160 W laser-diode end-pumped Nd:YVO4 slab laser with positive branch hybrid resonator
Authors: Yuan Jia-Hua  , Zhang Heng-Li  , Mao Ye-Fei  , Hao Xiang-Long  , Xing Ji-Chuan  , Xin Jian-Guo  ,   School of Optoelectronics, Beijing Institute of Technology, Beijing 100081, China
Published: December 2020

In this paper, we present an experiment on a continuous-wave Nd:YVO4 Innoslab laser diode-pumped at 808 nm. The LD stack is composed of six bars, with the central wavelength fixed at 808 nm by adjusting the cooling water temperature. The emission from each diode laser bar is individually collimated by micro lens, which is coupled into a coupling system. The coupling system includes a planar waveguide, four cylindrical lenses and a spherical lens. The planar waveguide is used to shape the emitting beams of LD to obtain uniform distribution. The coupling system leads to a pump power loss of ~12%. By the coupling system, we obtain a homogeneous pumping line of ~0.4 mm22 mm coupled into the 0.3 at. % Nd:YVO4 (22 mm10 mm1 mm) crystal. The Nd:YVO4 crystal is a-cut with c axis along 22 mm direction. Indium foil is used for uniform thermal contact and cooling. The laser crystal is mounted between two water-cooled copper heat sinks with two large faces 22 mm10 mm. The heat conduction inside the laser crystal is quasionedimensional. The two 22 mm1 mm surfaces are polished and antireflectioncoated for the pump light and the laser light. Temperature of LD stack and laser crystal are controlled by cooling circulating water. The resonator consists of the input mirror (M1) and the output mirror (M2). M1 is a concave mirror with a radius of R1=500 mm, which is coated for high refection (HR) at 1064 nm and high transmission (HT) at 808 nm. The output mirror (M2) is a cylindrical mirror with a radius of R2=-350 mm, which is coated for HR at 1064 nm. M2 is cut and polished at one edge where the large beam exits. M1 and M2 constitute a stable resonator in vertical direction and off-axis unstable positive confocal resonator in the horizontal direction. In theory, the length of the resonator is L=(R1+R2)/2=75 mm. In fact, the length of the resonator is the same as the theoretical value. The equivalent transmission of the resonator is T=1-|R2/R1|=30%. At a pumping power of 462 W, a maximum power of 160 W continuous wave laser output is obtained, with the stability being 2.6%. Considering 88% of the coupling efficiency and 95% of absorbed efficiency, the optical-to-optical efficiency and slope efficiency are 41.5% and 47.7%, respectively. When the output power is 145 W, the beam quality M2 factors in the stable direction and unstable direction are 2.21 and 1.37, respectively. With the help of the ANSYS software, the temperature distribution in the crystal at the pumped power of 462 W is demonstrated. The temperature distributions are analogous to exponential decays in the Z-direction and parabola decay in the Y-direction, respectively. The maximum temperature difference is 71.6 K in our experiment. The thermal lens is negligible in the unstable direction because the temperature distribution is uniform. That is why the Innoslab laser is beneficial to the power scaling, as it keeps the power density constant, and enlarges the size of gain medium in the unstable direction to inject bigger power to obtain a higher power output, and maintain the constant beam quality.