Mode Adapting Etched Air Taper Fiber Optic Pump Combiner

ABSTRACT: The semiconductor pumped fiber laser has generated considerable interest for High Energy Lasers (HELs) in a number of BMDS applications including long range sensing, communications and missile defense. The potential efficiency, compact size and low weight, reliability and ruggedness are attractive for these and other directed energy applications. A key requirement is the creation and demonstration of all-fiber semiconductor pumped fiber laser designs that in a single gain stage can produce narrow linewidth amplified signals at the multi kW power level with near diffraction limited beam quality. Central to the creation of these sources is the all fiber pump power combiner. Reliable operation at multi kW power levels and the need for efficient multi kW pump integration requires the use of active gain fibers with large pump cladding diameters. These large pump cladding diameters in turn require the combination of several pump fibers with an even larger input bundle diameter. In addition the core must be transferred through the combiner, adapting to the larger core of the active fiber while preserving the polarization extinction ratio of the input signal. Finally the combiner needs to be integrated with advanced gain fibers in a near diffraction limited counter pumped configuration, and packaged in a robust manner that is thermally robust and stable. While all-fiber combiners exist commercially, they are of low input bundle diameters, are co-pumped, cannot provide the mode adaption necessary for large gain cores, and cannot handle multi-kW power levels. What is truly needed is a systems approach to DE fiber laser pump integration that seamlessly integrates multi kW levels of pump light simultaneously in both the co and counter pumped directions in both LMS step index fiber and Photonic Crystal Fiber Optical Engines has accomplished many different things during the phase 1 program including delivering 2 counter pumped fiber combiners and has made numerous improvements to the underlying processes and materials. This effort has created a foundation to develop a counter pumping combiner efficiently interfacing to high performance advanced PCF gain fiber. BENEFIT: Creating a counter pumped combiner for PCF fiber will improve the SBS and MI performance of a kW amplifier by over 3dB for both cases. This will allow for higher powers to be achieved per channel, lowering the number of channels required by the system and greatly reducing the complexity of the system resulting in substantial SWaP reductions and cost savings.