Although the fiber loss limits the transmission distance, the need for longer fiber optical transmission link seems never ending. In the pursuit of progress, several kinds of optical amplifiers are published to enhance the signals. Hence, longer fiber optical transmission link with big capacity and fast transmission rate can be achieved. As the EDFA and Raman amplifiers are the two main options for optical signal amplification. which one should be used when designing long fiber optical network? What are the differences of the two optical amplifiers? Which one would perform better to achieve the long fiber optical link? And which one is more cost effective? Let’s talk about this topics.
What’s EDFA Amplifier?
EDFA (Erbium-doped Fiber Amplifier), firstly invented in 1987 for commercial use, is the most deployed optical amplifier in the DWDM system that uses the Erbium-doped fiber as optical amplification medium to directly enhance the signals. It enables instantaneous amplification for signals with multiple wavelengths, basically within two bands. One is the Conventional, or C-band, approximately from 1525 nm to 1565 nm, and the other is the Long, or L-band, approximately from 1570 nm to 1610 nm. Meanwhile, it has two commonly used pumping bands, 980 nm and 1480 nm. The 980nm band has a higher absorption cross-section usually used in low-noise application, while 1480nm band has a lower but broader absorption cross-section that is generally used for higher power amplifiers.
The following figure detailedly illustrates how the EDFA amplifier enhance the signals. When the EDFA amplifier works, it offers a pump laser with 980 nm or 1480 nm. Once the pump laser and the input signals pass through the coupler, they will be multiplexed over the Erbium-doped fiber. Through the interaction with the doping ions, the signal amplification can be finally achieved. This all-optical amplifier not only greatly lowers the cost but highly improves the efficiency for optical signal amplification. In short, the EDFA amplifier is a milestone in the history of fiber optics that can directly amplify signals with multiple wavelengths over one fiber, instead of optical-electrical-optical signal amplification.
What’s Raman Amplifier?
As the limitations of EDFA amplifier working band and bandwidth became more and more obvious, Raman amplifier was put forward as an advanced optical amplifier that enhances the signals by stimulated Raman scattering. To meet the future-proof network needs, it can provide gain at any wavelength. At present, two kinds of Raman amplifiers are available on the market. One is lumped Raman amplifier that always uses the DCF (dispersion compensation fiber) or high nonlinear fiber as gain medium. Its gain fiber is relatively short, generally within 10 km. The other one is distributed Raman amplifier. Its gain medium is common fiber, which is much longer, generally dozens of kilometers.
When the Raman amplifier is working, the pump laser may be coupled into the transmission fiber in the same direction as the signal (co-directional pumping), in the opposite direction (contra-directional pumping) or in both directions. Then the signals and pump laser will be nonlinearly interacted within the optical fiber for signal amplification. In general, the contra-directional pumping is more common as the transfer of noise from the pump to the signal is reduced, as shown in the following figure.
EDFA vs Raman Optical Amplifier: Which One Wins?
After knowing the basic information of EDFA and Raman optical amplifiers, you must consider that the Raman amplifier performs better for two main reasons. Firstly, it has a wide band, while the band of EDFA is only from 1525 nm to 1565 nm and 1570 nm to 1610 nm. Secondly, it enables distributed amplification within the transmission fiber. As the transmission fiber is used as gain medium in the Raman amplifier, it can increase the length of spans between the amplifiers and regeneration sites. Except for the two advantages mentioned above, Raman amplifier can be also used to extend EDFA.
However, if the Raman amplifier is a better option, why there are still so many users choosing the EDFA amplifiers? Compared with Raman amplifier, EDFA amplifier also features many advantages, such as, low cost, high pump power utilization, high energy conversion efficiency, good gain stability and high gain with little cross-talk. Here offers a table that shows the differences between EDFA and Raman optical amplifiers for your reference.
|Property||EDFA Amplifier||Raman Amplifier|
|Wavelength (nm)||1525-1565, 1570-1610||All Wavelengths|
|Gain (dB)||> 40||> 25|
|Noise Figure (dB)||5||5|
|Pump Power (dBm)||25||> 30|
|Cost Factor||Relatively Low||Relatively High|
Considering that both EDFA and Raman optical amplifiers have their own advantages, which one should be used for enhancing signals, EDFA amplifier, Raman amplifier or both? It strictly depends on the requirement of your fiber optical link. You should just take the characteristics of your fiber optical link like length, fiber type, attenuation, and channel count into account for network design. When the EDFA amplifier meets the need, you don’t need the Raman amplifier as the Raman amplifier will cost you more.