Large mode area double-clad ytterbium-doped spun tapered fiber

We performed a comparative study of Yb-doped double-clad large mode area (MFD=31µm) spun tapered fibers with different pitch lengths. A novel type of spun tapered fibers with constant pitch lengths of 7.5, 15, and 30 mm, internal birefringence as low as ∼10−8, and perfect output beam quality (M2<1.2) has been manufactured and investigated. We demonstrated the efficient direct amplification of a picosecond pulsed signal (95 ps/100 MHz, 1064 nm) in the spun active tapered fiber with optimized geometry providing an average output power of 71 W (gain 32 dB, 74% slope efficiency). Efficient green light generation (λ=532nm) with 17 W average power (1.7 µJ) and 80 pm linewidth was demonstrated using the developed master oscillator power amplifier system.