Rare-earth-doped Fibers

Author: the photonics expert Dr. Rüdiger Paschotta

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 specialty fibers are often called active fibers or laser and amplifier fibers. They are gain media with a particularly high gain efficiency, resulting mainly from the strong optical confinement in the fiber's waveguide structure.

This article discusses only aspects which are specific to rare-earth-doped fibers; see the articles on fibers and passive fibers for more general aspects. Also see the article on fiber fabrication, discussing different fabrication technologies.

Common Types of Rare-earth-doped Fibers

The following table shows the most common laser-active ions and host glasses and also typical emission wavelength ranges of rare-earth-doped fibers:

Table 1: Common laser-active ions and host glasses and important emission wavelengths.

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.

Importance of the Host Glass

The chemical composition of the glass for the laser-active fiber core has many important influences on the possible performance and practical use of an active fiber:

• The limited transparency range may exclude the use of certain laser transitions. For example, mid-infrared lasers cannot be realized with silicate fibers, which are strongly absorbing for wavelengths above ≈2 μm.
• The glass composition strongly influences the maximum concentration of the dopant ions that can be incorporated without excessive clustering, which would resulting in quenching effects and possibly increased propagation losses.
• It also influences in various ways the optical transitions of the rare earth ions, in particular the emission and absorption cross-sections, absorption and emission bandwidth, total transition rates and thus the metastable level lifetimes, etc.
• The rate constants for energy transfers between different ions also depend on the chemistry.
• Mainly the maximum phonon energy of the host glass determines the rate of multi-phonon transition processes, thus the speed of non-radiative transfers between certain levels. This effect can be strong: certain levels may be long-lived (multiple milliseconds) in heavy-metal fluoride glasses, but very short-lived (few microseconds) in silicate glasses.
• Some glasses (e.g. fluoride glasses) tend to be difficult and expensive to fabricate and handle. Clean fiber cleaves are not always easy to obtain, and often require modified methods.
• Some glasses are photosensitive, allowing the fabrication of fiber Bragg gratings with ultraviolet light. The photosensitivity can strongly depend on certain dopants.
• Glasses differ very much in their optical nonlinearities and optical damage threshold.

For rare-earth-doped fibers, the core composition is normally modified substantially by additional dopants. For example, one rarely uses pure fused silica, which is not suitable for incorporating substantial concentrations of rare earth ions. Instead, one creates aluminosilicate, germanosilicate, or phosphosilicate glass by adding substances like alumina (Al₂O₃), germania (GeO₂), phosphorus pentoxide (P₂O₅) or phosphorus oxychloride (POCl₃) to silica, the starting material. Specifically, alumina has been found (originally, in secret research at Polaroid) to much improve the solubility of rare earth ions and thus allow for higher rare-earth doping concentrations without quenching of the upper-state lifetime. Other codopants can have additional beneficial effects, such as improving the gain bandwidth, influencing the rate of energy transfers, or modifying the refractive index . The articles on rare-earth-doped laser 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). Such fluoride fibers also exhibit good transmission in the mid-infrared and are therefore used for mid-infrared laser sources.

Simulations on Active Fibers

For designing a fiber amplifier or laser system, a suitable simulator is essential to have. For example, simulate pump absorption, gain saturation effects, ASE, pulse distortions and many other aspects. That way, you get complete insight into how it works and how it can be optimized. The RP Fiber Power software is an ideal tool for such work.

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 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 therefore be used for fiber devices with much shorter lengths, such as distributed-feedback lasers. 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 for, e.g., 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 codoping 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 [8]. 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.

Particularly for mitigating nonlinear effects at high peak power levels, large mode area fibers are used, which often are double-clad fibers at the same time.

Case Studies

Case Study: Designing a Double-clad Fiber Amplifier

We develop a double-clad fiber amplifier with high gain, where we have to care about limiting losses by ASE.

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 by 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.

As an alternative method, so-called Giles parameters can be specified, which depend on the doping concentration, effective mode area and effective transition cross-sections.

For such characterization, a variety of measurement techniques are 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 the reciprocity method (→ McCumber theory) or the metastable level lifetimes (→ Füchtbauer–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.

Further characterization may be required for quantifying effects such as photodarkening, which can sometimes seriously degrade the efficiency of active fiber devices.

Predicting the Performance of Fiber Devices

In principle, it is easy to obtain some amplification of a signal by sending it through an active fiber which at the same time is subject to some pump light. However, the detailed performance is often hard to anticipate due to various complications, which can lead to surprising behavior. Some examples:

• Pump intensities are often well above the pump saturation power, causing strong pump saturation effects. One may require a far longer fiber for efficient pump absorption than one would naively estimate based on ground-state absorption. Often, the appropriate fiber length depends very much on the operation conditions.
• Gain saturation effects can also be very strong, and caused not only by signals but also by amplified spontaneous emission. The latter is often strong in fiber devices as a large gain is easily achieved.
• The quasi-three-level characteristics, which are encountered for most of the used laser transitions, often have profound effects. For example, the shape of the net gain spectrum can substantially depend on the average degree of excitation of the laser-active ions, and reabsorption effects are highly relevant for the optimization of fiber length.

For such reasons, the efficient development of devices utilizing rare-earth-doped fibers usually requires laser and amplifier models with which many performance aspects of fiber lasers and amplifiers can be simulated. Such work requires (a) suitable simulation and design software and (b) data of comprehensively characterized fibers.

More to Learn

Tutorials:

• Tutorial on Fiber Amplifiers
• Tutorial on Fiber Amplifiers, Part 1: Rare Earth Ions in Fibers
• Tutorial on Modeling and Simulation of Fiber Amplifiers and Lasers, Part 4: The Laser-active Ions

Case studies:

• Case Study: Designing a Double-clad Fiber Amplifier
• Case Study: Erbium-doped Fiber Amplifier for a Long-wavelength Signal
• Case Study: Erbium-doped Fiber Amplifier for Multiple Signals
• Case Study: Erbium-doped Fiber Amplifier for Rectangular Nanosecond Pulses
• Case Study: Ytterbium-doped 975-nm Fiber Lasers

Encyclopedia articles:

• active fibers
• rare-earth-doped laser gain media
• fibers
• fiber fabrication
• fiber core