低重复频率被动锁模半导体碟片激光器

Low repetition rate passive mode-locked semiconductor disk laser

锁模半导体碟片激光器(semiconductor disk laser,SDL)兼具输出功率高与光束质量好的优点,但半导体增益介质ns量级很短的载流子寿命限制了锁模脉冲重复频率的降低,因而在一定程度上限制了锁模脉冲峰值功率的提高.本工作中增益芯片内较浅的量子阱所对应的载流子寿命相对较长,结合特殊设计的较小饱和通量的半导体可饱和吸收镜(semiconductor saturable absorption mirror,SESAM),获得了低重复频率、高峰值功率的被动锁模SDL.当温度为12℃时,利用六镜谐振腔产生的被动锁模激光脉冲重复频率低至78 MHz,为迄今为止在SESAM锁模SDL中所获得的最低重复频率.锁模SDL的平均输出功率为2.1 W,脉冲宽度为2.08 ps,对应的脉冲的峰值功率12.8 kW,为已有报道最高值的近2倍.

Semiconductor disk lasers(SDLs)have advantages of high output power and good beam quality.Their flexible external cavity provides convenience for inserting additional optical element to start mode locking and produce ultra-short pulse train with duration from picosecond to femtosecond.However,the very short lifetime in a range from about a few nanoseconds to tens of nanoseconds of the carrier in semiconductor gain medium limits the decrease of pulse repetition rate,thus restricting the increase of peak power of the mode-locked laser pulse to some extent.In this work,by using the relatively shallow In_(0.2)GaAs quantum wells,which have a relatively long carrier lifetime in the active region of gain chip,as well as the particularly designed semiconductor saturable absorption mirror(SESAM)that has a relatively small saturation flux,a passively mode-locked SDL with low repetition rate and high peak power is demonstrated.The used six-mirror cavity has a spot radius of about 200μm on the chip and a 40μm spot on the SESAM,and the total cavity length is about 1.92 m.The SESAM passively mode-locked SDL produces a stable pulse train with a lowest repetition rate of 78 MHz.When the temperature is 12℃ and the transmittance of the output coupler is T=3%,an average output power value of 2.1 W and a pulse duration of 2.08 ps are achieved.The corresponding pulse peak power reaches 12.8 kW,which is about twice the reported highest peak power in an SESAM mode-locked SDL.When T=2% and T=5%,the obtained average output power values are 1.34 W and 1.62 W respectively,and the corresponding pulse peak power values are 8.17 kW and 9.88 kW.Based on the values reported in the literature and the results of pulse repetition rate in our experiments,the estimated lifetime of the carriers of the In0.2GaAs quantum wells in the active region of the gain used chip is 16.4 ns.This high peak power mode-locked semiconductor disk laser has important potential applications in biomedical photonics,chemistry,and nonlinear microscopy.