High-energy nanosecond pulse extraction from a Tm$^{3+}$-doped photonic crystal fiber laser emitting at 2050 nm with narrow linewidth

A nanosecond-pulsed diode-pumped Tm$^{3+}$-doped fiber laser (TDFL) operating at
2050 nm is reported based on a flexible photonic crystal fiber (PCF) with a core diameter of 50 μm.
Analytical evaluation of reabsorption effects and optimization of the fiber length allow enhanced
pulse energy extraction at long wavelengths. This enables the generation of Gaussian-shaped
pulses with pulse energies of 1.9 mJ, pulse widths of 116 ns, and peak powers of 15.4 kW from
the TDFL. The measured output spectrum shows a single peak at 2050 nm with a 3-dB-linewidth
of 130 pm and a 10-dB-linewidth of 370 pm. With a slope efficiency of 44 %, the average output
power of the pulsed TDFL is scaled up to 114 W. To the best of our knowledge, this represents
the first demonstration of a Tm$^{3+}$-doped PCF at a long emission wavelength of 2050 nm and the highest pulse energies from a TDFL based on a flexible fiber.