Suppression of electron spin-echo envelope modulation peaks in double quantum coherence electron spin resonance

We show the use of the observer blind spots effect for the elimination of electron spin-echo envelope modulation (ESEEM) peaks in double quantum coherence (DQC) electron spin resonance (ESR). The suppression of ESEEM facilitates the routine and unambiguous extraction of distances from DQC-ESR spectra. This is also the first demonstration of this challenging methodology on commercial instrumentation.     

Generation of anti-Stokes radiation upon amplification of stokes pulses in transient stimulated Raman scattering

An analytical solution is obtained for equations of transient stimulated Raman scattering for short samples and a weak seed Stokes wave in the approximation of a given pump field, and a numerical solution for more extended media or more intense input Stokes pulses with allowance for pump depletion. The dependence of the anti-Stokes wave energy on its spatial mismatch with the Stokes and laser waves is studied. The optimum angle of the anti-Stokes generation is found as a function of the length of the system, transverse relaxation rate, laser pulse energy, and intensity ratio of the Stokes and pump pulses at the entrance of the sample.     

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.     

Analyzing the Switching Characteristics of a NOLM Demultiplexer with on Analytic and Numerical Methods

The switching characteristics of a NOLM demultiplexer are investigated by using an analytic and numerical methods. The results reveal that the switching characteristics of a NOLM demultiplexer depend on parameters such as the power coupling ratio of the coupler,the peak power of control pulses,the walk-off and the initial time delay between signal pulses and control pulses,etc. The efficiency of NOLM is dependent on the peak power of control pulses. However,the switching window profile begins to oscillate as the power exceeds a critical value and this will be detrimental to the demultiplexed signals. The signal-control pulse walk-off can broaden the switching window of NOLM,make the window profile deviate from its central position,and decrease its switching efficiency.     

Transform-limited behavior from the synchronously pumped cw dye laser

We report for the first time transform limited behavior from the picosecond synchronously pumped cw dye laser. This is accomplished by the use of an inexpensive, low-loss intra-cavity etalon. The Fourier relationship between pulse frequency width and time duration is shown to be obeyed quantitatively at three different wavelengths. The pulse profile changes as a function of wavelength and does not follow an analytical functionality at any wavelength studied. For each wavelength, the optical cavity length producing transform limited pulses is shown to be the same.     

Continuum generation of the third-harmonic pulse generated by an intense femtosecond IR laser pulse in air

The continuum generation by intense femtosecond IR laser pulses focused in air including the effect of third-harmonic generation is investigated. We have used a theoretical model that includes the full spatio-temporal dynamics of both the fundamental and the third-harmonic pulses. Results of our numerical calculations show that a two-color filamentation effect occurs, in which the third-harmonic conversion efficiency remains almost constant over the whole filament length. It is found that this effect is rather independent of the wavelength of the input beam and the focal geometry. During the filamentation process the third-harmonic pulse itself generates a broad continuum, which can even overlap with the continuum of the fundamental pulse for the longer pump wavelengths. In consequence, the continuum generation generated by intense IR laser pulses is further extended into the UV. PACS 42.65.Jx; 42.65.Ky; 52.35.Mw     

Femtosecond direct space-to-time pulse shaping in an integrated-optic configuration

We demonstrate femtosecond operation of an integrated-optic direct space-to-time pulse shaper for which there is a direct mapping (no Fourier transform) between the spatial position of the masking function and the temporal position in the output waveform. The apparatus is used to generate trains of more than 30 pulses as an ultrafast optical data packet over approximately an 80-ps temporal window.     

Analyzing the Switching Characteristics of a NOLM Demultiplexer with on Analytic and Numrical Methods

1IntroduCtionAllopticalswitchesarekeycOrnpoentsinfuturehigh-speedOPticaltimediVisi0nmultiPleking(0TDM),yst.ms[i].Asasort0fall-OPticalsWtchingdevices,nonllnear0Pticalboharrors(NOLMs)areverysultablef0rall-0PticaldemultiplexinginultrahighsPeedOTDMsystemsbecause0ftheirsimPleconfiguratdri,stableandreliableperformance,andultrafastresP0ndingspeed['J.HOWever,asN0LMutilizestheinherentnonlinearity(crossphasemodulation:XPM)0ffibertoperf0rmswttching,weaknonlineareffects0ffibercausethatl0nger…     

Time–space transformation of femtosecond free‐electron laser pulses by periodical multilayers

A scattering scheme to probe the time evolution of femtosecond pulses of a soft X‐ray free‐electron laser (FEL) in a multilayer structure is presented. The response of periodic multilayers (MLs) with low and high absorption and various numbers of bi‐layers to a pulse train of Gaussian‐shaped sub‐pulses is calculated. During the passage of the incident pulse the interaction length increases and the scattering profile changes as a function of the spatial position of the pulse within the sample. Owing to stretching of the reflected pulse compared with the incident pulse, the time‐dependent scattering evolution in the ML can be visualized along a spatial coordinate of a position‐sensitive detector. Using a scattering geometry where the mean energy of the incident pulse train is slightly detuned from the energy of maximum reflectivity at the first‐order peak, the response of the ML shows an oscillator behaviour along this spatial coordinate at the detector. For a FEL wavelength of 6.4 nm this effect is promising for MLs with low absorption, such as La/C for example. On the other hand, the oscillations will not be present for MLs with high absorption. Therefore a low‐absorbing ML is a sensitive tool for studying the possible change of sample absorption caused by femtosecond‐pulse interaction with matter.     

Inversion in an extended three-level medium produced by adiabatic population transfer

Features of the adiabatic population transfer are studied with the spatial evolution of interacting pulses propagating in an optically dense medium of three-level Λ-atoms taken into account. A self-consistent analytical solution describing the spatial-temporal dynamics of interacting short pulses under the conditions of adiabatic population transfer is constructed in the adiabatic approximation with consideration for the first nonadiabatic correction. Practically complete inversion on a forbidden transition determined by coherent (adiabatic) population transfer is shown to take place over a length of the medium, which may exceed the absorption length of a weak probing pulse in the absence of control radiation on the adjacent transition by several orders of magnitude.     

Performances of Switching Window of Ultrafast Nonlinear Interferometer (UNI) Demultiplexer

In this paper, the theoretical model of the UNI is proposed in detail, which counts on the ultrafast nonlinearities and longitudinal effects of semiconductor optical amplifier (SOA). The transfer function of UNI is also obtained. The switching window, which is a key parameter for demultiplexer, is then analyzed. It's shown that the switching window properties are determined by the length of SOA, the length of the birefringence fiber (BRF), and the parameters of control pulses such as control pulse energy, pulsewidth, power.     

Filamentation length of powerful laser pulses

From a theoretical model for the propagation of high-power laser pulses in air over long distances, we derive analytical estimates for the length of an infrared light filament as a function of the pulse duration and beam energy. For a fixed energy per pulse, a maximum filamentation length is shown to be obtained for a specific pulse duration. These estimates are in agreement with the results of numerical simulations and measurements available in the literature.     

Femtosecond optical packet generation by a direct space-to-time pulse shaper

We demonstrate femtosecond operation of a direct space-to-time pulse shaper in which there is direct mapping (no Fourier transform) between the spatial position of the masking function and the temporal position in the output waveform. We use this apparatus to generate trains of 20 pulses as an ultrafast optical data packet over an ~40-ps temporal window.     

Cascaded compression of the first and second Stokes pulses during forward transient stimulated Raman amplification

The results of numerical modelling of cascaded compression of the first and second Stokes pulses during regenerative regime of the forward transient stimulated Raman amplification are presented for the case when the walk-off length of the first Stokes pulse due to group velocity mismatch is shorter than the length of the nonlinear medium. The influence of the initial amplitudes of the seed first Stokes pulses, its durations and its time delay with respect to the pump pulse, the Kerr nonlinearity of the medium on the conversion efficiency, duration and propagation factor M² of the first and second Stokes pulse are studied. It is demonstrated that for the pump pulse duration of 1 ps the duration of the compressed second Stokes pulses in a KGW crystal near the beam axis may be approximately 14 times shorter than the pump pulse duration. It is shown that the propagation factor of the compressed pulses increases significantly because of complex spatial-temporal dynamics of compression and the influence of Kerr nonlinearity of Raman medium.     

Pulse-length dependence of cellular response to intense near-infrared laser pulses in multiphoton microscopes

The influence of the pulse length, tau , of ultrashort laser pulses at 780 and 920 nm on cell vitality and cellular reproduction has been studied. A total of 2400 nonlabeled cells were exposed to a highly focused scanning beam from a mode-locked 80-MHz Ti:sapphire laser with 60-micros pixel dwell time. For the same pulse energy, destructive effects were more pronounced for shorter pulses. The damage behavior was found to follow approximately a P(2)/tau dependence (P , mean power), indicating that cell destruction is likely based on a two-photon excitation process rather than a one- or a three-photon event. Therefore, femtosecond as well as picosecond pulses provide approximately the same relative optical window for safe two-photon fluorescence microscopy.     

A phase cycle scheme that significantly suppresses offset-dependent artifacts in the R2-CPMG 15N relaxation experiment

R2-CPMG 15N relaxation experiments form the basis of NMR dynamics measurements, both for analysis of nano-pico second dynamics and milli-micro second dynamics (kinetics). It has been known for some time that in the practical limit of finite pulse widths, which becomes acute when using cryogenic probes, systematic errors in the apparent R2 relaxation behavior occur for spins far off-resonance from the RF carrier. Inaccurate measurement of R2 rates propagates into quantitative models such as model-free relaxation analysis, rotational diffusion tensor analysis, and relaxation dispersion. The root of the problem stems from evolution of the magnetization vectors out of the XY-plane, both during the pulses as well as between the pulses. These deviations vary as a function of pulse length, number of applied CPMG pulses, and CPMG inter-pulse delay. Herein, we analyze these effects in detail with experimentation, numerical simulations, and analytical equations. Our work suggests a surprisingly simple change in the phase progression of the CPMG pulses, which leads to a remarkable improvement in performance. First, the applicability range of the CPMG experiment is increased by a factor of two in spectral width; second, the dynamical/kinetic processes that can be assessed are significantly extended towards the slower time scale; finally, the robustness of the relaxation dispersion experiments is greatly improved.     

两束同色激光场和中红外场驱动氦原子在等离激元中产生的单个阿秒脉冲

通过数值求解一维氦原子由两束同色激光场和中红外形成组合场中的含时薛定谔方程,研究了氦原子在纳米等离激元中发射高次谐波的性质以及合成阿秒脉冲的特点.研究表明,在等离激元中氦原子在组合场驱动下发射的高次谐波相对于均匀场情况下截止位置会得到明显扩展,但等离激元对处在连续态电子的吸收效应会对高次谐波截止位置影响较大,通过改变激元的相对位置能明显提高其中一个轨道对谐波的贡献,抑制另一些电子轨道的贡献.经典分析表明,两个电子轨道发生并合,从而实现单个阿秒脉冲的输出.与原子在均匀场驱动的情况相比,阿秒脉冲的宽度明显缩短,最短可实现28 as的单个脉冲输出.     

新型全光纤高能量飞秒光脉冲源的数值模拟

数值模拟了自相似脉冲的产生与压缩,得到一种产生高能量飞秒光脉冲的新方法.结果表明:利用掺铒光纤对光脉冲进行自相似传输,可得到含线性频率啁啾的高能量自相似光脉冲;自相似光脉冲经过空芯光子带隙光纤的一级线性压缩和高非线性光纤的二级非线性压缩,可获得高峰值功率的飞秒光脉冲;压缩过程中存在最佳光纤长度;喇曼自频移和自陡效应对脉冲压缩产生不利影响.     

输入光特性对超快非线性干涉仪开关窗口的影响

就超快非线性干涉仪(UNI)的输入光特性对其开关窗口的影响进行了数值模拟和实验研究,输入光包括控制光脉冲和探测光脉冲。在数值模拟中,调节控制光脉冲和探测光脉冲的功率及脉宽,功率越高和脉宽越窄,窗口的形状越好。在调节过程中发现控制光脉冲和探测光脉冲都存在一个最佳的功率点使窗口的形状达到最优。如果继续增大控制光脉冲功率,会使窗口的顶部倾斜,窗口形状恶化;而继续增大探测光脉冲功率,窗口的消光比开始下降。在数值模拟的基础上进行了10Gb／s的超快非线性干涉仪全光开关实验,在实验中用连续光代替探测光脉冲以观察窗口形状。通过改变控制光脉冲和连续光功率来验证它们对超快非线性干涉仪开关窗口的影响。实验表明,应选用短而强的控制光脉冲和最优功率点的连续光,这与模拟结果吻合。     

Space-time dynamics of ultrashort pulses in vacuum

An analytical study is made of the evolution of spatially bounded pulses whose length amounts to several periods of the field oscillations. An equation is analyzed that describes unidirectional (reflectionless) propagation of light pulses in vacuum. The method of moments is used to find the variations in length, effective width of the wave field, and other characteristic averaged parameters of a pulse along its propagation path. A broad class of self-similar solutions describing the focusing of the light pulses is found. Finally, by direct integration of the starting equation it is shown that a horseshoe-shaped precursor forms near the leading edge of the pulse. Zh. éksp. Teor. Fiz. 116, 35–46 (July 1999)