Fiber Optic Amplifiers
Fiber optic amplifiers re-amplify an attenuated signal without converting the signal into electrical form. Common fiber optical amplifiers include erbium doped fiber amplifiers (EDFA), Raman fiber amplifiers, and silicon optical amplifiers (SOA). Fiber amplifiers are developed to support dense wavelength division multiplexing (DWDM) and to expand to the other wavelength bands supported by fiber optics. EFDA fiber optic amplifiers function by adding erbium, rare earth ions, to the fiber core material as a dopant; typically in levels of a few hundred parts per million. The fiber is highly transparent at the erbium lasing wavelength of two to nine microns. When pumped by a laser diode, optical gain is created, and amplification occurs.
When light is transmitted through matter, part of the light is scattered in random directions. A small part of the scattered light has frequencies removed from the frequency of the incident beam by quantities equal to the vibration frequencies of the material scattering system. Raman fiber optic amplifiers function within this small scattering range. If the initial beam is sufficiently intense and monochromatic, a threshold can be reached beyond which light at the Raman frequencies is amplified, builds up strongly, and generally exhibits the characteristics of stimulated emission. This is called the stimulated or coherent Raman effect.
Silicon or semiconductor fiber optic amplifiers (SOA) function in a similar way to a basic laser. The structure is much the same, with two specially designed slabs of semiconductor material on top of each other, with another material in between them forming the ‘active layer.’ An electrical current is set running through the device in order to excite electrons which can then fall back to the non-excited ground state and give out photons. Incoming optical signal stimulates emission of light at its own wavelength. SOA can be classified into two groups, Fabry-Perot Amplifiers (FPA) and Traveling Wave Amplifiers (TWA). The difference is the reflectivity coefficient value of both mirror surfaces.