Last updated on 05/7/2021
Fluoride fiber is multi-composite glass optical fiber composed of several heavy metal fluorides, and many different types of fluoride fibers exist depending on the material composition. Fluoride fibers have many unique characteristics that are not enabled by silica fiber, such as a wider operating wavelength range, as well as a higher emission efficiency when doped with rare-earth elements.
In this article, two major compositions of fluoride fibers, ZBLAN fiber (ZrF4-BaF2-LaF3-AlF3-NaF) and AlF3-based fiber (AlF3-BaF2-SrF2-CaF2-MgF2-YF3), are explained.
Figure 1 shows loss spectra of three different kinds of glass fibers, ZBLAN, AlF3, and silica. Among the three, ZBLAN fiber has the widest transmission window ranging from 0.4 to 4 μm. ZBLAN fiber is thus suitable as a wide-band optical guiding medium in the near infrared (NIR) and mid infrared (MIR), e.g. for spectroscopy.
Figure 1: Loss spectra of three kinds of fiber.
ZBLAN fiber is also an efficient medium for MIR supercontinuum generation, when pumped at a wavelength near the zero-dispersion wavelength (ZDW) of the fiber. Figure 2 displays the material dispersion of ZBLAN glass, showing that the material dispersion becomes zero at around 1.7 μm. As a result, a ZDW of ZBLAN fiber is typically located at around 1.7-1.9 μm, where high-power pulsed laser sources are commercially available.
Figure 2: Material dispersion of ZBLAN glass.
Another huge advantage of ZBLAN fiber lies in its excellent emission characteristics when doped with rare-earth elements. Figure 3 shows some examples of visible fluorescence from ZBLAN fibers doped with Tm, Er, and Nd. The strong visible fluorescence is characteristic to ZBLAN fiber, because ZBLAN fiber is less influenced by non-radiative transition by phonons (than silica fiber) due to its low phonon energy.
Figure 3: Visible fluorescence from rare-earth-doped ZBLAN fibers, Tm-doped, Er-doped, and Nd-doped (from left to right).
Figure 4 shows various emission wavelengths of rare-earth-doped ZBLAN fiber in the visible, NIR, and MIR. Also marked in the figure are four major emission wavelengths enabled by rare-earth-doped silica fiber. There are many spectral regions that only ZBLAN can cover.
For example, emissions at around 1.31 and 1.45 μm in the NIR, that are of significant importance in optical communication, can be obtained by ZBLAN fibers. Emission in the MIR is also characteristic to ZBLAN. Such exotic characteristics of ZBLAN enable us to produce light sources (fiber amplifiers, ASE light sources, fiber lasers) in various spectral regions.
The emission efficiency is also affected by slight differences in glass composition and fiber design.
Figure 4: Emission spectra from rare-earth doped ZBLAN fiber
IR transmission range of AlF3-based fiber is not as wide as that of ZBLAN fiber, and its emission property is not as exotic as that of ZBLAN fibers. AlF3-based fiber, however, has the following advantages over ZBLAN: (1) lower optical loss at 2.94 µm, (2) higher laser damage threshold, (3) better mechanical properties, and (4) better durability against moisture (see water solubility in Table 1). AlF3-based fiber has thus been used for MIR light delivery up to 3 µm, e.g. Er:YAG laser guide for laser dentistry.
In addition, these four advantages have also made AlF3-based fiber a popular choice for endcapping a high-power ZBLAN fiber laser in the MIR. The endcapping is done by splicing a short piece of AlF3 fiber to the output endface of a ZBLAN fiber laser, thereby protecting the ZBLAN from being directly exposed to moisture.
|Properties||AlF3 glass||ZBLAN glass|
|Optical||Transmission range||0.3~3.5 µm||0.35~4.0 µm|
|Refractive index (nd)||1.46||1.51|
|Thermal||Glass transition temperature (Tg)||367°C||265°C|
|Thermal expansion (a)||186×10-7/°C||200×10-7/°C|
|Chemical||Water solubility (Dw)||0.27wt%||29.2wt%|
|Acid solubility (Da)||0.69wt%*||32wt%*|
|Physical||Density||3.85 g/cm3||4.50 g/cm3|
|Mechanical||Young’s modulus (E)||66 GPa||53 GPa|
|Knoop hardness (HK)||3.1 GPa||2.2 GPa|
Table 1: Comparison of AlF3-based glass and ZBLAN glass
Fabrication of low-loss fluoride fibers is not an easy task. This is mostly because the chemical-vapor deposition (CVD) technique – by which low-loss silica fibers are made – cannot be used for fluoride-fiber fabrication due to the lack of gaseous raw precursors.
FiberLabs has long been working on the loss reduction, and has established its own techniques to manufacture both kinds of fluoride fiber, ZBLAN fiber and AlF3-based fiber with low loss (less than 0.1 dB/m). We will continue our R&D activity to improve our own manufacturing technique to achieve a further low loss value by removing impurities and other origins.
Figure 4: Fiber manufacturing at FiberLabs.
In addition to our internal production purposes, FiberLabs has been supplying a wide range of fluoride fibers for many external projects, mainly in the area of mid-infrared fiber laser research.
Click here for a list of publications where FiberLabs’ fluoride fibers are referred to. Please feel free to contact us for inquiry on both standard-lineup and custom-made fluoride fibers for your research project and product development. We also provide bulk fluoride glass for academic research purposes.
Please visit the following product pages if you are interested in our fluoride fiber products.