Research On Passive Mode-locked Fiber Lasers Based On Novel Optical Filter

In recent years,passive mode-locked fiber lasers have been extensively used in industrial applications and scientific research.In addition,center wavelength-,pulse-tunable,and multi-wavelength mode-locked fiber lasers have important application value in wavelength division multiplexing optical fiber communication,optical fiber sensing,biological imaging,and terahertz generation,which has received considerable research interest.Effectively regulating the wavelength of passive mode-locked fiber lasers has important research value.It is well known that spectral filtering can greatly affect the pulse pattern in a laser cavity.Therefore,adding a tunable filter device to a laser cavity is a simple and stable way of effectively regulating the wavelength.Faced with the complex laser structures and inconvenient tuning functions currently available on the market,this thesis proposes two novel types of tunable lasers based on the Lyot birefringent filter effect and Mach-Zehnder interferometer.Different tuning functions are achieved,and the tuning functions of the lasers were studied and analyzed from both experimental and theoretical perspectives,further optimizing the output scheme of the laser.The main research contents of this paper are as follows:(1)A flexible wavelength-,pulse-controlled mode-locked all-fiber laser based on a novel fiber optic Lyot filter is studied.Firstly,a novel all-fiber filter is constructed by splicing a single-mode fiber between two segments of polarization-maintaining fiber with the polarization-maintaining fiber axes spliced at 45°.The center wavelength and filter bandwidth can be adjusted by adjusting the polarization controller on the single-mode fiber.Then,the transmission characteristics of the filter are analyzed theoretically,which agrees well with the experimental results.The tuning of the filter bandwidth is achieved by the birefringence effect of polarization-maintaining fibers instead of changing their length.Finally,the designed novel Lyot filter was added to the erbium-doped passive mode-locked fiber laser based on carbon nanotubes,by appropriately adjusting the polarization controller,laser can deliver pulses with the spectral bandwidth and pulse duration from 0.9 nm～6.3 nm and 2.9 ps to ～0.623 ps,respectively.Additionally,the wavelengths of pulse can be tuned in a range of approximately～32 nm.(2)A dissipative soliton molecule mode-locked fiber laser with controllable separation is studied.Firstly,a dissipative soliton passive mode-locked fiber laser is designed based on nonlinear polarization rotation technology,and the dissipative solitons was obtained with the center wavelength of 1565 nm.Then,input the obtained dissipative solitons into the novel Mach-Zehnder interferometer which is constructed by two 50:50 optical coupler and a tunable optical time delay line,the separation of the dissipative soliton molecule can be adjusted with the optical path difference of the Mach-Zehnder interferometer,by adjusting the time delay line.In the experiment,the dissipative soliton molecules with the pulse separation of 0.80 ps,1.30 ps,2.37 ps,4.25 ps,6.24 ps,9.40 ps,and 15.30 ps are observed,respectively,the corresponding spectrum separation are 8.30 nm,6.10 nm,3.83 nm,1.88 nm,1.27 nm,0.85 nm,and 0.52 nm,respectively.Finally,the generation mechanism of the soliton molecule by the Mach-Zehnder interferometer is analyzed theoretically,which agrees well with the experimental results.