A 2.05 μm holmium-doped yttrium lithium fluoride (Ho:YLF) master oscillator power amplifier system with both high average power and high pulse energy operating at a 1 kHz repetition rate is demonstrated, achieving a maximum output power of 280 W with a pulse width of 14.5 ns. The system comprises three-stage amplifiers, boosting a 20 W seed laser to output powers of 110, 205 and 280 W, corresponding to extraction efficiencies of 46.1%, 45.0% and 34.9%, respectively. At maximum output, the system exhibits excellent beam quality (Mx2 = 1.22 and My2 = 1.23) and power stability (root mean square = 0.5% over 30 min). To the best of our knowledge, this work reports the highest pulse energy (280 mJ) achieved for a 2 μm laser operating at a kHz repetition rate. In addition, a slice model of an end-pumped quasi-three-level laser amplifier was developed to analyze the output limitations of multi-stage Ho:YLF amplifiers based on rod geometry, providing theoretical support for the experimental results.

We present a study of second harmonic generation (SHG) and third harmonic generation (THG) in lithium triborate (LBO) crystals using a high-energy, 10-J-class, 10 Hz ytterbium-doped yttrium aluminum garnet laser system. We achieved high conversion efficiencies of 75% for SHG and 56% for THG for Gaussian-like temporal pulse shapes and top-hat-like beam profiles. The angular and temperature dependence of the LBO crystals was measured and validated through numerical simulations. The SHG process exhibited an angular acceptance bandwidth of 1.33 mrad and a temperature acceptance bandwidth of 2.61 K, while the THG process showed 1.19 mrad and 1.35 K, respectively. In addition, long-term stability measurements revealed root mean square energy stabilities of 1.3% for SHG and 1.24% for THG. These results showcase the reliability of LBO crystals for high-energy, high-average-power harmonic generation. The developed system offers automated switching between harmonics provided at the system output. The system can be easily adapted to neodymium-doped yttrium aluminum garnet based pump lasers as well.

The Mamyshev oscillator (MO) is well-known for its high modulation depth, which provides an excellent platform for achieving both high average power and short pulse durations. However, this characteristic typically limits the high-repetition-rate pulse generation. Herein, we construct an MO that achieves a gigahertz (GHz) repetition rate through harmonic mode-locking. The laser can reach up to the 93rd order, which corresponds to the repetition rate of 1.6 GHz. The maximum achieved output average power is 3 W at a repetition rate of 1.2 GHz (69th order), with the corresponding pulse duration compressed to 51 fs. To our knowledge, this is the first time that the GHz repetition rate in an MO has been obtained simultaneously with the recorded average power and pulse duration.

Mamyshev oscillators (MOs) exhibit the potential for generating high average power and ultrashort pulses. Herein, we construct an MO using flexible double-cladding ytterbium-fiber with a fusion-spliced-combiner pumped scheme. Consistent with the most reported research results, the offset filter separation significantly affects the pulse characteristics (spectrum, pulse duration, etc.). Notably, in comparison with red-shifting, blue-shifting the peak spectral emission of the grating filter relative to a constant central wavelength of the bandpass filter substantially enhances the laser output characteristics. This phenomenon, which has not been previously reported, results in an average power up to 2.23 W and a pulse duration as short as 49 fs. To our knowledge, this is the highest average power achieved in sub-50 fs pulse duration in the nonlinear polarization rotation-assisted mode-locked MO laser architecture. The presented technique offers unique scientific proof for developing ultrafast laser sources with higher average power and shorter pulse duration.

A high-power all polarization-maintaining (PM) chirped pulse amplification (CPA) system operating in the 2.0 μm range is experimentally demonstrated. Large mode area (LMA) thulium-doped fiber (TDF) with a core/cladding diameter of 25/400 μm is employed to construct the main amplifier. Through dedicated coiling and cooling of the LMA-TDF to manage the loss of the higher order mode and thermal effect, a maximum average power of 314 W with a slope efficiency of 52% and polarization extinction ratio of 20 dB is realized. The pulse duration is compressed to 283 fs with a grating pair, corresponding to a calculated peak power of 10.8 MW, considering the compression efficiency of 88% and the estimated Strehl ratio of 89%. Moreover, through characterizing the noise properties of the laser, an integrated relative intensity noise of 0.11% at 100 Hz−1 MHz is obtained at the maximum output power, whereas the laser timing jitter is degraded by the final amplifier from 318 to 410 fs at an integration frequency of 5 kHz to 1 MHz, owing to the self-phase modulation effect-induced spectrum broadening. The root-mean-square of long-term power fluctuation is tested to be 0.6%, verifying the good stability of the laser operation. To the best of our knowledge, this is the highest average power of an ultrafast laser realized from an all-PM-fiber TDF-CPA system ever reported.

We report on frequency doubling of high-energy, high repetition rate ns pulses from a cryogenically gas cooled multi-slab ytterbium-doped yttrium aluminum garnet laser system, Bivoj/DiPOLE, using a type-I phase matched lithium triborate crystal. We achieved conversion to 515 nm with energy of 95 J at repetition rate of 10 Hz and conversion efficiency of 79%. High conversion efficiency was achieved due to successful depolarization compensation of the fundamental input beam.