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Definition: optical glass fibers which are doped with rare-earth ions
Fiber lasers and fiber amplifiers are nearly always based on glass fibers which are doped with laser-active rare-earth ions (normally only in the fiber core). These ions absorb pump light, typically at a shorter wavelength than the laser or amplifier wavelength (except in upconversion lasers), which excites them into some metastable levels. This allows for light amplification via stimulated emission. Such fibers are often called active fibers. They are gain media with a particularly high gain efficiency, resulting mainly from the high optical confinement in the fiber's waveguide structure.
This article discusses only aspects which are specific to rare-earth-doped fibers; see the article on fibers for more general aspects.
Common Types of Rare-earth-doped Fibers
The following table shows the most common laser-active ions and host glasses as well as typical emission wavelength ranges of rare-earth-doped fibers:
| Ion | Common host glasses | Important emission wavelengths |
|---|---|---|
| neodymium (Nd3+) | silicate and phosphate glasses | 1.03-1.1 μm, 0.9-0.95 μm, 1.32-1.35 μm |
| ytterbium (Yb3+) | silicate glass | 1.0-1.1 μm |
| erbium (Er3+) | silicate and phosphate glasses, fluoride glasses | 1.5-1.6 μm, 2.7 μm, 0.55 μm |
| thulium (Tm3+) | silicate and germanate glasses, fluoride glasses | 1.7-2.1 μm, 1.45-1.53 μm, 0.48 μm, 0.8 μm |
| praseodymium (Pr3+) | silicate and fluoride glasses | 1.3 μm, 0.635 μm, 0.6 μm, 0.52 μm, 0.49 μm |
| holmium (Ho3+) | silicate glasses, fluorozirconate glasses | 2.1 μm, 2.9 μm |
The technologically most important rare-earth-doped fibers are erbium-doped fibers for erbium-doped fiber amplifiers and ytterbium-doped fibers for high-power fiber lasers and amplifiers.
Particularly in the case of rare-earth-doped silica fibers, the core composition is very often modified by additional dopants. This means that effectively these are aluminosilicate, germanosilicate, or phosphosilicate glasses. Some codopants such as aluminum improve the solubility of rare-earth ions and thus allow for higher rare-earth doping concentration without quenching of the upper-state lifetime. (The articles on rare-earth-doped gain media, gain media, and fiber core give some more details.)
For many upconversion lasers and visible fiber lasers, some kind of fluoride glass is required where the phonon energies are lowered so that the metastable level lifetimes are long enough (→ no quenching via multi-phonon transitions).
Codoped Fibers
Some fibers are intentionally doped with two different kinds of rare-earth ions. Most popular is the combination of erbium and ytterbium (→ erbium-ytterbium fibers) – normally with a significantly higher concentration of ytterbium. When such a fiber is pumped e.g. around 980 nm, most of the pump light is absorbed by ytterbium ions (called sensitizer ions), bringing these into their excited states. From there, the energy can be quite efficiently transferred to the erbium ions, which then provide laser gain in the 1.5-μm spectral region. Compared with purely erbium-doped fibers, Er:Yb fibers offer much higher pump absorption per unit length and can thus be used for fiber devices with much shorter lengths. For example, this is useful for making robust single-frequency fiber lasers of a few centimeters length, or for double-clad fiber devices with a moderate length.
For the energy transfer to be efficient, the doping concentrations have to be well balanced, and the core composition must be suitable.
Ytterbium codoping can also be used for other gain systems, such as in praseodymium-doped upconversion lasers. This allows e.g. for red, orange or blue emission with single-wavelength pumping (instead of dual-wavelength pumping for purely praseodymium-doped fibers).
Codopants can also be used for quenching the lower-state population in gain systems with self-terminating laser transitions. For example, praseodymium doping allows for relatively efficient operation of 2.7-μm erbium fiber lasers.
Fibers for High-power Lasers and Amplifiers
For high-power fiber lasers and amplifiers, double-clad fibers are used. These have a highly multimode inner cladding, into which the pump light is launched, and a fiber core which is either single-mode or supports only a few modes. Only the core (or sometimes a ring around the core) is rare-earth-doped. Such fibers allow for a high beam quality of the laser or amplifier output, whereas the pump beam quality can be very low. The resulting devices are often called brightness converters, since the brightness of the output can be much higher than that of the pump source, despite the somewhat lower power.
Characterization of Rare-earth-doped Fibers
In addition to all the properties of a passive (undoped) optical fiber, such as the guiding properties (effective mode area, numerical aperture, cut-off wavelength, bend losses), nonlinearities, etc., active fibers can be characterized with respect to several other properties:
- One of the most important parameters is the rare-earth doping concentration, most often specified in ppm wt. (parts per million with respect to weight). A higher doping concentration allows for efficient pump absorption in a shorter length and thus also reduces the effect of nonlinearities in high peak power devices. However, it can also lead to concentration quenching.
- Wavelength-dependent effective absorption and emission cross sections (and possibly ESA cross sections) together with the upper-state lifetime (and possibly lifetimes of intermediate levels) are required for calculating the wavelength tuning behavior, power efficiency, etc.
- Parameters for quantifying the speed of energy transfer processes are important particularly for codoped fibers.
For such characterization, a variety of measurement techniques is used. White-light absorption spectra can be used for finding absorption cross sections (for known doping concentrations). Emission cross sections are obtained from fluorescence spectra, with scaling e.g. via reciprocity relations (→ McCumber theory) or the metastable level lifetimes (→ Fuchtbauer-Ladenburg equation). Upper-state lifetimes are often obtained from fluorescence measurements with pulsed pumping, and ESA parameters can be obtained in experiments with a modulated pump power.
The resulting set of data can be used e.g. in laser and amplifier models based on rate equations. Such models allow e.g. to predict or check the performance of fiber laser or amplifier devices, the effect of possible modifications, etc.
Further characterization may be required for quantifying effects like photodarkening, which can sometimes seriously degrade the efficiency of active fiber devices.
Bibliography
| [1] | S. B. Poole, D. N. Payne, and M. E. Fermann, "Fabrication of low loss optical fibers containing rare-earth ions", Electron. Lett. 21, 737 (1985) |
| [2] | S. B. Poole et al., "Fabrication and characterization of low-loss optical fibers containing rare-earth ions", J. Lightwave Technol. LT-4 (7), 870 (1986) |
| [3] | J. E. Townsend et al., "Solution-doping technique for fabrication of rare earth doped optical fibres", Electron. Lett. 23, 329 (1987) |
| [4] | M. E. Fermann et al., "Efficient operation of an Yb-sensitised Er fiber laser at 1.56 μm", Electron. Lett. 24, 1135 (1988) |
| [5] | W. J. Miniscalco, "Erbium-doped glasses for fiber amplifiers at 1500 nm", J. Lightwave Technol. LT-9, 234 (1991) |
| [6] | L. Wetenkamp et al., "Optical properties of rare earth-doped ZBLAN glasses", J. Non-Crystal. Solids 140, 35 (1992) |
| [7] | R. Paschotta et al., "Characterization and modeling of thulium:ZBLAN blue upconversion fiber lasers", J. Opt. Soc. Am. B 14 (5), 1213 (1997) |
| [8] | G. G. Vienne et al., "Fabrication and characterization of Yb3+:Er3+ phosophosilicate fibers for lasers", J. Lightwave Technol. 16, 1990 (1998) |
| [9] | S. Tanabe, "Rare-earth-doped glasses for fiber amplifiers in broadband telecommunication", C. R. Chimie 5, 815 (2002) |
| [10] | M. J. F. Digonnet, "Rare-earth-doped fiber lasers and amplifiers", 2nd edition, CRC Press, Boca Raton, FL (2001) |
See also: rare-earth-doped gain media, fibers, fiber fabrication, fiber core, fluoride fibers, double-clad fibers, fiber lasers, fiber amplifiers, erbium-doped gain media, erbium-doped fiber amplifiers, silica fibers, high-power fiber lasers and amplifiers, photodarkening
Categories: fibers and other waveguides, materials
This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) could become very valuable for your business!
