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Raman amplifier

Pronounced "ruh-mahn," it is a device that boosts the signal in an optical fiber by transferring energy from a powerful pump beam to a weaker signal beam. It relies on the interaction between light and atoms in the fiber. Unlike erbium-doped fiber amplifiers (EDFAs), which boost wavelengths in the 1530-1610 nm range, a Raman amplifier can increase the signal strength of any wavelength by pumping at 13THz more than the desired frequency.

Normally, Raman amplification is distributed along lengths of signal-transmitting fiber. A Raman pump laser is typically stationed at the end of a fiber run and pumps backward several kilometers to amplify signals through that region. See EDFA.


(Erbium-Doped Fiber Amplifier) A device that boosts the signal in an optical fiber. Introduced in the late 1980s, the EDFA was the first successful optical amplifier. It was a major factor in the rapid development of fiber-optic networks in the 1990s, because it extended the distance between costly regenerators. In addition, an EDFA amplifies all the channels in a WDM signal simultaneously, whereas regenerators require optical to electrical conversion for each channel.

A Laser Without Mirrors
Functioning like a laser without mirrors, the EDFA uses a semiconductor pump laser to introduce a powerful beam at a shorter wavelength into a section of erbium-doped fiber several meters long. The pump light excites the erbium atoms to higher orbits, and the input signal stimulates them to release excess energy as photons in phase and at the same wavelength. EDFAs boost wavelengths in the 1550 nm range, and the pump light is typically 1480 nm or 980 nm. See EDWA, WDM, Raman amplifier and optical amplifier.

Erbium-Doped Fiber Amplifier
EDFAs boost the input regardless of the number of wavelengths. In several meters of doped fiber, the pump laser excites the doped atoms to higher orbits, and the input signal stimulates them to release excess energy as photons in phase and at the same wavelength. (Illustration courtesy of Jeff Hecht.)

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