In optical transmission systems, there is loss when an optical signal traverses a fiber. A typical loss value is 0.22 dB/km. Thus, after tens of kilometres along a fiber, the optical signal can become very weak. To reboot the optical signal, signal amplification is required for a long-distance transmission.

Electronic signal repeaters

Under the traditional technology, electronic signal repeaters are often employed whenever an optical signal is transmitted more than 30-50 km. Basically, the electronic signal repeater uses an OEO conversion process to fulfil the signal amplification function. That is, a received (weak) optical signal is first converted into an electronic format, and then the latter is recovered in its shape, time, and amplitude in the electronic domain. Finally, the recovered electronic signal is used to modulate an output laser to generate new good-shape optical pulse signals. In addition to amplification, electronic signal repeaters can also provide advantages of signal reshaping and signal re-timing. Thus, it is also called 3R-regenerator. However, electronic signal repeaters are suffered from several disadvantages. First, the regeneration process is not signal or wavelength transparent. For different data rates, different signal repeaters should be deployed and normally the higher the data rate is, the more expensive the repeater will be. Second, the electronic signal repeaters are normally expensive, especially in a WDM transmission system, as each wavelength is need a dedicated signal repeater.

Optical amplifiers

In contrast, all-optical amplifiers such as EDFA and Ramam amplifiers do not need to go through an OEO conversion process. By employing some nonlinear effects in fiber communications, optical amplifiers can transfer optical power from a pump laser to an optical channel that carries user data. Moreover, optical amplifiers normally provide a wide amplification band, which is especially useful for a WDM transmission system in which a single all-optical amplifier is sufficient to amplify all the wavelengths channels within a transmission band. For example, an EDFA in C-band can cover up to around 30 nm band. A Ramam amplifier can cover even a larger band. Thus, compared to the electronic signal repeaters, optical amplifiers generally show advantage of lower cost as for each WDM system with tens of wavelengths requires only a single optical amplifier to amplify its signals, while the same number of signal regenerators is required under the transitional signal repeater case. Second, optical amplifiers are normally transparent to optical signals or wavelengths, thereby providing convenience for the future upgradation of a transmission system. For example, under the optical amplification, no system change, except at transceivers, is required if we wish to upgrade an optical transmission system from 2.5 Gb/s to 10 Gb/s. The major shortcoming of optical amplifiers is that they cannot help to reshape and retime optical signals, but provide capability of boosting optical signal amplitude only.

Optical amplifiers plus electronic signal repeaters

However, in an optical transmission system, there are other effects such as chromatic dispersion, nonlinear effects, and so on, which all degrade an optical signal. In order to maintain a good signal quality, the signal regeneration is still required after a couple of all-optical amplification spans. In this case, an electronic signal repeater is required to reshape and retime the signal. Thus, in a real optical transmission system, optical amplifiers and electronic signal repeaters often work together to enable the system to transmit signals over thousands of kilometres.