Research On Tunable Multi-Wavelength Dark-Bright Pulse Pairs Fiber Laser

Passively mode-locked fiber lasers can be used in optical sensing,fiber communication,laser processing,biomedicine and other fields.According to the law of energy distribution,mode-locked fiber lasers can be divided into bright pulses,dark pulses and dark-bright pulse pairs fiber lasers and so on.Compared with bright pulses,dark pulses and dark-bright pulse pairs have many advantages in anti-noise and suppression of nonlinear scattering.Compared with the single-wavelength dark-bright pulse pair,the tunable multi-wavelength dark-bright pulse pairs can improve the application of lasers in optical fiber sensing,optical communications,and wavelength division multiplexing systems,etc.,which has attracted wide attention from researchers in recent years.This dissertation mainly studies the dynamic characteristics of ytterbium-doped fiber lasers with tunable multi-wavelength dark-bright pulse pairs based on tapered fiber.The main research contents are as follows:1.Tunable multi-wavelength bright-dark pulse pairs and dark-bright pulse pairs have been achieved experimentally in an all-normal-dispersion Yb-doped fiber laser with a long cavity.Contribution to the “comb filtering” effect of the tapered fiber,the multi-wavelength bright-dark pulse pairs could be easily obtained by slightly adjusting the pump power and polarization state.Furthermore,the bright-dark pulse pairs could be converted into dark-bright pulse pairs with the unchanged wavelength positions.By using the tapered fiber to improve the gain competition between the generated wavelengths and control the cavity losses,the wavelengths for dual-wavelength pulse pairs and triple-wavelength pulse pairs could be tuned over a range of 14 nm and 4 nm,it should be noted that when the wavelength shift,the different spectral peaks would moving simultaneously,and the wavelength interval almost remain unchanged.The experiment results demonstrated that the tapered fiber was the compact,easy fabrication,low cost,and all-fiber adjustable device,and the tapered fiber can efficiently promote the realization of tunable multi-wavelength bright-dark pulse pairs and dark-bright pulse pairs fiber lasers.2.Tunable multi-wavelength dark-bright pulse pairs have been achieved experimentally in an Yb-doped fiber laser based on the Mach-Zehnder interferometer(MZI).The MZI is developed by means of fused tapered fiber technology,and its filtering characteristics are studied.By adjusting the pump power and the state of the polarization controller in the resonator,stable dual wavelength dark-bright pulse pairs were obtained.The state of the polarization controller in the resonator is fixed and the pump power is changed slowly,we can obtain the second,third,fourth and fifth order dual wavelength dark-bright pulse pairs.By using the comb-filtering effect of the MZI,tunable dual-wavelength and triple-wavelength dark-bright pulse pairs are obtained.The tunable range is 14 nm and 8 nm,respectively,and the wavelength spacing is 4.5 nm.3.The formation process of bright-dark and dark-bright pulse pairs in ytterbium-doped fiber lasers is numerically simulated by solving the Ginzberg Landau equation.The influences of gain fiber length,small signal gain and gain saturated energy on the formation of bright-dark pulse pairs and dark-bright pulse pairs are investigated.The results show that with the increase of the length of the gain fiber,the increase of the small signal gain and the increase of the gain saturation energy,the pulse width of bright pulse and dark pulse will be narrowed,and the peak power will be increased in bright-dark pulse pairs and dark-bright pulse pairs.In addition,the amplification process of the dark-bright pulse pair is investigated.With the decrease of the gain fiber length,the depth of the bright pulse and the dark pulse become greater,the dark-bright pulse pair become narrower.With the length of gain fiber is longer,the depth of bright pulse and dark pulse is smaller,and the width of dark-bright pulse pair is wider.