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What Are Optical Fiber Amplifiers And How Do They Work?

What Are Optical Fiber Amplifiers And How Do They Work?

Why do we need optical fiber amplifiers?

Basically what the optical fiber amplifiers solve is the traditional distance problem as in any long distance telecommunication system such as a trans-Atlantic link. As optical signals travel through the fiber, the signals become weaker in power. The farther you go, the weaker the signal become until it becomes too weak to be detected reliably.

Fiber optic communication systems solve this problem by using fiber amplifiers along the way. A repeater or amplifier is inserted into the system at a point where the signal has become weak, to boost the strength of the signal so it can be transmitted through another length of fiber cable. Many amplifiers or repeaters can be placed in sequence to keep the signal strong along the whole fiber link.

What are the differences between a repeater and an optical fiber amplifier?

Traditionally, electronic repeaters were used for optical signal amplification. A repeater is an opto-electro-opto device. It converts a weak optical signal into electronic signal, cleans up the electronic signal, and then converts the electronic signal back to optical signal with a lightwave transmitter. The lightwave transmitter emits much stronger power than the incoming optical signal and thus amplifies it.

However, this is an inconvenient and expensive process and which is why it has been replaced by the new optical fiber amplifiers technology.

An optical fiber amplifier is a purely optical device. It doesn’t convert the incoming optical signal to electronic signal at all. Basically, you can call it a in-line laser. And an optical fiber amplifier can simultaneously amplify dozens of optical channels since they do not convert each channel into electronic signals separately.

What are optical fiber amplifiers and how do they work?

Optical fiber amplifier is a section of optical fiber that is doped with a rare-earth element such as erbium or praseodymium.

The atoms of erbium or praseodymium can be pumped by high power light (pump laser) into excited state. But they are not stable in the excited state. When the optical signals that are to be amplified pass though the fiber, they stimulate the excited erbium atoms. The erbium atoms will jump from the high power level excited state into low power level stable state, and release their energy in the form of emitted light photons at the same time. The emitted photons have the same phase and wavelength as the input optical signal, thus amplify the optical signal.

This is a very convenient form of amplifier for an optical fiber communication system since it is an in-line amplifier, thus removes the need to do the optical-electrical and electrical-optical conversion process.

The pump laser wavelengths and the corresponding optical signal wavelengths are key parameters for operation of fiber amplifiers. These wavelengths depend on the type of rare-earth element doped in the fiber and also on the composition of the glass in the fiber.

Another important term in understanding fiber amplifiers is its “gain”. Gain measures the amplification per unit length of fiber. Gain depends on both the materials and the operating conditions, and it varies with wavelength for all materials.

For low input powers, the output power is proportional to the gains times the fiber length. Thus, P(output) = P(input) x Gain x Length

For high input powers, the gain saturation effect comes into play. So increment of input power produces less and less output power, which essentially means the amplifier has run out of the power it needs to generate more output.