Optical amplifier

An optical amplifier is a device which passes an optical signal amplified without having it converted to an electrical signal. The gain arises by means of stimulated emission, which is triggered by the signal to be amplified. An optical amplifier works in principle is like a laser, but without optical resonator. In contrast to regenerative amplifiers, they operate independently of the modulation of the optical signal and, therefore, sometimes also referred to as "transparent". This property and the large amplifier bandwidth is particularly important in the use of wavelength division multiplexing ( WDM). Optical amplifiers are, since a high reliability is guaranteed, used for all long-distance fiber links such as the transatlantic routes.

Most of the optical amplifier also emit spontaneous photons which are then also enhanced. This undesirable effect is called ASE (English amplified spontaneous emission, see also: superluminescent ). The ASE caused by radiation is incoherent and unpolarized. ASE is one of the factors that limits the number of cascaded amplifiers. The ratio of signal power to the noise power normalized to a narrow bandwidth (typically 0.1 nm) is referred to as an optical signal -to-noise ratio OSNR in dB normalized and, based on the specified range (for example, dB / 0.1 nm).

Erbium -doped fiber amplifier ( EDFA)

For erbium -doped fiber amplifiers ( erbium doped fiber amplifier Sheet, EDFA) is doped to a conventional glass fiber of a length of one to about 100 meters, with erbium. This fiber section is then optically pumped by a semiconductor laser.

Typical values ​​for commercial EDFAs:

• Operating wavelength C-band ( approximately 1530 to 1560 nm)
• L-band ( approximately 1570 to 1600 nm).
• S-band (below 1480 nm ) requires different dopants.

EDFA were first demonstrated in 1987 by a group at the University of Southampton to David N. Payne and at Bell Laboratories ( Emmanuel Desurvire and others). With the EDFA succeeded from the late 1980s, the breakthrough for fiber optic transmission of optical signals at very long distances.

Semi -conductor laser amplifier (SOA )

Semi -conductor laser amplifier ( Semiconductor Optical Amplifier Sheet SOA) are constructed as semiconductor laser, however, have at the end faces, in which the light exits, a anti-reflection coating so that no undesirable effects of resonance. They are usually operated with single-mode fibers. Semiconductor laser amplifier in terms of gain, noise, and polarization dependence inferior to the EDFAs, however, arises due to the strong integrability a price advantage.

Typical values ​​:

• Operating wavelength: mainly 1300 and 1500 nm ( active medium: InGaAsP )
• Gain: 30 dB, and 20 dB chip alone included loss of the contact surfaces
• Maximum optical power output: -5 dBm
• Bandwidth: 25 nm

Raman amplifier

In Raman amplifiers, use is made of the so-called Raman scattering. In light of an optical pump wave ( high intensity ) are scattered in quartz glass fibers, the photons to the silica molecules. A part of the energy goes into phonons, and the remainder is dispersed as lower energy photons. This process takes place first spontaneously, but can also be stimulated by the signal to be amplified wave. This is called stimulated Raman scattering.

Advantages over EDFA are:

• High bandwidth: C- and L-band at the same time
• Gain range adjustable via the pump wavelength
• Which over the entire fiber distributed gain leads to a better signal -to-noise ratio

The principle has been demonstrated already by Erich P. Ippen and Rogers H. Stolen 1973. The benefit for signal transmission over long distances has already been exploited in the 1980s (eg, Linn F. Mollenauer at Bell Laboratories ), but was initially pushed by the introduction of EDFAs in the background and experienced only in the course of the 1990s a boom.