Introduction to Infrared Fiber Optics

Abstract

Infrared (IR) optical fibers may be defined as fiber optics that transmit radiation with wavelengths greater than approximately 2 μm. The first IR fibers were fabricated in the mid-1960s from a rather special class of IR transparent glasses called chalcogenide glasses. It had been known for some time that blending chalcogen elements such as arsenic and sulfur can form a dark red-colored glass that is transparent well beyond 2 μm. Being an excellent glass former, arsenic trisulfide (As2S3) was a logical choice to be drawn into crude fiber using a simple fiber drawing apparatus. Kapany et al. reported on these first IR fibers in 1965, but the losses were very high. In fact, the losses were an untenable 10 dB∕m over the IR spectrum from 2 to 8 μm. A loss of 10 dB∕m means that a mere 1-m length of As2S3 fiber would transmit only 10% of the incident light apart from reflection losses from the fiber end faces. The reflection losses would amount to an additional 31%, as this glass has a high refractive index of about 2.3. On top of these problems, their As2S3 fiber was quite brittle. During the mid-1970s, the interest in developing an efficient and reliable IR fiber for short-haul applications increased, partly in response to the need for a fiber to link broadband, long-wavelength radiation to remote photodetectors in military sensor applications. In 1975, Hughes Research Laboratories (HRL) researchers in Malibu, CA, began to search for an IR fiber that could be used in a satellite application to link IR radiation in the 3- to 5-μm, 8- to 12-μm, and longer wavelength bands, which was incident on a surveillance satellite, to an interior IR detector array. The basic fiber/detector system as envisioned by HRL is shown schematically in Fig. 1.1. The idea was simple: Take an array of IR fibers and position them so that they would each intercept a large field of view, and then transmit that signal through an IR fiber for a distance of no more than about 50 cm to a mosaic IR detector array. This particular U.S. Army-funded program, called Mosaic Infrared Sensor Technology (MIST), would ultimately require a fiber array consisting of several hundred fibers, each interfaced with one detector element in the detector array. Initially the investigators at HRL experimented with the chalcogenide fibers, but they found the loss too high and the fibers too brittle for their application. Another unfortunate feature of chalcogenide fibers is that they are made from toxic materials.