Tunable Wavelength by Adjusting SPM and SRS in Pulse Non-Linear Post-Compression Scheme

Wavelength (WL) tunability in optical parametric amplification (OPA) or post-compression setups provides a versatile approach for tailoring ultrafast laser pulses. Pulse post-compression is typically achieved in two steps: first, non-linear spectral broadening via self-phase modulation (SPM)[1] or, more recently, stimulated Raman scattering (SRS) in hollow-core fibers (HCF)[2], [3] or multi-pass cells (MPC)[4] filled with noble gases (or molecular gases in the case of SRS). In this step broadband chirped pulses are generated. Second, the temporal chirp is compensated using a dedicated optical setup, such as negatively chirped mirrors, to achieve shorter pulses. For a given pulse duration, gas type, and interaction length (e.g., the length of the HCF), the extent of spectral broadening can be controlled, within certain limits, by adjusting the product of gas pressure and pulse energy. In the case of SPM, the spectral broadening is symmetric, preserving the central wavelength of the pulse. Conversely, in the case of SRS, the spectral broadening is asymmetric, favoring longer wavelengths and causing a net redshift of the pulse's central wavelength. In practice, spectral broadening and wavelength tunability in the vicinity of the fundamental can be achieved in a single, efficient step. However, this wavelength shift is intrinsically coupled to the degree of spectral broadening, meaning that these two parameters cannot be controlled independently. In this work, we propose and experimentally demonstrate the feasibility of tuning central wavelength and broadening independently, by balancing SPM and SRS in HCF.