⇦Chinese Physics Letters, 2019, Vol. 36, No. 4, Article code 044204 The 10 kW Level High Brightness Face-Pumped Slab Nd:YAG Amplifier with a Hybrid Cooling System Shuai Li (李帅)1,3, Ya-Ding Guo (郭亚丁)1,3, Zhong-Zheng Chen (陈中正)1, Lin Zhang (张林)1,3, Ke-Ling Gong (龚柯菱)1,3, Zhi-Feng Zhang (张志峰)1,3, Bao-Shan Wang (王保山)1, Jian Xu (徐健)1, Yi-Ting Xu (徐一汀)1**, Lei Yuan (袁磊)1, Yang Kou (寇洋)1, Yang Liu (刘杨)1, Yan-Yong Lin (林延勇)1,2, Qin-Jun Peng (彭钦军)1**, Zu-Yan Xu (许祖彦)1,2 Affiliations 1Key Lab of Solid State Laser, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 2Key Lab of Function Crystal and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 3University of Chinese Academy of Sciences, Beijing 100190 Received 31 January 2019, online 23 March 2019 **Corresponding author. Email: xuyiting1984@sina.com.cn; pengqinjun@163.com Citation Text: Li S, Guo Y D, Chen Z Z, Zhang L and Gong K L et al 2019 Chin. Phys. Lett. 36 044204    Abstract We demonstrate a high power, high brightness, slab amplifier based on face-pumped Nd:YAG slab gain modules, having a high efficient hybrid cooling system of the conduction cooling and forced convection cooling. Using a single gain module, a laser output power up to 4.5 kW with a remarkable optical-optical conversion efficiency of 51% is realized, indicating an excellent lasing performance of the Nd:YAG slab module. The amplifier operates at a repetition rate of 700 Hz and delivers a maximum average output power exceeding 10.5 kW with pulse duration of 150 μs. A good beam quality factor is measured to be $\beta=1.9$. To the best of our knowledge, this is the highest brightness for a 10 kW level Nd:YAG slab amplifier. DOI:10.1088/0256-307X/36/4/044204 PACS:42.55.Xi, 42.60.Da, 42.60.Lh, 42.15.Dp © 2019 Chinese Physics Society Article Text Microsecond solid-state lasers with high brightness and high peak power are especially expected in manufacturing fields and scientific researches.[1,2] However, the extensively used multi-rod-like lasers mostly only provide a power range of 1–2 kW, while further power scaling of the beam quality decreases nonlinearly due to the thermal effects for solid-state lasers.[3-5] The zigzag slab lasers offer several advantages in output power and beam quality compared with rod configuration because of their availability of higher cooling capability with larger aperture and averaging the thermo-optic effects in the direction perpendicular to total internal reflection (TIR) surface by zigzag optical path. Laser diode (LD) edge-, end- and face-pumped configurations are widely used as the pump schemes for the zigzag slab lasers. For instance, Rutherford et al. demonstrated an edge-pumped Nd:YAG zigzag slab laser with an optical-optical efficiency of 42.3% but a low output power of 127 W.[6] Liu et al. built an end-pumped Nd:YAG slab amplifier chain, delivering an output power of 5.2 kW with a beam quality of 4 times the diffraction limit at a repetition rate of 1 kHz and pulse width of 200 µs. However, its optical efficiency was only 29%.[7] Among these schemes, face-pumped slab has the advantage of large pump and cooling surface, enabling homogeneous pump and high efficient thermal management. In 2013, Chen et al. reported an LD face-pumped Nd:YAG zigzag slab laser with a 2.51 kW output power at 1064 nm corresponding to an optical efficiency of 37.5%.[8] In 2015, Chen et al. developed a master oscillator power amplifier (MOPA) system with a face-pumped Nd:YAG crystal slab, which created a maximum 8.2 kW of output power, operating at a repetition rate of 400 Hz with a pulse duration of 200 µs and beam quality factor of $\beta =3.5$.[9] To date, the face-pumped slab schemes for high power system are mainly using the forced convection cooling systems, i.e., the coolant flows over both the top and bottom large faces of the slab symmetrically.[8-11] However, the heat transfer capacity is limited by the heat transfer coefficient at the interface of coolant and slab, which imposes restrictions on power scaling potential of the laser gain module. Instead, the conduction cooling could provide a higher heat transfer capacity using a metal heat sink with much higher thermal conductivity. Also, to design face-pumped configurations, solutions to improve the pump uniform and to reduce the thermal of the active medium, careful management of the thermally induced slab aberrations and feasibility must be considered. In this Letter, LD face-pumped Nd:YAG crystal slab gain modules with a novel cooling system are developed firstly. The slab module, using both the conduction cooling and forced convection cooling, i.e., hybrid cooling approach, is optimized with uniform pump and uniform cooling simultaneously. Theoretical simulation on the temperature, thermal stress, and deformation distributions of the slab, as well as the beam wavefront distortion are performed with the finite element method (FEM) under the given pumping conditions. Based on a single slab module, a maximum laser output power of 4.5 kW is achieved, corresponding to an optical-optical conversion efficiency of 51%. As we know, this is the highest conversion efficiency for a single Nd:YAG slab module to date. Based on these modules, an MOPA system is built and delivers a maximum average power of 10.5 kW with a pulse duration of 150 µs at a repetition rate of 700 Hz. With the use of a dynamically feed-backed optical aberration compensation device (OACD) and an adaptive optics (AO) system, the beam quality factor is measured to be $\beta=1.9$.
cpl-36-4-044204-fig1.png
Fig. 1. Schematic diagram of the face-pumped slab gain module. LDA: laser diode arrays. Shaping elements: two aspherical cylindrical lenses.
Figure 1 shows the schematic diagram of the face-pumped slab gain module. In the experiment, the trapezoidal slab Nd:YAG crystal was 0.6 at.% doped, which has a top face size of 45 mm$\times$133 mm, a bottom face size of 45 mm$\times$139 mm for a thickness of 4 mm and two 56$^{\circ}$ cut edge facets. A hybrid cooling technique, i.e., the combination of the conduction cooling with forced convection cooling, was employed. A thin layer of cooling water was allowed to flow over the gap between top surface of the slab and a fused quartz plate along the length direction, and the pump light went through it. A silicone rubber O-shaped ring was used to seal the cooling water channel carefully with low stress, which is insensitive to the bulging of the slab crystal. The side faces of the slab crystal was bonded with the silicone layer to make thermal insulating, while the bottom surface of the slab was soldered to the micro-channel heat sink carefully with indium solder to maintain high efficient cooling, and the mechanical stability of gain module was improved. In addition, the indium solder could reduce the thermal stress and mitigate the end effect. The LD arrays (LDA) operating at 150 µs with a pulse repetition rate of 700 Hz can provide a total pump power of 10 kW at 808 nm. The pump light from LDA was reshaped by two aspherical cylindrical lenses as the shaping elements and passing through the fused quartz plate, leading to uniform distribution of the pump. The top surface of the slab as the pump surface was coated with antireflection (AR) at 808 nm, while the opposite bottom surface was coated with HR at 808 nm for the double-pass face-pump configuration. The face-pumped region was carefully designed, which has a size of 42 mm $\times$ 104 mm. Uniform distribution of the pump was identified by both the ray tracing method and experiments, as shown in Fig. 2. The intensity distribution of the shaped LD pump beam near the pumping surface of the slab was investigated by measuring a series of data for LD pump power of 8.8 kW. The normalized pump intensity along the length and width directions of the slab was depicted in Fig. 2(b). It was found that the rms fluctuation of the normalized intensity is less than 2%, which is in close agreement with the simulation value rms = 1% (Fig. 2(a)).
cpl-36-4-044204-fig2.png
Fig. 2. Pump intensity distribution near the pump surface of the slab for LD pump power of 8.8 kW: (a) simulated result, (b) measured normalized intensity distribution along the length direction and width direction, respectively.
The wavefront distortion of the laser beam in the slab is connected to thermal gradient, stress and slab deformation, and these can severely influence the laser output power and the beam quality. To assess the hybrid cooling system, the thermal gradient, stress and deformation of the slab were analyzed by use of FEM under a uniform pump condition. Moreover, a probe beam went through the Nd:YAG slab with a zigzag path as the laser beam was defined to evaluate the wavefront distortion with the ray tracing method. The heat generated in the slab was set to 26% under a pump power of 8.8 kW.[12-14] The forced convection heat transfer coefficient was set to 11.5 kW$\cdot$m$^{-1}\cdot$K$^{-1}$ according to experiments. The solder layers were 45 mm$\,\times 115$ mm with a thickness of 1.2 mm, and temperature of the cooling water was maintained at 19.5$^{\circ}\!$C. As a comparison, a conventional symmetrical forced convection cooling structure was also simulated under the same conditions as well. The simulated temperature, thermal stress and deformation distributions of the slab for the hybrid cooling and conventional symmetrical forced convection cooling are shown in Fig. 3. For hybrid cooling, as shown in Fig. 3(a), the average temperature in the pump region is about 45$^{\circ}\!$C, and the maximum stress is only 55 MPa with almost homogeneous stress distribution. The slab deformation is no more than 3.5 µm, which means that the end effect is mitigated. However, for the symmetrical forced convection cooling, as shown in Fig. 3(b), the average temperature is about 50$^{\circ}\!$C, the maximum stress is 70 MPa, and the inhomogeneous stress distribution occurs at the slab edges due to the mechanical seal. The maximum slab deformation is 7.4 µm and shows a serious end effect. The simulated results indicate that the proposed hybrid cooling approach maintains uniform cooling ensuring high efficiency. As a consequence, the wavefront distortion of the laser beam, arising from thermal gradient, thermal stress and deformation could be greatly reduced, leading to high beam quality and high efficiency lasing. Figures 4(a) and 4(b) show the calculated 3D wavefront of the probe beam through the Nd:YAG slab gain module at 8.8 kW pump power and the wavelength $\lambda$ is 1064 nm. Compared to the forced convection cooling design, the thermal induced wavefront distortion is improved significantly for hybrid cooling, particularly the PV and rms values reduced from 4.6$\lambda$ to 3.18$\lambda$ and from 1.14$\lambda$ to 0.76$\lambda$, respectively. All the above-mentioned results confirm that the uniform pump and the optimized hybrid cooling system lead to small slab deformation and lower thermal induced wavefront distortion of the laser beam, resulting in a promising power scaling potential with good beam quality simultaneously.
cpl-36-4-044204-fig3.png
Fig. 3. Simulated distributions of the temperature and stress over the middle cross section at half thickness of the slab, and the slab deformation distribution with (a) hybrid cooling and (b) forced convection cooling.
cpl-36-4-044204-fig4.png
Fig. 4. Calculated 3D wavefront of the probe beam through a Nd:YAG slab gain module at 8.8 kW pump power: (a) hybrid cooling, PV = 3.18$\lambda $ and rms = 0.76$\lambda $, (b) forced convection cooling, PV = 4.6$\lambda $ and rms = 1.14$\lambda$.
To evaluate the lasing performance of the face-pumped slab Nd:YAG module, we made a laser oscillator based on a single gain module. Figure 5(a) shows the schematic diagram of the laser oscillator with a cavity consisting of a high reflection (HR) plane-concave rear mirror ($R=-800$) and a plane output coupler ($T=60{\%}$). The two cut-edge facets were AR coated at 1064 nm. The laser beam traveled near normal incident to the edge facets and propagated along a zigzag path in the slab. About 90% of the pump light was absorbed in the double pass configuration. At the pump power of 8.8 kW, a maximum output power of 4.5 kW was obtained, corresponding to an optical-optical conversion efficiency of 51%. As far as we know, it is the highest conversion efficiency reported for a single slab Nd:YAG module oscillator. The measured power fluctuation was less than $\pm$0.5% over 30 min at full output power, as shown in the inset of Fig. 5(b).
cpl-36-4-044204-fig5.png
Fig. 5. (a) Schematic diagram of slab gain module test with a single module laser oscillator. (b) Measured laser output power versus LD pump power, and the upper left inset: output stability measurement at 4.5 kW.
cpl-36-4-044204-fig6.png
Fig. 6. Experimental setup of the slab Nd:YAG MOPA configuration. Seed, TEM$_{00}$ oscillator, RS, re-shaper, M1–M12, high reflection mirrors, SF1–SF5, spatial filters, Am1–Am4, slab amplification modules, OACD, optical aberration compensation device, AO, adaptive optics.
cpl-36-4-044204-fig7.png
Fig. 7. Measured MOPA output power versus total pump power. The upper left inset: output power stability of the MOPA system at 9.7 kW. The upper right inset: typical far-field beam intensity distribution of 10.5 kW. The lower right inset: pulse train of amplifier at 700 Hz.
Figure 6 shows the experimental setup of the 1064 nm Nd:YAG MOPA configuration, similar to the previous reports.[15-17] Briefly, the MOPA comprised of a TEM$_{00}$ mode Nd:YAG oscillator, two-stage single-pass rod Nd:YAG preamplifiers, four-stage slab Nd:YAG main amplifiers (Am1–Am4) by employing the home-made face-pumped slab Nd:YAG modules, twelve flat mirrors (M1–M12) coated with high-reflective film at 1064 nm, five spatial filters (SF1–SF5), a dynamically OACD and an AO system. Firstly, the 80 W seed was produced by a Nd:YAG oscillator. The seed laser operated at a repetition rate of 700 Hz with 150 µs pulse duration. Then, preamplifiers produced 180 W of output power under a pump power of 300 W. Following that, the pulse was amplified by the four stage Nd:YAG slab main amplifiers Am1 to Am4, successively. To improve the energy extraction efficiency and to compensate for the thermal effect, the first and second stages were operated with the double-pass amplification, then followed two-stage single-pass amplifiers. Figure 7 shows the MOPA output power measured by a calibrated power meter (OPHIR NOVAII/30K-W-BB-74). The amplified power increased with the pump power linearly, up to 10.5 kW under a pump power of 30.5 kW, corresponding to an optical-optical conversion efficiency of 35% and shows no saturation. The upper left inset of Fig. 7 shows the output power stability measurement at 9.7 kW and the output power fluctuation was less $\pm$1% over 500 s. The upper right inset in Fig. 7 shows the typical far-field intensity distribution of the amplified beam at 10.5 kW of output power and the $\beta$ factors were calculated to be $\beta=1.9$. As well know, the brightness is proportional to laser power $P$ and inversely proportional to the square of the beam quality factor.[4] The brightness achieved in this work was enhanced over more four times higher than the previous best result (8.2 kW output power and $\beta=3.5$).[9] The lower right inset of Fig. 7 shows the typical pulse trains, and the pulse width was found to be about 150 µs with a repetition rate of 700 Hz. In summary, an excellent Nd:YAG crystal slab gain module has been developed with uniform pump and uniform cooling for homogenized thermal stress and deformation. Theoretical simulations and experimental investigations have been performed, verifying the superiority of the hybrid cooling approach. A maximum output power of 4.5 kW is obtained with a single slab laser oscillator under 8.8 kW pump power, corresponding to an optical-optical conversion efficiency of 51%. With such high performance amplification gain modules, up to 10.5 kW of output power is achieved by an MOPA configuration. Furthermore, a good beam quality factor of $\beta=1.9$ was obtained with the OACD and AO system. To the best of our knowledge, this is the highest brightness reported for a 10 kW level face-pumped microsecond Nd:YAG slab amplifier. Further power scaling can be expected by optimizing pump source and the MOPA parameters. References Pulsed YAG laser welding of ODS alloysHigh-power QCW microsecond-pulse solid-state sodium beacon laser with spiking suppression and D_2b re-pumpingQuasi-continuous-wave birefringence-compensated single- and double-rod Nd:YAG lasersCompact high-efficiency 100-W-level diode-side-pumped Nd:YAG laser with linearly polarized TEM_00 mode outputA highly efficient and compact long pulse Nd:YAG rod laser with 540 J of pulse energy for welding applicationYb:YAG and Nd:YAG edge-pumped slab lasersThe 5.2 kW Nd:YAG Slab Amplifier Chain Seeded by Nd:YVO 4 Innoslab LaserHigh-efficiency high-power QCW diode-side-pumped zigzag Nd:YAG ceramic slab laser82 kW high beam quality quasi-continuous-wave face-pumped Nd:YAG slab amplifierDiode-double-face-pumped Nd:YAG ceramic slab laser amplifier with low depolarization lossDiode-pumped large-aperture Nd:YAG slab amplifier for high energy nanosecond pulse laserThermal optics distortion of face pump slab lasers with two pump arrangementsHeat generation in Nd:YAG at different doping levelsZigzag slab lasersActively compensation of low order aberrations by refractive shaping system for high power slab lasersSpatial filters for high-peak-power multistage laser amplifiersEnhancement of the beam quality of non-uniform output slab laser amplifier with a 39-actuator rectangular piezoelectric deformable mirror
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