The Study Of All-normal-dispersion Mode-locked Yb:fiber Laser

As the characteristics of mode-locked fiber laser continue to evolve towards higher power and shorter pulse-widths,their applications in various fields such as basic science research,biomedicine and coherent communications are becoming more widespread.To achieve higher and more stable output pulses,NPR and NALM which are classified as equivalent fast saturable absorbers mode-locking techniques are preferable to real saturable absorbers.On the other hand,dissipative solitons and self-similar solitons can withstand larger nonlinear phase shifts and are less likely to split at high power than conventional solitons and dispersion-managed solitons.Therefore,we plan to build an all-normal-dispersion mode-locked fiber laser capable of generating dissipative solitons based on NPR technique and NALM technique to directly output pulses with higher power.At the same time,ultrashort pulses can be achieved by designing shorter cavity lengths and using grating pairs outside the cavity to compress the pulse.The details of the experiment are as follows.Firstly,the NPR mode-locking mechanism and the effect of(non-)linear phase shift on mode-locking are analyzed.The NPR-based all-normal-dispersion Yb-doped fiber laser was then designed to achieve mode-locking.The spectrum of the mode-locked noise-like pulse has a center wavelength of 1045 nm,with a full width at half maximum of 35.9 nm and a spike pulse width of 138 fs,with a mode-locking range of 100 m W-400 m W,corresponding to a maximum output power of 70.1 m W.And it has a good output stability.By adjusting the pump power and the polarization controller,this laser achieves a dark pulse,a bright-dark pulse pair and a dissipative-soliton with a spectral full width at half maximum of 3.2 nm,3.1nm,and 6 nm,respectively.The laser can achieve switchable pulse for three types,providing a new solution for the study of lasers suitable for different application scenarios.Secondly,based on the theoretical analysis of principles of the NALM,the phase shifter,the birefringent filter and the grating pair compression,a single-mode all-normal-dispersion fiber laser based on a modified NALM was first designed.After the mode-locking,the total output power was 153.7 m W at maximum pump power,and the good performance of the laser can be seen from the SNR of 70 d B and the RMS value of 0.176%.The spectrum of the pulse from one of the output ports has a center wavelength of 1029 nm with a full width at half maximum of 12 nm,with a tuning range of 1029 nm-1039 nm.The pulse width after compression by the grating pair is 178 fs.The laser can be used as a well high-power ultrashort-pulsed seed source.Finally,a polarization-maintaining fiber laser having a similar structure with(2)was designed based on the stabilization of the polarization state due to the strong stress birefringence of the PM fiber.The laser has a repetition frequency of 100.5 MHz after replacing the fiber coupler with spatially solid devices,and a pulse which have a spectrum centered at 1040.4 nm with a full width at half maximum of of 15 nm and a pulse width of4.16 ps is achieved at one monitoring port.The corresponding total output power at maximum pump power was 264 m W and the output efficiency is approximately 31.5%.Besides,the second harmonic mode-locking was observed in the cavity,corresponding to a repetition frequency of 204 MHz.