Research On Mode Field Regulation Of Fiber Laser Based On Few-mode Fiber Grating

Fiber lasers are widely used in fiber optic communication,laser processing,laser medicine,laser radar and scientific research due to their small size,high beam quality,good stability,and the ability to generate ultrafast pulses with high peak power and short duration.Among them,the formation of ultrashort pulse lasers by means of mode-locking is developing very rapidly.Mode-locked pulsed fiber lasers with tunable wavelength and optical field have important applications in wavelength-division multiplexing systems,mode-division multiplexing systems,structured optical field generation,and multimode fiber nonlinear spatio-temporal modulation.The all-fiber structure generated by the cylindrical vector beam field has the advantages of small size and simple fabrication,and can be well applied to mode-locked fiber lasers.In this thesis,dynamic long-period fiber gratings and few-mode fiber filters are fabricated and used to investigate the dynamic tuning characteristics of wavelength and optical field in mode-locked fiber lasers,mainly as follows:1.Dynamic long-period fiber grating design and fabrication.The acousto-optic effect in optical fiber is introduced from the principle of acousto-optic action,and the mode conversion of different wavelengths of light under acousto-optic action is analyzed by using coupled-mode theory.The LP₀₁mode conversion condition to LP₁₁mode at different wavelengths of two-mode fiber is simulated and the two-mode fiber acousto-optic mode converter is built.The optimal length of the acousto-optical action zone under the mode conversion conditions and the relationship between acoustic wave frequency and conversion wavelength are investigated.Finally,a dynamically tunable long-period fiber grating with a mode conversion efficiency of 12 d B and a mode purity greater than 90%,i.e.,an acoustically induced mode converter,is achieved,which can operate in any optical band of the corresponding acousto-optical resonance.2.Mode-field regulation of mode-locked fiber lasers based on few-mode fiber gratings.The dynamic long-period fiber grating is combined with a few-mode fiber Bragg grating,where the dynamic long-period fiber grating uses RF signals to control different wavelength mode conversion,and the few-mode fiber Bragg grating achieves mode selection and spectral filtering.The passive mode-locked laser uses a semiconductor saturable absorber mirror to realize the optical pulse output.The laser achieves wavelength switching from 1551.52 nm to 1550.21 nm laser output under RF signal control,3 d B bandwidth changes from 0.056 nm to 0.11 nm,and pulse signal-to-noise ratio is kept above 75 d B with repetition frequency at 12.681 MHz.at the same time,the output mode is switched from LP₀₁mode to LP₁₁mode,and the purity of the generated column vector pulse beam is greater than 94.8%.3.Wavelength-tuned mode-locked fiber lasers based on long-period gratings in few-mode fibers.A comb filter is constructed by splicing a long-period grating with a single-mode fiber at different misalignment distances,and the comb filter spectrum is studied at different misalignment distances to obtain the optimal misalignment distance.An all-fiber laser was built to enable switchable single,dual,triple and quadruple wavelength continuous laser generation in the laser by adjusting the polarization state.In addition,the wavelength change from 1567.72 nm to 1571.04 nm and a 3 d B bandwidth change from 0.049 nm to 0.044 nm were achieved in the passive mode-locked fiber laser,with the optical pulse signal-to-noise ratio remaining above 61 d B and a pulse repetition frequency of 17.016 MHz.