N221-063 TITLE: Nonlinear Mitigated Gain Fiber Development for kW-class Fiber Lasers

OUSD (R&E) MODERNIZATION PRIORITY: Directed Energy (DE)

TECHNOLOGY AREA(S): Weapons

The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws.

OBJECTIVE: Develop Stimulated Brillouin Scattering (SBS) mitigated rare-earth-doped fibers supporting the advancement of narrow-linewidth kW-class fiber amplifiers essential for future high-energy laser (HEL) weapons integration with reduced cost and power scalability.

DESCRIPTION: The rapid development and deployment of HEL systems have enabled laser weapons in different platforms for DoD�s applications. Kilowatt (kW)-class single-mode fiber lasers have been the key engines for these HEL systems because of their inherent advantages including excellent beam quality, outstanding heat dissipation capability, high single-pass gain, hermetically guided laser beam, and high-power scalability. There is high demand for kW-class fiber amplifiers for narrow-linewidth HELs that are required for specific applications. However, kW-class fiber amplifiers always suffer from SBS, RAMAN scattering, and other nonlinear effects such as four-wave mixing (FWM), self-phase modulation (SPM), and stimulated RAMAN scattering (SRS), etc. Although several types of large-mode-area (LMA) fibers have been fabricated and used to demonstrate kW-class fiber amplifiers with mitigated nonlinear Kerr effects and SRS, SBS is still a major constraint on the power scaling of kW-class fiber amplifiers for narrow-linewidth lasers. Therefore, innovative rare-earth-doped fibers with mitigated SBS and other nonlinear effects are crucial for the development of narrow-linewidth kW-class fiber amplifiers. The U.S. Navy is searching for an innovative nonlinear mitigation kW class rare-earth doped fiber at 1 to 2 �m wavelength. Current commercially available kW class amplifier power is limited to approximately 2 kW per amplifier. This SBIR topic seeks innovative technology to increase the HEL SM (Single mode) narrow line width amplifier to approximately 4 kW. This topic seeks design, development, and fabrication of > 6 kW-class rare-earth optical fiber . Commercial state of the technology is only around 2 kW per amplifier due to SBS.

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 implemented and approved by the Defense Counterintelligence Security Agency (DCSA), formerly the Defense Security Service (DSS). The selected contractor 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 DCSA and NAVSEA 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: Develop a concept and demonstrate the feasibility of that concept to meet the Navy�s requirements as outlined in the Description. Demonstrate the power scalability of the new single mode SBS reduced optical fibers for narrow-linewidth laser amplification. Provide the design of kW-level fiber amplifiers and show path to reaching 6-kW laser output without mitigated SBS and other nonlinear effect. The Phase I Option, if exercised, will include the initial design specifications and capabilities description to build a prototype solution in Phase II.

PHASE II: Under Phase II base period develop the SBS reduced fiber that can meet the Navy requirements and in Phase II Option I use this type of SBS reduced doped fiber for > 5kW class amplifier design and demonstrate the performance to the Navy. And in Phase II deliver a prototype system for testing and evaluation based on the results of the Phase II base and Option I. Phase II Option II, deliver SBS reduced doped fiber > 200 meter to the Navy for the final evaluation done at a Navy lab. Results shall be used to optimize the design, modeling and fabrication of the rare-earth-doped optical fibers and provide a fiber amplifier prototype that can be used to achieve 6-kW narrow-linewidth (< 0.5 nm) laser at � 1 micro meter, 1.5 micro meter and 2 micro meter.

It is probable that the work under this effort will be classified under Phase II (see Description section for details).

PHASE III DUAL USE APPLICATIONS: Support the Navy in transitioning the technology to Navy use. Advanced rare-earth doped optical fibers with mitigated SBS and other nonlinear effects can be used in various HEL weapon systems for DoD applications, DOE�s accelerator lasers, the laser sources for free-space communication, kW class optical power delivery over > 300 meter, and remote sensing systems.

For commercial and industrial application; this technology could produce compact, efficient, kW class HEL systems for industrial material processing applications such as welding, cutting, soldering, marking, cleaning, etc., in the automotive and aerospace industries.

REFERENCES:

1. T. T. Alkeskijold, M. Laurila, L. Scolari, and J. Broeng, "Single-mode ytterbium-doped large-mode-area photonic bandgap rod fiber amplifier", Opt. Express 19 (8), 7398 (2011).
2. L. Dong et al., "Leakage channel optical fibers with large effective area", J. Opt. Soc. Am. B 24 (8), 1689 (2007).
3. D. Jain et al., "Demonstration of ultra-low NA rare-earth doped step index fiber for applications in high power fiber lasers", Opt. Express 23 (6), 7407 (2015).
4. F. Beier et al., "Narrow linewidth, single mode 3 kW average power from a directly diode pumped ytterbium-doped low NA fiber amplifier", Opt. Express 24 (6), 6011 (2016).
5. J. Olson, et al, "3 mJ All-Fiber MOPA With a Short-Length Highly Er3+-Doped Phosphate Fiber," IEEE Photon. Technol. Lett. 32 (23), 1481 (2020).

KEYWORDS: Stimulated Brillouin scattering; SBS; RAMAN scattering mitigation; Rare-earth-doped fibers; kW-class fiber lasers; Nonlinear effects; Large-mode-area fibers