Combiner Architectures for Maximum Brightness Fiber Laser Amplifier Pumping

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Directed Energy (DE) 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: State of the art Yb doped fiber amplifiers (YDFAs) with low size, weight, and power (SWaP) depend heavily on the availability of low SWaP pump diodes to meet their size, weight, and power targets. High power YDFAs have historically used (6+1)x1 fiber taper bundle combiners in a co-pumped configuration, but there is evidence that this area of fiber amplifier power scaling could be refined to bring lower cost and/or higher power. Current topics in combiner design include: (1) counter-pumped combiners to reduce amplified spontaneous emission (ASE), spread the inversion profile of the gain fiber and/or support bidirectional gain fiber pumping. (2) tandem pumped co-pumped and counter-pumped combiner configurations to spread the inversion of the gain fiber across multiple fibers and reduce gain per stage. (2) combiners with higher channel count (e.g. (7+1)x1 and (8+1)x1) to reach record powers. Etendue conservation calculations suggest that channel count might be considerably increased from (6+1)x1 without a significant increase in loss. Both approaches lead to increased power, should be further explored, and are mutually inclusive. Due to the manufacturing challenges and risks of making high power pump diodes with many single emitters, higher channel count means that an amplifier could make use of more inexpensive pump diodes. Such a combiner design may lead to lower overall system cost at state-of-the-art performance. An optimized bidirectional combiner design which is also optimized to maximize brightness conservation is likely to benefit both next generation directed energy weapons at 1.03-1.07 um wavelength as well as the commercial laser material processing industry. Cost per watt and material processing speed are key metrics for commercial systems using high power YDFAs. DESCRIPTION: Demonstrate a production prototype of a combiner architecture for a fiber laser amplifier for use with existing commercial off the shelf (COTS) pump diodes. The combiner architecture should support an amplifier design with at minimum 50% more pump power in the gain fiber than existing state-of-the-art Yb laser systems (see references) and operate with minimum loss of brightness in the combiner. PHASE I: The awardees will provide a set of combiner designs, a trade space survey, and simulation justifying a design for a (N+1)x1 combiner architecture in the context of an amplifier which can meet the high power constraint laid out in the Topic Description. The awardees will also provide a plan to produce the (N+1)x1 combiner(s) necessary to employ the architecture. PHASE II: The awardees will develop manufacturing for and demonstrate a (N+1)x1 combiner handling power consistent with the Topic Description with less than 0.1 dB insertion loss. The performer will also demonstrate combiner handling of 15% of forward power in the reverse direction. The awardees will conduct experiments detailing the forward and backward power handling limitations of the combiner, separating manufacturing challenges from fundamental physical limits. The net loss of brightness will be examined and performance limits will be explicitly discussed. Finally, the awardees will deliver four prototype combiners to AFRL/RDLT. PHASE III DUAL USE APPLICATIONS: Awardees will commence packaging and development work to refine thermal handling and increase the manufacturability of the (N+1)x1 combiner to commercially viable levels, delivering at least 10 units to AFRL/RDLT. REFERENCES: 1. H. Wu et al., Bidirectional tandem-pumped high-brightness 6 kW level narrow-linewidth confined-doped fiber amplifier exploiting the side-coupled technique, Optics Express, vol. 30, no. 12. Optica Publishing Group, p. 21338, May 31, 2022. doi: 10.1364/oe.459850.; 2. R. Li, H. Wu, H. Xiao, J. Leng, L. Huang, and P. Zhou, More than 6 kW near single-mode fiber amplifier based on a bidirectional tandem pumping scheme, Applied Optics, vol. 61, no. 23. Optica Publishing Group, p. 6804, Aug. 02, 2022. doi: 10.1364/ao.465076.; 3. Y. Wang et al., 8-kW single-stage all-fiber Yb-doped fiber laser with a BPP of 0.50 mm-mrad, Fiber Lasers XVII: Technology and Systems. SPIE, Feb. 21, 2020. doi: 10.1117/12.2545832.; 4. B. Yang et al., Mitigating transverse mode instability in all-fiber laser oscillator and scaling power up to 25 kW employing bidirectional-pump scheme, Optics Express, vol. 24, no. 24. The Optical Society, p. 27828, Nov. 21, 2016. doi: 10.1364/oe.24.027828.; 5. Q. Fang et al., 5 kW Near-Diffraction-Limited and 8 kW High-Brightness Monolithic Continuous Wave Fiber Lasers Directly Pumped by Laser Diodes, IEEE Photonics Journal, vol. 9, no. 5. Institute of Electrical and Electronics Engineers (IEEE), pp. 1 7, Oct. 2017. doi: 10.1109/jphot.2017.2744803.; 6. B. Mete, A. Yeniay, F. N. Ecevit, and S. K. Kalyoncu, High brightness in-band pumped fiber MOPA with output power scaling to >4.6k W, Applied Optics, vol. 61, no. 34. Optica Publishing Group, p. 10121, Nov. 23, 2022. doi: 10.1364/ao.47936 KEYWORDS: High Power Combiner; Combiner Architecture; Brightness optimization; Directed Energy; Fiber Optics; Combiner;