To get all the photons to the receiving end, you might think you'd raise the input power of the laser, thereby increasing the number of photons that reach the other end. This would be true if fiber optics worked in a linear fashion, but they are nonlinear. To keep input power and output power as close to linear as possible, most single-mode lasers are kept at about 0.5 watts. Increasing the power can decrease the output power because of electromagnetic forces in the core.
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Regeneration is therefore the common method of extending the reach of the photons in fiber. Depending on the wavelengths used, regenerating an optical signal can take two forms: OEO (optical/electrical/optical) or FA (fiber amplifiers). OEO systems, also called optical repeaters, take the optical signal, demultiplex it and convert it to electrical pulses. The electrical signal is amplified, groomed to remove noise and converted back to optical. It is then multiplexed back on the line and sent on its way. OEO can be used across the spectrum and is commonly used for CWDM and some WDM transmissions.
Fiber amplifiers offer a more elegant solution without converting the photons to an electrical signal. The most common amplifier is the EDFA (erbium-doped fiber amplifier). Although fiber amplifiers commonly use erbium, other elements, such as praseodymin and ytterbium, can provide the same results. A better term would be REDFA (rare-earth-doped fiber amplifier), which operates by doping a section of fiber with a rare earth element, such as erbium. Doping is the process of adding impurities during manufacture; fiber-optic cable already has almost 10 percent germanium oxide as a dopant to increase the reflective index of the silica glass.
The doped section of fiber combines the signal from the transmitter with that coming from a pump laser. For erbium, the pump laser operates at 980 nm. The pump laser excites the erbium atoms, and when they are struck by photons from the original signal, some energy is transferred to the transmitted signal, amplifying it. A typical REDFA has a coil of 10 meters of doped fiber for amplification. The pump laser can be located locally to the doped fiber or remotely, as long as the strength of the light has not degraded too badly.
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