A metallic hollow waveguide with inner dielectric multilayers is proposed for the transmission of infrared light and its basic transmission characteristics are fully analyzed by using a two-dimensional slab waveguide model. The power loss in the waveguide is shown to be extremely reduced by coating dielectric layers properly. Simple loss formulas are also presented which consider absorptions of dielectric materials, and minimum possible losses are theoretically estimated. Finally, relations to bending losses in the waveguide are discussed.

Transmission characteristics of dielectric-coated metallic waveguide for infrared transmission: Slab waveguide model

An infared image guide made of bundled As–S glass fiber cores has been developed. The transmission range of this fiber was 2–6 μm. At the 3.3- and 4.8-μm wavelengths the minimum optical loss was 0.6 dB/m. With this image guide efficient transmission of the thermal image was experimentally established.

Infrared image guide with bundled As-S glass fibers.

Various waveguides were proposed for transmission in the middle infrared for applications such as laser surgery or communications. Infrared fibers made of crystalline halide salts, chalcogenide and fluoride glasses, as well as hollow core fibers are reviewed here. We fabricated AgCl and AgBr fibers by extrusion, and used these fibers for direct detection and heterodyne detection in a simple communication channel. The AgCl fibers have also been used for infrared radiometry.

Long wavelength infrared optical fibers

Optical and mechanical properties have been investigated for As/sub 2/S/sub 3/ and As/sub 2/Se/sub 3/ glass fibers and a KRS-5 polycrystalline fiber. A bending effect was serious for a KRS-5 fiber but not for chalcogenide fibers. In an aging test, a KRS-5 fiber showed a deterioration of transmissivity due to adsorption of moisture as well as plastic deformation of the crystal. Optical loss spectra of chalcogenide fibers were observed to change characteristically due to environmental factors such as humidity, high temperature, and ultraviolet radiation. The potential usefulness of these fibers is discussed. >

Optical and mechanical properties of infrared fibers

The infrared optical constants of a number of commercially available glasses and ceramics have been determined by Kramers-Kronig analyses of the reflectance spectra of the materials at near normal incidence. The data derived, the refractive indexn, and extinction coefficientK, have been further analysed to provide an assessment of these materials as hollow waveguides at mid-infrared frequencies. The results demonstrate that the frequency dependence of waveguide transmission is a feature of anomalous dispersion. Furthermore, in the case of beryllia and alumina ceramics, the high transmissions predicted in straight guide at 10.6μm can be attributed to the low refractive indices measured for these materials. Some experimental data are presented which substantiate the frequency characteristics of the predicted waveguide transmission.

Infrared optical constants for CO2 laser waveguide materials

Polycrystalline KRS-5 optical fibers were experimentally demonstrated to have high-power transmission capabilities for a cw CO2 laser beam. A typical sample of fibers provided by extrusion, 1 mm in diameter and 0.87 m in length, carried a laser power of 68 W to its output end and was found to remain free from damage at an incoming laser intensity up to 30 kW/cm2.

T. Morikawa

Papers

KRS-5 optical fibers capable of transmitting high-power CO 2 laser beam