Raman fiber amplifier in the application of long-distance network – fiber amplif
Raman fiber Amplifiers The working principle is based on the quartz fiber of stimulated Raman scattering (SRS) effect, manifested in the form of light is pumped Raman gain bandwidth of the weak signal and strong pump light transmission in optical fiber at the same time, so that Weak signals are amplified light. Compared with erbium-doped fiber amplifier, Raman fiber amplifier significantly different: a, in theory, as long as the appropriate Raman pump source, it can be any wavelength within the optical window to amplify the signal. 2, we can use optical fiber as the gain medium itself, and has a very broad gain spectrum and lower equivalent noise index. 3, can be adjusted by each pump power to dynamically adjust the signal gain flatness. Raman fiber amplifier is a future high-speed, high-capacity optical fiber communication systems one of the key technologies.
Raman fiber amplifier in the SDH system is mainly for long span optical fiber transmission systems, such as cross-sea communications, terrestrial long haul transmission. In these cases, the distributed Raman fiber amplifier used as a pre-fiber amplifier, with its low noise characteristics, improve the system's overall receiver sensitivity, thus extending the transmission distance or improve the system design margin. As the special nature of the distributed Raman fiber amplifier, we must consider the safety, reliability, reasonable. View from the requirements of actual use, should meet these requirements: 1, with signal power Monitor And no light automatic shutdown function; 2, the pump output power stability, maximize the use of hardware circuits dead Cycle Control of constant power output mode; 3, in the signal wavelength has enough effective gain. 4, should fully consider the actual Cable Attenuation coefficient and the cross-section of attenuation.
Long span of SDH optical transmission system plan
1, long-span optical fiber transmission issues to be considered
For 2.5Gbit / s long span optical fiber transmission systems, engineering design, the general need to consider the following a few basic areas:
A, limited availability of optical fiber attenuation problem? Combined power fiber amplifier, line optical amplifier, pre-fiber amplifier, optical transmission line can be solved generally limited loss problem.
B, with or without nonlinear problems, in particular, stimulated Brillouin scattering (SBS) of the problem? Using optical amplifiers, if the signal is very narrow spectrum of light (such as the use of EA modulation optical transmitter module), stimulated Brillouin scattering (SBS) is easy to produce, limiting the entry of optical fiber optical power amplifier (such as 13dBm) in the transmission fiber certain circumstances, can only use low-frequency disturbances imposed on the sending laser, spectral width broadening signal suppression methods.
C, with or without dispersion limited problem? Depends on fiber dispersion and limited Launch Signal itself, in some cases optical fiber through the use of different modules to avoid dispersion of emission limits.
D, with or without noise-constrained problem? Optical transmission line noise sources: the quality of emitted light signal (SNR, extinction ratio, spectral width, etc.), optical amplifier noise (noise figure), fiber nonlinear effects caused by noise, the receiver characteristics of the system itself ( sensitivity, power bandwidth, noise, etc.). As the pre-optical amplifier input signal power is small, the introduction of the signal band and the band noise power is obvious, reflected into the optical receiver before the signal to noise ratio (SNR) of the decline in order to improve the SNR, in the pre-fiber fit within the integrated amplifier bandwidth optical filter is necessary.
2, long-span optical fiber transmission line according to the actual program, we put forward the following programs, applications of Raman amplifiers to achieve long-span SDH optical transport, use this program instead of pre-pre-Raman fiber amplifier EDFA .
3, the improvement of receiver sensitivity
In this scenario, we have resolved the SBS and dispersion problems, without considering their impact, optical fiber amplifier into the power of 19dBm. As the optical signal itself, and after sending optical amplifier, the signal's optical signal to noise ratio is sufficiently high (better than 30dB), pre-fiber amplifier to become a major part of the introduction of the noise, the noise from the main signal and stimulated emission (ASE) between The shot noise, preamplifier noise levels can be built by the noise index and jointly determine the bandwidth of optical filters. Consistent optical filter, amplifier gain enough former case, the pre-optical amplifier noise figure directly determine the system's receiver sensitivity (ie, into the front end optical fiber amplifier of optical power value).
The current front-doped fiber amplifier noise figure, the average level of 4.5dB, while the Raman fiber amplifier noise figure, the average level of 0dB. Thus, the visual can be roughly determined using the Raman fiber amplifier, fiber amplifier as a pre-reception system and the use of pre-EDFA optical amplifier system as compared to the system can be improved receiver sensitivity of 4.5dB.
RFA- PA Have good results, simply because of the noise index RFA-PA EDFA-PA than the much lower noise figure. Actual system is the choice for pre-EDFA optical amplifier or RFA depends on the actual line is the signal power (gain) on the sensitivity of large or optical amplifier noise characteristics of a great impact on the sensitivity? This is the level of the receiver and the specific use of environment-related gain of the amplifier.
4, signal power detection and safety control
Distributed Raman fiber amplifier pump power into the fiber may be more than 500mW, need to take the necessary safety measures. In addition to ensuring clean fiber connector end face outside the Raman amplifier itself should be able to detect signal power to ensure that no light in the fiber-optic cables or broken, the automatic shut down pump lasers, signal recovery can work. Here, we repeatedly emphasized the detection of signal power, not including the total power noise power. This is because the existence of a distributed amplifier noise from the stimulated emission, noise level increased with the increase of gain, how to accurately detect background noise signal power is a technical problem, is also a factor affecting the application. Our approach is in line with fixed wavelength transmitter optical signal transmission, Raman amplifiers by detecting the signal wavelength of two different fixed bandwidth the total power, signal power to calculate the actual value.
This way, remove the signal at the bottom of the impact of noise power, field application, we can detect the-45dB
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