FOT Lesson: Structure of EDFA

Structure of EDFA

  The structure of the erbium-doped fiber amplifier is shown in the figure below. Its key components are erbium-doped fiber, high-power pump source, wavelength division multiplexer (combiner) that is multiplexed as signal light and pump light, and light feedback and Reduce system noise, optical isolator used at the input and output. From a functional point of view, a practical EDFA is composed of two parts: an optical path and an auxiliary circuit. The optical route is composed of an erbium doped fiber, a pump light source, a fiber coupler, an optical isolator, and an optical filter. The auxiliary circuit has a power supply and the microprocessor automatically Control detection systems, alarms and protection circuits.

FOT Blog: Typical structure diagram of EDFA

Erbium Doped Fiber

  Erbium-doped fiber (EDF) is the core component of the EDFA. Quartz fiber is used as the substrate. When the fiber is drawn, a certain proportion of rare earth cerium ions (Er3+) are incorporated into the silica core. When a certain power is injected into the erbium-doped fiber as pump light, Er3+ is excited from a low energy level to a high energy level. Since the lifetime of Er3+ at high energy levels is very short, it quickly jumps to a lower intermediate energy level and forms a population inversion distribution between this energy level and a low energy level. The key to the optical amplification of erbium-doped fiber amplifiers is the ytterbium-doped fiber with gain amplification. Therefore, optimizing the design of ytterbium-doped fibers has always been the key to the technology. The gain of the EDFA is related to many parameters, such as the concentration of erbium ions, the length of the amplifier erbium-doped fiber, the diameter of the core, the power of the pump light, the wavelength of the pump light, and the like. The rare earth-doped optical fiber and other manufacturing methods include a dissolution method, a carrier container method, a melting method, a vapor deposition method, and the like.

EDFA pump source

  Another component of the EDFA is the pump light source, which provides sufficient energy guarantee for signal amplification and is also a necessary condition for the working substance to reach the population inversion distribution. Therefore, the pump light source is an important part of the EDFA. The EDFA has two basic requirements for the pump light source. One is that the pump light source has an emission wavelength corresponding to the peak absorption band of the doped fiber in order to increase the absorption efficiency; rather, a larger output power is required. Since the pump light source directly determines the performance of the EDFA, it is required that the pump light source must be stable and reliable and have a long life. The EDFA’s pump light source is mainly a semiconductor laser diode, and its pump wavelengths are 820 nm, 980 nm, and 1480 nm. The 1480nm InGaAs multi-quantum well (MOW) laser was first used as a pump source. Its output power was up to 100mW and the pump gain coefficient was high. The bandwidth of the EDFA was matched with the existing practical InGaAs laser. However, in the commercial-grade EDFA, the most widely used is the 980 nm InGaAs pump light source that has made great strides and has output power of up to several hundred milliwatts.
  The pump source can adopt three methods for EDFA pumping: forward pumping, reverse pumping, and bidirectional pumping.

Other components

1. The combiner

  The combiner (wavelength division multiplexer) is another component of the EDFA. Its function is to combine the signal light with the pump light and send it to the erbium-doped fiber. The pair combiner pair has a small pair of insertion loss at the signal light and the pump light, and is insensitive to the polarization of the light, and the combiner is sometimes referred to as a wavelength division multiplexer.

2. Optical isolators and optical filters

  The optical isolator makes the light transmission unidirectional, passes the light in the forward direction with extremely low loss, and suppresses the reflected light with high loss (more than 40 dB). Optical isolator needs to be inserted at the input and output ends of the optical amplifier to suppress the reflection in the optical path. The requirements for the isolator are low insertion loss and large reverse isolation, so that the optical amplifier is not affected by the radiation, ensuring the stable operation of the system.
  The role of the optical filter is to filter out the noise in the optical amplifier and improve the signal to noise ratio S/N of the system. The following table shows the technical requirements for the components of each light path of a small, practical EDFA.

ComponentsTechnical Requirement
Pump laser diode0.98 μm LD, Lens Coupling Efficiency -2.9 dB
Fiber Optic CouplerWDM type fused tapered fiber coupler, loss less than 0.2 dB, crossover ratio of 98%
Erbium-doped fiberEr/Ge fiber, Er3+ concentration is 80*10^(-6), λc=0.85 μm, fiber length is 37m
Optical isolatorPolarization insensitive, isolation ratio 47dB
Input/output fiber connectorSG type connector, reflection loss 47dB

3. Auxiliary Circuit

  In the auxiliary circuit, the requirements of the system for the power supply are relatively high, and should have the characteristics of high stability, low noise, long service life, etc., and generally adopt a switching power supply. The microprocessor-based control and detection system detects the working state of the pump laser and the intensity of the input/output optical signals, and appropriately adjusts the working state parameters of the pump source so that the optical amplifier operates in an optimal state. The auxiliary circuit also includes automatic temperature control and automatic power control protection circuit. Some auxiliary circuits also have a computer communication protocol to complete the man-machine conversation and monitor the optical amplifier network.

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