Basic working principle:

The erbium-doped fiber amplifier (EDFA) is a medium that uses helium ions to achieve energy conversion. The energy amplification window has an operating wavelength window of 1 550 nm and a width of 50 am, which is consistent with the low loss window of the fiber. The energy injection window is 980 nm and 1 480 nm. Generally, an erbium-doped ion fiber is prepared as an EDFA amplification core, that is, an active medium. The amplification system is a laser three-level system, the injected light energy of 980 nm is absorbed by the helium ions to the high energy level 4", and the transition level 4n of the laser is transitioned by the relaxation oscillation. Due to the long life of the energy level, a large amount of accumulation The activated particles, which reserve a large amount of energy, and then pass the stimulated radiation with the signal light, obtain multiplied signal of the same frequency and the same phase, and return the particles to the ground state. The induced noise in the amplification process is spontaneous radiation ( Amplified Spontaneous Emission (ASE), which is related to the wavelength of the pump. In general, the 980 nm laser pump is low in efficiency and low in noise, while the 1 480 nm laser is highly efficient and noisy. In the design process, the general pre-fiber amplifier EDFA uses 980 nm pumping; the booster Booster EDFA at the transmitting end uses a hybrid pumping method of 980 nm and 1 480 nm, and specially designed media according to DWDM requirements for optical equalization filters. Diaphragm flat filter.

The basic structure of an erbium doped fiber amplifier (FDFA):

A typical EDFA consists of an erbium-doped fiber, a pump source, a wavelength division multiplexer, an optical isolator, and an optical filter. The erbium-doped fiber provides amplification, the pump source provides sufficient pump power, and the wavelength division multiplexer combines the signal light and the pump light into the erbium-doped fiber. The optical isolator ensures one-way transmission of light to prevent Light reflections form optical oscillations and feedback light causes disturbances in the operational state of the signal laser. The role of the optical filter is to filter out the ASE noise in the optical amplifier and improve the signal-to-noise ratio of the EDFA. Usually EDFA has three pump types: co-directional pump, reverse pump and two-way pump. In order to ensure that the EDFA‘s amplification is constant (ie, the linear amplifier of the preamplifier and the line) or the output power is constant (ie, the saturating power amplifier at the transmitting end), it is necessary to design an auxiliary circuit to monitor the input and output power of the EDFA, as well as the pumping source. Work status is monitored and controlled. According to the monitoring results, the working parameters of the pump light source are appropriately adjusted to make the EDFA work at the optimum state. In addition, the auxiliary circuit section also includes circuits for protection functions such as automatic temperature control and automatic power control.

Basic performance of erbium doped fiber amplifier (EDFA):

The basic performance of EDFA is reflected in gain, output power and noise, as well as bandwidth and equalization.

1. Gain characteristics The gain characteristics represent the amplification capability of the ratio of the output power of the optical amplifier to the input power. It is related to various factors, generally expressed in dB, and the commonly used amplification factor is 15 to 40 dB. In general, the gain is directly related to the pump power and also to the length of the erbium-doped fiber. The best value can be found by experiment.

2. Output Power Characteristics For an ideal linear optical amplifier, the optical signal can be amplified and output at the same gain regardless of the input optical power. In order to ensure this condition, generally only when a small optical signal is input, the output of the optical signal amplified by sufficient gain is insufficient to reduce the number of particles of the energy level of the pump power injected into the laser. However, when the input optical power is sufficiently large, the injected power is insufficient to compensate for the output power after amplification, so that the number of inverted particles is saturated and reduced, and thus the output optical power is decreased, which affects the decrease of the amplification factor, that is, the gain saturation. , so that the amplification enters the nonlinear amplification saturation region. The maximum output power of the EDFA is usually expressed as 3 dB saturated output power, which corresponds to the output power when the saturation gain drops by 3 dB, reflecting the maximum power output capability of the EDFA. The saturation output characteristics of the EDFA are related to the pump power, the length of the erbium doped fiber, and the structure. The higher the pump optical power, the greater the 3 dB saturated output power; the longer the erbium-doped fiber length, the greater the 3 dB saturated output power.

3. Noise characteristics The optical noise introduced by the EDFA during the amplification process is mainly the spontaneous radiated optical power in the activated erbium-doped fiber, and then amplified by the active region of the erbium-doped fiber, which is an amplified spontaneous emission optical noise. . There are four main sources of noise: shot noise of signal light, shot noise of amplified spontaneous emission ASE, beat noise between white light emission ASE spectrum and signal light, and beat noise between spontaneous emission ASE spectra. . Among them, the latter two have the greatest influence, and the beat noise between the spontaneous emission ASE spectrum and the signal light is the main factor determining the performance of the EDFA. The noise characteristic of the EDFA can be expressed by the noise figure NF, which is the ratio of the input signal to noise ratio of the EDFA to the output signal to noise ratio, expressed in dB. It is closely related to the spontaneous emission spectral density and amplifier gain of the in-phase transmission, and is related to input signal power, pump power, and pumping mode. Under the input of a small optical signal, the noise figure NF of the optical amplifier increases the stimulated radiation as the power of the input signal increases, and the ratio of the spontaneous radiation is weakened, thereby reducing the noise figure NF. Under the input of large optical signal, the noise figure NF of the optical amplifier decreases with the increase of the input signal power, and the optical power of the spontaneous radiation increases, so that the noise figure NF increases. The noise figure decreases as the pump power increases. The noise power of the EDFA consists of two parts, one is the white-emitting radiation generated by each small-length fiber, and most of it is the amplification of the spontaneous radiation generated by the fiber to the front part of the fiber, that is, the amplified spontaneous emission. The larger the pump power, the smaller the proportion of the former part, because although the output noise power increases with the increase of the pump power, the signal also gains, so the proportion of spontaneous radiation generated by each fiber is higher. Small, so the overall signal-to-noise ratio is improved, that is, the noise figure NF is lowered.