Silver Halide Fibers Under Development for Mid-IR Imaging

Daniel S. Burgess

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. 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.