Ultrafast Mode-locked Thulium-doped Fiber Laser Based On Nonlinear Polarization Rotation

Ultrashort pulse fiber lasers have many advantages,including narrow pulse duration,high average power and pulse energy,such that they play an irreplaceable role in ultrafast optics,materials processing,communications,biomedicine and so on.Thulium(Tm)-doped pulse sources operate around 2 ?m wavelength,which is in the range of eye-safe wavelength region and some atmospheric windows.They become hotspots in laser researches due to their specific applications in atmospheric remote sensing,laser medicine,mid-infrared supercontinuum generation and so on.Though the quality and stability of ps-scale Tm-doped fiber lasers have been greatly improved,the fundamental mode-locking technology behind ultrashort pulse lasers still should be further studied for power scaling and performance improvements of fs-scale fiber lasers.Mode-locked fiber lasers based on nonlinear polarization rotation(NPR)become perfect ultrashort pulse resources because of their simple structures,short response time and abilities to export pulse lasers with extremely high peak powers and narrow pulse durations.Our researches focus on the ultrafast Tm-doped fiber laser mode-locked by NPR technology.We have studied the working principles and properties of the passively mode-locked fiber laser firstly,including analyzing the transmission functions through the deduction of Jones matrix when pulses propagate in the fiber laser resonant cavity.It can be found that tunable filtering effect exists in the NPR resonant cavity,which is caused by the optical birefringence of fibers and can be used to realize multi-wavelength fiber lasers.In this paper,numerical modelings and experiments have been performed to study the tunable filtering effect.The results show that periodic filtering bandwidths are decided by the lengths and birefringence coefficients of fibers,while their central wavelengths are deeply influenced by the phase difference between different polarization components of light fields.The main achievements in all-fiber Tm-doped fiber laser modelocked by NPR are as follows:Through carefully optimizating the resonant cavity of the Tm-doped mode-locked fiber laser,soliton pulses with pulse duration of 350 fs and pulse energy of 7.8 nJ have been generated.The central wavelength of the Tm-doped fiber laser is 1.89 ?m.It works in the all anomalous dispersion region and no optical device for dispersion compensation is inserted.The short pulse duration and high power are much better than that in other Tmdoped mode-locked fiber lasers.The good performance of our fiber laser should be attributed to the use of bi-directionally pump and the optimization of fiber lengths as well as the coupling ratios of output coupler.Secondly,the average power and peak pulse power of the output pulses are 90 mW and 22.3 kW,respectively.The high power of the stable single pulses is due to the high dispersion and low nonlinearity of SMF when the laser operates in 2 ?m region.On the other band,the high-order soliton generation also contributes to the high power.The spectrum of soliton pulses is smooth without any sideband,which is attributed to the NPR filtering effect.The NPR effect breaks the phase-matching condition of Kelly sidebands so that the sidebands are well suppressed.Our researches on the femtosecond Tm-doped mode-locked fiber lasers show the key technologies of ultrashort pulses generation,which have provided theoretical analyses and experimental experiences for the practical applications of ultrafast fiber lasers.