment on the temperature of laser diodes. Three different configurations of cavity optics were designed and the output performance using these cavities was investigated. In Q-switching operation, 73.2 W TEM00 mode average power at 650 kHz was obtained. The stable Q-switching range was from 80 kHz to 650 kHz with the pulse duration increasing from 17.5 ns to 80 ns. In CW operation, 78 W TEM00 mode and 93 W multi-mode output power was achieved with different cavity designs, corresponding to the optical-optical efficiency of 46.5% and 52.2% respectively.

Thermal properties measurement and laser operation of a new Yb: Gd0.68Lu0.32VO4 crystal

A new series mixed vanadate crystals, Yb:Gd(x)Lu(1-x)VO(4) (x=0.2, 0.35, 0.53, 0.68 and 0.84), were grown by the Czochralski method. The thermal conductivities of these crystals were measured from room temperature to 280?C, and the thermal conductivities of Yb:Gd(0.68)Lu(0.32)VO(4) at room temperature are 6.4 and 6.9 W m(-1)K(-1) along a- and c-axis, respectively. The material constants of Yb:Gd(x)Lu(1-x)VO(4) series crystals were estimated. The properties of Yb:Gd(x)Lu(1-x)VO(4) series crystals were compared with those of Yb:LuVO(4) and Yb:GdVO(4) crystals. Laser output power of 1.25 W at 1031 nm was obtained with a slope efficiency of 26% by use of diode pumping for Yb:Gd(0.68)Lu(0.32)VO(4) crystal.

Analytical solution of the heat equation in a longitudinally pumped cubic solid-state laser

Knowledge about the temperature distribution inside solid-state laser crystals is essential for calculation of thermal phase shift, thermal lensing, thermally induced birefringence, and heat-induced crystal bending. Solutions for the temperature distribution for the case of steady-state heat loading have appeared in the literature only for simple cylindrical crystal shapes and are usually based on numerical techniques. For the first time, to our knowledge, a full analytical solution of the heat equation for an anisotropic cubic cross-section solid-state crystal is presented. The crystal is assumed to be longitudinally pumped by a Gaussian pump profile. The pump power attenuation along the crystal and the real cooling mechanisms, such as convection, are considered in detail. A comparison between our analytical solutions and its numerical counterparts shows excellent agreement when just a few terms are employed in the series solutions.

Continuous-wave laser performance of Nd:LuVO4 crystal operating at 1.34 microm

A laser-diode-array end-pumped Nd:LuVO4 crystal continuous-wave (cw) laser operating at 1.34 microm has been demonstrated. The maximum cw output power of 1.85 W was obtained at the incident pump power of 17 W for a c-cut 0.5 at. % Nd-doped LuVO4 crystal sample, giving the corresponding optical conversion efficiency of 10.88% and a slope efficiency of 13.5%. Laser experiments of Nd3+ concentration of 0.5 and 0.9 at. % a-cut crystal LuVO4 samples were also investigated; due to the strong excited-state absorption of Nd:LuVO4 at 1.34 microm, the output power was limited.

High-power efficient continuous-wave TEM00 intracavity frequency-doubled diode-pumped Nd:YLF laser

We describe experimental results with a diode-pumped, intracavity-doubled cw Nd:YLF laser in multilongitudinal mode and TEM00 spatial transverse mode with a critical phase-matched lithium triborate crystal. Taking into account the thermal effects of Nd:YLF, energy-transfer upconversion, and the thermal fracture limit, we set up a power-scaling model to optimize and design a fundamental diode-pumped Nd:YLF laser. A highly efficient second-harmonic laser was achieved, based on the optimized cavity design. A second-harmonic-generation output power of 20.5 W at a wavelength of 527 nm was obtained at an incident pump power of 60 W, corresponding to an optical-to-optical efficiency of 34.2%. The TEM00 mode green laser operates at a measured M2 parameter of 1.2. The instability of the green laser power is less than +/- 1% RMS.

Characterization and laser performance of a new material: 2 at. % Nd:YAG grown by the Czochralski method

We report on the optical quality and laser performance of Czochralski-grown 2-at. %-doped Nd:YAG. Using a diode pumped laser in an end pumped configuration, we compare the laser performance of this material with the performance of 1-at. %-doped Nd:YAG and 0.7-at. %-doped Nd:YVO4 crystals. Experimental results show the superior performance of 2-at. % Nd:YAG over Nd:YVO4. With a pump power of 25.7 W, a maximum output power of 12.3 W with a slope efficiency of 57% and an optical-to-optical efficiency of 48% were achieved.