Keywords: 
• 光纤光栅 /
• 掺铥光纤激光器 /
• 纳秒锁模 /
• 石墨烯

Abstract: The Tm-doped mode-locked pulsed fiber lasers,which are known for their wide applications in optical communication,laser medical system and special material processing,have attracted considerable interest as novel laser sources.Up to now,many reported Tm-doped mode-locked fiber lasers focused on emitting picosecond or femtosecond pulses at a few megahertz (MHz) repetition rate.Actually,due to the strong chirp,large pulse width,low peak power and little nonlinear phase accumulation characteristics in the process of power amplifier,nanosecond mode-locked fiber laser is a representative of ideal seed source in the chirped pulse amplification (CPA) system.However,nanosecond mode-locked fiber lasers are generally implemented with the kilometerlong cavity length,corresponding to the fundamental repetition rate of hundreds of kilohertz.Usually,fiber lasers with such a low repetition rate are not desirable in applications of laser material processing,nor medical treatment nor scientific researches.In this paper,we report a nanosecond mode-locked Tm-doped fiber laser with MHz repetition rate based on graphene saturable absorber (SA).As the SA,graphene has excellent optical properties,such as optical visualization,high transparency,ultra-fast relaxation time and nonlinear absorption.It is not limited by the band gap either because of its zero-band-gap structure.Therefore,graphene can be used as fast SA,with wide spectral range operated.Generally,graphene suitable for mode-locked fiber lasers can be produced by using chemical vapor deposition (CVD),liquid phase exfoliation and mechanical exfoliation.Since the CVD technique can obtain high-quality graphene with precisely controlled number of layers,it is always the first choice for the manufacture of graphene.In our work,monolayer graphene layers are grown on copper foils by CVD,and then transferred onto the end face of the fiber connector three times.Meanwhile,a narrow-band fiber Bragg grating is used to constrain longitudinal modes of the laser intra-cavity.By simply adjusting the pump power and the polarization angle of polarization controller,stable 2 μm nanosecond mode-locked pulses are obtained in a wide range from 3.8 ns to 94.3 ns at 3.8 MHz repetition rate.We believe that the results obtained will be helpful for investigating the CPA system at 2 μm.