Study On High Power Laser Oscillators Based On Distributed Side-coupled Cladding-pumped Fiber

In commercial and military fields,such as industrial manufacturing and material processing,high power fiber lasers are considered as a good energy source because of their excellent performance,and favored by researchers in recent years.For kilowatt-level fiber lasers,all-fiber oscillators are preferred choices because of the advantages of their simple configurations and stable performances.Currently,fiber laser oscillators mostly utilize large-mode-area double cladding Yb-doped fibers as the gain medium and inject pump light from the end facets of the double cladding fibers(DCFs).Recently,fiber laser oscillators develop rapidly and over 2 kW output power has been achieved.However,further power scaling of the oscillators based on DCF is difficult as results of the limited pump ports and heavy thermal load at the end facet causing by the pump light.The distributed side-coupled cladding-pumped(DSCCP)fiber provides a promising scheme of power promotion,because of its distributed pumping scheme which is in form of evanescent wave coupling and massive pump ports by cascading DSCCP fibers.However,present experimental studies of DSCCP fiber laser were limited and mainly focused on fiber amplifiers.Reports of DSCCP fiber oscillators are rare and only can be found in the dissertation of Dr.Huang Zhihe.Therefore,this paper mainly focuses on experimental study on high power DSCCP fiber oscillators,and then proposes the schemes of kW-level singlemode DSCCP fiber oscillators on the basis of experimental results.1.Experimental study on DSCCP fiber oscillatorsFirstly,one-section,two-section and three-section DSCCP fiber oscillators have been established respectively.The output power of the one-section DSCCP fiber oscillator has reached 760 W with the slope efficiency of 72.8% under a co-pumping scheme.It demonstrates advantages of co-pumping scheme for power scaling,and indicates the advantage of suppressing residual pump light of the counter-pumping scheme.The output power of the two-section DSCCP fiber oscillator has reached 1140 W with 68.1% slope efficiency.It demonstrates the advantage of power scaling of DSCCP fibers by cascading DSCCP fibers,and vertifies the advantage of suppressing residual pump light of the final-section counter-pumping scheme in cascaded DSCCP fiber oscillators.The output power of the three-section DSCCP fiber oscillator has reached 1969 W with 72.2% slope efficiency,which,to the best of our knowledge,is the highest output power achieved in DSCCP fiber-based oscillator and is the first demonstration of 2 kW-level fiber oscillators with 250-?m cladding diameter gain fiber.The beam quality have also been studied,and schemes for improving beam quality are proposed.2.Stimulations on DSCCP fiber oscillatorsCascaded DSCCP fiber oscillators model is proposed for the first time,to the best of our knowledge.Oscillators under experimental setup are tested with a theoretical model of DSCCP fiber oscillators.The simulation results match the experimental results well,which demonstrates the correctness of the theoretical model.Then,the scalability of cascaded DSSCCP fiber oscillators is studied with considerations of stimulated Raman scattering effect.Finally,schemes of kW-level singlemode DSCCP fiber oscillators are proposed,and simulated with the verified theoretical model.The simulation results imply the feasibility of mode-control of oscillators by decreasing the core diameter of the gain fiber.