Optical fibers tackle the mid-infrared

14 Oct 2004

Singlemode fibers that transmit across the 4 to 18 microns range become commercially available.

European scientists claim to have fabricated the first singlemode optical fibers for guiding mid-infrared light. The fibers have an attenuation of less than 1.5 dB/m (>70% transmission per meter) between 5 and 17 µm with a minimum loss of about 0.5 dB/m (89% transmission per meter) at 10.6 microns, the wavelength of a CO₂ laser.

Fiber attenuation

The polycrystalline infrared (PIR) fibers have been developed by a collaboration between A.R.T. Photonics of Berlin, Vienna University of Technology and EADS Astrium, a German maker of space equipment.

A.R.T is now supplying lengths of up to 20 m in a wide range of fiber diameters including square cross-sections for making tightly packed fibre bundles. "PIR fibers are non-toxic, very flexible, transparent across a broad spectral region of 4 to 18;;microns and are capable of operating over a wide temperature range of -270 °C to +140 °C," Viacheslav Artyushenko told Optics.org. "Our hot news is the first singlemode PIR fibre fabrication, up to now only multimode core/clad fibers have been fabricated."

The PIR fibers are made from AgCl:AgBr solid solution crystals which have a refractive index of 2.2 and unlike silica fibres cannot be drawn like molten glass. Instead, they are fabricated using a vacuum extrusion method before being protected by a polymer jacket.

Applications for the fibers include everything from flexible delivery of CO₂ laser beams and infrared spectroscopy to infrared imaging systems and remote pyrometry in the 100 to 600 K range. "They also open the possibility to make heterodyne IR detection systems and CO₂ laser Doppler Velocimeters in the range 4 to 18 µm," commented Artyushenko.

The fibers were developed for the European Space Agency(ESA) for its DARWIN program which is searching for extraterrestrial life and Earth-like planets. The idea is to use satellites to collect mid-infrared radiation from space that corresponds to an Earth-like temperature of around 300 K and to analyse it for presence of atmospheric gases. In order to make the project feasible it is important to filter out strong star light that corresponds to a much higher temperature of around 6000 K. The answer is to equip the satellite with an interferometer featuring an optical wavefront filter.

"The singlemode fiber is the best way to do [create a filter] this but for a long time no such fiber was available in the wavelength range of 4 to 20 µm," explained Reinhold Flatscher of EADS Astrium. "This mid-infrared range is interesting as we expect atmospheric tracers like CO₂, H₂0 and ozone in this range."

Author
Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.