Effective method of characterization of the phase and intensity profiles of asymmetrically distorted light pulses in optical fiber systems

We address the problem of characterization of light pulses that propagate in long-haul high-bit-rate optical communication systems, under strongly perturbed conditions. We show that the conventional technique for characterization of the phase and intensity profile of such pulses becomes qualitatively inconsistent when the pulse’s profile is asymmetrically distorted with respect to its center-of-mass. We resolve these inconsistencies by partially reformulating the conventional technique by means of appropriate pulse parameters, which we call upgraded parameters, which allow a fair characterization of the intensity and phase of all types of light pulses, including those which are asymmetrically distorted. We illustrate the effectiveness of the upgraded parameters by applying them to a meticulous characterization of light pulses in a dispersion-managed optical fiber system in which third-order dispersion is acting as a strong perturbation.     

Amplitude autocorrelation of femtosecond laser pulses using linear photogalvanic effect in sillenite crystals

We demonstrate excitation of the linear photogalvanic current in a Bi₁₂TiO₂₀ crystal by two orthogonally polarized femtosecond laser pulses with detecting the electrical current via charge accumulation on the sample electrodes. Such a setup was used to implement an interferometric autocorrelation technique for characterization of ultrashort light pulses. Integration of the detected current in femtosecond time domain leads to vanishing of a bipolar component of the photogalvanic current which arises due to a pulse chirping. The advantage of the proposed technique is that it produces the electric field correlation function directly without the need for data processing using a compact, robust, and non-expensive detector in the form of a photoconductive cell of a non-centrosymmetric crystal.     

Generation of excellent self-similar pulses in a dispersion-decreasing fiber

Pulses compression has been widely studied for a long time. In order to generate excellent self-similar pulses in a dispersion-decreasing optical fiber with normal group-velocity dispersion, the influence of initial pulse parameters on the properties of the self-similar parabolic pulses interaction are firstly investigated in the paper. We find that the phase of sinusoidal fit and asymptotic dark soliton change according to the changing of initial phase difference. Meanwhile, increasing the full-width at half-maximum and input energy or decreasing time-delay properly, the interaction between self-similar pulses enhances accordingly, which makes the interaction length shorter and reduce the lose of energy, resulting in high energy output. The results are beneficial in experimental studies by adjusting the initial parameters of pulses to generate high-quality self-similar pulse. It is important for Dense Wavelength Division Multiplexing (DWDM) transmission system which is in heavy demands of light source in wide-range wavelength.     

Characteristics of microwave magneto-optic modulation based on magnetostatic waves

According to the magneto-optic (MO) perturbation theory, the coupled-mode equations for guided optical waves (GOWs) with microwave magneto-static waves (MSWs) in MO film waveguides are presented, which cannot only be used to analyze the MO modulation induced by MSW pulses, but also explain the optical pulse compression caused by chirped MSWs. The rectangular pulse modulation of continuous guided optical waves by magnetostatic forward volume waves (MSFVWs) excited under normal bias magnetic field is theoretically studied in detail. It is shown that: (1) the diffracted light pulse approximates to an isosceles trapezoid, which is in agreement with the experimental results, and the duration time and the flat-top response time are equal to the sum and difference between the width and transit time of the rectangular MSFVW pulse, respectively; (2) utilizing a small duty factor of MSFVW pulse helps to improve the modulating data rate, which is in inverse proportion to the transit time, and the relative peak intensity of the diffracted light pulse is less than the corresponding Bragg diffraction efficiency when the MSW duty factor is less than 0.5.     

POTDR分布式光纤传感及其在安防监测中的应用

提出了一种基于偏振光时域反射计分布式光纤传感技术的新型安防监测系统.该系统使用偏振分束器获取到具有正交偏振态的两路偏振光时域反射计信号,对比验证以确认是否出现了造成偏振态变化的扰动,提高了系统对扰动远程定位检测的准确度,大幅度减小了误判率和漏报率.同时在偏振光时域反射计系统的扰动探测中创造性的采用差值法和相对坐标相结合的方法,首次可以明显并且直观地检测和定位偏振光时域反射计系统中的扰动.最后采用Visual C++开发了一套实时的数据采集与处理软件.实验结果表明该系统具有较高的灵敏度,并在2 km和14 km处获取到具有较高信噪比的扰动信号.     

Transit time of chirped pulses through one-dimensional, nonabsorbing barriers

Experiments show that the transit times of chirped, narrow-band pulses that move across nonabsorbing, one-dimensional barriers are modified dramatically by the interplay between the chirp and the transmission function of the sample. In an experiment we monitored 0.9-ns chirped, nearly Gaussian pulses as they traversed a 450-mum GaAs etalon. At certain wavelengths pulse transit times can be superluminal or even negative. To explain these phenomena we have proposed a generalization of the transit time for chirped pulses that is still meaningful even when the transit times are superluminal or negative. Our predictions agree well with the experimental results.     

Heterodyne detection scheme for light-induced transient grating experiment

A novel heterodyne detection scheme for light-induced transient grating technique is presented. The heterodyne detection is achieved coherently mixing the picosecond pulses of diffracted and scattered light. A phase difference between these pulses is controlled by moving a holographic beam splitter along its grating vector. This approach is employed to study a competition of coexisting electronic and thermal optical nonlinearities in CVD grown diamond films in sub-nanosecond time domain and to determine carrier parameters.     

Optical coherence pulsed interferometry: shaping probe pulses in time-domain interferometry

The autocorrelation of a modulated coherent light source is used as a probe pulse in a time-domain interferometry scheme. With respect to conventional techniques, higher flexibility in selecting the shape of the probe pulse can be achieved by simply acting on the modulation parameters. The complex amplitude of short pulses propagating through a generic optical device can be directly measured, with no need for fast sampling and time synchronization. The potentialities of the technique are shown by reporting measurements of amplitude distortion, group delay, and frequency chirp of optical pulses transmitted through integrated ring resonators.     

Driftgeschwindigkeiten von Ionen und Elektronen in Argon und Wasserstoff

Short light sparks release 10 nsec pulses of 10⁵ electrons each which initiate electron avalanches in high-purity argon and hydrogen. The transit times of the avalanches are measured and thus the drift velocities of ions and electrons derived. — There is general agreement with available results of other authors.     

Spectral line narrowing in a gas by atoms trapped in a standing light wave

This paper presents results of a theoretical analysis of a new method for eliminating the Doppler broadening of spectral lines and the broadening by the transit time of atoms through a light beam. The atomic motion in a one-dimensional standing wave is studied and the conditions for translational-to-vibrational motion transformation are found. The variation in the Doppler contour by the trapping effect is investigated. It is illustrated, in particular, that the width of the narrow peak at the line centre depends mainly on the finite transit time of the atoms through the light beam. Next it is shown that, by accumulating slow atoms in a three-dimensional standing wave, it is possible, in principle, to observe narrow peaks with their widths determined only by the natural line width. The possibility of experimentally detecting of the phenomenon is discussed.     

不同激光等离子体条件下的阿秒光脉冲产生

通过一维粒子模拟研究了利用相对论少周期强激光与固体密度等离子体表面相互作用实现单个孤立阿秒光脉冲产生的参数条件。主要研究描述相互作用的多维参数,如激光强度、入射角和等离子体标尺长度等,对相对论高次谐波能量转换效率和孤立阿秒光脉冲分离度的影响。研究发现,虽然激光等离子体参数对阿秒光脉冲产生的影响是复杂的,但是存在着能够实现大能量孤立阿秒光脉冲的最佳等离子体标尺长度和最佳入射角。当其他相互作用条件确定时,使用中等强度的相对论强激光可以在较宽的参数范围内实现孤立的阿秒光脉冲。大角度入射时,孤立阿秒光脉冲的分离度较高,能够实现孤立阿秒光脉冲的相互作用参数范围也较宽。     

Experimental investigation of slow light propagation in degenerate two-level system

Slowing a light pulse in a degenerate two-level system is observed with a double-frequency sweeping technique. The effects of coupling beam intensity, cell temperature and frequency detunings of the coupling and probe beams in resonance, on the slowing of light propagation in such a system are investigated. It is found that group velocities depend strongly on polarization combinations. A group velocity $v_{\rm g}$=6760m/s of light pulses in caesium vapour is obtained under the optimal parameters.     

Detection of scattered light pulses at femto-Joule level by using a picosecond BBO optical parametric amplifier

We report a technique for detecting weak scattered light pulses based on a 532 nm pumped picosecond β-barium-borate collinear optical parametric amplifier. The measured maximum slope gain factor of the amplifier was found to be around 10⁷, and the energy detection limit was of the order of fJ/pulse for the signal of 730 nm and the idler of 1.5 μm at a pumping intensity of 2.83 GW/cm². The linearity of the gain for this amplifier was found to be excellent for a seeding level of lower than 420 fJ per pulse. The maximum gains and the energy detection limits for the scattered light pulses from various scattering targets were found to be poorer than that from the reflective mirror, owing to the degraded beam quality and the depolarization of the scattered light. A reduction of the maximum gain for the scattered light with the increase of the angle of incidence, which causes pulse broadening and reduces the photon flux of the signal, was investigated. The feasibility of detecting weak scattered light in the infrared by using idler-to-signal frequency up-conversion was also demonstrated, in which the infrared seeder located in the idler branch of the amplifier was injected as the seeding beam and was then parametrically up-converted into the visible signal branch, with an even higher gain.     

Characterization of the train-average time-frequency parameters inherent in the low-power picosecond optical pulses generated by the actively mode-locked semiconductor laser with an external single-mode fiber cavity

The specific approach to characterizing the train-average parameters of low-power picosecond optical pulses with the frequency chirp, arranged in high-repetition-frequency trains, in both time and frequency domains is elaborated for the important case when the semiconductor laser is matched by an external single-mode fiber cavity and operates in the active mode-locking regime. This approach involves the joint Wigner time-frequency distributions, which can be created for those pulses due to exploitation of a novel interferometric technique. Practically, the InGaAsP/InP-heterolaser generating at the wavelength 1320 nm was used during the experiments carried out and an opportunity of reconstructing the corresponding joint Wigner time-frequency distributions was successfully demonstrated.     

Entangled light pulses from single cold atoms

The coherent interaction between a laser-driven single trapped atom and an optical high-finesse resonator allows one to produce entangled multiphoton light pulses on demand. The mechanism is based on the mechanical effect of light. The degree of entanglement can be controlled through the parameters of the laser excitation. Experimental realization of the scheme is within reach of current technology. A variation of the technique allows for controlled generation of entangled subsequent pulses, with the atomic motion serving as intermediate memory of the quantum state.     

飞秒时间分辨实验中相关函数和时间零点的确定

结合飞秒激光在研究分子激发态弛豫动力学中的应用,介绍了几种飞秒时间分辨实验中确定泵浦激光脉冲与探测激光脉冲的相关函数和时间零点的方法.对于波长在可见波段的泵浦和探测激光脉冲,我们可以利用非线性光学的技术手段,探测泵浦光与探测光的和频光的强度随二者间的时间延迟的变化来确定相关函数和时间零点;对于波长在紫外甚至更短的波段的泵浦和探测激光脉冲,由于单脉冲能量比较低,目前还很难利用技术手段来测定泵浦激光与探测激光的相关函数及时间零点,可以利用某些原子气体(如Xe)或某些具有短寿命态的分子作平行实验进行间接测量.     

The comparison study of diagnostics of light filaments in air

Long plasma channels in air induced by femtosecond laser pulses are investigated using three different methods, including the cross-section imaging, resistivity measuring and acoustic diagnostics. These methods are based on different properties of the light filaments. A comparison of the three diagnostics shows that the imaging method is the most precise one in studying the filaments distribution and evolution, that the sonographic method is the most convenient approach to detecting long plasma channels by detecting the acoustic wave generation, and that the resistivity measurement can only be applied for giving a rough estimate. The diagnostics of filaments allow us to choose appropriate detecting methods and provide further insight into the dynamic evolution of the light filaments in air.     

Self-generation of two-dimensional spin-wave bullets

The experimental observation of self-generation of two-dimensional, self-focusing nonlinear spin wave packets-spin wave bullets-in an active ring is reported. The ring is composed of a ferrite film with two antennae for excitation and detection of the wave packets, and a microwave amplifier connecting the antennae and closing the ring. Experimental observation has been made by using the time and space resolved Brillouin light scattering technique. The parameters of spin wave bullets self-generated from noise in an active ring are similar to those of bullets coherently excited by external microwave pulses. The observed self-generation process provides unambiguous evidence that wave bullets are intrinsic excitations of a two-dimensional nonlinear medium with dissipation that is focusing in both directions.     

基于亚毫米尺度金属包覆波导的脉冲展宽

利用自由空间耦合技术,用超短脉冲激发亚毫米尺度对称金属包覆波导中的超高阶导模,提出一种脉冲展宽的新方法。由于超高阶导模的强色散性质,经过短距离的传输,即可使脉冲宽度迅速展宽。理论分析表明,1 ps的光脉冲在光波导中传输毫米量级距离后,脉冲展宽因子可达到1000倍。     

基于光脉冲诱导快相与弛豫荧光的光合作用参数测量技术

为了更准确获取反映植物生理状态的荧光动力学曲线,基于光合作用电子传递过程研究了植物光合作用参数测量技术.采用可变光脉冲技术将植物光合作用过程分段为快相与弛豫过程,并测量激发光诱导产生的荧光动力学曲线.对激发光带宽与响应时间进行了定量分析;对I-V转换单元与MFB滤波器进行了设计与仿真分析,获取快相荧光动力学信息;采用同步脉冲采样积分技术,对微弱弛豫荧光进行积分,实现了快相与弛豫荧光动力学曲线的完整测量,并结合非线性拟合算法获取光合作用参数.测试结果表明,系统信噪比达到23.8 dB;暗适应与光适应下,本系统所测Fv/Fm与Water-PAM测量结果的线性相关系数分别达到0.980和0.997.该研究结果为植物光合作用研究及过程参数测量提供了一种测量手段.