Continuing their efforts to develop fibers that transmit mid-IR radiation for endoscopic imaging applications in medicine and industry, a team of scientists at Tel Aviv University in Israel has fabricated 2- and 0.7-mm-diameter bundles of silver halide fibers with a single-fiber attenuation of 2.3 and 9.4 dB/m, respectively.
The researchers note that there currently are no fibers suitable for the transmission of 3- to 30-µm mid-IR radiation, so thermal imaging has been restricted to line-of-sight situations. Although bundles of small-bore, hollow-core waveguides have been proposed as a solution, their transmission loss to date has proved too high and their flexibility too low. Similarly, chalcogenide glasses transmit IR radiation, but their performance suffers for wavelengths longer than 8 µm. Moreover, they can be brittle and toxic, limiting medical applications.
Previously, the scientists had demonstrated that bundles of flexible, nontoxic and nonhygroscopic silver halide fibers could transmit mid-IR radiation (see “Silver Fiber Transmits IR Images,” Photonics Spectra, November 2000, page 38). The fibers with low transmission losses, however, were too thick and inflexible for endoscopy, and thin, flexible fibers were too lossy.
In the improved fabrication process, the researchers extrude a meters-long core-clad fiber of single-crystal silver halide with an outer diameter of 700 µm. They cut the fiber into segments that are several centimeters in length and place 100 segments in a square arrangement in an AgCl tube to form a preform.
Another extrusion step yields an ordered bundle of silver halide fibers several meters in length — in the current round of experiments, a 2-mm-diameter bundle of 45-μm-diameter fibers and a 0.7-mm-diameter bundle of 30-μm-diameter fibers.
To characterize the performance of the fiber bundles, they measured single-fiber attenuation by comparing the power injected into a fiber using a CO₂ laser with that emerging from the end of the fiber. Substituting a thermal camera for the power meter enabled them to measure crosstalk between fibers, which was approximately 25 percent for the 2-mm bundle and less than 1 percent for the 0.7-mm bundle.
The scientists found no bending loss for radii between 60 and 5 mm, and calculate that bending losses should be negligible for radii greater than 1 mm. Resolution was two lines per millimeter.

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