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Raman amplifier is another important optical amplifier other than EDFA. An optical signal can be amplified in the optical domain without being converted to electronic domain and back.

As shown in the lower left figure, an input signal ωs can be amplified while travelling in the same direction or opposite direction with a pump laser ωp. The wavelength of ωs is typically a few tens of nanometers shorter than ωp.

The working principle behind Raman Amplifier is called Stimulated Raman Scattering. The process is shown in the left picture.

Stimulated Raman Scattering is an nonlinear effect in optical fibers and it requires optical power to be higher than a threshold to happen, usually at least 500 milliwatts. The photon of pump beam ωp is scattered by molecules in the fiber medium, and becomes a low energy photon ωs, the balance of the energy becomes vibration and dissipated in the fiber medium.

For intense optical power, this nonlinear effect can transfer most of the pump power ωp into signal power ωs.

The frequency difference between ωp and ωs has to match a relationship in order to fully make use of this nonlinear effect. This is shown in the Raman Gain Coefficient graph.

The horizontal axis is the frequency difference between the pump and signal, marked in THz. The vertical axis is the normalized gain coefficient. We can see that the peak gain occurs when the frequency difference is between 10 THz and 15 THz.

The top right picture shows a pump laser at 1535nm, which has much higher power than the data signals. And the pump laser power is transferred to the signal power as shown in the bottom right picture.

There are two types of Raman amplifiers -- Distributed Raman Amplifier and Discrete Raman Amplifier.

In a distributed Raman amplifier, the optical fiber link itself is used as the amplification medium. A high power pump laser is injected at the far end and travels backward in the fiber to amplify the signal.

In a discrete Raman amplifier, a coil of dedicated fiber is used together with pump laser.

In the real world, Raman amplifiers are used with EDFA amplifiers together to fully utilize their respective advantages.

As shown in the top picture, Distributed Raman amplifiers amplify the signal in the backward direction, and EDFA amplifiers amplify the signal in the forward direction.

The corresponding signal power level is shown at the bottom picture, the original signal first gets weaker because of fiber loss, but later it is gradually boosted up by the backward pumped distributed Raman amplifier, and right after the Raman amplifier, the signal is immediately boosted again by the EDFA amplifier. The overall signal is the dotted line as DRA Amplified Transmission Signal. This is a very typically real world configuration.

So there you have it. Please leave your comment below if you'd like to see other topics.

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