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Abstract
High average power lasers with a high pulse energy are of considerable interest in various fields such as high-energy-density physics. Light-absorbing edge cladding is effective in suppressing parasitic oscillations in high-pulse-energy disk lasers; however, the large amount of heat generated from the cladding can affect the laser medium. We develop an improved conduction-cooled active-mirror laser with a double-sided cooled-edge cladding. A stable laser output with a pulse energy of 10 J at a repetition rate of 100 Hz was achieved using six liquid-nitrogen-cooled active-mirrors in the main amplifier. This study shows that aggressive cooling of the edge cladding is highly effective in decreasing the temperature rise and controlling the temperature distribution in the laser medium.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High average power lasers with a high pulse energy are of considerable interest, not only to the science community working in the field of high-energy-density physics [1], but also in advanced industrial and medical applications such as material processing [2], surface treatment [3], creation of new materials [4], and medical therapies requiring radiation or particle beam generation [5]. Diode-pumped solid-state lasers (DPSSLs) are among the most promising candidates owing to their high energy-conversion efficiency, high beam quality, and compact size. As gain media, amplifier geometry, thermal management, and pumping architecture are important aspects for the development of a power-scalable laser system, various materials and amplification designs have been explored in many projects. The Mercury project demonstrated 61 J at 10-Hz laser amplification using a room-temperature-cooled multi-slab Yb:S-FAP amplifier [6]. The HAPLS project achieved 70 J at 3.3-Hz laser amplification using a helium gas-cooled multi-slab Nd:APG-1 amplifier [7]. The LUCIA project reported 14 J at 2 Hz laser amplification using an active-mirror Yb:YAG ceramic amplifier [8]. In addition, several 10-J, 10-Hz class nanosecond lasers have been developed worldwide [9]. The lasers were cooled using refrigerants at room temperature.
The improvement in the thermal properties of Yb:YAG laser crystals at low temperatures has led to the development of lasers that use low-temperature gases as refrigerants. The Bivoj/DiPOLE system recently demonstrated 105 J at 10-Hz laser amplification using a cryogenic gas-cooled multi-slab Yb:YAG ceramic amplifier [10]. The TACMI consortium in Japan demonstrated 117 J at 0.05-Hz laser amplification with a cryogenic gas-cooled multi-slab Yb:YAG ceramic amplifier [11]; however, the repetition rate was limited to 10 Hz owing to the poor heat removal by the helium-gas-cooled slabs. Giesen proposed a practical conduction-cooled disk laser [12]. In thin-disk lasers, metals such as copper with high thermal conductivity are used as heat sinks, and the gain medium is made thinner to reduce thermal resistance. Therefore, the heat removal capability is high, and Negel et al. achieved an average power of 1.1 kW (1.38 mJ, 800 kHz) [13]. In addition, Nagel et al. achieved 10 kW under CW operation [14]. Meanwhile, a conduction-cooled laser using liquid nitrogen has the potential to realize high-energy, high-repetition lasers. We proposed an improved thin-disk concept for high-energy high-average-power lasers with a total reflection active-mirror architecture, achieving 1 J and 100 Hz [15]. 1-kHz high-repetition-rate DPSSL was developed at the Colorado State University using cryogenic active-mirror amplifiers at 1 J [16]. In our previous study, a DPSSL was developed with four cryogenic active-mirrors, and 9.3 J pulses at 33.3 Hz were demonstrated using this system [17]. In this case, the considerable heat at the Cr:YAG edge cladding heated the Yb:YAG disk, decreasing the laser gain. Light-absorbing edge cladding is effective in suppressing parasitic oscillations in high-pulse-energy disk lasers; however, the large amount of heat generated from the cladding can affect the laser medium. Liu et al. reduced the heat flux from the cladding to the gain medium by creating a gap between the gain medium and cladding, and cooling it with water [18]. Varying the shape of YAG to emit fluorescence without cladding was also found to be effective [15,19]. Therefore, the suppression of ASE and the heat generated by ASE are important design items for disk-type lasers [20,21].
Herein, we propose a new active-mirror laser with a cooling jacket that provides direct conduction cooling to the top cladding surface. Numerical calculations show that active cooling of the edge cladding is highly effective in reducing the temperature increase of the gain medium and controlling the temperature distribution. Furthermore, we demonstrate that the repetition rate can be successfully increased to 100 Hz, making it one of the best performing lasers in the world, with high average power (1 kW) and high energy (10-J) pulses. This indicates that the developed laser has an excellent heat-removal capability for the active-mirror cooled by the attached cryogenic metal heat sink.
2. Experiment setup
Figure 1(a) shows the schematic of the laser system. The seed laser is a Yb-doped CW single-mode DFB fiber laser (Koheras ADJUSTIK, NKT Photonics) with a narrow bandwidth (<20 kHz), 5 mW output power, and a 1029.5 nm center wavelength. To obtain the desired temporal profile with a pulse duration of 10 ns and repetition rate of 100 Hz, the electro-optical modulator was controlled by an arbitrary waveform generator (AWG70002A, Tektronics). The pulse shape was adjusted using the AWG to prevent an increase in the peak power at the pulse front due to saturation amplification. The chopped pulses were amplified to pulses with 3 mJ of energy using a cryogenically cooled Yb:YAG regenerative amplifier, comprising a Yb:YAG single crystal pumped by a fiber-coupled 940 nm LD with a 1 ms pulse duration. The crystal was cooled to 77 K using a closed-loop Stirling engine cooler (SC-UF01, Twinbird). The output pulse from the regenerative amplifier had a Gaussian beam profile with a diameter of 1 mm at 1/e2. This pulse was passed through a Faraday isolator (ISO), and its diameter was expanded to 7 mm by a Galilean telescope before it passed through a λ/2 waveplate. Finally, it was injected into the multipass amplifier. Free space propagation further increased the beam diameter to 10 mm (1∕e2). The multipass amplifier consisted of a cryogenically cooled Yb:YAG single-crystal rod (20 mm diameter and 15 mm thickness) and was pumped by a fiber-coupled 940 nm LD with a 0.5 ms pulse duration. The crystal was cooled to less than 77 K using a Gifford-McMahon cooler (AL300, CRYOMECH). The amplifier amplifies the pulse energy to 1 J at 100 Hz. The beam was spatially expanded by a Galilean telescope, and its circular shape was converted to a hexagonal shape with a flat-top intensity profile using a 32-mm hexagonal serrated aperture. A deformable mirror was used to compensate for the wavefront of the main amplifier. A seed pulse energy of 530 mJ was obtained before the final amplifier. The main amplifier had six Yb:YAG ceramic active-mirror heads in a vacuum. The Yb3+ ion doping concentrations of YAG ceramic disks (Konoshima Chemical Co., Ltd.) were 1.0 and 0.6.%. The thickness of the ceramics was 7 mm, and their diameters were 45 mm (heads 1–4) and 50 mm (heads 5 and 6). 0.25% Cr4+-doped absorptive edge claddings with widths of 7.5 mm and 5 mm, respectively, helped prevent parasitic oscillations. A 7-mm thick molybdenum plate was sandwiched between the YAG ceramic disk and a copper heat sink (Fig. 1(b)). The high Young's modulus of molybdenum and its thermal expansion coefficient, which is close to that of YAG, reduce the internal stress of the YAG disk at cryogenic temperatures. The Cr:YAG edge cladding of each disk was covered with a copper jacket for intensive cooling. The heat sink and jacket were cooled by liquid nitrogen with total flow rate of 6 L/min. The temperature of the side surface of the Cr:YAG was monitored during operation using thermocouples; the maximum recorded temperature without diode pumping was 74 K. The total peak power of the three 940 nm laser diode arrays (PM72-12, Lastronics GmbH) was 83 kW during 0.5 ms at 100 Hz. After pre-compensation of the residual wavefront distortion of the main amplifier using a deformable mirror, the hexagonal seed pulse entered the active-mirror chain at an incidence angle of 15°. Two 90° quartz rotators were used to compensate for the birefringence. After single-pass amplification with six active-mirrors, the seed pulse was amplified again by retro-reflection using an end mirror via a quarter-wave plate. Finally, the amplified pulse was reflected by a thin-film polarizer.
Fig. 1. (a) System layout of ∼10-J Yb:YAG cryogenically cooled active-mirror amplifier (DFB: Distributed feedback, EOM: Electro-optic moderator, AWG: Arbitrary Waveform Generator, BE: Beam expander, SA: Serrated aperture, VSF: Vacuum spatial filter, TFP: Thin film polarizer). (b) 3D model of active-mirror laser head and photograph of Cr:YAG/Yb:YAG ceramic.
3. Numerical simulations of heat generation for active-mirrors
The advantages of the cooling jacket were confirmed through fluid numerical simulations using the finite element method. Figures 2(a) and 2(b) show the calculation model, results of numerical simulations of the laser medium temperature, and the temperature distribution of the pumped laser medium cross-section with and without the cooling jacket, assuming a 10-J pulse and 100-Hz laser operation. In Yb:YAG, approximately 10% of the heat generation occurred because of Stokes shift. Furthermore, more than 60% of the fluorescence generated in Yb:YAG was absorbed by Cr:YAG, as calculated by ray-tracing simulations, and more than 85% of the absorbed energy (at 77 K) heated the Cr:YAG [22]. To achieve an average power of 1 kW, a total average pump power of 4.24 kW is required, assuming a laser efficiency of approximately 20%. The heat generated by Cr:YAG was estimated to be 2.13 kW and that by Yb:YAG was estimated to be 382 W. The thermal resistance from the gain medium to the heat sink was experimentally measured and calculated using a value of 0.05 K/W. This thermal resistance was approximately one-sixth that of resin-based adhesives. Without the cooler jacket, the Yb:YAG temperature was 107 K owing to the heat inflow from the Cr:YAG, as shown in Fig. 2(a). With the cooler jacket, the Yb:YAG temperature could be reduced to 98 K, as shown in Fig. 2(b). The total heat flux of the back surface of Yb:YAG and Cr:YAG was estimated to be more than 10 W/cm2, which is more than 10 times higher than that of gas-cooled slabs [10]. Figure 2(c) shows the average temperature distribution in the cross section of the pumped Yb:YAG. The heat flow from the Cr:YAG to the pumped Yb:YAG was reduced by the heat flow into the cooler jacket. Without the cooler jacket, the temperature difference from the center to the outside of the pumped YAG was 13.2 K, while with the cooler jacket, the temperature difference could be reduced to 4.3 K.
Fig. 2. Calculation model and numerical simulation results: laser medium temperature and temperature distribution of the cross section of the pumped laser medium (a) with and (b) without Cr:YAG cooler jacket (assuming that the thermal resistance across the interface between the laser medium and molybdenum is 0.05 K/W). (c) Average temperature distribution in the cross section of pumped Yb:YAG.
4. Experiment results and discussion
To confirm the effect of the cooling jacket, the cladding temperatures (Cr:YAG side surface temperature) with and without the cooling jacket were compared, as shown in Fig. 3. The gain medium was pumped at an average power of approximately 700 W per head at a wavelength of 940 nm, with a repetition rate of 100 Hz for 20 s. The cladding side surface temperature was measured using a resistance-temperature detector. Without the cooling jacket, the temperature increased to approximately 120 K, and with the cooling jacket, it reached approximately 100 K. With the cooling jacket, the cooling capacity was improved by a factor of approximately two.
Fig. 3. Dependence of the measured and calculated Cr:YAG side surface temperature on the average pump power.
Figure 4 shows the dependence of the small-signal gain on the pumping energy at 100 Hz operation. The gain medium was pumped by a total energy of 41.6 J in 0.5 ms with a repetition rate of 100 Hz for 20 s. The input pulse energy was attenuated to 0.6 mJ. The input and output energies were measured using a commercial energy meter (QE65LP-H-MB-D0, Gentec). The small-signal gain was measured at repetition rates of 10 and 100 Hz. Theoretically, the small-signal gain at 10 Hz varied linearly with the pumping energy. The small-signal gain G0 at 100 Hz decreased slightly after pumping, and in terms of the logarithmic gain log(G0), it was estimated to decrease by approximately 10% compared to that at 10 Hz. This decrease is due to the change in the stimulated emission cross section and reabsorption caused by the increase in the temperature of the gain medium, and the temperature of the gain medium was estimated to have increased to 98 K with the cooler jacket. This agrees well with the results of the thermal calculations (Fig. 2(b)). The small-signal gain at 100 Hz in a previous experiment without a cooling jacket showed a sharp decrease soon after pumping, and the temperature of the gain medium was estimated to have reached approximately 115 K in approximately 10 s [17]. Therefore, the use of a cooling jacket improved the cooling capability of the laser head, resulting in a small decrease in the gain.
Figure 5 shows the dependence of the optical-to-optical efficiency of the output energy on the pump power. Using six active-mirror laser heads with total pump energy of 41.6 J in 0.5 ms at 100 Hz, and an input seed energy of 530 mJ, we achieved lasing with pulse energy of 10.2 J corresponding to an average power of 1.02 kW with an optical-optical efficiency of 23.6%. To the best of our knowledge, this is the highest value ever reported.
Figure 6 shows the typical output near-field and far-field profiles measured at 10 J and 100 Hz. The flatness factor of the near-field profile was 50.5%.
The spot diameter of the far-field profile without deformable mirror was 780 µm in the x direction and 560 µm in the y direction, corresponding to over 2.4 times the diffraction limit and with deformable mirror was 300 µm in the x direction and 313 µm in the y direction, corresponding to approximately 1.3 times the diffraction limit. The total change in the wavefront distortion (the difference from the non-pumping wavefront) was 1.2 λ peak-to-valley at the maximum output. This wavefront change is expected to be caused by the change in shape of the laser head, owing to the temperature increase from pumping.
Figure 7 shows the output energy stability at 10 J and 100 Hz. The monitored temperature of the Cr:YAG side surface reached a steady state at 106 K, approximately 20 s after the operation commenced. At this temperature, the gain reduction of Yb:YAG was predicted to be small. As a result, the energy stability was recorded as 0.7% RMS for 2000 shots.
Fig. 7. Energy stability at 100 Hz (2000 shots). Inset: typical temporal temperature profile of the Cr:YAG side surface.
5. Conclusion
We developed a new 60-mm aperture conduction-cooled cryogenic active-mirror using double-side-cooled, direct-conduction cooling. Using six active-mirrors in the main amplifier, we achieved lasing with a pulse energy of 10 J at a repetition rate of 100 Hz, corresponding to an average power of 1 kW. With this active-mirror design, easy energy scaling is possible by enlarging the aperture size and using higher pump energy. In the future, this system can be upgraded for 100 J operation by combining large-aperture laser heads.
Funding
JST-Mirai Program (JPMJMI17A1).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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