Distributed Amplifiers

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

For longer fiber-optic links as used for long-haul data transmission, one or several fiber amplifiers are usually needed for obtaining a sufficiently high signal power at the receiver and maintaining a high enough signal-to-noise ratio for the required bit error rate. In many cases, such amplifiers are discrete amplifiers, realized with a few meters of some rare-earth-doped fiber, which is pumped with a fiber-coupled diode laser and used as part of the transmitter, or just before the receiver, or somewhere between parts of the transmission fiber. However, it is also possible to employ so-called distributed amplification in a long length of the transmission fiber itself, even though this not very common.

Kinds of Distributed Amplifiers

Distributed amplifiers can be based on two different operation principles:

Distributed Laser Amplifiers

One could use a transmission fiber which contains some rare-earth dopant such as erbium (Er³⁺), but with a much lower doping concentration than a regular amplifier fiber. Although the material of silica fibers, as normally used for transmission, exhibits a low solubility for rare earth ions, a low concentration can be incorporated without quenching effects. However, it is difficult to optimize the fiber also for a large gain bandwidth, as the transmission fiber is subject to further constraints. In particular, any dopants need to be avoided which substantially raise the propagation losses, whereas in a short discrete amplifier these are typically not a serious issue.

Note also that the pump light for a distributed amplifier needs to be delivered over a substantial length, and is therefore also subject to propagation losses – even more than the signal light if the pump wavelength is significantly shorter than the signal wavelength. A long distributed erbium amplifier should thus be pumped around 1.45 μm rather than the otherwise often used wavelength of 980 nm. This introduces further restrictions on the spectral shape of the amplifier gain. Even with a long pump wavelength, the pump losses lead to the requirement of a higher pump input power, compared with that of a discrete fiber amplifier.

For the explained reasons, and because already laid down transmission fibers usually have no dopant, this approach is not common.

Distributed Raman Amplifiers

Another (more common) type of distributed amplifier is the Raman amplifier, where no rare earth dopant is required, and stimulated Raman scattering is used for amplification. Again, the transmission fiber can hardly be optimized for Raman amplification, as the propagation losses need to be low, and the pump light is also subject to propagation losses. Therefore, substantial pump powers are needed.

The gain spectrum achieved with a single pump source is essentially determined by the chemical composition of the fiber core. Broader gain spectra, possibly with a tailored shape, can be achieved by using some combination of different pump wavelengths.

Maintaining the Signal Power

Such a distributed amplifier may have a similar overall gain like an ordinary (discrete) fiber amplifier, but a much lower gain per unit length. It is meant to approximately maintain a reasonable signal power level in the presence of propagation losses, rather than increasing the power level by tens of decibels. Note that a telecom signal becomes sensitive to noise when its power level gets too low.

General Advantages and Disadvantages

An advantage of using distributed amplifiers is that this approach normally leads to less variation of signal power. That way one can have a low accumulation of amplifier noise within the link by avoiding that the power drops to too low levels. At the same time, the maximum signal power level can actually be reduced without obtaining excessive amplifier noise. This also reduces the potentially detrimental effect of fiber nonlinearities.

An important disadvantage is that distributed amplifiers generally require higher pump powers. This applies to both Raman amplifiers and rare-earth-doped amplifiers, as explained above.

Another problem is a laser safety issue: When a transmission fiber is broken, the relatively high emerging pump power may endanger eyes of a nearby person. Of course, that risk strongly depends on the pump wavelength.

The detailed advantages of different types of amplifiers depend on the type of transmission system and its characteristics. For example, there are specific aspects which are relevant only for soliton-based systems, and the wavelength region and signal bandwidth are also important factors to be considered.

More to Learn

Suppliers

The RP Photonics Buyer's Guide contains 14 suppliers for Raman amplifiers. Among them:

TOPTICA Photonics

Our new highly reliable Raman fiber amplifiers are based on patented new technology. With their high power up to 30 W the amplifiers cover the wavelength range from 1120 to 1370 nm that is not accessible by Yb or Er fiber amplifiers. With a tuning range of 10 nm and a relative intensity noise <1% r.m.s., TOPTICA offers its own portfolio of RFAs that can be seamlessly integrated with TOPTICA lasers as seeders and frequency converters to reach visible and UV wavelengths.

MPB Communications

MPBC’s Network-ready subsystems feature a variety of distributed Raman and patented super Raman amplifiers, delivering the highest level of sensitivity improvement in the industry for OPGW, terrestrial, and submarine networks.

Custom Raman fiber amplifiers (RFA) are also available for sodium Laser Guide Stars (LGS).

MPBC’s Single-frequency Raman fiber amplifiers are designed to provide optical gain in spectral bands not covered by rare-earth amplifiers for amplification of narrowband single-frequency sources.

Bibliography

(Suggest additional literature!)

Questions and Comments from Users