In the optical communication network, the signal is transmitted through the optical fiber for a long distance without obvious attenuation. However, when the signal needs to be transmitted for hundreds of kilometers, the signal needs to be amplified during the transmission process.
EDFA (Erbium Doped Fiber Amplifier) was commercialized in the early 1990s and became a key enabling technology for optical communication networks. It enables the optical signal in the optical fiber to be directly amplified in high bit rate systems exceeding terabits
How does signal amplification happen?
Erbium-doped fiber (EDF) is the core of EDFA technology. It is a traditional erbium-doped silica fiber. When erbium is irradiated with light energy of the appropriate wavelength (980 nm or 1480 nm), it will be excited to a long-lived intermediate state , And then decay back to the ground state by emitting light in the 1525-1565nm band (see the figure below).
If the light energy already exists in the 1525-2565nm band, for example due to the signal channel through the EDF, then this will stimulate the attenuation process (so-called stimulated emission), thereby generating additional light energy. Therefore, if the pump wavelength and the signal wavelength At the same time propagating through the EDF, the energy will be transferred from the pump wavelength to the signal wavelength through the erbium, resulting in signal amplification.
Components and roles in EDFA design
In its most basic form, EDFA consists of a section of EDF (usually 10-30m), a pump laser, and a component (usually called WDM) that combines the signal and pump wavelength. In principle, EDFA can be designed to pump energy in the same direction as the signal (forward pumping), the opposite direction to the signal (backward pumping), or both directions. The pump energy can be 980nm pump energy, 1480nm pump energy or a combination of the two. The most common EDFA configuration is a forward pump configuration using 980nm pump energy, as shown in the figure below. This configuration can make the most effective use of cost-effective, reliable and low-power 980nm diode pumped laser diodes.
In addition to the three basic components mentioned above, the figure also shows other optical and electronic components used in a basic single-stage EDFA. The signal enters the amplifier through the input port and then through a tap that is used to transfer a small portion of the signal power ( Usually 1-2%) is transferred to the input detector, then the signal passes through the isolator, and then combines with the pump energy emitted by the 980nm pump laser diode. The combined signal and pump energy propagate along the EDF, where signal amplification occurs, Then the amplified signal leaves the EDF and passes through the second isolator. The two isolators only allow light to pass in one direction. The purpose is to ensure that the EDF does not generate laser light.
EDFA operating mode
EDFA usually operates in one of two modes of operation: AGC (Automatic Gain Control) or APC (Automatic Power Control).
In AGC mode, the amplifier gain remains constant, while in APC mode, the amplifier output power remains constant. Although APC mode is used for some single-channel applications, AGC mode is more common and is always used for multi-channel WDM applications.
The figure below shows the AGC in the EDFA. The information from the input and output detectors is used to calculate the actual gain, and then it is compared with the desired gain. Based on this comparison, the pump current is then adjusted to change the actual gain to the desired Gain, which is a classic feedback control loop, can be implemented using analog or digital circuits.
This article briefly introduces EDFA, which is one of the most important components in WDM communication. Among the various technologies that can be used for optical amplifiers, EDFA technology is by far the most advanced, so most of the optical amplifiers deployed so far are based on the technology.
(Click on the picture below to learn more EDFA product information)