Near-Infrared Mode-Locked Fiber Lasers Based On Novel Optical Modulator

Mode-locked fiber lasers own the advantages of good heat dissipation,easy to achieve high power output,good beam quality,easy coupling,miniaturization and compact structure,having been widely applied in various fields,such as laser medicine,laser communication,laser processing and laser weapons.Among them,Er-doped fiber lasers for optical communication and Yb-doped fiber lasers for high energy output are the most widely used.At present,many advances have been made in the research and development of new optical modulator and pulse dynamics in ultrafast fiber lasers.In this paper,the research and development of new acoustooptic modulation devices in actively mode-locked lasers,as well as the bound state solitons,harmonic mode-locking,continuous wave and mode-locked soliton coexistence in passively mode-locked lasers are carried out.The main contents are as follows:1.Two kinds of fiber-coupled acousto-optic modulators based on novel molybdenum/tellurate crystals are designed and fabricated,and are applied to fiber lasers.Firstly,a novel all-fiber acousto-optic modulator based on α-phase barium molybdate tellurate（α-BTM）is designed and applied to 1.5 μm and 1.0 μm Q-switched and mode-locked fiber lasers.In order to further improve the performance of the acousto-optic modulator,the titanium tellurate（TTO）crystal with faster sound speed and greater acousto-optic merit is used as the modulator.The acousto-optic modulator based on TTO crystal is applied to the Yb-doped fiber lasers.The actively mode-locked output of 1-18 orders,continuously adjustable repetition rate of 356 kHz to 6.4 MHz,and pulse width less than 8 ns is realized,which provides a new scheme for the construction of the acousto-optic modulation fiber lasers.2.In₂Se₃ saturated absorber based on microfiber is prepared by magnetron sputtering method,and the nonlinear optical parameters at 1.5 μm are measured by I-scan technology.The modulation depth and the saturation intensity of In₂Se₃ saturated absorber is 0.48%and 25.5 MW/cm²,respectively.The output of traditional soliton and bound state soliton is realized by applying In₂Se₃ saturated absorber in Er-doped fiber lasers with large negative dispersion.With the increase of pump power,the number of pulses increases from 23 to 337,which realizes the output of bound state soliton with the largest number of pulses in the fiber lasers based on real saturated absorber.The results show that the modulation depth of the saturated absorber and the dispersion of the resonator are important factors affecting the number of bound pulses.In addition,the internal dynamics of the bound state pulse generation is theoretically studied,which is of great significance for further studying the bound state pulse.3.The harmonic mode-locking operation with the repetition frequency of 581.4 MHz is obtained by using the excellent optical modulation performance of In₂Se₃ saturated absorber at 1.5 μm,which is the highest repetition frequency mode-locking pulse obtained based on In₂Se₃ saturated absorber at present.The Kelly sideband on both sides of the spectrum widens gradually with the increase of pump power,indicating that the dispersion wave and gain loss and recovery work together in the harmonic mode-locking process.In addition,the modelocked state with the coexistence of continuous wave and mode-locked solitons is observed.There are some additional sidebands on both sides of continuous wave spectrum which do not conform to either Kelly sideband rule or modulation instability sideband rule.After theoretical calculation and analysis,it is found that the phase difference of 2kπ appears between the continuous wave and the sideband after one cycle in the cavity,which indicates that the sideband is induced by the soliton and continuous wave.The fiber dispersion is calculated according to the Sellmeier equation,and the theoretical model is modified to reduce the maximum error of the sideband order by one order of magnitude,only 0.076.