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Efficient Compact Diode-Pumped High-Power Fiber Coupled Laser Modules

Description:

TECHNOLOGY AREA(S): Sensors, Electronics, Battlespace 

OBJECTIVE: Develop a highly efficient, high-power density, kilowatt-level, diode pumped fiber coupled laser module. 

DESCRIPTION: High-energy laser (HEL) systems represent a revolutionary advancement in naval warfare. Laser weapons are often described as having an unlimited “magazine depth,” being instantly available, and as not suffering from the issues associated with explosive ordnance storage and handling. However, the lethal power of laser weapons fundamentally derives from the ship’s onboard power. Furthermore, the ship must supply cooling to the laser, in essence, shifting the combat burden onto the ship’s auxiliary systems. As ship’s power (which also fundamentally limits cooling capacity) is a precious resource, deployment of a laser weapon will be greatly facilitated by optimizing the system’s efficiency. More efficient laser technology does not so much reduce the cost of the laser weapon itself but rather reduces the cost associated with shipboard prime power and cooling. Targeting and control of the laser represent a trivial portion of the system’s overall power budget. The power consumed by the system is predominantly used to produce the desired high-power laser output. The cooling required by the system is then directly determined by the overall power conversion efficiency. Even though the laser weapon is highly complex, system efficiencies can be allocated to three basic areas: 1) conversion of prime power to usable voltage levels, 2) conversion of electrical power to light, and 3) losses in the optical system. Prime power conversion (i.e., DC to DC and AC to DC conversion) is an area that has been well addressed elsewhere. The losses in the optical transmission system, which are largely constrained by system design trade-offs, are second-order effects. The fundamental efficiency of the individual laser module, the heart of the system, primarily determines overall efficiency. Under current laser weapons programs, the high-power laser output is achieved by a combination of multiple diode-pumped fiber coupled modules (FCMs) operating at a center wavelength of 976 nm. The FCM is the basic building block of the laser system and therefore largely determines system performance but is constrained by system-level design. A key constraint is the fiber coupling that imposes a fiber core diameter of 225µm and a numerical aperture (NA) of 0.22. In order to limit the number of FCMs that must be combined, the minimum output (mode stripped) optical power is 1000W. However, higher single-FCM output powers present an obvious advantage, provided the efficiency can be optimized at that higher output power. Finally, maximizing optical power density (FCM power output per unit volume in W/cm3) is an additional objective. Even though shipboard applications are not critically sensitive to the FCM weight, optimization of the power density makes the technology viable for airborne applications, thereby reducing cost through commonality and increased transition opportunities that enable economies of scale in production. Restricting the minimum power density also serves to preclude solutions that are overly complex or focus on improvements extraneous to the core FCM technologies. Although FCM technology is continually advancing, current technologies do not exist that solve the Navy’s need. The Navy seeks development of a highly efficient FCM technology meeting the objectives described above. Efficiency is the overriding objective and efficiency exceeding 60% (DC to optical output at the fiber exit) is the minimum expectation. Input voltage is system defined at 18VDC and any further power conversion required by the proposed technology should be included in the efficiency calculation. Minimum mode stripped power output is 1kW although obtaining peak efficiency at a higher output power is highly desirable. Consequently, proposed solutions shall consider the efficiency trade-space for output power up to 5kW. Power density is an important consideration; second only to efficiency and desirable for the reasons explained above. A power density of 1.0 W/cm3 is therefore a threshold requirement. The intended application requires that the technology meet shipboard storage temperature requirements of -51°C to 40°C. 

PHASE I: Define and develop a concept for a diode-pumped fiber coupled laser module as defined in the description. Prove the feasibility of the concept in meeting Navy needs and establish the feasibility of producing the FCM. Feasibility will be established by some combination of initial concept design, analysis, and modeling. The Phase I Option, if awarded, will include the initial design specifications and capabilities description to build a prototype in Phase II. Develop a Phase II plan. 

PHASE II: Based on the Phase I results and the Phase II Statement of Work (SOW), design, develop, test, and deliver prototype FCMs for evaluation. Approximately six prototypes are desired in order to assess stability and demonstrate suitability for optical combining; however, the exact number should be proposed by the company based on the projected cost of development. Demonstrate that the prototypes meet the parameters in the description. Demonstrations will take place at a Government- or company-provided facility. A manufacturability analysis will propose best-practice manufacturing methods to prepare the FCM technology for Phase III transition. The company will prepare a Phase III development plan to transition the technology for Navy laser systems production and potential commercial use. 

PHASE III: Support the Navy in transitioning the technology to Navy use. Further refine the FCM technology according to the Phase III development plan for evaluation and testing to determine its effectiveness and reliability in an operationally relevant environment. Perform test and validation to certify and qualify initial production units for Navy use. The FCM should be a fully functional and packaged sub-assembly, ready for insertion into a laser weapon system. Produce the final product (or under license) and transition to the Government directly through technology upgrades to the Surface Navy Laser Weapon System (SNLWS) program or through insertion into new program baselines in partnership with the SNLWS prime contractor. High power laser technology has a multitude of military, industrial, and scientific applications such as electro-optical countermeasures, materials processing, laser cutting, and materials research. Advances resulting from this topic have wide application in these fields. 

REFERENCES: 

1: Zervas, Michalis N. and Codemard, Christophe A. "High power fiber lasers: a review." IEEE J. Selected Topics in Quantum Electronics, 20, September/October 2014, article sequence number 0904123, 23 pages. http://ieeexplore.ieee.org/abstract/document/6808413/?reload=true

2:  McNaught, Stuart J., et al., "Scalable coherent combining of kilowatt amplifiers into a 2.4-kW beam." IEEE J. Selected Topics in Quantum Electronics, 20, September/October 2014, article sequence number 0901008, 8 pages. http://ieeexplore.ieee.org/abstract/document/6732914/

KEYWORDS: High-Energy Laser; Laser Weapons; Fiber Coupled Module; Optical Power Density; Fiber Coupling; Fiber Core 

CONTACT(S): 

John (Tom) Schriempf 

(202) 781-1196 

john.schriempf@navy.mil 

Lawrence Dressman 

(812) 854-4804 

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