The short-wave infrared (SWIR) spectrum within the 1.4 - 3 μm range is highly valuable. It can be utilized to generate mid-infrared radiation and terahertz (THz) waves, as well as to study photoionization mechanisms. Longer wavelengths within this band offer higher phase-matching cutoff energies, which are beneficial for generating high-order harmonics. The main approaches for generating SWIR include Optical Parametric Chirped Pulse Amplification (OPCPA) and Chirped Pulse Amplification (CPA) systems. These systems typically use Holmium (Ho)-doped or Thulium (Tm)-doped materials as gain media. OPCPA is capable of producing pulses with durations shorter than 20 femtoseconds (fs) and power exceeding 100 Watts (W). However, its conversion efficiency peaks at only 16%. Due to the limited bandwidth of the gain media, CPA systems generally produce pulses with longer durations. To address this, post-compression techniques such as solid thin films, gas-filled hollow-core fibers, and Multi-Pass Cells (MPCs) are employed in experiments to shorten the pulse width. MPCs offer several advantages, including high efficiency, excellent beam quality, uniform spectral broadening, and insensitivity to variations in beam pointing and shape. When compared to the 1 μm band, MPCs operating in the 2 μm band have a larger intrinsic mode area, which reduces the risk of cavity mirror damage. Additionally, the higher ionization threshold at longer wavelengths prevents photoionization at the focal point. In this study, an MPC was utilized to achieve 2 μm pulses with an average power of 144 W, a pulse width of 20 fs, and a repetition rate of 101 kHz [1].

Figure 1: Experimental Setup [1] Figure 1 presents the experimental setup. At the front end, there is a four-way coherent synthesis Tm-doped fiber system. This system can deliver pulses with an average power of 178 W, a pulse energy of 1.76 mJ, and a repetition rate of 101 kHz, centered at a wavelength of 1930 nm, with a pulse duration of 80 fs. Before entering the MPC cavity, the beam undergoes mode matching through a lens assembly. The cavity mirrors have a radius of curvature of 750 mm and are spaced 1443 mm apart. The Group Delay Dispersion (GDD) of these cavity mirrors is less than 25 fs² within the wavelength range of 1675 - 2125 nm. The beam has a focal diameter of 0.59 mm, and the spot diameter at the cavity mirrors is 3.03 mm. Given the narrow input pulse width, only 15 passes through the MPC are required to achieve sufficient spectral broadening. The MPC device is housed in a vacuum-sealed container, and experiments are conducted at pressures below 1 bar. Krypton (Kr) is used as the nonlinear gas within the cavity at a pressure of ≤550 mbar. Additionally, 350 mbar of Helium (He) is introduced to enhance heat dissipation and prevent potential thermal drift. The beam is collimated using a mirror with a radius of curvature of 4500 mm and exits the system through a 5 mm fused silica window. A small portion of the beam is reflected by a wedge-shaped fused silica plate for detection purposes. The sampled beam passes through a 1 mm-thick fused silica plate before entering the detector, while the power of the transmitted beam is measured through a 5 mm-thick fused silica plate. The compressor in the system consists of fused silica and provides -518 fs² of second-order dispersion before the sampled beam reaches the detector, having passed through a total of 6 mm of fused silica.

Figure 2: (a) Spectrum (b) Pulse Temporal Shape (c) M² Factor (d) Power Stability [1] When the system is filled with 200 mbar of Kr and 350 mbar of He, the input pulse (with a duration of 86 fs, an average power of 178 W, and a pulse energy of 1.76 mJ) is compressed by the MPC system. The output power reaches 165 W, achieving an efficiency of 93%. The output spectrum and the compressed pulse width of the MPC are shown in Figures 2(a) and (b), respectively. The spectral bandwidth at -20 dB is 341 nm, and the compressed pulse width is 30 fs, which is close to the transform-limited pulse. The peak power of the compressed pulse can reach 37 Gigawatts (GW). The M² factors of the output beam in the x and y directions are 1.3 and 1.1, respectively. Over a 2-hour period, the Root Mean Square (RMS) of the average power variation is 0.25%.

Figure 3: (a) Spectrum (b) Pulse Temporal Shape [1] When the Kr pressure is increased to 550 mbar, with an input pulse power of 150 W, an energy of 1.49 mJ, and a pulse width of 97 fs, the MPC-compressed output power is 144 W, achieving an efficiency of 96%. The output spectrum is shown in Figure 3(a), with a bandwidth of 536 nm at -20 dB. Using 4 mm of fused silica for compression, the pulse can be compressed to the transform-limited width of 20 fs (Figure 3(b)). This study represents the first achievement of high-average-power pulse compression to 20 fs at 2 μm in a gas-filled MPC. It has obtained an average power exceeding 144 W and a pulse energy of 1.43 mJ. This marks a nearly 10-fold increase in pulse energy and a 3-fold increase in average power compared to existing MPC results. It provides a high-quality light source for future experimental research, such as the generation of high-order harmonics. References: Lucas Eisenbach, Ziyao Wang, Jan Schulte, et al. Highly efficient nonlinear compression of mJ pulses at 2μm wavelength to 20 fs in a gas-filled multi-pass cell[J]. 2024 J.Phys.Photonics 6 035015