A Primer on Optical Amplifiers

When there is too much attenuation, the bit-error rate rises rapidly. Ideally, optical signals should be transmitted at the maximum possible data rate that offers a tolerable bit error rate, typically between 10–9 and 10–12. Bit-error rate issues can be avoided by either amplifying or regenerating the signals during transmission. Regeneration converts a digital optical signal into an electrical one, transmits it, and then restores it to an optical signal at its destination. While in its electrical form, the signal is re-shaped and re-timed to correct for signal dispersion. This approach can be costly. However, in some instances, optical amplification will achieve the same result.
 
Optical amplifiers increase the amplitude of a weak optical signal without the costly process of regeneration. An amplifier has fewer parts than an electronic regenerator and is protocol independent, thereby eliminating the need for optical-to-electronic conversion. Their operation is based on lasing principles, in which photons passing by an atom with an electron are pumped into a higher energy orbit. The photon will stimulate the electron to drop to a lower energy orbit and the energy difference between the two orbits will be the energy of an identical photon. Doubling the number of photons will result in a power gain of 3 dB. These two photons also may stimulate other photons, which will further increase the gain.
 
There are three types of optical amplifiers that are commonly used:

• Erbium-doped fiber amplifiers (EDFAs)
• Raman fiber amplifiers
• Semiconductor optical amplifiers (SOAs)

 
The most commonly implemented is the EDFA. An EDFA contains an optical fiber that has been doped with the rare earth element erbium, which can be pumped using either a 980-nm or 1480-nm pump laser for optical gain. Most EDFA amplifiers operate in the low loss C-band (1530 to 1565 nm) and typically have a signal gain in the range of 15 to 25 dB, depending on the power of the pump laser and the length of the erbium doped fiber. The gain can be adjusted by varying the output power emitted by the pumping laser. This pumping optical power in the EDFA will cause the erbium electrons to be pumped into a greater energy higher orbit. Once in the higher orbit, the electrons can be stimulated by a photon to emit an identical wavelength photon in the same direction.
 
EDFAs were originally developed for oceanic long-haul and dense wavelength division multiplexing applications. However, their highly desirable operating characteristics quickly forced their migration towards terrestrial long-haul, metropolitan area networks, cable television, and fiber to the home—where analog transmission mandates the low attenuation of the 1550-nm wavelength.
 
The second type of amplifier, the Raman amplifier, involves a non-linear interaction called Raman scattering that occurs between light and the atoms of the glass transmission fiber itself. Stimulated Raman scattering is a similar process where photons of an applied optical signal stimulate the scattering process. This interaction has the effect of shifting energy from a strong pump beam to a weaker signal beam as both pass along the length of a fiber, resulting in amplification of the signal.
 
The energy shift depends on the type of glass used, but the wavelength depends on the pump beam, so Raman amplification can be used across a wide range of wavelengths by changing the pump wavelength. Use a different pump wavelength, and you can amplify a different set of signal wavelengths. Furthermore, multiple pump beams can be used to create any desired gain profile, thus allowing broadband amplification across multiple bands.
 
Unlike EDFAs, where the gain spectrum is constant and determined by the erbium atoms, the Raman amplification gain-spectrum depends on the pump wavelength. The maximum gain occurs when the pump wavelength is approximately 100 nm shorter than the signal wavelength. For example, in silica glass fibers, a 1550-nm signal is amplified through absorption of pump energy at approximately 1450 nm.
 
While conventional EDFA technology can be used at the terminal sites for links shorter than about 160 kilometers, longer spans will require Raman amplification. For extreme distances, both EFDAs and Raman amplification may be used.
 
Other applications for Raman amplifiers include high-loss spans in regional, long haul, and ultra long-haul systems with span lengths greater than 1000 kilometers. They are also ideal for applications where commercial, legal, or security constraints make intermediate amplification sites impractical or impossible.
 
The third common type of amplifier is the semiconductor optical amplifier (SOA). An SOA is simply a laser diode that is operated without a resonant cavity to perform stimulated emission for signal amplification. Fiber pigtails are bonded to each end of the amplifier chip. When electrically pumped with a forward bias current, incoming photons stimulate the emission of identical photons, providing signal amplification. The device is operated below threshold to prevent the cleaved facets at each end of the chip from forming a resonant cavity resulting in laser oscillation. This amplification process is bidirectional and independent of the type of signal being carried.
 
SOAs typically do not provide the performance that EDFA and Raman amplifiers do. They have lower gain (between 10 to 15 dB), low output power (less than 13 dBm), and a higher noise figure (typically about 6-9 dB). But unlike EDFAs and Raman amplifiers, SOAs are available in small 14-pin dual-inline packages and have much lower power requirements, making them a good low-cost choice for some applications. They also may be used as a pre-amp to increase the sensitivity of receivers.