What are the main development directions of fiber amplifier EDFA.

Understand the OFA: Optical Fiber Amplifier (OFA: Optical Fiber Amplifier) first understand the role of Optical amplifiers in Optical communications. In Fiber-optic communication, light causes loss and dispersion during transmission, which can weaken the signal over long distances. The traditional method is to use the optical-electric-optical relay to compensate for the loss of optical signal, to extend the communication distance. Opto-electro-optical is the conversion of optical signals into electrical signals, electrical signals through amplification, processing, and then converted into optical signals, so that it can achieve long-distance transmission. But this method requires the use of optical receivers and optical transmitters, equipment cost high optical power direct amplification mode in general, at the same power and transmission distance, originally, in order to achieve long-distance transmission, a photoelectric repeater was added in the middle of the distance to convert the optical signal into an electrical signal at the transmitting end and an electrical signal into an optical signal at the receiving end, the energy of the optical signal is maintained and transmitted without being weakened. The role of optical amplifier is to replace the original photoelectric repeater, become all-optical repeater, through the signal amplification process to extend the transmission distance. Understand the Erbium-Doped Fiber Amplifier EDFA (Erbium Doped Fiber Amplifier) an EDFA is an optical Fiber in which Erbium ions are Doped into the Fiber to form an Erbium Doped Fiber, and the Erbium ions are activated by pumping the signal to achieve the gain effect. It works in the same way as an erbium-doped semi-conductor laser. The reason it amplifies the optical signal is simply that, under the action of a pumping source, there is an inversion of the particle number distribution in the erbium-doped fiber, which produces stimulated radiation, this amplifies the optical signal. EDFA is the most widely used WDM system, the most important feature is that it can amplify multiple optical signals at the same time, and can be easily combined with WDM technology. Typically, it is used in the C-band and l-band, almost in the range of 1530 to 1565 nm, consistent with the fiber minimum loss window.

Erbium-doped fiber is a kind of fiber with a small amount of rare earth (25mg/kg) erbium (Er) ions in a length of about 10 ~ 100m quartz fiber, it's the heart of the Erbium-doped fiber amplifier. Simply put, erbium-doped fiber, as a gain medium, plays a major role in signal amplification. Optical couplers are Passive optical device that combine the input optical signal with the output wave of a pump light source, usually using a wavelength division multiplexer. The optical isolator is a device which can prevent the reflected light from affecting the stable operation of the optical amplifier and ensure that the optical signal can only be transmitted forward. Gain flattening filter (GFF) , also known as gain equalization filter, is used to smooth or vary the signal strength in a specified wavelength range. This unequal signal strength usually occurs after the amplification stage. Pumping is the process of using light to raise (or"Pump") an electron from a lower energy level in an atom or molecule to a higher energy level. In EDFA, the pump light source is usually semi-conductor laser, the output power is 10-100mw, and the operating wavelength is about 980nm or 1480nm. According to the pump mode of the pump light source, EDFA has the same pump, reverse pump and two-way pump. Co-pumping: erbium-doped fiber is injected from the same end of the pump and signal, and the pump wave propagates in the same direction as the signal wave. Co-pump reverse pump: pump light and signal light pump from the opposite direction, pump wave and signal wave propagation in the opposite direction. Reverse pump bidirectional pump: two pump light sources pump forward and backward respectively. The pump wave and signal wave can be bidirectional. The main reason why two-way pumping is different is that erbium-doped fiber is injected at different stages before, after, or both before and after optical signal amplification, different conversion efficiency and output power of pump light source have different effect on noise performance.

Which one is chosen mainly from the analysis of the structure and characteristics of EDFA, the synthesis of key performance parameters and economic cost and other factors, compared with EDFA applied in different occasions, can adopt different structural design. The application of EDFA EDFA can be used in large-capacity, high-speed and long-distance optical communication systems. The use of EDFA can reduce the number of repeaters, compensate transmission loss of lines and reduce construction costs. DWDM system and CATV system are typical application scenarios. 1) the application of DWDM in dense wavelength division multiplexing (DWDM) system in long-distance transmission, DWDM system in optical fiber transmission system has become the mainstream technology, the main role of EDFA is to extend the middle distance, EDFA is commonly used in C-band and l-band, can amplify multi-channel optical signal at the same time, there is enough gain bandwidth, and can be easily combined with WDM technology to achieve ultra-large capacity, ultra-long-distance transmission

Application in CATV systems EDFA plays an important role in CATV cable television systems, as shown below. In CATV systems, EDFA can significantly reduce the use of optical transmitters, at the same time, it can compensate the loss of the link and reduce the cost of the system. If the DWDM system is applied at the same time, the capacity of the system can be increased effectively.