Erbium-Based Gas-Cooled Disk Laser

Transcription

1 1 of 1 Erbium-Based Gas-Cooled Disk Laser John Vetrovec a Aqwest, LLC P.O. BOX 468, Larkspur, CO 8118, USA ABSTRACT We report on a novel resonantly-pumped, erbium (Er)-based, gas-cooled disk laser (GCDL) scalable to highaverage power (HAP). The GCDL uses edge-pumped composite laser disks known for their near perfect pump uniformity and compact configuration. Edge pumping enables a long path for pump absorption, which permits using low Er concentration and limits upconversion losses. Resonant operation reduces the waste heat load and enables the use of gas rather than liquid for disk cooling. These attributes make it possible to engineer a lightweight and compact laser device operating at eye-safer wavelengths. This paper presents a GCDL concept design and evaluates its performance for several host materials and operating conditions. Keywords: Solid-state laser, disk laser, gas cooling, edge pumping, erbium, eye safe 1. INTRODUCTION AND BACKGROUND Solid-state lasers (SSL) generating high-average power (HAP) output at near 1 μm wavelength are becoming increasingly more important tools for a variety of industrial [1] and military applications [2]. The utility of HAP SSL can be greatly broadened by operating at wavelengths with reduced susceptibility to eye damage. Such reduced-eye-hazard wavelengths (REHW) are generally considered to be greater than 1.3 μm. In many industrial applications, REHW lasers would greatly improve workers safety and make unnecessary many currently used and costly safety measures such as laser booths. REHW lasers would also improve operational safety of many military systems including high-resolution active sensing, target illumination, target designation, and other non-lethal interactions. In high-energy lasers (HEL), especially those engaging ground targets, operation at REHW would greatly reduce the likelihood of collateral damage. Solid-state disk lasers are known for their good beam quality (BQ) and scalability to HAP. In recent years, a liquid-cooled disk laser has demonstrated multi-kilowatt output with excellent BQ [3] and became an important industrial tool [4]. However, engineering a compact and lightweight laser system using multiple liquid-cooled disk lasers appears to be very challenging. Recent developments in pump diode technology and laser materials make it possible to operate certain lasant ions, e.g., Yb 3+ and Er 3+, under resonantly pumped conditions. Resonant pumping reduces waste heat and makes it more practical to cool the laser gain medium with gas rather than liquid coolant. Gas cooling is particularly suitable for applications requiring very simple and lightweight laser devices for intermittent or time-limited operation. This paper describes a novel erbium (Er)-based GCDL concept using an edge-pumped composite disk operated under resonant conditions. Edge-pumping provides near-perfect pump uniformity while allowing unobstructed access to disk faces making it easier to design a gas-cooling solution. Gas cooling also benefits from novel concepts for enhancing heat transfer from the disk and for economizing the consumption of coolant gas. This article includes the study that considered ceramic YAG and fluoro-phosphate host materials, two approaches to gas cooling, and continuous or intermittent modes of operation. With the proper selection of design parameters and operating condition, it is possible to develop an Er-based GCDL that is lightweight, compact, simple, rugged, and operates at or 1644-nm wavelengths, which offer greatly reduced eye hazard. a jvetrovec@aqwest.com; tel. (33)

2 2 of 1 2. MOTIVATIONS FOR GCDL GCDL leverages several important technologies: disk lasers for good BQ, edge pumping for low upconversion losses in erbium, resonant pumping for low waste heat, and gas cooling for lightweight and compact laser device. Disk-type SSLs enjoy inherently low susceptibility to thermo-optical distortions and have demonstrated lasing at HAP with outstanding BQ [5,6]. Their large, round aperture reduces diffraction and beam clipping losses experienced by other SSL configurations. In a disk laser, transverse temperature gradients are reduced because waste heat is extracted from the gain medium in the direction parallel to laser beam axis. One other advantage of a disk laser is its scalability over a broad range of average laser powers. With each disk producing kilowatts of laser power, power scaling can be accomplished by changing the disk size and/or by changing the number of disks in a resonator. A disk laser may use a reflective configuration shown in Figure 1a or a transmissive configuration shown in Figure 1b. In a reflective disk, also known as active mirror, the back surface of the disk is available for liquid cooling [7]. The advantage of liquid cooling is fast removal of waste heat at very high fluxes, up to about 1 W/cm 2 [8], which permits operation at very high-power densities. However, the front surface of the disk is not actively cooled and the resulting thermally induced stress tends to bulge the disk toward the laser beam. To maintain proper optical figure, this tendency must be overcome by attaching the disk either mechanically (with a solder) [3] or hydrostatically [9] onto a rigid heat sink. A laser resonator with multiple liquid-cooled disks has a folded beam path (Figure 2a), which results in a rather large separation between the end and the outcoupling mirrors, beam mode size variation, and increased susceptibility to misalignments. Overcoming these problems requires corrective optical elements, large support structures, optical benches and active cooling. In addition, the susceptibility of the intracavity beam to aerothermal perturbations must be mitigated. As a result, complexity, weight, volume, and cost of the laser device are greatly increased while the robustness is compromised. Laser Beam Δ T Laser Beam Δ T Reflective Coating on Back Surface a) reflective disk (aka active mirror) b) transmissive disk Figure 1: Architectures for pumping and cooling disk lasers Disk Module (typ.) Disk Module (typ.) a) For liquid-cooled disks b) For gas-cooled disks Figure 2: Typical layout of disk laser resonators In a transmissive disk, waste heat is removed by flowing gas over the disk faces. Since both disk faces are cooled at the same time, thermomechanical distortions are largely avoided. Aerothermal effects in the optical

3 3 of 1 path are very small because the cooling gas flows at very high velocity in relatively thin sheets. Waste heat removal by the flowing gas is limited to rather low fluxes; typically <1 W/cm 2 [1]. This means that a gascooled disk is most practical when the time-averaged heat load is rather modest, such as offered by resonant pumping and/or by intermittent laser operation. In many applications, such operational constraints are more than offset by the GCDL simple resonator, lightweight, compactness, and robustness to misalignment, Figure 2b. 3. EDGE-PUMPED COMPOSITE DISK The GCDL uses edge-pumped composite laser disks made of laser host material suitable for fabricating large components. Each disk has a central portion doped with Er ions and a perimetral (non-lasing) edge, Figure 3. The perimetral edge improves coupling between the pump diodes and the laser medium, guides the pump radiation, and outcouples amplified spontaneous emission (ASE). Pump diodes at 147 nm are arranged around each composite disk and point toward its center. Diode power is injected into the perimetral edge and channeled into the lasing (central) portion of the disk where it is gradually absorbed. Superposition of diode beamlets is balanced by absorption in the disk to provide uniform gain over the entire lasing volume [11,12]. The edge-pumped disk laser was test-verified in a DARPA-funded project, which showed that 96% pump uniformity is achievable under realistic conditions [13]. Edge pumping is very advantageous because it provides a long absorption path for the pump, which allows low Er doping, and thus avoids significant upconversion losses. To keep upconversion losses low, erbium doping in YAG should be less than.5% at. or preferably <.25% at. [14]. To achieve good pump absorption, this translates to a disk diameter of at least a few centimeters. An edge-pumped disk has unobstructed access to both disk faces, which allows for flowing gas coolant. Er Composite Laser Disk (doped center, undoped edge) Laser-Active Center (Er-doped) COTS Pump Diodes Tapered Perimetral Edge (Outcouples ASE) Amplified laser beam extracts power from >95% of pumped volume COTS diode arrays (multiple vendors) Diode-to-gain medium coupling efficiency > 95% Pump absorption efficiency ~95% Gain uniformity ~96% Extraction from > 95% of pumped volume ASE outcoupling via tapered disk edge Round aperture for excellent mode matching Disk cooled by gas flow Very simple, ultra-compact, and lightweight packaging Figure 3: Edge-pumped disk laser amplifier module uses COTS diodes closely coupled to the disk edge for high efficiency, uniform gain, and very compact packaging (after [15]) The primary choice of material for the composite disk is ceramic YAG (C-YAG). Composite C-YAG preforms up to 1 cm thick and with transverse dimensions of up to 12 cm can be readily fabricated [16]. Figure 4 shows a cross-section of the composite disk and diode array. The disk edge is tapered to further concentrate the pump radiation and to outcouple ASE. The traditional method for controlling ASE to an acceptable level uses cladding of the gain medium with material absorbing at the laser wavelengths. This approach is impractical in a resonantly operated, edge-pumped disk because the material for absorbing Er emission at 1535 or 1644 nm must be also transparent at the pump wavelength of 147 or 153 nm. This situation is resolved by tapering the disk edge for ASE outcoupling. A 3-D ray-tracing analysis using the Lightools model showed that a tapered disk edge reduces about 3 times the ASE feedback compared to a traditional square disk edge, Figure 5 [17].

4 4 of 1 In some applications, phosphate and fluorophosphates glass is advantageous for fabricating composite disks, Figure 6. Using glass has several practical benefits. First, the optical quality of glass is superior to that of crystals and ceramics, which translates to lower scattering losses. Fabricating composite glass disks is at least 1 times less costly than comparable C-YAG composite disks. Er:glass has broader and smoother absorption and emission features than C-YAG, which respectively translate to relaxed temperature control for pump diodes and laser tunability. The latter is particularly important for avoiding atmospheric absorption lines. Finally, optical glass can be made athermal, which means that the coefficient of thermal expansion is balanced by a negative dn/dt to make the coefficient of optical path essentially invariant with temperature. This translates to greatly reduced susceptibility to thermally-induced optical path difference (OPD). Studies show that despite its lower thermal conductivity, athermal glass has a nearly 5% lower OPD per unit of laser power produced than a comparable YAG material [18]. Coolant Outlet Manifold and El. Terminal 2 Lensing Surface Laser Beam Diffusion Bond Tapered Section Undoped Edge Doped Disk Coolant Inlet Manifold Diode Bar & El. Terminal 1 Stack Coolant Figure 4: Configuration of the composite disk and pump diode arrays Straight Edge Disk 4% of incident rays are reflected back into gain medium and re-amplified Tapered Edge Disk Only 1.4% of incident rays are reflected* *) Fresnel reflection at edge assumed (worst case), for AR-coated edge reflection is less than 1% Figure 5: ASE reduction by tapered disk edge compared to square disk edge [17] Figure 6: Yb-doped composite laser disk made of phosphate glass [13] 4. RESONANTLY PUMPED ERBIUM FOR LOW WASTE HEAT Several SSL lasants can operate at REHW; namely erbium, thulium, and holmium, see Figure 7. Of these lasants, trivalent erbium (Er 3+ ) offers the combined advantages of REHW, far-field intensity, well-established physics backed by over 2 years of experience with Er-based range finders [19], favorable cross-sections and lifetimes, availability of efficient and robust pump diodes from several commercial sources, use of conventional transmissive optics and coatings, and good atmospheric propagation. The use of Er for HAP SSL was previously considered impractical due to the large Stokes shift when pumped at 94 nm via energy transfer from Yb 3+. Waste heat caused by the large Stokes shift drives thermo-optical

5 2 F 5/2 4 I 11/2 5 of 1 distortions in the gain medium making it more difficult to maintain BQ. In addition, the electro-optical efficiency of the Er:Yb laser is very limited, which necessitates large electric power supplies and thermal management system. In recent years, high-power semiconductor laser diodes capable of operation near 1.5 μm became efficient, robust, and commercially available. This new technology enables resonant pumping of the Er 3+ ion that yields ultra-low heat fraction and greatly increases electro-optical efficiency [2, 21]. Figure 8 is a diagrammatic representation of the resonant pump architecture. Figure 9 compares the properties of Er:YAG and Er:glass materials. Laser Ion Wavelength [μm] Relative Eye Hazard (based on [22]) Far-Field Intensity (in vacuum, diffraction limited) Basis for Comparison Nd in YAG host in glass host.48 Lasants for Operation Er 3+ ~ in YAG host.41 at Reduced Eye Tm in YAG host ~.5.28 Hazard Wavelength Ho in YAG host ~.5.26 Figure 7: Comparison of candidate lasants for operation at REHW η Stokes = 61% η Stokes = as high as 96% 4 I 13/2 Traditional Pumping via Yb 94 nm (Er:Yb:Glass) 4 I 13/2 Lasing ~1.535nm (Er:Glass) Resonant Pumping 147 or 153 nm (Er:YAG) 148 nm (Er:Glass) Lasing nm (Er:YAG) 1.535nm (Er:Glass) 2 F 7/2 4 I 5/2 Er 3+ Yb 3+ Er 3+ a) b) 4 I 5/2 Figure 8: Pumping of Er: a) traditional enery transfer via Yb, and b) resonant pumping Parameter Er:YAG Er:glass [19] Laser Wavelength Peak [nm] Wavelength for Resonant Pumping [nm] Theoretical Fraction 147 nm pump 1525 nm pump 3.6% Measured Fraction TBD 18% (phosphate) [23] ~9% (fluoro-phosphate)* [23] Emission Cross-Section [x 1-2 cm 2 ] 2.8 Fluorescence Lifetime [ms] Thermal Coef. of Opt. Path [1-6/ C] 12.5 (Q98 glass) Laser Saturation Intensity, Isat [W/cm 2 ] Laser Sat. Energy Density, Jsat [J/cm 2 ] 6 23 *) Estimated, measurements in-progress Figure 9: Selected properties of Er:YAG and Er:glass materials 5. GAS COOLING CONCEPTS Gas-cooled disk lasers have been investigated since the 197s [24,25]. Gas cooling enables a simple, compact, and lightweight thermal management system, which has no moving parts or startup inertia to overcome. In a gas-cooled laser, the amount of waste heat that can be removed from the gain medium is of

6 6 of 1 central importance to the laser design, Figure 1. This study considered continuous and intermittent modes for both heating and cooling. Figure 11 shows a section of an amplifier configured for continuous cooling consisting of two gas-cooled composite laser disks interspaced by spacer disks. The gaps between the laser and spacer disk form narrow (.5-1 mm) channels for coolant gas flow. System Mode of Operation (application driven) Continuous Intermittent Time-limited Pulse format Waste Removal Disk thickness Disk material Stress limits Er 3+ temp. limits Coolant temperature xfer coefficient Scattering loss Pump Operating Mode Laser Operating Mode Resonator Design Waste heat fraction Gain medium physics ASE limits Cw, q-cw, q-switched Figure 1: Methodology for gas-cooled laser design Disk heating (by deposition of waste laser heat) and cooling can be carried out simultaneously. Once a thermal equilibrium is reached, all the laser waste heat is carried away by the gas coolant. Because the gas gradually heats up as it traverses across the disk face, a thermal gradient transverse with respect to laser axis is induced in the disk. To mitigate the resulting OPD, gas in adjacent channels is flowing in antiparallel directions. Figure 12 shows results of computer simulations using the ANSYS model for a laser disk made of athermal glass and cooled by helium at 76 Torr flowing at M =.2 [15]. The calculated OPD profile has less than.2 μm variation over 95% of the lasing aperture in laser disk and an associated spacer disk. One disadvantage of the continuous cooling configuration is the large number of optical surfaces (laser and spacer disks) in the resonator, which translates to increased scattering losses. Spacer Disk Amplified Laser Beam Holder OPD [meters] 1.7E-7 1.6E-7 1.5E-7 1.4E-7 1.3E-7 1.2E-7 1.1E-7 95% of Disk Volume.2 λ over 95% of Aperture (λ = 1.3 μm) Composite Laser Disk Diodes Anti- Parallel Gas Flow 1.E-7 9.E-8 8.E Normalized Radial Location Figure 11: Two-disk amplifier Figure 12: OPD predicted by 3D CFD model for 1 laser disk and 1 section for continuous cooling spacer disk; disk temperature profile inserted (from [15]) Figure 13 shows an amplifier section configured for intermittent lasing and cooling consisting of two composite laser disks and four cooling blocks. During the lasing mode, the blocks are retracted from the beam path and waste heat is stored as sensible heat in the laser disks. During the cooling mode, the cooling blocks are positioned in proximity of the laser disk faces and a gas is injected into the narrow gap between them. Gas transports the heat from the laser disk across the gap and into cooling blocks where it is deposited

7 7 of 1 into a phase change material. Because the gas temperature does not significantly change as the gas flows through the gap, thermally induced OPD in the laser disk is largely avoided. Composite Laser Disk Pump Diodes Composite Laser Disk Pump Diodes Cooling Block (phasechange material) Laser Beam Cooling Block (phasechange material) During Lasing Helium Gas (anti-parallel flow) During Cool-Down Figure 13: Two-disk amplifier section for intermittent lasing and cooling The choice of the coolant gas includes helium, nitrogen, and air. Helium has the advantage of high thermal conductivity and low scattering loss, which must be traded against the wide availability and lower cost of nitrogen and air [1]. A high gas flow velocity, typically M =.2.3 at 76 Torr is required to generate acceptable heat transfer coefficient, typically 7-1 W/m 2 - K. transfer can be further enhanced by increasing the gas pressure [1]. The gas can be provided by an open-loop system supplied by a highpressure reservoir. Coolant consumption can be reduced by a factor of 4 to 5 by a recirculation loop shown in Figure 14. The loop consists of an ejector, a feedback line, and a back pressure valve. The feedback line directs a large portion of the gas exhausted from the disk amplifier into the suction chamber of the ejector. High-pressure gas is accelerated in the motive nozzle of the ejector to supersonic velocities, entrains the gas in the suction chamber, and pumps it into the disk amplifier. Excess gas is exhausted from the loop through the backpressure valve. The advantage of the recirculation loop is reducing the overall gas consumption without compromising gas flow-rates and heat transfer in the laser amplifier. Gas Reservoir Ejector Feedback Line Laser Disks Back Pressure Valve Exhausted Flow Figure 14: Gas cooling system with recirculation loop for reduced gas consumption [26] 6. THERMAL ANALYSIS AND SIMULATIONS A model was developed to simulate the temperature profile in a gas-cooled disk during the continuous and intermittent heating and cooling modes. The model tracks the temperature profile in the disk (see geometry in Figure 15) starting from a predetermined initial temperature using a temperature-dependent thermal

8 8 of 1 conductivity and temperature-dependent specific heat of the laser disk material, and a prescribed timedependent internal heating and surface cooling. Laser Beam Axis -L/2 Disk Mid- Plane L/2 Position Inside the Disk Figure 15: Geometry for thermal analysis Figures 16a and 16b, respectively, show the temporal and spatial evolution of temperature in a continuously cooled.5-cm-thick C-YAG disk with a continuous heat load of 28 W/cm 3 and an intermittent 5-second heat load of 45 W/cm 3 applied with a 5% duty factor. With a typical center-to-surface temperature difference around 9 K, this material operates at less than 1% of its thermal fracture limit. However, the effects of cyclic stresses must be taken into account Position 6 5 Inside the Time [s] Disk [cm] Temperature [K].25 Position Inside the Disk [cm] Time [s] a) Continuous heat load of 28 W/cm 3 b) Intermittent 5-s heat load of 45 W/cm 5% duty Figure 16: Temporal and spatial evolution of temperature in a continuously cooled.5-cm-thick C-YAG disk Figure 17 shows the temporal and spatial evolution of temperature in a continuously cooled.5 cm-thick fluoro-phosphate glass disk with an intermittent 5-second heat load of 25 W/cm 3 applied with a 5% duty factor. With a typical center-to-surface temperature difference around 65 K, this material is operating at about 3% of its thermal fracture stress and the cyclic nature of this stress may be a limiting factor. Figure 18 shows the temporal and spatial evolution of temperature in an alternately heated and cooled.5-cm-thick C- YAG disk with an intermittent 5-second heat load of 22 W/cm 3 applied with 5% duty factor. In both cases, the cooling gas was injected with 23 K temperature. Average temperature in the disks was about 3 K and the peak disk temperature never exceeded 32 K. Temperature [K]

9 9 of Position 6 5 Inside the Time [s] 2 Disk [cm] Figure 17: Temporal and spatial evolution of temperature in a continuously cooled.5-cm-thick fluoro-phosphate glass disk with intermittent 5- second heat load of 25 W/cm 5% duty Temperature [K] Temperature [K] Time [s] Position Inside the -.25 Disk [cm] 1 Figure 18: Temporal and spatial evolution of temperature in an alternately heated and cooled.5-cm-thick C-YAG disk with intermittent 5-second heat load of 22 W/cm 5% duty. Results of these simulations indicate that both Er:C-YAG and Er:glass disks can generate several kilowatts of laser power and can be used as a building block for a HAP SSL operating at REHW. 7. CONCLUSION We presented a novel resonantly-pumped, Er-based, GCDL concept scalable to HAP. The GCDL uses edgepumped composite laser disks for a long path for pump absorption which permits using low Er concentration to limit upconversion losses. Resonant operation reduces waste heat load and makes it possible to use gas rather than liquid for disk cooling. Concepts for continuous and intermittent cooling by gas were presented and evaluated for C-YAG and glass host materials under continuous and intermittent heat loads. A method for reducing the consumption of coolant gas in an open-loop system was introduced. Results of thermal simulations show that gas cooling allows for efficient operation of both Er:C-YAG and Er:glass disk lasers at high average power. REFERENCES 1. D. Belforte, Laser Focus, January 2 2. J. Vetrovec, Solid-state high-energy laser, SPIE vol. 463, C. Stewen, K. Contag, M. Larionov, A. Giesen, and H. Hugel, "1-kW CW thin disk laser," IEEE J. Selected Topics in Quantum Electr., vol. 6, no. 4, pp , July/August 2 4. R. Brockmann and K. Mann, High-power disk laser, SPIE vol. 6871, G. F. Albrecht, B. Comaskey, and L. Fury, "A 1.4 kj Solid-state power oscillator with good beam quality," UCRL-JC , August L. Hackel, "The Mercury laser, a diode-pumped solid-state laser driver for inertial fusion, is activated," UCRL-TB , February D. C. Brown, J. H. Kelly, and J. A. Abate, Active-mirror amplifiers: Progress and prospects, IEEE J. of Quantum Electron., vol. 17, no. 9, 1755, L. Zapata, R. Beach and S. Payne, "Composite thin-disk laser scalable to 1 kw average power output and beyond," in the Technical Digest from the Solid State and Diode Laser Technology Review, held in Albuquerque, NM., June 5-8, 2 9. J. Vetrovec, Active mirror amplifier for high-average power, SPIE vol. 427, G. Albrecht, S. Sutton, H. Robey, and B. Freitas, Flow, heat transfer and wavefront distortion in a gascooled disk amplifier, UCRL-142, January 1989

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