Ultrafast Spectral Analysis Of Nonlinear Polarization Rotation Mode-Locking Dynamics

Real-time detection,analysis and processing of optical signals play an important role in high-speed spectral analysis.Spectral dynamic phenomena in systems such as femtosecond pulse laser and on-chip micro-ring have attracted much attention.Traditional spectral measurement is limited by scanning speed or the rate of detector arrays,so the ultrafast spectral dynamics cannot be captured in real time.Therefore,the ultrafast spectral analysis technology arises at the right moment.On the premise of ensuring the spectral resolution,it overcomes the speed limitation brought by spatial dispersion elements,and has greatly improved the frame rate.The technologies of time stretch and time-lens focusing extend the concept of spatial dispersion commonly used in spectral measurement into time domain,therefore the spectral information can be directly received by single-pixel photodetector and real-time oscilloscope,and the frame rate mostly depends on the repetition rate of the light source used in the experiment.Femtosecond mode-locked laser has been widely used in optical frequency comb,laser ranging,ultrafast temporal imaging and other fields.Mode-locked fiber laser based on nonlinear polarization rotation has a compact all-fiber structure,which can be tolerant to perturbation and can realize stable self-starting.Therefore,it has become a common configuration for femtosecond pulse light source.The mode-locked process of the laser can reveal the complex dynamics mechanism of solitons in the system.The ultrafast spectral measurement of this process has become an important tool to analyze the evolution of soliton and can improve the performance of laser.The main content of this project can be summarized as follows:(1)The basic structures and principles of time-stretch dispersive Fourier transform and parametric spectro-temporal analyzer are discussed,respectively.The formation of femtosecond pulses and the spectral evolution process of mode-locked fiber lasers based on nonlinear polarization rotation are analyzed.(2)We built a reconfigurable ultrashort mode-locked twisted light laser with spatial high-order orbital angular momentum mode output,and it contains a free-space and fiber hybrid cavity,to generate stable twisted single solitons with the pulsewidth of ?600 fs and a repetition rate of 25.33 MHz.The spatial and spectral coherence of the laser system are tested.The real-time spectral measurement of mode-locking process are also carried out.(3)Based on time-stretch dispersive Fourier transform,the spectral evolutions of three typical positions in one roundtrip have been simultaneously observed in real time to investigate intracavity transient phenomena.Some preliminary experimental results have been acquired,and the intracavity spectral differences during the buildup of single solitons are analyzed in a lateral manner.The spectral modulation period in the spectral beating stage is also explored and analyzed.