Theoretical Analysis And Experimental Study On Passive Mode-Lock Fiber Lasers

Passive mode-locked fiber lasers are widely used in biomedical applications,metal cutting and ultra-fast detection because of their ability to generate ultra-narrow pulse width,ultra-high energy pulses,compact structure,and no need to add any modulator.Therefore,in this paper,three kinds of implementation mechanism of passively mode-locked fiber laser,nonlinear optical fiber loop mirror(NOLM),the effect of nonlinear polarization rotation(NPR)and real saturable absorber(SA)conducted a series of theoretical analysis and experimental research,in this three kinds of clamping mechanism under stable mode-locked laser output,and design the Sagnac loop comb filter structure,combined with NPR mode-locked fiber laser,multiple wavelength mode-locked fiber laser output is realized.The details are as follows:(1)The research background and application scenario of fiber laser are introduced,and the implementation mechanism,development and application status of passive mode-locked fiber laser in recent years are listed,followed by an overview of the research content of this paper.(2)A passive mode-locked fiber laser based on NOLM is designed.NOLM acts as an equivalent saturable absorber and is composed of an unbalanced coupling(OC)and a segment of single-mode fiber(SMF).Firstly,the mode-locking mechanism of NOLM is theoretically analyzed,and the influence of different coupling coefficients and different SMF lengths in NOLM on NOLM output characteristics is analyzed through MATLAB simulation.Then,the stable output of the mode-locked laser is obtained through experiments,and the factors affecting the output characteristics of the mode-locked laser are explored in combination with simulation analysis.(3)A passive mode-locked fiber laser based on NPR effectis designed,the effects of the first theoretical analysis of the NPR clamping mechanism,and through MATLAB simulation analysis about the role of NPR structure principle.Then,the stable output of mode-locked laser is obtained through experiments,and the relationship between cavity length and output characteristics of mode-locked fiber laser is explored.Finally,Sagnac ring structure is designed as a comb filter.By combining Sagnac comb filter with NPR mode-locked fiber laser,tunable multiwavelength mode-locked laser output is achieved.(4)The characteristics and mode-locking mechanism of real saturable absorber materials are theoretically analyzed,and the coating method of integrating saturable absorber materials with optical fiber devices is introduced in detail.Single wall carbon nanotube(SWCNT)was selected as the saturable absorber.SWCNT material was coated on the end face of the fiber by direct dip coating method under laboratory conditions to make a saturable absorber.Then the real saturable absorber fiber laser was experimentally tested and stable mode-locked laser output was obtained.