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New process boosts fiber production

25 Jan 2008

A hollow-core optical fiber that can be produced in a day instead of a week suffers no loss in quality according to UK researchers.

A team from the University of Bath has made a hollow-core photonic bandgap fiber (HC-PBGF) that it claims is superior in virtually every respect compared with previous versions of the technology. What's more, they claim that simplifying the manufacturing process means that the fibers can be produced quicker than ever before reducing the overall cost of fabrication. (Optics Express 16 1142)

"Our 7-cell HC-PBGFs are able to guide light over a broader spectral window, present comparable or lower attenuation and the lowest dispersion yet reported (better by almost a factor of almost two compared to previous fibers)," Rodrigo Amezcua-Correa, a researcher at the University of Bath, told optics.org. "These features are important for applications in high-power, ultrashort pulse compression and delivery such as spectroscopy, biomedical and surgical optics, laser machining, the automotive industry and space science."

The team achieved a minimum attenuation loss of 15 db/km and less than 50 db/km over 300 nm for a fiber operating at 1550 nm. A low-loss transmission window of approximately 350-400 nm for a bandgap centered at 1550 nm can be obtained.

Until now, the performance of HC-PBGFs has been limited by the existence of high-attenuation surface modes localized to the core wall. "Surface modes couple with the core-guided mode inside the bandgap, which decrease the effective bandwidth of the fibers and increase the dispersion," explained Amezcua-Correa. "In order to remove their impact, one needs to fabricate a fiber in which the cladding structure terminates as naturally as possible at the core/cladding interface."

Conventional hollow-core fibers are fabricated using a stack-and-draw technique, which involves creating an array of capillaries within a silica tube and drawing this preform down to form a fine fiber. In contrast, Amezcua-Correa's fiber core was created by simply omitting seven central capillaries without using an extra tube. "This reduced the core wall thickness from 100 nm to 40 nm, suppressing surface modes and increasing bandwidth," he explained.

An added advantage to this method is that it leaves out some of the most difficult steps in the fabrication procedure, reducing the time required to make the fibers from around a week to a single day.

Although the team currently has no plans to commercialize the fibers, they do intend to extend the design for fibers with larger and smaller core sizes. "For example, 19-cell HC-PBGFs free of surface modes would have a greatly extended useable bandwidth since their transmission spectra has been completely dominated by surface mode crossings," concluded Amezcua-Correa. "The larger core size and greatly reduced dispersion slope in the new fibers would enable a new regime of ulstrashort-pulse solutions and high-power beam delivery."

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