摘要
报道了激光二极管(LD)直接泵浦的吉赫兹(GHz)重复频率克尔透镜锁模Yb∶KGW激光器。采用976 nm单模光纤耦合LD作为泵浦源,在基于Yb∶KGW的“蝴蝶形”结构环形腔中,实现了1 GHz重复频率的克尔透镜锁模激光输出,平均功率为90 mW,脉冲宽度为265 fs。进一步利用979.5 nm单模光纤耦合LD作为泵浦源,将锁模脉冲的平均功率提高到了151 mW,对应的脉冲宽度为249 fs、中心波长为1045 nm。
Objective Optical frequency combs based on femtosecond lasers are powerful tools for the development of precision metrology.Femtosecond lasers with high repetition rates are particularly important when optical-frequency combs are applied to spectroscopy.However,it is difficult to realize high-repetition-rate Kerr-lens mode-locking because of the weak Kerr nonlinear effect.Due to the mode matching limitation of soft aperture Kerr-lens mode-locking,a single-mode fiber-coupled laser diode(LD) is usually used as the pump source,and an output coupling(OC) mirror with low transmittance is chosen to improve the intracavity peak power,resulting in a low output power.Output power is an important parameter when a high-repetition-rate laser is used for optical frequency comb generation.Therefore,it is essential to improve the output power of GHz-mode-locked lasers.Methods The Yb∶KGW crystal is an excellent candidate for Kerr-lens mode-locking because of its high nonlinear refractive itndex of 2×10~(-15) cm~2/W.It has a large absorption cross section(1.22×10~(-19) cm~2) at 980 nm,which additionally supports its high pumping efficiency.Moreover,the emission cross section at 1023 nm is as high as 2.8×10~(-20) cm~2,meaning it has a high gain,which is expected to achieve a high power output.Based on the above properties,we demonstrate a high-power Kerr-lens mode-locked Yb∶KGW femtosecond laser with 1 GHz repetition rate.The experimental setup is shown in Fig.1.Single-mode fiber-coupled LDs with center wavelengths of 976 nm and 979.5 nm are selected as the pump sources.To improve the mode matching,the pump light is collimated and focused through a coupling system with an imaging ratio of 1∶3,and the radius of the focused pump spot is 11μm.A 2-mm long Yb∶KGW crystal coated with an antireflection film@980-1100 nm on both transparent surfaces is used as the gain medium.During the experiment,the crystal is wrapped with indium foil and mounted on a water-cooled copper heat sink block to ensure effective heat dissipation.The resonant cavity is a four-mirror ring cavity with a total cavity length of 274.5 mm,corresponding to a repetition rate of 1.093 GHz.M_1 and M_2are concave mirrors with a radius of curvature(ROC) of 50 mm,and M_2 and M_3 are interferometer(GTI) mirrors providing-1600 fs~2(1035-1055 nm) group delay dispersion(GDD) in total per round trip.An OC with a transmittance of 0.4%is selected to ensure a sufficiently high power in the cavity.Results and Discussions First,using the 976 nm single-mode LD as the pump source,mode-locking with an average power output of 90 mW at a center wavelength of 1045 nm and a full width at half maximum(FWHM) of 4.5 nm is achieved[Fig.2(a)],and the corresponding pulse duration is 265 fs[Fig.2(b)].To increase the output power of mode-locking,the pump source is replaced with a 979.5 nm single-mode LD,whose wavelength is closer to the main absorption peak wavelength of the crystal.In addition,the polarization of the pump light is adjusted using a half-wave plate to further improve the absorption efficiency of the crystal.Consequently,the average power of unidirectional mode-locking is increased to 151 mW.The FWHM of the optical spectrum is approximately 4.2 nm at a center wavelength of 1045 nm[Fig.3(a)],and the corresponding pulse duration is 249 fs[Fig.3(b)].In the laboratory environment,mode-locking can be self-started only by increasing the pump power,and the root mean square(RMS)value of power fluctuations within 24 h is 0.76%[Fig.4(b)].The power jitter in the vicinity of 3 h and 6 h is because of environmental vibration and other factors that cause the laser to move out of the stable mode-locked state;however,it can restart in a short time.The signal-to-noise ratio of the 1.029 GHz fundamental frequency signal in radio frequency spectrum is 62 dB[Fig.4(a)].Conclusions We report a 1 GHz Kerr-lens mode-locked Yb∶KGW laser pumped by a laser diode.A 976 nm single-mode fibercoupled laser diode is used as the pump,and a stable mode-locked operation at a repetition rate of 1 GHz with an output power of90 mW is obtained in a bow-tie ring cavity with a pulse duration of 265 fs.By using a 979.5 nm single-mode laser diode as the pump source and adjusting the polarization of the pump beam,the output power is increased to 151 mW at a center wavelength of 1045 nm,and the corresponding pulse duration is 249 fs.
作者
陈月航
郑立
麦海静
于洋
田文龙
朱江峰
魏志义
Chen Yuehang;Zheng Li;Mai Haijing;Yu Yang;Tian Wenlong;Zhu Jiangfeng;Wei Zhiyi(School of Optoelectronic Engineering,Xidian University,Xi an 710071,Shaanxi,China;Academy of Advanced Interdisciplinary Research,Xidian University,Xi an 710071,Shaanxi,China;Beijing National Laboratory for Condensed Matter Physics,Institute of Physics,Chinese Academy of Science,Beijing 100190,China)
出处
《中国激光》
EI
CAS
CSCD
北大核心
2023年第2期38-42,共5页
Chinese Journal of Lasers
基金
国家自然科学基金(11774277)
科技部重点研发计划(2017YFB0405202)
陕西省自然科学基础研究计划(2019JCW-03)
西安市科技创新计划(202005YK01)。
