Research On Repetition Frequency Lifting Mechanism Of Mode-locked Fiber Laser Based On Nonlinear Amplifying Ring Mirror

Mode-locked fiber lasers attract much attention these years since they can generate optical pulses with high peak power and short pulse duration,which can be used as ideal light sources for future ultra-high-speed,high-capacity,long distance fiber-optic communication systems.The stability of the laser and the repetition frequency of the output pulse are important indicators for measuring the performance of the laser.In the common passive mode-locked lasers,the mode-locked laser based on the nonlinear amplifying ring mirror is insensitive to the external environment because it can realize the polarization-maintaining fiber structure.This type of laser can meet the conditions for outputting stable pulses.However,in the direction of increasing the repetition frequency of the output pulse of the nonlinear amplified ring mirror(NALM)laser,a polarization-maintaining figure-of-8 cavity laser having a high(>20MHz)repetition frequency has not been reported so far.Therefore,in order to further improve the repetition frequency of the figure-of-8 cavity fiber laser,the feasibility of improving the laser output pulse repetition frequency by shortening the laser cavity length is studied firstly.Secondly,a dispersion-compensating fiber with a large positive dispersion coefficient near the wavelength of 1550nm is introduced into the NALM ring of the figure-of-8 cavity laser.Finally,a stable pulse output with a repetition frequency of 20.38MHz and a pulse width of 590fs is successfully obtained.The research we conducted and the experimental results obtained are as follows:1.The mode-locking principle of nonlinear amplifying ring mirror is analyzed.Based on this model,a dispersion-compensating fiber based on figure-of-8 cavity is built by adding dispersion compensation fiber to the main ring in the optical path structure,a stable pulse output with a repetition frequency of 12MHz and a pulse width of 1.6ps was obtained.2.In order to study the scheme of increasing the pulse repetition frequency of the laser output,the repetition frequency is improved by using the scheme of shortening the cavity length on the pulsed laser based on the saturable absorber mode-locking.The repetition rate can be effectively increased to 1.02GHz with a pulse width of 379fs.However,experiments have found that such lasers are susceptible to material degradation and lack long-term stability.3.In order to realize a stable laser with polarization maintaining structure,the experimental scheme for increasing the repetition frequency of the output pulse of a passive mode-locked laser with figure-of-8 cavity continues to be studied.The experimental model is optimized on the basis of 1 and the dispersion compensation is introduced in the secondary ring.After shortening the cavity length,finally a stable pulse output with a spectral width of 7nm,a repetition frequency of 20.3 8MHz,and a pulse width of 1.2ps was obtained.The pulse width outside the cavity can reach 590fs.