erbium-ytterbium-doped laser gain media (original) (raw)

Definition: laser gain media which are doped with both erbium (Er) and ytterbium (Yb)

Categories: optical materials, laser devices and laser physics

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Contents

Erbium-ytterbium-doped laser gain media are laser gain media which are doped with both erbium (Er) and ytterbium (Yb) — more precisely, they contain the corresponding trivalent ions Er3+ and Yb3+. They are used for fiber lasers and fiber amplifiers, and sometimes in diode-pumped bulk lasers, where the emission or amplification from erbium is required (usually in the 1.5-μm spectral region), and particularly where efficient pump absorption in a short length of material is needed.

Energy Transfer from Ytterbium to Erbium

Figure 1: Energy transfer from Yb3+ to Er3+, followed by non-radiative decay of erbium to the upper laser level.

Basically always, erbium-ytterbium-doped gain media are pumped with wavelengths around the 975-nm absorption peak of Yb3+ ions. This leads to the excitation of the ytterbium ions. Thereafter, the excitation energy may be transferred to nearby erbium ions — see the article on energy transfer and Fig. 1. After a successful energy transfer, one first has an Er3+ ion in the 4I11/2 state (more precisely, a Stark level manifold), which has a similar excitation energy as the Yb3+ ions. From that state, the ions normally undergo a rapid non-radiative transition to the state 4I13/2, which is often used as the upper laser level. Such non-radiative transitions occur based on multi-phonon emission, provided that the phonon energy of the material is high enough (which is the case for silica fibers, for example).

It is possible that energy is transferred back from erbium ions in 4I11/2 to ytterbium ions. That would be quite unwanted, particularly because energy losses by spontaneous emission are about an order of magnitude faster in ytterbium, compared with the 4I13/2 level of erbium. However, such a back-transfer may be largely suppressed if the mentioned non-radiative decay to 4I13/2 is very fast. From there, a back-transfer is no longer possible, since the required energy is lacking.

For obtaining the best efficiency of a laser or amplifier, one needs to optimize the doping concentrations of both types of ions. For too low doping, the energy transfer may not occur at a sufficiently high rate, and excitation energy is lost by spontaneous emission of Yb3+. Also, the pump absorption (see below) may then not be strong enough. On the other hand, too high doping concentrations can lead to clustering, which can cause severe energy losses, and even without clustering to an increased tendency of additional (unwanted) energy transfer processes.

Additional dopants such as alumina can help to incorporate more active ions without clustering, and also to assist in the energy transfer. At the same time, they can change the absorption and radiation properties of the ions, which can e.g. lead to a change in the gain spectrum of an amplifier. There are also additional side effects, for example concerning radiation sensitivity.

For such reasons, a careful optimization of the overall chemical composition of the material is important. The detailed composition used is often not revealed by suppliers.

Note that the quantum defect is not larger for Er/Yb-doped media than for purely Er-doped gain media, given that about the same pump wavelength is used (not considering in-band pumping of erbium). However, the quantum efficiency can be significantly lower, since the energy transfer process may not work perfectly.

Enhanced Pump Absorption

The essential advantage of using Yb codoping is that this way one can achieve a substantially improved efficiency of pump absorption in a short piece of a gain medium — for two reasons:

• The absorption transition cross-section of Yb3+ is substantially higher than that of Er3+.
• The common silica fibers (more precisely, fibers based on some kind of silicate glass) can incorporate substantially more ytterbium than erbium, which has a higher tendency of clustering. The latter would introduce various additional energy transfer processes, which are detrimental for the laser or amplifier efficiency.

The ytterbium ions are sometimes called sensitizer ions because they make the gain medium in some sense more sensitive to the applied pump radiation.

Unwanted Emission of Ytterbium; ASE Problem

Spontaneous emission of excited Yb3+ ions constitutes an energy loss and is therefore unwanted. Another possibly quite detrimental effect is that those excited ions lead to laser gain mostly in the spectral region around 1030 nm to 1080 nm. Particularly in fiber devices, that gain can get so high that a substantial amount of power is lost through amplified spontaneous emission (ASE) and/or parasitic lasing. The power losses through ASE may be further increased by the multimode nature of the fiber core at the pump wavelength.

Such problems can be mitigated by optimization of the composition of the gain medium and the system design, including the pumping conditions and the length of fiber. In ideal cases, there is not much ytterbium excitation because the energy is quickly transferred to erbium and cannot get back from there. That may not be possible to achieve, however, for example if a high level of erbium excitation is needed, so that many excited erbium ions cannot accept energy from the ytterbium ions.

Er/Yb-doped Laser Crystals

In the case of bulk laser crystals, the low pump absorption achievable with erbium alone can be a particularly severe problem. The same holds to an even larger extent when amplifier gain is needed in micro-lasers [13]. Therefore, Er/Yb doping can be quite helpful in this area. However, the availability of such laser crystals is quite limited.

Er/Yb-doped Fibers

In the area of rare-earth-doped fibers, Er/Yb doping is more common. Erbium–ytterbium-doped fibers are used e.g. for erbium-doped fiber amplifiers and for short fiber lasers.

In the case of lasers, it is sometimes important to have a rather short active fiber, particularly to achieve single-frequency operation. For a longer laser resonator, the free spectral range — the mode spacing in frequency space — gets rather small, and it is more difficult to suppress mode hopping. For example, distributed-feedback lasers (DFB lasers) based on purely erbium-doped fiber exhibit very inefficient pump absorption, while with Er/Yb doping a substantially better power conversion efficiency is possible.

For fiber amplifiers, it can also be beneficial to improve pump absorption, particularly if cladding pumping of double-clad fibers is applied, where the limited core/cladding overlap typically leads to weak pump absorption. For the amplification of short light pulses with high peak power, it is important to keep the amplifier fiber short to limit nonlinear optical effects. For core-pumped amplifiers, several decibels of amplifier gain per centimeter length can be achieved.

The fiber core usually needs to contain substantial concentrations of phosphorus, which raises the refractive index. That leads to a high numerical aperture, which particularly for large mode area fibers makes it more difficult to obtain single-mode guidance. A partial solution can be to create a region which raises the refractive index around the core (called a pedestal), but that region as a whole can still guide light. That can lead to increased higher-order mode content of the output.

The relevant physical processes in Er/Yb-doped crystals and glasses — pump absorption, energy transfer, spontaneous and stimulated emission etc. — can be quantitatively investigated with mathematical models, usually of numerical type. While the complexity of such models is only moderately increased by the presence of two different types of ions and the energy transfers, it becomes substantially more challenging to acquire a complete set of spectroscopic data:

• One needs to measure the relevant wavelength-dependent transition cross-sections of Er3+ and Yb3+.
• Second, one requires the upper-state lifetimes — in the case of ytterbium, the theoretical value without the energy transfer. One will usually calculate that from the emission cross-sections, if those are available with absolute scaling, or possibly take a value from a similar glass without ytterbium.
• The rate of energy transfers in both directions are rather difficult to measure. These parameters may be calibrated by comparing experimental results with a model.

A complete spectroscopic characterization requires a quite sophisticated set of measurements and calculations; that is often not done. In particular, commercial suppliers usually do not offer such data, also because sales volumes for such specialty fibers are relatively small. In effect, it is usually difficult to estimate the achievable performance numbers before ordering the parts. This can be considered as a severe practical disadvantage of erbium/ytterbium-doped laser gain media.

Frequently Asked Questions

What are erbium-ytterbium-doped gain media?

They are laser gain media, such as optical fibers or crystals, that are doped with both erbium (Er³⁺) and ytterbium (Yb³⁺) ions. They are primarily used for lasers and amplifiers operating in the 1.5-μm spectral region.

Why is ytterbium added to erbium-doped media?

Ytterbium is added as a sensitizer to dramatically increase the absorption of pump light, typically around 975 nm. This allows for much more efficient pump absorption in a shorter length of the gain medium.

How does the energy transfer from ytterbium to erbium work?

Pump light excites the ytterbium (Yb³⁺) ions. This excitation energy is then transferred to nearby erbium (Er³⁺) ions, which then quickly undergo a non-radiative transition to the upper laser level (4I13/2).

What are the main advantages of using Er/Yb co-doping?

The main advantage is substantially stronger pump absorption. This results from the higher absorption cross-section of Yb³⁺ and the ability to incorporate much more ytterbium than erbium into silica glass without detrimental clustering.

Are there any disadvantages to using Er/Yb doping?

Yes. Excited ytterbium ions can cause unwanted gain and amplified spontaneous emission (ASE) near 1 μm, creating a loss channel. Also, imperfect energy transfer can lower the quantum efficiency, and the complexity (and frequently a lack of data) makes accurate modeling difficult.

What are common applications of Er/Yb-doped fibers?

They are often used for high-power erbium-doped fiber amplifiers (EDFAs), especially cladding-pumped ones, and for compact fiber lasers where a short gain medium is essential, such as in single-frequency distributed-feedback (DFB) lasers.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains ten suppliers for erbium-ytterbium-doped laser gain media. Among them:

⚙ hardware

Exail (formerly iXblue) offers a wide range of erbium/ytterbium codoped optical fibers for high power CW or pulsed fiber amplifiers and lasers. They have been optimized to address the specific requirements of high efficiency and low noise for high power fiber lasers and amplifiers, with applications in LIDAR, CATV, and space.

Benefits and features:

• broad choice of designs: double-clad, double-cladd all-glass, and tripe clad versions
• high efficiency, low background losses
• high and consistent pump absorption
• weak parasitic 1-μm emission
• high-brightness single-mode core

Bibliography

(Suggest additional literature!)

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