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Germanate Fibers

Definition: optical fibers based on germanate glasses

More general term: optical fibers

German: Germanatfasern

Category: fiber optics and waveguides

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URL: https://www.rp-photonics.com/germanate_fibers.html

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 in order 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.

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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), doi:10.1063/1.353922
[2]H. Li et al., “Thermal sensitivity of tellurite and germanate optical fibers”, Opt. Express 15 (14), 8857 (2007), doi: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), doi: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), doi: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), doi:10.1038/srep20344

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See also: fibers, silica fibers, infrared optics, mid-infrared fibers
and other articles in the category fiber optics and waveguides

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