Infrared (IR) optical fibers may be defined as fiber optics transmitting radiation with wavelengths greater than approximately 2 µm. The first IR fibers were fabricated in the mid-1960's from chalcogenide glasses such as arsenic trisulfide with losses in excess of 10 dB/m.¹ During the mid-1970's, 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 addition, there was an ever-increasing need for a flexible fiber delivery system for transmitting CO₂ laser radiation in surgical applications. Around 1975, a variety of IR materials and fibers were developed to meet these needs. These included the heavy metal fluoride glass (HMFG) and polycrystalline fibers as well as hollow rectangular waveguides. While none of these fibers had physical properties even approaching that of conventional silica fibers, they were, nevertheless, useful in lengths less than 2 to 3 m for a variety of IR sensor and power delivery applications.²

IR fiber optics may logically be divided into three broad categories: glass, crystalline, and hollow waveguides. These categories may be further subdivided based on either the fiber material or structure or both as shown in Table 1. Over the past 25 years many novel IR fibers have been made in an effort to fabricate a fiber optic with properties as close to silica as possible, but only a relatively small number have survived. A good source of general information on these various IR fiber types may be found in the literature. ^3,4,5,6 In this review only the best, most viable and, in most cases, commercially available IR fibers are discussed. In general, both the optical and mechanical properties of IR fibers remain inferior to silica fibers and, therefore, the use of IR fibers is still limited primarily to non-telecommunication, short-haul applications requiring only tens of meters of fiber rather than kilometer lengths common to telecommunication applications. The short-haul nature of IR fibers results from the fact that most IR fibers have losses in the few dB/m range. An exception is fluoride glass fibers which can have losses as low as a few dB/km. In addition, IR fibers are much weaker than silica fiber and, therefore, more fragile. These deleterious features have slowed the acceptance of IR fibers and restricted their use today to applications in chemical sensing, thermometry, and laser power delivery.

Table 1 - Categories of IR fibers with a common example to illustrate each subcategory.

A key feature of current IR fibers is their ability to transmit wavelengths longer than most oxide glass fibers. In some cases the transmittance of the fiber can extend well beyond 20 mm, but most applications do not require the delivery of radiation longer than about 12 µm. In Figure 1 we give the attenuation for some of the most common IR fibers as listed in Table 1. From the data it is clear that there is a wide variation in range of transmission for the different IR fibers and that there is significant extrinsic absorption which degrades the overall optical response. Most of these extrinsic bands can be attributed to various impurities, but, in the case of the hollow waveguides, they are due to interference effects resulting from the thin-film coatings used to make the guides.

Some of the other physical properties of IR fibers are listed in Table 2. For comparison, the properties of silica fibers are also listed. The data in the table and in Figure 1 reveal that, compared to silica, IR fibers usually have higher loss, larger refractive indices and dn/dT, lower melting or softening points, and greater thermal expansion. For example, chalcogenide and polycrystalline Ag-halide fibers have refractive indices greater than 2. This means that the Fresnel loss exceeds 20% for two fiber ends. The higher dn/dT and low melting or softening point leads to thermal lensing and low laser induced damage thresholds for some of the fibers. Finally, a number of these fibers do not have cladding analogous to clad oxide glass fibers. Nevertheless, core-only IR fibers such as sapphire and chalcogenide fibers can still be useful because their refractive indices are sufficiently high. For these high index fibers, the energy is largely confined to the core of the fiber as long as the unprotected fiber core does not come in contact with an absorbing medium.⁷

Figure 1 - Composite loss spectra for some common IR fiber optics: ZBLAN fluoride glass,¹²
SC sapphire,¹⁹ chalcogenide glass,¹⁷ PC AgBrCl,²³ and hollow glass waveguide.³⁷

Table 2 - Selected physical properties of key IR fibers compared to conventional silica fiber.

The motivation to develop a viable IR fiber stems from many proposed applications. A summary of the most important current and future applications and the associated candidate IR fiber that will best meet the need is given in Table 3. We may note several trends from this table. The first is that hollow waveguides are an ideal candidate for laser-power delivery at all IR laser wavelengths. The air core issive materials. The high refractive index of chalcogenide fibers is ideal for chemical sensing via evanescent wave coupling of a small portion of the light from the core into an IR absorbing medium. For the measurement of temperature through the simple transmission of blackbody radiation, IR fibers which transmit beyond about 8 µm, such as the Ag halide, chalcogenide, and hollow waveguides, are excellent candidates for use in measuring temperatures below 50 ^oC. This is because the peak for room temperature blackbody radiation is about 10 µm.

Table 3 - Examples of IR fiber candidates for various sensor and power delivery applications.

IR Fibers | Introduction | Non-oxide & H-M oxide glass | Crystalline | Hollow | Conclusions | References
 

Return to the Specialty Fiber homepage

Please report broken links on this page to the
Specialty Fiber Webmaster