Silver Fiber Transmits IR Images

Michael Moss

TEL AVIV, Israel -- Scientists at Tel Aviv University are developing an infrared endoscopy technique that employs bundles of silver-halide fibers. Infrared endoscopy could detect cancerous tumors or monitor laser surgery and may find a place in surveillance applications for monitoring activity outside a vehicle or building.

Today endoscopy is limited to the visible light range. The silica-based or polymer fibers in an endoscope that transmit images from inside the body to a CCD detector are opaque to mid-infrared wavelengths, which makes it impossible to carry out thermal imaging.

Researchers at Tel Aviv University are using silver halide fibers to transmit IR images. A cooled thermal camera directly images a tungsten wire heated to 40°C (left); the same image, transmitted to the camera through a 900-fiber bundle (right), lacks resolution but holds promise for endoscopy.

The team, whose method is detailed in the April 3 issue of Applied Physics Letters, decided to look to silver halide. Silver halide has many advantages over other IR-transmitting materials such as chalcogenide, including good room-temperature transmission between 3 and 30 µm. It is also nontoxic and nonhygroscopic.

The researchers produced the fiber bundles using multiple extrusion. They inserted an AgBr rod in AgCl, which has a lower index of refraction, and extruded the coated rod to form a 0.7- to 0.9-mm-diameter clad fiber. They cut the fiber into shorter lengths, and then ordered and again extruded these segments to form a 900-fiber bundle.

Several problems still remain. The silver-halide bundles are flexible only when their diameter is less than 1 mm, but the team estimates that approximately 10,000 fibers are needed in a bundle to achieve acceptable resolution. The group has produced bundles of 100 to 9000 elements, with outer diameters of 2 to 8 mm.

Work is under way to produce bundles with smaller diameters, higher density of fibers and lower transmission losses.