At near-IR to IR wavelengths (2.1 to 20mm), fused silica does not transmit light particularly well due to multiphoton (atomic vibrational) absorbance. However, there are several materials that do transmit well at these wavelengths. Chalcogenide glasses, fluoride glasses, polycrystalline metal halides, and some germanate oxides perform well beyond 2.1mm and have all been fabricated into optical fibers. Unfortunately, these materials tend to be very difficult to process, and have inferior mechanical and durability properties when compared to fused silica.  This has limited their widespread use, but they are still utilized in specialized applications.

There is also a class of optical fiber known as hollow waveguides (HSW). This type of waveguide can effectively transmit wavelengths in the IR (out to 20mm). A HSW can be one of two types of light guides:

1) “leaky” guide, or
2) attenuated total reflectance guide (ATR)

The ATR guides perform the same as a solid core optical fiber, having a core of higher refractive index than the clad. In the ATR case, the core is not solid, but a gas (i.e. air) with a lower refractive index tube surrounding it. The most common material used for this type is sapphire crystal. The fibers tend to be stiff and relatively expensive.

The “leaky” guide consists of a hollow tube coated on its inner surface with a reflective metal (e.g. Ag). This coating may or may not be followed by dielectric coating which serves to improve reflectivity and provide protection to the metallic surface. This type of waveguide has been produced using plastic, metal, and glass capillary tubing. Tubing produced from synthetic fused silica is most effective due to its tight dimensional control, smooth surface properties, high strength, and lower cost. HSW fabricated with synthetic fused silica tubing approach the theoretical limits of transmission for this type of waveguide and are finding increased utilization in Sensing and in Er:YAG (2.9mm) and CO2 (10.6mm) laser systems in the medical and industrial fields. Please refer to our Product Reference section for more information on HSW.

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