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Principle and Structure of Erbium Doped Fiber Amplifier

Principle and Structure of Erbium Doped Fiber Amplifier

The erbium doped fiber amplifier is an optical amplifier made of fiber optic doped with rare metal erbium. The erbium doped fiber amplifiers have become very important devices in today's high-speed communication systems and wavelength division multiplexing systems, working in the 1550nm window. The erbium doped fiber amplifier has been a breakthrough achievement of optoelectronic technology in the information field in the world in the past ten years. Its appearance has led to the revolution of optical communication technology. It is the most important part of the three core technologies including the edfa erbium doped fiber amplifier, wavelength division multiplexing and fiber dispersion compensation technology.


Ⅰ. The principle of erbium doped fiber amplifier


When the laser medium is supplied with energy to make it in the excited state, it will produce the phenomenon of stimulated radiation of light. If the conditions for continuous stimulated radiation can be met, and the input light is used to sense, then stronger output light can be obtained so as to play an amplifying effect. The amplification effect of the erbium doped fiber amplifier is generated by the interaction of Er3+ (erbium ions) through the transmission of signal light with a wavelength of 1550nm in the erbium doped fiber optic. The energy state of Er3+ in erbium doped fiber optic cannot be continuously evaluated. It can only be in a series of discrete energy states. These energy states are called energy levels. When Er3+ is not excited, it is in the lowest energy level, which is the ground state E1.


Injecting sufficiently strong pump light into the erbium doped fiber optic can pump most of the E1 ions in the ground state to the high-energy state E3, and the Er3+ ions in the E3 are quickly transferred to the metastable state E2 without radiation. The Er3+ ion has a longer life time of the level in the metastable state. Due to the continuous pumping, the number of E2 particles increases continuously, so that the number of E1 and E2 particles is reversed, that is, the number of particles in E2 is more than that in E1. When the signal photon passes through the erbium doped fiber optic, it interacts with Er3+ ions to produce a stimulated emission effect. The Er3+ ion of E2 transitions to E1 and generates photons exactly the same as the photons in the incident signal light, thereby greatly increasing the number of signal photons and playing an amplifying effect.


The metastable state and ground state of Er3+ ion has a certain width, so that the amplifying effect of erbium doped fiber amplifier has a certain wavelength range, and its typical value is 1530 to 1570nm. When Er3+ ion is in E2, in addition to stimulated radiation and stimulated absorption, spontaneous radiation is also generated, which spontaneously transitions from E2 to E1, and emits a photon with a wavelength of 1550nm. This photon is different from signal light and constitutes the noise of erbium doped fiber amplifier. If the input optical power of the erbium doped fiber amplifier is low, the strong self-excited radiation will produce great noise.


Ⅱ. The structure of erbium doped fiber amplifier


The erbium doped fiber amplifiers generally consist of five basic parts, namely, erbium doped fiber optic, pump light source, optical passive device, control unit, and monitoring interface. The function of the optical coupler is to combine the signal light and the pump light, which is generally realized by a wavelength division multiplexer. The function of the optical isolator is to suppress light reflection to ensure the stable operation of the fiber amplifier. It must have low insertion loss, be independent of polarization, and have an isolation better than 40dB. The function of the optical filter is to reduce the influence of the noise generated by the spontaneous radiation on the system. The control unit controls the work of the fiber optic booster amplifier in real time, and the monitoring unit provides working status information.


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