Principle and application of EDFA erbium-doped fiber amplifier

Principle and application of EDFA erbium-doped fiber amplifier

The basic structure of EDFA erbium-doped fiber amplifier, which is mainly composed of an active medium (dozens of meters long doped quartz fiber, core diameter 3-5 microns, doping concentration (25-1000)x10-6), pump light source (990 or 1480nm LD), optical coupler and optical isolator. Signal light and pump light can propagate in the same direction (co-pumping), opposite direction (reverse pumping), or both directions (bidirectional pumping) in the Erbium fiber. When the signal light and pump light are injected into the erbium fiber at the same time, the erbium ion is excited to the high energy level (three-level system) under the action of the pump light, and soon decayed to the metastable level. When it returns to the ground state under the action of the incident signal light, the photon corresponding to the signal light is emitted, so that the signal is amplified. Its amplified spontaneous emission (ASE) spectrum has a large bandwidth (up to 20-40nm) and has two peaks corresponding to 1530nm and 1550nm respectively.

The main advantages of EDFA amplifier are high gain, large bandwidth, high output power, high pumping efficiency, low insertion loss, and insensitivity to polarization states.

The working principle of erbium-doped fiber amplifier

The Erbium-doped fiber amplifier（EDFA Optical Amplifier） is mainly composed of an erbium-doped fiber (about 10-30m in length) and a pump light source. The working principle is that the erbium-doped fiber generates stimulated radiation under the action of the pumped light source (wavelength 980nm or 1480nm), and the radiated light changes with the change of the input light signal, which is equivalent to amplifying the input light signal. The results show that the gain of Erbium-doped fiber amplifier is usually 15-40db, and the relay distance can be increased by more than 100km. So, people can’t help but ask: why did scientists think of using doped erbium in fiber amplifier to increase the intensity of light waves? We know that erbium is a rare earth element, and rare earth elements have their special structural characteristics. Doping rare earth elements in optical devices has been used for a long time to improve the performance of optical devices, so this is not an accidental factor. In addition, why the wavelength of the pump light source is chosen at 980nm or 1480nm? In fact, the wavelength of the pump light source can be 520nm, 650nm, 980nm, and 1480nm, but practice has proved that the wavelength of 1480nm pump light source laser efficiency is the highest, followed by the wavelength of 980nm pump light source.

Physical structure

Basic structure of erbium-doped fiber amplifier（EDFA Optical Amplifier）. There is an isolator at the input end and the output end, the purpose is to make the optical signal one-way transmission. The pump exciter has a wavelength of 980nm or 1480nm and is used to provide energy. The function of the coupler is to couple the input optical signal and the pump light into the erbium-doped fiber, and transfer the energy of the pump light to the input optical signal through the action of the erbium-doped fiber, so as to realize the energy amplification of the input optical signal. In order to obtain higher output optical power and lower noise index, the Erbium-doped fiber amplifier used in practice adopts the structure of two or more pump sources with isolators in the middle to isolate each other. In order to obtain a wider and flatter gain curve, a gain flattening filter is added.

The EDFA consists of five main parts: Erbium-doped fiber (EDF), Optical coupler (WDM), optical isolator (ISO), Optical Filter, and Pumping Supply. Commonly used pump sources include 980nm and 1480nm, and these two pump sources have higher pumping efficiency and are used more. 980nm pump light source noise coefficient is lower; The 1480nm pump light source has higher pumping efficiency and can obtain larger output power (about 3dB higher than the 980nm pump light source).

advantage

1. The operating wavelength is consistent with the minimum attenuation window of single-mode fiber.

2. High coupling efficiency. Because it is a fiber amplifier, it is easy to coupling with the transmission fiber.

3. High energy conversion efficiency. The core of EDF is smaller than that of transmission fiber, and the signal light and pump light are transmitted simultaneously in EDF, so the optical capacity is very concentrated. This makes the interaction between light and the gain medium Er ion very full, coupled with the appropriate length of erbium-doped fiber, so the conversion efficiency of light energy is high.

4. High gain, low noise index, large output power, low crosstalk between channels.

5. Stable gain characteristics: EDFA is not sensitive to temperature, and gain has little correlation with polarization.

6. The gain feature is independent of the system bit rate and data format.

shortcoming

1. Nonlinear effect: EDFA amplifies the optical power by increasing the optical power injected into the fiber, but the bigger the better. When the optical power is increased to a certain extent, the nonlinear effect of optical fiber will be produced. Therefore, when using optical fiber amplifiers, attention should be paid to the value of controlling the single-channel incoming fiber optical power.

2. The gain wavelength range is fixed: the working wavelength range of C-band EDFA is 1530nm~1561nm; The working wavelength range of L-band EDFA is 1565nm~1625nm.

3. Uneven gain bandwidth: The gain bandwidth of EDFA erbium-doped fiber amplifier is very wide, but the gain spectrum of EDF itself is not flat. The gain flattening filter must be adopted to flatten the gain in WDM system.

4. Light surge problem: When the light path is normal, the erbium ions excited by the pump light are carried away by the signal light, thus completing the amplification of the signal light. If the input light is truncated, because the metastable erbium ions continue to accumulate, once the signal light input is restored, energy will jump, resulting in light surge.

5. The solution to the optical surge is to realize the automatic optical power reduction (APR) or automatic optical power off (APSD) function in EDFA, that is, EDFA automatically reduces the power or automatically turns off the power when there is no input light, thereby suppressing the occurrence of the surge phenomenon.

Application mode

1. The booster Amplifier is used to boost the power of multiple wavelength signals after the booster wave, and then transmit them. Since the signal power after the booster wave is generally large, the noise index and gain of a power amplifier are not very high. Has relatively large output power.

2. Line-amplifier, after the power Amplifier, is used to periodically compensate the Line transmission loss, generally requiring a relatively small noise index and a large output optical power.

3. Pre-Amplifier: Before the splitter and after the line amplifier, it is used to amplify the signal and improve the sensitivity of the receiver (in the case that the optical signal-to-noise ratio (OSNR) meets the requirements, the larger input power can suppress the noise of the receiver itself and improve the receiving sensitivity), and the noise index is very small. There is no great requirement on the output power.