Research On Soliton States Of Thulium-doped Mode-locked Laser Based On The Nonlinear Multimode Interference Effect

Benefiting from ultrafast temporal resolution,as well as high peak power,passively mode-locked fiber lasers have exhibited great potential applications at fundamental sciences,industrial and military.As nonlinear system containing complex interactions from gain,loss,nonlinearity,dispersion,etc.,ultrafast fiber lasers deliver not only conventional solitons but also bound solitons and dissipative solitons.The research of soliton states in fiber lasers is of vital importance for in-depth understanding of the nonlinear interaction mechanism and further exploration for high-capacity fiber-optic communications.Compared with the traditional saturable absorbers,such as semiconductor saturable absorber mirrors(SESAMs),grapheme,single-walled carbon nanotubes(SWCNTs),and the two dimensional(2D)nanomaterials,the saturable absorber based on the nonlinear multimode interference effect of multimode fiber has important research value owing to the advantages of all-fiber structure,convenient preparation and high damage threshold.In our research work,we report a passively thulium-doped mode-locked fiber laser based on a stretched GIMF SA,which can realize the output of conventional solitons,bound solitons as well as dissipative solitons.The main works are as follows:1、The nonlinear optical properties of saturable absorber based on single mode fiber-graded-index multimode fiber-single mode fiber structure are studied.The saturable absorber has been prepared by using a length of 15.05 cm graded-index multimode fiber.The modulation depth,non-saturable loss and the saturation intensity of the SA have been measured to be 23%,16.14%,64.89μJ/cm~2,respectively.2、By using a home-made graded index multimode fiber SA,a passively thulium-doped mode-locked fiber laser was built.The stable output of traditional solitons were realized with a central wavelength of 1960.2 nm.The 3 d B bandwidth of optical spectrum,the full width at half-maximum(FWHM)of pulses,the fundamental frequency and the signal-to-noise ratio(SNR)were 3.55 nm,1.756 ps,19.97 MHz and exceeding 65 d B,respectivley.At the same time,traditional single solitons are stable with continuously tunable wavelength from 1950 nm to 1965.3 nm.3、Apart from the output of the traditional solitons,the experiment of bound solitons have also been studied.The stable bound soliton has been realized with spectral period of 2.74 nm,central wavelength of 1959.6 nm,pulse interval of 4.36 ps and signal-to-noise ratio(SNR)of 66 d B.The wavelength of the bound soliton is tunable in the range of 1959.5 nm to 1962.2 nm.In addition,the output of bound soliton pulses with different frequency spectrum modulation periods has been realized by carefully adjusting the PC.4、Based on the previous research,a section of 50m dispersion-compensating fiber has been introduced into the fiber laser to compensate for negative dispersion in cavity.Then the total dispersion value of the laser cavity was to be 4.2 ps~2.The experimental exploration of dissipative solitons of thulium-doped mode-locked fiber laser has been carried out.The output of dissipative solitons with spectral bandwidth of 2.4 nm,wavelength range from 1901.2 nm to 1903.6 nm and pulse duration of 1.7ps have been realized.