Design considerations for large-mode-area polarization maintaining double clad fibers

Large-mode-area double clad fibers offer excellent efficiency and beam quality, high output power as well as lightweight, robust and reliable packaging. The addition of polarization maintaining property though use of well-know Panda-structure has further increased the interest in double clad fibers, especially in fibers doped with ytterbium (Yb). Many material processing, military and R&D applications benefit from wavelength conversion by nonlinear effects, from IR through UV, of 1064nm Q-switched pulses through polarization maintaining large-mode-area double clad Yb-fiber amplifiers. The possibility of power scaling through coherent beam combining has also been identified by the military. The design of a polarization maintaining large modea area double clad fiber for the above mentioned applications must address several key performance parameters: provide large mode area (>300μm), high efficiency (>80% slope PCE), high average power (>100W), high birefringence (>2*10-4) and offer good beam quality (M <1.5), short fiber length (<3m), as well as high reliability and good usability. Further optimization of the fiber design must take into consideration the impairment of the fiber by thermal loading as well as coiling of the fiber for elimination of higher order modes. This paper presents the key design considerations of such fibers for high-average-power pulsed amplifiers and provides the latest experimental techniques to verify the results. The design and results on high performance highly Yb-doped polarization maintaining large mode area fiber manufactured by the Direct Nanoparticle Deposition technology are presented and possibilities and opportunities brought by this technology are discussed.