Hollow-core fibers revolutionize green laser technology

Efficient and flexible light delivery has been dominated by solid-core silica glass optical fibers, especially in telecommunications and industrial lasers. But high-power laser light delivery, vital for industrial applications, faces significant challenges with traditional fibers due to nonlinear processes like the Kerr effect and stimulated Raman scattering in the core, along with the damage threshold of silica glass. These factors greatly limit the deliverable power densities.

In hollow-core fibers (HCFs), >99.99% of the guided light is contained within the central air (or vacuum) filled core, bypassing the limitations of solid-silica core or traditional fibers. In 2022, the Southampton team successfully demonstrated the advantages of a new HCF design, transmitting 1 kW of continuous-wave near-infrared light (~1 μm) through a 1-km length, showcasing the technology’s potential.

In the latest work, the team has expanded these capabilities into the green wavelength, crucial for numerous industrial applications, by transmitting kilowatt-peak power 520-nm laser pulses through 300 m of HCF.

Developing HCFs for visible wavelengths presents fabrication challenges due to the small structural features.

The team also conducted a comprehensive nonlinearity study on practical air-filled, long-length HCFs. Nonlinearity in HCFs in the visible relative to the infrared region is significantly higher, which is attributed not only to the reduced fiber core size but also to the shorter operation wavelength. This advancement is a crucial step towards using green laser light in high-precision, efficient material processing, which will benefit sectors like e-mobility manufacturing, notably in battery production.

Hollow-core fiber for green laser power delivery

The HCF used in this work guides light via anti-resonance, in which a series of thin-glass membranes surround the fiber’s core and confine the guided light within it. This is achieved via a single ring of seven cladding capillaries (seven represents an excellent balance between loss, bend-loss, and modality).

The fiber is fabricated by the stack-and-draw method using Heraeus F300 fused silica glass, and has a ~20.7-μm core diameter, a mode-field diameter of ~14.5 μm, and guides light from 515 nm to 618 nm with <30 dB/km loss (see Fig. 1). The reported fiber is 300 m long, but the team in Southampton regularly produces multi-kilometer lengths via this process. The fiber is also relatively bend-loss insensitive, with <0.1 dB/m loss for bends with diameters >13 cm at the operation wavelength of 520 nm.

Power delivery results

The power delivery experiments reported use an in-house-made 15.5-W, 520-nm, frequency-doubled ytterbium-doped fiber laser, producing 1.6 MHz repetition rate, ~520 ps pulses with ~18 kW peak power. The laser beam is focused to a 15-µm mode-field diameter to match the fibers, and results in a coupling efficiency of ~86%. HCF lengths of 2, 100, and 300 m were able to deliver average powers of 13.2, 6.7, and 3 W, corresponding to peak powers of 15.9, 8, and 3.6 kW, respectively (see Fig. 2).