A direct-liquid-cooled side-pumped Nd:YAG multi-disk QCW laser resonator is presented, in which the oscillating laser propagates through multiple thin disks and cooling flow layers in Brewster angle. Twenty Nd:YAG thin disks side-pumped by LD arrays are directly cooled by flowing deuteroxide at the end surfaces. A laser output with the highest pulse energy of 17.04 J is obtained at the pulse width of 250 μs and repetition rate of 25 Hz, corresponding to an optical-optical efficiency of 34.1% and a slope efficiency of 44.5%. The maximum average output power of 7.48 kW is achieved at the repetition rate of 500 Hz. Due to thermal effects, the corresponding optical-optical efficiency decreases to 30%. Under the 12.5 kW pumping condition while not oscillating, the wavefront of a He-Ne probe passing through the gain module is as low as 0.256 μm (RMS) with the defocus and tetrafoil subtracted.
© 2016 Optical Society of America
Cascading multiple thin disks with high gain and weak wavefront aberration is an effective way to obtain high power output and good beam quality from a solid state laser (SSL) resonator [1, 2]. With the direct-liquid-cooled configuration, the linear arrangement of multiple thin disks is can be realized since the oscillating laser passes through dozens of thin disks transmissively [3,4], while a high pump intensity is allowed due to its efficient thermal management . Furthermore, a flexible support is easy to realize and the awful wavefront aberration existing in the conventional thin disks that are welded on coolers can be avoided [6–8]. The study of direct-liquid-cooled multi-disk SSL has attracted much attention [9, 10].
Recently there are a few published reports presenting the development of high power SSL with direct-liquid-cooled multi-disk resonator. The most impressive case is the “liquid laser” scheme with a 150 kW output power by General Atomics Corp, but its detail has not been disclosed yet . In 2014, M. Gong et al. reported an elastically supported liquid–cooled multi-slab laser resonator with an output power of 3006 W using Nd:YAG as the gain medium and deuteroxide (D2O) as the coolant . Based on this work, a design and simulation of 30-kW class laser resonator was presented in 2015 . Z. Cai et al. had developed an end-pumped direct-liquid-cooled Nd:YLF thin disk laser resonator working at kW-level (CW) using the refractive-index-matching liquid as coolant to restrain loss at the interface between gain medium and the coolant . The reports [12–14] are all based on the end-pumped structure, in which a uniform pumping distribution is relatively easy to obtain, thereby the wavefront distortion is restrained. However, there is an overlap of the laser beam with the pumping beam outside the gain module in the end-pumped configuration, which is hard to be avoided. It increases the complexity of the system [12, 13], and even induce additional loss due to the insertion of dichroic mirrors . Moreover, thin disks at different doping levels should be adopted to realize an equal gain in every disk [9, 14]. With the increase of laser power scaling, the difficulty of fabrication increases with the increasing disk quantity as a result of an improving doping requirement. The problems above can be avoided by side-pumped structure, in which the pumping lights enter the gain module from the side surfaces. In a side-pumped configuration, an equal gain in every disk can be achieved at a same doping level, while there is no beam overlap outside the gain module. In addition, the area of pumping surface increases with the increase of disk quantity and will not be the limit of achieving a higher output power.
In this paper, we present a 7kW direct-liquid-cooled side-pumped Nd:YAG multi-disk laser resonator using D2O as the coolant. Pumping light is confined in the thin disks as a result of total reflection and the oscillating laser propagates through multiple thin disks and D2O layers in Brewster angle. Interface loss induced by the change of Brewster angle between Nd:YAG and D2O owing to thermal effects has been calculated, the calculation results proves that its influence is quite slight. A homogenizer has been designed to homogenize the flow coming into the cooling channel, and the validity has been verified by computational fluid dynamics (CFD) simulation. The resonator operates at QCW mode with a pulse width of 250 μs. Numerical simulations of the pumping, temperature and stress distribution have been carried out. The results prove that the laser operates in safe status under the given operating conditions. By using the output coupler (OC) with a transmittance (TOC) of 55%, the maximum output pulse energy of 17.04 J is obtained at the single-pulse pumping energy (Epump) of 49.9 J and a pumping frequency (fpump) of 25 Hz, corresponding to the highest optical-optical efficiency (ηo-o) of 34.1%. The highest average output power of 7.48 kW is obtained at the fpump of 500 Hz, corresponding to a comparatively high optical-optical efficiency of 30%. The result reveals the feasibility in high power operation. The wavefront of a He-Ne beam passing through the gain module is as low as 0.256 μm with the three major low-order terms subtracted, which reveals it has the potential of obtaining a good beam quality once the unstable cavity and deformation mirror are used.
2. Design and experimental setup
The configuration of direct-liquid-cooled side-pumped Nd:YAG multi-disk laser resonator is shown in Fig. 1. The gain module (GM) consists of 20 pieces of thin disks, 22 cooling channels and 4 fused silica windows, as shown in Fig. 1(a). Nd:YAG thin disks with a tilt angle of 48.5°, thickness of 2mm and doping concentration of 0.25 at. % are immersed in the cooling liquid and linearly arranged with a distance of 0.5 mm. The cooling channel between the two thin disks is filled with D2O, which flows in the direction vertical to the laser beam and pumping beam, as shown in Fig. 1(b). The oscillating laser passes through the interface between the thin disks and D2O in Brewster angle. The dimension of the end surfaces of each disk is 28 mm × 38 mm (height × width), corresponding to the clear laser aperture of about 27 mm × 23 mm. All the surfaces of the thin disks are uncoated, in which the two end surfaces (for laser transmission) and the two side surfaces (for pumping transmission) are polished, the other two surfaces are rough in order to restrain the formation of parasitic oscillation. The laser window has one side (besides the liquid) uncoated, and the other side (besides the air) antireflection coated for 1064 nm, while the pumping window has one side (on the liquid side) uncoated, and the other side (on the air side) antireflection coated for 808 nm. The overall dimension of the GM is 150mm × 96mm × 88mm. Four laser diode stacks (LDS) are used to pump the GM at the side surfaces. As shown in Fig. 1(c), most of the pumping light reach the side surfaces of thin disks and totally reflects inside them, the other transmits through D2O and reach the end surfaces of the thin disks, then passes through multiple thin disks and cooling flow layers. The plano-concave stable cavity is adopted, which consists of an output coupler (OC) and a high reflector (HR, Rs > 99.9% @ 1064 nm, RHR = 2 m). The cavity length is about 350 mm.
2.1 Interface loss
In order to reduce the loss due to the reflection, the light path is optimized so that the oscillating laser propagates through the interface between the GM and the cooling liquid in Brewster angle. As a result, only p-polarized laser can oscillate in the resonator. Nd:YAG crystal is chosen as the laser material, which is widely used in SSL due to its excellent thermal, mechanical, optical performance and laser properties . D2O with the deuteration degree of 99.8% is used as the cooling liquid owing to its low absorption coefficients at both the pump wavelength (808 nm) and laser wavelength (1064 nm) .
The interface loss always exists owing to the misalignment of optical axis although the laser passes through the interface between Nd:YAG and D2O in Brewster angle. The blue line in Fig. 2(a) shows the interface loss as a function of the offset angle at one interface. The inset shows the interface loss versus the number of thin disks when the laser passes through multiple disks and liquid layers. The loss is very small when the laser passes through only one interface and becomes considerable with the increase of the disk quantity and offset angle. However, it is only 0.27% at the disk number of 20 and the excursion angle of 1°, which is still very small. In this case, twenty pieces of Nd:YAG thin disks are adopted in the GM. The single-pass interface loss versus offset angle is shown in Fig. 2(a) with the red line.
The change of Brewster angle from D2O to Nd:YAG with different temperature increase is shown in Fig. 2(b). At the temperature increase of 50 K, the refractive index of D2O decreases by 6 × 10−3 and the refractive index of Nd:YAG increases by 3.65 × 10−4, corresponding to an optical axis offset of 0.12°. Referring to Fig. 2(a), the single-pass interface loss is lower than 0.01% in this case, which can be neglected.
2.2 Cooling flow
Fully developed pipe flow is used to cool the Nd:YAG thin disks. The velocity profiles in the cross section of fully developed pipe flow at different positions along the flow channel are similar and the heat transfer coefficients are similar too . Therefore, the wavefront aberration induced by nonuniform heat transfer can be avoided.
The device shown in Fig. 3(a) is used to achieve a fully developed pipe flow. Its structure is with reference to our previous work . The entrances of the homogenizer are sharp so that the impact of incoming flow and the turbulence intensity are weakened. The Reynolds number of the flow channel is defined as:
The numerical simulation of the flow field in cooling channel is discussed as following. K-ε model is chosen because the flow in the channel is in turbulent state (as its Reynolds number is higher than the critical Reynolds number values of 2300). The results demonstrate that a fully developed velocity profile can always be obtained by utilizing the homogenizer in the case of different incoming velocity distribution. Figure 3(c) shows the normalized RMS of the velocity in the cross section along the flowing direction, revealing a decrease and convergence of velocity unevenness. It can be inferred that the flow has become fully developed before flowing into the laser aperture, where the heat transfer coefficient is equal. The velocity distribution in the cross section of the fully developed flow at the entrance of laser aperture is shown in Fig. 3(b).
2.3 Pumping system
As shown in Fig. 1, four LDSs are used to pump the GM at the side surfaces. Each LDS contains 300 LD bars in an array of 5 × 60. With the fast axis collimated by microlenses, each bar has a divergence angle of 4-6° along the fast axis and 7-9° along the slow axis at the maximum peak output power of 200 W and a maximum duty cycle of 12.5%. Two 1/2 wave plates, polarization beam combiners and cylindrical lens groups are used to reshape the pumping beam. Considering the divergence angle of laser diode (shown in Fig. 1(c)), the pumping beam transmits to the side surfaces of GM at an angle of 13°.
The location and structural parameters of the optical elements are optimized using Zemax software. Synthesizing the influence of unabsorbed pumping beam and the distribution of absorption, the absorption efficiency is set to be 95%, and the corresponding doping concentration of the disk is 0.25 at. %. Figure 4(a) shows the simulated average pumping intensity distribution at the side surface of the GM. The efficiency of pumping coupling is about 91.4% and the pumping uniformity is 59.8%. Figure 4(b) shows the intensity distribution at the plane 5mm away from the side surface in the GM. It can be inferred that the total reflection of pumping is obtained in the disk. The normalized gain distribution of one disk in the laser aperture in parallel with the end surfaces of the thin disk is shown in Fig. 4(c). The gain is high near the entrance and low in the middle region along the pumping direction.
2.4 Thermal calculation
Besides the typical stress fracture limit of the laser medium, the safe operation of the direct-liquid-cooled laser resonator is also limited by the boiling point of cooling liquid.
The Fluent software has been used to calculate the temperature distribution of the central disk (shown in Fig. 4(b) with red mark) in the pumping case mentioned in section 2.3. The experiential heat generation ratio is 38% , and the corresponding total thermal power is 672 W. The initial temperature is 15 °C. Two end surfaces of the disk are cooled by flowing D2O with a flow velocity of 5m/s and the other four side surfaces are adiabatic. As shown in Fig. 5(a), the temperature is high at edge and low in the middle along the pumping direction. The highest temperature of 47.3 °C, which is far lower than the boiling point, appears at the center region of pumping surface with a little offset to the obtuse angle (shown in Fig. 5(a) with a black circinal mark).
The finite element method is adopted to calculate the thermal stress in the disk based on the temperature distribution. As shown in Fig. 5(b) with a black circinal mark, the maximum stress of 40.5 Mpa (far lower than the typical stress fracture limit of 130 MPa ~260 MPa) appears at the edge of the sharp angle, where the maximum temperature gradient exists. The results of thermal calculation confirm the safety of operating in the given pumping conditions.
3. Experimental results
The detailed configuration of the direct-liquid-cooled side-pumped Nd:YAG multi-disk laser resonator is expressed in Section 2. D2O was kept circulating in the cooling channel with Vflow = 5 m/s at 15 °C.
Two LDSs on one side were used to measure the absorption efficiency of the GM at the highest Epump and a fpump of 25 Hz. The average power of pumping was 639.5 W and 16 W before and after the GM respectively, with a corresponding absorption efficiency of about 97.5% and highest Epump of 24.94 J.
Four LDSs were used to pump the GM and the laser beam oscillated in the plano-concave cavity. The laser resonator worked at QCW mode with a pulse width of 250 μs. The gain and ASE/PO characters of the GM have been evaluated at a low fpump of 25 Hz. Figure 6 shows the near-field profile of the laser output at Epump = 30 J. The intensity distribution is similar to the gain distribution as shown in Fig. 4(c).
The output energy versus the pumping energy at different output transmittance is shown in Fig. 7 The optimum transmittance was around 55%, which indicated a high gain in the resonator. Using the OC with TOC = 55%, the highest output energy of 17.04 J was obtained at the highest Epump of 49.9 J, corresponding to ηo-o = 34.1% and ηslope = 44.5%. It can be inferred that ASE/PO in the resonator was not serious on account of the comparatively high ηo-o and ηslope . A photoelectric detector was used to detect the parasitic lasing emitted from the GM under the non-lasing condition. No obvious light beam was observed at the highest Epump of 49.9 J with an output transmittance of 55%.
The average output power and optical-optical efficiency as a function of the pumping frequency at Epump = 49.9 J, TOC = 55% is shown in Fig. 8. The inset shows a decrease of the output energy as the pumping frequency increases, which is mainly caused by thermal effects. The highest average output power of 7.48 kW was achieved at the average pumping power of 24.95 kW (fpump = 500 Hz), corresponding to the ηo-o of 30%. The power stability and the waveform of a single pulse at the highest output power have been tested, as shown in Fig. 9. No obvious decrease was found in the operating duration of 10 s after the thermal relaxation.
When the GM was pumped at the power of 12.5 kW, which is much higher than the experiential heat generation of 9.48kW at the highest pumping power of 24.95 kW, the optical path difference (OPD) information of the GM was measured by a Hartman-Shack (HS) sensor and a He-Ne laser under the non-lasing condition. The result is shown in Fig. 10(a). The distribution of OPD is similar to that of temperature (shown in Fig. 5). The coefficients of the Legendre polynomial expansion are shown in Fig. 10(c). The Y direction defocus, X direction defocus and tetrafoil account for the major proportion. Figure 10(b) shows the wavefront with the three major low-order terms subtracted, in which the RMS is 0.256 μm, revealing a weak wavefront distortion of the GM. It can be inferred that a good beam quality will be obtained once the unstable cavity and deformation mirror are used.
In summary, a direct-liquid-cooled double-side-pumped Nd:YAG multi-disk QCW laser resonator using D2O as the cooling liquid has been presented. In this resonator, pumping light totally reflects inside the thin disks, while the laser transmits through multiple thin slabs and coolant layers in Brewster angle. The structure of laser is quite compact with a dimension of 150 mm × 96 mm × 88 mm. At Epump = 49.9 J and Vflow = 5 m/s, the maximum output energy of 17.04 J has been obtained, corresponding to the comparatively high ηo-o of 34.1% and ηslope of 44.5%. The highest average power of 7.48 kW has been obtained at fpump = 500 Hz, corresponding to an optical-optical efficiency of 30%. The RMS of OPD is as low as 0.256 μm with the three major low-order terms subtracted at the average pumping power of 12.5 kW under non-lasing condition, revealing the potential of obtaining a good beam quality. Experiment results demonstrated the validity and feasibility of the novel configuration in high power operation.
The device in this work exhibits an outstanding scaling ability, mainly reflects in that the output power increases with the increasing laser aperture and disk quantity, while the total pumping power is not limited by the pumping area since it increases with the rising disk quantity. Hundreds of thin disks can be used in one GM in the case of an effective control of the absorption and scattering losses. The laser aperture can be very large with the improving fabrication techniques of the thin disk. With the disk quantity increasing to 100 and the aperture increasing to 4 times of the present, the expected output power would reach 100 kW level. However, there are still many problems to be solved, including the control of wavefront aberration, the design of flow field at a higher output level and so on, which will be the maincontent of our future work.
References and links
1. A. Giesen and J. Speiser, “Fifteen years of work on thin-disk lasers: results and scaling laws,” IEEE J. Sel. Top. Quantum Electron. 13(3), 598–609 (2007). [CrossRef]
2. R. Brockmann and D. Havrilla, “Disk laser: a new generation of industrial lasers,” Proc. SPIE 7193, 71931R (2009).
3. P. Li, Q. Liu, X. Fu, and M. Gong, “Large-aperture end-pumped Nd:YAG thin-disk laser directly cooled by liquid,” Chin. Opt. Lett. 11(4), 041408 (2013). [CrossRef]
4. R. Nie, J. She, P. Zhao, F. Li, and B. Peng, “Fully immersed liquid cooling thin-disk oscillator,” Laser Phys. Lett. 11(11), 115808 (2014). [CrossRef]
5. J. R. Wang, J. C. Min, and Y. Z. Song, “Forced convective cooling of a high-power solid-state laser slab,” Appl. Therm. Eng. 26(5), 549–558 (2006). [CrossRef]
6. H. Okada, H. Yoshida, H. Fujita, and M. Nakatsuka, “Liquid-cooled ceramic Nd:YAG split-disk amplifier for high-average-power laser,” Opt. Commun. 266(1), 274–279 (2006). [CrossRef]
7. P. Li, X. Fu, Q. Liu, and M. Gong, “Analysis of wavefront aberration induced by turbulent flow field in liquid convection-cooled disk laser,” J. Opt. Soc. Am. B 30(8), 2161–2167 (2013). [CrossRef]
9. A. Mandl and D. E. Klimek, “Textron’s J-HPSSL 100 kW ThinZag laser program” in Conference on Lasers and Electro-Optics (Optical Society of America, 2010), paper JThH2. [CrossRef]
10. M. D. Perry, P. S. Banks, J. Zweiback, and R. W. Schleicher, “Laser containing a distributed gain medium,” U.S.Patent 7,366,211 (April 29, 2008).
11. “Gen 3 High Energy Laser Completes Beam Quality Evaluation,” http://www.ga-asi.com/gen-3-high-energy-laser-completes-beam-quality-evaluation.
14. Z. Ye, C. Liu, B. Tu, K. Wang, Q. Gao, C. Tang, and Z. Cai, “Kilowatt-level direct-‘refractive index matching liquid’-cooled Nd:YLF thin disk laser resonator,” Opt. Express 24(2), 1758–1772 (2016). [CrossRef] [PubMed]
15. W. Koechner, Solid-State Lasers Engineering (Springer, 2006).
16. S. Patankar, Numerical Heat Transfer and Fluid Flow (Hemisphere, 1980).
17. H. Su, Y. Wei, X. Wang, and C. Tang, “Modal instability in high power solid-state lasers with an unstable cavity,” Opt. Commun. 341(1), 37–46 (2015). [CrossRef]