3.3 Optical amplifiers

Figure 22 shows in outline one possible structure for an Erbium-doped fibre amplifier (EDFA).

Erbium-doped fibre amplifier

A length of erbium-doped fibre is spliced in line with the fibre carrying the signal, and high-power, unmodulated light at a specific, shorter, wavelength (the ‘pump’ wavelength) is coupled into the same fibre. The erbium-doped fibre has the same physical structure as standard single-mode fibre but the glass it is made of has erbium atoms added (it is ‘doped’ with erbium), and the presence of the erbium has the effect of transferring energy from light at the pump wavelength to the signal wavelength. In this way the (modulated) signal at around 1550 nm emerges from the erbium-doped fibre at a higher power, having taken the power from the pump.

The wavelength of the light used for the pump can be either 980 nm or 1480 nm. Sometimes both pump wavelengths are used simultaneously to get maximum gain.

While EDFAs are simple in principle and have had a dramatic effect on optical-fibre communications, there are a number of considerations that must be taken into account when they are used, including:

• Cost: Both the wavelengths used in the pump source are difficult to generate and the lasers used to produce them are expensive.

• Noise: In addition to amplifying the input signal, EDFAs (like all amplifiers) also generate a noise signal, in this case known as amplified spontaneous emission (ASE).

• Gain-flatness: In an ideal amplifier a plot of gain against wavelength would be flat across the range of wavelengths for which it is to be used. In a real amplifier the gain depends to some extent upon the precise input wavelength. This can be a problem in systems carrying WDM (wavelength division multiplexed) signals since some wavelengths will be amplified more than others.

In addition, as already explained, EDFAs only work in the 1550 nm window.

Other technologies are available for manufacturing optical amplifiers. For example, semiconductor optical amplifiers (SOAs), also called semiconductor laser amplifiers (SLAs), can be made for use in both the 1550 nm and 1300 nm windows. They use the same technology as the laser diodes that provide the light source at the transmitter, but have a different physical structure. Although SOAs have been available for some time and are potentially cheaper than EDFAs, their performance is not as attractive as EDFAs and they have not been very widely used.

At the time of writing Raman amplifiers have just started to appear commercially. Raman amplifiers use stimulated Raman scattering (SRS) to transfer power from light at a pump wavelength to the signal. This effect is similar to what happens in an EDFA, but is caused by a different physical property leading to different characteristics. Whereas EDFAs transfer power from one or both of two specific pump wavelengths to a specific range of signal wavelengths, with SRS the importance lies in the difference between the pump wavelength and the signal wavelength. For both the 1300 and 1550 nm windows a pump at any wavelength will amplify light with a wavelength about 100 nm longer. Thus if you want to amplify light at, say, 1330 nm you need a pump with a wavelength of about 1230 nm. By having several pumps at different wavelengths, it is possible to amplify over a large range of signal wavelengths.

When considering optical amplifiers it is convenient to refer to the frequency ranges available for optical-fibre transmission defined by Ramaswami and Sivarajan (2002):