In this work, we explore a novel theoretical approach to enhance our understanding of ultrashort laser pulse compression in gas-filled hollow-core fibers. These pulses are essential in ultrafast science, where they are used to study atomic and molecular dynamics on extremely short timescales. However, compressing these pulses to very short durations without the appearance of relevant secondary structures (pre-pulses and/or post-pulses) is a significant technical challenge. To address this challenge, we apply a method called “reverse nonlinear propagation,” which allows us to predict the ideal shape of the input pulse to achieve an optimal compressed pulse at the output. The key to this approach is that, instead of directly designing the input pulse, we simulate what an ideal pulse would look like at the output and reverse its propagation in the fiber to determine the characteristics the initial pulse should have.

The process of pulse compression typically involves using a hollow capillary filled with gas in which a laser pulse is coupled to broaden its spectrum during propagation, mainly due to self-phase modulation, a nonlinear effect that generates new frequencies very efficiently. Then, the phase of the new spectrum obtained at the output of the capillary is adjusted in an external compressor, composed of dispersive elements, to shorten the temporal duration of the pulse. The problem is that compressed pulses often exhibit undesired secondary structures, such as additional peaks that distort the pulse shape. Our method allows us to design an input pulse that minimizes or eliminates these secondary structures.

One of the most interesting findings is that the ideal pulse predicted by the reverse propagation technique has a characteristic profile: its spectrum always presents small modulations around the main peak. Our simulations demonstrate that these initial spectral modulations allow compensating for the nonlinear effects that occur inside the capillary during the pulse propagation to produce the clean pulse at the output.

The reverse propagation method is not new, but its application in this context presents particular challenges due to the high energy losses of hollow-core fibers and the symmetries of the equation that describes the nonlinear propagation of ultrashort pulses. Despite these complications, we demonstrate that it is possible to numerically reverse the pulse propagation and accurately predict the characteristics of the input pulse required to obtain optimal compression.

Furthermore, our study highlights the high sensitivity of the compression process to small changes in the phase and amplitude profile of the input pulse. Even slight variations in the initial phase or amplitude can lead to significantly different results at the output, underlining the importance of controlling both aspects in the design of experiments.

In summary, this work proposes a new theoretical tool that can guide the design of ultrashort pulse compression experiments in laboratories. While some of our results still need to be experimentally validated, we believe that this method opens the door to generating clean and ultrashort pulses that could improve applications in ultrafast spectroscopy, strong-field physics, and other areas of ultrafast science.

More information in:

F. Galán, E. C. Jarque, and J. San Roman, “Reverse design of the ideal pulse for hollow capillary fiber post-compression schemes,” Phys. Rev. Res. 6(2), 023111 (2024). https://doi.org/10.1103/PhysRevResearch.6.023111