| Ultrafast fiber laser has been one of the hot topics in laser research because it can compress the laser pulse on time scale simply and effectively,and output the laser pulse with high time resolution,high spatial resolution and high peak power.Intracavity all-fiber nonlinear polarization rotation(NPR)mode-locked Raman laser can generate pulses in the wavelength range that the rare earth ion doped laser cannot reach,and has the advantages of high integration,high stability,no calibration,low maintenance cost,high beam quality,and is an ideal platform for the study of nonlinear effects such as Raman effect,so it is an ideal light source to realize special band ultrafast laser.In this paper,the intracavity all-fiber NPR mode-locked Raman lasers are studied.The detailed research contents and results of this paper are as follows:The pulse transmission equation in fiber and the numerical method of Split-step Fourier Method(SSFM)are overviewed.The mathematical model of NPR mode-locking mechanism is established,its working principle and mode of action are analyzed,and the influence of the interaction between the mode-locking mechanism and laser parameters on the output characteristics of the laser is studied.The generation of dissipative solitons in NPR modelocked fiber lasers is simulated successfully by appropriate system parameters setting.By adjusting the parameters of gain,dispersion and nonlinear effect,their influences on the output characteristics of the laser and the action mechanism are statistically analyzed,which provide certain theoretical reference for the construction of NPR mode-locked fiber laser and the analysis of the output characteristics in experiments.Based on NPR mode-locked Yb-doped intracavity all-fiber Raman laser,the output characteristics are studied and analyzed.With the increase of pump power,the intensity decreasing of first-order Stokes waves and second-order Stokes waves and the narrowing of pulse is found for the first time,and the reason of this unusual evolution is analysed.Based on the unusual evolution,a flat wideband spectrum with a 3-dB linewidth of 48.5 nm and a 10-dB linewidth of 83.45 nm is obtained.The pulse burst phenomenon occurs with the spectrum of Raman Scattering,and the pulse number in the pulse groups is up to 8.The formation process of pulse burst caused by spectral filtering and peak power clamping is explained in the allnormal dispersion(ANDi)fiber laser.It is speculated that the increasing Raman scattering is the main reason for the broken of pulse burst phenomenon under high pump power.Using bend loss,with common commercial single mode Nd-doped fiber(NDF)as the gain medium,0.9 μm NPR mode-locked Nd-doped intracavity all fiber Raman laser is constructed.Using single-circle SMF fiber ring of 10 mm diameter to introduce bending loss to filter out 1.06 μm laser,suppressing gain competition of 1.06 μm strong radiation and 0.9 μm laser in NDF.When the length of SMF is～110 m,Noise like pulses(NLP)with a pulse width of 194.70 fs,repetition frequency of 1.73 MHz and single pulse energy of 2.03 nJ are obtained in the mode-locked Raman laser with first-order Stokes wavelength of 932.59 nm.When the length of SMF is～410 m,the second-order cascade Raman pulses were obtained at the maximum pump power of 391 mW.The central wavelength of first-order Stokes wave is 942.37 nm and the central wavelength of second-order Stokes wave is 983.03 nm. |
PDF Full Text Request