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The application of porous foam structure cooling arrangement system for a thin disk laser
Journal Pre-proof The application of porous foam structure cooling arrangement system for a thin disk laser Rui Liu, Yong Tan, Faquan Gong, Xiang Li, Longhui Dai, Junyan Yang, Song Xu, Gang Li
PII:
S0030-4026(19)31321-X
DOI:
https://doi.org/10.1016/j.ijleo.2019.163423
Reference:
IJLEO 163423
To appear in:
Optik
Received Date:
13 June 2019
Accepted Date:
13 September 2019
Please cite this article as: Liu R, Tan Y, Gong F, Li X, Dai L, Yang J, Xu S, Li G, The application of porous foam structure cooling arrangement system for a thin disk laser, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.163423
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
The application of porous foam structure cooling arrangement system for a thin disk laser Rui Liu 1, 2 Yong Tan1* Faquan Gong 2 Xiang Li2 Longhui Dai2 Junyan Yang3 Song Xu3 Gang Li 2* 1School
of Science, Changchun University of Science and Technology, Changchun，Jilin,130039,China
2Key
Laboratory of Chemical lasers, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning, 116024, China 3Shanghai
Aerospace Control Technology institute，Shanghai 201109,China
Abstract: We have studied a porous foam structure cooling arrangement system that has a better
-p
ro of
uniformity than an ordinary mini channel cooling system. It can be seen from the finite element analysis model that the results of disk crystal temperature distribution during the operation of the thin disk laser (TDL) demonstrate a significant advantage in terms of uniformity and heat transfer coefficient. In the comparative experiment, we realized a 150W continuous wave pump with a 24-multipasses pumping structure; the optical-to-optical efficiency reached 49.3%. This proves that the porous foam structure cooling arrangement system has better heat exchange effect and uniformity than the cooling system without porous foam.
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Key Word： Thin disk laser, porous foam, cooling arrangement system，mini channel
1 Introduction
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Owing to quantum defects (e.g. Yb:YAG, 8.7%@1030 to 940) and absorption/scattering losses, high-power solid-state lasers (SSLs) are always accompanied with energy deficiency and thermal effects. In high power SSLs systems, thermal-induced wave front distortion and thermal lensing will degrade the beam quality and ultimately put a limit on their power scaling and laser conversion efficiency. SSLs generate waste heat during the optical pumping. They mainly come from [1]: 1) the energy difference between the pump photons and the emitted fluorescent photons, also known as quantum losses. 2) The quantum efficiency of fluorescence emission is smaller than that of part of the inversion particles activated from low energy level to high energy level during non-radiative relaxation process. 3) The pump source emits a broad spectrum and some of the energy is not absorbed by the gain medium. How to get high average output power with good beam quality is still an open question for these new cooling configurations in the SSLs. The amplified spontaneous emission (ASE) of crystal can be reduced through changing the gain medium shape in SSLs. The TDL[2]is one of the most promising techniques to realize high output power [3], high optical-to-optical efficiency[4], and good beam quality simultaneously[5]. Nowadays, the output power for four disks has scaled to more than 16 kW.[6]Highest output power from single disk is 11.2 kW.[7]In the future, with an output efficiency of 50% and a round-trip loss of 0.25, it is expected to achieve 1 MW laser power output on one disk[8]. But many engineering problems need to be solved. However, the current development of high-power TDL is somehow limited by the cooling structure. The cooling system plays a very important role in determining the implementation of high power SSLs. Spray cooling, 1
microchannel heat sink cooling and jet impingement cooling are three typical cooling methods for high-power SSLs. The research on heat sink microchannel mainly focuses on the optimization design of microchannel structure, size and shape [9]. Chang H.Oh and others [10] studied the jet cooling scheme for high heat flux devices. Fabbri[11] and others used water and FC-40 as cooling medium, and proposed a single-phase cooling scheme. Evelyn N.Wang[12] and others proposed a two phases microfluidic array cooling scheme for VLSI chips. The heat transfer surface is designed as a pit surface which can promote the formation of turbulence, make the boundary layer thinner and increase the heat transfer capacity when the pit is impacted. As a rule of thumb the maximum temperature in the disk should be kept well below 200℃ to
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minimize the re-absorption losses from the lower laser level and keep the small-signal gain at a high enough value. Therefore, a new cooling-configuration is essential. Although the jet-cooling technology can offer higher heat transfer coefficient, it has a large inhomogeneity, especially it is not applicable for the uniform pumped disk laser. For these problems, TDL output power lasers are expected to be improved by mini channels cooling technology. In this paper, the effects of porous foam mini channels cooling system are tested by simulation analysis and experimental research. The results show that the porous foam structure cooling arrangement system of the disk laser can provide better uniformity，it plays role in the TDL with
the large area pump. In addition, the system can achieve a maximum output power of 79.8 W and an optical-to-optical efficiency of 49.3 %.
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2 Simulation section
2.1 Analytical Model（Brinkman-Forchheimer model[13]）for the cooling arrangement system
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We have found that the porous foam has excellent physical properties, and the ratio of the volume of all the pores in the porous body to the total volume of the porous body is over 90%, and has a certain strength and rigidity. After the porous foam is filled in the mini channels, the flow pressure drop is significantly increased due to the intricate microstructure of the foam itself. When the fluid flows out of the porous foam, the fluids mix with each other, and the disturbance area is increased by the influence of the solid skeleton. In turn, the nonlinear effects of the flow increase and the pressure drop increases. This effect becomes more apparent as the velocity of flow increases. The addition of porous foam in the mini channels can effectively enhance the mixing characteristics of the fluid, thereby enhancing the convective heat transfer of the fluid in the channels.
Fig.1. TDL Yb:YAG crystal with porous foam mini channels cooling arrangement system.(a) The porous foam structure cooling arrangement system. (b) Direction of water flowing through the porous foam and the mini channels The Yb:YAG crystal front surface is an anti-reflection (AR) film of 940-1030 nm, back surface is a high reflection (HR) film of 940-1030 nm. It means that the backside of Yb:YAG disk is covered by HR coating . The HR film which is banded to the front of the heat sink is the mirror of 2
the cavity. The metal indium solder can effectively band the HR coating on the back side of the crystal to the diamond heat sink（from Hebei Laser Research Institute，China）. The thickness of the porous foam（Silicon carbide alloy，porosity: > 98 %，volume density: 3.2 g/cm3，heat transfer coefficient : k= 83.6 W／m·K）is 50 mm, the length of the min channel is 50 mm, the width is 2.5 mm, and the heat sink shelter are 34 mm in diameter, the thickness of indium solder is 10 um. The fluid velocity of the porous foam structure cooling arrangement system is 7.566 m / s. In the case of continuous wave pump, the thermal conduction of the crystal is steady-state. In the cylindrical coordinate system, the three-dimensional steady-state heat conduction equation of the heat source in the laser medium is:
(1)
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Here，T is the temperature distribution of the crystal, Kc is the thermal conductivity of the crystal, and q is the heat source density. Due to the circular symmetry of the laser crystal and the heat source, the temperature distribution on the crystal also has the axis symmetry, (2)
If the heat transfer structure is determined ，the temperature distribution and heat transfer
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coefficient on the crystal can be obtained by finite element analysis. Finite element analysis method and finite difference method are two commonly used methods in numerical simulation. In this paper, finite element analysis software ANSYS is used to analyze the model. 2.2 Finite-element analysis for the cooling arrangement system Under the uniform pumping condition, the heat flow density of the crystal is shown as: [16-17]
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(3)
Here, Pabs is the crystal absorption pump power, spot radius,
is the heat rate of crystal,
is the crystal pump
is the crystal length. And the concentration of Yb3+ ion doped in Yb:YAG is
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10%.Table 1 shows the boundary conditions of finite element simulation. After analyzing by finite element, Fig. 2 (a) is a common mini channels cooling arrangement system. It can be seen that the heat transfer direction is transmitted from the right side to the left side. The heat transfer performance of the inlet port is better than that of the water outlet. Fig. 2(b) shows the structure of the mini channels cooling system with the porous foam. In the Fig.2, heat transfer coefficient is (a) between 1.17e5 to 5.05e4 without foam and (b) between 1.026e5 to 2.025e4 with foam. Therefore, with foam heat transfer coefficient is closed to each other. But the cooling arrangement system with the porous foam has better uniformity. Therefore, it can be proved that the foam is of great significance for the improvement of the heat exchange uniformity of the heat sink [18-19].
3
Fig.2. Distribution of the heat convection coefficient（a）Mini channels cooling system without
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porous foam（b）Mini channels cooling system with porous foam Thickness of flow boundary layer decrease can decrease in the mini channels, with filling the porous foam. Therefore, porous foam can enhance the fluid flow blending and improve the uniformity of heat exchange. At the same time, the flow resistance of the fluid in the min channels also increases rapidly, resulting in a relative decrease in the average heat transfer coefficient. The intricate microstructure of the foam itself caused the phenomenon. In order to prove that the cooling device with the porous has excellent performance, we used 200 W thermal load to analyze the temperature distribution of the Yb:YAG crystal. As shown in Fig. 3 temperature distribution of the disks with different thermal load.
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Fig.3 Temperature distribution of the disks with different thermal load As the simulation shows, under the same pump power, the crystal surface temperature of the porous foam mini channels cooling arrangement system is much lower. For a quasi-three-level laser system, the lower the crystal temperature is, the higher optical-optical efficiency will be
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achieved. As Fig.3 shows，With the thermal load increasing, the porous foam cooling arrangement can obtain lower temperature. To verify the correctness of the model for simulation, the performance of the cooling arrangement system was demonstrated by a disk laser system experiment based on a multi-passes TDL platform.
4
3 Experimental
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Fig.4.The experimental equipment TDL module allows 24-mutilpasses pump to achieve sufficient absorption within the thin active medium. As shown in Fig. 4, the pump laser beam from LD with a homogenizing rob (core diameter is 200 μm and numerical aperture is 0.22) passes the four lenses and a parabolic mirror with a focal length of 74.5 mm. It abstain pump spot on the thin disk crystal, which is 2.0 mm in diameter. For the resonator design, a thin disk and an output coupler have already been provided. The thin disk is a flat optic (R = ∞) and the output coupler is a plano-concave 25.4mm diameter lens. The output coupler has a 341 mm radius of curvature with a 97.5 % reflective coating centered at 1030 nm. For optimal results, the resonator is designed to a stable resonator configuration by using a 45mm length linear cavity.
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Fig.5 The experimental measurement results As shown in Fig. 5, when the pump power is increased to 153.5 W, the maximum output power of the porous foam structure cooling arrangement system is 75.8 W and the optical-optical efficiency is 49.3 %. The mini channels cooling system without the porous foam has 61.9 W output power. But it can be seen from the output power curve that when the pump power is about 60 W, optical-to-optical efficiency of the output laser is close. As the pump power increases, the optical-to-optical efficiency of the mini channels heat sink system decreases significantly, which is caused by the excessive decreases temperature of the Yb:YAG crystal. As can be seen from the Fig.5, the TDL with porous foam structure cooling arrangement systems have more excellent performance. Under the action of the porous foam structure cooling arrangement system, the TDL can operate with a high optical-to-optical efficiency.
4 Discussion and conclusions This paper researches the application effect of the porous foam structure cooling arrangement system operating in CW Yb:YAG TDL. The simulation results show that the heat sink with the 5
foam structure has excellent uniformity and good heat transfer performance. It is significant , especially for the end-pumped laser such as the TDL. Ultimately，the experimental and simulation results show that the porous foam structure cooling arrangement system has better uniformity. The cooling arrangement system with the foam mini channels structure obtains 49.3% optical-to-optical efficiency. In the future, higher power pump experiments will be tested under the new cooling arrangement system. After further optimizing the experimental parameters, it is expected to realize a high-power, high-efficiency laser through a single crystal. Acknowledgments: We thank Pingwei Zhang for help with the diamond heat sink for support. This work was supported by Hebei Laser Research Institute，China
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References:
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[1] Le Garrec, B. J., et al. "High-average-power diode-array-pumped frequency-doubled YAG laser."Opt. Lett. 21.24: 1990-1992. (1996) [2] Giesen, Adolf, et al. "Scalable concept for diode-pumped high-power solid-state lasers." Appl. Phys.b 58.5: 365-372. (1994) [3] Blázquez-Sánchez, David, et al. "Improving the brightness of a multi-kilowatt single thin-disk laser by an aspherical phase front correction." Opt. Lett.36.6: 799-801. (2011) [4] Ahmed, Marwan Abdou, et al. "High-power radially polarized Yb: YAG thin-disk laser with high efficiency." Opt. Express. 19.6: 5093-5103. (2011) [5] Piehler, Stefan, et al. "Highly efficient 400 W near-fundamental-mode green thin-disk laser." Opt. Lett..41.1: 171-174. (2016)
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[6] Mende, Jens , et al. "Thin-disk laser – Power scaling to the kW regime in fundamental mode
na
operation." Proc. SPIE the International Society for Optical Engineering.7193.1:50-56. (2009) [7] Feuchtenbeiner, Stefanie , et al. "New generation of compact high power disk lasers." Solid State Lasers XXVII: Technology and Devices . (2018) [8] Speiser J. ''Scaling of thin-disk lasers—influence of amplified spontaneous emission. '' J. Opt. Soc. Am. B. 2009, 26: 26-35. (2009) [9] Husain, Afzal, and Kwang-Yong Kim. "Shape optimization of micro-channel heat sink for micro-electronic cooling." IEEE Trans. Compon. Packag. Technol. 31.2: 322-330. (2008)
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[10] Oh, Chang H., et al. "Liquid jet‐array cooling modules for high heat fluxes." AIChE J. 44.4:
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769-779. (1998) [11] Fabbri, Matteo, Shanjuan Jiang, and Vijay K. Dhir. "A comparative study of cooling of high power density electronics using sprays and microjets." J. Heat Transfer 127.1: 38-48. (2005) [12] Wang, Evelyn N., et al. "Micromachined jets for liquid impingement cooling of VLSI chips." J. Microelectromech. Syst. 13.5: 833-842. (2004) [13] Vafai, K., and C. L. Tien. "Boundary and inertia effects on convective mass transfer in porous media." Int. J. Heat Mass Transfer 25.8:1183-1190. (1982) [14] Vretenar, Natasa. "Room temperature and cryogenic Yb: YAG thin disk laser: single crystal and ceramic." (2012) [15] Shang, Jianli, Xiao Zhu, and Guangzhi Zhu. "Analytical approach to thermal lensing in end-pumped Yb: YAG thin-disk laser." Appl. Opt. 50.32: 6103-6120. (2011)
6
[16] Vorholt, Christian, et al. "Measurement of temperature-dependent absorption and emission spectra of Yb:YAG, Yb:LuAG, and Yb:CaF2 between 20 C and 200 C and predictions on their influence on laser performance." J. Opt. Soc. Am. B 29.9:2493-2502. (2012) [17] H. Ohashi et al., “Enhancement of emitting power density with abeam-shaping technique for a high-power laser-diode array stack,”Opt. Eng. 43(10), 2206–2207 (2004)
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[18] Liu R, Zhang X, Gong F, et al. Research on the adjusting technology of the thin disk laser[J]. Optik. 2017, 157. [19] [1] Li C, Liu R, Gong F, et al. Theoretical calculation and experimental study on absorption properties of Yb: YAG crystal applied in thin disk laser[J]. Optik. 2016: 511-516.
7
Table1. The boundary conditions of finite element simulation Description Numerical Value 20
Diamond thickness/mm Indium tin solder thickness/um
2 10
YAG（ASE cap）thickness/mm
1
Cooling water pressure/bar Air convection coefficient/(W/m2K) Pump power intensity/(kW/cm2) Residual heat power intensity/(kW/cm2) Diameter of the sink /(mm)
2.5 25 4 2 34
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Environment temperature/℃
8
PII:
S0030-4026(19)31321-X
DOI:
https://doi.org/10.1016/j.ijleo.2019.163423
Reference:
IJLEO 163423
To appear in:
Optik
Received Date:
13 June 2019
Accepted Date:
13 September 2019
Please cite this article as: Liu R, Tan Y, Gong F, Li X, Dai L, Yang J, Xu S, Li G, The application of porous foam structure cooling arrangement system for a thin disk laser, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.163423
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
The application of porous foam structure cooling arrangement system for a thin disk laser Rui Liu 1, 2 Yong Tan1* Faquan Gong 2 Xiang Li2 Longhui Dai2 Junyan Yang3 Song Xu3 Gang Li 2* 1School
of Science, Changchun University of Science and Technology, Changchun，Jilin,130039,China
2Key
Laboratory of Chemical lasers, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning, 116024, China 3Shanghai
Aerospace Control Technology institute，Shanghai 201109,China
Abstract: We have studied a porous foam structure cooling arrangement system that has a better
-p
ro of
uniformity than an ordinary mini channel cooling system. It can be seen from the finite element analysis model that the results of disk crystal temperature distribution during the operation of the thin disk laser (TDL) demonstrate a significant advantage in terms of uniformity and heat transfer coefficient. In the comparative experiment, we realized a 150W continuous wave pump with a 24-multipasses pumping structure; the optical-to-optical efficiency reached 49.3%. This proves that the porous foam structure cooling arrangement system has better heat exchange effect and uniformity than the cooling system without porous foam.
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Key Word： Thin disk laser, porous foam, cooling arrangement system，mini channel
1 Introduction
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Owing to quantum defects (e.g. Yb:YAG, 8.7%@1030 to 940) and absorption/scattering losses, high-power solid-state lasers (SSLs) are always accompanied with energy deficiency and thermal effects. In high power SSLs systems, thermal-induced wave front distortion and thermal lensing will degrade the beam quality and ultimately put a limit on their power scaling and laser conversion efficiency. SSLs generate waste heat during the optical pumping. They mainly come from [1]: 1) the energy difference between the pump photons and the emitted fluorescent photons, also known as quantum losses. 2) The quantum efficiency of fluorescence emission is smaller than that of part of the inversion particles activated from low energy level to high energy level during non-radiative relaxation process. 3) The pump source emits a broad spectrum and some of the energy is not absorbed by the gain medium. How to get high average output power with good beam quality is still an open question for these new cooling configurations in the SSLs. The amplified spontaneous emission (ASE) of crystal can be reduced through changing the gain medium shape in SSLs. The TDL[2]is one of the most promising techniques to realize high output power [3], high optical-to-optical efficiency[4], and good beam quality simultaneously[5]. Nowadays, the output power for four disks has scaled to more than 16 kW.[6]Highest output power from single disk is 11.2 kW.[7]In the future, with an output efficiency of 50% and a round-trip loss of 0.25, it is expected to achieve 1 MW laser power output on one disk[8]. But many engineering problems need to be solved. However, the current development of high-power TDL is somehow limited by the cooling structure. The cooling system plays a very important role in determining the implementation of high power SSLs. Spray cooling, 1
microchannel heat sink cooling and jet impingement cooling are three typical cooling methods for high-power SSLs. The research on heat sink microchannel mainly focuses on the optimization design of microchannel structure, size and shape [9]. Chang H.Oh and others [10] studied the jet cooling scheme for high heat flux devices. Fabbri[11] and others used water and FC-40 as cooling medium, and proposed a single-phase cooling scheme. Evelyn N.Wang[12] and others proposed a two phases microfluidic array cooling scheme for VLSI chips. The heat transfer surface is designed as a pit surface which can promote the formation of turbulence, make the boundary layer thinner and increase the heat transfer capacity when the pit is impacted. As a rule of thumb the maximum temperature in the disk should be kept well below 200℃ to
ro of
minimize the re-absorption losses from the lower laser level and keep the small-signal gain at a high enough value. Therefore, a new cooling-configuration is essential. Although the jet-cooling technology can offer higher heat transfer coefficient, it has a large inhomogeneity, especially it is not applicable for the uniform pumped disk laser. For these problems, TDL output power lasers are expected to be improved by mini channels cooling technology. In this paper, the effects of porous foam mini channels cooling system are tested by simulation analysis and experimental research. The results show that the porous foam structure cooling arrangement system of the disk laser can provide better uniformity，it plays role in the TDL with
the large area pump. In addition, the system can achieve a maximum output power of 79.8 W and an optical-to-optical efficiency of 49.3 %.
-p
2 Simulation section
2.1 Analytical Model（Brinkman-Forchheimer model[13]）for the cooling arrangement system
Jo
ur
na
lP
re
We have found that the porous foam has excellent physical properties, and the ratio of the volume of all the pores in the porous body to the total volume of the porous body is over 90%, and has a certain strength and rigidity. After the porous foam is filled in the mini channels, the flow pressure drop is significantly increased due to the intricate microstructure of the foam itself. When the fluid flows out of the porous foam, the fluids mix with each other, and the disturbance area is increased by the influence of the solid skeleton. In turn, the nonlinear effects of the flow increase and the pressure drop increases. This effect becomes more apparent as the velocity of flow increases. The addition of porous foam in the mini channels can effectively enhance the mixing characteristics of the fluid, thereby enhancing the convective heat transfer of the fluid in the channels.
Fig.1. TDL Yb:YAG crystal with porous foam mini channels cooling arrangement system.(a) The porous foam structure cooling arrangement system. (b) Direction of water flowing through the porous foam and the mini channels The Yb:YAG crystal front surface is an anti-reflection (AR) film of 940-1030 nm, back surface is a high reflection (HR) film of 940-1030 nm. It means that the backside of Yb:YAG disk is covered by HR coating . The HR film which is banded to the front of the heat sink is the mirror of 2
the cavity. The metal indium solder can effectively band the HR coating on the back side of the crystal to the diamond heat sink（from Hebei Laser Research Institute，China）. The thickness of the porous foam（Silicon carbide alloy，porosity: > 98 %，volume density: 3.2 g/cm3，heat transfer coefficient : k= 83.6 W／m·K）is 50 mm, the length of the min channel is 50 mm, the width is 2.5 mm, and the heat sink shelter are 34 mm in diameter, the thickness of indium solder is 10 um. The fluid velocity of the porous foam structure cooling arrangement system is 7.566 m / s. In the case of continuous wave pump, the thermal conduction of the crystal is steady-state. In the cylindrical coordinate system, the three-dimensional steady-state heat conduction equation of the heat source in the laser medium is:
(1)
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Here，T is the temperature distribution of the crystal, Kc is the thermal conductivity of the crystal, and q is the heat source density. Due to the circular symmetry of the laser crystal and the heat source, the temperature distribution on the crystal also has the axis symmetry, (2)
If the heat transfer structure is determined ，the temperature distribution and heat transfer
re
-p
coefficient on the crystal can be obtained by finite element analysis. Finite element analysis method and finite difference method are two commonly used methods in numerical simulation. In this paper, finite element analysis software ANSYS is used to analyze the model. 2.2 Finite-element analysis for the cooling arrangement system Under the uniform pumping condition, the heat flow density of the crystal is shown as: [16-17]
lP
(3)
Here, Pabs is the crystal absorption pump power, spot radius,
is the heat rate of crystal,
is the crystal pump
is the crystal length. And the concentration of Yb3+ ion doped in Yb:YAG is
Jo
ur
na
10%.Table 1 shows the boundary conditions of finite element simulation. After analyzing by finite element, Fig. 2 (a) is a common mini channels cooling arrangement system. It can be seen that the heat transfer direction is transmitted from the right side to the left side. The heat transfer performance of the inlet port is better than that of the water outlet. Fig. 2(b) shows the structure of the mini channels cooling system with the porous foam. In the Fig.2, heat transfer coefficient is (a) between 1.17e5 to 5.05e4 without foam and (b) between 1.026e5 to 2.025e4 with foam. Therefore, with foam heat transfer coefficient is closed to each other. But the cooling arrangement system with the porous foam has better uniformity. Therefore, it can be proved that the foam is of great significance for the improvement of the heat exchange uniformity of the heat sink [18-19].
3
Fig.2. Distribution of the heat convection coefficient（a）Mini channels cooling system without
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porous foam（b）Mini channels cooling system with porous foam Thickness of flow boundary layer decrease can decrease in the mini channels, with filling the porous foam. Therefore, porous foam can enhance the fluid flow blending and improve the uniformity of heat exchange. At the same time, the flow resistance of the fluid in the min channels also increases rapidly, resulting in a relative decrease in the average heat transfer coefficient. The intricate microstructure of the foam itself caused the phenomenon. In order to prove that the cooling device with the porous has excellent performance, we used 200 W thermal load to analyze the temperature distribution of the Yb:YAG crystal. As shown in Fig. 3 temperature distribution of the disks with different thermal load.
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Fig.3 Temperature distribution of the disks with different thermal load As the simulation shows, under the same pump power, the crystal surface temperature of the porous foam mini channels cooling arrangement system is much lower. For a quasi-three-level laser system, the lower the crystal temperature is, the higher optical-optical efficiency will be
Jo
achieved. As Fig.3 shows，With the thermal load increasing, the porous foam cooling arrangement can obtain lower temperature. To verify the correctness of the model for simulation, the performance of the cooling arrangement system was demonstrated by a disk laser system experiment based on a multi-passes TDL platform.
4
3 Experimental
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Fig.4.The experimental equipment TDL module allows 24-mutilpasses pump to achieve sufficient absorption within the thin active medium. As shown in Fig. 4, the pump laser beam from LD with a homogenizing rob (core diameter is 200 μm and numerical aperture is 0.22) passes the four lenses and a parabolic mirror with a focal length of 74.5 mm. It abstain pump spot on the thin disk crystal, which is 2.0 mm in diameter. For the resonator design, a thin disk and an output coupler have already been provided. The thin disk is a flat optic (R = ∞) and the output coupler is a plano-concave 25.4mm diameter lens. The output coupler has a 341 mm radius of curvature with a 97.5 % reflective coating centered at 1030 nm. For optimal results, the resonator is designed to a stable resonator configuration by using a 45mm length linear cavity.
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Fig.5 The experimental measurement results As shown in Fig. 5, when the pump power is increased to 153.5 W, the maximum output power of the porous foam structure cooling arrangement system is 75.8 W and the optical-optical efficiency is 49.3 %. The mini channels cooling system without the porous foam has 61.9 W output power. But it can be seen from the output power curve that when the pump power is about 60 W, optical-to-optical efficiency of the output laser is close. As the pump power increases, the optical-to-optical efficiency of the mini channels heat sink system decreases significantly, which is caused by the excessive decreases temperature of the Yb:YAG crystal. As can be seen from the Fig.5, the TDL with porous foam structure cooling arrangement systems have more excellent performance. Under the action of the porous foam structure cooling arrangement system, the TDL can operate with a high optical-to-optical efficiency.
4 Discussion and conclusions This paper researches the application effect of the porous foam structure cooling arrangement system operating in CW Yb:YAG TDL. The simulation results show that the heat sink with the 5
foam structure has excellent uniformity and good heat transfer performance. It is significant , especially for the end-pumped laser such as the TDL. Ultimately，the experimental and simulation results show that the porous foam structure cooling arrangement system has better uniformity. The cooling arrangement system with the foam mini channels structure obtains 49.3% optical-to-optical efficiency. In the future, higher power pump experiments will be tested under the new cooling arrangement system. After further optimizing the experimental parameters, it is expected to realize a high-power, high-efficiency laser through a single crystal. Acknowledgments: We thank Pingwei Zhang for help with the diamond heat sink for support. This work was supported by Hebei Laser Research Institute，China
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References:
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[1] Le Garrec, B. J., et al. "High-average-power diode-array-pumped frequency-doubled YAG laser."Opt. Lett. 21.24: 1990-1992. (1996) [2] Giesen, Adolf, et al. "Scalable concept for diode-pumped high-power solid-state lasers." Appl. Phys.b 58.5: 365-372. (1994) [3] Blázquez-Sánchez, David, et al. "Improving the brightness of a multi-kilowatt single thin-disk laser by an aspherical phase front correction." Opt. Lett.36.6: 799-801. (2011) [4] Ahmed, Marwan Abdou, et al. "High-power radially polarized Yb: YAG thin-disk laser with high efficiency." Opt. Express. 19.6: 5093-5103. (2011) [5] Piehler, Stefan, et al. "Highly efficient 400 W near-fundamental-mode green thin-disk laser." Opt. Lett..41.1: 171-174. (2016)
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[6] Mende, Jens , et al. "Thin-disk laser – Power scaling to the kW regime in fundamental mode
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operation." Proc. SPIE the International Society for Optical Engineering.7193.1:50-56. (2009) [7] Feuchtenbeiner, Stefanie , et al. "New generation of compact high power disk lasers." Solid State Lasers XXVII: Technology and Devices . (2018) [8] Speiser J. ''Scaling of thin-disk lasers—influence of amplified spontaneous emission. '' J. Opt. Soc. Am. B. 2009, 26: 26-35. (2009) [9] Husain, Afzal, and Kwang-Yong Kim. "Shape optimization of micro-channel heat sink for micro-electronic cooling." IEEE Trans. Compon. Packag. Technol. 31.2: 322-330. (2008)
ur
[10] Oh, Chang H., et al. "Liquid jet‐array cooling modules for high heat fluxes." AIChE J. 44.4:
Jo
769-779. (1998) [11] Fabbri, Matteo, Shanjuan Jiang, and Vijay K. Dhir. "A comparative study of cooling of high power density electronics using sprays and microjets." J. Heat Transfer 127.1: 38-48. (2005) [12] Wang, Evelyn N., et al. "Micromachined jets for liquid impingement cooling of VLSI chips." J. Microelectromech. Syst. 13.5: 833-842. (2004) [13] Vafai, K., and C. L. Tien. "Boundary and inertia effects on convective mass transfer in porous media." Int. J. Heat Mass Transfer 25.8:1183-1190. (1982) [14] Vretenar, Natasa. "Room temperature and cryogenic Yb: YAG thin disk laser: single crystal and ceramic." (2012) [15] Shang, Jianli, Xiao Zhu, and Guangzhi Zhu. "Analytical approach to thermal lensing in end-pumped Yb: YAG thin-disk laser." Appl. Opt. 50.32: 6103-6120. (2011)
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[16] Vorholt, Christian, et al. "Measurement of temperature-dependent absorption and emission spectra of Yb:YAG, Yb:LuAG, and Yb:CaF2 between 20 C and 200 C and predictions on their influence on laser performance." J. Opt. Soc. Am. B 29.9:2493-2502. (2012) [17] H. Ohashi et al., “Enhancement of emitting power density with abeam-shaping technique for a high-power laser-diode array stack,”Opt. Eng. 43(10), 2206–2207 (2004)
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[18] Liu R, Zhang X, Gong F, et al. Research on the adjusting technology of the thin disk laser[J]. Optik. 2017, 157. [19] [1] Li C, Liu R, Gong F, et al. Theoretical calculation and experimental study on absorption properties of Yb: YAG crystal applied in thin disk laser[J]. Optik. 2016: 511-516.
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Table1. The boundary conditions of finite element simulation Description Numerical Value 20
Diamond thickness/mm Indium tin solder thickness/um
2 10
YAG（ASE cap）thickness/mm
1
Cooling water pressure/bar Air convection coefficient/(W/m2K) Pump power intensity/(kW/cm2) Residual heat power intensity/(kW/cm2) Diameter of the sink /(mm)
2.5 25 4 2 34
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Environment temperature/℃
8