How to use Raman optical amplifier?

With the rapid growth of the demand for communication services, the requirements for the capacity and non-relay transmission distance of the optical fiber transmission system are getting higher and higher. The rate and bandwidth of Dense Wavelength Division Multiplexing (DWDM) communication systems continue to increase, and DWDM systems based on 10Gbit/s or even higher rates will inevitably become the mainstream optical transmission system. Erbium-doped fiber amplifiers (EDFA) have been unable to fully meet the requirements of the development of optical communication systems due to their flat gain and noise limitations. Compared with erbium-doped fiber amplifiers, Raman optical amplifiers have the advantages of larger gain bandwidth, flexible gain spectrum, good temperature stability, and low spontaneous emission noise of the amplifier. Raman optical amplifiers are the only ones that can operate at 1292~1660nm. A device that magnifies the spectrum. Moreover, the Raman scattering effect exists on all types of optical fibers and has good compatibility with various types of optical fiber systems, including various fiber links that have been laid and newly built. Raman optical amplifier and new type of large effective area transmission fiber, high spectral efficiency modulation code pattern, and forward error correction technology are known as the four key technologies of modern large-capacity, long-distance fiber transmission.

Here is the content list:
Classification of Raman optical amplifiers.

Application and progress of Raman optical amplifier.

The current problems.
Classification of Raman optical amplifiers
(1) Distributed Raman optical amplifier

The distributed Raman optical amplifier is based on the fiber stimulated Raman scattering (SRS) effect, and generally adopts the reverse pumping method. The realization method is as follows: inject the high-power continuous-running laser from the output end of the fiber span into the transmission fiber, and the pump The transmission direction of light is opposite to the transmission direction of the signal light. The wavelength of the pump laser is about 100nm shorter than the signal light. The high-power optical field pumps the constituent substances in the fiber to produce virtual excited states; electrons transition from these virtual excited states to the ground state, thereby achieving an optical signal gain. The distributed Raman optical amplifier transmission fiber itself is a gain medium, and the signal is amplified while being transmitted in the fiber so that the equivalent noise index of the Raman optical amplifier is negative. The distributed Raman optical amplifier with a low noise figure can effectively overcome the influence of non-linear effects such as four-wave mixing and improve the optical signal-to-noise ratio (OSNR) of the system.

(2) Discrete Raman optical amplifier

The amplifying medium used in the discrete Raman optical amplifier is usually dispersion compensation fiber or highly nonlinear fiber, such as DCF fiber or tellurium-based fiber. At present, the Raman gain coefficient of DCF fiber is about 10 times higher than that of SMF. As a Raman gain medium, it can also form a dispersion compensation module (DCM). Using tellurium-based fiber, its Raman gain coefficient is 16 times higher than that of quartz fiber, and its peak value reaches 55W/km.

Application and progress of Raman optical amplifier
At present, distributed Raman optical amplifiers are progressing rapidly. Many long-distance, ultra-large-capacity DWDM optical communication systems (DWDM) use optical amplifiers mostly distributed 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, to reduce system performance degradation caused by four-wave mixing and inter-channel crosstalk due to nonlinear effects. However, the gain of the Raman optical amplifier is low (no more than 16dB when used in the actual line), and although the noise index of the EDFA is not as good as the Raman amplifier, the small-signal gain can exceed 30dB, so the Raman optical amplifier and EDFA are combined Hybrid amplifier is an ideal application form. The C-band is amplified by 980nm pumped EDFA, and the 1497nm Raman pump source is responsible for the L-band amplification. The gain spectrum line has three gain peaks near 1535 (generated by EDFA), 1560 (generated by superimposition), and 1600nm (generated by Raman amplification), the size is 1.5~2dB, and there are two 0dB or so near 1540 and 1560. Bottom of the valley. After adopting GFF, the gain of all signals is controlled at about 0dB, thus achieving 80nm bandwidth and 256×10Gbit/s×11000km transmission.

The current problems
In the process of in-depth research on Raman optical amplifiers, the selection and configuration of the pump source, noise control, etc. are all urgent problems to be solved. Among them, the chromatic dispersion characteristics of the optical fiber will cause interference between the front and rear codes in transmission, that is, inter-code interference, which limits the transmission code rate and transmission distance. Given the large dispersion problem of the G652 single-mode fiber laid on the current transmission line, DCF fiber can be used as the dispersion compensation and dispersion slope compensation part of the G652 fiber to form a compensating Raman optical amplifier.

In addition to complex and difficult engineering design, to obtain an ideal gain effect distributed Raman optical amplifiers often use amplifiers exceeding 1W (>30dBm). Therefore, the optical transmission system has high requirements on the quality of the optical fiber connector and the optical fiber joint near the Raman optical amplifier to minimize the side effects of reflection and loss on the Raman gain mechanism. At the same time, to prevent possible damage to engineering maintenance personnel caused by high-energy lasers, automatic optical power shutdown (ALS) and special personnel training are indispensable.

At last
Raman optical amplifiers are widely used in dense wavelength division multiplexing (DWDM) communication systems. If you need, please visit our company’s official website to contact us.

Post time: Dec-06-2021