Research On Tunable Fiber Lasers Based On Polarization Controlling

Tunable fiber lasers are of important research value due to their extensive applications in optical fiber communication,remote sensing and biomedicine,This dissertation is to carry out research on time-domain tunable square-wave pulse fiber laser operating in dissipative soliton resonance(DSR)region and frequency-domain tunable multi-wavelength fiber laser based on two distinct mechanisms.The energy of generated rectangular temporal pulse increases linearly with the pump power without suffering wave breaking,which is able to break through the energy limitation of conventional soliton.On the other hand,by suppressing mode competition,the multiwavelength fiber lasers are improved in spectral range,fluctuation and stability.In this dissertation,the main contents and achieved results are presented as follows:1.We propose the generation of wide-tunable square pulses in an anomalous dispersion passively mode-locked erbium-doped fiber laser based on dispersion-imbalanced nonlinear optical loop mirror(DI-NOLM)and nonlinear polarization rotation(NPR).Firstly,the advantages and mechanisms of the proposed laser are analyzed.Then the stable mode-locked square pulse with pulse width tunable from 135 ns to 2272 ns is achieved.As the evidence of DSR,the square pulse has the typical characteristics of DSR soliton.In addition,single pulse energy up to 236.8 nJ has been obtained at a maximum pump power of 508 mW.Meanwhile,the maximum pulse width of square-wave pulse is 2272 ns.2.The square pulse fiber lasers based on DSR in anomalous-dispersion and normal-dispersion region are theoretically and experimentally studied.Numerical simulation is conducted to analyze the evolution of square-wave pulse as well as the influences of cavity length and nonlinearity on square-wave pulse in NPR based anomalous dispersion erbium-doped mode-locked fiber laser.The experimental results show that the pulse width is 119.5 ns,which is in accord with simulation results.On other hand,the NPR based normal dispersion ytterbium-doped mode-locked fiber laser is studied experimentally.The 431 ns wide square pulse is obtained.3.A novel ?-cavity multi-wavelength erbium-doped fiber laser based on four wave mixing(FWM)in dispersion flattened high nonlinear fiber(DF-HNLF)is studied.We demonstrate the advantages of the proposed multi-wavelength fiber laser in improving performance of multi-wavelength output spectrum by comparing it with lasers based on different structure and fiber.The channel spacing and 3 dB bandwidth of obtained multi-wavelength output is 12.5 GHz and 11.1 nm,respectively.In comparison with those fiber lasers based on ring-cavity structure and standard HNLF,the spectrum bandwidth of this ?-cavity multi-wavelength erbium-doped fiber with DF-HNLF is 1.79 and 2.85 times as wide as those,respectively.Compared with other results,the performance of the proposed multi-wavelength fiber laser is improved both in spectral range and flatness.4.A novel tunable L-band multi-wavelength fiber laser based on polarization rotation in SOA is studied.The intensity dependent loss is induced through the polarization rotation effect,which effectively alleviates the mode competition and improves the spectrum flatness.The channel spacing of obtained multi-wavelength output is 25 GHz,with the 3 dB spectral coverage reaching 21.2 nm,corresponding to 101 wavelengths.All results show good stability over time,as no obvious peak power fluctuation is observed and wavelength shift is only about 0.01 nm after 90 min with standard deviation below 0.2 dB.The tunability is studied and the advantage of our multi-wavelength fiber laser is verified through the comparison between those lasers based on various structures,polarization and driving currents.