What is a Raman Fiber Amplifier

The Raman Fiber Amplifier (RFA) is operated by the third-order nonlinear effect of the strong laser in the fiber-stimulated Raman Scattering (SRS).

If a weak signal light is transmitted in a fiber at the same time as a strong pump light, and the wavelength of the weak signal light is within the Raman gain bandwidth of the pump light, the energy of the strong pump light is coupled to the fiber silicon material through the SRS. The oscillating mode is then emitted at a longer wavelength, which is the wavelength of the signal light, thereby amplifying the weak signal light to obtain a Raman gain.

Classification of Raman fiber amplifiers

RFA is divided into two main categories: discrete (or centralized) RFA and distributed RFA. They have their own characteristics and are suitable for different application areas.

The amplification medium used by discrete RFAs is typically a dispersion compensating fiber (DCF) or a highly nonlinear fiber. The gain fiber used is relatively short, typically a few kilometers. Due to the low efficiency of the distribution amplification, high power is required, and the pump power is required to be relatively high, and the cost is correspondingly increased.

Distributed RFA utilizes ordinary single-mode fiber as the gain medium. The gain fiber used is very long, usually several tens of kilometers, and the pump power can be reduced to several hundred milliwatts, mainly in conjunction with the EDFA to improve system performance.

Advantages of Raman fiber amplifiers

RFA is an ultra-wideband fiber amplifier: the low loss range of ordinary fiber is 1270 nm to 1670 nm, EDFA can only work in the range of 1525 nm to 1625 nm, and RFA can be amplified at full wavelength. Multiple pumps can be utilized, and a wide choice of pump wavelengths and power can be used to achieve a wider flat gain spectrum.

The RFA gain medium is the transmission fiber itself. Fiber Raman amplifiers do not require special doped fibers as the amplifying medium like EDFA. The amplification medium is the transmission fiber itself. There is no doubt that this has greatly reduced costs.

Low noise figure: Used in conjunction with EDFA, the system noise figure can be reduced, which can increase the non-relay distance. .

Distributed amplification can be realized: long-distance transmission and remote pump spectrum are realized, which is especially suitable for occasions where it is not convenient to set up a repeater such as a seabed or a desert optical cable communication. In addition, because the amplifier is distributed along the fiber rather than concentrated, the signal power around the fiber is relatively small, which can reduce the nonlinear effect, especially the four-wave mixing effect.

Disadvantages of Raman fiber amplifiers

The gain is not high: the gain of the general RFA is less than 15 dB

The gain has a positive correlation: the gain of RFA is closely related to the polarization state of light.

Pumping efficiency is low: generally only 10% to 20%.

Raman fiber amplifier application

Increase system capacity: When the transmission rate is constant, the system capacity can be increased by increasing the number of channel multiplexing. Opening up a new transmission window is a way to increase the number of channel multiplexing, and RFA's full-band amplification just meets the requirements.

Extend spectrum utilization and increase transmission system speed. RFA's full-band amplification capability allows it to operate throughout the low-loss region of the fiber, greatly expanding spectrum utilization and increasing transmission system speed.

Increase the example of no relay transmission. The non-relay transmission distance is mainly determined by the signal-to-noise ratio of the optical transmission system. The equivalent noise figure of the distributed RFA is extremely low (-2 dB~0 dB), which is 4.5 dB lower than the noise figure of the EDFA.

Compensate for DCF losses. The loss factor of DCF is much larger than that of single-mode fiber and non-zero dispersion-shifted fiber, and the Raman gain coefficient is also large. The combination of DCF and RFA can compensate for dispersion and loss, and also improve the signal-to-noise ratio.

Communication system upgrade. Under the premise of the receiver's performance unchanged, if the transmission rate of the system is increased, to ensure that the bit error rate of the receiver is unchanged, the signal-to-noise ratio of the receiver must be increased. Using RFA combined with a preamplifier to improve the signal-to-noise ratio is one of the ways to achieve system upgrades.

RFA has been widely used in recent years due to its advantages of full-band amplification, low noise, suppression of nonlinear effects, and dispersion compensation. RFA is mainly used as a distributed amplifier to assist the EDFA in signal amplification. It can also be used alone to amplify the band that EDFA cannot amplify, while overcoming the shortcomings of EDFA cascade noise and limited amplification bandwidth. At present, the status of RFA transmission in long-distance backbone networks and submarine cables has been recognized; in metropolitan area networks, RFA also has its value. The use of communication band extension and dense wavelength division multiplexing technology has brought a broad application prospect to RFA. This series of advantages of RFA makes it possible to become the mainstream of next-generation optical amplifiers.