Research On Key Technology Of Fiber Lasers In Optical Fiber Transmission Systems

Since the end of the twentieth century, with the wide range of application anddevelopment of optical fiber communication systems, ultra-fast optoelectronics,nonlinear optics and optical sensing applications have attracted more and moreattention. The current optical fiber communication systems can not meet the growingdemand for communication capacity. At present, the methods used to improve thecommunication capability of optical fiber communication systems, including theusage of optical time division multiplexing （OTDM）, orthogonal frequency divisionmultiplexing （OFDM） and code division multiple access （CDMA） technologies toimprove the communication capacity and the usage of high-power laser light sourceand optical fiber amplifier to increase the relay distance. Fiber lasers which have theadvantages of absolute ideal beam quality, high conversion efficiency, completelymaintenance-free, high stability and small size have become the emerging field ofoptical communications technology. Fiber lasers can be used in the currentcommunication system to support higher transmission speeds, the high rate densewavelength division multiplexing systems and coherent optical communication andare the basis of optic communication system in the future. In this paper, we will studythe application technology of fiber lasers in optical fiber communication systems.In the present optical fiber communication systems, optical fiber loss anddispersion are the main factors which limit the transmission capacity and distance.Due to the development of optical fiber manufacturing technology, fiber loss isreduced to the degree which is close to the theoretical limit therefore dispersionbecomes a major obstacle to achieve large capacity and ultra-long-haul optical fibercommunication. The role of dispersion in the fiber will result in the broadening of theoptical pulse, and eventually the pulse will vanish completely. The study found thatanother effect in the fiber named nonlinear self-phase modulation will result in thecompression of optical pulse signal. When dispersion and nonlinear self-phasemodulation effect are balanced, a kind of special optical pulse named soliton will begenerated, which can always maintain its shape unchanged during process of thetransmission. The usage of optical soliton in the optical communication system hasthe advantages of high transfer rate, long distance relay, building maintenance and low cost, therefore the solution based optical communication system is widelyconsidered to be the most promising new generation optical communication system.In this paper, we study the key technologies of fiber lasers in optical fibercommunication systems, including the basic principles of fiber lasers in optical fibercommunication systems and transmission experiments, the theory of mode-lockedfiber laser and output soliton characteristics of optical soliton communication system.The main contents and research completed as follows:1. Study the basic theory of the fiber laser, including the emission and absorptionof the laser, pump and gain medium. We analyze the structure of erbium-doped fiberand level structure of the erbium ion. The structure of L-band fiber ring laser isthoroughly analyzed, including the working principle of erbium-doped fiber in L-bandand the basic principles of the L-band fiber ring laser.2. Based on42km optical transmission links, the multi-speed receiving and clockdata recovery were experimented by using the L-band fiber laser. This systememployed NRZ format, LiNbO₃electro-optic modulator, modulation rate from622Mbps to2.7Gbps. Center wavelength of L-band fiber lasers used in theexperiment is1610.28nm, the linewidth is0.1nm, SMAR is more than45dB, andoutput power stability is better than0.02dB. The multi-rate receiving eye-diagram wastested. The stretching of the eye-diagrams was ideal and the eyelid was thin. Thecoding jam and signal aberrance were not detected, and signal-noise ratio was fine.The sensitivity of receiver is about-30.62dBm when BER is10-12, and the overloadpower is-4.1dBm. The factors of influencing system transmission quality wereanalyzed. The problem of the non-synchronization between signal and clock recoveryin high speed modulation was studied.3. We elaborated the transmission characteristics of the optical soliton and derivethe basic transmission equation of the optical soliton in the single-mode fiber. Usingthe equation, we studied the role of the group-velocity dispersion and nonlinearself-phase modulation, which can affect the optical soliton propagation, and then westudied the propagation characteristics of high-order soliton in the single-mode fiber.4. We deeply discussed the classification of the passively mode-locked fiber laserand comparatively analyzed the advantages and disadvantages of differentmode-locking technology. This paper focused on the saturable absorber basedpassively mode-locked erbium-doped fiber laser, established the system model, andsimulated the process of the soliton generation in the mode-locked fiber laser systemsthrough the step-by-step Fourier method （SSFM）.5. Through simulation experiments, we studied the system parameter optimization of the passively mode-locked erbium-doped fiber laser. Currently themost commonly used optical soliton in the communication field is the ground statefirst-order optical soliton. To obtain the first-order optical soliton, it is critical toensure the system parameter N is as close as possible to1, otherwise, when thedeviation between N and1is greater than0.5, the first-order optical soliton can not beformed. Even if the deviation is less than0.5, but the deviation is comparatively toolarge, the pulse energy dispersive wave will be generated, which can result in thepulse energy loss and the instability of the first-order optical soliton. Therefore, thispart of the paper focuses on the parameter optimization of the passively mode-lockederbium-doped fiber laser. The research goal is to make N infinitely close to1byadjusting the system parameters of the mode-locked fiber laser, so that the output lightpulse is as close as possible to the ground state soliton, thus ensuring the efficiencyand stability of the optical soliton communication system.Theoretical research and experimental results have important guidingsignificance for the practical application of fiber laser in optical fiber communicationsystem.