OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Directed Energy (DE) OBJECTIVE: Develop a robust, spectrally stabilized, continuous wave fiber-laser system with < 15 GHz spectral bandwidth that is free from stimulated Brillouin scattering and thermal mode instability at kW power levels. DESCRIPTION: Fiber-laser sources are highly desired for high-energy laser (HEL) applications due to their compactness and robustness. The performance of high-power fiber lasers is hindered by two instabilities: stimulated Brillouin scattering (SBS) and thermal mode instability (TMI). SBS manifests as a reduction of output power coincident with a large backward propagating power that damages upstream components causing catastrophic failure. TMI manifests as significantly degraded beam quality, reducing power on target and HEL lethality. Increasing HEL power requires combination of multiple beams through either spectral or coherent combination. Spectral beam combination (SBC) is viewed as the next step in fieldable laser weapons with significantly increased power levels and range. SBC requires that each source be a specific and separate wavelength with a sufficiently narrow bandwidth to allow dense spectral packing of sources and mitigate spectral beam dispersion. However, techniques to mitigate SBS and TMI instabilities for scaling to multi-kW powers from a single fiber-laser source element require broadened spectral linewidths that are far beyond SBC requirements. New fiber-laser systems are required that can overcome these limitations. Most current solutions for mitigating SBS and TMI are extrinsic, requiring additional subsystems and controls that add complexity and increase the number of failure modes of the system. Intrinsic mitigation methods are fewer but tend not to lead to additional failure modes. In addition to overcoming both SBS and TMI, the desired fiber laser should be able to cover the 40 nm bandwidth in the ytterbium doped fiber spectrum, with an individual channel spectral bandwidth of < 15 GHz and less than 1% of the power outside the spectral band. Center wavelength long-term stability should be less than 50 MHz. Output power should be > 1 kW with high beam quality of M2 < 1.3. PHASE I: For a Direct to Phase II topic, the Government expects that the small business would have accomplished the following in a Phase I-type effort and developed a concept for a workable prototype or design to address, at a minimum, the basic requirements of the stated objective above. The below actions would be required in order to satisfy the requirements of Phase I: Provide a conceptual solution that is suitable for conventional spectral beam combining that can meet the stated requirements. Modeling and/or results of risk reduction experiments that validate the concept should be provided, along with a preliminary failure mode and effects analysis (FMEA). FEASIBILITY DOCUMENTATION: Offerors interested in participating in Direct to Phase II must include in their response to this topic Phase I feasibility documentation that substantiates the scientific and technical merit and Phase I feasibility described in Phase I above has been met (i.e., the small business must have performed Phase I-type research and development related to the topic NOT solely based on work performed under prior or ongoing federally funded SBIR/STTR work) and describe the potential commercialization applications. The documentation provided must validate that the proposer has completed development of technology as stated in Phase I above. Documentation should include all relevant information including, but not limited to: technical reports, test data, prototype designs/models, and performance goals/results. Work submitted within the feasibility documentation must have been substantially performed by the offeror and/or the principal investigator (PI). Read and follow all of the DON SBIR 22.2 Direct to Phase II Broad Agency Announcement (BAA) Instructions. Phase I proposals will NOT be accepted for this topic. PHASE II: Develop and optimize an innovative prototype fiber-laser system suitable for conventional spectral beam combining that can demonstrate the following requirements: (a) high optical output power > 1 kW, (b) good beam quality M2 < 1.3, (c) narrow spectral bandwidth < 15 GHz, (d) percentage of output power out of spectral band < 1%, (e) center wavelength long-term stability < 50 MHz, (f) complete mitigation of SBS, (g) complete mitigation of TMI. Characterize the system optical bandwidth, spectral stability, beam quality, and total pump power to signal power efficiency, all at maximum power level. Demonstrate ability to span the 40-nm wavelength range required for SBC. Validate the absence of SBS and TMI at maximum power level. Perform a preliminary failure mode and effects analysis (FMEA) for the proposed design. Project manufacturability of the system, highlighting COTS versus custom components and subsystems. PHASE III DUAL USE APPLICATIONS: Provide demonstration of a full SBC laser system. Transition the technology to a major demonstration program such as an ONR-funded Future Naval Capability (FNC) or Innovative Naval Prototype. Although the primary applications for the improved fiber laser would be for military laser systems, fiber lasers are routinely used in applications such as laser welding and cutting. There may be certain welding and cutting applications that may be improved with higher power fiber lasers that would result from the elimination of SBS and TMI in the fiber lasers. REFERENCES: 1. Naderi, N. A., Dajani, I., & Flores, A. (2016). High-efficiency, kilowatt 1034 nm all-fiber amplifier operating at 11 pm linewidth. Optics letters, 41(5), 1018-1021. https://doi.org/10.1364/OL.41.001018 2. Brilliant, N. A. (2002). Stimulated Brillouin scattering in a dual-clad fiber amplifier. JOSA B, 19(11), 2551-2557. https://doi.org/10.1364/JOSAB.19.002551 3. Eidam, T., Wirth, C., Jauregui, C., Stutzki, F., Jansen, F., Otto, H. J., Schmidt, O., Schreiber, J. L., & T nnermann, A. (2011). Experimental observations of the threshold-like onset of mode instabilities in high power fiber amplifiers. Optics express, 19(14), 13218-13224. https://doi.org/10.1364/OE.19.013218 4. Augst, S. J., Goyal, A. K., Aggarwal, R. L., Fan, T. Y., & Sanchez, A. (2003). Wavelength beam combining of ytterbium fiber lasers. Optics letters, 28(5), 331-333. https://doi.org/10.1364/OL.28.000331 KEYWORDS: optical fiber; fiber laser; high energy laser; spectral beam combination; directed energy
