High-power, highly repetitive ytterbium-doped ultrafast lasers are of great value for applications in research and industry. However, the narrow spectral bandwidth (10 nm) of this laser system has led to the emergence of numerous post-compression techniques based on self-phase modulation to broaden the spectrum. The compression efficiency of multipass cavity techniques can exceed > 90%, producing high-energy, high-average power ultrashort pulses with uniform spatial distribution.
In this paper, we numerically simulate the compression process in a multipass cavity and show how to optimize the system parameters so that the broadened spectrum has a smooth phase and clean compressed pulses are obtained.
The authors used a distributed Fourier numerical method to simulate the propagation of the pulse inside the multipass cavity. Effects such as diffraction, dispersion, self-phase modulation, and self-steepening are taken into account in the simulation, and the gas inside the multipass cavity is an inert gas so that Raman effects can be neglected. The pulse intensity in the cavity is controlled below the ionization threshold, so the ionization effect can also be ignored. The actual multi-pass cavity system needs to satisfy four conditions: (1) the optical length inside the cavity is larger than the nonlinear length and smaller than the dispersion length, i.e., < L <; (2) the upper limit of the soliton order is less than 10, i.e., N = √ < 10; (3) avoiding self-focusing, <; and (4) avoiding ionization. Satisfying the above four conditions at the same time, the input pulse center wavelength is 1030 nm, the pulse width is 150 fs, the curvature of the multi-pass cavity lumen is 40 cm, the distance of the cavity lumen is 40 cm, and the pulse goes back and forth within the cavity 20 times. At this time to meet the actual multi-pass cavity needs of the pressure and pulse energy range shown in Figure 1 light blue region.
Fig. 1 Multi-pass cavity parameter region.
The spectral characteristics of the output pulse are measured by two parameters, the half-height full width and the spectral cleanliness C. The spectral width of the output pulse is the limit of compression. The spectral width demonstrates the limiting pulse width of the pulse compression, while the spectral cleanliness C characterizes the cleanliness of the compressed pulse (high percentage of main peak energy and low intensity of secondary pulses). At C > 0.9 the compressed pulse has a primary peak energy share of >98% and a secondary pulse intensity of <0.5%. Figure 2 shows the spectral half-height widths of the multi-pass cavity with different parameters and the spectral cleanliness C. It can be seen from the figure that wide and clean spectra can only be obtained when the pressure and energy satisfy certain conditions.
Fig. 2 Pulse cleanliness in the energy-pressure diagram.
From Fig. 2, it can be seen that better compression results can be obtained when the pulse energy is 100 μJ and the pressure is 10 bar, and the related simulation results are shown in Fig. 3. The spatial uniformity of the spectra is analyzed in Fig. 3(a) and Fig. 3(b), and it can be seen that the x-axis and y-axis spectra are exactly the same, and the spatial uniformity is good. Figures 3(c) and 3(d) show the pulse widths and spectra, from which it can be seen that the spectra have a large bottom trailing and a smooth parabolic phase, which corresponds to a transform limit pulse of 14.2 fs.
Fig. 3 Spatial spectral distributions on the x-axis (a) and y-axis (b), as well as the pulse width (c) and spectral (d) distributions, for a pulse broadening and compression result with an energy of 100 μJ in an MPC filled with 10 bar argon.
Figure 4 shows in detail the spectral and spot variations for each round trip of the pulse through the multi-pass cavity device. Figure 4(a) 1/spectrum is consistent with the change in spectral cleanliness parameter and the spectral half-height width remains constant after 10 round trips, but the 1/spectrum increases and the spectrum appears to have a larger base. The plot in Fig. 4(b) shows the final output spot as a perfect Gaussian. Fig. 4 (c) demonstrates the evolution of the spot size, which changes smoothly throughout, ensuring the compressibility of the subsequent pulses.
Fig. 4 (a) shows the evolution of the spectral spread after each round trip; (b) shows the spatial pattern at the end of the propagation; and (c) shows the comparison of the transverse beam sizes during propagation without gas (blue line) and with gas (dots)
In this paper, it is demonstrated through numerical simulation that when using a multi-pass cavity to compress the pulse, a wide and clean spectrum and a high-quality compressed pulse can be obtained by jointly optimizing the pulse energy and the gas pressure, which will provide a guide for the subsequent construction of a practical multi-pass cavity system.
