Theoretical Analysis And Experimental Study Of Fiber Lasers Based On Novel Sagnac Loop Structure

The fiber lasers have broad application prospects in the field of optical communication system,optical sensing,laser engraving and medical instrument because of their numerous advantages of compact structure,easily maintainability,small size,high optical conversion efficiency and so on.To solve the problems of the existing fiber lasers based on Sagnac loop structure,this thesis proposes several fiber lasers with the novel Saganc loop structure.The properties of the proposed fiber lasers are comprehensively discussed.The details are summarized as follow:(1)The research background and significance of the fiber lasers are presented at first,then the research status and progress of two fiber lasers with great application value,multi-wavelength fiber lasers and mode-locked fiber lasers,are listed.(2)The Jones matrix theory to analyze the transmission characteristics of the polarization dependent systems is presented.The transmission functions of the Sagnac loop filter and Mach-Zehnder(M-Z)interferometer is derived by adopting Jones matrix theory.The influences of the parameter settings of two filters on the transmission characteristics are studied.Four wave mixing(FWM)effect and polarization hole burning(PHB)effect are studied and analyzed in detail.The transfer function of the nonlinear polarization rotation(NPR)effect is derived by using Jones matrix theory.The simulation results show that the NPR effect can not only be used in the frequency stabilization technology of the multi-wavelength fiber lasers,but also can be used to produce mode-locked pulses.(3)The double Sagnac loop filter based on two segments of the polarzation-maintaining fibers(PMF)is studied.The transmission function of double Sagnac loop filter is analyzed by using Jones matrix theory and numerical simulation method,the simulation results indicate that double Sagnac loop filter have the characteristics of the channel-spcing tunability and polarization independence.Then,the transmission spectra of the double Sagnac loop filter are measured and the measurement results are entirely consistent with the simulation.The interval-adjustable multi-wavelength erbium-doped fiber laser(MWEDFL)is proposed by the combination of double Saganc loop filter and NPR effect.Then,the channel-spacing tunable MWEDFL is achieved successfully by using FWM effect to alleviate the mode competition.The laser output the multi-wavelength lasings with the channel spacing of 0.9 nm or 0.35 nm.(4)The Sagnac loop filter embedded with M-Z interferometer is studied.The transfer function of the Sagnac loop filter embedded with M-Z interferometer is theoretically derived by adopting Jones matrix theory,and the influence of the length of PMF on the transfer function is analyzed.On this basis,the MWEDFL based on the Sagnac loop filter embedded with M-Z interferometer is proposed.In the experiment,the output characteristics of tunable multi-wavelength lasings is comparatively analyzed with different length of PMF.The experimental results show that the multi-wavelength fiber laser have better output characteristics when the length of PMF is set to 0.15 m.(5)The bidirectional fiber laser with switchable generation of mode-locked pulses and multi-wavelength lasings is proposed.The laser is divided into two loops,which is generate multi-wavelength lasings and mode-locked pulses respectively.Multi-wavelength loop(MWL)can generate the tunable single-,dual-wavelength lasing and triple-wavelength lasing with different wavelength interval by adjusting the polarization controllers(PC)and the variable optical attenuator(VOA).Mode-locked loop(MLL)can emit stable square wave pulses and harmonic mode-locked pulses by finely adjusting PC and VOA.The experiment also explores the influences of pump power on pulse width and spectral characteristics.