Experimental And Theoretical Studies On Mode-locked Erbium-doped Fiber Lasers

Mode-locked erbium-doped fiber (EDF) lasers have been in-depth investigatedfor their important applications in fields of fiber optic communications, materialsprocessing, frequency comb, biomedicine, and nonlinear optics. In comparison withothers lasers, fiber lasers exhibit inherent advantages of high gain coefficient, wideoperating wavelength range, excellent heat dissipation, and freedom frommisalignment. Additionally, fiber laser also offers an attractive platform to study thedynamic evolution of nonlinear optical waves. As a result, the study of mode-lockedEDF laser not only has a great theoretical importance but also can promote itspractical applications. In this dissertation, we have experimentally and theoreticallyinvestigated the performances of mode-locked EDF lasers. Several different types ofultra-short pulses have been achieved, and their formation mechanisms, outputcharacteristics, as well as dynamic evolutions were in-depth analyzed. The mainresearch contents of this dissertation are as follows:1. By using the nonlinear polarization rotation (NPR) technique, two differenttypes of dual-wavelength pulses are obtained in mode-locked fiber lasers. Theformation mechanism of the dual-wavelength pulses is analyzed. One type of pulseexhibits typical characteristics of conventional soliton (CS) with spectral sidebandsand picosecond duration. Attributing to the peak-power-clamping effect of the lasercavity, two pulses at different wavelengths possess the same intensity and energy. Asthe net cavity dispersion is non-zero, two pulse trains have different group velocitiesand round-trip times. During the collision, two pulses simply pass through each otherand maintain their soliton nature. The other dual-wavelength pulses show distinctintensities and durations. Due to the combined effects of dispersion and fibernonlinearity, two pulses adhere to each other and exhibit a step-like temporal profile.Our results demonstrate that the formation of dual-wavelength pulses arises from thefiltering effect induced by the fiber birefringence and the uneven gain spectrum ofEDF.2. We investigate the generation, compression, and interaction of the dissipativesoliton (DS). By exploiting the NPR technique for mode locking, DS with the spectralwidth of16nm and the duration of23ps is obtained in a normal-dispersion fiber laser.The DS can be further compressed to290fs with a section of SMF. The pulse fissionand interaction of the DS are experimentally observed and numerically simulated. It isdemonstrated that the phase difference between two pulses is π and the direct pulse-pulse interaction dominates the formation of DS molecules. Besides, weexperimentally observe that two unequal pulses coexist in the same DS fiber laser.The two types of pulses exhibit distinct optical spectra, durations, pulse energies, andformation mechanisms.3. Two high-energy rectangular pulses and a broadband pulse are obtained infiber lasers. Experimental results show that the pulse duration of rectangular pulseincreases linearly while the intensity keeps unchanged with the enhancement of pumppower. As a result, the rectangular pulse can accumulate high energy and eliminatewave breaking effect induced by the fiber nonlinearity. The rectangular pulse in theanomalous dispersion regime can be amplified to2μJ without distortion by an EDFamplifier (EDFA). By injecting the amplified pulse into a100-m highly nonlinearphotonic crystal fiber (PCF), a broadband supercontinuum covering the range from1200to1800nm is achieved. The rectangular pulse in the normal dispersion regime isshaped into a Gaussian pulse and exhibits a red-shift on the spectrum afterpropagation through anomalous-dispersion single-mode fibers (SMFs). The numericalsimulations show that the intensity ratio of two orthogonal-polarized components islimited at8~65. The high-energy broadband pulse is formed in a DS fiber laser withlarge normal dispersion. The bandwidth and energy of the pulse reach83nm and75nJ, respectively. Our experimental observations show that the polarization state of thepulse is nonuniform, i.e., the polarization state is diferernt in each part of the pulse.The pulse outputted from polarization sensitive isolator is linearly polarized, and itevolves to partially-polarized after amplification with an EDFA. The broadband pulseis a novel type of pulse, and the polarization evolution is different from that of CS,continuous wave, as well as DS. Our results demonstrate that the nonuniformpolarization state results from the intensity-dependnt nonlinear phase shift during theamplification process.4. By exploiting the semiconductor saturable absorber mirror as a mode locker,group-velocity-locked vector soliton (GVLVS) is obtained in a near zero dispersionregime. The formation mechanism of the GVLVS is numerically simulated by solvingthe coupled nonlinear Schrodinger equations (NLSEs). It is indicated that the cavitydispersion, fiber birefringence, and saturable absorption effect mainly contribute tothe formation of vector soliton in the fiber laser. Based on a chirped fiber Bragggrating and a four-port circulator, we construct an all-fiber laser to implementsimultaneous CS and DS mode locking. The bandwidth and duration of the CS are 0.28nm and15.1ps, respectively. However, the giant-chirp DS has a bandwidth of9.5nm and a temporal duration of7.5ps. After propagation through a section of SMF,the CS almost keeps unchanged while the DS can be compressed to0.55ps.5. Repetition-rate controllable, duration tunable ultrashort pulses are achieved inan all-fiber laser by exploiting an intracavity Mach-Zehnder (M-Z) interferometer andan optical time delay line. By reducing the optical path difference between the twoarms of the M-Z interferometer, the repetition rate of the pulses can be flexibly tunedfrom about7to1100GHz. Meanwhile, the spacing between adjacent peaks of thespectrum increases from0.06to9.5nm. Interestingly, by tuning the intracavitypolarization controller, the pulse duration can be adjusted continuously from10.1to0.55ps. We numerically investigate the formation mechanism of the pulses based onthe NLSE. Our simulation results show that the filter-driven four-wave mixing effect,induced by the M-Z interferometer, is the main mechanism that governs the formationof the high-repetition-rate pulses.