Research On Novel Passively Mode-locked Fiber Laser

Within the past few years, passively mode-locking fiber laser and the technology of the ultra-short pulse are currently being developed for applications in the science research and industrial production, such as the optical coherence tomography, micro-machining, and the optic communications. Offering the advantages of compact, portability, a high efficient and low cost, passively mode-locking fiber lasers have the ability of generation ultra-short pulse with a high peak power. In this thesis, the novel passively mode-locking fiber lasers are investigated, including the passively mode-locked fiber laser with normal dispersion cavity, nanosecond square pulse fiber laser, and the the passively mode-locked fiber laser with ultra-low repetition rate.The major results in the thesis are obtained as follows.1. All-normal dispersion passively mode-locking fiber laser is experimentally studied. Based on the ytterbium-doped and erbium-doped fiber, the passively mode-locking fiber laser operating at the 1.06μm and 1.5μm band are obtained. The generated mode-locking optical spectrums, in a rectangular shape, are broad and flat, and have 3dB width of 8nm and 28nm respectively. The difference between mode-locking in normal dispersion and abnormal dispersion fiber cavity is disccussed in the numerically calculation, and the result is according with the experiment results. This kind of all-normal dispersion mode-locking fiber lasers has potential application in some important areas, including the high energy pulse generation, the optical frequency combs and super-continuum light generation.2. A nanosecond square pulse fiber laser based on the nonlinear amplifying loop mirror (NALM) is proposed and numerically analyzed by using the nonlinear Schrodinger equation. From our analysis, the fiber cavity with a NALM has a tendency to provide pulse shaping effect with a nonlinearity increasing in the NALM and the nanosecond square pulse is generated by the pulse shaping effect. The numerical results show that the stable square pulse can be obtained when the parameters of the NALM are chosen appropriately. This is also demonstrated in the experiment. With a specially designed NALM in our experiment, self-start and stable nanosecond square pulse is obtaind. The generated square pulses have flat top without internal structure, and the rise time is shorter than 70 ps. The pulse width can be turned from Ins to 100ns. The center wavelength is at 1.06μm, and a bandwidth wider than lnm is implemented by the dispersion manage in the cavity. Additionally, the chirp of the square pulse fiber laser is also studied experimentally. This kind of square pulse fiber laser based on the NALM will be used in the laser micro-machining, high power laser system and some other fields.3. Study two kinds of low repetition rate passively mode-locking fiber laser operating at 1.0μm band:a. mode-locking fiber laser with the repetition of 1MHz based on the NPR and F8L structure respectively. B. mode-locking fiber laser with the repetition of 126kHz based on the NPR structure, which generates mode-locking pulse with the energy of 77nJ. The result is the most low repetition rate and the highest pulse energy mode-locking fiber laser as we know.4. A high speed signal collection system based on the time-stretched technique in the microwave-photonic is investigated. A ultra-short electrical pulse signal in two nanoseconds is modulated into a dispersion stretched laser pulse by a Mach-Zehnder electro-optic modulator. After being further stretched in fiber, this modulated optical pulse is captured by a 10G bandwidth photodetector and digitized by a digital oscilloscope, then the signal pulse can recovery. The results indicate that an electrical signal in nanoseconds can be captured by this time-stretched technique and the equivalent sample rate of the digital oscilloscope increases from 5 GS/s to 20 GS/s.The innovative work and results in this thesis are as follows:1. Based on the pulse shaping effect of the NALM, a nanosecond square pulse fiber laser working at 1.06μm band is proposed and demonstrated. The square pulse has flat top and no internal structure, with a short rise time(< 70 ps) and a tunable width (1ns-100ns).2. A theoretical model of the nanosecond square pulse fiber laser based on the NALM is built and a numerically analysis is made by using the nonlinear Schrodinger equation. The numerical result shows that the stable square pulse can be obtained when the parameters of the NALM are chosen appropriately and the width of the square pulse varies with the NALM length.3. A ultra low repetition of 126kHz mode-locking fiber laser is obtained by a optimization of the NPR structure.The output pulse energy is higher than 70 nJ.