Fluoride glass optical waveguides - Technology and applications

The fluoride glasses discussed in this article are zirconium, indium/gallium or aluminum fluoride glasses, with the exception of beryllium fluoride glasses, which have been known for decades but are unusable in practice due to their hygroscopicity and toxicity.

The discovery of zirconium fluoride-based lenses in the 1970s, on the other hand, led to numerous technological developments in the field of optics.

Fluoride glasses are characterized by an infrared transmission limit shifted by several μm towards longer wavelengths, compared with the reference glass, silica. This characteristic lowers the minimum attenuation of fluoride fibers by an order of magnitude compared with silica, in theory.

In practice, despite the considerable resources deployed worldwide by laboratories and companies to improve the chemical purity of fluoride glasses, this very low level of optical loss has never been achieved.

While fluoride glasses can't compete with silica in terms of losses, and therefore in terms of long-distance transmission for telecommunications, they are unavoidable in the field of doped earth-rare laser fibers.

The infrared transmission limit is directly related to the fundamental frequencies of vibration between the elements making up the glass. These frequencies, or phonon energies, are about a factor of 2 lower in fluoride glasses than in oxides.

The result is a very significant reduction in non-radiative processes, and therefore a remarkable increase in the luminescence quantum yields of rare-earth ions in fluorides.

Fluoride glasses are widely used in the form of laser fibers emitting at wavelengths ranging from visible to mid-infrared, using various rare-earth ion dopings. Undoped, fluoride glass fibers are also used for infrared signal transmission (optical losses from 2 to 30 dB/km and mechanical strength from 50 to 500 kpsi).

Other types of waveguide, such as confined planar waveguides for integrated optics and microspheres, are also being extensively studied.

The aim of this article is to review the state of the art in fluoride glass optical waveguides, starting with the fundamentals of glass chemical formulations and their physical properties, essentially thermal, mechanical and optical. It also describes the techniques used to produce different types of waveguides: fibers, planar guides and microspheres.

As far as possible, the article situates the properties of fluorinated glasses and waveguides in relation to those of better-known silica-based glasses. This is particularly true of infrared transmission, luminescence and laser emission properties, in connection...