How do Erbium Doped Fiber Amplifiers Work?

Figure 1: Gain Flattened Erbium Doped Fiber Amplifier C-Band Pre Amplifier EDFA-PA-GF-25-SM 25dB

The erbium doped fiber amplifiers are the most widely used optical amplifiers in optical fiber communications. It has the advantages of high coupling efficiency, high energy conversion efficiency, high gain, low noise figure, and high output power, which are closely related to the characteristics of the doped erbium ions. This article will introduce the working principle of erbium doped fiber amplifiers according to the characteristics of erbium ions.

Characteristics of Erbium Ions

Erbium-doped fiber amplifiers are doped with erbium, a rare earth element, in the core of the silica fiber. The erbium ion has three working energy levels, namely E1, E2 and E3. Among them, the E1 energy level has the lowest energy and the largest number of particles, while the E3 energy level has the highest energy and the least number of particles. Among these three energy levels, because the E1 level has the lowest energy level, the largest number of particles and is the most stable one, it is called the ground state, i.e., the most stable state. The E3 energy level has the highest energy, and its particles are the most unstable, also known as the excited state.
The E2 energy level is between the ground state and the excited state, and we call it a metastable state. It is more active than the ground state and stabler than the excited state.

The number of particles in the metastable state is relatively stable and can maintain a stable state for a period of time.

The Working Principles of Erbium Doped Fiber Amplifiers

The erbium doped fiber amplifier also includes a pump light source whose function is to input energy to the material. In this way, the particles of the low energy level E1 absorb the energy of the pump light and then transition upward to the E3 energy level, i.e., the excited state. We call this process the transition of absorption of pump light.

Figure 2: The working principle of erbium doped fiber amplifier.

The particles at the E3 level are in an excited state and are very unstable. Therefore, in the absence of external particle excitation, it will transition downward to the E2 energy level, which is a metastable state. We call this process a radiationless transition. Without the excitation of external particles, high-energy-level particles will automatically and spontaneously transition down to low-energy levels.

Since the E2 energy level is in a metastable state, the particles at this energy level can be maintained for a period of time. Thus, the inversion distribution state of the number of particles is realized between E2 and E1.
At this time, if there is an excitation of an extraneous photon, this light is the optical signal to be amplified. The light signal at this time is weak light, and the number of photons contained in it is relatively small. Injected in this way, the photon, as an external excitation photon between E2 and E1, will make the process of stimulated emission greater than stimulated absorption. From this, new identical photons are generated, thereby realizing the amplification of light. Then the light output from matter is strong light. This is how erbium doped fiber amplifiers work.

Related Info