Research On Laser Pulse 2-Micron Multimode Fibers

The research of rare-earth-doped single-mode pulsed fiber laser has been very well developed.The pulse energy and peak power of these fiber lasers are mainly restricted by various strong nonlinear effects happened due to the small cross-sectional area of single-mode fiber.In order to get higher single pulse energy and peak power,the simplest way is to enlarge the fiber core area,using a large mode area fiber.In this paper,we focus on experimental research about 2μm thuliumdoped multimode fiber laser.Through design and optimization of laser cavity,pulse generation and related dynamics of 2μm multimode fiber laser has been studied in depth,and transverse mode-control has been realized,improving the capability of 2μm fiber lasers.First,mode-locking of 2μm step-index multimode fiber laser is achieved based on modulation with a semiconductor saturable absorber mirror(SESAM).Maximum average output power,single-pulse energy and peak power of 10.6W,530 nJ and 17 kW have been achieved,and novel dynamics of coupling between longitudinal modes and transverse modes has been explored.Secondly,an optical fiber Bragg grating(FBG)has been introduced into the laser cavity for further exploring the relationship between longitudinal modes and transverse modes and realizing transverse-mode control.It is found that there is a one-to-one relationship between transverse modes and longitudinal modes based on the unique spectral reflection of the multimode FBG.Through combining the mode-selection characteristics of the FBG and the spatial mode filtering of the feedback mirror,transverse mode-control of multimode fibers can be realized.Based on the research under mode control,single higher-order mode Q-switching or mode-locking can be realized through SESAM modulating the multimode fiber laser.In Q-switched operation,the fiber laser generates 5.58 W of average power,46.10μJ of pulse energy and 57.63 W of peak power.Through tuning the laser,single higher-order-mode mode-locking of the fiber laser can be observed with 38.8nJ of pulse energy,9.88 kW of peak power,and pulse width of 3.93 ps.This kind of pulsed higher-order-mode fiber laser has much higher performance than that based on single mode fibers,showing great potential in improving pulse energy and peak power of fiber lasers.At the same time,intermodal nonlinear interaction and the unique mode-selecting feature of multimode FBGs provide an excellent platform for further studying nonlinear dynamics of multimode fiber solitons and exploring novel nonlinear optical phenomena.