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High energy, multi-component, optical isolator for 2 μm fiber lasers


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: Advancements in portable high energy pulsed fiber lasers will have a broad impact on the DOD and for industrial applications including medicine, material processing and remote sensing. Substantial R&D by industry in fiber technologies, thermal management, and nonlinear mitigation techniques has led to advancements in high energy, narrow-linewidth thulium-doped fiber amplifiers operating in the 2 μm regime. These amplifiers continue to show great promise for improved scalability, nonlinear performance, thermal management, and atmospheric transmission. Currently, master oscillator power amplifier (MOPA) architectures using all-fiber designs are being developed and scaled to high average powers and high pulse energies. This architecture consists of multiple amplifier stages, with fiber-coupled isolators separating each stage. To reach commercial use power levels, a fiber amplifier typically consists of multiple pre-amplifier stages, which are spliced together with fiber-coupled isolators. Current isolators either have low isolation or low power handling that is not suitable for high power pulsed fiber amplifiers. Additionally, fiber amplifier designs often include other components (such as tap couplers, mode field adapters, and wavelength division multiplexers) between pre-amplifier stages which can contribute to deleterious nonlinearities such as stimulated Brillouin scattering and modulation instability. The objective will be to develop an innovative fiber-coupled isolator that will handle high average and peak powers found in 2 μm pulsed fiber amplifiers. This high-power isolator should also incorporate multiple common fiber components such as tap couplers and mode field adapters in a single portable package. DESCRIPTION: Demonstrate a production prototype version of a fiber-to-fiber isolator for pulsed fiber lasers operating from 1.9 to 2.1 μm with the following capabilities: 1. average power handling greater than 100 W, 2. peak power handling greater than 100 kW, 3. pulse energy handling greater than 10 mJ, 4. high optical isolation greater than 35 dB, 5. low insertion loss less than 0.5 dB, 6. a B integral less than 1.5 rad., 7. maintain fundamental mode operation with near diffraction limited beam quality from output fiber (M2 < 1.2), 8. insensitive to input polarization, Additionally, the prototype package for the fiber-coupled isolator should be no larger than 80 mm x 50 mm x 50 mm (excluding fiber pigtails) and the design must support the inclusion of common fiber components such as fiber tap couplers and spectral filters. PHASE I: The criteria for substantiating the proposer’s technology is at an acceptable stage when a report is provided with a prototype design and sufficient evidence that the design can support the capabilities as outlined in the description. The report will include: 1.) description and design of the multi-component, fiber-coupled isolator, 2.) list of components and fibers selected to build the prototype, 3.) theoretical, experimental or a combination of both, results supporting that the prototype design can meet the desired capabilities listed in the Topic Description. Such results may include: a.) damage threshold calculations and measurements supporting that the components can handle the peak powers and energies, b.) thermal lensing calculations and measurements justifying the selection of the Faraday rotator, c.) calculations and measurements of optical nonlinearities (such as self-focusing, B integral for modulation instability, and stimulated Brillouin scattering) for components and fiber pigtails, d.) calculations and measurements that the prototype package can the heat loads due to high average powers. PHASE II: The awardee will demonstrate the performance of a prototype, multi-component, fiber-coupled isolator that meets the specifications in the Topic Description. The intent is to deliver the working prototype to AFRL/RDLT for assessment and testing PHASE III DUAL USE APPLICATIONS: The awardee will work with RDLT and industry partners to make their isolator design available to the DoD customer base as well as DoD industrial partners. REFERENCES: 1. G. Stevens, A. Robertson, "Fibre laser component technology for 2-micron laser systems," Proc. SPIE 9135, Laser Sources and Applications II, 91350N (1 May 2014). KEYWORDS: optical isolator; Faraday rotator; fiber laser components
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