Hello, everyone!
This post is dedicated to the Erbium-doped fiber amplifier(EDFA). I hope you’re interested.
Working Principles of EDFAs
Erbium-doped fibers are the key components of EDFAs and contain Er3+ ions of a certain concentration. Before illustrating the working principle of EDFAs, this course introduces the energy level diagram of erbium ions. The outer-shell electrons of an erbium ion occupy three energy levels (E1, E2, and E3). E1 is the ground state, E2 the metastable state, and E3 is the high-energy state.
When a high-energy pump laser is used to simulate an erbium-doped fiber, a large number of bound electrons of the erbium ion are simulated from the ground state to high-energy state (E3).
However, the high-energy state is unstable, and therefore the erbium ion soon undergoes radiationless decay (photons are not released) and enters the metastable state E2.
E2 is a metastable energy state, at which a particle can exist for a long time. The particles simulated by the pump laser continuously assemble to this state in radiationless transition mode, achieving distribution for populationinversion.
When optical signals with a 1550 nm wavelength traverse the erbium-doped fiber, particles at the metastable state transit to the ground state in stimulated radiation mode and generate the same photons as incident signal photons. In this way, photons in the signal light are increased, and signals are continuously amplified when traversing the erbium-doped fiber.
EDFA Structure
EDFA: It is an indispensable key component in a large-capacity DWDM system. Its internal structure is as follows:
Erbium-doped fiber: It is one of the most important parts of the EDFA. The fiber length is about 10 m to 30 m. During manufacturing, erbium is doped to the quartz fiber core. Therefore, the fiber is called an erbium-doped fiber.
Isolator (ISO): One isolator is configured before the erbium-doped fiber and another is configured after the erbium-doped fiber to transmit optical signals in a single direction.
Pump light source: 980 nm and 1480 nm pumping supplies are the most common ones(Important knowledge points). This is because the 1480 nm pump light source has the highest laser efficiency, and the 980 nm pump light source has a low noise figure and the second highest efficiency. The function is to enable the erbium ion to transit from the low-energy state to the high-energy state.
Coupler: It combines signal light and pump light and injects them into the erbium-doped fiber.
Photoelectric detector: It converts optical/electrical signals. There are two kinds of commonly used semiconductor photodetectors: PIN photodiode and APD avalanche photodiode.
EDFA Characteristics(Important knowledge points)
Advantage
1. Operating wavelength consistent with the minimum attenuation window of single-mode fibers.
The low-loss wavelength window used in DWDM systems is 1550 nm, and the EDFA works at this wavelength accessory. It’s like a gift from heaven to mankind.
2. High coupling efficiency.
Because it is a fiber amplifier, it is easier to couple with the transmission fiber.
3. High energy conversion efficiency.
The diameter of the erbium-doped fiber core is smaller than that of the transmission fiber core, and both the signal light and pump light are transmitted over the erbium-doped fiber. Therefore, the light energy is concentrated, enabling adequate interaction between light and gain medium (Er ions). With the erbium-doped fiber of a proper length, the light energy conversion efficiency is high.
4. High gain, low noise figure, and high output power.
The gain of the EDFA can reach more than 30 dB, which cannot be achieved by the Raman amplifier.
5. Good gain stability.
EDFAs are insensitive to temperature, and the gain is independent of polarization. The gain is independent of system bit rates and signal formats.
Disadvantage
1. Fixed gain range
Due to the differences between energy levels of Er ions, the operating wavelength range of EDFAs can only be in the 1550 nm window.
2. Gain unflatness
The gain bandwidth of EDFAs is large but not flat. During application in WDM systems, a special technology must be used to optimize the gain flatness.
3. Optical surge problem
The input optical power can be increased quickly using EDFAs, but the dynamic gain of the EDFAs changes slowly. Therefore, optical surge may occur in case of a step change of the input signals. That is, an input optical power peak occurs. Optical surge is more severe when EDFAs are cascaded. The peak optical power can be several walts, which may damage O/E converters and optical connector end faces.
Automatic Gain Control (AGC)
Gain locking of the EDFA is an important function. In a multi-wavelength system, if the number of wavelengths changes, the gain of each wavelength remains unchanged.
There are many technologies for locking the FDFA gain. The typical method is to control the gain of a pump lights source. The internal monitoring circuit of the EDFA monitors the ratio of the input power to the output to control the output of the pump source. When some signals of the input wavelengths are lost, the input power decreases and the ratio of the output power to the input power increases. Through the feedback circuit, the output power of the pump source is reduced to keep the EDFA gain (output/input) unchanged. In this way, the total output power of the EDFA is reduced, and the output signal level is stable.
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