Research On Mode-locking Mechanism And Technology Of Fiber Lasers Based On Graded-index Multimode Fibers

The rapid development of ultrashort pulse laser technology has continuously promoted the emergence of new materials,new technologies and new industries,which has effectively promoted the development of frontier research fields such as optical communication,optical fiber sensor,biomedicine,optical frequency combing,micromachining and so on,and which provides powerful tool for exploring unknown areas.Compared with traditional solid-state lasers,all-fiber lasers have been rapidly developed due to their unique advantages such as high integration,low cost,and diverse operation modes.As an important research direction in the field of ultrafast,fiber lasers involve many new research fields such as mode-locking technology and the development of new mode-locking materials,and the development of high-energy all-fiber components and so forth.Especially,the emergence of graded-index multimode fiber as an effective saturable absorber overcomes the drawbacks of traditional saturable absorbers in mode-locked fiber lasers,which greatly promotes the process of all-fiber mode-locked fiber lasers,and has potential application value and extensive research significance.However,there are few reports on the gradient-index multimode fiber-mode-locked fiber lasers.There are still many new phenomena and new mechanisms waiting to be explored and explained.In this thesis,we carry out experimental studies on multimode fiber saturable absorber mode-locked fiber lasers in detail.By effectively regulating the dispersion and nonlinear parameters of the fiber laser,diverse characteristics ultrashort pulse all-fiber mode-locked fiber lasers with high stability and high power are realized.The main research content of this dissertation is carried out in the following aspects:1.The transmission characteristic of light in single mode fiber and multimode fiber is explained.The linear dispersion effects and nonlinear effects in the fiber such as self-phase modulation,cross-phase modulation and four-wave mixing are summarized.Especially based on the multimode fiber model given by Poletti and Horak,this thesis carries out a lot of research based on multimode fiber.At the same time,the basic concept of multimode interference is introduced.The basic principle of multimode interference is detailly explained by introducing some numerical results.Finally,a numerical model for the transmission of optical pulses in a multimode fiber is given.2.The various mode-locking phenomena based on the hybrid structure of graded-index multimode fiber and step-index multimode fiber saturable absorber are studied and discussed.In the anomalous dispersion region,we research the effects of multimode fiber length,bending degree and mode field diameter on mode-locking behavior in details to solve the technology problem of low transmission power in fiber laser.In the experiment,the center wavelength of the conventional soliton is 1598 nm,the spectral bandwidth of 3 dB is 3.4 nm,the pulse width is 960 fs,and the average output power is 24 mW,which corresponds to the single pulse energy of 2.44 nJ.It is the first time to achieve soliton pulse energy up to nJ level based on hybrid structure of multimode fiber.3.A stretched pulse fiber laser based on hybrid structure of multimode fiber is proposed and realized.Unlike the conventional solitons in the negative dispersion region and the dissipative solitons in the normal dispersion,the stretched pulse spectrum of the zero dispersion region exhibits a smooth Gaussian profile.When the length of graded-index multimode fiber is chose to be 30 cm,the center wavelength of mode-locked pulse is 1595 nm,the pulse width is 550 fs,and 3 dB spectral width is 8.02 nm.It is found that in the same oscillator,the operating state of the mode-locked fiber laser can be changed from the conventional soliton mode-locking to the stable stretched pulse mode-locking state by simply increasing the pump power or changing polarization controllers.When the length of graded-index multimode fiber is 20 cm,the center wavelength of the pulse is 1603 nm,3 dB spectral width is 14.2 nm,and the pulse width is 310 fs.310 fs pulse duration is the shortest pulse obtained in an erbium-doped fiber laser based on a hybrid structure of multimode fiber saturable absorber so far.By increasing the pump power,the mode-locked fiber laser evolves from conventional soliton to stable stretched pulse operation.The results show that the transition between conventional soliton and stretched pulse is attributed to the change of nonlinear effect and dispersion induced by the graded-index multimode fiber when the pump power is increased.4.We systematically study the generation,interaction of bound state solitons in mode-locked fiber based on hydrid structure of multimode fiber saturable absorber.In the design of erbium-doped fiber oscillator,by optimizing the nonlinear parameters and the dispersion in the laser cavity,stable bound state conventional solitons are generated for the first time based on the nonlinear multimode interference principle of the graded-index multimode fiber.The modulation depth of anti-phase bound state solitons is 25 dB,the spectrum modulation period is 2.08 nm with the pulse separation of 4.93 ps.The modulation depth of in-phase bound state solitons is 27 dB,the spectrum modulation period is 1.66 nm.When the pump power is increased slightly,the stable bound state soliton mode-locking operation evolves into the conventional soliton operation.This means that the nonlinear effects and dispersion effects in the cavity will break the balance of interaction between the stable bound states solitons with increasing the incident pump power.Analysis on the results shows that the formation of bound states solitons stems from the direct interaction between solitons.The formation of stable bound state solitons further validates the application of multimode fiber as a bandpass filter in fiber lasers.5.The generation of pulses in Ybium-doped fiber lasers in normal dispersion regions is studied by using hybrid structure multimode fiber mode-locking technique.For the first time,Q-switching,Q-switched mode-locking and mode-locking operation are achieved in the same oscilator.As the pump power increases,the repetition frequency of the Q-switched pulse output varies from 52.288 kHz to 72.88 kHz,and the corresponding single pulse width is decreased from 4.61 ?s to 3.937 ?s.The maximum average output power and single pulse energy are 3.07 mW and 42.1 nJ,respectively.The center wavelength of the mode-locked fiber laser is 1064 nm,the maximum spectral width is 6.4081 nm,and corresponding pulse width is 78.1 ps.