Millifocal less-cycle laser sources in the short-wavelength infrared can drive two-color plasmas to produce terahertz pulses at higher efficiencies, as well as optically aberrated mid-infrared femtosecond pulses >5 μm in non-oxide crystals. Thulium-doped fiber laser system can produce hundred femtosecond pulses with a central wavelength near 2 μm. The Limpert group in Jena, Germany, coherently synthesized the outputs of four thulium-doped fiber amplifiers in 2022 [1], and finally obtained 85 fs pulses with a pulse energy of 1.65 mJ and a repetition frequency of 100 kHz, which breaks through the limitations of a single optical fiber on the single-pulse energy and the average power. The device is shown in Fig. 1.
Figure 1 Schematic diagram of the coherent synthesis device with four thulium-doped fibers.
In order to further shorten the pulse width, Limpert's group will use the above device as a front-end in 2023, and use hollow-core optical fiber for compression. The structure of the compression device is shown in Fig. 2, including two vacuum chambers for input and output, respectively, and a high-voltage chamber for nonlinear broadening, which is filled with argon gas. In order to reduce water vapor absorption, the air pressure in both vacuum chambers is kept <1 mabr. The bottom side of the high-voltage chamber is equipped with water cooling to dissipate the heat, avoiding harmful thermal effects at high power. The hollow core fiber is placed on a long straight V-groove to avoid bending loss. The core diameter of the hollow core fiber is 500 μm, the length is 1.05 m. The internal nonlinear gas is selected as argon, and the theoretical maximum passing efficiency is 89.5%.
Figure 2 Schematic diagram of the hollow core fiber compression device
Gradually increasing the gas pressure inside the cavity, the corresponding output results are shown in Figure 3. When the gas pressure is lower than 3 bar, the output power is around 139 W and the beam quality remains good (Fig. 3a). When the air pressure is higher than 3 bar, the output power starts to decrease and the beam quality deteriorates significantly, and the spot has deviated from the Gaussian beam at 4.25 bar, as shown in Fig. 3b. Figure 3c analyzes the spectral widths of the outputs under different air pressures. After the air pressure exceeds 3 bar, the spectra no longer broaden significantly with the increase of air pressure, and the corresponding transform limit pulses remain basically unchanged. The authors considered the above factors and finally chose 3 bar air pressure for the subsequent experiments.
Fig. 3 Output results of different air pressures in a hollow-core fiber
The spectra and autocorrelation curves measured at 3 bar air pressure are shown in Fig. 4, with the spectra covering 1.2 μm-2.4 μm. After using a pair of chirped mirrors to compensate for the dispersion, the pulse width is reduced to 10.2 fs, the average power is 132 W, and the main peak energy of the pulses accounts for 66% of the energy, with the peak power as high as 80 GW. Fig. 5 demonstrates the results of the stability test, and the relative intensity noise of the front-end output is 0.75%. that is concentrated in the frequency range of 20 Hz to 50 kHz. After nonlinear pulse compression, the main noise contribution is in the low frequency range as low as 2 kHz, which comes from the mechanical vibration of the water cooling and vacuum pump, proving that there is no introduction of additional noise during the compression process, ensuring the stability of the light source.
Fig. 4 Spectral and autocorrelation measurements at 3 bar air pressure
Fig. 5 Short-term stability test
In this paper, a high-energy femtosecond pulse with a center wavelength of 1.9 μm, a width of 10.2 fs, a pulse width of less than two cycles, a pulse energy of 1.3 mJ, and a peak power of 80 GW are obtained by using a hollow-core fiber compression.The average power of this light source is 132 W, which is the highest power level of the less-cycle pulses working in the short-wavelength infrared region, and this high-energy, high-power driven light source will certainly vigorously This high-energy, high-power drive light source will vigorously promote the development of laser technology in the mid-infrared band.
