distributed amplifiers (original) (raw)

Definition: fiber amplifiers in fiber-optic data links, where the amplification occurs within a large length of transmission fiber

Categories: article belongs to category fiber optics and waveguides fiber optics and waveguides, article belongs to category lightwave communications lightwave communications

optical amplifiers
- amplifier chains
- distributed amplifiers
- multipass amplifiers
- fiber amplifiers
- ultrafast amplifiers
- regenerative amplifiers
- semiconductor optical amplifiers
- power amplifiers
- high-power amplifiers
- tapered amplifiers
- optical parametric amplifiers
- Raman amplifiers
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DOI: 10.61835/otn Cite the article: BibTex BibLaTex plain text HTML Link to this page! LinkedIn

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What are Distributed Amplifiers?

For longer fiber-optic links (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, dopants that substantially raise the propagation losses need to be avoided, , 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 as 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.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is a distributed amplifier?

A distributed amplifier is an optical amplifier where the gain medium is a long section of the transmission fiber itself. It is used to counteract signal power loss along the fiber, rather than providing a large gain at a single point.

How does a distributed amplifier differ from a discrete amplifier?

A discrete amplifier uses a short, highly doped fiber to provide a large, localized gain. In contrast, a distributed amplifier provides a small amount of gain per unit length over a long distance, aiming to maintain a relatively constant signal power level.

What are the main types of distributed amplifiers?

The two primary types are distributed laser amplifiers, using transmission fiber lightly doped with rare-earth ions (e.g., erbium), and distributed Raman amplifiers, which utilize stimulated Raman scattering in standard transmission fiber.

What is the main advantage of distributed amplification?

Its main advantage is a smaller variation in signal power along the fiber. This improves the signal-to-noise ratio by keeping the signal from becoming too weak and reduces nonlinear effects by avoiding excessively high power levels.

What are the disadvantages of using distributed amplifiers?

Distributed amplifiers require higher pump powers compared to discrete amplifiers. This can also pose a laser safety risk if the transmission fiber breaks. Furthermore, the transmission fiber cannot be fully optimized for gain.

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Bibliography

[1]	A. Hasegawa, “Numerical study of optical soliton transmission amplified periodically by the stimulated Raman process”, Appl. Opt. 23 (19), 3302 (1984); doi:10.1364/AO.23.003302
[2]	L. F. Mollenauer et al., “Soliton propagation in long fibers with periodically compensated loss”, IEEE J. Quantum Electron. 22 (1), 157 (1986); doi:10.1109/JQE.1986.1072858
[3]	M. Nakazawa et al., “Ultralong dispersion-shifted erbium-doped fiber amplifier and its application to soliton transmission”, IEEE J. Quantum Electron. 26 (12), 2103 (1990); doi:10.1109/3.64345
[4]	E. Desurvire, “Analysis of distributed erbium-doped fiber amplifiers with fiber background loss”, IEEE Photon. Technol. Lett. 3 (7), 625 (1991); doi:10.1109/68.87934
[5]	S. Wen, “Distributed erbium-doped fiber amplifier for soliton transmission”, Opt. Lett. 19 (1), 22 (1994); doi:10.1364/OL.19.000022
[6]	Y. Emori et al., “100 nm bandwidth flat-gain Raman amplifiers pumped and gain-equalized by 12-wavelength channel WDM laser diode unit”, Electron. Lett. 35, 1355 (1999); doi:10.1109/OFC.1999.766052
[7]	V. E. Perlin and H. G. Winful, “On distributed Raman amplification for ultrabroad-band long-haul WDM systems”, J. Lightwave Technol. 20 (3), 409 (2002)
[8]	T. Zhang et al., “Distributed fiber Raman amplifiers with incoherent pumping”, IEEE Photon. Technol. Lett. 17 (6), 1175 (2005); doi:10.1109/LPT.2005.846479

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