Research On Techniques Of 1.5?m Band Tunable Mode-locked Fiber Laser

Fiber lasers are widely used in precision machining,defense weapons,fiber optic sensing,medical treatment,and lidar because of their advantages such as good beam quality,excellent heat dissipation,no need for collimation,compact structure,and high light-to-light conversion efficiency.As a branch research field of fiber laser-tunable ultra-short pulse fiber laser.The tunable ultra-short pulse fiber laser light source can be applied to many fields such as high-speed wavelength division multiplexing / optical time division multiplexing(DWDM / OTDM)communication systems,fiber optic sensing,optical coherence tomography light sources,and metrological testing.In recent years,wavelength-tunable passive mode-locked fiber lasers have become research hotspots due to their advantages such as high output beam quality,good heat dissipation performance,high conversion efficiency,compact structure,and strong reliability.The realization of tunable ultra-short pulse fiber lasers is mainly divided into two parts: wavelength tunable method and ultra-short pulse generation method.Commonly used to achieve tunable methods are the use of wavelength selection devices such as gratings;the use of nonlinear effects to change the polarization state,fiber birefringence;external modulation devices.In addition,the main method to achieve ultra-short pulse laser is passive mode locking.Passive mode-locking can be realized by saturable absorber mode and artificial saturable mode.The saturable mode can use the non-linear transmittance of optical materials to achieve mode-locked pulse output.Low and complex to manufacture,saturable absorber-like mode-locking is widely used.Commonly used saturable absorber mode-locking methods include non-linear polarization rotation and non-linear amplification ring mirrors.This paper focuses on the pulse generation method and pulse tunable method in all-fiber tunable ultra-short pulse fiber lasers.The main research contents include:(1)Summarize the traditional ways of mode-locking mode and tuning,and introduce a new mode-locking method: multimode interference mode-locking.This mothod is simple and convenient in construction.It is only necessary to fuse a short piece of graded-index multimode fiber in a single-mode fiber laser,and use the mode interference effect of single-mode multimode single-mode(SMS)structure to achieve saturable absorption mechanism.And the advantages and disadvantages of the reported mode interference mode-locked lasers are analyzed and summarized.(2)In order to realize the mode-locking of the SMS structure,it is necessary to precisely control the length.We propose to coil the SMS structure into the polarization controller.By theoretically deriving polarization controller to adjust the phase of transmission light in a multimode fiber,the saturable absorption effect can be achieved.And the I-scan test of the SMS found that the polarization controller can change the transmittance and coupling efficiency of the transmitted light,so that the SMS structure has multiple modulation depths.In addition,since the polarization controller can change the phase relationship of the transmitted light in the SMS,resulting in a change in the transmission spectrum,the filtering effect based on the SMS structure is deduced and verified.(3)A fiber laser based on the SMS structure was constructed,and it was verified that the phase relationship of the transmission light in the multimode fiber modulated by the polarization controller can achieve the mode-locked pulse of 559 fs.Explore the filtering characteristics of SMS,and build dual SMS laser structures to achieve mode-locking and filtering functions,respectively,to achieve wavelength tunable of 40 nm.The mode-locked fiber laser with positive dispersion based on SMS was built.The mode-locking instability phenomenon appeared in the experiment.The low-pass filtering characteristics of SMS were explored theoretically and experimentally,and the loss of pulse instability was suppressed by introducing SMS structure for intra-cavity loss feedback.Achieve stable dissipative mode-locked output.