| In the past two decades,mode-locked lasers have been widely used in remote sensing[1],material processing[2],nonlinear optics[3],military and other fields,attracting the attention of a large number of researchers.Mode-locking is a traditional technique for generating ultrashort pulses using active or passive modulation.Mode-locking technology can be divided into passive mode-locking and active mode-locking.Compared with active mode-locking in lasers,passive mode-locking based on saturable absorber has the advantages of compact structure,low cost and simple structure,which has become the mainstream of research.However,the development of passive mode-locking is largely dependent on the development of saturable absorbers,which play an important role in the generation of laser pulses.Since the first successful application of SESAM in 1992[4],the number and types of SAs have grown rapidly.To date,many SAs,such as carbon nano tubes(CNTs)[5,6],graphene[7,8],topological insulators[9,10],transition metal chalcogenides[11],black phosphorus(BP)[12,13],and other similar 2D materials,have been successfully demonstrated in pulsed lasers.However,these SAs suffer from drawbacks that limit their operation.As a result,new materials with lower cost,easier to manufacture and superior performance are still attractive.With the rapid development of nanotechnology,magnetic fluids,a new functional material composed of Fe3O4 nanoparticles,have attracted extensive attention of researchers because of their excellent physical properties.When the size of Fe3O4 particles is gradually reduced to nanometer scale,the ratio of surface area to volume(specific surface area)will increase significantly in inverse proportion to the radius of the particles.This makes the atoms on the surface of Fe3O4 nanoparticles extremely active,resulting in a larger surface effect.And the larger surface effect makes it have larger optical nonlinearity and larger relaxation time.These characteristics of FONPs make it possible for magnetic fluids to be used in lasers to achieve pulse output.In this paper,a new type of magnetic fluid-SA is used to realize continuous mode-locked operation in all-solid-state Nd:YVO4 laser for the first time.The main contents of this paper are as follows:1.In the first chapter,the characteristics and applications of ultra-fast lasers are introduced in detail.The development of ultrafast lasers is summarized from dye mode-locked lasers,semiconductor saturable mirror mode-locked lasers and two-dimensional material mode-locked lasers.The advantages and disadvantages of the above different passive mode locking methods are compared.The development status of pulsed laser based on magnetic fluid is described in detail.2.In chapter 2,the optical properties of magnetic fluid are studied.The principle and process of saturation absorption of magnetic fluid are analyzed in detail.Finally,the magnetic fluid used in the experiment is characterized by linear optical properties and nonlinear optical properties.3.The third chapter focuses on all-solid-state mode-locked laser based on magneto-fluid saturable absorber.Firstly,the theory involved in laser design is analyzed and studied.Then the laser components involved in the experimental research are introduced,which are gain medium,resonant cavity,pump source,magnetic fluid,and the design process and method of each part are introduced in detail.Finally,the influence of different parameters of magneto-fluid on mode-locked laser is studied by comparative experiments.Finally,we first realized the continuous mode-locked laser operation based on magnetic fluid saturable absorber in a self-designed Z-type folded cavity using self-made magnetic fluid samples.The output continuous mode locking pulse width is 463ps.the repetition rate of the pulse is 125MHz.The maximum average power is 856mW and the corresponding peak power is 14.79W. |
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