DWDM is a transmission technology in optical fiber communication. It uses one optical fiber to transmit multiple optical carrier signals of different wavelengths at the same time, and divides the wavelength range used by the optical fiber into several channels. The channel transmits optical signals. Therefore, DWDM has greatly improved the transmission capacity of the optical communication system. The appearance of Erbium-doped fiber amplifier (EDFA) makes DWDM optical signal transmission farther.

Erbium (Er) is a rare earth element. When making an optical fiber, a certain proportion of erbium element is added to form an erbium-doped optical fiber, which has the effect of amplification. Erbium ions have three working energy levels: E1, E2 and E3. Among them, E1 has the lowest energy level and is called the ground state; E2 is the metastable state; E3 has the highest energy level and becomes the excited state. In the case of not being excited by any light, it is at the lowest energy level E1. When the laser of the pumping light source is used to continuously excite the erbium-doped fiber, the particles in the ground state will jump to a higher energy level when they gain energy. If it transitions from E1 to E3, since the particle is unstable at the energy level of E3, it will quickly transition to the metastable energy level without radiation, and the life of the particle at this energy level is relatively long, due to the pump light source With continuous excitation, the number of particles on the E2 energy level will continue to increase, while the number of particles on the E1 energy level will decrease. When the photon energy of the input optical signal is exactly equal to the energy level difference between E2 and E1, the particles in the metastable state It will transition to the ground state in the form of stimulated radiation, and radiate the same photons as the photons in the input optical signal, thereby greatly increasing the number of photons, making the input signal light become a strong output in the erbium-doped fiber The optical signal realizes the direct amplification of light.

Of course, this also has certain requirements on the working wavelength of the pump light source. The above figure shows the absorption spectrum of erbium ions, from which it can be seen that there are absorption bands at wavelengths of 650nm, 800nm, 980nm, and 1480nm, and can be used in these frequency bands. It is regarded as the working wavelength of the EDFA pump light source. However, after comparing factors such as efficiency, 980nm and 1480nm semiconductor lasers are more suitable as pumping light sources for EDFA. Compared with 1480nm, 980nm has high gain and low noise, and is currently the preferred pumping wavelength for fiber amplifiers. There are many types of pumping methods used in EDFA. It is mainly named whether the energy output from the pump light source is injected into the erbium-doped fiber in the same direction as the input optical signal energy. It can be divided into forward pumping and backward pumping. way, and two-way pumping way.

The bidirectional pumping method has the advantages of forward pumping and reverse pumping, so this method can not only make the pump light evenly distributed in the optical fiber, but also from the perspective of output power, the output power of single pumping is 14dBm, and the output power of bidirectional pumping is 14dBm. The pump can reach l 7dBm. Moreover, bidirectional pumping has the best amplification efficiency, and co-directional pumping has the lowest noise.

At the receiving end of wavelength division multiplexing, in order to compensate for the insertion loss of the demultiplexer and the decrease in receiver sensitivity due to the increase in the rate, a low-noise optical preamplifier must be configured before the demultiplexer. At the receiving end, EDFA is used as a preamplifier for PIN and APD. When the channel rate is as high as 2.5Gbit/s, compared with no optical preamplifier, the sensitivity of the receiver can be increased by about 10 times.

EDFA is used as a booster amplifier, which has the advantages of large output power, stable output, low noise, wide gain frequency band, and easy monitoring. It is placed at the optical transmitter to directly amplify the signal of the optical transmitter. Power amplifiers can be used alone or in combination. After being amplified by EDFA, the output power of the sending end can be increased by about an order of magnitude, which greatly improves the power entering the fiber.

In optical communication systems, dispersion and loss limit the distance and capacity of communication. In order to reduce the influence of dispersion, dispersion compensation can be performed. After adding the dispersion compensating fiber, the insertion attenuation is significantly increased, so the insertion attenuation must be compensated with an optical amplifier. Using EDFA as a line amplifier can significantly increase the regeneration distance, and can also replace expensive optical repeaters.

DWDM is a special multiplexing method in the optical fiber communication system. This method can make full use of the wide low-loss area of the optical fiber, and can easily double the increase without changing the existing installed optical fiber communication lines. The capacity of fiber optic communication systems. Now EDFA and DWDM have become the mainstream of high-speed optical fiber communication network development, representing a new generation of optical fiber communication technology.