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Optical Networks, Optical Fiber, Wavelength Division Multiplexing (WDM), Laser BasicsImagine the scene. You are a “bit” of information traveling through an optical fiber. When you were born at the laser, all those milliseconds ago, you were strong, on top of your game, and ready to take on the world. But then, a few tens of kilometers later, the optical fiber has sapped most of your strength, and you are only a fraction of the optical pulse that you used to be. You need a quick pick-me-up so you enter a mysterious place called the “Amplifier.” This small box that you travel through gives you the vital boost that you so desperately needed, and you exit the amplifier as bold as you were at birth. So what happened in that little box?

You may like to think of an amplifier as one of those drinks tables you see during marathons. A traditional amplifier only has stocks of Electrical Holy Water, and so, as an agnostic pulse of light, you first have to be converted to the religion of electricity before you can take aboard the much-needed boost to your energy. But there is no place for religion within optical fiber, and so you have to be put back into the irreligious optical state again before you leave the amplifier, fresh and boosted after the water has cleansed your soul. As you may imagine, this process is far from ideal. It is lengthy and expensive, involving many stages of conversion, missions to pagan outposts, and many Sunday mornings spent repenting.

To avoid this unnecessary expense and complication, you may wish instead to visit an optical amplifier drinks table. This one is handing out bottles of Optical Energy Juice that requires no religious conversion before consumption. You simply run up to the table as a weakened pulse of light, take a sip of the magical juice as you travel through, and then continue through the fiber refreshed. You must be a little careful however, as your own signal is not the only thing to be boosted. Your breath, a mephitic stench after such a long distance of travel, is made even worse with that isotonic drink aftertaste in your mouth. This amplification of the “noise” within you, although not too serious at the moment, could affect your performance further down the fiber.

The amount of amplification that takes place is known as “gain” and is usually given in units of dB (dB means decibel and is calculated as one tenth of the logarithm of the output power divided by the input power). Just as with runners, there are several positions at which an optical signal may need an energy boost. At the start of the race, the runner may have an energy drink, along with some early morning exercises to get the blood pumping. Optical signals can also get a boost just after they have left the laser, which is known as “power amplification.” An amplifier in the middle of the link (like a drinks table) is known as a “line amplifier.” A final boost may be required as the runner crosses the finishing line and as the optical signals reach the detectors: Such amplification at the end of the system is called “pre-amplification.”

Key Points

Further Reading

Erbium Doped-Fiber Amplifiers (EDFAs), Raman Amplification, Semiconductor Optical Amplifiers (SOAs)