What are fiber optic amplifiers and how do they work?
Why do we need fiber optic amplifiers?
Basically, what fiber optic amplifiers solve is the problem of traditional distance as in any long distance telecommunication system like a transatlantic link. As the optical signals travel through the fiber, the signals become weaker. The further you go, the weaker the signal will become until it becomes too weak to be reliably detected.
Fiber optic communication systems solve this problem by using fiber amplifiers on the go. A repeater or amplifier is inserted into the system at a point where the signal has weakened, to increase the strength of the signal so that it can be transmitted through another length of fiber cable. Many amplifiers or repeaters can be placed in sequence to keep the signal strong throughout the entire fiber link.
What are the differences between a repeater and a fiber optic amplifier?
Traditionally, electronic repeaters were used for the amplification of optical signals. A repeater is an opto-electro-opto device. It converts a weak optical signal to electronic signal, cleans the electronic signal, and then converts the electronic signal back to optical signal with a light wave transmitter. The light wave transmitter emits much stronger power than the incoming optical signal and therefore amplifies it.
However, this is an inconvenient and costly process and has therefore been superseded by newer fiber optic amplifier technology.
A fiber optic amplifier is a purely optical device. It does not convert the incoming optical signal to electronic signal at all. Basically, you can call it laser online. And a fiber optic amplifier can simultaneously amplify dozens of optical channels, since they do not convert each channel into electronic signals separately.
What are fiber optic amplifiers and how do they work?
The fiber optic amplifier is a section of fiber optic that is doped with a rare earth element such as erbium or praseodymium.
Erbium or praseodymium atoms can be pumped by high power light (pump laser) to the excited state. But they are not stable in the excited state. When the optical signals to be amplified pass through the fiber, they stimulate the excited erbium atoms. Erbium atoms will jump from the high-power excited state to the low-power steady state and release their energy as photons of light emitted at the same time. The emitted photons have the same phase and wavelength as the input optical signal, thus amplifying the optical signal.
This is a very convenient form of amplifier for a fiber optic communication system, as it is an in-line amplifier, thus eliminating the need for the optical-electrical and electrical-optical conversion process.
The wavelengths of the pump laser and the corresponding wavelengths of the optical signal are key parameters for the 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 to understand fiber amplifiers is their “gain”. Gain measures amplification per unit length of fiber. Gain is dependent on both materials and operating conditions, and varies with the wavelength of all materials.
For low input powers, the output power is proportional to the gains multiplied by the length of the fiber. Therefore, P (output) = P (input) x gain x length
For high input powers, the gain saturation effect comes into play. So increasing the input power produces less and less output power, which essentially means that the amplifier has run out of the power it needs to generate more output.