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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.

Erbium Doped Fibre Amplifier 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.

image 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.”

image 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

  • Few meters of regular fiber doped with a tiny amount of erbium
  • Signal passes through this fiber along with light from pump laser
  • Pump laser excites erbium ions, which give extra energy to signal
  • Amplification possible at many wavelengths around 1550nm
  • Pumping with 980nm laser is more effective than 1480nm pumping
  • Commonly used in submarine systems, and increasingly on land


Further Reading

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

COMMENTS Add Comment
eekhm 12/5/2012 | 12:16:33 AM
re: Erbium Doped-Fiber Amplifiers (EDFAs) The reference article is really helpful !!
optigirl 12/4/2012 | 11:00:35 PM
re: Erbium Doped-Fiber Amplifiers (EDFAs) A) Your wife is right. We always are. :)

B) You could probably get away with maybe $15-20K for a LH and a metro for $6-7K. HFC I don't know really and I am sure that others out there will have some other numbers or estimates. These are always a function of volume, contracts, a slow quarter or how badly you did at the golf course (ha ha)

optodunce 12/4/2012 | 11:00:35 PM
re: Erbium Doped-Fiber Amplifiers (EDFAs) I am working on a paper for my MBA course and am soliciting some desperate help...I am looking for the average price of a Metro EDFA, HFC EDFA and a Long Haul EDFA. I am putting together a sample business plan and wish to make the numbers as realistic as possible...assume cost in quantity. I sincerely appreciate in advance your help in assisting me in expanding my mind...although at this point my wife believes nothing can be retained in a black hole...

Thanks Opto the Dunce
optodunce 12/4/2012 | 11:00:29 PM
re: Erbium Doped-Fiber Amplifiers (EDFAs) Thank You very much OptiGirl, I sincerely appreciate your reply.

Bluebeam 12/4/2012 | 10:47:59 PM
re: Erbium Doped-Fiber Amplifiers (EDFAs) Just thought it would be helpful to share a couple of links to articles about 980 vs 1480 pumps

http://lw.pennnet.com/Articles...
http://lw.pennnet.com/Articles...

bb
Petabit 12/4/2012 | 7:51:41 PM
re: Erbium Doped-Fiber Amplifiers (EDFAs) I'm not sure that I completely agree with the beer analogy, but I'll let that one slide.

Of more concern is the incorrect statements at the end of the article:

Both 980nm and 1480nm pumps are used in modern EDFAs because they have different characteristics. 980nm pumps provide lower noise amplification, but they are less efficient than 1480nm pumps. You tend to see 980nm pumps at the front of the EDFA, with 1480nm pumps in the latter stages of the amplifier.

980nm pumps suffer from several non-linearities (such as pump inhomogeneity) beacuse of the narrow erbium absorbtion window at 980nm. 1480nm pumps suffer from much fewer problems, and higher power 1480nm pumps are being developed for Raman amplification. The general trend is to see fewer 980nm pumps being used, and more 1480nm pumps.

P.
optodunce 12/4/2012 | 7:47:06 PM
re: Erbium Doped-Fiber Amplifiers (EDFAs) Peter, in a recent news analysis you described several amplifier technologies for the metro environment. In the article you indicate low cost amplifiers such as EDWA type or SOA based. What is considered low cost?

In future articles it would be helpful to the optical novice who is interested in learning more about this arena, to get a frame work of reference on what the technology and cost impacts are.

Thank you
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