Novel High-performance All-fiber Mode-Locked Yetterbium-doped Lasers

As an essential avenue to generate optical pulses,mode-locked lasers boast excellent output characteristics such as narrow pulse width,high peak power and broad spectrum,thus being widely applied in the sphere of high-speed optical communication,laser micromachining,optical frequency combs,biomedicine and nonlinear optics,to name but a few.Albeit manifold advantages including compactness,robust environmental stability and maintenance-free,mode-locked fiber lasers have some performance deficiencies in critical metrics compared with their solid-state counterparts.With new mode-locking mechanism proposed and implemented in recent years,pulse energy of mode-locked fiber lasers have approached the level of solid-state ones and even surpass them.Nonetheless,sacrifice in the structural superiority was spawned by introducing many free-space optical elements in these fiber lasers.Considering this,the main research aim of this thesis is to make the best of advantageous full-fiberized laser structure,promoting further the key performance indicators of mode-locked fiber lasers,such as pulse energy,peak power,average power and tunability in both time and frequency domain.The major research contents of this thesis are listed as follow:A discrete model of NOLM-based all-normal-dispersion Yb-doped mode-locked fiber laser was established,under which an intensive simulation of diversified modelocking states in laser parameter space was conducted.Distribution of mode-locked states including dissipative soliton,dissipative soliton resonance and the unstable mode-locking region was explicitly revealed in the parameter space.Researching the evolution of the distribution and its features lends possibility to induce the properties of these modelocking states,their evolution laws as well as how they were affected by the variation of laser parameters.The formation mechanism of different mode-locked states was briefly analysed;An investigation on the causes of multi-pulsing which was detrimental for improving pulse energy was conducted.In the light of this,a scheme implementing largemode-area fibers to build a all-fiber mode-locked laser was proposed to increase the generation threshold of multi-pulsing.In view of the inherent shortcomings of traditional mode-locked fiber lasers,a new mode-locked laser: dumbbell-shaped mode-locked lasers was proposed.This novel mode-locked fiber laser has many advantages,such as simple and compact structure,no optical isolator,high design flexibility,strong anti-interference ability,and high potential in the output pulse performance enhancement.A series of international leading experimental results have been obtained in the preliminary experimental study on the dumbbell-shaped mode-locked lasers.Based on different laser parameter settings,various high-performance pulses were directly generated from all-fiber dumbbell-shaped modelocked lasers,including noise-like pulses with pulse energy of 0.4 ?J,peak power of 7 kW,3 dB bandwidth of 76 nm-165 nm(the pulse energy was about one order of magnitude higher than similar fore-reported results);Ultrafast dissipative solitons with pulse energy of 80 nJ and after-compression pulse width of 1.2 ps(the pulse energy was about 1.6 times similar fore-reported results);Dissipation soliton resonance pulse with average power of 10 W-level and pulse energy of 2.3 ?J(the average power was 4.5 times fore-reported results).As an application example,an ultra-compact flat watt-level visible supercontinuum source based on noise-like pulse from dumbbell-shaped mode-locked fiber laser was designed and built.It was the first time to realize Watt-level visible supercontinuum generation,which was directly pumped by an all-fiber resonator.The structure was extremely simple and compact,thus having broad application prospects.To further improve the reliability and anti-interference ability of dumbbell-shaped mode-locked lasers,the key problems of dumbbell-shaped mode-locked lasers were analyzed,and the basic principle and structure of all-polarization-maintaining dumbbell-shaped mode-locked lasers were proposed,and experimentally studied.Ultrafast pulses with pulse energy of 6.7 nJ,after-compression pulse width of 176 fs and the repetition rate of 29.4 MHz were obtained,which were the most energetic 100 fs-level pulses generated from an all-polarization-locked ytterbium model-locked laser.An allpolarization-maintaining dumbbell-shaped mode-locked fiber laser based on structure of nonlinear amplifying optical loop and passive nonlinear amplifying optical loop was realized and studied.It could generate pulses with adjustable pulse width ranging from 352 fs to 404 ps,and the output pulse repetition rate was 37.1 MHz,the maximum output power of 2.1 W,the corresponding pulse energy was as high as 56 nJ.It was the first realization of generating femtosecond to sub-nanosecond pulses from an all-polarizationmaintaining all-fiber ytterbium-doped mode-locked laser.Based on pulse-modulated active harmonic mode locking technique,we realized generating optical pulses,which were widely tunable in the time domain and operating wavelength,from all-normal-dispersion mode-locked fiber lasers.Pulse-modulation and narrow-band filtering were combined to realize genrationg optical pulses with independently tunable pulse-width and repetition rate.The tuning range of pulse widths was 33 ps to 910 ps,and the repetition rate tuning range was 13 MHz to 6.2 GHz.Moreover,the pulse width tuning range could be further extended to the span of 890 ps to 90 ns by the combanition of pulse-modulated active mode-locking and passive modelocking based on nonlinear polarization rotation.Finally,through the employment of ultra-long cavity design and broadband tunable filter in pulse-modulated active modelocked fiber laser,the laser could be continuously tunable in repetition rate from 226 kHz to 6.25 GHz at an interval of 226 kHz,and in operating wavelengths from 1005 nm to 1100 nm.The laser refresh the best records of all-fiber mode-locked lasers in terms of repetition rate range,tuning accuracy,wavelength adjustment range,and harmonic mode-locked order(up to 27655 harmonic mode-locking order).As an application example,by exploiting the wavelength tunability characteristics of the developed active mode-locked laser,the effects of seed laser wavelength on the supercontinuum generation process in fiber amplifier were studied experimentally and theoretically.