| Optical fiber communication based on light waves as the carrier frequency, and a new ways of communication with fiber transmission channel.Optical carrier used in near infrared region of the electromagnetic spectrum, the frequency is very high (1014 ~ 1015Hz), so that the communication capacity is very huge.So from the outset showed unparalleled advantages, has aroused great interest and concern, in just 30 years, it has achieved rapid development. Optical fiber communications,satellite communications and mobile communications field parallel to the three pillars of modern communication. With a variety of data services on the growing bandwidth demand, how to use the existing optical fiber transmission system, and improve the communication capacity to meet the expanding needs of optical communication has become a hot issue.Wavelength division multiplexing (WDM) technology can significantly improve the system transmission capacity, so in recent years, erbium-doped fiber amplifier and wavelength division multiplexing (WDM + EDFA) technology has become a mainstream of a high-speed and high-capacity optical fiber communication systems.But in the long-distance, high-speed WDM systems using cascade EDFA is also facing some problems. For example, the conventional bandwidth of WDM system can not meet the requirements of transmission capacity, network management, such as WDM on the down line, network reconfiguration, equipment upgrades, and protection switch problems caused by gain clamping. It is because in WDM systems, many channels are shared by optical active devices. If a channel on the link has been adjusted, it will affect the other channel power and noise parameters, which requires automatic gain clamped to EDFA. In addition, there are gain flatness problems, because of restrictions of erbium ion level structure, absorption and emission index are not the same in different wavelengths, so that the different wavelengths of signal gain is not exactly the same, which result in rough gain. Therefore, a gain flattening filter (GFF) of the EDFA is used in gain spectrum correction. But in the long-distance high-speed WDM systems, optical fiber on different wavelengths of light loss value is not the same in C-band (1530nm ~ 1560nm).As the wavelength increases, the link loss became lower. Even if a certain flatness of the optical signal is reached from the EDFA, after long distance transmission, the short-wavelength gain will be less than the long-wavelength. Transmit farther, the error higher, and it will accumulate and eventually lead to optical signal distortion and bit error rate increased. To reduce the impact of optical signals in EDFA amplification process, EDFA gain control over the tilt is exactly necessary. Around this problem, a theoretical research is done to identify the factors affecting the gain tilt. And in the laboratory these factors are studied by simulation software, and the results has once again proven the theoretical research. The work provide a variety of reference methods in the production of EDFA gain tilt adjustment, which are used in the long distance, high-speed WDM systems. In this thesis, First, we discussed the background of this topic and its significance, and more comprehensive description of the optical communication in a variety of optical amplifiers is presented. Several major optical amplifier some of that basic principles, types, characteristics and applications are analysed. Second, the development of the existing EDFA history and research at home and abroad were introduced. The EDFA rate equations and propagation equations for more detailed theoretical was studied, highlighting the most widely used Saleh model and Giles model.For EDFA in WDM systems in the presence of the gain flatness problem, we found several factors that affect the gain tilt theoretically: erbium-doped fiber length, input power and pump power. Then, from the experimental and simulated methods, the influence of the EDFA gain tilt factors were in-depth analysis. According to schematic of experiment, the gain changes ware researched and discused, respectively erbium-doped fiber length, input power and pump power, Through the experiment, we obtained EDFA gain spectra and gain tilt. And we got that the shorter erbium fiber length, the lower the gain tilt, even to negative. With the input signal power increases, the gain tilt is almost linearly increased. With the increase of pump power but reduces in gain tilt. Experimental phenomena and theoretical explanations was also got. Finally, OptiSystem simulation what compared with experimental data was done, and verify the dependencies better between them. Our experimental results and related analysis can provide some guidance on the future control of the gain tilt.Finally, the article is summarized, and points out the direction of future studies. The next step could be quantitative research EDF length, input power and pump power on the impact of the gain tilt to find a more suitable material or device structure optimization of EDFA in order to achieve better improvement in EDFA gain tilt. So that we could achieve gain intelligentized control, applicate these improvements in the WDM system, and further improve the communication capacity expansion to meet growing demand. |
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