6 Regeneration of signals in optical networks by using link EDFAs
6.1 Erbium Doped Fibre Amplifiers

Doped fibres are often used as optical amplifiers within optical networks. Erbium is the most common dopant - it follows that Erbium Doped Fibre Amplifiers (EDFAs) are the most common optical amplifiers of this type. EDFAs are pumped with a diode laser with a central wavelength of 980 nm that is multiplexed with the signal-carrying fibre. The pump radiation excites the dopant ions along the fibre amplifier.

The excited erbium ions then undergo a rapid, non-radiative relaxation to a lower energy band, after which they experience a slow, radiative relaxation to their ground state. The relative decay times of these states are critical to the achievement of population inversion – a prerequisite for high stimulated to spontaneous emission ratio, which, in more practical considerations, manifests itself as a high signal-to-noise ratio and greater amplification efficiency.

EDFA operating as in-line amplifier.
Operating wavelengths and gain of EDFA.
Energy model of EDFA.

The solution comprises of a distributed feedback (DFB) laser source, a 980 nm pump module, a multiplexer, an EDFA and spectral analyser. The DFB laser source and the pump module’s outputs were combined using the multiplexer and then transmitted through the EDFA.

## Advantages:

- Broad operating range (C+L band – 1530 nm to about 1680 nm).
- Huge gain 20 to 50 dB (nowadays gain larger than 20 dB is no longer required for the reasons of exciting undesired nonlinear phenomena, such as Four Wave Mixing (FWM) in very high-speed networks).
- Construction – a fibre using optical pump.
- Relatively flat gain for transparent optical networks using WDM (Wavelength Division Multiplexing).
- Low price.
- Application as in-line amplifier.

## Disadvantages:

- Significant ASE noise (Amplified Spontaneous Emission).
- Cannot be used as a booster because of gain suppression (saturation).
- Cannot be used as a preamplifier without special filters.