Fiber Laser Based On Multimode Fiber/Erbium-Ytterbium Co-Doped Fiber

Fiber lasers are characterized by the use of rare earth-doped glass fibers as gain media,which are used in a wide range of applications,including fiber communication,fiber sensing,and pumping sources.Random fiber laser is a new type of cavity-less fiber laser that utilizes random distributed Rayleigh scattering in the fiber as feedback.This cavity-less fiber laser has the characteristics of no output longitudinal mode,good output stability,and single transverse mode.It can be designed as a tunable wavelength,multi-wavelength output,narrow line width output,and cascaded output,which has good application prospects.Early research on fiber lasers was based on single-mode fiber(SMF),but the small core diameter of single-mode fibers limits the development of high-power fiber lasers,and the lower backward Rayleigh scattering coefficient makes high lasing threshold,difficult cascade output,and low output efficiency.Therefore,the research on new fibers in fiber lasers requires in-depth research.In this thesis,we used multimode fiber(MMF)and Erbium-ytterbium co-doped fiber for fiber laser to achieve new performance different from traditional fiber laser,and realize cladding pumped Erbium-ytterbium co-doped random fiber laser and spectral tailoring.Finally,we also use multimode fiber to achieve a low threshold,high beam quality fiber laser.Firstly,we designed a combination of Erbium-Ytterbium co-doped fiber and SMF,which provide Raman gain and random distribution feedback,respectively.As a result,stable 2.14 W of 1550 nm random lasing at 9.8 W of 976 nm LD pump power and a linear output with the slope efficiency as 22.7%are generated successfully.A fiber loop mirror(FLM)incorporating multimode interference filter(MMIF)with SMF-MMF-SMF structure realize spectral tailoring.By varying the length of MMF,either single wavelength random lasing with specific wavelength or multi-wavelength random lasing with selective wavelength spacing can be generated in MMIF-FLM based random fiber laser.Due to the wide wavelength range of the MMIF,cascaded multi-wavelength random lasing can also be realized.The spectrum adjustable all fiberized random fiber laser based on the MMIF shows distinct advantages such as easy fabrication,wide operating wavelength range,and low-cost.Furthermore,we used the pumping total reflection cavity constructed by a dichroic mirror to and the beam cleaning effect in multimode graded-index fiber to reduce threshold and purify the multimode lasing respectively.As a result,stable 6.28 W of 1020 nm random lasing at 41.5 W of 976 nm LD pump power,a linear output with the slope eff-iciency as 41%and the beam quality as 4.5 are generated successfully.The threshold is 26.5 W.In addition,we proposed all fiber multi-mode LD-pumped multimode fiber cascading Raman random laser and simulate the power output curves of first-order and second-order random lasing with different lengths of multimode graded-index fibers.The results indicate the corresponding relationship between the output power,threshold and the length of the fiber.The proposed simple and novel random fiber laser based on cladding pumped Erbium-Ytterbium co-doped fiber can provide a promising way to develop high-power 1.5?m light source,and is widely used in fiber sensing and optical communication.The proposed MMIF-FLM has good power handling capability by combining MMIF-FLM and short length cavity,and the forward pump random fiber laser based on MMIF has both tunable spectral characteristics and great potential for producing high power,highly efficient random lasers.The proposed LD direct pumped Raman fiber laser based on multimode gradient index fiber provides a new idea for developing high power,high beam quality lasers.