超短光脉冲波形对门模单光子探测的影响

采用两种不同的皮秒光脉冲波形进行了门模单光子探测实验.测量了单光子探测效率随探测器和超短脉冲光源之间的同步延迟细微调节时的变化曲线.结果表明：光脉冲波形直接影响光子到达时间与门脉冲时间窗口之间的精确同步和探测效率,使用其中一种脉宽较短的皮秒光脉冲时探测效率比使用另一种脉冲提高了约9%.     

Preparation of inverted medium and processing in the inverted medium

The processing of weak optical signals in spatial-spectral holographic (SSH) materials coherently inverted with optical frequency chirped pulses were investigated. Simulations and experimental studies in Tm³⁺:YAG were conducted to characterize the parameters of the frequency chirped laser pulse used to invert the SSH material in order to obtain high photon echo efficiency for SSH lidar processing. Collinear and angled beam geometries and single shot and accumulated processes were investigated. Echo efficiencies as high as 450% were measured, significantly higher than the typical stimulated photon echo efficiency of 10%.     

基于单光子探测及脉冲位置调制的激光通信关键技术研究

为了实现高灵敏度的空间激光通信,并提高传输信道的抗干扰能力,将单光子探测技术和脉冲位置调制技术相结合,采用门控电路与反馈淬灭电路相结合的方式淬灭单光子探测器雪崩,设计了插入帧头法用于脉冲位置调制解调。用现场可编程门阵列进行了脉冲位置调制解调过程的仿真,验证了插入帧头法的有效性与可行性。在此基础上搭建了1 550 nm的脉冲位置调制激光通信实验,同时测试了单光子探测器在不同参数下的性能。结果表明,当探测效率为25%,触发延时为8.00 ns,门宽为5.0 ns,死时间为0.1μs时,单光子探测器性能最佳。最后测试了不同调制速率下单光子探测器的探测灵敏度,结果表明,当通信码速率为1 Mbps时,通信灵敏度为-51.8 dBm;当通信码速率为4 Mbps时,通信灵敏度为-41.0 dBm,实现了高灵敏度的空间激光通信。     

Streaking at high energies with electrons and positrons

A detection scheme for characterizing high-energy γ-ray pulses down to the zeptosecond timescale is proposed. In contrast to existing attosecond metrology techniques, our method is not limited by atomic shell physics and therefore capable of breaking the MeV photon energy and attosecond time-scale barriers. It is inspired by attosecond streak imaging, but builds upon the high-energy process of electron-positron pair production in vacuum through the collision of a test pulse with an intense laser pulse. We discuss necessary conditions to render the scheme feasible in the upcoming Extreme Light Infrastructure laser facility.     

Talbot effect under illumination of double femtosecond laser pulses

The Talbot effect under illumination of double femtosecond laser pulses has been reported. Spectrums of double femtosecond laser pulses with phase differences are quite different from that of one single femtosecond laser pulse. Therefore, the Talbot images of the double femtosecond laser pulses with phase differences are different from that of one single femtosecond laser pulse. Specifically, for the phase difference corresponding to π, the Talbot image shows the largest difference from that of one single pulse. Experimental results are in good agreement with the theoretical analysis. The behaviors of Talbot images under double femtosecond laser pulses illumination cannot be obtained under one femtosecond laser pulse, monochromatic or polychromatic light illumination. Therefore, it is a new interesting optical phenomenon for the Talbot effect which should have potential applications.     

A fiber-based differential phase shift quantum key distribution scheme with higher key creation efficiency

In this paper, a differential phase shift key distribution system with a selectable delay unit is proposed, in which higher key creation efficiency can be achieved. A photon from a single-photon source is split into four pulses with the same time interval T through two cascaded Mach-Zehnder interferometers. In Bob’s site, a 4T delay loop with an optical switch is employed to reuse the two end pulses discarded in the conventional differential phase shift scheme, and then entire superposition of the all single photon pulses is achieved. The utilization ratio of photons can reach 100% in perfect condition.     

基于超导纳米线单光子探测器深空激光通信模型及误码率研究

高速深空通信是深空探测的关键技术之一,具备单光子灵敏度的激光通信系统将大大提高现有的深空通信速度.然而,单光子条件下的激光通信不仅需要考虑传输环境的影响,还需要考虑实际单光子探测器性能和光子数量子态的分布.本文在不考虑大气湍流影响的情况下,以光电探测模型为基础,引入超导纳米线单光子探测器(SNSPD)系统的探测效率和暗计数,建立了反应系统差错性能的数学模型,提出了系统误码率的计算公式.先对公式中的光强和激光脉冲重复频率对误码率的影响进行仿真,再通过实验结果验证仿真模型.结果表明,光强对误码率的影响最明显,随着光强从0.01光子/脉冲到1000光子/脉冲的增加,误码率从10~(-1)到10~(-7)量级明显下降;激光脉冲重复频率对误码率的影响受到不同光强的制约,但都随着脉冲重复频率的增加呈下降趋势.与此同时,当增加光强或者提高速度时,误码率高于仿真结果,约在10~(-4)量级,其原因可能是实际通信中调制光信号的消光比不足和光纤引入背景噪声提高了系统暗计数.以上模型和实验结果为进一步开展基于SNSPD的月球-地球、火星-地球等高速深空激光通信奠定了基础.     

Effect of the amplitude of excitation pulses on the shape of photon-echo signals in two-level systems

It is theoretically established that multiple photon-echo signals reflect the oscillatory structure of the primary echo signal, which arises under the conditions of collinear excitation of two-level active centers by laser pulses having the same width but different amplitudes. It is shown that the oscillatory structure of the primary photon echo exists within the intervals (?t ₁, t ₁) and (?2t ₁, 2t ₁), where t ₁ is the pulse width. It is found that, when the pulse width is of the same order of magnitude as the delay between the pulses and the amplitudes of the excitation pulses obey certain relations, the oscillatory structure of the primary photon-echo signal becomes asymmetric. In the mode of quadratic detection of the primary photon-echo signal, the asymmetric oscillatory structure of the primary echo manifests itself as a manifold of isolated signals (multiple photon echoes), with the time intervals between these signals being equal to the pulse width.     

Mobile source of high-energy single-cycle terahertz pulses

The Teramobile laser facility was used to realize the first mobile source of high-power THz pulses. The source is based on a tilted-pulse-front pumping THz generation scheme optimized for application of terawatt laser pulses. Generation of 50-μJ single-cycle electromagnetic pulses centered at 0.19 THz with a repetition rate of 10 Hz was obtained for incoming 700-fs 120-mJ near-infrared laser pulses. The corresponding laser-to-THz photon conversion efficiency is approximately 100%.     

Spectral density matrix of a single photon measured

We propose and demonstrate a method for measuring the spectral density matrix of a single photon pulse. The method is based on registering Hong-Ou-Mandel interference between a photon to be measured and a pair of attenuated and suitably delayed laser pulses described by a known spectral amplitude. The density matrix is retrieved from a two-dimensional interferogram of coincidence counts. The method has been implemented for a type-I down-conversion source, pumped by ultrashort laser pulses. The experimental results agree well with a theoretical model which takes into account the temporal as well as spatial effects in the source.     

Generation of femtosecond vacuum-ultraviolet pulses

High power femtosecond pulses in the Vacuum Ultra Violet (VUV) have been generated through the nonlinear interaction of femtosecond KrF pulses with xenon and argon gas. Under near resonant two photon excitation of xenon by a femtosecond KrF laser, parametric four wave mixing processes lead to VUV pulses at 147 and 108 nm with pulse energies in the 10 µJ range. Tuning is demonstrated by mixing the KrF pulse with a 500 fs dye laser pulse at 497 nm, resulting in 165 nm emission. In argon, a three photon resonance leads to third harmonic generation at 83 nm and micro joule level pulses near 127 nm generated by a six wave mixing process. Since the spectra of the VUV pulses show an ionization-induced blue shift with increasing KrF laser intensity, the VUV pulses can be shown to have temporal duration less than the pulse width (450 fs) of the KrF laser. Blue shifting of the third harmonic of the KrF laser in argon is dominated by a reduction in the neutral gas density rather than by an increase in the electron density.     

Ultra-low power anti-crosstalk collision avoidance light detection and ranging using chaotic pulse position modulation approach

A novel concept of collision avoidance single-photon light detection and ranging(LIDAR) for vehicles has been demonstrated, in which chaotic pulse position modulation is applied on the transmitted laser pulses for robust anti-crosstalk purposes. Besides, single-photon detectors(SPD) and time correlated single photon counting techniques are adapted, to sense the ultra-low power used for the consideration of compact structure and eye safety. Parameters including pulse rate, discrimination threshold, and number of accumulated pulses have been thoroughly analyzed based on the detection requirements, resulting in specified receiver operating characteristics curves. Both simulation and indoor experiments were performed to verify the excellent anti-crosstalk capability of the presented collision avoidance LIDAR despite ultra-low transmitting power.     

Differential phase shift quantum key distribution

A novel quantum cryptography scheme is proposed, in which a single photon is prepared in a linear superposition state of three basis kets. A photon split to three pulses is sent from Alice to Bob, where the phase difference between sequential two pulses carries bit information. Bob measures the phase difference by passive differential phase detection. This scheme is suitable for fiber transmission systems and offers a key creation efficiency higher than conventional fiber-based BB84.     

预报单光子源诱骗态量子密钥产生率及数值计算

独立推导预报单光子源诱骗态量子密钥分发的密钥产生率计算公式,讨论密钥产生率和发送端探测效率的关系;进行弱相干光和预报单光子源诱骗态量子密钥分发的最优强度估计和密钥产生率数值计算.结果表明,预报单光子源诱骗态量子密钥分发的密钥产生率随着发送端探测效率的增加而增加,其安全通信距离与完美单光子源的通信距离一致;诱骗态量子密钥分发可提高安全通信距离和密钥产生率;预报单光子源由于减少了暗计数的影响,进一步提高了安全通信距离.     

Modelling of experimentally measured Q-switched pulsations in InGaAs/GaAs diode lasers

A numerical calculation of a passively Q-switched two-section ridge-waveguide InGaAs/GaAs diode laser is presented in this study. The author has modelled the output power-current (L-I) variation under cw conditions, time evolution of photon numbers under transient conditions and Q-switched pulses of the device for various reverse bias voltages to the absorber section. Resulting simulations of the L-I characterisation and Q-switched pulsations are compared quantitatively with experimental results. Simulated Q-switched pulse profiles have been obtained in the absence and presence of noise. In both cases, proposed model shows that a tail occurring at the end of the Q-switched pulse is eliminated at −7.5 V reverse bias voltage, which is confirmed by experiment. As a result, experimentally obtained tail-free and single peak picosecond Q-switched pulses with peak powers of ∼1 W and durations of typically tens of picoseconds are also demonstrated theoretically. Simulations show consistency with the experimental data.     

强激光等离子体相互作用驱动高次谐波与阿秒辐射研究进展

对超快过程的探测和控制决定了人类在微观层面认识和改造物质世界的能力.阿秒光源可完成对组成物质的电子运动及其关联效应进行超高时空分辨的探测和操控,为人类认识微观世界提供了全新手段,被认为是激光科学史上最重要的里程碑之一.世界主要科技强国都将阿秒科学列为未来10年重要的科技发展方向.利用强激光与物质相互作用产生高次谐波是突破飞秒极限实现高亮度阿秒脉冲辐射的重要方案之一,成为了近年来激光等离子体领域的研究热点.本文聚焦强激光与等离子体相互作用中的高次谐波和阿秒脉冲辐射,主要介绍其产生机制、研究进展和前沿应用,并对未来的发展趋势和创新突破进行展望.     

Chirp-dependent ionization of hydrogen atoms in the presence of super-intense laser pulses

We perform a theoretical study on dynamic interference in single photon ionization of ground state hydrogen atoms in the presence of a super-intense ultra-fast chirped laser pulse of different chirp types(equal-power and equal-FWHM laser pulses) by numerically solving the time-dependent Schr ¨odinger equation in one dimension. We investigate the influences of peak intensity and chirp parameters on the instantaneous ionization rate and photoelectron yield, respectively. We also compare the photoelectron energy spectra for the ionization by the laser pulses with different chirp types. We find that the difference between the instantaneous ionization rates for the ionization of hydrogen atom driven by two different chirped laser pulses is originated from the difference in variation of vector potentials with time.     

Theory of coherent luminescence of inverted systems

A study of a system of N two-level molecules excited by a series of resonant laser pulses is made. Upon inversion such a system may give rise to coherent photon avalanches. If the system is irradiated with a subsequent pulse one should observe “light echoes” whose intensity is proportional to the square of the density of dipole moments of the system.     

Generation and detection of tunable ultrashort infrared and far-infrared radiation pulses of high intensity

We report on generation and detection of intense pulsed radiation with frequency tunability in the infrared and far-infrared spectral regions. Infrared radiation is generated with a transversally electrically excited high pressure CO₂ laser. A laser pulse of a total duration of about 300 ns consisted, due to self mode locking, of a series of single pulses, some with pulse durations of less than 450 ps and peak powers larger than 20 MW. Using these pulses for optical pumping of a Raman D₂O laser, trains of short far-infrared pulses with durations less than 400 ps were obtained. For detection a new ultrafast superconducting detector was used.     

Resonant multiphoton ionization of Li

We present calculations of three photon ionization of Li produced by 3-ps laser pulses within a single electron local model potential using the dressed state picture. The laser frequency ranges from 15 000 to 18 400 cm⁻¹. We have found that the measured ionization signal as a function of photon frequency results from ionization processes in a region where the laser intensity is not homogeneous. We assume a Gaussian shape for the light pulse in the interaction volume. Our results are shown to be in good agreement with experiment. We propose that free electrons are submitted to a ponderomotif potential to interpret experimental results. This revised version was published online in August 2006 with corrections to the Cover Date.