Research Of Multi-wavelength Thulium-doped Fiber Laser Based On Birefringent Filtering Effect

Multi-wavelength fiber lasers have the advantages of compact structure and multiple output channels,which can replace traditional semiconductor lasers as the ideal light sources for wavelength division multiplexing(WDM)systems.Since the output wavelength is in the 2 ?m band,multi-wavelength thulium-doped fiber laser(TDFL)has important applications in free-space optical communication and laser medicine.At present,this type of laser still has some disadvantages,such as complex fabrication of filters,expensive and unfavorable to the realization of all-fiber structure.Based on the above problems,this thesis uses the birefringent filtering effect for wavelength selection in TDFL to achieve continuous multi-wavelength and multi-wavelength bright-dark pulse pairs in the 2 ?m band,respectively.The main research contents of this thesis are as follows.1.First,the theoretical analysis and numerical simulation of the Sagnac ring filter based on the birefringent filtering effect were carried out.Then a continuous multi-wavelength TDFL based on the Sagnac ring filter was proposed and implemented.When the lengths of the polarization maintaining fibers(PMFs)in the laser cavity were4.9 m and 9.9 m,the laser obtained 7 and 10 emission wavelengths in the full width at half maximum(FWHM)range with wavelength intervals of 2.3 nm and 1 nm,respectively.Meanwhile,the output spectra of the laser were scanned continuously within1 hour,and the results showed that the maximum wavelength drift were 0.15 nm and 0.25 nm,respectively,which indicats that the laser could operate stably for a long time.Finally,the output spectra of the laser were studied at different pump powers.2.Then,in order to study the feasibility of pulsed multi-wavelength TDFL,a multi-wavelength mode-locked TDFL based on the nonlinear amplifier loop mirror(NALM)and PMF was proposed and implemented.2 ?m multi-wavelength bright-dark pulse pairs with a repetition frequency of 638.9 k Hz and a signal-to-noise ratio of about45 d B were experimentally obtained.Then,the evolution of the time-domain waveforms and spectra of this laser at different pump powers were experimentally studied.3.Finally,in order to further study the characteristics of multi-wavelength bright-dark pulse pairs under different mode-locking mechanisms,a multi-wavelength mode-locked TDFL based on a hybrid structure of nonlinear polarization rotation(NPR)and Sagnac ring filter was proposed and implemented.The output characteristics of the laser with different lengths of PMFs in the cavity were experimentally studied.Since the total cavity length of the laser was kept essentially constant when adding different lengths of PMFs to the cavity,the repetition frequencies of the bright-dark pulse pairs were all located around 746 k Hz with signal-to-noise ratios of 39,40,41,40,and 39 d B,respectively.The experimental results show that both two different mode-locking techniques can obtain multi-wavelength bright-dark pulse pairs in the 2 ?m band,which is of great significance to expand the application of mode-locking techniques in the research of 2 ?m bright-dark pulse pairs.