| Ultrashort pulse fiber laser is widely used in communication,industry and medical fields due to its advantages of low transmission loss,high compatibility and simple system structure.As the working medium in laser cavity,rare earth doped glass fiber is beneficial to realize the high efficiency of ultrashort pulse output.Erbium-doped fiber laser(EDFL)emits laser at 1.55 μm wavelength,which is also a low loss window for optical communication.And it can be used as an important light source for long-distance large-capacity soliton communication and optical time division multiplexing communication,special significance to the development of all-optical network.Ultrashort pulses in fiber lasers rely on the rapidly developing technology of passive mode-locking,which uses saturable absorbers(SAs),either artificial SAs or natural SAs,to achieve nonlinear absorption.In this paper,three typical passive mode-locking techniques are selected to experimentally investigate the ultrashort pulses in the order of picosecond or femtosecond in EDFL.The specific contents include:1.Many factors involved in pulse transmission such as loss,dispersion and nonlinearity are analyzed in detail.Based on the theory,the transmission mechanism of optical pulses in the optical fiber resonator and its influence are analyzed.Then it provides the theoretical basis for the formation and interpretation of soliton pulses in subsequent experiments.In addition,based on the basic theoretical knowledge of mode-locking technology,the differences of ultrafast fiber lasers realized by different mode-locking technologies are introduced,especially the basic structure and mode-locking mechanism of different SAs devices.2.To explore the nonlinear polarization rotation(NPR)mode-locked technology applied in the window of EDFL.The output of stable traditional soliton and bound soliton pulses is obtained,respectively,and the excellent performance of pulse duration as short as hundreds of femtoseconds is achieved.In addition,after adjusting the pump power and the polarization state in the cavity,the interaction force between solitons changes,thus realizing the transition from the tightly bound state to the loosely bound state.3.Copper oxide(CuO)material were prepared by liquid phase precipitation,and the morphological characteristics and component purity of CuO material were determined by various typical material characterization methods.The Cuo-SA device was synthesized by coupling CuO material with taper fiber by optical pulse deposition technology.A double-balance measurement system was established to characterize the saturation absorption capacity of the SA device.The saturation strength of 13.1 MW/cm2 and the modulation depth of 6.82%also clearly reflect the excellent nonlinear optical performance of the Cuo-SA device.By embedding the CuO-SA device in the all-fiber ring laser and adjusting the intensity of the pump source and polarization controller,stable traditional soliton and bound soliton pulse outputs were obtained.In addition,by introducing a high reflectivity fiber Bragg grating(FBG)into the above-mentioned EDFL,a narrow linewidth fiber laser with a linewidth as narrow as 0.05 nm was successfully realized.4.Combined with the two different mode-locking effects of NPR and CuO-SA,the performance optimization and continuous extension of hybrid mode-locking technology in ultrafast fiber lasers are explored through experiments.The typical output of conventional soliton pulses was observed in the experiment,and the pulse duration was significantly shorter,only 357 fs.In addition,the pump power and polarization state are the main factors affecting the soliton state,and the evolution from a single soliton to soliton molecules with different soliton numbers is realized after adjustment,including soliton pairs and rare three-soliton and four-soliton pulses.Combined with the nonlinear polarization rotation and CuO-SA two kinds of different locking mode effects,the performance optimization and continuous technical extension of hybrid mode-locking technology applied in ultrafast fiber lasers are explored through experiments. |
