飞秒泵浦不同锥长微结构光纤超连续谱的产生

摘要

Tapered microstructure fibers with different taper lengths and waist diameters are pumped with femtosecond laser for supercontinuum generation .With“fast and cold tapered method” ,home made microstructure fiber with air‐hole pitch Λ=6.53μm and normalized air‐hole diameter d/Λ=0.79 were tapered to 6 ,8 ,10 mm taper length while keeping d/Λunchanged .Nu‐merical simulations show that the zero dispersion wavelength shifts to blue when the taper waist shrinks .The zero dispersion wavelengths for untapered and 6 ,8 ,10 mm length tapered fiber were 1 029 ,885 ,806 ,and 637 nm ,respectively .In the experi‐ment ,120 fs pulses centered at 810 nm ,which is generated by mode‐locked Ti:sapphire laser at a repetition rate of 76 M Hz ,is coupled into the tapered microstructure fiber .With the tapered length of 6 mm ,the center wavelength of the pump light locates in the normal dispersion region of the fiber and near the zero dispersion wavelength of the tapered waist .The main factors causes spectra broaden are intrapulse Raman scattering and cascaded four‐wave mixing .When the pump power reaches 450 mW ,con‐tinuous spectra with -5 dB flatness are generated at 390~461 and 1 134~1 512 nm .With 500 mW pump power ,supercontinu‐um spans from 366 to 2 450 nm ,which has already covered ultraviolet ,visible ,near‐infrared and mid‐infrared .This broadband spectrum almost reaches the red and blue edges of the microstructure fiber’s transmission bandwidth .With 8mm tapered length and 450 mW pump power ,the blue edge of the continuous spectrum shifts down to 366 nm as a result of group velocity match and group acceleration mismatch ,a 9 nm deeper blue shift compared to results from 6mm tapered length .With the tapered length of 10 mm ,because the zero dispersion wavelength of the waist also moves to visible region ,phase matching condition can still be satisfied in that region .Due to the effect of cascaded four‐wave mixing ,the frequency up conversion is realized in visible region .When pump power reaches 500mW ,up conversion frequency lies in 30 nm band from 382 to 412 nm ,the conversion effi‐ciency is up to 27.7% .%对飞秒脉冲泵浦下，不同锥长及锥腰直径的微结构光纤的超连续谱产生进行了实验研究。采用“快速低温拉锥方法”，在保持d／Λ不变的情况下，对实验室自制的空气孔间距Λ＝6.53μm，归一化孔径d／Λ＝0.79的微结构光纤进行了拉锥，分别得到6，8，10mm等不同锥长微结构光纤。理论计算表明，随着锥长变长，锥腰直径变小，锥腰处零色散波长向短波移动：未拉锥及6，8和10mm锥微结构光纤锥腰处零色散波长分别为1129，885，806和637nm。利用中心波长为810nm，重复频率76MHz，脉宽120fs的钛蓝宝石飞秒激光器对拉锥后微结构光纤进行了实验研究：锥长为6mm时，泵浦光中心波长位于整根光纤的正常色散区，锥腰的零色散点附近，内脉冲拉曼散射和级联四波混频是光谱初始展宽的主要因素。泵浦功率达到450mW时，在可见波段390～461nm及红外波段1134～1512nm形成－5dB的平坦宽带连续光谱。泵浦功率达到500mW时，出现366～2450nm覆盖紫外、可见、近红外、中红外的超连续谱，其光谱红蓝移边缘已经接近实验用微结构光纤的传输带宽。锥长为8mm、泵浦功率为450mW时，在群速度匹配和群加速度失配的共同影响下，连续谱蓝移边缘达到366nm，比6mm锥时蓝移9nm；锥长为10mm时，由于锥腰处零色散点移动到可见光区域，可见区光谱仍能满足相位匹配条件。通过级联四波混频效应，在可见区域实现了频率上转换及光谱蓝移。泵浦光功率达到500mW时，在382～412nm得到谱宽仅为30nm，转换效率达到27.7％的频率上转换。