What Is an Optical Amplifier?
Optical amplifier is used to provide gain for optical signals. The core of the optical amplifier is the gain medium. The fiber optic amplifier is pumped to realize population inversion and gain. The gain is generally associated with the gain medium, pump source, and input signal. The function of the pump source is to make the gain medium activated to produce the population inversion. And the signal generates stimulated emission through the activated gain medium to finally realize amplification. A basic structure of an optical amplifier is shown as follows:
Basic Optical Amplifier Structure
The typical types of optical amplifiers include three types as follows.
Types of optical amplifier
• Semiconductor Optical Amplifier(SOA)
• Erbium-Doped Fiber Amplifier(EDFA)
• Raman Fiber Amplifier(RFA).
Optical amplifier brings a qualitative leap to optical fiber communication technology. With the rapid commercial application of the optical amplifiers, WDM technology came into high maturity. The combination of the optical amplifier and WDM technology can flexibly and conveniently realize the capacity expansion of the optical communication system without a large cost.
EDFA amplifier is the most widely commercially type today due to its excellent performance, but it also has limitations, such as its narrow bandwidth limits people’s pursuit of high broadband and all-optical amplification. So RFA amplifiers, as an all-optical amplifier, are also considered potential in the future. Next, this article will give a detailed introduction to the most commercially EDFA amplifier.
What Is an EDFA?
As mentioned above, EDFA is a kind of optical amplifier, which stands for Erbium-Doped Fiber Amplifier. EDFA amplifier is one of the greatest inventions in optical fiber communication, which greatly increases the capacity of optical fiber communication. EDFA is typically used in long-distance fiber optical communication systems to realize optical amplification and decrease optical attenuation.
The following figure shows the basic structure of EDFA.
EDFA Structure Diagram
A typical EDFA amplifier mainly consists of the following components:
• Erbium-Doped Fiber: Core of the EDFA. It is made by doping the rare earth element Erbium into quartz optical fiber.
• Pumping Laser: Used to raise energy for Erbium. 1480nm wavelength is proved to work best, followed by 980nm wavelength.
• Isolator: Used to restrain the optical lights from reflecting and ensure the optical amplifier works stably.
• Coupler: Used to couple optical lights and pump lights into Erbium-doped fiber.
• Optical Filter: Narrow band-passed optical filter with bandwidth within 1nm. Used to eliminate the spontaneous emission light of the EDFA amplifier to reduce the EDFA noise.
How the EDFA Fiber Amplifier Works
An EDFA is mainly composed of the Erbium-doped fiber (10~30 meters) and the pumping laser (980nm or 1480nm wavelength). After being stimulated by the pumping laser, the Erbium-doped fiber comes into a metastable high excited state and generates stimulated emission, so as to amplify the input optical signals. Generally, an EDFA can get 15-40dB gain, and the relay distance can extend more than 100km.
Questions About EDFA You May Concern
1. Why Er-doped fiber amplifier is most widely used? Which rare-earth ions can also be used in the optical amplifier except for Erbium?
Except for the Er-doped fiber amplifier, there are also Pr-doped, and Tm-doped fiber amplifiers.
• EDFA (Er-doped Fiber Amplifier) Works at 1550nm wavelength.
• PDFA (Pr-doped Fiber Amplifier) Works at 1300nm wavelength.
• TDFA (Tm-doped Fiber Amplifier) Works at 1400nm wavelength.
The working wavelength of EDFA is exactly the minimum-loss window of optical communication, which is one of the reasons why EDFA is most widely used.
2. Why 980nm and 1480nm are chosen as the pump source?
The wavelengths of the source can be 520nm, 650nm, 980nm, and 1480nm, but the practice has proved that the pump efficiency of 980nm and 1480nm is higher than others. The pump efficiency of EDFA pumped at 1480nm is higher than that at 980nm. However, the 980nm pump source has a lower noise figure, so if you need better noise performance, 980nm is more recommended.
Advantages of EDFA
EDFA amplifier is the most widely used optical amplifier, which has many advantages as follows:
• Works at the minimum-loss window of optical communication. EDFA works at 1550nm wavelength, which is exactly consistent with the minimum attenuation window of the single-mode fiber.
• High coupling efficiency. The coupling loss of EDFA and transmission fiber is very low, even lower than 0.1dB.
• Low noise figure. 4~7dB.
• High Gain. 20~40dB
• High output power: Bidirectional pumping can reach up to 20dBm.
• Wide frequency bandwidth. 20~40nm wide frequency bandwidth at 1550nm supports simultaneously amplify of multi-path signals.
• Low required pump power. Only about 10mW.
• All-fiber structure. Fully compatible with fiber optical communication system.
• High stability. The gain of EDFA is insensitive to temperature and polarization independence.
EDFA Application: EDFA + WDM
Since the EDFA amplifier has many advantages as mentioned above, it has been widely used in many applications as follows:
• WDM System
• Optical Soliton Communication
• Distribution Network
In recent years, the combination of WDM ((Wavelength Division Multiplexing)) technology and EDFA amplifier has become the mainstream in high-speed and large-capacity optical fiber communication systems. From the general WDM system structure below, we can see that the wavelengths transmitted in the optical fiber increased many times with the combination of WDM technology and the optical amplifier.
WDM System Structure
EDFA fiber amplifier used in the WDM system is mainly to compensate for the fiber transmission loss. EDFA amplifiers can be divided into three types according to different locations and functions in the WDM systems:
1. Booster-Amplifier. Located behind the multiplexer, is used to increase the power of multiple wavelength signals after multiplexing. High output power is required.
2. Line-Amplifier: Located behind the booster amplifier, is used to periodically compensate for the transmission loss. A low noise index and high output power are required.
3. Pre-Amplifier: Located before the demultiplexer and behind the line amplifier, is used to amplify signals and increase the receiving sensitivity of the receiver. A very small noise index is required.
Optical amplifier, especially EDFA is important contributors to the optical communication system. EDFA works at 1550nm wavelength. The commercial EDFA amplifier has the advantages of low noise index, good gain curve, large amplifier bandwidth, compatibility with wavelength division multiplexing (WDM) system, high pump efficiency, and stable performance. It is favored in modern long-distance and high-speed optical communication systems. At present, "EDFA+DWDM" is becoming the main technical solution for international long-distance and high-speed optical fiber communication transmission.
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