In a longer fiber-optic line, DWDM Erbium-doped Fiber Amplifier are installed at specified distances for the purpose of ensuring the recovery of signals weakened by the fiber. Erbium-doped fiber amplifier EDFA can amplify any form of optical signal, and, at the same time, to recover a large number of independent WDM channel signals. However, Erbium-doped fiber amplifiers, which provide power amplification of the transmitted optical signal, accumulate distortion, which occurs in various parts of the DWDM network. This distortion results in a lower OSNR (Optical Signal to Noise Ratio) at the receiving end of the system.

Requirements of WDM system for EDFA
In order to ensure the transmission quality of the WDM system, the EDFA used in the WDM system should have sufficient bandwidth, flat gain, low noise figure and high output power.

1. EDFA gain bandwidth
At present, the available gain spectrum range of EDFA is l530-l565nm, and the gain bandwidth is about 35nm, which can satisfy the WDM system of 4-32 channels. If you want to further increase the bandwidth to utilize wavelength resources, you must develop a new type of optical amplifier.

2. WDM system requirements for EDFA gain flatness
Gain flatness (GF) of EDFA refers to the difference between the gain of the maximum gain wavelength point and the gain of the minimum gain wavelength point within the bandwidth of the entire available gain. In the WDM system, the smaller the GF of the EDFA is required, the better. Generally, EDFA has a certain gain fluctuation in its working band, that is, the gain obtained by different wavelengths is different. Although the gain difference is not large, when multiple EDFAs are cascaded, the gain difference will accumulate linearly. In serious cases, after the signal reaches the receiving end, the received optical power of some high-gain channels is too high to overload the receiver, while The received optical power of some low-gain channels is too small to reach the receiver sensitivity. Therefore, the gain of the amplifier must be flat in order to keep the gain deviation on each channel within the allowable range. There are two ways to flatten the gain of fiber amplifiers: one is gain equalization technology; the other is fiber optic technology.

Advantages and disadvantages of EDFA
1. The working wavelength is consistent with the minimum loss window of optical fiber, which can be widely used in optical fiber communication.

2. High coupling efficiency. Because it is a fiber-optic amplifier, it is easy to connect with fiber coupling, and it can also be welded together with the transmission fiber by fusion splicing technology, and the loss can be reduced to 0.1dB.

3. High energy conversion efficiency. The working matter of the laser is concentrated in the fiber core, and concentrated in the near-axis part of the fiber core, while the signal light and pump light are also strongest in the near-axis part, which makes the light and matter fully interact.

4. High gain and low noise. The output power is large, the gain can reach 40dB, the output power can reach 14dBm when unidirectional pumping, 17dBm or even 20dBm when bidirectional pumping, when fully pumped, the noise figure can be as low as 3~4dB, and the crosstalk is also low very small.

5. The gain characteristic is not sensitive. Firstly, the gain of EDFA is not sensitive to temperature, and the gain characteristic is stable within 100°C. In addition, the gain has nothing to do with polarization.

6. Transparent transmission of signals can be realized, that is, analog signals and digital signals, high-speed signals and low-speed signals can be transmitted simultaneously in the wavelength division multiplexing system. When the system is expanded, only the terminal machine can be changed without changing the line.

EDFA also has inherent disadvantages:

1. The wavelength is fixed, and it can only amplify light waves around 1.55 μm. When using optical fibers with different substrates, the energy level of erbium ions can only change slightly. The adjustable wavelength is limited, and only other elements can be used;

2. The gain bandwidth is not flat, and special means are required to compensate the gain spectrum in the WDM system.