Unified and vector theory of Raman scattering in gas-filled hollow-core fiber across temporal regimes

Y.-H Chen,  F. Wise, “Unified and vector theory of Raman scattering in gas-filled hollow-core fiber across temporal regimes,” APL Photonics 9, 030902 (2024)

Raman scattering is an important physical phenomenon that enables wavelength conversion beyond what can be achieved with natural materials, nonlocal interactions between pulses for material examinations, and so on. However, it hasn’t been well-studied. In particular, there is no a single theory that can cover various temporal regimes for ultrashort pulse applications. Here, we develop a theory to comprehensively study the Raman scattering, which also make connections of several physical phenomena across different regimes that were used to be treated as different phenomena. This theory constitutes a fundamental yet crucial cornerstone of Raman theory.

Femtosecond long-wave-infrared generation in hydrogen-filled hollow-core fiber

Yi-Hao Chen and Jeffrey Moses and Frank Wise, “Femtosecond long-wave-infrared generation in hydrogen-filled hollow-core fiber,” J. Opt. Soc. Am. B 40, 796-806 (2023)

This paper is chosen as Spotlight on Optics.

Generating femtosecond long-wave-infrared (LWIR) pulses is currently restricted to CO₂ lasers and solid-state frequency converters, but waveguide-based Raman red shifting offers a promising alternative. In this study, we used a hydrogen-filled hollow-core fiber to generate LWIR pulses and found that a waveguide structure allows for tailored Raman gain. Using a two-pulse scheme with a two-color source, we achieved a numerical generation of clean 88-fs pulses at 12 μm with 41% total quantum efficiency. Our simulations also shed light on the nonlinear dynamics of the Raman gain, emphasizing the importance of a phonon amplifier for optimal performance.

In this work, especially its supplemental document, we have provided a complete introduction to the gas nonlinearity and its modeling detail. Please read it if you’re interested.

Efficient soliton self-frequency shift in hydrogen-filled hollow-core fiber​

Y.-H. Chen, P. Sidorenko, E. Antonio-Lopez, R. Amezcua-Correa, and F. Wise, “Efficient soliton self-frequency shift in hydrogen-filled hollow-core fiber,” Opt. Lett. 47, 285–288 (2022)

SSFS has been considered as a good option for fiber sources with tunable wavelengths for a long time. Particularly, nonlinear microscopy, such as three-photon imaging, requires high peak power at 1300 and 1700 nm to overcome the depth limit of two-photon imaging. During the process, the Raman soliton gradually shifts towards the red color as it moves forward. Although several methods have been utilized to achieve these wavelengths, it seems challenging to increase the pulse energy in solid-glass fibers. Here, we have shown that SSFS can occur efficiently and cleanly in a hydrogen-filled anti-resonant hollow-core fiber. By using hydrogen and short input pulses, we have demonstrated continuous tuning of the wavelength between 1080 and 1600 nm. We have obtained pulse energies in the range of 20 to 110 nJ and durations below 50 fs over this spectral range.