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Germanate Fibers

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Definition: optical fibers based on germanate glasses

Category: article belongs to category fiber optics and waveguides fiber optics and waveguides

DOI: 10.61835/o9r Cite the article: BibTex plain text HTML Link to this page! LinkedIn

Germanate fibers are optical fibers based on germanate glasses, i.e., glasses containing germanium oxide (GeO₂) as a major constituent. They should not be confused with germanosilicate fibers, where usually (but not always) only a smaller part of the fused silica is replaced with germania to somewhat raise the refractive index. In contrast, germanate fibers usually do not contain a substantial amount of silica.

Chemically, germania is similar to silica; note that germanium is just the next group-IV element after silicon in the periodic system of chemical elements, and both are oxides. Because germanium is heavier than silicon, the vibration frequencies of the Ge–O bonds are correspondingly lower, and this is the reason for the much extended transmission in the infrared spectral region. Good transmission can be achieved for wavelengths up to roughly 3 μm or even 4 μm. That makes germanate fibers suitable, for example, for transmitting 2.9-μm radiation from Er:YAG lasers. However, the vibration frequencies are still substantially larger than those for certain fluoride glasses, tellurite glasses and chalcogenide glasses. Also, the infrared transmission can be substantially degraded by hydroxyl content (particularly around 2.4 μm to 3.6 μm) – a common problem with oxide glasses –, which therefore needs to be minimized with suitable fabrication techniques if optimum infrared transmission is needed. For example, one can make fluorogermanate glasses with rather low OH⁻ content for mid-infrared fibers.

For making optical fibers, one typically realizes a fiber core with somewhat increased refractive index by doping the germanate glass with lead and/or other heavier chemical constituents. Both single-mode and multimode fibers can be fabricated that way. Further, the fiber core can be doped with laser-active ions – for example with Tm³⁺ for realizing fiber amplifiers and lasers operating at wavelengths around 1.7–2.1 μm.

Because of the low glass transition temperature of germanates (belonging to the “soft glasses”), one can also employ extrusion methods for producing photonic crystal fibers with air holes. That way, one can obtain a rather small effective mode area without substantial lead doping. In combination with the high nonlinear index of germanate glass, that leads to strong nonlinear effects, e.g. for supercontinuum generation or Raman scattering.

More to Learn

Suppliers

The RP Photonics Buyer's Guide contains one supplier for germanate fibers. Among them:

Le Verre Fluore

LVF offers a new range of high quality GeO₂ (germanate) multimode fibers for wavelengths around 2.7 µm to 3 µm that exhibit a high power handling and chemical stability. These fibers are the fibers of choice for power handling and pigtailing of Er:YAG and Er:YSSG lasers, which are extensively used in medical operations (dentistry, dermatology, ophthalmology, surgery…).

Bibliography

[1]	J. Wang et al., “Fabrication and optical properties of lead-germanate glasses and a new class of optical fibers doped with Tm³⁺”, J. Appl. Phys. 73, 8066 (1993); https://doi.org/10.1063/1.353922
[2]	H. Li et al., “Thermal sensitivity of tellurite and germanate optical fibers”, Opt. Express 15 (14), 8857 (2007); https://doi.org/10.1364/OE.15.008857
[3]	J. Wu et al., “Highly efficient high-power thulium-doped germanate glass fiber laser”, Opt. Lett. 32 (6), 638 (2007); https://doi.org/10.1364/OL.32.000638
[4]	H. T. Munasinghe et al., “Lead-germanate glasses and fibers: a practical alternative to tellurite for nonlinear fiber applications”, Opt. Mater. Express 3 (9), 1488 (2013); https://doi.org/10.1364/OME.3.001488
[5]	X. Wen et al., “Highly Tm³⁺ doped germanate glass and its single mode fiber for 2.0 μm laser”, Sci. Rep. 6, 20344 (2016); https://doi.org/10.1038/srep20344

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Questions and Comments from Users

2023-01-15

Does germanate fiber have any advantages over silica for transmission of NIR laser radiation in a medical application, say: 755, and 1024 nm along with a 650-nm guide beam?

The author's answer:

I don't think so – silica fibers perform well at such wavelengths.

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