氩团簇高信噪比13—23nm软x射线辐射谱实验观察

用150fs的掺钛蓝宝石激光系统, 在功率密度约为5×10¹⁵ W/cm²时 激励氩(Ar)团簇，利用具有空间分辨能力的平场光栅谱仪观察到13—23nm波段Ar的软x射线谱，并观察到Ar的11阶离子谱线.在较宽的激光脉宽和较低的激光功率密度情况下，通过激励Ar团簇，获得 了Ar的高阶电离度的实验结果，且谱线的信噪比明显好于光场感应电离的情况，说明团簇的 形成大幅度地提高了激光能量的吸收效率. 关键词： Ar团簇 超短强激光 软x射线辐射     

用腔倒空Nd：YAG激光器对fs超短脉冲的同步放大

实现了用腔倒空Nd:YGA激光器对fs超短脉冲放大器的同步泵浦。由碰撞锁模环形染料激光器产生的80fs脉冲经二级染料放大后,实现放大倍数6.2×10~4,输出超短脉冲峰值功率～2×10~7W。     

Hollow-fiber compression of visible, 200 fs laser pulses to 40 fs pulse duration

We demonstrate the use of a very simple, compact, and versatile method, based on the hollow-fiber compression technique, to shorten the temporal length of visible laser pulses of 100-300 fs to pulse durations shorter than approximately 50 fs. In particular, 200 fs, frequency-doubled, Nd:glass laser pulses (527 nm) were spectrally broadened to final bandwidths as large as 25 nm by nonlinear propagation through an Ar-filled hollow fiber. A compact, dispersive, prism-pair compressor was then used to produce as short as 40 fs, 150 microJ pulses. A very satisfactory agreement between numerical simulations and measurements is found.     

Nonlinear optical properties of an azo-based dye irradiated by picosecond and nanosecond laser pulses

The nonlinear optical properties of an azo-based dye were investigated using Z-scan technique employing 38 ps pulses at 532 and 1064 nm, and 6 ns laser pulses at 532 nm. Large nonlinear absorption and nonlinear refraction were observed at both ps and ns 532 nm in the azoic dye. When excited at ps 1064 nm, this dye displayed a large two-photon absorption cross-section (σ₂=1810 GM). Meanwhile, the optical nonlinearity mechanism was discussed in terms of molecular structure, excitation wavelength, and pulse width.     

Ultrafast thermalization characteristics in Au film irradiated by temporally shaped femtosecond laser pulses

The ultrafast thermalization processes of Au film irradiated by multi-pulse sequences with variable temporal separation were investigated by numerical simulations. Two temporally sequential thermal relaxation mechanisms including the two temperature relaxation and the thermal diffusion relaxation were demonstrated. With inclusion of the electron ballistic effect, we obtained the full 2-D temperature fields evolution dominated by the two distinct relaxation mechanisms. It is proposed that the laser thermalization processes can be greatly promoted through choosing the optimized temporal separation of the pulse train. We also found that the two temperature relaxation period can be modulated by changing the pulse train parameters such as the temporal separation, the pulse durations and the pulse number in a train.     

Sub-100 fs SDPF optical soliton compressor for diode laser pulses

We report our studies on a fiber-optic soliton compressor for generation of sub-100 femtosecond (fs) optical pulses out of picosecond (ps) diode laser pulses. The soliton compressor is rather simple and composed of a 15 ～ 20 m-long step-like dispersion profiled fiber (SDPF) in conjunction with a single Er-doped fiber amplifier (EDFA). Careful design of such a SDPF compressor was performed, leading to demonstration of 20-fs class compression performance, and experimental investigation was carried out in detail on the optical pulse propagation in the compression processes. In addition, nonlinear fiber loops were applied to suppression of pulse pedestals, resulting successfully in high quality optical pulses of the 100-fs range.     

Ultrafast optical switching of an ionized medium by interfering ultrashort laser pulses

Multibeam ionization of gas and condensed-phase media with interfering ultrashort laser pulses can help to substantially increase the maximum field intensity and the density of free electrons attainable in the focus of the laser field relative to the regime of single-beam ionization. Multibeam ionization schemes offer new solutions for laser micro- and nanomachining, micro- and nanosurgery, spectral and temporal transformation of ultrashort light pulses, as well as remote sensing of the atmosphere. Subfemtosecond changes in the local refractive index induced in the regime of multibeam tunneling ionization enable the high-speed switching of optical signals.     

输出13.5fs激光脉冲的非共线式光参量放大器

利用BBO晶体在400nm的抽运光与信号光夹角为37°时,可同时在可见光范围内很宽的光谱范围里实现相位匹配的特点,实现了在可见光范围内具有极宽输出光谱宽度的飞秒光学参量放大器(OPA)的稳定运转,其输出脉冲能量大于15μJ,转换效率超过15%.并且在仅采用啁啾镜作为脉冲压缩器的情况下,获得了输出脉冲宽度为135fs的极窄光脉冲,其脉冲宽度压缩比高达22倍 关键词： 非共线相位匹配 光学参量放大 啁啾镜     

Ultrafast optical nonlinear properties of metal nanoparticles

The physical origin and the dynamics of the ultrafast optical nonlinear response of noble metal nanoparticles are analyzed around the surface plasmon resonance frequency using extension of the bulk metal electron kinetics and band structure models. The computed spectral and temporal responses are found to be in very good agreement with the measured ones in silver when taking into account the impact of electron excitation on both the interband absorption and electron optical scattering rate. A good reproduction of the strong excitation regime experimental results is also obtained in the case of gold, with a dominant contribution of the interband effect. Received: 4 July 2001 / Published online: 10 October 2001     

Synthesis and optical properties of basic fuchsin dye-doped PMMA polymeric films for laser applications: wide scale absorption band

Basic fuchsin dye-doped poly(methyl methacrylate) polymeric films were sensitized with various dye concentrations ranging from 0.0833 to 1.667 wt% of basic fuchsin. Their structure, linear absorption, and optical limiting properties were examined. The films were prepared using a simple and fast casting technique dissolved in chloroform for both the dye and the polymer. Structural characterizations were achieved by XRD, and the films showed an amorphous hump supporting the noncrystalline structure of studied polymeric composites. Spectrophotometer measurements were used to estimate the spectral absorption measurements of the films such as transmittance, absorbance with the calculations of absorption index (k), and optical energy band gap (E _g ) in the wavelength region from 190 to 2500 nm. Results show that the optical constants change with increasing the dye doping concentrations. It has been found that optical energy gap (E _g ) appearing that, both direct and indirect optical transitions are conceivable for these films. Optical limiting properties of the films with various dye concentrations were studied using a continuous wave He–Ne laser operating at 632.8 nm. The results appeared that the sample has an obvious optical limiting effect. The designed BF/PMMA composites can be applicable in wide-scale applications.     

Compression of high-energy laser pulses below 5 fs

High-energy 20-fs pulses generated by a Ti:sapphire laser system were spectrally broadened to more than 250 nm by self-phase modulation in a hollow fiber filled with noble gases and subsequently compressed in a broadband high-throughput dispersive system. Pulses as short as 4.5 fs with energy up to 20-microJ were obtained with krypton, while pulses as short as 5 fs with energy up to 70 microJ were obtained with argon. These pulses are, to our knowledge, the shortest generated to date at multigigawatt peak powers.     

SPM induced limitations for 40 Gbps chirped Gaussian pulses in optical channel

An analysis of self-phase modulation (SPM) induced nonlinearities in a 40 Gbps link with chirped Gaussian pulses of different duty cycles has been reported in this paper. In the present analysis only SPM effect has been considered to control the pulse propagation behavior through the fiber by appropriate selection of pulse width, peak power and channel length to suppress the other channel impairments caused by group velocity dispersion (GVD) and third order dispersion (TOD). The effect of SPM on the Q-factor for the transmission of 40 Gbps chirped pulses with different initial pulse widths and input peak powers has been investigated. It has been observed that a wider pulse having maximum negative chirp can withstand the nonlinear effect of SPM for relatively higher ranges of input peak power.     

非线性克尔效应对飞秒激光偏振的超快调制

研究了近红外飞秒激光的偏振在太赫兹频率的超快调制.利用抽运-探测光谱技术,通过改变两个脉冲之间的延迟时间可以控制光脉冲的旋转角.在Li：NaTb（WO₄）₂磁光晶体中观察到探测光的偏振随延迟时间变化的高速振荡,振荡信号的中心频率为0.19 THz.这种超快偏振调制现象可以解释为,抽运-探测实验构置中,前向传播的抽运光诱导的光学克尔非线性引起被晶体远端表面所反射的背向传播的探测光脉冲偏振面的额外旋转.通过改变抽运光的圆偏振旋性可以控制探测光调制信号的相位和振幅.实验结果表明,非线性光学克尔效应可以作为一种全新的手段,在磁光晶体中实现近红外飞秒激光以太赫兹频率的超快偏振调控.这将在超快磁光调制器等全光器件中得以应用.实验结果将有助于偏振依赖的超快动力学过程的研究.     

500 fs超短脉冲激光对CCD探测器的破坏效应

 采用脉宽500 fs,脉冲能量250 μJ的超短脉冲激光辐照线阵CCD探测器,观察到了CCD从线性响应到像元饱和、饱和串音直至硬损伤的整个过程,并着重对两种辐照能量密度下的硬损伤机理进行了理论分析。结果表明：激光能量密度为0.45 μJ/cm²时,达到像元饱和;能量密度为0.14 J/cm²时,辐照6个脉冲后实现了CCD器件的硬损伤,硬损伤源于晶格被加热并汽化形成等离子体;能量密度为1.41 J/cm²时,单个脉冲就使CCD器件的输出波形无法辨认,2个脉冲后CCD器件没有任何信号输出,硬破坏源于电荷分离形成的电场库仑力。     

330 MHz,sub 50 fs Yb: Fiber ring laser

We demonstrate 330 MHz repetition rate operation in a ring cavity Yb:fiber laser with an innovative wavelength-division-multiplexing collimator to raise the repetition rate. The spectral bandwidth of the pulse is 30 nm and the dechirped pulse width is 48 fs. The output power is 70 mW with 600 mW, 975 nm pump laser diode.     

5fs驱动激光脉冲的高次谐波选择性优化

研究了在紧聚焦实验条件下, 采用5 fs激光脉冲与氩气相互作用产生的高次谐波特性. 通过优化系统色散、气体靶气压与位置等参数, 观察到在60–73 eV波段范围内高次谐波光谱接近一个量级的增强. 进一步通过对单原子模型和传播方程的数值求解及模拟相位匹配实验参数下高次谐波的产生过程, 发现相位匹配在所观察到的实验现象中起着关键作用, 得到了理论上与实验规律一致的结果. 关键词： 极紫外激光 高次谐波 超快光谱     

飞秒激光在晶体中传输的级联非线性过程

系统地回顾了有关级联过程导致的各种非线性效应的工作,指出了级联效应可导致不同光谱成分间出现能量转移现象,且二阶级联过程中产生的白光还能够提供种子光进而得到基于参量放大过程的宽带锥形辐射。在此基础上,进一步研究了入射的基频波长对于超连续白光能量转移效果及产生倍频效率的影响以及二阶级联过程引起的偏振调制现象,指出当入射波长越靠近相位匹配波长时倍频效率越高,发现二阶级联效应还可以引起显著的基频光偏振调制。     

飞秒激光在晶体中传输的级联非线性过程

系统地回顾了有关级联过程导致的各种非线性效应的工作,指出了级联效应可导致不同光谱成分间出现能量转移现象,且二阶级联过程中产生的白光还能够提供种子光进而得到基于参量放大过程的宽带锥形辐射。在此基础上,进一步研究了入射的基频波长对于超连续白光能量转移效果及产生倍频效率的影响以及二阶级联过程引起的偏振调制现象,指出当入射波长越靠近相位匹配波长时倍频效率越高,发现二阶级联效应还可以引起显著的基频光偏振调制。