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Study On Single Longitudinal-mode Er3+/Yb3+ Co-doped Fiber Laser

Posted on:2009-12-06Degree:MasterType:Thesis
Country:ChinaCandidate:M J JiangFull Text:PDF
GTID:2178360242481659Subject:Communication and Information System
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With the development of optical communication technology, Light source in optical communication system have to meet the increasing requirements, optical fiber lasers with significant advantages has received great attention from the world. In order to increase the bit rate and make full use of 1.55μm wave band in WDM fiber optical communications system, the study on lasers of accurate wavelength, narrow linewidth, and single longitudinal mode has always been the subject of close attention to the people.The nature of EYDF makes it be the ideal gain medium of fiber laser. And the ring-cavity fiber laser with a closed waveguide structure, has resistance to electromagnetic interference, high stability, small size and produced easily, so it has high practical value. Therefore, ring-cavity Er3+/Yb3+ co-doped fiber lasers will be a strong competitor to semiconductor lasers even Er3+-doped fiber lasers(EDFL) in the applications where the wavelength selection and wavelength stability is demanding, (such as DWDM systems, optical fiber, coaxial cable systems, interference measurements and in some other areas such as spectroscopy). At present, there have been few domestic researches on ring-cavity Er3+/Yb3+ co-doped fiber laser. Based on this background, the ring-cavity Er3+/Yb3+ co-doped fiber laser has been studied in this thesis.First, the paper has introduced the development history and the prospects of fiber lasers, then the structure, characteristics and classification of it has been presented. There were detailed description of the advantages of fiber lasers compared with the semiconductor lasers, its applications and development in fiber-optic communication systems has also been narrated.Second, from the three elements of fiber lasers which are gain medium, resonator cavity and the pump source, the paper focused on the emitting principle of Er3+-doped fiber, as well as the effect pair-induced quenching and homogenous cooperative up-conversion on the performance of EDFL. Analysis of the EYDF, as well as a wide cross-section of energy absorption, and inhibition principle of self-pulsation has been given in this paper. It has also presented the energy level diagram and the rate equation of EYDF. Through the choice of the resonant cavity and pump source, the establishment of a ring-cavity EYDFL model was completed, and using software to make simulation of a simple model in order to provide theoretical foundation of the overall structure of the laser and verify its legitimacy.Finally, on the basis of the theoretical design, the necessary equipment, apparatus, and all kinds of components for the experiment were actively prepared and set up. Having been used of isolators, polarization, circulator, saturable absorber, the optimization of ring resonator structure has been accomplished. The experiment structure of ring-cavity Er3+/ Yb3+ co-doped fiber laser was established.The main technological indexes of laser output were measured for giving the overall appraisal of the ring-cavity Er3+/ Yb3+ co-doped fiber laser.The ring-cavity EYDFL put forward in this thesis used 1064nm Nd:YAG solid State Laser as its pump, and the gain medium was a 4.1m-long EYDF, the concentration ratio of Er3+/Yb3 + is 45:1. The single longitudinal-mode operation is realized by introducing a fiber Bragg grating (FBG) with peak reflectivity being 99% as narrow wavelength-selective element, and a section of EDF not-pumped as saturable absorber to produce standing-wave saturation effects in the cavity. The effect for adding isolators is to inhibit the transmission of reverse laser, and the polarimeter is for choosing one between two orthogonal polarization modes, through those the fiber laser resonator has been optimized, and the stable laser output with single longitudinal- mode was got .In the simulation and experimental study, the difference of fiber laser output power using output coupler in different coupling ratio has been studied. The optimal coupling ratio testified in experiment was near 80 percents, in this condition, the EYDF was in the highest conversion efficiency and the laser in the maximum output power. Through extensive collection of data, the difference between simulation and experiment, and the reasons for the existence of coupling ratio was analyzed.The ASE spectrum of 4.1m-long EYDF has been measured in the experiments, the central wavelength is 1535.04nm, 3dB linewidth is 2.24nm. The peak wavelength of fiber laser chosen by FBG is 1544.82 nm; side mode suppression ratio is greater than 50 dB; 3dB linewidth is 0.08 nm, 25dB linewidth is 0.2 nm; the maximal output power is 8.5mW and the efficiency is 4%; the degree of instability is 1.42% when the input pump power is 120mW; a more stable single longitudinal- mode is observed, noise of laser output is less than -100 dBm.A fiber-coupling scanning Fabry–Perot (F-P) interferometer was used to measures fiber laser longitudinal-mode at 1.55μm. Overall length of the laser cavity is 20m and the laser longitudinal-mode spacing is 10MHz. The single longitudinal-modem was observed by the interferometer with free spectrum range (FSR) of 7.5GHz and 1.5GHz. The longitudinal-mode fabric is analyzed by the interferometer with FSR of 1.5GHz. The linewidth of longitudinal-mode was measured as 30MHz. The experiment approves the single longitudinal-mode of fiber laser precisely. Simultaneity, the interference effect arising from the laser oscillation in the saturable absorber was experimented. The influence of different lengths, concentration of saturable absorber on characteristics of output laser mode is analyzed.The thesis explored the structure design the ring-cavity EYDFL, optimized cavity parameters, improving the output performance. The ring-cavity EYDFL developed can emit single-mode laser, with accurate central wavelength, low noise, and the laser couples with optical system efficiently, small size, low cost. These researches provide the evidence for the further study and the popularization of the ring-cavity EYDFL.
Keywords/Search Tags:fiber laser, Er3+/Yb3+ co-doped, ring-cavity, single longitudinal-mode, saturable absorber
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