N_2O的近红外腔增强吸收光谱技术研究

以外腔式可调谐、窄线宽近红外半导体激光为光源,以一对曲率半径r=1 000mm的宽带高反射率平凹镜(反射率R=99.97%)构成的腔长为650mm的对称高精度光学稳定腔,建立了腔增强吸收光谱系统。详细研究了纯净N2O气体、以及N2O和N2的混合气体在不同浓度和不同气压下、中心波长位于6 561.39cm-1的腔增强吸收光谱、光谱强度和谱线宽度,该腔增强吸收光谱系统的有效吸收光程可达1 460km。获得了光谱线宽与气体压强的关系曲线,导出了波数6 561.39cm-1处N2对N2O的压力展宽系数为(0.114±0.004)cm-1·atm-1。采用该腔增强吸收光谱系统,开展了N2O气体检测研究,建立可用于定量检测的N2O气体腔增强吸收光谱强度与气体浓度关系曲线,获得了2.34×10-7cm-1的检测灵敏度,多次重复测量的相对标准偏差(RSD)为1.73%,在微量N2O气体检测中具有很好的应用前景。     

基于半导体激光的腔增强吸收光谱技术研究

用近红外可调谐分布反馈(DFB)半导体激光器作光源,用反射率为99.7%左右的平凹镜组成的稳定光学谐振腔作吸收池,建立了一套腔增强吸收光谱(CEAS)系统.根据系统工作时激光器与谐振腔的工作状态,将CEAS技术分为三类:控制波长法,控制腔长法及同时扫描波长和腔长法.以二氧化碳(CO2)和甲烷(CH4)为主要样品气体,用控制腔长法和控制波长法CEAS技术对CO2分子在1.573μm附近的吸收光谱做了测量;用扫描腔长和波长法CEAS技术对CH4气体在1.316μm附近的吸收光谱做了测量;考察了三种方法的探测灵敏度和在定量分析方面的能力.实验结果表明,CEAS技术是一种装置简单,操作方便,灵敏度高,稳定性好的定量吸收光谱技术,探测灵敏度达1.15×10-7cm-1.     

白噪声扰动在波长调制离轴积分腔输出光谱技术中的应用研究

离轴积分腔输出光谱技术是痕量气体检测的重要方法,这种测量方法的检测限容易受到残余腔模式噪声和背景噪声的影响。通过注入射频白噪声到激光器的调制电流中,以减小离轴积分腔输出光谱中的残余腔模式噪声,同时利用波长调制技术抑制了背景信号的影响,进一步提高了基于离轴积分腔输出光谱技术的甲烷传感系统的信噪比。首先,详细研究了不同功率射频白噪声对空气中甲烷吸收光谱的影响,并对吸收谱的线宽进行了分析,计算出了不同功率噪声扰动下的吸收谱对应的最佳调制幅度。随后,研究了不同功率的射频白噪声对2f信号的影响。结果表明,随着扰动噪声功率的增加,基线噪声水平和2f信号幅值同时减小。对几组2f信号的信噪比进行分析,确定了射频白噪声提高系统信噪比的最佳功率为-25 dBm。最后,研究了0.05～2.2×10~(-6)浓度范围内,甲烷浓度与2f信号之间的对应关系,结果表明:在甲烷浓度小于1.0×10~(-6)时,甲烷浓度与2f信号之间的线性度为0.999 6;在甲烷浓度为0.1～2.2×10~(-6)时,甲烷浓度与2f信号之间呈曲线关系,二阶多项式拟合的相关度为0.999 89。此外,对浓度为2.2×10~(-6)的甲烷气体进行了长时间的测量,并利用Allan方差对系统的稳定性进行了分析,分析结果表明系统的最佳积分时间为1 250 s,系统的可探测极限约为1.2×10~(-9)。最后,使用建立的甲烷气体探测系统,对大气环境中的甲烷气体浓度进行了长达两个昼夜的检测,结果显示甲烷浓度的昼夜变化规律是昼降夜升,浓度昼夜波动范围在2.02～2.3×10~(-6)范围内,平均浓度为2.14×10~(-6)。本研究为离轴积分腔输出光谱技术在痕量气体测量方面的应用提供了一定的参考,对高精密的原位痕量气体测量仪器的研发具有重要的指导价值。     

磁旋转腔增强光谱技术

为了提高吸收光谱的探测灵敏度,在弱吸收或短光程吸收的情况下实现高灵敏探测,将腔增强光谱技术与磁旋转光谱技术有效地结合起来,发展了高灵敏的磁旋转腔增强吸收光谱技术,并通过测量O2 的三重禁戒跃迁谱线验证了该技术的探测灵敏度。实验采用环型增强腔,以避免光束的返回对激光器的干扰。给出了腔的耦合匹配条件,以及镜面反射率、腔损耗对增强因子的影响;同时也给出了在实验中对光谱信号的处理方法。测量结果表明,在谐振腔精细度为F=48,腔内总损耗为13%,以及腔镜的耦合效率为 95%的情况下,对 O2 分子最小相对吸收度约为4.5×10-8(1 s积分时间)。     

基于腔增强激光光谱的水中溶解甲烷传感系统

为探测水中甲烷气体浓度,研制了一种基于离轴积分腔输出光谱的水中溶解甲烷传感系统。系统由分布式反馈激光器(中心波长为1 653 nm)、激光器温度控制模块、激光器电流驱动模块、谐振腔/气室、光电探测器、数据采集模块、数据处理模块和气液分离模块构成。利用配备的甲烷气体样品和纯氮气(N2),分别开展系统有效光程标定、直接吸收光谱信号的标定和稳定性测试等实验。使用浓度为10×10-6的甲烷气体样品标定了系统有效光程,约为1 906 m。将纯氮气作为目标气体,测量系统的稳定性。Allan方差分析结果表明,当积分时间为2 s时,系统检测灵敏度为92.8×10-9,当积分时间增加到134 s,系统的灵敏度可提高到13.2×10-9。利用该系统开展自来水、雨水和湖水样品中溶解甲烷的浓度检测实验,结果证实了该技术及系统的工程实用价值。该研究及相关结果为水质检测和天然气水合物等清洁能源的勘探开发奠定了良好的基础。     

基于标准样品回归算法和腔增强光谱的NO2检测方法

腔增强吸收光谱技术作为一种高灵敏的痕量气体测量技术,其吸收光谱的浓度反演是极其关键的环节.为消除因吸收截面和仪器响应函数的不确定性引入的测量误差,本文提出了一种基于标准样品吸收光谱的浓度回归算法,该方法在浓度反演过程上进行优化,采用标准气体样品吸收光谱直接拟合未知浓度气体吸收光谱.采用中心波长在440 nm处的蓝色发光二极管(LED)作为光源,建立了一套非相干光腔增强吸收光谱技术(IBBCEAS)系统,实测腔镜反射率为99.915%,利用NO₂气体的实测吸收光谱对该算法的有效性进行了验证.与常规吸收截面回归算法比较,结果表明本文提出的标准样品回归算法具有显著的优越性,测量精度提升约4倍.利用改进的算法结合标准样品配制的多个NO₂气体对实验系统性能进行了深入评估,测量结果与理论值具有很好的一致性.Allan方差分析显示在积分时间为360 s的情况下,NO₂检测限可达到5.3 ppb(1 ppb=10^–9).     

采用离轴入射的腔增强吸收光谱研究

为了改善腔增强吸收光谱的探测灵敏度和光谱分辨率,通过将激光光束离轴注入光学谐振腔中,减小了谐振腔内振荡,提高了模式密度,使得谐振腔被近似看作为“怀特池”。采用记录波长在扫描过程中实际变化的方法,对光谱数据进行了非线性校正,省去了参考光路的布置,简化了实验光路。对二氧化碳在1．572μm处的一条弱吸收谱线进行了测量,得到了大小为1．98×10^-7cm^-1的最小探测灵敏度。实验结果表明同等实验条件下,光束离轴入射时比正轴入射时的腔增强吸收光谱具有更高的信噪比和光谱分辨率。     

基于离轴腔增强吸收光谱双组分CH4/H2O高灵敏度探测研究

H₂O和CH₄在气候变化过程中起着关键作用,实时在线测量H₂O和CH₄浓度一直都是国内外学者研究的热点问题之一。利用1.653 μm可调谐半导体激光器作光源,结合反射率为99.997 6%的两片高反射镜组成离轴腔增强吸收光谱装置,开展了H₂O和CH₄的高灵敏度测量研究。离轴腔增强系统的有效吸收光程通过吸收面积-浓度关系法来标定,吸收面积-浓度关系法的可行性首先通过已知光程的光学吸收池进行验证,确定有效后用于标定离轴腔增强系统的有效光程。结果表明,基长为21 cm的离轴腔增强系统的有效吸收光程达到了8 626.3 m。当谐振腔内压力为5.06 kPa时,利用7组不同浓度的CH₄标准气体（0.2～1.4 μmol·mol-1）对系统进行了线性响应标定测试,得到了CH₄吸收的积分面积与浓度拟合关系曲线。系统的稳定性、可实现的最小探测灵敏度等信息通过Allan方差进行分析,结果表明系统对探测CH₄的最佳平均时间为100 s,最小可探测浓度极限为7.5 nmol·mol-1;系统对探测H₂O的最佳平均时间为200 s,最小可探测浓度极限为55 μmol·mol-1。对提高系统测量精度的数据处理方法也进行了分析研究,结果表明相比于多次平均方法,Kalman滤波能显著的提高测量精度,而且缩短了系统的响应时间。最后,利用搭建的离轴腔增强实验系统结合Kalman滤波数据处理方法对实际大气中CH₄和H₂O浓度进行了连续两天的测量,CH₄每天平均的浓度分别为2.1和2.08 μmol·mol-1,H₂O每天平均的浓度分别为11 515.6和11 628.6 μmol·mol-1,由此可知建立的离轴腔增强吸收光谱装置能够用于大气CH₄和H₂O的测量,另外建立的系统也可用于相关工业领域的高灵敏度CH₄和H₂O监测。     

多光程吸收的频率调制光谱

采用可连续调谐的半导体二极管激光器作探测光源,将多光程吸收光谱技术、频率调制光谱技术和相敏检波技术相结合,建立了一套具有高检测灵敏度和分辨率的测量气态分子光谱和进行微量分析的研究装置并利用该装置研究了动态分压强为40Pa和120Pa时,H₂O分子在7462cm^-1和7473cm^-1附近的近红外吸收光谱,检测灵敏度远高于传统吸收光谱技术,为气态分子的光谱研究和微量分析提供了一种高灵敏度的光谱检测方法.     

基于椭圆腔共振的石英增强光声光谱理论研究

石英增强光声光谱技术作为一种新型的光学检测技术,已被广泛应用于痕量气体检测场合.其中声波共振增强性能是决定检测灵敏度的重要因素.为提高光声光谱检测系统的信噪比和检测极限,提出一种新型的椭圆腔共振石英增强光声光谱检测方法,建立了其声学特征模型并利用有限元分析方法对光声腔内部声学特性进行仿真研究.研究结果表明,椭圆腔的特征模态在(2,1)模态下长轴两端声压达到最大值.通过对椭圆腔的尺寸和形状进行优化,建立实验装置,得到目标气体硫化氢检测极限为6.3 ppm(parts per million),相应的归一化噪声等效吸收系数为2.02×10~(-9)cm~(-1)W/Hz~(1/2).     

Guided-Wave Two-Dimensional Acousto-Optic Scanner Using Proton-Exchanged Lithium Niobate Waveguide

Two-dimensional scanning of a 0.6328 mum guided-light beam has been realized using noncolinear acousto-optic (AO) coplanar Bragg diffraction together with colinear AO guided-mode to substrate radiation-mode conversion in a Z-cut Xpropagation LiNbO3 proton-exchanged (PE) waveguide. The two surface acoustic (SAW) waves utilized are at the center frequencies of 500 and 200 MHz, propagating in the Y and X axes, respectively. Two-dimensional scanning of approximately 720 resolvable light beam spots, namely, 18 40 (horizontal vertical) scanning, has been demonstrated using a light beam of 1.0 mm aperture. The total number of resolvable beam spots can be greatly increased from 720 by simply utilizing SAW transducers of larger bandwidth and a light beam of greater aperture. It should also be possible to significantly increase the diffraction efficiency from 3 % by optimizing the parameters of the PE waveguide and the SAWs.     

Hybrid Wavelength and Code Division Multiple Access in Optical Networks

A review of schemes for multiple access in fiber optic networks shows that a hybrid of wavelength and code division multiple access (WCDMA) combines the best features of both. In particular, the hybrid scheme retains the large information carrying capacity of wavelength division multiple access (WDMA) and flexibility of code division multiple access (CDMA). In this paper WDMA, optical CDMA (OCDMA), and WCDMA networks are discussed. In OCDMA networks, concept of incoherent and coherent coding including inverse decoding and matched filter is introduced. The delay performance of both WDMA and WCDMA networks, under the simple suboptimum access protocols based on cyclic search, is computed. It has been shown quantitatively that tuning delay significantly affects the delay performance of both WDMA and WCDMA networks. Futhermore, delay performance of WCDMA networks is always better than the WDMA networks for the same tuning delay, load, and number of users.     

Femtosecond Transform-Limited Pulse Generation by Compensating for the Linear Chirp of SPM Spectra in Dispersion Shifted Fibers

A simple technique of pulse compression, based on the linear chirp compensation of self-phase modulation (SPM) spectra in dispersion shifted fibers, is demonstrated. The optimization procedure is carried out, for a short span of a single-mode fiber, using a parabolic law, which describes the behavior of the squared output pulse width versus the pump peak power in the case of Gaussian pulses. The experimental results give a minimum pulse duration of 233 fs, which is in good agreement with the model. Shorter and coherent pulses, down to 90 fs, have been obtained by inserting an interference filter at the optical output.     

Effect ofγ-radiation on the spectral luminescence characteristics of impure cadmium tungstate crystals

Results of a study of the effect of γ-radiation on the spectral luminescence properties of cadmium tungstate crystals doped with silver, bismuth, and molybdenum cations are presented. Spectral characteristics of the nondnnnoped crystals are briefly described. Absorption and photo and X-ray luminescence spectra of the crystals taken before and after exposure to γ-radiation (5.5·10⁴ Gy) are compared. It is found that the spectral characteristics of the crystals doped with silver, bismuth, and molybdenum cations do not change markedly after the exposure. The relation between the type of impurity-induced defects, individual characteristics of the impurity cations, and the character of the effect of γ-radiation on the spectral luminescence properties of impure crystals is analyzed (preliminarily). Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 55–60, January–February, 1997.     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Research on beamforming method for dual-line array with space mismatch between the two line arrays

The performance of port/starboard discrimination of dual-line array will be seriously degraded when the space between the two line arrays of the dual-line array is mismatched. The directivity function of the delay-sum and delay-subtract beamforming is deduced and the port/starboard discrimination of the delay-sum and delay-subtract beamforming is also studied both under the ideal status and the status under which the space between the two line arrays of the dual-line array is mismatched. Analysis results show the delay-subtract beamforming has better performance of port/starboard discrimination. At last, the sea trial results demonstrate the theoretical results.     

Sorting of bending magnets for the SSRF booster

The Shanghai Synchrotron Radiation Facility(SSRF)booster ring,a full energy injector for the storage ring,is deigned to accelerate the electron beam energy from 150MeV to 3.5GeV that demands high extraction efficiency at the extraction energy with low beam loss rate when electrons are ramping.Closed orbit distortion(COD)caused by bending magnet field uniformity errors which affects the machine performance harmfully could be effectively reduced by bending magnet location sorting.Considering the affections of random errors in measurement,both ideal sorting and realistic sorting are studied based on measured bending magnet field uniformity errors and one reasonable combination of bending magnets which can reduce the horizontal COD by a factor of 5is given as the final installation sequence of the booster bending magnets in this paper.