Research On The Output Characteristics Of A New Type Of Passively Mode-locked Fiber Laser

Passively mode-locked all-fiber laser based on nonlinear multimode interference effect of graded refractive index multimode fiber is a new type of passive mode-locked fiber laser.As a mode-locking device,the saturable absorber(SA)is composed of single-mode fiber(SMF)and graded refractive index multimode fiber(GIMF)(for example,SMF-GIMF-SMF).By bending or stretching the SA to a certain state,a mode-locked pulse laser with a pulse width of hundreds of femtoseconds can be obtained.This saturable absorber device has good generality and simplicity.When combined with a large mode area double-clad fiber,the possibility of acquiring high pulse energy laser makes this SA becoming highly attractive in ultra-fast photonics.The thesis mainly includes the following three parts:1.The development,characteristics and classification of the fiber laser are reviewed,the function of the saturable absorber in the mode-locked fiber laser is explained,the working principle of the structure-type saturable absorber based on the GIMF and the saturable absorption characteristic are introduced.The principle of the output of the controlled-bound solitons,tunable and double-wavelength interference solitons in a new passive mode-locked pulse laser is presented.2.By adding a small band of coreless fiber(NCF)between GIMF and SMF,a stronger nonlinear effect is excited,and a stable soliton molecular output is obtained in the laser.The experimental results show that the bound state belongs to the bound soliton pairs with phase difference of ?.By simply stretching the SA structure,the tunable tightly soliton pairs with continuous variation of central wavelength in the range of 1567.48 ? 1576.20 nm are obtained.At the same time,the loosely soliton pairs are obtained by adjusting the intracavity birefringence.The tunable time distances between the solitons in the loosely soliton pairs are realized by stretching the SA,and its variation characteristics are also studied.The experimental results deepen the understanding of solitons on bound states and have certain reference value for the research of large capacity optical fiber communication.3.By the introduction of loss controller into the resonator,the wide band tunable wavelength and coherent dual wavelength mode-locked pulses are realized.For the single wavelength state,the intracavity loss is changed by adjusting the variable optical Attenuator(VOA),and the C&L band continuous tunable wavelength mode-locked pulses with a wavelength tuning range of 46.7 nm,a stable bandwidth of about 5 nm,and a pulse width of 850 fs are obtained.For the dual wavelength state,the two solitons show rare time coherence at different wavelengths.By increasing the intracavity loss,the spectral spacing of dual wavelengths can be tuned from 11 nm to 33.01 nm while maintaining coherence.This kind of time coherent soliton has high application potential in the study of dual frequency comb and multicolor pulse in nonlinear microscope.