Actively pulse shaped linearly polarized nanosecond hybrid Ho$^{3+}$ and Tm$^{3+}$ -doped silica fiber & Ho$^{3+}$ :YAG MOPA at 2048 nm

Ho$^{3+}$: YAG amplifiers are able to efficiently scale pulse energy at wavelengths far shorter than the main
emission peak. For 24 W input power at 2048 nm a signal output power of 81.6 W is reached, resulting in 5.3 dB
gain. Diode seeded master oscillator power amplifiers (MOPA) based on Tm3+ and Ho$^{3+}$ -doped silica fibers are able to
create arbitrary shaped nanosecond-pulses over a large wavelength span reaching beyond 2 μm which renders
them ideal sources for applications dependent on narrow linewidth, exact wavelength adjustability, pulse shape
and linear polarization. Nevertheless, the pulse energy / peak power of fiber based MOPAs is capped; the small
mode field area compared to solid-state lasers limits the saturation energy and leads to low nonlinear thresholds
which ultimately prevents the peak power scaling (degenerate four-wave mixing / modulation instabilities (MI),
stimulated Brillouin scattering, stimulated Raman scattering). Solid state crystal lasers based on Ho$^{3+}$ :YAG offer
the possibility to further increase the peak power but are mainly used at the emission peaks at around 2090 nm [1]
and 2121 nm [2]. ... mehrIn this work we demonstrate for the first time that Ho 3+ :YAG crystal amplifiers can be used for
efficient power and pulse energy scaling at 2048 nm, at a wavelength far shorter than the main Ho$^{3+}$ :YAG emission
peak, while still achieving a gain of 5.3 dB. Our approach offers a pathway to efficiently scale the pulse energy
over a broad wavelength span ranging from 2015 nm up to 2125 nm, especially when linewidth, pulse shape, and
peak power do matter. The experimental setup is shown in figure 1. A laser diode emitting at 2048 nm is used as a seed, creating pulses at a repetition rate of 50 kHz. By current modulation a linear increasing pulse shape is achieved, also leading to a
change of the charge carrier density and thereby the refractive index of the semiconductor material resulting in a
20 GHz frequency chirp along the pulse (280 pm 3 dB linewidth). The following first stage high gain amplifier
based on a polarization-maintaining Ho$^{3+}$ -doped silica fiber provides around 40 dB gain. The pulses get further
amplified in a booster stage based on a Tm$^{3+}$ -doped double-clad silica fiber. By filtering the signal, the share of
output power accounting to the MI related sidebands is reduced, ensuring the spectral quality in the output
spectrum. The final fiber amplifier stage is based on a Tm$^{3+}$ -doped double-clad silica fiber pumped in the cladding
by two 793nm diodes [3]. Subsequently, the 2048 nm emission is launched into a free space isolator to prevent
back reflections from entering the fiber laser amplifier chain. The following attenuator and half-wave plate enable
the adjustability of the power and the state of polarization. A Tm$^{3+}$ -fiber laser emitting at 1908 nm is used to pump
the Ho$^{3+}$ :YAG crystal amplifier. The signal and pump beams are combined by a 55° dichroic polarizer and are
focused down to a 400 μm beam waist at the first crystal interface, adjusted for matched beam diameter, position
as well as angle of both signal and pump beam.