Erbium Doped-Fiber Amplifiers (EDFAs)

Before reading this you may find the following tutorials useful:
Optical Amplification, Optical Fiber, Wavelength Division Multiplexing (WDM)

An erbium doped fiber amplifier (EDFA) consists of a few meters of optical fiber doped with a few parts per million of the rare earth element erbium. The optical signal is injected into this fiber, along with the light from a special “pump” laser that is designed to excite the erbium ions. Let's continue the beverage theme from the Optical Amplification tutorial in explaining just how these erbium ions are excited, and then how they give a boost to the optical signal.

Let us think of regular optical fiber as a sensible and studious college student who drinks only mineral water. We can then consider an erbium-doped fiber a beer-swilling frat boy who drinks regularly. He has a slight difference to his composition (erbium doping) that makes him enjoy drinking to the point where he will have repeated bouts of physical sickness and no doubt accost innocent bystanders. So when you pump this guy with, say, a pint of beer, he is excited into a higher state of drunkenness, just as the erbium ions are excited into higher energy states when pumped by a laser. If you continue the pumping, feeding the student with beer and the fiber with laser light, both become excited to the point where they can be excited no more. An incoming optical signal can now be thought of as a double whisky. The double whisky goes into the student, but instantly comes back out, magnified several times with a flurry of liquid from the earlier pumping. And so the optical signal exits the EDFA having been increased in intensity several times over.

Erbium has several energy levels, but its ions are usually in the ground state (unexcited, alcohol free). The ions can be excited with a 1480-nanometer pump laser into the first excited state. If left there for long enough, they will fall back down to the ground state, just as the effects of alcohol will wear off after a while. When falling back to the ground state, the ions have some extra energy to get rid of, which they each give out as a photon (a single “particle” of light). Think of this as the student relieving himself in a dumpster if you must. This is called spontaneous emission because the ions fall back to the ground state and give out photons without any aid whatsoever. Such spontaneous emission can build up in the amplifier and is known as “amplified spontaneous emission” or ASE. ASE is an undesirable effect and adds “noise” to the amplifier system.

If an optical signal is incoming at around 1550nm however, it can cause some of those excited ions to fall down to the ground state and give out a photon each. This is stimulated emission because the signal is directly causing the photons to be emitted. The emitted photons are at the exact same wavelength as the signal and so are now a part of the signal. The signal now has more photons representing it than before, so it has been amplified. This process can continue down the few meters of this fiber, until lots of photons have joined the signal photons and the signal has been greatly amplified. This can happen at several wavelengths around 1550nm, and amplification can be achieved via fancy EDFA designs for signals between around 1530nm and 1580nm, which is known as “C-band” (Conventional-band) amplification. EDFAs can also be designed to give amplification between around 1580nm and 1610nm, which is known as the L-band (Long-band). The amount of amplification at different wavelengths can vary, and there is much effort put into EDFA designs to achieve similar levels of amplification at all wavelengths, known as “gain flattening.”

If you think of the 1480nm pump laser as a pint of regular beer, then you can think of a 980nm laser pump as a pint of super-strength lager. The 980nm pump excites the erbium ions into a much higher state than the 1480nm pump. However, the ions only stay in that higher state for a very short period of time (maybe nanoseconds) before moving down to the next state. Once there, they stick around for several milliseconds, which is much longer than ions excited by the 1480nm pump. The longer they remain in the excited state, the more likely it is that the signal will come along and cause stimulated emission. This also reduces the unwanted spontaneous emission that adds to the noise in the system. Therefore 980nm pumps give greater amplification efficiency and are the preferred pump method for EDFAs. Just as super-strength lager is generally the most efficient solution for students.

EDFAs are commonly used in submarine systems where signals often have to travel thousands of miles under the world's oceans. They can be made in compact, water-tight packages that will be placed every 50 miles or so along the length of the system. For such applications reliability is essential, and so submarine EDFAs tend to be of very simple design. Land-based EDFAs are becoming more popular now, as optical networks spread over wider distances on dry land. Such systems could incorporate more elaborate gain flattening schemes and other advanced features, as might future submarine systems, should the capacity demands require this improved performance.

Key Points

Further Reading

Raman Amplification, Semiconductor Optical Amplifiers (SOAs) , Submarine Systems