Research On Tunable And Switchable Ultrashort Pulse Fiber Laser

Ultra-short pulse has the advantages of high peak power,broad bandwidth,precise temporal and spatial resolution,have found widerspread applications in various fields.Passive mode-locked fiber laser,characterized by simple structures,compactness,and high stability,have become an effective approach for generating femtosecond pulses,and have thus attracted significant attention and research.With the development of multi-functional fiber lasers,such as single-cavity dual-comb,tunable and switchable,and multi-wavelength mode-locking,has further expanded the application areas of passive mode-locked fiber lasers to high-precision and fast measurement,ultra-high-speed and large-capacity optical communication,microwave photonics,and other fields.This thesis presents a theoretical analysis based on the passive mode-locking mechanism and filtering effect,with a focus on experimental research of tunable and switchable mode-locking functions of polarization multiplexed dual-frequency pulse lasers based on carbon nanotubes,erbium-doped fiber mode-locking lasers with nonlinear polarization rotation,and erbium-doped fiber lasers with nonlinear amplification optical fiber-loop mirror and nonlinear polarization rotation hybrid mode-locking.The main research contents and results are as follows:1.The transmittance function of Lyot filter and Sagnac loop filter is derived by Jones matrix theory,and the numerical simulation is carried out by Matlab software to analyze the various factors affecting the transmittance curve of the filter.2.The research of polarization multiplexing mode-locked fiber laser based on carbon nanotubes is carried out,and the sub-picosecond pulse performance characteristics of laser output with self-made carbon nanotubes as mode-locked devices were investigated.By changing the pump power and the state of the intra-cavity polarization controller,the single/dual-frequency pulse switchable output and the dual-frequency pulse switchable output with a repetition rate difference of 88-305 Hz are realized.3.The tunable and switchable mode-locking function of nonlinear polarization rotation mode-locked fiber laser is studied.The nonlinear polarization evolution and tunable Lyot filtering effect are introduced by using the polarization dependent loss device with polarization maintaining fiber and the intra-cavity polarization controller.The laser achieves tunable wavelength and pulse-width,as well as harmonic mode-locking function.In addition,a fast wavelength-switching nonlinear polarization rotation mode-locked fiber laser is developed using a 1×2 electro-optic switch,and a fast wavelength switching scheme is proposed and demonstrated,and the shortest switching-time can reach 0.06 s.This scheme significantly improves the convenience and speed of switching laser wavelengths,without changing the laser cavity status or manually adjusting the mode-locking state during the switching process.4.The numerical simulation of mode-locking mechanism of nonlinear amplification fiber-loop mirror and the experimental investigation of laser tunability are carried out.An Er³⁺-doped fiber laser based on nonlinear amplification fiber-loop mirror and nonlinear polarization rotation hybrid mode-locking is proposed and realized.The laser adopts a“8”-shaped cavity structure with a short cavity length.Its amplification fiber-ring has a nonlinear amplification fiber-ring mirror mode locking mechanism and Sagnac filtering effect,while the unidirectional ring has a nonlinear polarization rotation mode locking mechanism and Lyot filtering effect.The laser can output high-energy femtosecond pulses with a spectral-width of 10.59 nm,which is different from traditional solitons.By adjusting the pump power,the traditional soliton multi-pulse mode-locking with different pulse numbers can be realized.Furthermore,by tuning the polarization state in the cavity,the wavelength tuning operation in the range of 33 nm can be realized,and the pulse-width can also be tuned.At the same time,the tunable and switchable output function of wide spectrum pulse and traditional soliton pulse can also be realized.