Mid-infrared Fibers

Already since the mid 1960's [1], various types of chalcogenide glasses are used, which are sulfides, selenides or tellurides e.g. of arsenic (As) or germanium (Ge) and have substantially lower vibration frequencies (thus also lower phonon energies) than silica due to the higher mass of chalcogenide ions (comparing with oxygen ions). Consequently, their infrared absorption starts at longer wavelengths:

• Sulphide glasses like arsenic trisulfide (As₂S₃) are suitable e.g. for wavelengths of 1 to 6 μm, but can exhibit strong absorption peaks within that wavelength range, e.g. at ≈4 μm (see below).
• Selenide glass (As₂Se₃) can be used up to ≈9 μm.

Extrinsic propagation losses are usually strong; for example, hydrogen as an impurity causes absorption in the region around 4 μm related to the vibration of S–H bonds, and hydroxyl (OH⁻) groups cause additional absorption lines. Therefore, the transmission spectra are often quite structured. The propagation losses are much higher e.g. than those of fluoride fibers (see below).

Chalcogenide glasses have a low band gap energy and are thus not suitable for guiding shorter-wavelength light. (For that reason, such glasses also look colored, e.g. orange.) Infrared fibers based on chalcogenide glass are sometimes labeled as CIR = chalcogenide-based infrared fibers. They are fairly stable, e.g. insensitive to moisture, in contrast to some other mid-IR materials.

Germanate glasses are chemically similar to silica glasses, just containing germanium (Ge) instead of silicon (Si). An increased refractive index for the fiber core can be achieved by doping with PbO.

The transmission range extends to roughly 3 μm, to be compared with only ≈2 μm for silica. The transmission losses are of the order of 0.2 dB/m, and it is possible to transmit laser beams with more than 10 W of optical power through multimode fibers.

Some heavy metal fluoride glasses (HMFG) can be used for mid-infrared fibers [5]. The most common one is ZBLAN, chemically ZrF₄-BaF₂-LaF₃-AlF₃-NaF. It has a loss minimum around 2.5 μm, where propagation losses of a few dB/m can be reached. Unfortunately, these fluoride glasses are rather fragile, having a failure strain which is more than an order of magnitude lower than for silica, and the fragility can further increase due to the influence of moisture. There is also a tendency for surface crystallization. The small glass softening temperature range makes it relatively difficult to draw fibers from such materials.

Variants of such glasses exist (for example AlF₃-ZrF₄-BaF₂-CaF₂-YF₃) which can exhibit an improved durability, but possibly at the cost of higher propagation losses.

There are also indium fluoride fibers with improved infrared transmission at longer wavelengths.

See also the article on fluoride fibers.

Various halide materials exhibit excellent infrared transmission, but have a crystalline nature; one cannot make glasses out of them. Some of them are suitable for fiber fabrication by hot extrusion; one can press them through a die at a temperature near the melting temperature and obtains a polycrystalline structure with a grain size of the order of 10 μm. A fiber cladding can be produced in a second step with the rod-in-tube technique, or with a refined extrusion process. Silver halide fibers (the ternary alloy AgBrCl) are nowadays most common and allow transmission ranges from roughly 3 μm to 18 μm [3, 8]. In the optimum wavelength region around 8–14 μm, the propagation losses can be well below 1 dB/m. However, the propagation losses tend to increase with time due to growth of the microcrystals, possibly also due to exposure to visible light (photodarkening), which leads to the formation of silver particles. Also, excessive bending can permanently increase the propagation losses.

Thalide fibers have also been developed but are not widely used because of the high toxicity of thallium (Tl) and their inferior mechanical properties.

Optical fibers can also be made of the monocrystalline sapphire (→ single-crystal fibers), which is aluminum oxide (Al₂O₃). Common fabrication methods are edge-defined, film-fed growth and laser-heated pedestal growth, possibly followed by an annealing process for reducing the propagation losses.

The transmission range extends up to roughly 3.2 μm, and the low propagation losses and tight robustness of that material allows the use with fairly high optical average powers of more than 10 W. For example, sapphire fibers are used for transmitting the output of Er:YAG lasers at 2.94 μm [17].

A completely different approach is to use hollow waveguides [13, 14, 19], made e.g. of glass or sapphire [10], where most of the optical power propagates within an air core (typically a few hundred micrometers in diameter), and the light field has only little overlap with the solid materials. Therefore, substantial absorption in the used materials can be tolerated without obtaining high propagation losses for the guided light, and a very wide transmission wavelength range can be achieved. Also, end reflections are very weak. Typically, one has single-mode or few-mode propagation, which implies a high beam quality at the output, but also a high beam quality requirement for the input.

One can in principle simply use a hollow glass piece, but the performance can be optimized with additional layers deposited on the inner surface, e.g. a thin metallic layer reflecting the light, further assisted with a dielectric layer (e.g. AgI on Ag).

There are cases where the refractive index of the used solid material is below 1 at the operation wavelength, so that the operation principle of a standard fiber is effectively used; an example for this are sapphire hollow fibers for transmitting 10.6-μm light from CO₂ lasers [12]. In other cases, one has leaky waveguides.

The propagation losses can be quite small (of the order of 0.1 dB/m) if the air core is made large enough. (The propagation losses in leaky waveguides are usually proportional to the inverse third power of the air core diameter.) Very high optical powers can be transmitted, often several kilowatts. Unfortunately, the losses can rise steeply when such a fiber is bent.

Hollow waveguides can also be made using plastic or metallic materials for the basic structure. The optical properties are determined by layers of other materials deposited on the inner side.

Although most photonic crystal fibers are made of silica, research is underway on such fibers based on chalcogenide glasses, e.g., aiming at infrared transmission. Supercontinuum generation has been demonstrated [22], where one profits from the high nonlinear index of chalcogenide glasses, while the chromatic dispersion can be optimized through the photonic crystal structure design.

In various respects, mid-infrared optical materials are substantially less suitable for fiber fabrication than silica. For example, some of them are quite fragile and not as easily drawn into fibers. The technology for making such fibers out of ultra-pure materials is also far less developed than that for silica fibers. (Substantial investments have been made into the development of high-quality telecom fibers, which are essentially always made of silica.) Therefore, mid-infrared fibers are typically much more expensive, mechanically less robust and exhibit substantially higher propagation losses, compared with silica in its optimum wavelength region around 1.5 μm. Also, their optical damage threshold in terms of peak power or average power is often substantially lower, although some of them do allow the transmission of high-power infrared light. In some fibers, thermal lensing effects are strong due to the relatively high absorption and high dn / dT values. The refractive index of such glasses can be quite high (often well above 2), leading to strong Fresnel reflections at glass–air interfaces.

Mid-infrared fibers are available as bare fibers, but also as fiber patch cables, i.e., having additional protection and fiber connectors at the ends. Many of them are multimode fibers, either due to difficulties of making single-mode fibers or because multimode fibers are appropriate for typical applications. Hollow waveguide fibers, however, can be used for single-mode transmission.

There are many applications where some amount of mid-infrared light needs to be transmitted through a fiber; a few examples are given in the following:

• The light output of mid-infrared laser sources (e.g., quantum cascade lasers, optical parametric oscillators or fiber lasers) often needs to be transported to some application in transport fibers. For example, high-power mid-infrared light is used for military countermeasures, i.e., for distracting heat-seeking rockets which can attack airplanes. An optical fiber with a suitable transmission band and high-power capability can be used to transport such light from the source to the optical beam collimation system.
• In other cases, infrared light needs to be transported to some device analyzing it, e.g. a spectrometer or an imaging device. In the mid infrared, there are many characteristic absorption lines for identifying chemical species.
• Evanescent wave fiber sensors are mostly based on chalcogenide fibers. Here, the high refractive index of such a fiber is beneficial.
• For thermal imaging, bundles of many mid-infrared fibers can be used, where each fiber transmits light from some source area to the corresponding detector.

In the past, there were plans to use mid-infrared fibers as telecom fibers. This was because theoretically it should be possible to achieve even lower propagation losses than in silica fibers – in a longer wavelength range, where the unavoidable level of Rayleigh scattering is lower but silica would strongly absorb light. However, it has so far not been possible to realize that theoretical potential, because extrinsic loss mechanisms could not be reduced as much as required. The best results were achieved with heavy metal fluoride glasses, which can get down to a few dB/km around 2.5 μm, to be compared with 0.2 dB/km for silica around 1.5 μm.

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