报道了一种基于石墨烯可饱和吸收体的纳秒锁模掺铥光纤激光器.该激光器采用环形腔结构,利用自制的三层石墨烯薄膜作为可饱和吸收体实现锁模.同时在腔内插入一个窄带光纤光栅,约束腔内起振的纵模数,适当调节抽运功率和偏振控制器的角度,得到了重复频率为3.8 MHz、脉宽在3.8-94.3 ns之间灵活可调的2μm纳秒锁模脉冲输出,整个脉宽调节范围超过90 ns.此外,由于获得的兆赫兹纳秒锁模脉冲时间带宽积在49-1119范围内,即存在强烈的啁啾,因而可作为2μm波段啁啾脉冲放大系统中的种子源使用.%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.
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