In telecommunication, an optical amplifier is a device that amplifies an optical signal directly, without the need to convert it to an electrical signal, or amplify it electrically, and reconvert it to an optical signal.

Doped fiber amplifiers

Fiber amplifiers are optical amplifiers which use a doped optical fiber, which bears the communication signal, and is optically pumped with a laser having a high-powered continuous output at an optical frequency slightly higher than that of the communication signal. The pump lasers operate either at a wavelength of approx. 910 nm or at a wavelength of approx. 1480 nm. Because neither optical-electrical conversion nor electrical amplification takes place, this type of amplifier is well suited for a wide variety of applications, both digital and analog.

Because this type of amplifier does not require extraordinary frequency (wavelength) control of the pumping laser, it is relatively simple.

A fiber amplifier is capable of amplifying a complete wavelength-division multiplexed set of signals as a single wideband optical signal. In this way, an all-optical infrastructure equipped with fiber amplifiers is capable of supporting future modulation and WDM standards, in a way that systems with optical-electrical-optical regeneration cannot. This has led to all-optical infrastructure becoming the standard for long-haul networks, wherever physically possible.

Care must be taken to avoid saturating fiber amplifiers, as this will cause incorrect operation. To prevent this, the power levels of all the signals in a WDM group must be kept carefully balanced.

Erbium-doped fiber amplifiers (EDFA)

The most common dopant used in fiber amplifiers is erbium, and such a fiber amplifier is known as an erbium-doped fiber amplifier, or EDFA.

Typical values for commercial EDFAs :-

• Frequency of operation: C and L band (approx. 1530 to 1605 nm).
  • For S-band (below 1480 nm) other dopants are necessary.
• low noise with noise figure 3-6dB
• high gain(20-40dB) and less sensitivity to polarization of the light signal.
• Max. optical outut power: 14 - 25 dBm
• Internal gain: 25 - 50 dB
• Gain variation: +/- 0.5 dB
• Length of the active fiber: 10 - 60 m for C-band EDFAs and 50 - 300 m for L-band EDFAs
• Quantity of pump lasers: 1 - 6
• Pump wavelength: 980 nm or 1480 nm

Semiconductor Optical Amplifier (SOA)

Semiconductor Optical Amplifiers have a similar structure as FP(Fabry-Perot) laser but with anti-reflection(AR) design at the endfaces. The recent AR design includes AR coating, tilted waveguide and window regions to get rid of endface reflection almost perfectly and it prevents the amplifier from acting as a laser.

The semiconductor optical amplifier is of small size and electrically pumped. It can be potentially less expensive than EDFA and can be integrated with semiconductor lasers, modulators, etc. But the actual performance is still not competible with EDFAs. It has high noise, less gain, medium polarization dependence and high nonlinearity with fast transient time. Nonlinearity problem is the most severe problem in optical communication usage. Recently 'Linear Optical Amplifier' with gain-clamping technique was developed and commercially available.

High optical nonlinearity makes semiconductor amplifiers attractive for the optical signal processing like all-optic switching and wavelength conversion. There have been many researches on semiconductor optical amplifiers as optical computing components.

Raman amplifier

The signal is intensified by Raman amplification. Unlike EDFA and SOA, it is based on nonlinear-optical interaction between optical signal and pump light of high power. So the installed optical fiber itself can be gain medium of Raman amplfication. Or lumped amplifier can be made of specially designed highly nonlinear fiber with high doping and small core size.

The pump light is coupled into the transmission fiber either in the same direction as the transmission signal (co-directional pumping) or is coupled into the transmission fiber in the opposite direction (contra-directional) pumping. Contra-directional pumping is more usual because co-directional pumping has the problem of nonlinear amplification. High gain can be obtained in 13THz lower (approx. 100nm longer) than pump wavelength.

Pump power should be very high up to 1W (up to 1.2 W for L-band on SSMF). More than 2 laser diodes are usually used. But the noise is very small especially used with EDFAs.

Note: The text of an earlier version of this article was taken from the public domain Federal Standard 1037C.