2 μm可调谐高重复频率主动锁模光纤激光器

本文报道了一种2 μm波段可调谐主动锁模光纤激光器,增益介质为4 m铥钬共掺光纤。主动锁模脉冲采用强度调制方式实现,调制信号为高频正弦信号。可调谐窄带宽光滤波器用来窄化激光线宽,抑制噪声,同时可实现波长调谐。实现了稳定的2.2 GHz重复频率主动锁模脉冲输出,对应649阶谐波锁模脉冲序列,脉冲宽度约为200 ps,频谱信噪比可达68 dB,激光线宽为0.05 nm,光谱调谐范围为1907 nm~1927 nm。     

量子保密通信用单光子探测系统的设计初探

设计了以1310nm激光波长作为光源的单光子探测装置系统,对铟镓砷雪崩光电二极管(InGaAs-APD)作为量子保密通信单光子探测器件时的偏压生成、低温控制以及单光子信号检测进行了综合考虑,用脉冲发生器的单稳态电路等效地实现了直流电平叠加脉冲的“光子门”,用跨阻式前置放大器和精密的比较甄别器来改善系统的探测灵敏度。初期探测实验表明,能观察到-93dBm光信号的波形。     

Standoff detection of chemical and biological threats using laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is a promising technique for real-time chemical and biological warfare agent detection in the field. We have demonstrated the detection and discrimination of the biological warfare agent surrogates Bacillus subtilis (BG) (2% false negatives, 0% false positives) and ovalbumin (0% false negatives, 1% false positives) at 20 meters using standoff laser-induced breakdown spectroscopy (ST-LIBS) and linear correlation. Unknown interferent samples (not included in the model), samples on different substrates, and mixtures of BG and Arizona road dust have been classified with reasonable success using partial least squares discriminant analysis (PLS-DA). A few of the samples tested such as the soot (not included in the model) and the 25% BG:75% dust mixture resulted in a significant number of false positives or false negatives, respectively. Our preliminary results indicate that while LIBS is able to discriminate biomaterials with similar elemental compositions at standoff distances based on differences in key intensity ratios, further work is needed to reduce the number of false positives/negatives by refining the PLS-DA model to include a sufficient range of material classes and carefully selecting a detection threshold. In addition, we have demonstrated that LIBS can distinguish five different organophosphate nerve agent simulants at 20 meters, despite their similar stoichiometric formulas. Finally, a combined PLS-DA model for chemical, biological, and explosives detection using a single ST-LIBS sensor has been developed in order to demonstrate the potential of standoff LIBS for universal hazardous materials detection.     

Thin-Wire Bolometer for Measuring the Pulse Energy of Laser Radiation

A method of measuring the pulse energy and coefficient of nonuniformity of the distribution of strong and wide-aperture laser radiation by means of a thin-wire grating bolometer through simultaneous detection of the maximal value and pulse area of its output signal is evaluated.     

Sensitive in situ detection of chlorinated hydrocarbons in gas mixtures

We detect chlorinated hydrocarbons (CHC's) in gas mixtures by dissociating the CHC's with a 193-nm laser and measuring the subsequent concentration of the CCl fragmentation by means of laser-induced fluorescence. Sub-ppm detection, where ppm indicates parts in 10(6), is achieved for C(2)H(5)Cl with a 10-mm(3) measurement volume and integration over 50 laser shots. Every other CHC tested is also detectable, with the same or better detection limits. The CCl forms promptly during the fragmentation laser pulse through unimolecular dissociation of the parent CHC's. The technique should be a useful diagnostic for CHC incineration systems.     

Laser-enhanced ionization of mercury atoms in an inert atmosphere with avalanche amplification of the signal

A new method for laser-enhanced ionization detection of mercury atoms in an inert gas atmosphere is described. The method, which is based on the avalanche amplification of the signal resulting from the ionization from a selected Rydberg level reached by a three-step laser excitation of mercury vapor in a simple quartz cell, can be applied to the determination of this element in various matrices by the use of conventional cold atomization techniques. The overall (collisional + photo) ionization efficiency is investigated at different temperatures, and the avalanche amplification effect is reported for Ar and P-10 gases at atmospheric pressure. It is shown that the amplified signal is related to the number of charges produced in the laser-irradiated volume. Under amplifier noise-limited conditions, a detection limit of ～15 Hg atoms/laser pulse in the interaction region is estimated.     

2.07μm光纤激光在弱湍流条件下的传输特性研究

本文报道了一种2.07μm波段可调谐主动锁模光纤激光并在室内模拟大气湍流条件下进行传输特性研究。增益介质为1.5 m长的掺钬光纤,主动锁模通过LiNbO3强度调制器在腔内引入周期强度调制实现。腔内引入非线性偏振旋转效应实现波长2058.4nm^2078.6nm可调谐。实验获得了稳定的基频锁模脉冲和10阶,24阶,48阶谐波锁模脉冲,对应频谱信噪比为66.79 dB、61.37 dB、54.82 dB和49.66 dB。锁模脉冲经过数字调制后在实验室内大气湍流模拟池中进行传输,分别获得了?T为70℃,140℃和210℃时三种湍流强度和背对背条件下的眼图;与背对背条件相比,在?T=210℃时光信噪比降低了9.14 dB。     

Effective laser-induced breakdown spectroscopy (LIBS) detection using double pulse at optimum configuration

A short laser pulse is irradiated on a sample to create a highly energetic plasma that emits light of a specific peak wavelength according to the material. By identifying different peaks for the analyzed samples, their chemical composition can be rapidly determined. The characteristics of the laser-induced breakdown spectroscopy (LIBS) plasma are strongly dependent on the ambient conditions. Research aimed at enhancing LIBS intensity is of great benefit in advancing LIBS for the exploration of harsh environments. By using double-pulse LIBS, the signal intensity of Al and Ca lines was enhanced by five times compared to the single-pulse signal. Also, the angles of the target and detector are adjusted to simulate samples of arbitrary shape. We verified that there exists an optimal angle at which specific elements of a test sample may be detected with stronger signal intensity. We provide several optimum configurations for the LIBS system for maximizing the signal intensity for the analysis of a nonstandard aluminum sample.     

Influence of the laser pulse duration on spectrochemical analysis of solids by laser-induced plasma spectroscopy

Quantitative analysis of aluminum and copper alloys by means of laser-induced plasma spectroscopy (LIPS) has been investigated for three representative laser pulse durations (80 fs, 2 ps, and 270 ps). The experiments were carried out in air at atmospheric pressure with a constant energy density of 20 J/cm2. Because the decay rate of the spectral emission depends on the laser pulse duration, the optimum detection requires an optimization of the temporal gating acquisition parameters. LIPS calibration (sensitivity and nonlinearity) and the limit of detection (LOD) are discussed in detail. While the LOD of minor elements embedded in alloy samples obtained by sub-picosecond or sub-nanosecond laser pulses are both time and element dependent, provided an appropriate temporal window is chosen, the optimum LODs (several parts per million (ppm)) prove to be independent of the laser pulse duration. Finally, it is found that for elements such as those detected here, gated LIPS spectra using picosecond or sub-picosecond laser pulses provide much better LOD values than non-gated spectra.     

Optically controlled thermal management on the nanometer length scale

The manipulation of polymers and biological molecules or the control of chemical reactions on a nanometer scale by means of laser pulses shows great promise for applications in modern nanotechnology, biotechnology, molecular medicine or chemistry. A controllable, parallel, highly efficient and very local heat conversion of the incident laser light into metal nanoparticles without ablation or fragmentation provides the means for a tool like a 'nanoreactor', a 'nanowelder', a 'nanocrystallizer' or a 'nanodesorber'. In this paper we explain theoretically and show experimentally the interaction of laser radiation with gold nanoparticles on a polymethylmethacrylate (PMMA) layer (one-photon excitation) by means of different laser pulse lengths, wavelengths and pulse repetition rates. To the best of our knowledge this is the first report showing the possibility of highly local (in a 40?nm range) regulated heat insertion into the nanoparticle and its surroundings without ablation of the gold nanoparticles. In an earlier paper we showed that near-infrared femtosecond irradiation can cut labeled DNA sequences in metaphase chromosomes below the diffraction-limited spot size. Now, we use gold as well as silver-enhanced gold nanoparticles on DNA (also within chromosomes) as energy coupling objects for femtosecond laser irradiation with single-and two-photon excitation. We show the results of highly localized destruction effects on DNA that occur only nearby the nanoparticles.     

宽带信号检测中自适应门限设定方法

在水声定位中,常常需要估计声波从发射端到接收端的传输时间。但由于传输距离的影响,接收到的声波信号强度变化范围很大,给检测带来不便。基于线性调频脉冲信号相关检测,提出了一种自适应门限设定方法,可以在一定的条件下替代自动增益控制电路。这一方法根据发射信号、传输信道、发射系统和接收系统的参数,依据声信号到达接收端的时刻,自动调整相关峰检测时的门限值,简便易行。仿真和湖上试验结果表明,这一方法可以有效地保证计算信号到达时刻的准确性。     

Hybrid amperometric and conductometric chemical sensor based on conducting polymer nanojunctions

We report on a hybrid chemical sensor that can perform either amperometric or conductometric detection alone or simultaneously. It consists of an array of electrode pairs in which the two electrodes in each pair are separated with micrometer to nanometer-scale gaps. The gaps are bridged with conducting polymer (polyaniline) so that one can measure the conductance of the polymer bridge like a conventional Chem-FET. The electrode geometries are designed to allow simultaneously detection of electrochemical current like a conventional microelectrode amperometric sensor. The hybrid device provides increased selectivity for detection of analytes in complex matrixes and may provide new insights into the electrochemical reaction mechanisms of analytes. As an example, we have demonstrated the detection of dilute neurotransmitter (dopamine) in the presence of its concentrated major physiological interferent, ascorbic acid, which is not possible using either the amperometric or the conductometric techniques alone.     

Origin and effect of high-order dispersion in ultrashort pulse multiphoton microscopy in the 10 fs regime

Short pulses can induce high nonlinear excitation, and thus they should be favorable for use in multiphoton microscopy. However, the large spectral dispersion can easily destroy the advantages of the ultrashort pulse if there is no compensation. The group delay dispersion (GDD), third-order dispersion, and their effects on the intensity and bandwidth of second-harmonic generation (SHG) signal were analyzed. We found that the prism pair used for compensating the GDD of the two-photon microscope actually introduces significant negative high-order dispersion (HOD), which dramatically narrowed down the two-photon absorption probability for ultrashort pulses. We also investigated the SHG signal after GDD and HOD compensation for different pulse durations. Without HOD compensation, the SHG efficiency dropped significantly for a pulse duration below 20 fs. We experimentally compared the SHG and two-photon excited fluorescence (TPEF) signal intensity for 11 fs versus 50 fs pulses, a pulse duration close to that commonly used in conventional multiphoton microscopy. The result suggested that after adaptive phase compensation, the 11fs pulse can yield a 3.2- to 6.0-fold TPEF intensity and a 5.1-fold SHG intensity, compared to 50 fs pulses.     

CW脉冲回波信号的最小方差无畸变响应自适应检测

建立了适用于解析时间序列信号检测的最小方差无畸变响应(Minimum Variance Distortionless Response,MVDR)自适应处理模型(Time Series MVDR,TSMVDR),并针对CW声纳脉冲回波信号,通过构造解析信号和分段近似处理给出了相应算法(Algorithms of TSMVDR on CW pulse signal,AWMVDR),并进行了数值仿真研究。研究结果表明:对于CW脉冲信号:(1)在较高采样率的条件下,AWMVDR可以实现CW脉冲信号稳定的自适应检测;(2)AWMVDR较常规线性相关检测具有更高的脉冲参数估计精度和时间分辨力;(3)AWMVDR的实现需要付出较高采样率和更大计算量的代价。     

Focus-engineered coherent anti-Stokes Raman scattering microscopy: a numerical investigation

The coherent anti-Stokes Raman scattering (CARS) signal is calculated as a function of focal-field distributions with engineered phase jumps. We show that the focal fields in CARS microscopy can be shaped such that the signal from the bulk is suppressed in the forward detection mode. We present the field distributions that display enhanced sensitivity to vibrationally resonant object interfaces in the lateral dimension. The use of focus-engineered CARS provides a simple means to detect chemical edges against the strong background signals from the bulk.     

Chemical-assisted laser parallel nanostructuring of silicon in optical near fields

The authors present a simple and efficient technique for producing hexagonal arrays of nanostructures on silicon surfaces in chemical solutions. It utilizes the effect of optical near-field enhancement by self-assembled particle-lens arrays and a thermally induced chemical reaction with an alkaline solution. About 10(8) features can be produced simultaneously by one single laser pulse. Furthermore, the shape of the structures was found to be controllable, from concave holes to convex bumps, by means of a post-etching process, in the same chemical solution.     

Frequency encoding of resonant mass sensors for chemical vapor detection

Chemical vapors can be detected by a resonant mass sensor array with selective absorption coatings implementing a frequency encoding method. The sensor array consists of sensor elements with different frequencies for their identifications in the frequency response obtained with a pulse Fourier transform detection scheme. Zero-loading resonance frequencies are chosen so that frequency shift due to absorption is bounded within a predefined region so that there is no overlap of peaks and all peaks can be assigned to the correct elements at any operation conditions. Mechanical oscillations of all or selected numbers of the sensor elements are excited by application of an excitation signal. Free oscillation decay signals from all or selectively excited sensor elements are detected and digitized. The free oscillation decay signal is subjected to a spectral analysis routine converting into a frequency spectrum, in which frequency shifts due to absorption of chemical vapors can be obtained. The implementation of the frequency encoding method with pulse Fourier transform detection to resonant mass sensors allows simultaneous multisensor detection, fast data acquisition speed, high signal-to-noise ratio by coaddition of raw data, flexible excitation, reduced complexity of electronic hardware, application of advanced data/spectral analysis algorithms, and realization of many other advantages by the introduction of the pulse Fourier transform method. A practical chemical vapor sensing system is demonstrated experimentally by use of nine frequency-encoded and polymer-coated sensors.     

脉冲激光光谱测试仪的设计

设计了一种基于ISP技术的线阵CCD的驱动电路和高速脉冲光谱数据采集卡,建立了一套智能化脉冲激光光谱测试系统。该系统主要由分光系统、CCD传感器、光脉冲同步信号发生器、数据采集卡及计算机等部分组成。CCD将光强信号转换为电信号输出,经A/D转换器转换后的数字信号存入数据采集卡的帧存储器中,由计算机控制进行光谱数据的分析、处理。提出了用重心法对脉冲激光器的重复频率进行评估的方法,使测量精度达到了亚像素级。对系统所用脉冲激光器谱线峰值波长的复现性进行了测量,其标准差为0.0327nm。     

UV excitation thermal lens microscope for sensitive and nonlabeled detection of nonfluorescent molecules

An ultrasensitive and nonlabeled detection method of nonfluorescent molecules on a microchip was developed by realizing a thermal lens microscope (TLM) with a 266-nm UV pulsed laser as an excitation light source (UV-TLM). Pulsed laser sources have advantages over continuous-wave laser sources in more compact size and better wavelength tuning, which are important for microchip-based analytical systems. Their disadvantage is difficulty in applying a lock-in amplifier due to the high (>10(4)) duty ratio of pulse oscillation. To overcome this problem, we realized a quasi-continuous-wave excitation by modulating the pulse trains at approximately 1 kHz and detecting the synchronous signal with a lock-in amplifier. The optimum pulse repetition frequency was obtained at 80 kHz, which was reasonable considering thermal equilibrium time. Furthermore, a permissible flow velocity in the range of 6.6-19.8 mm/s was found to avoid sensitivity decrease due to photochemical reactions and thermal energy dissipation. Under these conditions, we detected adenine aqueous solutions on a fused-silica microchip without labeling and obtained a sensitivity that was 350 times higher than that in a spectrophotometric method. The sensitivity was enough for detection on a microchip with an optical path length that was 2-3 orders shorter than that in conventional cuvettes. Finally, the UV-TLM method was applied to liquid chromatography detection. Fluorene and pyrene were separated in a microcolumn and detected in a capillary (50-microm inner diameter) with 150 times higher sensitivity than a spectrophotometric method. Our method provides highly sensitive and widely applicable detections for various analytical procedures and chemical syntheses on microchips.     

Dependence of the laser two-photon ionization process in solution on the laser pulse width

The laser two-photon ionization process has been investigated using simultaneous irradiation of an UV beam and an IR beam. When the laser pulse width was 300 ps, it proceeded through a geminate pair state (a stepwise process), as indicated by the signal enhancement with simultaneous irradiation of the two laser beams. Although no signal enhancement was observed when the laser pulse width was 100 fs, and because molecules with no absorption at the laser wavelength showed an intense signal, the two-photon ionization excited by a femtosecond laser should proceed through a simultaneous two-photon process. The detection limit was quite susceptible to the laser fluctuations.