Raman amplifier is an all-optical amplifier realized by using the stimulated Raman scattering effect and using optical fiber as the gain medium.
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- Raman Amplifier
In the future, Raman amplifiers will mainly be used as distributed amplifiers to assist the signal amplification of EDFAs, but Raman fiber amplifiers can also be used alone to amplify bands that EDFAs cannot, and are suitable for It can also be used as a discrete amplifier with optical fiber.
Various design schemes of Raman fiber amplifiers are currently being deeply investigated to promote the development of fiber optic communication systems.
- Raman Amplifier in Optical Fiber
Raman amplifiers are mainly used as distributed amplifiers to assist EDFA in signal amplification in the future, but Raman fiber amplifiers can also be used alone to amplify bands that EDFA cannot amplify, and can also be used as discrete amplifiers with appropriate optical fibers.
Now people are deeply exploring various design schemes of Raman fiber amplifiers to facilitate the development of fiber optic communication systems.
Raman amplifier’s gain bandwidth is very wide
Up to 40 THz, and the available flat gain range is 20-30 nm. Theoretically speaking, as long as there is a high-power pump source of suitable power, it can amplify signals of any wavelength. In addition, raman amplifier has the advantages of low noise and can use transmission fiber for online amplification, etc., and has become an important member of the optical amplifier family.
According to different pumping methods, Raman fiber amplifiers can be divided into three structures:
Forward pumping, backward pumping and bidirectional pumping. Among them, the transmission of pump light and signal light in the same direction is called co-direction pumping, and vice versa is called backward pumping.
Compared with forward pumping, the use of backward pumping can make the noise of the amplifier lower, and at the same time, its polarization dependence is also smaller. When the Raman gain is large, the optical power of the signal at the fiber entrance or exit is large, and the nonlinear effect is serious. Therefore, the performance of the bidirectional pumped Raman fiber amplifier is better than that of the unidirectional pumped Raman fiber amplifier. .
There are two types of fiber Raman amplifiers
Namely centralized and distributed Raman amplifiers, each with its own characteristics. The centralized fiber Raman amplifier separates the Raman fiber amplifier from the transmission line as an independent component.
The gain fiber used in the centralized type is relatively short, and the pump power is very high, which can produce high gain above 40 dB, and amplify the band that EDF A cannot amplify. The distributed optical fiber Raman amplifier uses the transmission fiber itself as the gain medium. Compared with the centralized type, it has a lower noise figure. It is mainly used in conjunction with EDF A to improve the overall performance of the system.
(1) Raman amplification is a non-resonant process, and its gain response depends only on the wavelength of the pump light and its bandwidth, and the amplification of any wavelength can be obtained by selecting a suitable pump source. It has an irreplaceable role in developing the entire low-loss region of optical fiber.
(2) The gain medium is the transmission fiber itself, which has good compatibility with the fiber system, and it can utilize the G. 6 5 2 or G． 6 5 5 Optical fiber is used as a gain medium to perform distributed amplification of optical signals, thereby realizing long-distance non-relay transmission and remote pumping, especially suitable for occasions where it is inconvenient to set up repeaters such as submarine optical cable communications.
(3) Small crosstalk, good temperature stability, and low noise index.
(4) The saturation power of the fiber Raman amplifier is high, the adjustment method of the gain spectrum is direct and diverse, and the amplification time is short, which can realize the amplification of ultrashort pulse.
Fiber Raman amplifiers also have some disadvantages:
A very high-power pump source is required, which is a relatively strict requirement; another point, that is, it is more sensitive to the polarization state of light, which can be solved by adding a polarized optical coupler;
secondly, the Raman gain is amplified by the Rayleigh of spontaneous emission Restricted by backscattering and double Rayleigh backscattering of the signal, it will cause multi-point reflection and multi-path interference, generate inter-symbol interference, and reduce the signal-to-noise ratio.
( 1 ) Increase system capacity.
When the transmission rate remains unchanged, the system capacity can be improved by increasing the channel multiplexing number. The low-noise characteristics of the distributed optical fiber Raman amplifier can reduce the channel spacing, improve the multiplexing degree of optical fiber transmission, and thus increase the transmission capacity.
(2) Expand spectrum utilization and increase transmission system speed.
The full-band amplification characteristics of the fiber Raman amplifier enable it to work in the entire low-loss region, which greatly improves the spectrum utilization. Distributed optical fiber Raman amplifier is one of the key devices to upgrade the transmission rate of the existing system to 4 0 Gbit/s. It can fill the areas where E DFA cannot be applied.
( 3 ) Increase the transmission distance without repeaters.
The equivalent noise figure of the distributed fiber Raman amplifier is extremely low, 4 . 5 dB, it is mainly used for long-span transmission.
The research on fiber Raman amplifiers is still in its infancy, and some major technical problems need to be solved in order to improve performance.
( 1 ) Selection of pumping source.
Since stimulated Raman scattering requires strong pump power, the pump source becomes the primary problem to be solved by fiber Raman amplifiers. At present, there are three solutions, one is high-power LD and its combination; the other is Raman fiber laser; the third is semiconductor solid-state laser.
( 2 ) Achieve gain flatness.
At present, the gain flattening methods of optical fiber Raman amplifier mainly adopt two types of multi-wavelength pumping and gain equalizer. Broadband and flat gain curves can be obtained by using multiple wavelengths, and the total pump power required is relatively small, but precise design of wavelength intervals and distribution of pump power at different wavelengths are required.
( 3 ) How to suppress the noise of fiber Raman amplifier.
The noise research of optical fiber Raman amplifier is also a hot research topic at present. In addition, reducing crosstalk and improving distributed amplification characteristics are also problems that need to be solved. With the development of optical networks, the use and control of optical fiber Raman amplifiers in optical networks also needs to be further studied.
At present, distributed optical fiber Raman amplifiers are developing rapidly.
Most of the optical amplifiers used in foreign long-distance and ultra-large-capacity dense wavelength division multiplexing optical communication systems (Raman DWDM) are distributed optical fiber Raman amplifiers, which can not only make full use of Optical fiber resources reduce costs, and can reduce the optical density in the gain medium, so as to reduce system performance degradation caused by four-wave mixing and inter-channel crosstalk caused by nonlinear effects.
Regarding edfa and raman amplifier
However, the gain of the Raman amplifier is low (not more than 16dB when used in the actual line), and although the noise figure of the EDFA is not as good as that of the Raman amplifier, the small signal gain can exceed 30dB, so the hybrid amplifier that combines the Raman amplifier with the EDFA It is an ideal form of application.
The 980nm pumped EDFA is used to amplify the C-band, and the 1497nm Raman pump source is used to amplify the L-band. The gain spectrum line has three gain peaks at 1.5~2dB near 1535 (generated by EDFA), 1560 (generated by superposition) and 1600nm (generated by Raman amplification), and two 0dB peaks appear around 1540 and 1560 valley bottom. After adopting GFF, all signal gains are controlled at about 0dB, which realizes the transmission of 80nm bandwidth and 256×10Gbit/s×11000km.
At present, fiber Raman amplifiers have been developed mainly in the many aspects.
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Optical fiber Raman amplifiers use the stimulated Raman scattering effect in optical fibers to achieve optical amplification.
They have the characteristics of wide bandwidth, low noise index, and distributed amplification. They are very suitable for broadband and high-speed optical fiber communication systems.
Especially in the DWDM system, the broadband effect of the fiber Raman amplifier can amplify multiple channels with different wavelengths simultaneously, which makes the fiber Raman amplifier become one of the research hotspots in the field of optical communication.
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Photonic crystal fiber technology developed by HYD TECHNOLOGY in recent years is a highly nonlinear fiber that compensates for the small Raman gain coefficient of ordinary dispersion compensating fiber.