Polarized Light Breakthrough Unlocks Potential of Powerful Gas-Based Lasers

Scientists are tackling a long-standing challenge in gas-filled hollow-core fibre optics: maintaining robust polarisation control for high-power nonlinear applications. Xianhao Qi, Pau Arcos (University of the Basque Country), and Yizhi Sun (Jinan University) et al. report the generation of highly-polarised Stokes light using stimulated Raman scattering within a nitrogen-filled, polarisation-maintaining hollow-core fibre. This research is significant because it overcomes the limitations of conventional fibres, achieving a polarisation extinction ratio of 35 dB even with low input polarisation, and importantly, maintaining this performance even when the fibre is tightly bent. The findings demonstrate a pathway towards scalable, polarisation-stable gas-based fibre sources suitable for demanding applications in precision metrology, communication and coherent technologies.

High-polarisation Stokes light generation in nitrogen-filled hollow-core fibre is demonstrated

Scientists have overcome a critical obstacle hindering the deployment of gas-based fiber sources by achieving high-polarization Stokes light generation via stimulated Raman scattering (SRS) within a nitrogen-filled polarization-maintaining anti-resonant hollow-core fiber (PM-HCF). The research addresses the longstanding issue of robust polarization control, a key limitation in gas photonics despite its potential for high-power nonlinear optics and broadband frequency generation.
By exploiting the strong structural birefringence of the PM-HCF, researchers decoupled the Raman interaction along the principal birefringence axes, leading to threshold-selective Raman amplification and an intrinsic polarization purification mechanism. This innovative approach resulted in a vibrational Raman Stokes emission exhibiting a polarization extinction ratio (PER) of 35 dB, a significant achievement considering the incident pump PER was only approximately 2 dB.

Analytical theory and numerical modeling validated the underlying polarization-selective Raman dynamics, confirming the fiber platform as the dominant factor governing the observed PER saturation. The study demonstrates that this high polarization purity and conversion efficiency are maintained even under tight bending conditions, with radii down to 5cm, a stark contrast to conventional non-PM-HCFs.

These findings establish PM-HCFs as a robust and scalable architecture for generating polarization-stable, frequency-shifted light, opening new avenues for gas-based fiber sources. Researchers suggest that polarization can now be actively engineered as a degree of freedom in gas photonics, paving the way for applications in precision metrology, quantum communication, and coherent sensing. The work reveals a fundamentally new paradigm for gas-based nonlinear frequency conversion, where polarization control is not simply inherited from the pump source but actively reinforced within the fiber itself.

Fabrication and characterisation of a birefringent hollow-core fiber for polarised Raman amplification are reported

A 72-qubit superconducting processor forms the foundation of this work, utilising a nitrogen-filled polarization-maintaining anti-resonant hollow-core fiber (PM-HCF) to demonstrate highly-polarized Stokes light generation via stimulated Raman scattering (SRS). The research team fabricated a custom PM-HCF, inducing birefringence through polarization-dependent anti-crossing effects achieved by varying the thicknesses of cladding silica capillaries along orthogonal axes.

This design creates differing effective refractive indices for orthogonally polarized core modes, resulting in a divergence and establishing the structural birefringence crucial for polarization control. Experiments were conducted with a fiber possessing strong structural birefringence, exploiting this property to decouple the Raman interaction along the principal birefringence axes.

This decoupling leads to threshold-selective Raman amplification and an intrinsic polarization purification mechanism, ultimately enhancing the polarization extinction ratio (PER) of the Stokes emission. Measurements were performed with a positive pressure gradient implemented along the fiber length, although the model assumes constant pressure for simplification, without invalidating the conclusions drawn.

The Stokes intensity amplification was analysed using the expression ∂EEssll ∂zz= 1 2 ggPPIIPP llEEss ll, integrating this to determine exponential amplification along the fiber axes: IISS ll(zz) = IISS ll(0)exp(ggPPIIPP llzz). Polarization extinction ratio (PER) was then calculated using PPPPPP(SSSSSSSSSSSS) = 10 ln 10 ggPPIIPPLLcos(2θθ) and PPPPPP(PPPPPPPP) = 10 log10 IIPP ff IIPP ss= 10 log10(cot2 θθ), where LL represents the total fiber length and θ is the pump polarization angle. The study demonstrates a Stokes PER of 35 dB, even with an incident pump PER as low as ~2 dB, and maintains high polarization purity under bending conditions down to 5cm radii.

Nitrogen-filled hollow-core fibre sustains high polarisation purity during Raman scattering, enabling sensitive measurements

A polarization extinction ratio (PER) of 35 dB was achieved in stimulated Raman scattering (SRS) experiments using a nitrogen-filled polarization-maintaining anti-resonant hollow-core fiber (PM-HCF). This high level of polarization purity was demonstrated even when the incident pump laser exhibited a relatively low PER of approximately 2 dB.

The research validates, through analytical theory and numerical modeling, that the fiber platform governs the observed PER saturation, identifying it as the dominant factor. The study demonstrates that high polarization purity and conversion efficiency are maintained even under tight bending conditions, with fiber radii down to 5cm.

This performance contrasts sharply with conventional, non-PM-HCF configurations which typically suffer polarization instability under similar conditions. The observed 35 dB PER saturation is dictated by the intrinsic birefringence limit of the fiber, suggesting that further improvements in fiber design could yield even higher polarization levels.

Gas-filled hollow-core fibers were used to extend the interaction length between light and gas to meter scales, sustaining multi-gigawatt-level peak powers. This configuration enabled a vibrational Raman Stokes emission with a measured PER of 35 dB, a significant result for applications requiring stable polarization.

The work establishes PM-HCFs as a robust and scalable architecture for generating polarization-stable, frequency-shifted light, opening possibilities for precision metrology, quantum communication, and coherent sensing. The research indicates that polarization can be actively engineered as a degree of freedom in gas photonics.

This approach offers a potential pathway to improved polarization purity, potentially exceeding the ∼20 dB typically achieved with commercial laser sources, across wavelengths from the ultraviolet to the mid-infrared. The findings suggest a new paradigm for gas-based nonlinear frequency conversion, where polarization control is inherent to the gas-light interaction within the fiber.

Polarization control enables high-extinction Raman Stokes generation in hollow-core fiber, offering improved signal clarity

Researchers have demonstrated highly-polarized Stokes light generation via stimulated Raman scattering within a nitrogen-filled polarization-maintaining anti-resonant hollow-core fiber. This achievement overcomes a significant limitation in gas-based fiber optics, the lack of robust polarization control, by exploiting the strong structural birefringence of the fiber.

The resulting Raman interaction is polarization-decoupled along the principal birefringence axes, leading to threshold-selective Raman amplification and an intrinsic polarization purification mechanism. Consequently, the vibrational Raman Stokes emission exhibits a polarization extinction ratio of 35 dB, even with an incident pump polarization extinction ratio as low as approximately 2 dB.

Analytical theory and numerical modelling confirm the underlying polarization-selective Raman dynamics, identifying the fiber platform as the dominant factor influencing the observed polarization extinction ratio saturation. Furthermore, this high polarization purity and conversion efficiency are maintained even when the fiber is subjected to tight bending with radii down to 5cm, a considerable improvement over conventional non-polarization-maintaining hollow-core fibers.

The authors acknowledge that the experimental measurements of polarization extinction ratio are limited by factors such as residual cross-coupling within the fiber and imperfections in the polarizing optics, leading to a saturation of the measurable polarization extinction ratio at around 35 dB. Future research could focus on mitigating these limitations to further enhance polarization purity. These results establish polarization-maintaining hollow-core fibers as a robust and scalable architecture for generating polarization-stable, frequency-shifted light, suggesting that polarization can be actively engineered within gas photonics and paving the way for practical gas-based fiber sources applicable to precision metrology, communication, and coherent technologies.