| Due to the development of optical fiber manufacturing technology,fiber laser has significant advantages in beam quality,manufacturing cost and threshold performance,etc.It is a research hotspot in the fields of photoelectric information,and possess broad research prospects.With the improvement of requirement for laser peak power,modelocked fiber laser,which produces ultrafast pulse,is developing rapidly and have an important impact in the fields of communication,sensing,industry,national defense and so on.In the past,the research of mode-locked fiber laser mainly focused on singlemode fiber.In recent years,multimode fiber lasers have gradually attracted the attention ofresearchers.Compared with the fundamental mode beam output by single-mode fiber laser,the cylindrical vector beam(CVB)and flat-top beam output by multimode fiber laser have unique advantages in laser processing,particle manipulation,laser communication and other application fields.In addition,multimode fiber with large mode field area breaks through the limitation of output power and pulse energy in single-mode fiber,and multimode fiber can support more transverse modes.The interaction between modes makes the nonlinear effect in multimode fiber more complex.In this context,we study the multimode fiber mode-locked laser with all-fiber structure,and obtain the mode-locked pulse of CVB and flat-top beam,which has unique application advantages in the fields of industrial,medicine and scientific research.The influence of nonlinear effect in multi-mode fiber on the transmission characteristics of higher-order mode is studied,which lays the foundation for the development of highpower multimode fiber laser.The main research results are as follows:1.Based on the mode superposition principle,a mode-locked pulse CVB fiber laser with all-polarization-maintaining fiber structure is realized.There is no need to add any polarization control device in the laser,and the all-polarization-maintaining fiber structure has strong anti-interference ability,which has a broad prospect in practical application.2.We simulate the superposition of fundamental mode beam and CVB in multimode fiber to obtain the flat-top beam.Based on this method,an all-polarizationmaintaining mode-locked pulse flat-top beam fiber laser is built.The normalized root mean square of the flat-top beam profile is about 0.069,indicating that the beam has high flatness.Due to the consistent repetition frequency of the output pulse in the same resonator,the superimposed flat-top beam pulses can completely overlap in the time domain,so as to obtain higher single pulse energy.3.For the mode-locked pulse with high peak power,the nonlinear effect of optical fiber becomes non-negligible.In order to study the nonlinear propagation evolution of transverse modes in optical fiber,we implement a beam propagation algorithm based on full vector finite element method.We use this algorithm to simulate the nonlinear propagation evolution of CVB in optical fiber,and prove that the transverse stability of CVB is poor,which leads to the breakup of ring beam field.Besides,we propose an approximate stability theory to analyze the stability of CVB in Kerr nonlinear media.The results of the theoretical analysis are in good agreement with the simulation results.The simulation and theoretical results confirm the generation of CVB spatial optical solitons under the condition of low nonlinearity,which is of guiding significance to the development of high-power CVB fiber laser.4.We build a spatiotemporal mode-locked laser with all fiber structure.When the laser operates in the mode-locked state,the beam quality is significantly improved.Then we amplified the spatiotemporal mode-locked pulse through the multimode fiber amplifier and observed the obvious mode self-cleaning phenomenon.Finally,we obtained the pulsed laser with high power and high beam quality in the all fiber system based on graded-index multimode fiber.All-fiber cavity design has the advantages of high stability,compact structure and low cost.We believe that this is one of the future directions in the field of high power ultrafast lasers based on multimode fiber.The innovation of this paper is as follows:1.Based on the mode superposition principle in polarization maintaining fiber,a mode-locked pulse CVB fiber laser with all-polarization-maintaining fiber structure is realized,which solves the problem of high-order mode coupling and obtains high stability mode-locked CVB laser.2.Using the incoherent superposition of three scalar modes in polarizationmaintaining fiber,the flat-top beam is obtained.Thus,the problem of low peak power of continuous wave operated flat-top beam in optical fiber is solved,and the high stability mode-locked flat-top beam is obtained.3.Based on full vector finite element method,a beam propagation algorithm is implemented to simulate the nonlinear propagation process of CVB,and the approximate stability analysis theory in Kerr nonlinear medium is proposed to discuss the stability of CVB.The highly consistent results of both simulation and theoretical will guide the development of high power CVB fiber lasers.4.A spatiotemporal mode-locked self-cleaning laser with all-fiber structure is realized,and high beam quality mode-locked pulses are obtained in multimode fiber. |
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