FOT Lesson: EDFA & Light Amplification Process

EDFA: Erbium-doped fiber amplifier

  It is necessary to increase the capacity of the communication line and minimize its cost. The wavelength division multiplexing (WDM) system is the preferred solution. The maximum rate of the WDM commercial system has reached 40*10 Gbit/s, and the WDM technology develops so rapidly. Thanks to the successful development and application of EDFA.
  In 1985, the University of Southampton developed the first-time erbium-doped fiber (EDF). After several years of research, low-loss Erbium-doped fiber technology has become quite mature. Its performance is very good, and it has reached a practical level in the field. Using a rare-earth ytterbium ion as the gain medium for the fiber amplifier EDFA, germanium ions are incorporated into the fiber core during the fiber manufacturing process, and the pump light source is used to directly amplify the optical signal to provide optical gain. The characteristics of the EDFA amplifier, such as operating wavelength, bandwidth, etc., are determined by the dopant. EDFA is more noticeable than other optical amplifiers because its operating wavelength is near the 1.55 μm wavelength region where the fiber loss is minimal.
  In the early 1990s, the successful development of erbium-doped fiber amplifiers (EDFAs) broke the limitation of optical fiber communication transmission distance by the loss of optical fibers, extended the all-optical communication distance to several thousand kilometers, and brought revolutionary changes to optical fiber communications. Changes have been hailed as a “milestone” in the development of optical fiber communications.

EDFA pump characteristics

  The gain characteristics of EDFA are related to pumping methods, fiber optic dopants, and the like. In an EDFA system, EDFAs can be pumped using a variety of light sources of different wavelengths, but pumping with 980 nm and 1480 nm semiconductor lasers is most effective and the pumping efficiency is highest. When using these two wavelengths of optically pumped EDFAs, only an optical gain of up to 30-40 dB can be achieved with a few milliwatts of pump power.

Light amplification process

  How does pump light transfer energy to signal light? The three-level diagram of the erbium-doped ions shows that the energy level E1 represents the ground state, the energy of the ground state is the lowest, the energy level E2 represents the intermediate energy level, E3 represents the excited state (E3 is different when the pumping light of different wavelengths is different), and its energy is the highest.

How Erbium Doped Fiber Amplifiers Work?

  If the photon energy of the “pumped light” is equal to the difference between the energy levels E3 and E1, the doped erbium ions absorb the pump light and then jump from the ground state E1 to the excited state E3. However, the excited state is unstable, and the erbium ions excited to E3 quickly radiate without energy to E2. If the “signal light” photon energy is equal to the difference between the energy levels E2 and E1, then the E2 erbium ion transition to the ground state E1 will produce exactly the same photon as the signal light, which is the result of the radiation, resulting in the signal light being enlarge. In order to increase the gain of the amplifier, ground-state helium ions should be excited to E3 as much as possible. From the above analysis, it can be seen that the difference between the energy levels E2 and E1 must correspond to the photon energy that needs to amplify the signal light, and the photon energy of the pump light must also ensure that the cesium ions transition from the ground state E1 to the excited state E3.

Conversion efficiency and absorption efficiency

  It is always desirable that the conversion efficiency of the pump light into signal light be as high as possible. The relationship between the output signal optical power of the EDFA and the pump power can be found in the related documents. In order to reflect the pump characteristics and light absorption characteristics, the above characteristics are reflected by the pump efficiency and the absorption efficiency, respectively.

Optical amplifier energy conversion efficiency

  1. Conversion (pumping) efficiency
    Pump efficiency refers to the ratio of the output power of the optical amplifier to the pump power.
  2. Absorption efficiency
    Absorption efficiency refers to the ratio of the static output power of the optical amplifier to the pump power.

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