Efficient shortening of self-chirped picosecond pulses in a high-power CO(2) amplifier

We report a factor-of-6 shortening of the 240-ps (FWHM) pulses in a triple-pass, 2.5-atm CO(2) amplifier. This technique is based on the self-phase modulation of a 10-mum pulse in a plasma after the first pass of amplification, followed by narrowing of this chirped pulse during further amplification. Subsequently, strong power broadening provides the necessary bandwidth to amplify 40-ps pulses to terawatt power levels.     

Compression of single-cycle mid-infrared pulses by Raman-active molecular modulators

We show theoretically how high-order stimulated Raman scattering in the impulsive pump-probe regime can be used for generation of single mid-infrared (MIR) single-cycle pulses. The propagation of MIR probe pulses in a hollow waveguide filled with a Raman-excited gaseous medium, with a probe delay in the maximum of the molecular oscillations, results in spectral broadening covering almost 2 octaves. The spectral phases of this broadening can be compensated for by use of an output glass window with anomalous dispersion in the MIR. The spectral and temporal characteristics of the output pulses and the mechanism of pulse compression are studied by use of numerical and analytical solutions, and compression of a 70-fs input pulse at 4 microm to a single-cycle 6.5-fs output pulse is shown.     

Third harmonic generation by ultrashort laser pulses tightly focused in air

The influence of focusing conditions (numerical apertures from 0.004 to 0.06) on absolute energetic characteristics of third harmonic generation (THG) in air was experimentally studied for pumping 1R (744 nm wavelength) femtosecond laser pulses. THG was observed both for sub-critical and super-critical laser pulses in the linear and non-linear propagation modes, respectively. The maximum THG efficiency of 1.6 × 10⁻³ was obtained in our experiments at the tight focusing conditions and the sub-critical pulse powers.     

皮秒光纤激光脉冲两个关键问题的研究

窄带耗散孤子锁模光纤激光器可以产生接近变换限制的皮秒脉冲，但受非线性相移的限制，输出脉冲重复频率不能通过增加腔长来降低，脉冲能量仅在0.1 nJ以下，严重制约着这类皮秒脉冲的实际应用。提出一种通过耦合器抽取腔内脉冲能量、抑制腔内非线性相移积累，进而允许增加腔长来降低窄带耗散孤子皮秒光纤激光脉冲重复频率的方法。运用该方法，成功地将激光器重复频率由35.2 MHz降低到了1.77 MHz，且脉冲时频特性保持不变。提出了一种基于级间FBG陷波滤波的抑制皮秒脉冲光纤放大中光谱展宽的方法。通过简单地使用级间陷波滤波器，既可窄化第一级光纤放大器后的输出脉冲谱宽，允许采用第二级光纤放大器进一步提升脉冲能量，而且，还可将脉冲重塑为近高斯形，利用高斯脉冲光谱展宽斜率小的特点，允许第二级光纤放大器将脉冲能量提升得更高。利用该方法，在RMS（均方值）谱宽保持0.4 nm以内的前提下，10 ps脉冲经标准单模光纤放大器后，能量可由0.2 nJ可提升到10 nJ以上。     

Significance of prechirping on long-haul path-averaged soliton impulse in re-circulating loop at 10 and 20 Gb/s with TOD

In this paper, we have investigated the performance of first- and second-order path-averaged soliton long-haul transmission link including the impact of third-order dispersion (TOD) at varied chirp. Here, the varied chirp is considered keeping in view the inadvertent frequency chirp imposed on all practical sources of short optical pulses. The propagation of strongly chirped pulses in loss-managed long-haul path-averaged soliton transmission network has been shown. The investigations reveal that in first-order (N=1) path-averaged soliton transmission link at 10 and 20 Gb/s, SPM effect on the rising and falling edges of a pulse results in spectral broadening for all values of induced chirp. On the contrary, spectral narrowing of the pulses is observed in second-order negatively chirped path-averaged soliton pulses. The effect of the nonlinearity changes from narrowing to broadening of pulses if the sign of the initial chirp is changed to positive. The results ascertain that the system is capable of transmitting a pulse up to the distance of 24,500 km at bit rates of 10 and 20 Gb/s. Investigations have been carried out by varying the chirp factor in the range −1 to 1 and −1 to 0.4] for 10 and 20 Gb/s, respectively, to demonstrate the robustness of the long-haul soliton link. The observations establish that the pulse width (full-width at half-maximum (FWHM)) remains within the optimal range even at the transmission distance of 24,500 km without and at discrete values of the chirp factor.     

利用脉冲负反馈压缩准分子激光脉冲

 分析了准分子激光脉冲在放大过程中的展宽现象。将饱和开关技术应用于放电泵浦激光器放大腔的放大压缩,发展了脉冲负反馈饱和压缩技术。利用该技术在天光一号系统前端LPX-150激光器上成功获得能量100 mJ、脉宽10 ns的脉冲输出,其空间分布均匀性保持在3%以内。这证明该技术可与无阶梯诱导非相干技术联合使用,在保证光束均匀性基础上进行脉冲压缩。     

粒子数密度对飞秒Gauss型脉冲传播及光谱特性的影响

利用预估校正-时域有限差分(PC-FDTD)法求解全波Maxwell-Bloch方程,研究介质粒子数密度(N)对飞秒Gauss型激光脉冲在Λ型三能级原子介质中传播及光谱特性的影响.结果表明:小面积2π脉冲在不同N介质中都不发生分裂,脉冲频谱基本没有新的高频成分产生,随N增大中心频率附近光谱强度明显减小.面积4π脉冲,在N较大的稀疏介质及稠密介质中都产生分裂,在稀疏介质中随N增大频谱展宽幅度及高频成分强度增大,但在稠密介质中频谱展宽变小且远小于N较大时的稀疏介质情况.大面积8π脉冲,脉冲分裂情况与4π脉冲情况相似,但随N增大频谱展宽幅度及高频成分强度单调增大,且在稠密介质中的频谱展宽幅度及高频成分强度远大于N较小的稀疏介质情况.     

Self-compression of millijoule pulses to 7.8 fs duration in a white-light filament

We demonstrate a novel technique for pulse compression of few-millijoule pulses with shorter than 10 fs duration. Our technique relies on spectral broadening in a white-light filament generated in a noble gas. In this filament we observe self-compression of 45 fs pulses down to below 8 fs duration without the need for any additional dispersion compensation. Using input pulses of 5 mJ, we generate compressed pulses with up to 3.8 mJ pulse energy. Therefore this method is much more efficient than previously demonstrated compression schemes. The generated peak powers of more than 100 GW at a kilohertz repetition rate open up a perspective for compression of few-cycle pulses with energies well beyond the capacity of hollow-fiber compressors.     

Slow light pulse propagation in dispersive media

We present a theoretical and numerical analysis of pulse propagation in a semiconductor photonic crystal waveguide with embedded quantum dots in a regime where the pulse is subjected to both waveguide and material dispersion. The group index and the transmission are investigated by finite-difference-time-domain Maxwell–Bloch simulations and compared to analytic results. For long pulses the group index (transmission) for the combined system is significantly enhanced (reduced) relative to slow light based on purely material or waveguide dispersion. Shorter pulses are strongly distorted and depending on parameters broadening or break-up of the pulse may be observed. The transition from linear to nonlinear pulse propagation is quantified in terms of the spectral width of the pulse. To cite this article: T.R. Nielsen et al., C. R. Physique 10 (2009).     

不同入射脉冲强度线性啁啾对BaF2晶体交叉偏振波输出特性影响的数值模拟研究

在超短飞秒激光技术中, 基于BaF₂晶体三阶非线性效应的交叉偏振波技术是目前最重要的提高脉冲时域对比度的手段之一. 针对不同入射脉冲强度下入射脉冲的线性啁啾对BaF₂晶体交叉偏振波的输出特性, 包括转换效率、光谱加宽和时域压缩的影响进行了详细的数值模拟和讨论.特别地, 详细对比研究了非饱和及强入射脉冲情况下线性啁啾对交叉偏振波输出脉冲光谱加宽的影响. 结果表明, 在强入射脉冲情况下, 交叉偏振波的光谱加宽量从非饱和情况下相对于零点线性啁啾时的对称结构, 变得不再具有对称性, 并对产生非对称性的原因进行了详细的物理解释.     

A time-compensated spectral filter for transform-limited tunable picosecond pulse generation from spectro-temporal-selection dye lasers

Diffraction and transform-limited picosecond tunable pulses are generated from Spectro-temporal-Selection (STS) dye lasers by using a new extra-cavity filter. This filter is based on a grazing-incident grating and arranged in the configuration of a folded dispersive delay line. Thus, it provides both high spectral selectivity and controllable temporal compensation for elimination of pulse broadening. Direct production of diffraction- and transform-limited picosecond dye laser (10 µJ, 50 ps) pulses spectrally adjustable between 398 and 702 nm is demonstrated in a compact device, with 8 ns pump pulses from a nanosecond nitrogen laser.     

Transmission Properties of the Femtosecond Pulse Through a Near-field Optical Fiber probe

The experimental results are given on the broadening characteristics of the time and frequency profile of the femtosecond pulse passed through a SNOM fiber probe. The power threshold for the spectral and temporal broadening was 158 μw for pulses with 250 fs width and the repetition rate of 250 kHz at 830 nm wavelength. The time broadening reached lps for 250 fs incident pulse of 500 μw. A finite-difference time-domain (FDTD) simulation was carried out to illuminate the transmission properties of ultrafast pulse through the SNOM probe. The broadening influence on temporal resolution spectrum and near-field fluorescence excited by two-photon technique are discussed.     

Ionization-assisted guided-wave pulse compression to extreme peak powers and single-cycle pulse widths in the mid-infrared

Ionization-assisted spectral broadening of high-energy 10.6 μm laser pulses in a gas-filled hollow waveguide is shown to yield single-cycle pulses with multiterawatt peak powers in the mid-IR. While the highest quality of pulse compression is achieved in the regime of weak ionization, careful management of complex ionization-assisted spectral broadening of guided-wave fields is the key to compressing the output of advanced high-power mid-IR laser sources to single-cycle pulse widths.     

单极性亚纳秒脉冲激励口径的辐射特性

 通过解析积分运算，获得了阶跃脉冲激励口径天线瞬态辐射场的解析表达式, 分析了辐射电场脉冲的持续时间与口径尺寸、观察距离及观察角度的关系。研究了在高斯脉冲激励下口径辐射场特性的分析方法，并探讨了脉冲口径天线主轴方向上辐射场的分区依据。     

锥形镀膜光纤探针中飞秒激光脉冲的传输

采用三维时域有限差分法对飞秒激光脉冲照明下锥形镀膜光纤探针内的光场进行了计算,研究了飞秒脉冲在探针中传输时的时域、频谱和相位特性,并分析了不同锥角和不同长度的探针对这些特性的影响.研究结果发现飞秒脉冲在探针中传输时出现了脉冲展宽、幅值振荡以及频谱分裂等现象,并且利用频谱和相位的变化特性初步解释了脉冲的展宽和脉冲振幅随时间周期性变化等现象. 关键词： 时域有限差分法 飞秒脉冲 光纤探针     

Femtosecond parabolic pulse nonlinear compression with gas-filled hollow-core fiber

We study theoretically the spectral intensity evolutions of the femtosecond Gaussian and parabolic pulses with different initial pulse energies and compare the nonlinear compressions of these pulses based on a meter-long hollow-core fiber filled with neon for different initial pulse durations. The pulses are first coupled into gas-filled hollow-core fiber for spectrum broadening, then compressed by the optimal chirp compensation. The parabolic pulse possesses a shorter pulse duration, larger peak power, and cleaner wings than Gaussian pulse. The properties are useful for compressing the pulses and thus generating the high-energy, short-duration pulses.     

Soliton Propagation in Birefringent Fibers

In this article, the propagation of solitons in a single mode fiber with polarization mode dispersion (PMD) is analyzed. In optical fibers, the randomly varying birefringence degrades soliton transmission system in two aspects. First, the dispersive waves cause pulse broadening. Second, the dispersive waves interact with other soliton pulses. Here we studied the effects of PMD on a single pulse and the variation of pulse broadening, energy decay, and degree of polarization on a single soliton pulse propagating over a very long distance.     

Laser fabricated microchannels inside photostructurable glass-ceramic

Microchannels have been fabricated by laser direct-write in photostructurable glass-ceramic (Foturan) for their application in 3D-microfluidic systems. A Nd:YAG laser delivering 10 ns pulses at 355 nm wavelength has been used for irradiation. Afterwards, thermal treatment and chemical etching have been required for channel formation. The kinetics of channel formation and the channel morphology have been studied by optical and electron microscopy. A minimum accumulated energy (pulse energy multiplied by the number of pulses in a same site) is required to induce channel formation. Channels with symmetric round apertures at both ends can be obtained when using low pulse energies. On the contrary, irradiation with too high energetic pulses produces direct material damage in Foturan and provokes the formation of non-symmetric channels. One millimetre long channels with a minimum radius of 15 μm can be opened through Foturan slides after 15 min of chemical etching.     

不同偏振飞秒激光经块状材料传输后的脉宽压缩

实验研究了线偏振和圆偏振状态下的飞秒强激光脉冲在块状材料中的传输过程。不同偏振的激光脉冲在传输过程中得到了不同程度的光谱展宽,经色散补偿后,脉冲时域宽度均得到了压缩。详细分析了压缩脉冲的脉宽以及啁啾情况与入射激光脉冲能量之间的关系,比较了飞秒激光在线偏振及圆偏振情况下的不同压缩效果。在线偏振入射光情况下得到了最短21fs的压缩脉冲宽度,在圆偏振情况下得到的最短脉冲宽度为22fs。实验结果表明,这种光谱展宽与色散补偿方式对圆偏振光同样适用,而且圆偏振的入射激光将更有利于对更高能量的脉冲进行压缩。在色散补偿量相同的情况下,压缩效果随入射脉冲能量变化的规律符合理论估计。     

Generation of intense ultrabroadband optical pulses by induced phase modulation in an argon-filled single-mode hollow waveguide

We experimentally demonstrate the generation of intense ultrabroadband optical pulses whose spectrum ranges from 300 to 1000 nm (700-THz bandwidth) with a well-behaved spectral phase and 23-muJ pulse energy by a novel, simple setup utilizing induced phase modulation (IPM) in an argon-filled single-mode hollow waveguide. Fundamental as well as second-harmonic pulses produced by one common femtosecond pulse from a Ti:sapphire laser-amplifier system are copropagated in the hollow waveguide. The effect of the delay time between the two input pulses on the IPM spectral broadening is clarified and confirmed to agree with the theoretical result. It is found that the compressed pulse duration from this pulse is 1.51 fs if its phase is completely compensated for.