Generation Of All-fiber Ultrashort Pulse And Vector Beam Based On Multimode Fiber

In recent years,fiber laser has broad application prospects in scientific research and industrial fields owing to its compact structure,good heat dissipation,superior stability,and low cost.Among them,high-power ultrashort pulse fiber laser and vortex beam fiber laser are the hot research directions for the majority of researchers.Chirped pulse amplification(CPA)is usually used to realize high-power ultrashort pulse,which not only avoids nonlinear distortion and optical damage of pulse,but also output high-power,ultrashort pulse with high beam quality.This technology greatly promotes the growth of the output power of fiber laser.CPA system mainly includes seed laser,pulse stretcher,optical amplifier,and pulse compressor.Among them,the seed laser is usually a mode-locked laser pulse,and the realization of mode-locked technology usually requires the use of saturable absorber(SA).The reliability of SA plays a decisive role in the stability of laser operation.Therefore,it is very important to find a stable,cheap and easy to make SA material.Based on the principle of multimode interference,the structure of single mode fiber-multimode graded index fiber-single mode fiber(SMS),as a SA,has the advantages of high stability,low cost,and compactness.However,in SMS structure,the length of multimode graded fiber needs to be strictly controlled to meet the interference conditions,and its length limit accuracy is in the order of tens of microns,which is difficult to achieve in the actual process.Therefore,if we want to use SMS structure as a saturable absorber,it need to solve the length limitation of multimode fiber.The main difference between the vector beam and the traditional beam is that the vector beam has an isolated singular field,and the transverse field is hollow.According to the types of singularities it carries,those carrying polarization singularities are called cylindrical vector beams(Cylindrical Vector Beams,CVBs),and those carrying phase singularities are called orbital angular momentum(Orbital Angular Momentum,OAM).Due to its unique polarization and phase characteristics,vortex beams are widely used in optical tweezers,surface plasma excitation,ultra-high resolution imaging,optical communication,and other fields.Generally,the vortex beam is generated and transmitted in few-mode fiber.However,the vortex mode cannot propagate stably in a common fiber owing to severe mode crosstalk.How to produce stable,compact,high efficiency,high purity vortex beam is a very worthy of exploration.This paper first introduces the application of ultrashort pulse laser,the principle of mode-locked technology to produce an ultrashort pulse,the principle of SMS structure as a SA,the manufacturing process of SMS structure,and the principle of CPA technology.Based on the numerical simulation results,the length limit of multimode fiber is solved by offset-splicing.Secondly,it introduces the application of vector beam fiber laser,the method of generating vortex beam,the fiber coupling theory and the mode distribution in the Polarization-maintaining fiber(PMF).Based on this,the PMF is proposed to solve the mode crosstalk problem,and the stable and efficient vector beam laser is realized.The main contents and results of the thesis are as follows:1.Based on the principle of multimode interference,the saturable absorption effect is realized in the single-mode fibr-multimode fiber-single-mode fiber(SMS)structure,and a method is proposed to solve the length limitation of the multimode fiber.We effectively solve the problem of strict length limit of multimode fiber by using two different core diameter multimode fiber with offset splicing.And the modulation depth of the SA can be adjusted by changing the length of offset-splicing the SA between two multimode fibers.Based on this scheme,a S A with a modulation depth of 15.28%is fabricated,and a mode-locked all-fiber laser is built.2.A mode locked fiber laser by both nonlinear polarization rotation effects and nonlinear multimode interference is constructed.The hybrid mode locked mechanism can effectively improve the stability of the laser and compress the pulse width.3.A mode-locked square pulse fiber laser based on the inverse saturable absorption effect of multimode interference is studied.By adjusting the polarization controller,the square-shaped and h-shaped pulse can be generated.The chirp characteristics of these two pulses are investigated by using the simple chirp measurement device proposed by our research group.The laser can output square and h-shaped pulses with a maximum pulse energy of 0.14 μJ and 23.8 nJ,respectively.4.Based on the CPA technology,using chirped fiber grating and blazed grating as pulse stretcher and pulse compressor respectively,a high-power ultrashort pulse laser system with all-fiber structure is realized,which can output an average of 4.7 W and a pulse-width of 1.6 ps.5.An all polarization-maintaining OAM mode fiber laser is built,which can effectively solve the problem of mode crosstalk.At the same time,it has the characteristics of debugging free and high stability,which can output 83 mW average power,93.6%purity and ± 1 order OAM beam.6.Based on the mode superposition principle,a TM01 mode with polarization-maintaining structure fiber laser is built.The laser has the advantages of debugging free,high stability and effective reduction of mode crosstalk.It can output average power of 19.20 mW and a purity of 91.8%TM01 mode.The innovative work in this thesis are as follows:1.The length limitation of multimode fiber in SMS structure is solved by using the splicing offset of multimode fiber,and the modulation depth of saturable absorber can be effectively adjusted by adjusting the offset length.2.A multi-shape mode-locked pulse laser are generated based on reverse saturable effect in multimode interference,and the chirp characteristics of pulse are measured respectively.3.A hybrid mode-locked laser based on nonlinear polarization rotation and multi-mode interference effect is designed,which can effectively improve the stability of the laser and the width of the output pulse.4.A vector beam laser with polarization-maintaining fiber structure based on the principle of mode superposition is designed,which can output vector beam with high purity,high stability.