Research On Mode-locked Raman Fiber Lasers And Its Dynamic Characteristics

With the rapid development of modern communication network and data transmission,the existing communication wavelength-band will be used up soon,and the research and development of the new wavelength-band becomes an urgent task.Mode-locked Raman fiber lasers based on the basic principle of stimulated Raman scattering(SRS)effect in fiber,which theoretically can obtain arbitrary wavelength laser output as long as we select an appropriate pump source.Due to the advantages of high power output,good beam quality,flexibly tunable lasing wavelength and higher conversion efficiency with simple and compact structure,Raman fiber laser can make up for main drawback of the semiconductor laser and become an ideal pump source in optical fiber communication system.The research work of this dissertation is done around mode-locked Raman fiber laser and multi-wavelength Raman fiber laser,which include related theory analysis,experimental research and optimization design in U-wavelength-band Raman fiber lasers.First of all,this dissertation provides an overview of fiber structure and its basic characteristics,and a review of five main nonlinear effects of self-phase modulation,cross-phase modulation,four-wave mixing,stimulated Raman scattering and stimulated Brillouin scattering in optical fiber.On this basis,we further analyze three common basic mode-locked methods of saturable absorber(SA),nonlinear polarization rotation(NPR)and nonlinear optical loop mirror(NOLM)in passive mode-locked Raman fiber laser,and the basic operational principle of Raman fiber laser.These analysis include Raman scattering theory,physical mechanism and gain of stimulated Raman scattering in optical fiber,several classification of Raman fiber laser and so on.Second,we introduce the basic principle of fiber mode in fiber core and cladding layer,and further elaborate the multi-wavelength generation principle based on nonlinear polarization rotation technology mechanism and the operating principle of the Mach-Zehnder interferometer filter.Then,by the multi-wavelength mechanism of the intensity dependent loss characteristics based on nonlinear polarization rotation technology,a multi-wavelength tunable Raman fiber laser is constructed and researched,which is based on Mach-Zehnder filter between fiber core and cladding of different fiber core diameter.In our experiment,not only the tunable of multi-wavelength number,location,and spacing is realized but also the multi-wavelength Raman spectra of up to 40 wavelength is obtained with the wavelength intervals of 0.8 nm and the center wavelength of 1650 nm.The signal-to-noise ratio of the Raman wavelength is up to 58 dB.The multi-wavelength tunable Raman fiber laser around 1650 nm wavelength-band is bound to have potential applications in many fields owing to simple structure and convenient tuning.And then,we researched and constructed a home-made continuous wave multilongitudinal mode broadband laser pumping light source.From threshold condition of passively mode-locked Raman fiber laser,further,we focuses on the analysis and discussion of basic principle of passively mode-locked Raman fiber laser pumped by a multimode laser.By the multi-longitudinal mode laser source,a U-wavelength-band passively mode-locked Raman fiber laser based on nonlinear polarization rotation(NPR)mode-locking mechanism with high single pulse energy is proposed and demonstrated,and their related properties are researched further.At around 1651.3 nm,a high-energy single pulse is obtained in our experiment.The laser can obtain a stable mode-locked pulse of 110 mW output power with the pulse duration of 890 ps.The output pulses were achieved with 290.7 nJ single pulse energy and 326.7 W peak power.The supermode-suppression-ratio of Raman laser cavity is up to 41.2 dB with the signal-to-noise ratio higher than 50 dB.Owing to its simple structure,large pulse energy and high peak power,our laser could has potential applications in many areas.Last,a ultrahigh order passively harmonic mode-locked(HML)Raman fiber laser is researched according to mode-locked mechanism of Light intensity related transmission characteristics based on nonlinear polarization rotation.Experimental results show that The laser can obtain stable optical pulses up to the 1552 nd order HML with the output power of 168.3 mW under the pump power of 1.5 W.As far as we know,this is the highest order harmonic obtained in a passively mode-locked Raman fiber laser.In the experiment,Different orders harmonic mode-locked can also be produced at the range from the first order to the 1552 nd order.The super-mode suppression of the Raman fiber laser is superior to 40 dB.Compared with the previous work,the laser provides an unprecedented combination of high harmonic order,high pulse energy and excellent pulse quality.Due to its simple structure and high repetition rate,the ultrahigh order harmonic mode-locked Raman fiber laser has potential applications in many fields.