A 967 W Single Mode All-Fiber PM Yb PCF Fiber Amplifier

We report on the development and performance of a monolithic, all-fiber, polarization maintaining (PM), Yb-doped photonic crystal fiber (PCF) type fiber amplifier. The key components of the amplifier are a novel multi fiber-coupled laser diode stack and a monolithic 6+lxl fiber pump/signal multiplexer. The precisely aligned 2-D laser diode emitter array found in laser diode stacks is utilized by way of a simple in-line imaging process with no mirror reflections to process a 2-D array of 450 elements into 3 400/440μm 0.22NA pump delivery fibers. The amplifier in this work utilizes two laser diode stacks, one at 915 nm and the other at 976 nm for six total delivery fibers, three per stack, with an aggregate available pump power of 1.65 kW. The fiber combiner is an etched air taper design that transforms low numerical aperture (NA), large diameter pump radiation into a high NA, small diameter format for pump injection into an air-clad large mode area PCF, while maintaining a constant core size through the taper for efficient signal coupling and throughput. The fiber combiner has 6 400/440/0.22 core/clad/NA pump delivery fibers and a 20/440 PM step-index signal delivery fiber on the input side and a 40/500 PM Yb doped PCF on the output side. The etched air taper transforms the six 400/440 μm 0.22 NA pump fibers to the 500 μm 0.55 NA core of the PCF fiber with a measured pump combining efficiency of 92% with zero brightness drop. The combiner also operates as a stepwise mode converter via a 30 μm intermediate core region in the combiner between the 20 μm core of the input fiber and the 40 μm fiber core of the PCF with a measured signal efficiency of 90% while maintaining polarization with a measured PER of 20 dB. A seed laser was formed using one of the 915 nm pump legs to end pump a PLMA 20/400 Yb-doped fiber laser which provided 80W of seed power at 1085nm. This seed signal was then fed through the fiber pump/signal combiner with the remaining 5 pump legs into the large Yb-doped PCF fiber. In this configuration a total power of 967 W was observed with a slope efficiency of over 75%. We report the signal coupling efficiency and power handling capability as well.