Composite frequency comb spanning 0.4-2.4 microm from a phase-controlled femtosecond Ti:sapphire laser and synchronously pumped optical parametric oscillator

A repetition-rate-stabilized frequency comb ranging from the violet to the mid-infrared (0.4-2.4 microm) is obtained by phase locking a femtosecond Ti:sapphire laser and a synchronously pumped optical parametric oscillator to a common supercontinuum reference. The locking results have bandwidths lower than 3 kHz. By changing the locking frequencies, different relative and absolute offsets of the constituent frequency combs are achievable.     

高激光损伤阈值Ge-As-S硫系玻璃光纤及中红外超连续谱产生

测量了Ge-As-S系列硫系玻璃在中红外波段的飞秒激光损伤阈值,研究了它与玻璃化学组成的关系.基于优化的玻璃组成,采用棒管法制备了芯径为15μm的阶跃折射率非线性光纤.采用飞秒脉冲抽运光纤,研究了光纤中超连续谱(supercontinuum,SC)的产生特性.在研究的Ge-As-S硫系玻璃中,具有化学计量配比的Ge0....     

飞秒平顶光束经微透镜阵列在熔融石英中的成丝及其超连续辐射

实验研究了平顶激光光束经微透镜阵列在熔融石英中成丝的演化以及超连续辐射的产生,并进一步与高斯光束的成丝和超连续辐射进行了对比研究.分别对这两种光束的多丝传输进行了横向和纵向成像.结果表明,使用平顶光束可以获得更为均匀的多丝分布,成丝的起点也更为一致;尤其重要的是,相对于高斯光束,平顶光束可以使用更高的入射激光脉冲能量而不会造成介质的损伤,从而可以获得更高脉冲能量和更高转换效率的超连续辐射.     

温度对飞秒激光脉冲在NaCl溶液中成丝产生的超连续谱的影响

利用飞秒激光脉冲在NaCl溶液中成丝,研究了溶液温度对飞秒激光成丝过程中产生的超连续谱的影响.发现在激光脉冲能量较低时,溶液温度对超连续谱的影响几乎可以忽略;而在激光脉冲能量较高时,随着NaCl溶液温度的升高,超连续谱呈现出被压缩的趋势.此外,在激光脉冲能量较高的情况下, NaCl中会产生大量的气泡.通过分析,得出了飞秒激光在溶液中成丝产生的气泡是影响超连续谱发射的主要因素.     

飞秒激光在光子晶体光纤中产生超连续光谱机制的实验研究

报道了利用零色散在780nm处的光子晶体光纤与纳焦耳量级的飞秒激光脉冲相互作用的实验结果.实验使用35fs，中心波长810—840nm，单脉冲能量可达14nJ的飞秒激光光源获得了超过一个倍频程的平坦超连续光谱（500—1100nm）.在不同功率、不同中心波长、不同啁啾和有无直流成分的多种飞秒脉冲激光的条件下，研究了超连续光谱的产生情况.并对一系列现象进行了对比，分析了超连续光谱产生的机制. 关键词： 光子晶体光纤 飞秒脉冲激光 超连续光谱     

飞秒激光成丝过程中由等离子体光栅引起的超连续谱增强与转移

在飞秒宽带激光脉冲成丝过程中, 由于双光束干涉效应而形成的空间等离子体光栅, 研究了空气中飞秒宽带激光脉冲成丝所产生的超连续谱增强效应, 并进一步观测到泵浦光脉冲成丝产生的超连续谱能量向较弱的未成丝的探测光脉冲转移, 有效拓展探测光脉冲带宽的新现象. 通过基于布拉格衍射特性的理论计算较好地解释了实验结果.     

基于多光子脉冲内干涉相位扫描法对飞秒激光脉冲进行相位测量和补偿的研究

研制了一套基于多光子脉冲内干涉相位扫描方法的可以同时对飞秒激光脉冲进行相位测量和补偿的实验系统装置.实验中,通过自主研发的LabVIEW程序控制液晶空间光调制器和光纤光谱仪,对待测飞秒激光脉冲施加相位扫描,并同时记录受到调制的飞秒激光脉冲的倍频光谱,得到了多光子脉冲内干涉相位扫描(MIIPS)轨迹图.通过MIIPS轨迹图的三次测量和迭代运算,还原出了经过预先啁啾调制的中心波长约为810 nm、重复频率为1 kHz的飞秒激光脉冲的光谱相位,测量精度在0.1 rad以内.根据测量结果,利用液晶空间光调制器对该飞秒激光脉冲进行相位补偿,得到了近似傅里叶变换极限的飞秒激光脉冲.这一装置将在多光子显微成像、脉冲整形、飞秒激光光谱学等众多领域发挥重要作用.     

1040nm泵浦的光子晶体光纤超连续光谱产生性能退化研究

光子晶体光纤已经被广泛应用于由飞秒脉冲激光源产生超连续光谱。当激光源的重复频率较低时,由光子晶体光纤产生的超连续光谱随时间的变化过程较为缓慢,通常不被注意到。而在天文光谱仪定标等应用中,需要使用GHz至几十GHz量级的高重复频率激光源。此时,可观察到光子晶体光纤的超连续光谱产生性能在有限时间内产生显著的退化。在1 040 nm飞秒激光泵浦条件下,通过测试三种不同气孔占空比的光子晶体光纤的超连续光谱产生性能演化,发现超连续光谱的退化进程随光纤气孔占空比的增大而加速。观察发生光谱退化后的光子晶体光纤样品,发现在光纤上超连续光被产生的区段出现多个不同颜色的亮点,呈现有方向性的光泄露现象。针对光泄露现象,通过测量光纤的吸收光谱线,证实了实验中超连续光谱退化的主因并非是光纤熔融石英材料中大量非桥氧色心产生。针对光泄露具有方向性这一特征,提出了经由多光子吸收作用在光纤纤芯中形成长周期光栅的理论。为探究影响光子晶体光纤超连续光谱产生性能的退化的因素,以达到光谱退化抑制的目的,首先通过改变了光纤的拉锥参数,期望增强光纤熔融石英材料的光子耐受性。实验结果证实了该方法的有效性较为有限。其次,从保持激光源的平均功率,降低激光脉冲的峰值功率和保持激光脉冲的峰值功率,降低激光源平均功率两个方面入手,对激光源进行调制。实验结果证明,光纤单位时间内接受的高峰值功率脉冲总量是影响其超连续光谱产生性能的最重要因素。在天文光谱仪定标的应用中,对超连续光谱光功率的需求并不高,使用斩波器降低光子晶体光纤入射光的平均功率是减缓超连续光谱产生性能退化过程的有效且简单可行的方法。     

All fiber supercontinuum light source using photonic crystal fibers pumped by nanosecond fiber laser pulses

A novel compact supercontinuum (SC) source using the single mode photonic crystal fibers (PCF) pumped with an all fiber MOPA fiber laser is demonstrated experimentally. A bandwidth of 700 nm is achieved by operating the pumping fiber laser at a wavelength of 1064 nm, pulse duration of 10 ns, repetition rate of 50 kHz and peak power of 1 kW. The SC generation is initiated through modulation instability (MI) which breakups the nanosecond pump pulses into picosecond or femtosecond pulses, and further broadened through nonlinear effects of PCF.     

Femtosecond supercontinuum characteristics control

Supercontinuum generation in the spectral range 530–1100 nm in series of photonic-crystal fibers pumped by femtosecond Ti:S laser Mira Oprima 900-F is achieved. The evolution of spectral characteristics of femtosecond supercontinuum is proofed to be dependent on pump pulse wavelength and power radiation. Polarization characteristics of supercontinuum spectral components are analyzed. We demonstrated experimentally the possibility of control of femtosecond supercontinuum generation.     

Backward supercontinuum emission from a filament generated by ultrashort laser pulses in air

Backward emission of the supercontinuum from a light filament induced by high-intensity femtosecond laser pulses propagating in air has been observed to be enhanced compared with linear Rayleigh-Mie scattering. This enhancement is interpreted as a nonlinear scattering process onto longitudinal refractive-index changes induced by the laser pulse itself. The spectral dependence of the supercontinuum angular distribution is also investigated.     

Formation of conical emission of supercontinuum during filamentation of femtosecond laser radiation in fused silica

The formation of conical emission of supercontinuum during filamentation of femtosecond laser pulses with central wavelengths in a wide range is studied experimentally, numerically, and analytically. The frequency-angular intensity distribution of the spectral components of conical emission is determined by the interference of supercontinuum emission in a filament of a femtosecond laser pulse. The interference of supercontinuum emission has a general character, exists at different regimes of group velocity dispersion, gives rise to the fine spectral structure after the pulse splitting into subpulses and the formation of a distributed supercontinuum source in an extended filament, and causes the decomposition of the continuous spectrum of conical emission into many high-contrast maxima after pulse refocusing in the filament. In spectroscopic studies with a tunable femtosecond radiation source based on a TOPAS parametric amplifier, we used an original scheme with a wedge fused silica sample. Numerical simulations have been performed using a system of equations of nonlinear-optical interaction of laser radiation under conditions of diffraction, wave nonstationarity, and material dispersion in fused silica. The analytic study is based on the interference model of formation of conical emission by supercontinuum sources moving in a filament.     

Supercontinuum generation under filamentation driven by intense femtosecond pulses in supercritical xenon and carbon dioxide

It is found that supercritical fluids are a unique source of multioctave supercontinuum radiation, which is generated upon filamentation of an intense femtosecond laser pulse. If the laser pulse power significantly exceeds the critical power of self-focusing, then a supercontinuum with a width of three and a half spectral octaves (from 350 to 2000 nm) is generated in supercritical xenon. The red wing of supercontinuum generated in supercritical carbon dioxide has the form of a plateau in the range from 1400 to 1900 nm, while the blue wing of the spectrum is almost completely attenuated.     

Interference model of femtosecond laser pulse conical emission in dispersive medium

A simple interference model is proposed for conical emission frequency-angular spectrum formation during the filamentation of femtosecond laser pulse in a nonlinear dispersive medium. The model allows to obtain analytical expressions for frequency-angular distributions of the supercontinuum spectral components of pulses at different wavelengths in media with arbitrary material dispersion law. The model reproduces the supercontinuum frequency-angular spectrum transformation for the case of laser pulse splitting into several subpulses and for multiple refocusing of the light field in filament. Frequency-angular spectra analytically calculated from the proposed interference model are in good agreement with the results of numerical simulations performed for the filamentation of femtosecond laser pulses in fused silica.     

飞秒激光离焦抽运熔融石英产生超连续白光的实验研究

利用飞秒激光在熔融石英介质中传输产生能量达数毫焦、波长范围覆盖400-900 nm 且光谱分布较为均匀的超连续白光,实验过程中将熔融石英介质离焦放置以避免被击穿. 研究了入射激光能量以及介质离焦距离对超连续白光特性的影响. 结果表明采用高能量的入射激光脉冲离焦抽运介质的方法能够有效避免介质 击穿损伤并提高超连续白光脉冲的能量输出.     

多色脉冲激光光源研究

采用钛宝石锁模飞秒激光器作为泵浦光源,利用1.8 μm纤芯的保偏光子晶体光纤的高非线性特性产生超连续谱,通过调节泵浦光源和光子晶体光纤之间的耦合,在可见光区,获得了中心波长分别在431 nm, 497 nm, 520 nm, 575 nm的mW量级的不同颜色的激光输出,并且测量输出的脉冲序列重复频率与泵浦光的一致,具有良好的稳定性.     

光子晶体光纤产生飞秒超连续谱的实验研究

研究了飞秒脉冲经过光子晶体光纤时超连续谱产生的物理机制。采用输出波长可调谐的钛宝石光参量放大器作为泵浦源,光纤光谱仪测量不同泵浦功率和不同泵浦波长条件下光子晶体光纤产生的超连续谱的光谱图,对进行了归一化处理后的不同泵浦功率和不同泵浦波长条件下的超连续谱进行对比,分析影响光子晶体光纤超连续谱差异的物理机制。实验结果表明,当泵浦波长不变时,随着入射泵浦脉冲平均功率的增大,波峰增多,谱宽也逐渐加宽并伴随着出现能量向短波方向集中的现象,泵浦功率到达一定强度时,超连续谱的宽度最后到达饱和,谱的包络趋于稳定;入射光功率稳定在300 mW时,超连续谱的宽度和形状皆受到泵浦波长影响,在760～840 nm范围内,泵浦波长越长,波峰数越多,泵浦脉冲波长离零色散点越近,光子晶体光纤产生的超连续谱谱宽会越宽,超连续谱的形状相对越平坦。     

对超短脉冲强激光在大气通道中产生的三次谐波偏振特性及白光光谱调制特性的研究

研究了超短脉冲强激光在大气中传输产生电离通道后辐射的三次谐波强度随激光偏振的变化.此外，还研究了产生的白光辐射在短波长上的有规律的谱调制，分析了白光谱调制的产生机理. 关键词： 超短脉冲激光 大气通道 超连续辐射 三次谐波     

Supercontinuum generation in short dispersion-shifted fiber by a femtosecond fiber laser

The supercontinuum generation has been obtained in short conventional dispersion-shifted fiber using the femtosecond pulses from a passively mode-locked erbium-doped fiber laser. In the experiment, the supercontinuum spectrum of >300 nm has been observed by injecting 70-fs pulses into a several-meter dispersion-shifted fiber. The simulation of the evolution in the fiber shows that spectral broadening arises from soliton dynamics when pumping using femtosecond pulses in the anomalous group velocity dispersion regime of the fiber.     

宽谱带飞秒激光超连续谱特性的研究

利用高增益、低色散飞秒激光放大器研究了多种非线性介质的超连续谱特性,获得宽谱带飞秒激光超连续谱。 关键词：