| With the development of science and technology,fiber laser has gained a large market share in the laser industry due to its advantages of high efficiency,multifunction and low cost of production and maintenance.In particular,mode-locked fiber laser,as a kind of laser,has initially realized artificial intelligence in 2019.In addition,its pulse duration and output power are gradually close to the solid-state laser,so it has been favored by more and more researchers.In the mode-locked laser,there is a kind of output mode called bound state pulse.Recently,scientists have explored the internal mechanism of bound state pulse by using the time stretched dispersion Fourier transform technology.The formation mechanism of low order bound state pulse is gradually clear.However,the formation mechanism of high-order bound state pulse has not been revealed.Therefore,it is of great significance to study the bound state pulse.Two dimensional materials are widely used as saturable absorbers in mode-locked fiber lasers due to their easy integration with fiber,low cost and large worked bandwidth.Among them,graphene,as an important member of two-dimensional materials,is the first two-dimensional material which has been discovered and studied most widely.At present,graphene has been widely used in high-temperature wear-resistant materials,flexible displays,high-performance thermal sensors,etc.However,the application of graphene in the field of electronics is limited by its zero band-gap width.Therefore,people turn their attention to other two-dimensional materials with like to structure of graphene.Tin disulfide nanosheet is an n-type semiconductor two dimensional material with adjustable band-gap.Its proper band-gap characteristic makes it have better optical and electrical properties.However,there are few studies on the interaction between tin disulfide and light.Therefore,it is necessary to study the application of tin disulfide as saturable absorber in fiber laser.In this paper,tin disulfide nanosheets were prepared by liquid phase exfoliation approach.X-ray diffractometer,Ultraviolet visible near-infrared absorption spectrometer,scanning electron microscope,transmission electron microscope,Raman spectrometer and energy dispersive spectroscopy were used to characterize the tin disulfide sample.Secondly,the tapered fiber with low loss was fabricated by melting taper method.The saturable absorber based on tin disulfide was fabricated by optical depositional method.An evanescent field is formed when the laser beam passes through the tapered region of the tapered fiber.The damage threshold of saturable absorber can be increased by using the characteristic that the evanescent field intensity is far less than the internal transmission intensity of optical fiber.In addition,a femtosecond erbium-doped fiber laser based on nonlinear polarization rotation technique is built to test the nonlinear performance of saturable absorber.The center wavelength and pulse width of the output pulse are 1568 nm and 680 fs,respectively.the modulation depth and saturation intensity of the SnS2 SA device are 2.2%and 29.5 MW/cm2,respectively.Finally,when the pump power is 186 MW,the stable mode-locked state is obtained in the Er-doped fiber laser cavity.The central wavelength and 3-dB spectral width of the pulse spectrum are 1562 nm and 2.7 nm,respectively.The pulse width is 1.06 ps after Gaussian fitting.The period of the output pulse is 139 ns.The signal-to-noise ratio is 67.3 dB,which indicating that the output pulse signal is very good.The time bandwidth product of the output pulse is about 0.76,which is close to the chirp transform limit(0.315).It shows that there is less chirp in the cavity.Beyond that we study the evolution process from the conventional soliton to the bound-state pulse,and obtain the 11th bound-state pulse with a spectral modulation period of 1 nm.To sum up,the main work of this paper is to use the optical deposition method to deposit the tin disulfide nanosheets around the tapered fiber to prepare the saturable absorber based on tin disulfide,and apply saturable absorber device to the erbiumdoped fiber laser.This method improves the damage threshold of saturable absorber and has low cost.In addition,we obtain the bound state pulse with adjustable order from the laser.It is a good preparation work for the study of high order bound state pulse. |
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