Extreme Raman red shift: ultrafast multimode non-linear space-time dynamics, pulse compression, and broadly tunable frequency conversion

TL;DR

This paper introduces a novel, efficient method for tunable laser wavelength shifting using stimulated Raman scattering in hollow core fibers, enabling high-quality, short pulses with high conversion efficiency over a broad wavelength range.

Contribution

The authors demonstrate a scalable, energy-efficient laser frequency shifter based on SRS in nitrogen-filled HCF, offering continuous tunability and high beam quality, surpassing traditional OPA methods.

Findings

Wavelength tunability from 1030 nm to 1730 nm with >70% efficiency.

Generation of ~20 fs pulses shorter than pump pulses (~200 fs).

Pulse energy scalable to tens of millijoules.

Abstract

Ultrashort high-energy pulses at wavelengths longer than 1 $\mu$m are nowadays desired for a vast variety of applications in ultrafast and strong-field physics. To date, the main answer to the wavelength tunability for energetic, broadband pulses still relies on optical parametric amplification (OPA), which often requires multiple and complex stages, may feature imperfect beam quality and has limited conversion efficiency into one of the amplified waves. In this work, we present a completely different strategy to realize an energy-efficient and scalable laser frequency shifter. This relies on the continuous red shift provided by stimulated Raman scattering (SRS) over a long propagation distance in nitrogen-filled hollow core fibers (HCF). We show a continuous tunability of the laser wavelength from 1030 nm up to 1730 nm with conversion efficiency higher than 70% and high beam quality. The highly asymmetric spectral broadening, arising from the spatiotemporal nonlinear interplay between high-order modes of the HCF, can be readily employed to generate pulses (~20 fs) significantly shorter than the pump ones (~200 fs) with high beam quality, and the pulse energy can further be scaled up to tens of millijoules. We envision that this technique, coupled with the emerging high-power Yb laser technology, has the potential to answer the increasing demand for energetic multi-TW few-cycle sources tunable in the near-IR.