Study On Pulse-shaping Mechanism Of Tapered Fiber Integrated Devices And Ultrafast Laser Application

Ultra-fast fiber lasers have the advantages of extremely short pulse width（picosecond or femtosecond-level）,wide wavelength range,easy integration,compact and simple structure.They are widely used in the fields of holographic projection,biomedicine,optical communication,microwave tuning and so on.As a special kind of optical fiber,fiber taper has the characteristics of flexible preparation,miniaturization of devices,high stability,evanescent field strength,all fiber structure that can realize laser cavity,and low insertion loss.In recent years,it has attracted extensive attention of researchers.In this study,using fiber tapers as the starting point,ultra-long-period fiber gratings composed of cascaded fiber tapers and pulse-shaping devices integrated with new two-dimensional material MoTe₂ and fiber tapers are prepared respectively,and their pulse-shaping characteristics and ultrafast laser applications are studied.The main research contents of this paper are as follows:1.According to the theory of nonlinear optics,the nonlinear Schrodinger equation is derived,and the traditional soliton solution with Kelly sideband is obtained.Then,the pulse-shaping mechanism of ultra-long-period fiber grating and MoTe₂-tapered fiber integrated device are analyzed theoretically.2.Seven fiber tapers cascaded ultra-long-period fiber gratings were fabricated by fused biconical taper technique.The pulse-shaping characteristics are studied by the balanced double detector method.It is found that the modulation depth is 1.16%and the saturated light intensity is 400 MW/cm².Then,the ultra-long-period fiber grating is transferred to the ring cavity fiber laser to study its ultra-fast laser application.In the experiment,three,four,five,six and seven wavelength mode-locked pulses are obtained for the first time,of which seven are the largest number of wavelengths so far.The maximum average output power and slope efficiency of these pulses are 8.4 m W and 2.03%,respectively.For three wavelength and four wavelength mode-locked pulses,soliton state pulses are obtained.Furthermore,the dispersion of the laser is controlled by changing the length of the single-mode fiber.Through this study,it is found that ultra-long-period fiber grating has the dual functions of mode locking and filtering.However,it is still difficult to obtain multi-wavelength mode-locked pulses with high average output power and large wavelength numbers（>4）due to the high insertion loss caused by the presence of a mode-locker and a comb filter in the laser cavity.The findings of this experiment solved this problem.3.Few-layer MoTe₂ nanosheets were prepared by chemical vapor deposition,and their morphology,number of layers and structure were characterized.Attaching MoTe₂ nanosheets to the fiber taper to obtain a MoTe₂-tapered fiber integrated device.The saturation absorption characteristics are studied by the balanced double detector method.The modulation depth is4.45%and the saturation light intensity is 40 MW/cm².Then,the MoTe₂-tapered fiber integrated device is inserted into the ring cavity fiber laser to study its application of bound soliton ultrafast laser.In the experiment,the cascaded bound solitons of double pulse,three pulse and four pulse are obtained for the first time.Among them,the pulse interval of two pulse bound solitons is 5.4 ps and the pulse width is 500 fs.