Quantum key distribution with high loss: toward global secure communication

We propose a decoy-pulse method to overcome the photon-number-splitting attack for Bennett-Brassard 1984 quantum key distribution protocol in the presence of high loss: A legitimate user intentionally and randomly replaces signal pulses by multiphoton pulses (decoy pulses). Then they check the loss of the decoy pulses. If the loss of the decoy pulses is abnormally less than that of signal pulses, the whole protocol is aborted. Otherwise, to continue the protocol, they estimate the loss of signal multiphoton pulses based on that of decoy pulses. This estimation can be done with an assumption that the two losses have similar values. We justify that assumption.     

Asymmetry in the strong-field ionization of Rydberg atoms by few-cycle pulses

We present measurements of the electron ejection direction in the ionization of high (n=90) Rydberg states of rubidium subjected to few-cycle radio-frequency (RF) pulses. For weak pulses we find a strong asymmetry for even (cosine) pulses and no asymmetry for odd (sine) pulses. This asymmetry disappears for pulses longer than four RF cycles. For strong pulses, very large asymmetry is found for both sine and cosine pulses that persists up to eight RF cycles and is attributed to initial state depletion effects within a cycle.     

High-resolution Raman spectra with femtosecond pulses: an example of combined time- and frequency-domain spectroscopy

Frequency-domain spectroscopy requires long pulses, whereas time-domain spectroscopy requires short pulses. This Letter demonstrates both theoretically and experimentally that simultaneous detection in frequency and time generates well-resolved spectra using intermediate-length pulses. In the case of coherent Raman spectroscopy, typical femtosecond pulses lie between the time and frequency domains. To demonstrate this method, a high-resolution Raman spectrum of nitrobenzene is obtained from 60 fs pulses. Phase control, pulse shaping, or pulses of widely differing duration are not required.     

Linear phase slope in pulse design: application to coherence transfer

Using optimal control methods, robust broadband excitation pulses can be designed with a defined linear phase dispersion. Applications include increased bandwidth for a given pulse length compared to equivalent pulses requiring no phase correction, selective pulses, and pulses that mitigate the effects of relaxation. This also makes it possible to create pulses that are equivalent to ideal hard pulses followed by an effective evolution period. For example, in applications, where the excitation pulse is followed by a constant delay, e.g. for the evolution of heteronuclear couplings, part of the pulse duration can be absorbed in existing delays, significantly reducing the time overhead of long, highly robust pulses. We refer to the class of such excitation pulses with a defined linear phase dispersion as ICEBERG pulses (Inherent Coherence Evolution optimized Broadband Excitation Resulting in constant phase Gradients). A systematic study of the dependence of the excitation efficiency on the phase dispersion of the excitation pulses is presented, which reveals surprising opportunities for improved pulse sequence performance.     

Generation of high-energy, sub-20-fs pulses in the deep ultraviolet by using spectral broadening during filamentation in argon

Generation of sub-20-fs UV pulses with more than 300 μJ energy at 268 nm is reported. First, the UV pulses are produced by successive second-harmonic and third-harmonic (TH) generation of 805 nm pulses of a 1 kHz Ti:sapphire laser amplifier. The spectral broadening of TH pulses is realized in a filament, generated in argon. The produced pulses are compressed in a simple double-pass prism-pair compressor. Starting from 100 fs pulses, we achieve a fivefold pulse shortening.     

Dispersion-insensitive low-coherent pulses emerging from nonlinear polarization switching

We have experimentally investigated low-repetition nanosecond pulses delivered from an erbium-doped fiber (EDF) laser operating in ultra-large anomalous dispersion regime. The output pulses with rectangular profile and Gaussian spectrum almost keep invariable when they propagate through either normal- or anomalous-dispersion fibers. After nanosecond pulses are amplified via a two-stage EDF amplifier, they are broken up and exhibited as flatly broadened supercontinuum from 1520 to 1700 nm if amplified pulses are launched into a 10-km single-mode fiber, whereas the pulses retain the same duration with a broadband supercontinuum from 1200 to 1750 nm if they are input into a 100-m highly-nonlinear low-dispersion photonic-crystal fiber (PCF). The experimental observations demonstrate that the nanosecond pulses result from nonlinear polarization switching and can be regarded as dispersion-insensitive low-coherent pulses rather than compressible pulses.     

All-optical control of the carrier-envelope phase with multi-stage optical parametric amplifiers verified with spectral interference

Intense ultrashort laser pulses with stabilized carrier-envelope phase (CEP) are generated at 800 nm by using multi-stage collinear and non-collinear optical parametric amplifiers (OPAs). The first-stage collinear OPA is directly pumped by the fundamental-wave pulses and tuned to generate idler pulses at 1600 nm, which are further amplified by a second-stage collinear OPA, and then frequency-doubled to generate CEP-stabilized pulses at 800 nm. A non-collinear OPA is used to amplify the CEP-stabilized pulses at 800 nm. The combination of different OPAs can generate and amplify CEP-stabilized pulses at 800 nm without any detrimental influence from the fundamental-wave pulses. The CEP stabilization is verified with a simple and robust spectral interference setup. The stable interference pattern is measured for every single pulse and compared with the unstable pattern from pulses of random CEP. PACS 42.65.Re; 42.65.Yj; 42.25.Kb     

All-optical pulse shaping for ultrawideband doublet pulses using nonlinear optical loop mirror with optical parametric amplification

We propose and experimentally demonstrate an alternative photonic scheme for the generation of ultrawideband (UWB) doublet pulses, which is based on an optical fiber-based nonlinear optical loop mirror (NOLM) incorporating the optical parametric amplification (OPA) effect. The proposed scheme uses both cross-phase modulation and OPA within an optical fiber-based NOLM to produce an ideal transfer function for the shaping of input soliton pulses into doublet pulses. Using the proposed scheme, a successful conversion of input soliton pulses into doublet pulses is readily demonstrated. The system performance of UWB doublet pulses is also assessed by propagating the 1.25?Gbit/s doublet pulses over a fiber link. Error-free UWB doublet signal transmission is demonstrated.     

Performance of frequency-modulated pulses in NQR

This paper presents an analysis of the performance of frequency-modulated pulses and of pulses modulated simultaneously in amplitude and frequency in the pure nuclear quadrupolar resonance (NQR) spectroscopy of spin 3/2 nuclei. Computer simulations are given of the offset compensation efficiency of such pulses as applied to single crystals. Spin evolution equations were solved numerically. Experimental measurements, using FM pulses, of the³⁵Cl NQR of sodium chlorate (NaClO₃) single crystal and of mercuric chloride (HgCl₂) powder are reported. The results suggest that frequency modulated pulses are alternative pulses to composite pulses in NQR.     

Sound generation in the atmosphere upon exposure to high-power milli- and microsecond laser pulses

A review of works on generation of acoustic pulses in the atmosphere upon exposure to high-power laser pulses is presented in this paper. Characteristics of sound pulses in the atmosphere accompanying the propagation of high-power milli- and microsecond laser pulses, including the peak sound pressure level and the shape and spectrum of acoustic pulses, are discussed. The special features of acoustic pulses from individual liquid and solid aerosol particles and ensembles of monodisperse particles and polydisperse atmospheric aerosol are discussed. The characteristics of acoustic signals from long laser sparks and collective optical discharges are considered.     

高能量无波分裂超短脉冲自相似传输的理论研究和数值模拟

从非线性薛定谔方程出发，对SPM作用引起传输脉冲的相位作了合理近似，引入了频谱展宽因子，发现在存在增益时正色散光纤的SPM作用加剧了脉冲频谱展宽幅度，并且脉冲中心和两翼部分的频谱展宽差异很大.给出了存在增益的正色散光纤中高斯型脉冲自相似传输的时域演化，揭示了脉冲展宽的中心部分对边翼部分挤压和覆盖及最终形成自相似脉冲的过程，提出了自相似传输现象的一种合理解释.并通过数值模拟三种不同形状初始脉冲的自相似演化过程对提出的理论进行了验证.     

A novel time-to-live countdown scheme based on asymmetric Mach-Zehnder interferometer and Fabry-Perot semiconductor optical amplifier

We propose a novel optical time-to-live (TTL) processing scheme using asymmetric Mech-Zehnder interferometer (AMZI) and Fabry-Perot semiconductor optical amplifier (FP-SOA). AMZI transfers M TTL pulses into M - 1 pulses and two residual pulses with 6-dB power difference. FP-SOA enhances the power difference between the M- 1 pulses to the residual pulses to more than 10 dB. A numerical model is established for verifying the feasibility of this scheme.     

Pattern pulses: design of arbitrary excitation profiles as a function of pulse amplitude and offset

A novel class of pulses is presented which can be regarded as a generalization of both frequency-selective pulses and B1-selective pulses. The excitation profile of these pulses forms a pre-defined pattern in two dimensions, which are spanned by pulse offset and radio-frequency (RF) amplitude. The presented pulses were designed numerically based on principles of optimal control theory. For simple test patterns, we demonstrate the flexibility of this approach by simulations and experiments. This previously unknown flexibility may trigger novel applications in NMR spectroscopy and imaging. As a first practical application, we demonstrate a direct approach for calibrating RF pulses.     

Generation of sub-mJ terahertz pulses by optical rectification

Recent theoretical calculations predicted an order-of-magnitude increase in the efficiency of terahertz pulse generation by optical rectification in lithium niobate when 500 fs long pump pulses are used, rather than the commonly used ~100 fs pulses. Even by using longer than optimal pump pulses of 1.3 ps duration, 2.5× higher THz pulse energy (125 μJ) was measured with 2.5× higher pump-to-THz energy conversion efficiency (0.25%) than reported previously with shorter pulses. These results verify the advantage of longer pump pulses and support the expectation that mJ-level THz pulses will be available by cooling the crystal and using large pumped area.     

REDOR with adiabatic dephasing pulses

The response of a spin (1/2) ensemble, at thermal equilibrium and experiencing chemical shift anisotropy (CSA), to the application of adiabatic inversion pulses has been studied under magic-angle spinning (MAS). Numerical simulations and experimental studies on such systems, carried out under slow spinning conditions, show that the response to adiabatic inversion pulses has much more favorable characteristics than the response to conventional rectangular pulses. We have also explored the possibilities of employing adiabatic 180 degrees pulses as dephasing pulses in rotational-echo double-resonance (REDOR) experiments. Our results show that it is indeed possible to employ such adiabatic inversion pulses conveniently in REDOR experiments to eliminate resonance offset and H(1) inhomogeneity effects which may arise from the usage of conventional rectangular 180 degrees pulses. Copyright 2000 Academic Press.     

Generation of femtosecond radiation at 211 nm by femtosecond pulse upconversion in the field of a picosecond pulse

We show that, in the case of sum-frequency mixing, one can alleviate group-velocity mismatch between IR and UV pulses by choosing different pulse widths, thus extending the interaction length of ultrashort pulses within nonlinear crystals. By fifth-harmonic generation with a Nd:glass laser, we demonstrate efficient frequency upconversion of 195-fs 264-nm pulses under the envelope of 0.9-ps 1055-nm pulses in beta-barium borate crystal, yielding <270-fs pulses with energy of up to 110muJ at 211 nm.     

光学材料的多脉冲激光损伤研究

研究了ＺＦ２玻璃多脉冲与单脉冲激光损伤，分析了多脉冲损伤阈值与脉冲数，脉冲间隔的关系，进行了多脉冲损伤机理的分析，得出多脉冲损伤是热和缺陷累积共同作用最终促成雪崩击穿的结果，并提出了提高抗多脉冲激光损伤的方法。     

Characterization of sub-two-cycle pulses from a hollow-core fiber compressor in the spatiotemporal and spatiospectral domains

We have post-compressed 25 fs (Fourier limit) amplified pulses in an argon-filled hollow-core fiber. The output pulses were compressed using a pair of wedges and chirped mirrors down to 4.5 fs (Fourier limit of 4.1 fs), which corresponds to less than two optical cycles. We then performed the characterization of the pulses by combining the d-scan and the STARFISH techniques. The temporal (and spectral) measurement of the pulses is done with d-scan, which is used as the reference to extend the characterization to the spatiotemporal (and spatiospectral) amplitude and phase of the pulses by means of STARFISH. The post-compressed pulses at the output of the hollow-fiber had an energy of 150 μJ. The analysis of the pulses revealed larger spectral broadening and blue-shift, and shorter duration at the center of the beam. For the first time, we demonstrate the complete characterization of intense ultra-broadband pulses in the sub-two-cycle regime, which provides an improved insight into the properties (space–time and space–frequency) of the pulses and is highly relevant for their applications.     

Shaping of few-cycle laser pulses via a subwavelength structure

We theoretically investigate the propagation of few-cycle laser pulses in resonant two-level dense media with a sub- wavelength structure, which is described by the full Maxwell-Bloch equations without the frame of slowly varying envelope and rotating wave approximations. The input pulses can be shaped into shorter ones with a single or less than one optical cycle. The effect of the parameters of the subwavelength structure and laser pulses is studied. Our study shows that the media with a subwavelength structure can significantly shape the few-cycle pulses into a subcycle pulse, even for the case of chirp pulses as input fields. This suggests that such subwavelength structures have potential application in the shaping of few-cycle laser pulses.     

Time-resolved shadowgraphs and morphology analyses of aluminum ablation with multiple femtosecond laser pulses

Aluminum ablation by multiple femtosecond laser pulses is investigated via time-resolved shadowgraphs and scanning electron microscope(SEM) images of the ablation spot. The spatial distribution of the ejected material and the radius of the shock wave generated during the ablation are found to vary with the increase in the number of pulses. In the initial two pulses, nearly concentric and semicircular stripes within the shock wave front are observed, unlike in subsequent pulses. Ablation by multiple femtosecond pulses exhibits different characteristics compared with the case induced by single femtosecond pulse because of the changes to the aluminum target surface induced by the preceding pulses.