Distribution laser amplifier

Definition: Fiber amplifier in a fiber optic data link, the amplification process that occurs over a very long transmission fiber.

For long fiber links used in long-distance data transmission, one or more fiber amplifiers are needed to ensure sufficient signal power at the receiver and to maintain a sufficient signal-to-noise ratio while ensuring a bit error rate. In many cases these amplifiers are discrete, implemented with a few meters of rare earth-doped fiber, pumped by a fiber-coupled diode laser, sometimes as part of the transmitter or just in front of the receiver, or in the middle of the transmission fiber used somewhere. A distributed amplifier in the transmission fiber itself can also be used. The pump light is usually injected at the receiver or transmitter port, or both ports are injected at the same time. This distributed amplifier may achieve similar overall gain, but the gain per unit length is much lower. This means that this can maintain a reasonable signal power level in the presence of transmission losses, rather than increasing the power by a few decibels.

Pros and cons:

One advantage of using distributed amplifiers is lower amplifier noise build-up on the link. This is mainly because the signal power is maintained all the time rather than to a very low degree, as is the case with discrete amplifiers. Peak signal power can then be reduced without adding amplifier noise. This actually reduces potentially detrimental fiber nonlinear effects.

A very big disadvantage of distributed amplifiers is the need for higher pump power. This applies to Raman amplifiers and rare earth doped amplifiers, discussed below.

The advantages of different types of amplifiers depend on the transmission system and its characteristics. For example, for systems based solely on solitons, important factors to consider are wavelength range and signal bandwidth.

Distributed laser amplifier

Distribution amplifiers can be implemented in two different forms. The first method is to use a transmission fiber that contains some rare earth doped ions, such as erbium ions, but the doping concentration needs to be much lower than that of ordinary amplifier fibers. Although silica fiber is commonly used for communications, its solubility in rare earth ions is very low, and low doping can avoid quenching effects. However, since transmission optical fiber also has some other limitations, it is difficult to optimize the optical fiber to have a large gain bandwidth. In particular, any doping will increase transmission losses, which is not a serious problem in short discrete amplifiers.

Since the pump light of the distributed amplifier also needs to be transmitted over a long distance, it will experience transmission loss. If the pump wavelength is much smaller than the signal wavelength, the loss is even greater than the signal light. Therefore, long distribution erbium-doped amplifiers need to use 1.45 micron pump light instead of the commonly used 980nm light. This in turn will put more restrictions on the spectral shape of the amplifier gain. Even with long pump wavelengths, the pump power requirement is higher due to pump losses compared to discrete fiber amplifiers.

Distributed Raman Amplifier

Another type of distributed amplifier is the Raman amplifier, which does not require rare earth doping. Instead, it uses stimulated Raman scattering to achieve the amplification process. Likewise, transmission fibers are difficult to optimize for Raman amplification processes because transmission losses need to be low and the pump light also experiences transmission losses. Therefore, very high pump power is required.

The gain spectrum of a pump source depends on the chemical composition of the fiber core. A tuned wider gain spectrum can be achieved by combining different pump wavelengths.