Description:
TECHNOLOGY AREA(S): Materials
OBJECTIVE: Development of fiber coupled optical isolators with high power handling capability, low insertion loss, and high isolation, while minimizing volume and weight over current state-of-the-art. Higher power fiber-coupled isolators are needed to reduce required gain at each stage, enabling multi-kW output fiber lasers and enabling amplifiers suitable for laser beam combining, thereby reducing the number of beams required to be combined, and making overall system Size, Weight, and Power (SWaP) requirement reductions possible.
DESCRIPTION: Scaling High Energy Laser (HEL) output power requires increasing the output power per fiber from the current values of approximately 1.5 kiloWatts (kW), and then combining several fiber laser sources to form a single monolithic laser beam. Multi-stage Master Oscillator Power Amplifier (MOPA) architectures have been developed that mitigate the risks of damage to optical components by increasing fiber core diameter in each stage. Fiber-coupled isolators, which are required to limit back reflections between stages, are currently limited to less than 30 Watts (W) forward and 3 W backward propagating power handling capability. This requires final stage amplifier gains of greater than 20 decibel (dB) as output power per fiber is scaled to the 2 to 3 kW range. This could lead to parasitic lasing, increased Amplified Spontaneous Emission (ASE), and increased risk of damage to optical components. Higher power capable fiber-coupled isolators are needed to reduce the required gain at each stage, enabling multi-kW output fiber lasers and amplifiers suitable for beam combining, hence reducing the number of combined beams, and overall system SWaP requirements. This solicitation seeks improvements in the backward power handling capability to reach values greater than or equal to (=) 40 W. Also desired is a reduction by half in mass and volume over current state-of-the-art (currently 20x14x50mm, 57g), with an insertion loss of less than (<) 1.5 dB, and isolation greater than or equal to (=) 25 dB. Work produced in Phase II may become classified. Note: The prospective contractor(s) must be U.S. Owned and Operated with no Foreign Influence as defined by DOD 5220.22-M, National Industrial Security Program Operating Manual, unless acceptable mitigating procedures can and have been be implemented and approved by the Defense Security Service (DSS). The selected contractor and/or subcontractor must be able to acquire and maintain a secret level facility and Personnel Security Clearances, in order to perform on advanced phases of this contract as set forth by DSS and ONR in order to gain access to classified information pertaining to the national defense of the United States and its allies; this will be an inherent requirement. The selected company will be required to safeguard classified material IAW DoD 5220.22-M during the advance phases of this contract.
PHASE I: Phase I will include development and/or use of detailed modeling tools to analyze the performance of fiber coupled isolator suitable for use in fiber lasers/amplifiers suitable for beam combining. Issues that impact and limit the performance of the fiber-coupled isolators will be examined to determine specific figures of merit that improve the power handling capability, while improving insertion loss, and isolation. The results of this modeling will be used to develop prototype fiber-coupled isolator’s designs, with recommendations for the down-selection of a specific design or designs. Consideration shall also be given to reducing the volume and mass of the device. The deliverables will be a detailed technical report of all analysis including discussions on the power scaling limits, and expected performance in terms of forward and backward power handling capability, insertion loss, and isolation. Computer models, including and developed or modified analysis analytical codes, will be delivered on an accompanying CD/DVD. The analysis should consider the practicalities of any proposed material processing required to produce prototype fiber-coupled isolators, or other fiber laser modifications required. Included with the report shall be a detailed fabrication plan for fiber-coupled isolators for Phase II, with alternatives considered with documentation for technical risks, cost, and schedule. Recommended quantities of fiber-coupled isolator designs to be prototyped shall be included, with plans for testing and verification of analytical results. The Phase I final report shall include the fiber-coupled isolator development plan, fabrication and testing timeline, with performance goals and key milestones, for Phase II.
PHASE II: In the first year, based upon the results of Phase I analysis and the development plan reported, fiber-coupled isolator samples will be fabricated and subjected initially to low power (approximately 5 to 10 W) evaluation. Careful measurements of insertion loss and isolation shall be collected and compared to previous results from Phase I, along with any associated thermal performance data. In the second year, high-power fiber-coupled isolators shall be evaluated at a level of approximately 40 W and, if possible, higher powers. Data on resulting power handling capability, insertion loss, isolation, and thermal performance of the fiber-coupled isolators shall be collected, compiled into a provided database, and reported. The goals will be increasing of power handling capability, improving insertion loss and isolation, while minimizing volume and mass. Stable device performance shall be demonstrated for operating times of ten (10) minutes or more at stable continuous-wave (CW) high power levels. The final report shall include all data collected, and a discussion of any remaining steps required to develop a commercial version of the device. It is probable that the work under this effort will be classified under Phase II (see Description section for details).
PHASE III: The primary applications of high power fiber lasers are defense related. However, the techniques employed in fiber laser amplifiers can find use in applications such as high-speed laser cutting and welding, broadband communication, and free space satellite data streaming utilizing lasers with consistently high power lasers with excellent beam quality. The contractor in Phase III shall support the transition of resulting components and design efforts to a ship based laser system and shall further develop the laser technology to support system integration for surface Navy shipboard implementation. A shipboard laser system comprised of multiple fiber lasers which are beam-combined into a single militarily useful laser beam at a very high power levels is expected.
REFERENCES:
1: Augst, S. J., Goyal, A. K., Aggarwal, R. L., Fan, T. Y., and Sanchez, A. "Wavelength beam combining of ytterbium fiber lasers"
2: Optical Society of America (OSA) Optics Letters
3: Vol. 28, Issue 5, pp. 331-333
4: 2003
5: https://doi.org/10.1364/OL.28.000331
6: Paschotta, R., Nilsson, J., Tropper, A. C., and Hanna, D. C. "Ytterbium-doped fiber amplifiers," in IEEE Journal of Quantum Electronics, vol. 33, no. 7, pp. 1049-1056, Jul 1997
7: doi: 10.1109/3.594865
8: Padula, C. and Young, C. "5.4 - Optical isolators for high-power 1.06-micron glass laser system," in IEEE Journal of Quantum Electronics, vol. 3, no. 11, pp. 493-498, November 1967
9: doi: 10.1109/JQE.1967.1074385
KEYWORDS: High Energy Lasers; HELs; Fiber Lasers; High Power Lasers; Fiber Amplifiers; Fiber Laser Coupled Isolators
CONTACT(S):
Peter Morrison
(703) 696-0553
peter.a.morrison@navy.mil
Sean Durrant
(540) 653-2246
Ron Flatley
(540) 653-0486
