Large-mode-area fibers operating near single-mode regime

Area of Science:

• Fiber optics
• Laser technology
• Materials science

Background:

• Traditional fiber NA is limited to 0.06 due to fabrication challenges.
• Transverse mode instability limits average power scaling in fiber lasers.
• Operating closer to the single-mode regime mitigates mode instability.

Purpose of the Study:

• Develop a fabrication process for low NA LMA fibers.
• Enable larger core diameters for nonlinear effect mitigation.
• Improve single-mode operation for high-power fiber lasers.

Main Methods:

• Developed a novel fabrication process for ytterbium-doped silica glass.
• Achieved high refractive index control and uniformity for LMA fibers.
• Utilized extensive post-processing of fiber preforms.

Main Results:

• Demonstrated 30/400 and 40/400 LMA fibers with NA of ~0.028.
• Achieved operation near the single-mode regime with second-order mode cutoff at 1.2µm and 1.55µm.
• Investigated bend loss issues associated with low NA fibers.

Conclusions:

• The new fabrication process supports large-volume production of LMA fibers.
• Low NA LMA fibers are critical for advanced fiber laser applications.
• Further optimization of LMA fibers for low NA is ongoing.