Solid-State Laser High-Energy Laser Solid-State Laser

Copyright  SPIE, Bellingham, WA 2002

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Solid-State Laser High-Energy Laser John Vetrovec* The Boeing Company 6633 Canoga Avenue, Canoga Park, Calif. 91309, U.S.A. tel. (818) 586-3101, fax (818) 586-3074 ABSTRACT This paper describes a high-energy laser (HEL) concept based on a disk-type solid-state laser operating in active mirror mode. The gain medium disks have high-performance real-time cooling that allows the laser to operate continuously. This configuration of the laser shows excellent scalability to high-average power required for directed energy applications and can be integrated into a simple, compact, lightweight, and affordable unit. The paper also discusses engineering concepts for integrated HEL, power-size-weight scaling model, as well as options for prime power and thermal management. Keywords: Solid-state laser, disk laser, active mirror, high-energy laser 1.

INTRODUCTION

There is a growing need for technologies suitable for lightweight, compact, and robust high-energy lasers (HEL) for a variety of directed energy weapon (DEW) applications. Key attributes of HEL are the promise of speed-of-light delivery, short engagement time, low cost of engagement, deep magazine, and reduced logistics support [1]. In addition to delivering lethal effects, HEL may be also used for a variety of ancillary missions including high-resolution surveillance and sensing, target illumination, target designation, and other non-lethal interactions. Platforms for HEL deployment may include land vehicles, naval vessels, tactical aircraft, unmanned air vehic le (UAV), and spacecraft. Many directed energy (DE) programs currently baseline chemical HEL because of their proven scalability to high-average power (HAP). Availability of suitable solid-state laser (SSL) technology could potentially change that. The HAP SSL holds the promise of scalability to compact, lightweight, rugged, affordable, electricallydriven HEL that can meet a broad variety of DEW applications. SSLs offer a broad selection of discrete wavelengths and, in some cases, wavelength tunability, which gives them a particular edge over alternate laser technologies. One potentially key factor for certain applications is the SSL capability to generate output in a short-pulse format. Increased peak power associated with short pulses can improve effectiveness of the laser beam on a target. Powered by electricity from conventional sources, SSLs can operate continuously for a long period. Figure 1 lists key benefits of SSL technologies to HEL. Lightweight

Scalable Rugged

Compact No Special Support Logistics

Solid-State Laser (SSL) Technologies

Electric Operation

*

No Chemical Hazards

Leverage Commercial Technologies

Deep Magazine

[email protected]

Preprint of a paper 4632-17 presented at the LASE'2002, Directed Energy, San Jose, CA, January 19-25, 2002


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Figure 1: Expected benefits of SSL technologies to HEL Another attribute of solid-state HEL (SSHEL) is a strong prospect for commonality of laser technologies and components (especially laser generator, power supply, and thermal management), which can be used to construct HELs for deployment on a variety of platforms and performing a broad range of missions. Such a commonality coupled with commercial availability of key components (such as pump diodes and laser crystals) provides a fundamental premise for reduced cost of weapon development, production, deployment, and operational support. 2.

CHALLENGES TO DEVELOPING HAP SSL

The key challenges to developing HAP SSL are thermomechanical distortions, which occur in the gain medium as a result of pumping. Even with pumping by the efficient semiconductor laser diodes, a significant portion of the pump energy is dissipated into heat, which is deposited into the SSL medium. This causes thermal lensing, mechanical stresses, and other effects, with likely consequences of degraded beam quality (BQ), reduced laser power, and possibly a fracture of the SSL medium. Popular SSL configurations, such as rods or slabs, are particularly susceptible to such thermal effects. Despite ingenious designs that recently appeared, attempts to further increase average power output from rod and slab lasers drive the design toward regimes of reduced electric efficiency and BQ. Disk-type SSLs enjoy inherently low susceptibility to thermo-optical distortions and have demonstrated lasing at HAP with outstanding BQ [2,3]. Their large, round aperture reduces diffraction and beam clipping losses experienced by other SSL configurations. In a disk gain medium, transverse temperature gradients are reduced because waste heat is extracted from the gain medium in the direction parallel to laser beam axis. Disk laser may use transmissive disks shown in Figure 2a or reflective disks shown in Figure 2b. In a transmissive disk, waste heat is removed by gas flowing through the optical path. In a reflective disk, also known as active mirror amplifier (AMA), the back surface of the disk is available for liquid cooling, which is more efficient than gas cooling and well suited for continuous operation at HAP. t

Hea

Hea

Las

∆

Laser Beam

t

T

t

a) Transmissive disk

Hea

er B eam

∆

Hea

T

Heat

Reflective Coating on Back Surface

t

b) Reflective disk (active mirror amplifier or AMA)

Figure 2: Architectures for pumping and cooling disk lasers Both disk laser types (transmissive and reflective) have been in development since the late 1960s especially as amplifiers for giant pulse lasers for inertial confinement fusion at the Lawrence Livermore National Laboratory (LLNL) and other research establishments [4]. Introduction of diode pumping made it possible to operate disk lasers at the very high-average power required for DEW applications. LLNL is now developing a largeaperture disk laser known as the heat capacity laser (HCL). The initial version of HCL completed in 2001 at LLNL uses large size Nd:glass disks and flash lamp pumping to produce over 10 kW of output power [5]. A diode-pumped version of HCL with Nd:GGG crystals is expected to produce around 100 kW of average power [6]. In HCL, waste heat is temporarily stored in laser crystals. In this mode the HCL can operate for a few seconds before the heat capacity of the crystals is "exhausted." Unfortunately, the need for frequent thermal recovery of the crystals limits the usefulness of the HCL to DEW. Figure 3 shows selected accomplishments and possible future trends in evolution of disk lasers DEW.



Disk lasers for laser fusion (e.g., Nova @ LLNL)

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Lamp-Pumped Heat Capacity Laser

Diode-Pumped Heat Capacity Laser

Diode-Pumped Active Mirror (e.g., CAMIL)

Real-Time Cooling for Continuous Operation

Figure 3 : Evolution of large-aperture disk SSL for very high average power

3.

CAMIL

Boeing is investigating an active mirror concept known as the Compact Active Mirror Laser (CAMIL) that effectively mitigates thermally induced deformations experienced in earlier disk lasers. In CAMIL, a gain medium disk ~2.5 mm thick and 5 to 15 cm in diameter is mounted on a rigid, cooled substrate. The substrate contains a built-in heat exchanger with microchannels on the front surface so that coolant can directly wet the back face of the disk. Except for the microchannel penetrations, the front surface (facing the disk) of the substrate is ground to optical flatness. The disk is attached to the substrate by a pressure differential between the surrounding atmosphere and the coolant fluid in the microchannels. As an alternate approach, we are also investigating the possibility of diffusion bonding the gain medium to the substrate. In either case, the substrate acts as a "backbone" that mitigates thermal deformations of the disk. Pump radiation is injected into the disk either through one of the large faces (face pumping, Figure 4a) [6] or through the peripheral edge (side pumping) [7]. Face-pumped CAMIL using Nd:YAG, Nd:GGG, Nd:glass, or Yb:S-FAP gain medium disks can be integrated into compact modules, which can be used to construct laser amplifiers and oscillators. A laser oscillator may typically use 10 or more CAMIL modules to obtain a round-trip gain sufficient for high outcoupling from an unstable resonator as shown in Figure 4b, for example. Diode Array and Focusing Optics

Alignment Block

Enclosure for Electrical Conenctions

CAMIL Module (typ.)

Outcoupled Beam

Manifold for Diode Coolant Gain Medium Disk

Microchannel Heat Exchanger Rigid Substrate (Optically Transparent )

Outcoupling Mirror

Collimator ASE Absober

a) CAMIL amplifier module

End Mirror

Feedback Mirror

b) Laser oscillator with 10 modules

Figure 4: Conceptual design of CAMIL oscillator showing a modularized design for greater scalability and compact packaging



4.

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AVERAGE POWER SCALING

One key advantage of CAMIL is its scalability over a broad range of average laser powers [8]. In particular, power scaling can be accomplished by increasing the aperture size and/or by increasing the number of modules in a laser oscillator, Figure 5. For any given aperture size, the maximum average laser power is also limited by thermal fracture of the gain medium and by amplified spontaneous emission (ASE) losses. Thus, with increasing aperture size, the laser gain must be reduced to avoid excessive loss to ASE and parasitic oscillations. The figure also shows scaling for a monolithic AMA disk for several gain medium materials.

Aperture Size

Avg. Laser Power Available per Disk [kW]

Number of Modules 350 Nd:YAG, 4.5 cm aperture Nd:GGG, 14 cm aperture Nd:Glass, 30 cm aperture Nd:Glass, 20 cm aperture

Avg. Power Output [kW]

300 250

Monolithic aperture

200 150 100 50 0 0

5

10

15

20

25

30

Number of Amplifier Modules

35

40

25 Nd:YAG 20

Nd:GGG Nd:Glass GGG crystal mfg. limit

15

10

YAG crystal mfg. limit

5

Monolithic aperture 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Disk Aperture Diameter [cm]

Figure 5: CAMIL scaling with disk aperture size and number of amplifier modules The size of a monolithic disk is generally limited by the maximum available size of a monolithic gain medium, which is ~5 cm for Nd:YAG and ~15 cm for Nd:GGG. Larger disks can be produced by diffusion bonding smaller crystals [9]. Nd:GGG is a very suitable choice of laser gain medium for a 100-kW-class CAMIL-based SSHEL. This material has properties comparable to Nd:YAG, but GGG crystals tolerate higher doping with Nd3+ ions and are available in better quality. With increasing aperture size, the heat flux in the gain medium is actually reduced to the point where it becomes practical to use glass as the host material. In contrast to crystals, Nd:glass is inexpensive and it is available in large sizes (up to 100 cm) in a superior quality. Furthermore, Nd:glass has a broader absorption line, which relaxes thermal control requirements for the pump diodes and allows for a more robust system. In a saturated regime, laser power also increases linearly with the number of modules inside the resonator. As seen in Figure 5, several hundred kW of average laser power could be obtained from a laser oscillator with 20 to 30 CAMIL modules using Nd:GGG or Nd:glass gain medium. Using segmented disks, laser power could be extended into MW range. Required segmented apertures of almost arbitrary size can be constructed from subapertures made of monolithic crystals or glass. Figure 6 shows a face-pumped CAMIL module with an aperture constructed from seven segments.



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Note that unlike in some other SSL configurations, the average power scaling of CAMIL does not significantly affect laser BQ. In fact, selecting a larger monolithic aperture for increased power output leads to reduced heat flux, proportionately smaller edge effects, and lower distortions of optical phase front. Pump Diode Arrays Segmented Gain Medium

Support Substrate

Collimator

Figure 6: CAMIL-FP using a gain medium with seven segments 5.

SYSTEM ARCHITECTURE

Principal elements of a SSHEL system are the SSL device, electric power supply (EPS), and thermal management system (TMS). In many DEW applications, SSHEL is expected to provide nearly continuous output for up to several hundred seconds. This is followed by a period of downtime that can be used for "recovery." During this time the power source is recharged and the thermal condition restored. These operational considerations are reflected in a generic SSHEL system architecture shown in Figure 7. Platform primary power during recovery

Power Supply Ele

Controller Batteries

PMU

ctr

ic P

Charger

Electric Power

Thermal Management

ow

er

Laser Device Laser Gain Gen.

nt ola o C

Optical Pump

Optical Bench

Other

Resonator

Fluid Circuit Fluid Pump

Heat Rejection Unit

Heat rejected to platform or environment

Figure 7: Functional diagram of a generic SSHEL system During lasing, the main EPS load is presented by the pump diodes. However, depending on system configuration, electric power requirements for TMS fluid pumps and other parts of the DEW system also could be very significant. In general, SSHEL platform has excess electric power available, but it may not be capable of directly driving the entire SSL load during lasing. This situation requires the EPS to contain an intermediate energy storage device. Of the many options available for energy storage (see e.g., [10, 11]), electrochemical batteries appear to suit most situations [12]. Thus the energy from the battery is drawn to operate the SSL device and the battery is recharged during the recovery period. Modern Li-ion batteries appear to be the best choice since they provide high energy density (~135 kJ/kg), high power density (~1.3 kW/kg), are compact, lightweight, durable, and have a long lifetime [13]. Because of the relatively short discharge time, the SSHEL Preprint of a paper 4632-17 presented at the LASE'2002, Directed Energy, San Jose, CA, January 19-25, 2002


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battery is likely to operate in a power-limited (rather than energy-limited) mode, i.e., it is sized to a design power load. This makes the specific power density of prime importance and its improvements subject to intense technology development. Projected improvements in Li-ion technology would provide ~3.3 kW/kg power density in the near-term and around 10 kW/kg in the far-term [ibid]. These new capabilities would further reduce weight and size of future DEW systems. The TMS receives waste heat from the laser device and rejects it to the platform or the environment. Either it can reject the heat in real time or it can temporarily store the heat and reject it during the recovery period. Importance of the TMS on the SSHEL is often overlooked. Despite using the efficient semiconductor diodes for pumping, operation of SSL still produces a significant amount of waste heat that must be rejected from the system. Two major heat loads are the pump diodes and the SSL gain medium. However, cooling also may be required for various optical elements (e.g., beam collimators and apertures) in the SSL and beam control system (BSC), as well as EPS elements (battery, power management unit - PMU, and charger). Assuming a 20% electro-optical efficiency for the SSL, for each Joule of laser energy produced, 4 Joules of heat must be removed from an SSL system and rejected to the environment. For example, SSHEL producing 100 kW of laser power for 100 seconds will generate around 40 MJ of waste heat. A system with a lower efficiency will generate comparably higher heat load. Since a TMS could represent a large portion of a SSHEL, a significant incentive exists to reduce its size and weight. TMS cooling fluid cycle can be either closed or open (i.e., with expandable coolant). Suitable closedcycle TMS could be realized with a thermal storage medium receiving SSL waste heat during lasing and transferring it to the platform or the environment during the recovery period [14]. Such systems can be sufficiently lightweight (specific thermal energies up to ~300 kJ/kg) and compact for SSHEL applications. Comparable open cycle TMS could be as much as ten times lighter, but may require some support logistics to replenish the coolant expended in operation. 6.

HARDWARE INTEGRATION

The Boeing SSHEL concept uses a laser oscillator with multiple CAMIL gain modules placed inside an unstable resonator [15]. To achieve high-level compactness, we developed a resonator with an axisymmetric layout shown in Figure 8. Analysis shows that CAMIL effectively mitigates thermo-optic distortions in the gain medium. If desirable, residual phase-front correction can be accomplished with intra-cavity adaptive optics. Resonator elements are mounted on a cylindrical optical bench providing sufficient stiffness to maintain optical alignment under operational dynamic loads. If necessary, residual beam jitter can be removed with a fast steering mirror. The assembly is further provided with power buses and coolant manifolds, and with electric and fluid connections for each CAMIL module. The resonator assembly is placed inside an enclosure protecting the components and maintaining the pressure required for attachment of CAMIL disk to the heat exchanger. As an illustrative example, we will outline a design for a 100 kW-class CAMIL-based SSHEL device [16]. The device would use 16 identical CAMIL modules each with a 15-cm-diameter Nd:GGG gain medium disk. The disks are doped with Nd3+ for 90% pump absorption in two passes and are diode-pumped in a long pulse mode to the regime limited by ASE and thermal fracture (Figure 5). Pump pulse duty cycle is conservatively chosen to keep the thermal load well below thermal fracture limit of the material. Using a simple onedimensional laser oscillator model, we calculated that 124 kW of laser power could be extracted from the system at about 20% electro-optical efficiency. The resulting SSHEL is sufficiently lightweight and compact for integration onto a medium-size land vehicle, Figure 9. Additional improvements are expected with the use of side-pumped CAMIL [7].



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••Self-contained Self-contained ••Compact Compact ••Lightweight Lightweight ••Rugged Rugged

Module

Electrical Feedtrough End Mirror

Optical Bench

Deformable Mirror

Resonator Layout

Outcoupling Mirror CAMIL Module Electric Power Bus

Resonator Integration

Window

Ouput Laser Beam

Coolant Feedthrough

Integrated Laser Device

Enclosure Coolant Manifold I&C Feedthrough

Figure 8: Approach to integration of CAMIL SSHEL device Beam Control System

Laser Device

Thermal Management System

Electric Power Supply

Integration Platform: HMMWV

Figure 9: Integration of SSHEL onto a HMWVEE 7.

SIZE AND WEIGHT SCALING

We have developed a model for scaling SSHEL weight and size with average power output and with operating time. For this purpose, we grouped all subsystems into two groups according to their scaling with 1) laser power only or with 2) a combination of laser power and operating time (i.e., laser energy). For this purpose, "descriptors" (sigma's) were assigned to the SSHEL system and each of its subsystems quantifying their weight and size scaling with laser power or with laser energy. These descriptors permit an easy way to calculate SSHEL weight and size, Figure 10a. Top-level descriptors used for the laser device, electric power supply, and thermal management system are shown in Figure 10b. Symbols P, E, w, and v shown in the figure respectively denote scaling with power, energy, weight, and volume. Symbols λ, ε, and τ, respectively designate the power or energy type as laser, electric, or thermal. The model calculates descriptors for the SSL system, its elements, Preprint of a paper 4632-17 presented at the LASE'2002, Directed Energy, San Jose, CA, January 19-25, 2002


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subsystems, and components. Fidelity of the model is established by anchoring to known hardware. We found the above representation of SSL system weight and size very useful for direct comparison of a wide range of laser, power supply, and thermal management technologies. SSHEL System Descriptors σ σP,w = specific laser power per unit weight [Wλλ/kg]

SSHEL Parameters

σ σP,v = specific laser power per unit volume [Wλλ/liter]

Weight = Laser power x (1/ σ σP,w + Run time / σ σE ,w )

σ σE,w = specific laser energy per unit weight [Jλλ/kg]

= Laser power x (1/σ σP,v + Run time / σ σE,v )

Size

σ σE,v = specific laser power per unit volume [J λλ/liter]

a) SSHEL System σ σP,w

Thermal Mgmt. σ σττP,w σ σττP,v σ σττE,w σ σττE,v η ηε−τ ε−τ

σ σP,v

σ σE,w

σ σE,v

Power Supply σ σεεP,w σ σεεP,v σ σεεE,w σ σεεE,v

Laser Device σ σλλP,w σ σλλP,v η ηε−ο ε−ο

b) Figure 10: a) SSHEL system descriptors, b) Element descriptors The model also estimates sizes and weights of various components in a CAMIL-based SSHEL device and calculates pertinent descriptors. This information is then used to calculate the distribution of sizes and weights for the key system elements and system weight scaling with run time, Figure 11.

Laser Device 15 cm Ø N d:GGG disks, η e-o = 20%, σ P,w = 172 W/kg, σ P,v = 68 W/liter

Coolant Pump and HEX (dry) 11% Power Conditioning 11%

Controls 4%

Amplifier Modules 23%

Optical Bench & Mounts 16%

Electrical Bus 7% Coolant Lines & Manifolds 4%

Resonator Optics & Mounts 7%

Enclosure 17%

a) Laser System (w/o Beam Control) 100 sec (up to continuous) lasing Wet Weight Distribution Laser device Thermal 27% mgmt. 39%

Power supply 34%

Volume Distribution Thermal mgmt. 21% Laser device 47%

Weight Volume

0

Power supply 32%

100

Run Time [seconds]

b) Figure 11: a) SSHEL device weight breakdown, b) Laser system weight and size distribution In addition, the model permits parametric studies, for example, such as scaling of system weight with SSL electro-optical efficiency (Figure 12a) or with recovery time (Figure 12b). Note that there is a strong incentive to keep the electro-optical efficiency well above ~15% and that efficiencies beyond ~25% provide diminishing



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returns. Also, in a stand-alone SSHEL system we investigated, recovery times below about 20 minutes appeared to be technically challenging and carried a substantial weight penalty. Laser Device σσ [W/kg] 43

86

129

172

258

215

16000

120 Thermal Mgmt. Power Supply 72 kJ/ kg 0.3 kW/ kg 143 kJ/kg 0.6 kW/kg 286 kJ/kg 1.3 kW/kg

SSHEL System Weight

14000 12000

100 80

10000 8000

Power

60

CAMIL

6000

Weight

40

4000

20

2000

0

0 5

10

15

20

25

0

30

0.2

0.4

0.6

0.8

1

Recovery Time [hrs]

Electro-optical Efficiency [%]

a) Weight scaling with electro-optical efficiency for some choices of EPS and TMS parameters

b) Weight and recharge power scaling with recovery time

Figure 12: Examples of SSHEL model outputs 8.

TECHNOLOGY HERITAGE

Over the last 30 years, the U.S. Government and private industry invested heavily in developing HAP lasers. Fortuitously, this investment created several key technologies that make CAMIL possible, namely high-quality laser gain medium available in large dimensions, microchannel heat exchangers, pump diode arrays, largeaperture disk lasers, large-aperture cooled optics and mounts, unstable resonators for extraction of diffractionlimited beams, methods for mitigating ASE, and intracavity beam control, Figure 13.

Diffusion Bonded µChannel HEX (Boeing)

Large-Aperture Disk Lasers (LLNL)

Large Diode Pump Arrays (Boeing)

The next step: Technology integration

Segmented Apertures

Large Laser Crystals Large-Aperture Cooled Optics & Mounts

(Litton)

(Boeing, Textron)

Figure 13: Key technologies that make it possible to develop CAMIL in the near term Preprint of a paper 4632-17 presented at the LASE'2002, Directed Energy, San Jose, CA, January 19-25, 2002


9.

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COMMERCIAL APPLICATIONS OF SSHEL TECHNOLOGIES

A variety of SSHEL technologies also shows good prospects for commercialization. HAP SSLs are particularly important for material processing. The sales of such lasers approached $400 million in 2000 [17] and are expected to grow significantly during this decade. At present, HAP SSLs have a very limited BQ. Commercial availability of a multikilowatt SSL with good BQ (i.e., high-brightness) will enable deep-penetration welding and precision cutting with only a very small heat-affected zone [18], Figure 14. Laboratory testing shows that such SSL would be very beneficial to the Department of Defense (DOD) production programs where they could be used for cutting, welding, and drilling in construction of aircraft, armored vehicles, and ships [19]. Other emerging applications include rock drilling for oil and gas, and dismantlement of nuclear reactors. Consequently, Government investment in a scalable SSL technology would develop a family of laser devices to meet diverse needs of the U.S. armed forces and greatly enhance the U.S. industrial base. Beam Features Beam Brightness Intensity Profile Depth of Field High Peak Intensity Long Depth of Field

High

Low Peak Intensity

Low

Short Depth of Field

Impact on Processing Quality Cutting Sharper cut, small kerf, less burr, faster processing

Large heat affected zone, uneven edge, burr

High-brightness ⇒ Deep penetration and narrow heat affected zone

Welding Faster production of high quality full penetrationwelds

Large heat affected zone, meniscus, weaker weld

Caterpillar Inc.

Low-brightness ⇒ Shallow penetration and wide heat affected zone

Figure 14: High-brightness SSL enable precision cutting and deep-penetration welding (photos courtesy of Caterpillar Inc.) 10. CONCLUSION We presented a HEL concept based on CAMIL, a disk-type SSL operating in active mirror mode. A suitable power oscillator can be constructed from CAMIL modules placed inside an unstable resonator. CAMIL shows excellent scalability to HAP required for DEW applications and can be integrated into a simple, compact, lightweight, and affordable unit requiring only very low logistics support. Analysis indicates that CAMIL can achieve high electric efficiency, which is very important to a HEL design because it also drives the size of the power supply and cooling system. Numerous key components for CAMIL are available commercially; for example, pump diode arrays, heat exchangers, and high-quality laser gain medium with appropriate dimensions. These attributes suggest that full-scale CAMIL devices could be developed in the near term. ACRONYMS AMA ASE BCS BQ CAMIL CAMIL-FP CAMIL-SP DE DEW DOD

- Active Mirror Amplifier - Amplified Spontaneous Emission - Beam Control System - Beam Quality - Compact Active Mirror Laser - Face-Pumped CAMIL - Side-Pumped CAMIL - Directed Energy - Directed Energy Weapon - Department Of Defense

EPS HAP HCL HEL LLNL PMU SSHEL SSL TMS UAV

- Electric Power Supply - High-Average Power - Heat Capacity Laser - High Energy Laser - Lawrence Livermore National Laboratory - Power Management Unit - Solid-State High Energy Laser - Solid-State Laser - Thermal Management System - Unmanned Air Vehicle



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REFERENCES 1.

2.

3.

4.

5.

6. 7. 8.

9. 10.

11.

12. 13. 14.

15. 16. 17. 18. 19.

M.J. Wardlaw, "Low, medium & high power laser applications," paper APP-5 in Technical digest from the 14th Annual Solid-State and Diode Laser Technology Review, held in Albuquerque, NM, May 2025, 2001 G. F. Albrecht, B. Comaskey, and L. Fury, "A 1.4 kJ Solid-state power oscillator with good beam quality," UCRL-JC-115127, August 1993, available from the Technical Information Dept. of the Lawrence Livermore National Laboratory, U.S. Dept. of Energy L. Hackel, "The Mercury laser, a diode-pumped solid-state laser driver for inertial fusion, is activated," UCRL-TB-136126-01-02, February 2001, available from the Technical Information Dept. of the Lawrence Livermore National Laboratory, U.S. Dept. of Energy J.L. Emmett, W.F. Krupke, and J.B. Trenholme, "The future development of high-power solid-state laser systems," UCRL-53344, November 1982, available from the Technical Information Dept. of the Lawrence Livermore National Laboratory, U.S. Dept. of Energy C. B. Dane, L. Flath, M. Rotter, S. Fochs, J. Brase, and K. Bretney, "Army solid-state laser program: Design, operation, and mission analysis for heat capacity laser," paper APP-2 in Technical digest from the 14th Annual Solid-State and Diode Laser Technology Review, held in Albuquerque, NM, May 2025, 2001 J. Vetrovec, “Active mirror amplifier for high-average power,” SPIE vol. 4270, pp. 45-55 (2001) J. Vetrovec, “Compact active mirror laser (CAMIL),” SPIE vol. 4630 (2002) J. Vetrovec, "Solid-state laser scalable to ultra-high average power," paper P-8 in Technical digest from the 14th Annual Solid-State and Diode Laser Technology Review, held in Albuquerque, NM, May 2025, 2001 A. Bayramian, et al., "Development of ytterbium doped Sr5(PO4)3F for the Mercury Laser Project," UCRL-JC-131875 M.R, Hallada and R.F. Walter, "Power system options for electrically-driven lasers," in Technical digest from the 13th Annual Solid-State and Diode Laser Technology Review, held in Albuquerque, NM, May 5-8, 2000 I.R. McNab and G. Frazier, "Recent developments in high-power from pulsed alternators, flywheels and batteries," 13 th Annual Solid-State and Diode Laser Technology Review, held in Albuquerque, NM, May 5-8, 2000 J.P. Fellner, "Electrochemical power/energy storage for diode lasers," in Technical digest from the 13th Annual Solid-State and Diode Laser Technology Review, held in Albuquerque, NM, May 5-8, 2000 T. Sack and T. Matty, "CHPS Li ion battery technology," in Technical digest from the 14th Annual Solid-State and Diode Laser Technology Review, held in Albuquerque, NM, May 20-25, 2001 D. McAllen, M. Hallada, R. Walter, and J. Bautch, "An evaluation of electric SBL feasibility and technology development requirements," in Technical digest from the 14th Annual Solid-State and Diode Laser Technology Review, held in Albuquerque, NM, May 20-25, 2001 J. Vetrovec, "Large-aperture disk lasers for DEW applications," in Proc. from the 4th Annual Directed Energy Symposium, Huntsville, AL, October 29 - November 1, 2001 J. Vetrovec, “Conceptual design of a 100 kW solid-state laser,” paper SS-1 in Technical digest from Solid-State and Diode Laser Technology Review, held in Albuquerque, NM, May 20-25, 2001 S.G. Anderson, "Review and forecast of the laser markets; Part I: Nondiode lasers," Laser Focus World, pp. 88-110, January 2001 J. Machan et al., "High-brightness, 3 kW diode-pumped, industrial laser," in Proc. from ICALEO'1999, San Diego, CA, November 15-18, 1999, 143-148 P. Denney, “Advances in laser beam welding technology,” in Proc from Laser Applications in DOD, PennState-ARL, State Collage, PA, September 2000
