Raman Amplification

Before reading this you may find the following tutorials useful:
Optical Amplification, Erbium Doped-Fiber Amplifiers (EDFAs), Nonlinear Effects

If you have ever passionately supported a sports team, you will know how demoralizing certain situations can be. Dramatic losses at the very last minute can disappoint and depress even Boston Red Sox fans. To avoid these emotional roller coasters, some supporters live in a constant state of pessimism, always expecting the worse so that if it does happen they are not emotionally affected.

Close your eyes and imagine that sadf khjasdfl jkashdfj klashdfasdf. Oh, sorry, maybe I should keep mine open. (Actually, so should you or your reading ability may be impaired.) Just imagine you are a happy, excited, buoyant supporter with lots of energy, on your way to a football match. Your team has the chance to win the league if they can just win this, the final game of the season. You are prepared for a night of raw emotion, unbelievable tension, and possibly, just possibly, the biggest celebration of your life. Packed in your bag is a thermos flask of soup to keep you warm in the cold winter night. You also have a few spare pair of underwear just in case the situation gets just a little too tense for you to handle. You are a passionate, lively, and energetic supporter on your way to the stadium, just like a Raman pump laser traveling down a length of optical fiber.

However, all around you are some of the more pessimistic supporters who are muttering doom and gloom and fully expecting your cherished side to lose and blow the biggest chance of their existence to win the league. You might say that these fans are less energetic than you. If they were light, the fact that they have less energy would mean that they have a longer wavelength. So let us say that they are a signal traveling down this optical fiber and they have a wavelength of around 1550nm. You have a higher energy than they do, so you have a shorter wavelength, let's say 1450nm. So now you are all walking down towards the stadium.

You get there to discover that your team's best player has a rare infection on his left buttock that rules him out of the game, the team's coach has been arrested the previous night after an incident in the local red light district, and your quarterback has broken his wrist in a similar but unrelated incident. Well, that does it. All of your vibrancy, enthusiasm, and energy have been lost at that stadium, and you just leave immediately with the same pessimistic fans that you arrived with. Now you have the same energy level as they do. You gave some of your energy to the stadium, which is like an atom within an optical fiber. You were then scattered away from the stadium along with the 1550nm fans, and as you lost energy you now join them as a new 1550nm fan. You have just experienced “stimulated Raman scattering.”

The principle of Raman scattering is that a lower wavelength pump-laser light traveling down an optical fiber along with the signal, scatters off atoms in the fiber, loses some energy to the atoms, and then continues its journey with the same wavelength as the signal. Therefore the signal has additional photons representing it and, hence, is amplified. This new photon can now be joined by many more from the pump, which continue to be scattered as they travel down the fiber in a cascading process.

Raman amplification requires no special doping in the optical fiber. It is usually accomplished as “distributed' amplification” — that is, it happens throughout the length of the actual transmission fiber, rather than all in one place in a small box (as with an EDFA for example). So into the same fiber that is carrying the signal, you can add a high-power pump wavelength (say of a few watts power), which will amplify the signal along many kilometers of fiber until the pump signal eventually fades away. If you insert the pump at the beginning of the fiber, that is known as forward pumping or co-pumping. Better performance can usually be achieved, however, by pumping from the far end of the fiber — known as backward pumping or counter-pumping — or by a combination of the two (co-counter pumping).If you have several different signal wavelengths in a WDM system, then usually several different pump wavelengths will need to be used together to achieve the required amplification at every wavelength. As with EDFAs, gain flatness is also an issue that needs careful design to achieve. Raman amplification is just now surfacing as a useful technology for optical networks. Several recent designs of submarine systems incorporate some form of Raman amplification, in order to extend the transmission distances possible before further amplification is required (from an EDFA, for example).

Key Points

Further Reading

Semiconductor Optical Amplifiers (SOAs) , Submarine Systems