The amplifying effect of EDFA is generated by the interaction of the signal light in the 1550nm band through the Erbium-doped fiber and the interaction of Er3+ ions. When light interacts with matter, light can be regarded as a particle beam composed of photons. The energy of each photon is: E=hv where E is the energy of the photon, v is the frequency of light, and h is Planck's constant.
The energy state of the Er3+ ion in the erbium-doped fiber cannot be continuously taken. It can only be in a series of discrete energy states to become energy levels, and these energy states become energy levels. When the energy of the photon transmitted in the erbium-doped fiber is equal to the energy difference between two energy levels of Er3+ ions, the Er3+ ions will interact with the photons to produce excitation radiation and excitation absorption effects. Stimulated emission refers to the transition of Er3+ ion and photon from high energy level to low energy level, emitting a photon that is exactly the same as the excited photon (the frequency, phase, propagation direction, and polarization state of the laser are the same); stimulated absorption refers to The Er3+ ion interacts with the photon to transition from a low energy set to a high energy level, and absorb the excited photon.
By injecting sufficiently strong pump light into the erbium-doped fiber, most of the Er3+ ions in the ground state can be pumped to the high-energy state, and the Er3+ ions in the high-energy state are quickly transferred to the metastable state without radiation. Since Er3+ ions have a longer energy level lifetime in the metastable state, it is easy to form a population inversion between the metastable state and the ground state, that is, the number of Er3+ particles in the metastable state is larger than the number of Er3+ particles in the ground state.
When signal photons pass through the ear-doped light string and interact with Er3+ ions to produce a stimulated emission effect, a large number of photons identical to themselves are generated. At this time, the signal photons transmitted through the ear-doped fiber rapidly increase, resulting in signal amplification; only a few are in the ground state The Er3+ ion team signal photons produce a stimulated absorption effect, which absorbs photons.
The metastable state and ground state of Er3+ ion have a certain width, so that the amplification effect of EDFA has a certain wavelength range, and its typical value is 1530-1570nm. When Er3+ ions are in a metastable state, in addition to stimulated emission and stimulated absorption, spontaneous radiation is also generated, that is, Er3+ ions stay in the metastable state for a short time and have no chance to interact with photons, and will spontaneously come from the substable state. The steady state transitions to the ground state and emits photons in the 1550nm band. This photon is different from signal light, and it constitutes the noise of EDFA. Since the spontaneous emission photons are also amplified when they are transmitted in the erbium-doped fiber, when the input optical power of the EDFA is low, greater noise will be generated.
