Study On Erbium-doped Fiber Lasers In Optical Communication Systems

Fiber lasers have a lot of outstanding merits and characteristics, such as good compatibility with fiber components and fiber systems; wide bandwidth; better beam quality and lower noise than semiconductor lasers; in addition, compact structure, portability, and so on. Consequently, there has been considerable interest in developing fiber lasers for optical communication, fiber sensors, spectroscopy, etc. And nowdays fiber lasers have become hot point over the world. In this dissertation, several key technologies of the fiber lasers have been studied theoretically and experimentally, such as continuous wave fiber laser, pulse wave fiber laser. The main contents of this dissertation are as follows:The multimode semiclassical Lamb method is applied to the analysis of injection-locked fiber ring lasers. Under weak signal limit, the problem of single-mode operation for homogeneously broadened fiber ring lasers is discussed, and some key parameters, which determine the performance of injection-locking, such as the locking bandwidth, the linewidth of output laser, and output power are analyzed and simulated respectively. Whereafter, the injection-locking for Erbium-doped fiber ring lasers (EDFRLs) is studied experimentally, and results show that the influence of the injected field on the cavity modes has a relation with some parameters such as detuning, pump power, injected intensity, etc. Besides, it is found that the linewidth broadening due to injection can be minimized if the parameters of the ring cavity and the injected power are chosen properly, thus linewidth narrowing can be realized.A single-longitudinal-mode (SLM) erbium-doped fiber laser with multiple ring cavities (MRC) is proposed and experimentally demonstrated. The EDFL is structured by inserting three different short passive subring cavities, which serve as mode filters, into the main cavity, and it is combined with a fiber Bragg grating(FBG), the MRC resonator make single-longitudinal-mode lasing. The resonant theory of multiple-ring cavities for SLM operation is discussed. And the experimental result show that a short-term linewidth of less than 500Hz is realized.The wavelength tuning and output stability of single-longitudinal-mode EDFRL are studied theoretically and experimentally. Firstly the output power characteristics of a tunable EDFRL are analyzed. The effects of total intracavity loss, erbium ion clustering, output coupling ratio, and active fiber length are investigated. The experimental results indicate that minimization of the intracavity lass and erbium ion clustering as well as optimization of the active fiber length are very important. After that, a novel method to stabilize the wavelength and output power of tunable single-longitudinal-mode EDFRL with compound cavity structure is proposed and experimentally demonstrated. One of the longitudinal modes is stimulated by the injected continuous wave(CW), So that this mode is able to win the competition to stabilize the system. The output power of >0.6dBm and the SNR of >43dB within the tuning range from 1527.4 to 1561.9nm can be achieved for this laser ,the wavelength variation of less than 0.01nm and the power fluctuation of less than 0.02dB, and linewidth of about 1.4kHz have also been obtained.Using the frequency domain analysis theory and the time domain analysis theory of the active mode-locked homogeneous lasers, we study the mechanism of the rational harmonic mode-locking fiber ring laser (RHML-FRL) pulse generation in detail. And the simulated results are in good accord with the experiments. The experimental results indicate that RHML not only can take place when the modulation frequency of the modulator f_m= (n+1/p)f_c but also can happen when f_m= (n+m/p)f_c, where m is unequal to 1 but an integer less than p. We call it arbitrary numerator RHML. In addition, a simple technique for the pulse-amplitude equalization of high repetition rate pulses generated from RHML-FRL is demonstrated through both simulation and experiment. Its principle is based on nonlinear modulation. By properly adjusting the bias level and modulation depth of modulator, amplitude equalized short pulses have been obtained.