Active Fibers

Definition: optical fibers containing laser-active dopants in the fiber core

More general term: optical fibers

More specific terms: rare-earth-doped fibers, double-clad fibers

Opposite term: passive fibers

German: aktive Fasern

Categories: fiber optics and waveguides, laser devices and laser physics

Author: Dr. Rüdiger Paschotta

Cite the article using its DOI: https://doi.org/10.61835/fro

Get citation code: Endnote (RIS) BibTex plain text HTML

Active fibers are optical fibers which have one or more laser-active dopants in the fiber core. In most cases, they are rare-earth-doped fibers, with dopants like ytterbium, erbium or thulium. Due to those dopants, they can be used as laser gain media, also for realizing fiber amplifiers. Only quite rarely, active fibers are made with transition metal ions.

Active fibers are generally made from some glass, in most cases from fused silica (→ silica fibers). However, there are also active single-crystal fibers.

Case Studies

Case Study: Erbium-doped Fiber Amplifier for a Long-wavelength Signal

Amplified spontaneous emission (ASE) turns out to be a limiting factor, requiring a dual-stage amplifier design.

Case Studies

Case Study: Erbium-doped Fiber Amplifier for Multiple Signals

We optimize an amplifier for equal output powers of signals spanning a substantial wavelength range. There is a trade-off between power efficiency and noise performance.

Besides the ability to amplify light (usually with high gain efficiency and often with a high power conversion efficiency and high beam quality of the output), active fibers usually exhibit higher propagation losses, which, however, are normally of little relevance, since one generally requires only a relatively short fiber. In most cases, they are single-mode or few-mode fibers, and sometimes large mode area fibers or polarization-maintaining fibers.

For more details, see the article on rare-earth-doped laser gain media, which also describes special variants such as double-clad fibers and triple-clad fibers.

Laser amplification is not the only possible physical mechanism to obtain gain in a fiber: stimulated Raman scattering (SRS) and the Kerr nonlinearity of the fiber can also be exploited for that purpose. In such cases, one may consider a fiber as active in the sense that it can amplify light.

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