1. Background
After the optical signal in the optical communication system passes a certain distance or some functional devices, the loss caused by it must be compensated for power before it can be correctly received at the receiving end. The device that completes these power compensations is the optical amplifier. The earliest optical amplifier is an optical-optical (OEO) method, that is, the received signal light is converted into a circuit signal, processed by the circuit, and then emitted through an optical transmitter. This type of amplification is limited by the frequency band of the circuit device, and the power of the amplification is not large. OEO amplification is basically not used in Wavelength Division Multiplexing (WDM) systems, but there are still such applications in some low-speed short-distance transmissions. The all-optical amplifier developed later, the signal light is directly amplified in the optical waveguide without circuit conversion, which solves the problem of OEO circuit constraints, and provides wide-spectrum multi-wave amplification, in the long-distance dense wavelength division multiplexing (DWDM) system , Has been widely used. After years of development, optical amplifiers mainly fall into the following categories:
(1) Inverted particle amplifier: doped ion fiber amplifiers, such as erbium-doped fiber amplifiers (EDFA), praseodymium-doped fiber amplifiers (PDFA), thulium-doped fiber amplifiers (TDFA), etc., semiconductor optical amplifiers are the reverse of direct electric pumping. Turn particle amplifier
(2) Non-linear scattering amplifier: for example: Raman amplifier, Brillouin amplifier;
(3) Nonlinear refraction amplifier: parametric amplifier;
The amplification band of the first type of amplifier depends on the amplifying medium doped ion or the type of material, and the second and third types of amplifiers have amplifying bands depending on the pump wavelength and material type.
Various types of optical amplifiers are used in different applications of optical communication systems. There are mainly three types of applications: post (power) amplifiers, line amplifiers, and preamplifiers. While these amplifiers provide amplification, they also bring amplified spontaneous emission (ASE) noise to the system. Design to reduce noise is an urgent requirement in recent high-speed systems.
2. Current status of optical amplifiers
The current mainstream of optical amplifiers is erbium-doped fiber amplifier (EDFA). Erbium-doped fiber amplifiers are suitable for CBand and LBand. The rate transparency, large gain, large bandwidth, low noise, and high power make it an ideal optical communication low-loss window. Amplifier. After years of development, EDFA has developed a variety of application forms, including single-wave applications, multi-wave applications, adjustable gain, reconfigurable and other forms; the miniaturization and density of EDFA are also increasing. In terms of miniaturization, from MSA Modules to HalfMSA modules or even Micro modules, array amplifiers range from 4Array to 8Array or even 16Array. Various system application forms have continuously promoted the development of the EDFA industry. In recent years, the development of ROADM and high-speed systems has made miniaturization and arraying a development trend. Raman amplifiers, mainly distributed Raman amplifiers (DRA), are used for long-distance transmission. Transmission fiber is used as amplifying medium. It can be amplified in any wavelength band of optical communication. It has large bandwidth and low effective noise. Compared with EDFA, it improves The signal-to-noise ratio of the system is improved. After years of development, Raman amplifier applications also have many forms, such as single-wave, multi-wave, forward, backward, distributed, discrete, etc. In recent years, with the increase of system speed, the system has become more and more sensitive to the signal-to-noise ratio, and the system has put forward higher and higher requirements for the noise of the amplifier. Using the low-noise characteristics of DRA, a hybrid amplifier of DRA and EDFA has been developed. The multi-stage distributed Raman amplifier further reduces noise and has been applied in 400Gbps systems.
The semiconductor amplifier (SOA) is the only electrically pumped amplifier among the amplifiers. It can be integrated with many devices to achieve functions such as amplification, switching, conversion and regeneration. The amplification wavelength range is wide, from 0.6um to 1.6um, and the bandwidth is relatively wide. However, SOA has low saturated output power, multi-wave crosstalk when saturated, large noise, and large polarization-dependent gain, which limit its application. In recent years, SOA has a variety of applications on PON, and the usage is gradually increasing, but the price is a problem.
3. Development of Optical Amplifier
With the development of optical communication, the growth of communication capacity is a never-ending pursuit. In recent years, with research on the order of 400Gbps, Tbps and even Pbps, a number of different optical amplifiers have emerged.
Optical parametric amplifiers (OPA), especially phase sensitive amplifiers (PSA), have appeared in some articles after 2010. PSA can provide a noise limit of 0dB while amplifying, making it an ideal amplifier in high-speed systems.
Space division multiplexing (SDM) amplifiers are divided into multi-core amplification and multi-mode amplification. These two kinds of amplification are the concept of parallel multi-channel amplification by a single amplifier, and each channel can also realize WDM. In 2012, there was a test report that through 12-core SDM+222-wave WDM, 52 kilometers of 1Pbps transmission can be achieved.
These two types of amplifiers are currently mostly in the experimental stage and have not yet been officially commercialized.
4 Conclusion
In optical communication technology, optical amplification technology is one of the supporting technologies, and optical amplifier technology continues to develop with the requirements of the system. OEO technology, doped fiber amplifier, SOA, Raman amplifier, parametric amplifier, these amplifiers are applied to different system nodes in various forms according to different system requirements. With the development of the information industry, amplification technologies suitable for different wavebands and different applications will inevitably bring about the continuous development and prosperity of the optical amplifier industry.
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