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Fiber Optic Amplifier Pump Combiners with Signal Feed Throughput


OBJECTIVE: Develop efficient high-power fiber-optic pump combiners to inject multimode laser pump power into large mode area fibers for kW all fiber amplifier architectures. DESCRIPTION: High power fiber master oscillator power amplifiers (MOPA) enable the development of Air Force Research Laboratory (AFRL) directed energy systems. The AFRL long term goal is to promote the readiness level of this technology so that powerful, lightweight and reliable amplifier units can be mounted onboard next generation aircraft. Reliable high power fiber lasers/amplifiers also spur commercial opportunities in laser well boring, hole punching, cutting, and welding other material processing amplification. The desired characteristics of a MOPA include monolithic construction; i.e., no free space optical components, an output power exceeding state-of-the-art at 1064nm, polarized emission, and fundamental transverse mode operation. This is typically implemented using several stages of optical amplification in Ytterbuim (Yb) doped double clad gain fiber. Clearly the final amplifier stage is the most critical since every component is subject to extremely high power levels. This topic focuses on one of these critical components, the all-fiber pump + signal combiner. The pump + signal combiner serves to inject multi-transverse mode pump light (976nm for Yb-doped fiber amplifiers) from multiple fiber-coupled diode laser banks into a single double clad and large-mode area fiber. The output is spliced to an ensuing gain fiber or the gain fiber can be directly integrated into the combiner. The injected pump light is subsequently absorbed by the Ytterbium ions embedded in the fiber core so as to amplify the seed signal. The typical combiner launches the pump light so that it propagates in the same direction as the 1064nm signal, co-pumping. However, for narrow spectral linewidth operation it is preferable to launch the pump light opposite to the signal. Counter-pumping may provide some mitigation to the onset of Stimulated Brillouin Scattering thus improving the power range of the amplifier. The ideal pump combiner minimizes the coupling losses between the pump diodes and the gain fiber. It also has minimal loss for the signal, and it preserves the polarization state of the throughput signal being amplified. Combiners are needed that are compatible with commercially available polarization-maintaining (PM) large mode area (LMA) fibers. Combiners for photonic crystal fibers (PCF) are needed for innovative concepts. In both cases, demonstration of a counter-pumped configuration is desirable. Each input fiber of the pump combiner must be able to handle greater than 300W with a 200m fiber within a numerical aperture of less than 0.22. The combiner must prove reliable and robust. It must be system integration ready into robust fiber amplifier architectures. Pump insertion loss (a goal of<0.25 dB), signal insertion loss (a goal of<0.4dB), and mode preservation should be addressed. PHASE I: Design and model concepts of high-power fiber laser pump combiners to establish fabrication and performance feasibility. Fabricate kilowatt capable power combiner prototypes for pump coupling to dual-clad Yb-doped large mode area fibers is required. Criteria for the design include brightness preservation, power handling capability, polarization preservation and robust packaging. PHASE II: Based on Phase I designs, models and prototype demonstrations, conduct in-depth characterization of hardware to prove maturity of technology toward potential military and commercial applications. The components and characterization data will be delivered to AFRL for insertion into a kilowatt-class fiber amplifier. PHASE III: In Phase III the kilowatt combiner will be built and reliability tested for military significant environments and packaged for system insertion and commercialization. A multi-kilowatt (>2kW) counter-pumped combiner utilizing advanced photonic-crystal signal fibers must be demonstrated and delivered. REFERENCES: 1. Andrea, B., Massimo, O., Alessandra, N., and Perrone, G.,"Fabrication of Pump Combiners for High Power Fiber Lasers,"Proc. SPIE 7914, 79142V, 2011. 2. Xiao, Q., Yan, P., He, J., Wang, Y., Zhang, X., and Gong, M.,"Tapered fused fiber bundle coupler capable of 1 kW laser combining and 300 W laser splitting,"Laser Phys. Vol. 21(8),pp. 1415-1419, 2011. 3. Muendel, H., et al.,"Fused fiber pump and signal combiners for a 4-kW ytterbium fiber laser,"Proc. SPIE 7914, 791431, 2011.
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