5 Optoelectronic Devices and Sensors
5.6 Optical amplifiers

In order to transmit signals over long distances (>100 km) it is necessary to compensate for attenuation losses within the optical fibre (optical transmission channel). This is the objective of optical amplifiers.

Typical optical fibre loss around 1.5 µm is in the range of 0.2 dB/km. It is possible to convert the optical signal to electrical signal and use conventional electronic amplifiers to compensate transmission losses and then convert back again the signal to optical. However, these signal conversions require costly and high speed electronic elements.

An optical amplifier amplifies an optical signal directly, without the need to first convert it to an electrical signal.

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Fig. 12. Optical amplifier scheme.

Main characteristics of optical amplifiers are: Gain (dB), range of operating frequencies or BW (bandwidth), gain saturation: Maximum output power, and output noise level. Gain is defined by the following equation

(018) , where Po and Pi are the output and input power respectively.

There are three main types of optical amplifiers: The EDFAs (erbium-doped fibre amplifiers), the SOAs (semiconductor optical amplifiers), and the fibre Raman amplifiers. In EDFAs the amplifying medium is a glass optical fibre doped with erbium ions that are optically pumped to a state of population inversion with a separate optical input. SOAs are pumped with electrical current and the gain medium is formed by undoped semiconductors. These optical amplifiers are very useful in local networks due to their relative low cost and enough gain for short distances.

In Raman amplifiers, the amplification is based on stimulated SRS (stimulated Raman scattering). Raman scattering is a process in which light is scattered by molecules from a lower wavelength to a higher wavelength.

Some types of optical amplifiers

Characteristics

Disadvantages

SOAs

400 – 2000 nm

Similar to laser cavities (semiconductor lasers).

Large BW and good gain

High noise figure and cross-talk levels.

Rare earth doped fibre amplifiers

erbium – EDFA

1500 nm

Praseodymium – PDFA

1300 nm

Amplification occurs primarily through the stimulated emission process.

The gain depends both on the frequency and on the local beam intensity

Relatively large devices

Cross-talk and gain saturation effects.

Spontaneous noise emission

Raman and Brillouin amplifiers

Does not require a population inversion.

The Pump and amplified signals are at different wavelengths.

High cost