| With the development of information era,the high capacity and high speed in optical time division multiplexing and wavelength division communication are necessary.As a result,the pulsed laser with the wavelength of 1.5 ?m,which is exactly in the communication band,has attracted wide attention in the field of optics.Compared to other lasers,the fiber laser exhibits its unique and magnificent advantage,such as high coupling efficiency,compact and simple structure,good beam quality,and convenient operate system.Besides,the two-demensional(2D)nanomaterials have been extensively investigated in ultrafast photonics for their optical properties,for example,the low insertion loss,simple fabrication technique,large nonlinear coefficient and the broadband saturable absorption band.In this dissertation,the output pulse characteristic in an erbium doped fiber(EDF)laser cavity are investigated based on passively optical modulation method.By designing the laser cavity and selecting suitable saturable absorber,the different type of output pulsed laser are achieved and analyzed.The details are as follows:1.High power Q-switched fiber laser based on black phosphors(BP)has been realized.The method by excapsulating BP with the polymer matrix(polyvinyl alcohol,PVA)is proposed to avoid its oxidation,which is benefitful to enhace its stability.The modulation depth and the saturation power intensity of the BP-PVA film are 8.3%and 7.9 MW/cm2,respectively.By increasing the pump power from 200 to 400 mW,the repetition rate raise from 64.51 to 82.64 kHz,while the pulse duration reduce from 3.39 to 1.36 ?s.The maximum single pulse energy is 148.63 nJ.It is noteworthy that the output spectra have a blue shift with the increase of pump power caused by the high nonlinearity of BP,and the tunable range is about 2.5 nm,from 1567.8 to 1565.3 nm,without a filter inserting.The results indicate BP can work as not only a Q-switcher,but also an artificial birefringence filter with a small bandwidth.2.A tri-wavelength mode-locked erbium-doped fiber laser has been demonstrated with BP saturable absorber,which is fabricated using a simple optical deposition method.The nonlinear optical properties of BP are investigated by Z-scan technique,with the modulation depth and saturation intensity are identified to be 12.4%and 2.16 ?J/cm2,respectively.By properly designing the laser cavity,the multi-wavelength ultrafast fiber laser is successfully realized with a total cavity length as long as 123 m.The emission spectrum has three peaks of 1557.2,1557.7,and 1558.2 nm and the pulse width is 9.41 ps.From the fine structure of the auto-correlation trace,the period of the beat pulses of 16.37 ps is observed,which was related to all the three wavelengths.The results prove that BP is not only a good saturable absorber,but also a frequency stabilizer that can effectively suppress the frequency competition within the EDF.3.A bright-dark pulse pair is observed with a new emerging transition metal dichalcogenids-rhenium disulfide(ReS2).First,the saturable absorbing ability of ReS2 is studied,with a modulation depth and saturation fluence of 6.9%and 27.5 ?J/cm2,respectively.Based on the ReS2 saturable sbsorber,a multi-wavelength bright-dark pulse pair from a mode-locked fiber laser has been observed experimentally.From output spectrum,Three spectral peaks are located at 1573.6,1591.1 and 1592.6 nm,respectively.The RF peaks are separated by 744 Hz and 80 Hz,respectively,for different wavelengths,which is in good agreement with the theoretical value.For further investigation,an external cavity PC and a tunable filter are used to survey the change of the pulse pair.The bright pulse and the dark pulse with different wavebands are separated.The results indicate the saturable absorbing ability of the ReS2 is attributed to the formation of the bright pulses and the dark pulses.While the cross-phase modulation(XPM)caused by different wavebands of bright pulse and dark pulse support the coexisting of the bright-dark pulse pair.4.With the nonlinear polarization rotation(NPR),the traditional soliton pulses are observed in mode-locked EDF laser.The pulse width is fetosecond level,and the output spectrum has obvious symmetric Kelly sidebands,which clearly indicates the pulsed laser is in the soliton regime.Besides,the long-term stability and good repeatability of the mode-locked fiber laser could be used as a pump source for measuring the saturation absorption characteristics of materials.We also theoretically simulate the dynamic evolution process of the soliton pulse.The split-step Fuorier method is used for numerical solution of coupled Ginzberg-landau equations.It is found that the stronger and stronger spectral sidebands are visible with the increase of pump power.The positions of the sidebands are closer to the spectrum peak.Morever,the evolution and mechanism of multisoliton caused by the pulse splitting are also theoretical simulated.5.The nonlinear optical properties of microfibers with different diameters are experimentally studied,and an ultrafast fiber laser using microfiber as saturable absorber has been achieved.By flame-brushing technique,microfibers with the taper waist diameter of 25,38,55,65 and 78 pm are fabricated,and we probe the characterization of structure and morphology.The nonlinear optical absorption of microfiber is measured by I-scan technique.The non-saturable loss and modulation depth both decreased with the increase of the diameter.According to the mode distribution of microfibers with a waist diameter of 25 mm,it could be supposed that the evanescent field effect may be useful for microfibers being saturable absorbers.Based on the 25 mm-diameter microfiber,an all-fiber-structure mode-locked fiber laser and third-order harmonic mode-locked fiber laser are achieved successfully with long term stability,which expand the application of microfibers in ultrafast optics. |
PDF Full Text Request