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Fluoride fibres guide mid-infrared light

17 Dec 2008

Researchers in Australia have fabricated what they say is the first large mode area microstuctured fibre for use in the mid-IR.

A microstructured optical fibre (MOF) fabricated from a fluoride-based glass could pave the way for such fibres to be used in a range of applications requiring mid-infrared light in the range of 2 to 5 microns. (Optics Letters 33 2861)

"Our fibre is the first fluoride MOF ever reported," Heike Ebendorff-Heidepriem of the University of Adelaide in Australia told optics.org. "This is a valuable first step towards a new generation of mid-infrared transmitting fibres that will benefit applications ranging from defence through to medicine and trace gas detection."

One alternative for high-power transmission of mid-infrared wavelengths between 2 and 5 microns is a step-index fibre consisting of a solid core and a solid cladding. However, such a fibre cannot support a large mode area with a good beam quality simultaneously, a key requirement for the team.

"Our target application of transmission of high-power mid-infrared light via a fibre requires both good beam quality and large mode area," explained Ebendorff-Heidepriem. "We are aiming for singlemode or few-mode guidance (i.e. a low M-squared value) and a mode area of the order of 1000µm2."

The group opted to use the fluoride-based glass known as ZBLAN due to its low nonlinearity and a high transmission in the mid-infrared. The trade-off is that processing fluoride glass is a complex task, which had to be addressed at all three stages of fabrication.

First, a bulk glass billet was produced from raw materials. "A good quality billet is required as this goes on to determine the transmission loss in the fibre," commented Ebendorff-Heidepriem. "We dedicated much time to fabricating glass billets of excellent optical quality and sufficient size for preform extrusion, step two of the process."

Extrusion involves heating up the billet and forcing it through an extrustion die to create a preform with macroscopic structure on the millimetre scale. "We adapted the extrusion technique to fluoride glass for the first time," commented Ebendorff-Heidepriem. "The use of graphite dies (in contrast to stainless steel dies used previously) was crucial to achieving structured fluoride preforms of good optical quality that were free of crystals."

Finally, the team optimized the fibre drawing conditions to overcome the issues of hole collapse and crystallization. The end result was MOFs between 10 and 50 meters in length containing seven holes with a mode area of 6600µm2 and a loss of 3 dB/m at 4 microns.

"We are working on increasing the billet size to increase the preform length and thus the fibre length," said Ebendorff-Heidepriem. "We also want to decrease the fibre loss, achieve singlemode guidance at 4 microns and coat the fibres to enhance their mechanical stability. All of these goals will require further optimization of fabrication conditions."

Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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