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UNCLASSIFIED, PUBLIC RELEASE
Polygon Ring Laser Amplifier (PRLA)
As military deployments move from stationary ground, mobile, shipboard, and airborne based platforms, low size, weight, and power (SWaP), and cost is becoming increasingly important for deployment of 100kW to >1MW high energy laser (HEL) output power for directed energy (DE) applications.
Toward a solution, the PRLA, conceived and designed by River Electro-Optics, LLC, is an innovative and potentially market disruptive solid state HEL amplifier. It is intended to compete with and replace fiber, slab, and thin disk-based HEL systems currently being used in the military, commercial and research markets.
This claim is based on the now dual patent pending Distributed Gain - PRLA (DG-PRLA) amplifier and Chemical Vapor Deposition (CVD) laser media / active mirror designs.
The PRLA is a solid state, monolithic, all ceramic, laser amplifier composed of a ring-shaped resonator cavity that exhibits natural polygon resonance when excited by an externally applied pump laser beam.
The equally important fabrication process involves CVD processing of the laser media (pseudo-thin disk / active mirror) on the inside diameter at the naturally occurring periodic reflection points of the ring. The resonator size can be increased to accommodate the required area/number of reflection points of the laser media. The laser media thickness can also be conveniently increased using CVD as determined by the output laser power requirement.
Because of the efficient cooling configuration and the flexibility in resonator and laser media size fabrication, laser output powers of 100s of kW to megawatt magnitudes are anticipated to be achievable in a low SWaP configuration. This capability will be further facilitated by current development of higher power pump laser diode technology.
The polygon ring construction makes access to whispering gallery mode (WGM) action at its center as a possible optical interface avenue that can exploited and will be investigated.
Current laser systems with beam control purchase costs to the military are roughly $0.8M / kW.
So, a 250kW directed energy laser system will cost ~$200M.[1][2][3]
It is known that the initial cost involved in developing military laser systems is high due to the need for sufficient time for R&D, infrastructure to develop laser systems from scratch, field testing, demonstration, and installation. Moreover, the development and production of military laser systems are complex and require a high technical expertise to design and manufacture, leading to high production costs. In addition, the high level of sophistication and precision required in these systems means that they need to be regularly maintained and upgraded in the field of deployment, further adding to the overall cost and making use of it more difficult for the warfighter.[2]
The costs associated with the PRLA modular and low part count resonator design and the CVD laser media processing promise to lower the manufacturing and maintenance costs of HEL systems.
It is anticipated that the low part count design will also lower the overall development costs.
The PRLA Distributed Gain / Polygon Ring Laser Amplifier (PRLA) with liquid / cryo/ air cooling capability is comprised of the following primary parts and processes.
PRLA Resonator
• Ring shaped all ceramic substrate- with diamond machined and ultra-polished resonator mirror form segments with astigmatism corrected confocal radii. The substrate is configured to be hermetically sealable both during the CVD coating process and ultimately during use as a laser. The amplifier body is designed to adhere to vacuum flange ISO-CF standards. This makes scaling up in size and interfacing to the amplifier easier and more predicatable during CVD processing and use as a laser.
• Distributed gain active mirrors- (pseudo thin laser disk) made up of: CVD multilayer broadband mirror, rare earth doped laser media, and anti-reflection coatings at each polygon mirror formed reflection point. CVD laser media lowers cost, allows application in areas not easily accessible, allows use of laser host and rare-earth (RE) doping materials not available in crystal grown methods. Use of RE doped sesquioxide ceramic host materials will facilitate laser outputs in the 1 to >5 um wavelengths feasible. Lutetium oxide (Lu2O3) is a high thermally conductive host material doped with Yb and Er RE are of high interest.
• Liquid cooling jacket - provides liquid (water and index matching fluid internally) cooling flow that can efficiently access all active mirror segments. The ISO-CF standard substrate facilitates this cooling capability by providing a easily implemented jacket design that is simply the next standard size up to that encases the substrate in a hemetically sealed cooling envelope.
• Laser Pump- High power laser diode seed pump operating at the RE doped laser media absorption wavelength band. this forms a distributed gain Master Optical Power Amplifier (DG-MOPA)
POLYGON RING OPTICS RESONANT MODES - Operating in one plane, convex and star polygon resonant modes rely on the fact that a laser beam entering a circular mirror, with a specular reflecting inside diameter, at an angle b with respect to the major diameter chord, can propagate along a star/convex polygon patterned resonant mode. These resonant modes are described by two parameters: the number of reflection points p and the density q of the polygon.
b = p/2 (1- 2*q / p)
Variable (p) defines the number of reflections that occur before reaching the input again. Variable (q) defines the relative size of the tangent central circle. It is identified by the number of points made counting CW after the entrance point to where the entrance ray strikes the circle first. Another simpler mode is the convex polygon where there are only reflection points along the inner circumference (whispering gallery) with no crossover points giving a density q = 1.
Shorthand for star polygons are written in the Schlafli notation where the number of points is written in the numerator and the density value is in the denominator e.g. a 5 point star polygon is thus written as Schlafli 5/2.
Prior Art - Thin - Disk multi-pass laser systems are typically designed around a single thin disk e.g. (10-25mm diam. 100 - 300um thick) to scale power output.[4]
This configuration, and others like it, are not easily scalable in terms of increasing the number of thin laser disks and cooling system in a single amplifier design. All optical parts such as the mirrors and thin laser disk, are individually constructed which increases cost considerably compared to PRLA CVD processed active mirrors.
Polygon multi-pass optical cells have only been used for metrological purposes such as laser absorption spectroscopy (gas spectrographic analysis) where the optical path length is extended to increase the sensitivity of the measurement system. [5] [6]
The PRLA makes novel use of the technologies noted above. As is immediately apparent, the quantity of laser media involved is substantially increased in a single PRLA amplifier by the distributed active mirrors at each reflection point along with high cooling efficiency. Due to the large surface-to-volume ratio, the heat dissipation from the active mirror into the heat sinking ceramic substrate is very efficient, thus allowing operation at extremely high-power densities.
Using CVD, obviously there is no soldering of the active mirror layer to the substrate heat sink necessary. For suppression of amplified spontaneous emission (anti-ASE), CVD deposition of an undoped host cap to each active mirror can be easily implemented without a power wasting bonding interface.
Other supportive laser elements can also be CVD processed such as graphene based saturable absorber passive mode locking for ultra-short pulse performance.
PRLA POWER SCALING CONFIGURATIONS
Vapor deposition of rare earth laser media on standardized ring substrates makes modularized laser resonators much simpler, more compact, and easier to cool. Both stacked and planar ganged amplifier configurations are facilitated. Active mirror diameters and thicknesses can be increased accordingly.
The polygon resonator modes suggest fractalized scalar configurations that can be explored including Fractal Stacked and Fractal Planar configurations.
Chemical Vapor Deposition (CVD)
The PRLA monolithic design prompted the need for vapor deposited laser media and its associated optical supportive coatings.
Current laser disk media is fabricated either by crystalline growth or hot pressing of polycrystalline ceramic media. Depending on how they are used (such as thin disk laser applications), the disks must be individually cut, polished, coated, etc. which make them labor intensive. Subsequently, they must be mounted and held in placed in the laser resonator. Disk size and material type is limited using the crystal growth method. The extremely high melting point of, for example, lutetium oxide (2490°C) has made crystalline growth of it difficult. Hot pressing methods of laser media are still being evaluated.[7]
A CVD method of creating rare earth doped lutetium oxide (Lu2O3) coatings has been evaluated and has been determined to be feasible.[8] Similar reasons to move away from hot pressing / sintering of individually fabricated Lu2O3 are echoed in the Topping / Sarin paper for scintillating detectors.
CVD has been chosen as the vapor deposition method to be used because of its capability of providing high deposition rates, coating purity, and its conformal nature so as to provide a uniform coating over a complex shape. This is specifically relevant to the PRLA mirror form.
To exploit the isolated individuality of each polygon deposition site, a right angled/spoked precursor transport system was designed to deliver the precursor mix only at each deposition site. The deposition site is not laminar but perpendicular to the transport flow. The intent is to minimize gas phase homogenous reactants which can be a deposition rate limiting process.
Plan for Prototype Fabrication and Verification
Phase I - includes the modeling, specification, fabrication, and verification of an all-ceramic disk shaped, ring cavity that supports polygon resonance on the inside diameter of the ring. Diamond turned and ultra polished mirror forms will be machined and polished at multiple / periodic locations on the inside diameter of the ceramic ring. These segmented locations are defined mathematically according to star and convex polygon resonance.
Phase II - Proves the patent pending ability to selectively apply via CVD, rare earth doped laser gain material to the segmented I.D. of the resonator. Includes the fabrication process performance verification of the applied laser gain medium and other supporting laser system elements. This verification process includes proving stimulated emission, a high-quality beam, and substantial laser efficiency. The DG-PRLA-MOPA must lase!
Fiber Laser Limitations
Fiber based lasers are the leading directed energy HEL technology. Scaling of single mode fiber laser systems requires beam combining optics to increase the laser power outputs to achieve the 300kW - 1MW energies required for present military requirements. High power scaling of the industry front runner single-mode fiber lasers to achieve these energies in a diffraction-limited output power face severe physical limitations.
These limits can be classified into four distinct categories: nonlinear effects (NLE), thermal issues, optical damage, and pump power limitations. Decreasing the fiber length and peak intensity are the very first general measures to mitigate the NLEs [9], however, this requires an increase in the number of fiber modules and the need for more coherent or incoherent combining that adds more size weight and complexity to an already high SWaP HEL system.
increases the SWaP considerably to achieve a higher power output. Thin disk and slab-based HEL have their own limitations as output power requirements go up including the cost of the fabrication, mounting, alignment, etc. of each disk or slab laser media.
The PRLA distributed gain configuration reduces the bulk of beam combining optics as each of the multiple active mirror segments within one resonator is equivalent to a single fiber laser amplifier module.
For example, a 250mm diameter PRLA substrate can accommodate up to eighteen 35mm diameter active mirrors, each of which contains the same rare ion doped laser gain volume of a single mode fiber 10m in length putting out ~10kW.
Therefore, under the same laser input pump environment, each active mirror segment can output the equivalent of an individual single mode fiber laser. Because of its pseudo thin laser disk configuration, it has been estimated that a single PRLA active mirror segment can output >40kW under ideal conditions [10]. The optimum design, of course requires a compromise between amplified spontaneous emission (ASE), overheating, and round-trip losses.
It is anticipated that the potential for higher output power per PRLA, with accordingly higher internal circulating power can be better tolerated because of the improved cooling efficiency of the monolithic and disk shaped amplifier configuration. The symmetry of the cavity should play a part in the maintenance of stability of the cavity resonsance during heat cycles and flow during the lasing process. Cooling fluids can envelope the entire amplifier externally and internally (with index matching fluids). Cryo-cooling of the PRLA - HEL system is also more easily achievable.
The PRLA active mirror segments are differentiated from the typical thin laser disk because of its chemical vapor deposition fabrication method vs. the individual thin disk component that is more difficult to manufacture, limited in size and materials it can be made from, must be soldered / bonded in place, individually cooled, and aligned, etc.
A single 10kW single mode laser system manufactured by one of the leading suppliers of fiber optic laser systems has the following specifications:
Using a 10kW basis, rough estimates of the beam combined outputs of single mode, present day technology fiber laser.
A 300kW output system can weigh up to 14 tons with a footprint area of 265ft2. A 500kW unit will be 23 tons with a footprint area of 442 ft2. These systems values include the internal DC power supplies. For comparison, a single 2005 Audi A6 car weighs ~ 2 tons (US).
Low SWaP HEL - DE systems are inherently relevant to future military missions.
The Value Proposition - It is suspected that the current front runner fiber optic laser technology is plateauing with the recently demonstrated 300kW HEL-DEL weapon by Lockheed Martin, Boeing (partnered with General Atomics), and others. The 300kW power level meets some of the mid-sized and short-range DEL Weapon target requirements
The recent Boeing / General Atomic-EMS partnership (slab laser media based resonator) is targeting the same 300kW level in the next year or two. The 500 - 600kW level is anticipated to occur within the next several years. A 1-megawatt HEL is estimated to emerge up to 8 years from now with current technology.
Lockheed Martin, in fact, has just announced (2023) its intent to develop a 500kW HEL on the heels of its recently delivered 300kW units. Unless some breakthrough fiber-based HEL technology is being used, it is assumed that spectral / incoherent beam combining will also be used to achieve that 500kW goal. Scaling up from 300 to 500kW will likely include a proportional increase in SWaP.
It is very apparent that a leap in HEL technology is needed to move directed energy HEL systems into warfighter's hands, airborne, shipborne, ground mobile and even stationary platforms that are not this unwieldy.
References:
[1] Why the Navy isn’t shooting down Houthi drones with lasers yet - Science & Technology
Editor, Defense One, P. Tucker, 1/2/2024
[2] US Navy Combat Lasers Are Too Expensive for Actual Deployment -, B. Wang, 4/27/23
[3] Navy Shipboard Lasers: Background and Issues for Congress Updated April 21, 2023
[4] COMPASS - Compact Multi-Pass Amplifier System: Powerful ultra-short pulse laser
amplifier (kW range), Technologie-Lizenz-Buro (TLB GmbH), patent US 10,574,024 B2
[5] Compact multi-pass optical cell for laser spectroscopy Béla Tuzson,1,* M. Mangold, 2013
[6] Multi-pass Optical Device and Process for Gas and Analyte Determination , Patent No.: US
7.876,443 B2, Jan. 25, 2011
[7] Synthesis of High Purity Yb31-Doped Lu2O3 Powder for High Power Solid-State
Lasers;Woohong Kim, Colin Baker et.al., Naval Research Laboratory, Washington, D.C., 2011
[8 ] CVD Lu2O3:Eu coatings For Advanced Scintillators Stephen G. Topping and V. K. Sarin
Materials Science and Engineering, Boston University, Brookline, MA 02446, USA
[9 ] Towards Ultimate High-Power Scaling: Coherent Beam Combining of Fiber Lasers, H.
Fathi, Mikko, R. Gumenyuk - December 2021
[10] Fifteen Years of Work on Thin-Disk Lasers: Results and Scaling Laws, A. Giesen, Jun. 2007
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