fluoride glasses (original) (raw)

Author: the photonics expert (RP)

Definition: glasses made from compounds of fluorine e.g. with zirconium, aluminum or indium

Category: optical materials

• optical materials
  • optical glasses
    * crown glasses
    * flint glasses
    * silicate glasses
    * fluoride glasses
    * phosphate glasses

Related: optical glassesfluoride fibersinfrared optics

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DOI: 10.61835/gop Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What Are Fluoride Glasses?

Fluoride glasses belong to the non-oxide glasses, made from compounds of fluorine with various other substances:

• There are fluorozirconate glasses, containing zirconium, and fluoroaluminate glasses with aluminum.
• The term fluoro-crown (or fluorine crown) glass is also common; these can be considered as crown glasses with some fluoride content.
• Fluoride glasses can also contain various other heavy metals like lead.
• Fluoroindate glasses (containing indium) exhibit improved infrared transmission.
• Mixtures of phosphate and fluoride glasses are called fluorophosphate glasses.

Among the fluorozirconate glasses for optical fibers, ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) is the most common. Such fibers can be doped with a number of rare earth ions for application in fiber lasers and amplifiers. Due to the low phonon energies, the lifetimes of various metastable electronic levels can be strongly increased compared with silica fibers, allowing the realization of certain lasers and amplifiers operating in the mid infrared. Also, there are fluoride-based upconversion lasers in the form of bulk lasers or fiber lasers. Such fluoride glasses can be doped with thulium (Tm3+) or praseodymium (Pr3+), for example.

Note that crystalline calcium fluoride (fluorite) is sometimes also used in high-quality optics, but is of course not a fluoride glass.

It may seem surprising that fluoride glasses have low maximum phonon energies despite fluorine atoms being quite lightweight. This can be explained with their weak ionic bonds. This weaker bond strength dominates over fluorine's low mass.

Optical Properties

Good Infrared Transmission

A special property of heavy-metal fluoride glasses is their good transmission in the infrared spectral region. (This results from the low phonon energies, which reduce the rates of multiphonon transitions.) Therefore, mid-infrared fluoride fibers were once thought to be a good candidate for long-distance optical fiber communications: the intrinsic scattering losses could in principle be lower than those of silica fibers because of the longer operation wavelengths which would be possible with fluoride fibers. However, silica fibers have firmly occupied this application area for various reasons like fabrication cost, mechanical strength, available light sources and fiber amplifiers. Still, fluoride fibers are needed in more specialized areas such as mid-infrared spectroscopy.

Low Chromatic Dispersion

Fluoride and fluorophosphate glasses can be used as extra-low dispersion (ED) glasses (with high Abbe number), e.g. for the construction of achromatic optics and even apochromatic optics with particularly high performance. Due to the high cost of fluoride optics, however, its use for such purposes is limited.

Difficulty of Fabrication and Use

Fluoride glasses are notoriously difficult to produce with high optical quality. They have a high tendency for partial crystallization during the glass transition, and the resulting microcrystallites can deteriorate the transparency. Therefore, the fabrication cost is relatively high.

Further, fluoride glasses are difficult to use, since they are mechanically quite weak (fragile) and chemically not resistant to moisture, for example.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What are fluoride glasses?

Fluoride glasses are non-oxide glasses made from metal fluorides. Prominent examples include fluorozirconate glasses like ZBLAN, fluoroaluminate glasses, and mixtures with phosphates called fluorophosphate glasses.

What is the main advantage of fluoride glasses for infrared applications?

Their primary advantage is excellent transmission in the mid-infrared spectral region. This is a result of their low phonon energies, which reduce absorption caused by multiphonon transitions.

Why are fluoride fibers used for some fiber lasers and amplifiers?

The low phonon energy of fluoride glass increases the upper-state lifetime of certain electronic levels in doped rare-earth ions. This enables efficient lasers and amplifiers at mid-infrared wavelengths which are not accessible with silica fibers.

What are the main challenges when producing and using fluoride glasses?

Fluoride glasses are difficult and costly to produce with high optical quality due to a strong tendency for crystallization. They are also mechanically fragile and chemically sensitive to moisture.

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