空间外差干涉光谱仪仪器线型函数测量新方法研究

空间外差干涉光谱技术是近年发展起来的新型静态超分辨光谱分光技术,仪器线型函数是其基本性能参数之一,代表了仪器的光谱分辨能力,需要精确表征。在分析仪器线型函数影响因素(切趾、有效视场角与离轴像元效应)以及测量方法与测量光源等特殊性要求基础上,提出了一种可调波长单色面光源的全新测量方法,并利用可调谐激光器与消散斑积分球等设备搭建了测量实验装置。在仪器线型函数测量实验中,通过选取光谱范围内的典型谱段进行高光谱(0.1 nm步长)扫描,经过干涉数据误差修正、光谱复原以及坐标归一化等数据处理过程,获取了仪器线型函数的能量分布形式。此外,利用全光谱范围内扫描的干涉数据,得到仪器线型函数的全峰半宽随波长的变化规律曲线。最后,将本方法获取的实测仪器线型函数与模拟光谱(LBL计算)进行卷积获取理论谱,并与地基探测实验获取的实测大气CO₂吸收光谱进行比对分析,两者吻合一致,验证了本方法获取的仪器线型函数具有较高的精度。     

激光外差光谱仪的仪器线型函数研究

激光外差是一种基于相干探测原理的高灵敏度光谱检测技术,因其同时具有很高的光谱分辨能力,被广泛应用于诸多研究领域.在光谱测量过程中,仪器线型函数对吸收谱线的平滑作用,会对气体浓度的反演结果产生影响.为了获取激光外差光谱仪的仪器线型函数,基于激光外差原理和信号处理过程,对影响仪器线型函数的射频滤波带宽和积分时间等参数进行了分析,获得了仪器线型函数表达式.利用自行建立的激光外差光谱仪,多次测量了3.53μm波段内水汽、甲烷的吸收谱线,分别将射频滤波频域响应函数和本文获得的仪器线型函数耦合进水汽、甲烷柱浓度的反演.结果表明,射频滤波带宽为30 MHz、积分时间分别为10 ms和100 ms时,光谱仪的实际分辨率分别约为0.005 cm~(-1)和0.025 cm~(-1);使用仪器线型函数对积分时间为100 ms时测量的数据进行反演,透过率残差平方和与甲烷吸收峰值处的残差分别减小16%和100%,提高了气体浓度反演的准确度.     

干涉型红外光谱辐射计仪器线型函数仿真及校正

针对大口径干涉型红外光谱辐射计,分析了不平行于主光轴的入射光,对理想仪器线型函数的影响.本文系统介绍了对仪器线型函数产生影响的截断效应、有限视场效应、离轴效应和离焦效应,并通过HITRAN数据库对理想的水汽吸收光谱进行仿真,建立了仪器线型函数误差与光谱畸变的定量关系.根据仿真结果,提出了因子权重校正算法.利用水汽吸收的仿真数据对因子权重校正算法进行验证,光谱漂移从0.51 cm–1降低到0.01 cm–1以下.通过自研干涉型红外光谱辐射计对标准黑体的观测实验,验证因子权重校正算法的准确性,实测数据的光谱漂移从0.226 cm–1降低到0.012 cm–1,校正后的光谱数据更为准确.     

基于谱线线型函数的气体浓度检测技术

快速、精确地测量微量气体浓度的技术在大气质量分析、环境污染检测等领域具有广泛的用途。在红外光谱检测技术中,气体吸收光谱的谱线线型函数是定量测量气体浓度的一个重要参量,而如何准确和快速地得到气体谱线线型函数值是气体浓度测量中的一个关键问题。首先从理论上分析了谱线线型函数,得出计算谱线线型函数的一般方法及探讨了气体浓度与谱线线型函数峰值之间的关系。然后,利用可调激光器及光谱仪检测系统测量了乙炔在1 515～1 545 nm波长范围内的吸收光谱,再通过Lambert-Beer定律计算得到在不同频率下的谱线线型函数峰值,最后利用程序拟合出该波段内气体的谱线线型函数峰值分布曲线,并与Voigt线型函数理论计算值进行了比较,发现理论计算的谱线线型函数峰值仍存在一定的偏差。相比理论计算结果,所提出的检测方法得到的乙炔浓度与真实的乙炔浓度值更加吻合,表明了通过实验确定的谱线线型函数的经验公式可以更好地用于气体浓度的检测。由于利用实验测量值获得了谱线线型函数峰值分布的拟合曲线,因此可以快速准确地计算出所对应的谱线线型函数峰值,从而大大地简化了线型函数的计算过程。实验所获得的数据可应用于光学遥测乙炔气体浓度,且提供的方法也可以应用到其他气体的谱线线型函数峰值的测量中。     

新型超分辨干涉型光谱仪光谱定标研究

针对综合光栅衍射与空间干涉与一体的新型空间外差光谱(SHS)技术开展了光谱定标技术研究。从SHS干涉机理入手,列举了SHS与传统超光谱仪器光谱定标的差异,开展了仪器线性函数、光谱分辨率和光谱范围等光谱特性数据的定标原理研究,设计可调谐激光积分球光谱定标法,并且针对超光谱数据特点探讨了谱峰定位等数据处理算法。利用上述光谱定标方法对CO2空间外差光谱仪样机开展了光谱定标实验,并利用镁元素灯进行了定标精度验证,结果表明该光谱定标方法能够满足SHS光谱定标要求,样机实测光谱定标数据与理论设计值吻合。     

光纤受激布里渊增益谱线型特性分析

光纤受激布里渊效应是一种非常重要的光纤非线性效应,受激布里渊增益谱的线型特性,对于光谱分析、微波光子学、光纤传感等领域的应用,至关重要。针对增益谱线型特性研究,现有方法难以获取高信噪比的增益谱数据,介绍了一种采用两个窄线宽激光器的增益谱测量方法,其中一个激光器作为探针信号,另一个可调谐激光器作为泵浦信号在宽波段范围内进行推扫。得益于激光器的高信噪比单谱线特性,并利用偏振跟随特性,实现了高信噪比的受激布里渊增益谱测量。测量并分析了不同泵浦功率,对于谱型和线宽等受激布里渊增益特性的影响,特别是,直接实验测量了增益谱型从洛伦兹线型到高斯线型的演化,揭示了泵浦功率等影响受激布里渊增益水平的因素,对于增益谱线型的影响。实验发现,当泵浦功率超过15 dBm时,增益谱出现饱和效应,导致测量增益谱线宽开始随泵浦功率提高而展宽,难以直接测量得到高泵浦功率时的高斯线型的增益谱。提出通过归一化等校正处理,以获取受激布里渊增益谱与待测信号光谱的有效卷积信息,从而得到了近似为高斯线型的增益谱型;针对中间增益区域增益谱型较为复杂,缺少合适增益谱线型演化数学模型的难题,给出了一种基于k次洛伦兹函数拟合的谱型模型,描述从洛伦兹线型到高斯线型的完整演进过程。实验表明拟合效果良好,是一种行之有效的受激布里渊增益谱线型数学模型,可完整描述增益谱在不同增益时的线型。最后,探索了受激布里渊效应在光谱分析方面的应用,通过合理选择泵浦功率等手段,增益谱型为高斯线型时的光谱分光效果最佳,获取了0.2 pm分辨率的6314CA稳定光源的光谱,显示了光纤受激布里渊效应在超高分辨率光谱分析领域的巨大技术优势,而这对于新一代光网络的验证评估至关重要。     

基于空间调制的星体观测张角强度关联干涉测量

强度关联干涉测量利用光场的高阶关联特性获取星体空间角度信息,有望实现脉冲星角位置的高精度测量。然而常规的强度关联测量需要满足相干探测条件,这对探测器的时间分辨提出了极高的要求。提出基于空间调制的星体观测张角强度关联干涉测量方法,在探测器前放置调制屏以对光场进行空间调制,通过旋转调制屏获取二阶干涉条纹。理论推导了当两条光路的调制屏存在角度差时二阶关联函数的表达式,并基于理论推导结果设计双反射镜实验方案并进行相关可见光实验验证,所得实验结果与理论分析结果相符。该方法大幅降低了探测器的时间分辨要求,对于我国未来实现航天器的自主导航具有重要意义。     

去卷积和高斯拟合方法在FTIR光谱处理中的应用

讨论了FTIR光谱产生的机理,并分析了影响其光谱质量的两种原因:(1)干涉图的截断;(2)仪器采样率低.为了改善由上述两种原因引起光谱畸变,我们对其进行了仪器线型函数去卷积和高斯拟合,使其更接近实际光谱,从而使由其计算出的物理量更接近实际值,结果更精确.     

可调谐掺铥光纤激光器线宽压缩及其超光谱吸收应用

可调谐二极管激光吸收光谱技术是一种应用非常广泛的吸收光谱测量技术.利用宽带可调谐窄线宽光源进行吸收光谱测量的超光谱吸收技术可以在单次扫描中获取一段连续光谱的所有吸收数据,可大大提高可调谐二极管激光吸收光谱技术的数据信息容量和光谱诊断能力.分析了在2μm波段对水进行超光谱吸收测量时对激光器输出线宽的具体要求.利用掺铥光纤在2μm波段较宽的发射谱,采用可调谐法布里-珀罗滤波器和光纤可饱和吸收体相结合的技术方案搭建了一台宽带调谐窄线宽的2μm光纤激光器.获得了1840—1900 nm约60 nm范围的调谐光谱输出,激光器静态线宽仅为0.05 nm.利用该光源对空气中水在2μm波段的吸收光谱数据进行了超光谱吸收测量,在1856—1886 nm约30 nm的光谱范围内分辨了35条水的吸收谱线.通过对不同线宽条件下1870—1880 nm范围内的理论吸收光谱数据进行对比发现,测量数据无法有效分辨分别位于1873 nm和1877 nm处与强吸收线相邻的两条吸收谱线,且测量结果与激光线宽在0.08 nm条件下的HITRAN2012光谱数据库最为接近.这表明,在动态扫描过程中激光器的线宽得到了展宽.     

基于ARTS的傅里叶红外高光谱计算模型研究及其影响因素分析

在基于红外高光谱辐射数据进行大气遥感方面的研究中,准确模拟红外高光谱数据是很重要的一步。分析了红外高光谱辐射仪的测量原理,建立了基于Atmospheric Radiation Transfer Simulator(ARTS）的考虑仪器干涉图截断与离散化处理过程的正向模型。在该正向模型中,首先采用高光谱辐射传输模式ARTS模拟得到离散化理想光谱,通过逆傅里叶变换将理想光谱转化为干涉图,对干涉图加窗截断处理,模拟仪器响应函数对干涉图的影响,最后采用傅里叶变换得到仪器测量光谱。在这一过程中,窗口函数的选择取决于仪器的干涉图截断方式。未经过切趾处理的仪器,其对应的窗口函数为矩形窗口;经过切趾函数处理,可以减少干涉图截断造成的能量泄露现象。逆傅里叶变换与傅里叶变换过程中必须满足Nyquist采样定律。基于已建立的正向模型,模拟了Atmospheric Emitted Radiance Interferometer (AERI)在Southern Great Plains (SGP)站点的108组晴空辐射数据,并与AERI的实测结果进行比较分析,结果发现理想光谱与AERI实测光谱在吸收线上差异较大,最大残差达到35 mW·sr-1·m-2·(cm-1)-1（简称RU）以上,增加干涉图截断过程后,模拟光谱与实测光谱的最大残差减小到10 RU以内。截断过程的增加对模拟光谱的精度有明显提高,尤其在吸收线上,模拟光谱明显被平滑,模拟精度显著提高。进一步分析六种常用窗口函数截断处理的结果与AERI实测数据的残差,结果发现,模拟过程中选择窗口函数为矩形窗口时,模拟光谱与AERI实测数据残差最小,基本可以约束在5 RU以内,确定了AERI的干涉图截断方式可以近似看作矩形截断。另外,在理想光谱转换为干涉图的过程中,理想光谱分辨率的选择决定了干涉图信息的采样率以及ARTS的计算效率,因此综合考虑模型计算精度和模型计算效率,确定最佳的理想光谱分辨率对于提高模型计算效能是非常必要的;基于此,本文模拟了不同理想光谱分辨率下的仪器测量光谱,对比分析了模拟光谱与AERI实测光谱的残差分布,并讨论了光谱分辨率对模型计算耗时的影响。结果表明,对于AERI,在对应的正向模型中设置理想光谱分辨率为0.241 1 cm-1时,可在保证模型准确度的前提下,最大化模型计算效率。     

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.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Propagation of Helmholtz-Gauss beams in weak turbulent atmosphere

Based on the Rytov approximation of light propagation in weak turbulent atmosphere,the closed-form expressions of field and average irradiance of each one of the four fundamental families of Helmholtz-Gauss (HzG)beams:cosine-Gauss beams,stationary Mathieu-Gauss beams,stationary parabolic-Gauss beams,and Bessel-Gauss beams,which are propagating in weak turbulent atmosphere,are obtained.The results show that the field and average irradiance can be written as the product of four factors:complex amplitude depending on the z-coordinate only,a Ganssian beam.a factor of complex phase perturbation induced by atmospheric turbulence,and a complex scaled version of the transverse shape of the non-diffracting beam.The effect of weak atmospheric turbulence on irradiance distribution of the HzG beam can be ignored.     

An effective method for reducing speckle noise in digital holography

An effective method for reducing the speckle noise in digital holography is proposed in this paper.Different from the methods based on classical filtering technique,it utilizes the multiple holograms which are generated by rotating the illuminating light continuously.The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns.Hence by properly averaging the reconstructed intensity fields,the speckle noises can be reduced greatly.Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.     

Influence of purity of HfO2 on reflectance of ultraviolet multilayer

The impurities in two kinds of HfO2 materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum technology and secondary ion mass spectrometry(SIMS) equipment respectively.It was found that ZrO2 was the main impurity in the two kinds of HfO2 either in the original HfO2 materials or in the electron beam deposited films.In addition,the difference of Zr content in the two kinds of HfO2 single laver films was much laxger than that of the other impurities such as Ti and Fe.which showed that it was just ZrO2 that made the difference between the optical performance of the film products including the two kinds of HfO2.With these two kinds of HfO2 and the same kind of SiO2.we deposited HfO2/SiO2 multilayer reflective coatings at the wavelength of 266 nm.Experimental results showed that the reflectances of these two mirrors were about 99.85% and 99.15% respectively,which agreed well with the designed results what were based on the optical constants obtained from the corresponding single layer thin films.     

Speech enhancement by array crosstalk resistant ANC and spectrum subtraction

Microphone array-based speech enhancement has great importance for speech communications and speech recognition. To reduce the aperture of the microphone array and to increase the effect of the speech enhancement will greatly broaden the application areas of the microphone array. An array crosstalk resistant adaptive noise cancellation method is therefore presented. And then an improved spectral subtraction algorithm is further cascaded to obtain better enhancement results. Theoretic analysis and experiments indicate that the proposed scheme needs only a very small microphone array while it simultaneously achieves a higher SNR improvement. Besides, the proposed scheme can be used in many noisy environments and is easy for real-time implementation.