全光纤双通道大气温室气体激光外差光谱探测技术研究

激光外差光谱技术是近年迅速发展的一种高光谱分辨率遥感探测技术,其装置具有体积小、光谱分辨率高等特点,适用于大气温室气体浓度的探测。各种观测实验已证明其是一种切实、有效的探测手段,在地球大气探测领域具有很大的应用前景和潜力。在现有激光外差光谱仪器的基础之上,提出了一种新型的仪器结构。采用光纤光开关对直射的太阳光进行调制与分束,实现利用单个光纤光开关进行两个波段激光外差信号的同时探测。为全光纤激光外差光谱仪的下一步系统集成和多波段激光外差光谱仪的构建提供了新方法。从激光外差探测的原理出发,分析了激光外差光谱探测技术的优势和关键参数,并结合自行研制的高精度太阳跟踪仪,搭建了一套近红外双通道全光纤式激光外差辐射计的原理样机。详细地阐述了激光外差辐射计原理样机中各功能模块的作用和参数,着重描述了光纤光开关在其中的作用原理和功能,详细地讨论了激光外差光谱仪的波长扫描模式和波长标定方法。在此基础上,讨论了激光外差辐射计的相关参数设置依据和仪器函数的测量方法,并给出了激光外差装置的仪器函数以及相应的光谱分辨率(0.004 4 cm-1)。利用搭建的激光外差光谱仪在合肥地区(31.9°N, 117.166°E)进行了实际的大气探测,同时在波段(6 056.2~6 058.1 cm-1)和(6 035.6~6 036.5 cm-1)分别获取了CH₄和CO₂分子的激光外差吸收信号,并对吸收信号进行了波长标定和归一化,得到了CH₄分子和CO₂分子的整层大气透过率谱,测量光谱信号的信噪比分别为197和209,同时也对分子吸收信号的光谱特征进行了分析。本文的测量实践表明光纤光开关可以用于激光外差光谱系统的结构优化,实现多通道多波段的激光外差光谱同时测量,拓展激光外差光谱仪在大气探测领域的应用。     

超声换能器对线性调频信号脉冲压缩性能的影响分析*

该文研究了不同编码带宽下超声换能器对编码信号脉冲压缩后信噪比与轴向分辨率的影响关系。将脉冲压缩后的时域峰值转化为频域积分的形式，得到考虑换能器影响的脉冲压缩信噪比公式。以线性调频信号为例，仿真与实验结果表明，编码激励相对于方波激励的信噪比增益随编码带宽的减小而增大，因为受换能器带宽限制，编码激励的轴向分辨率随编码带宽的增大先减小后趋于稳定。该研究为编码激励方法更有效地应用于超声检测的背景中提供了参考。     

基于EMCCD的AOTF成像光谱仪噪声分析与信噪比研究

声光可调谐滤波器(AOTF)成像光谱仪是一种新型高光谱成像系统。针对系统存在的波段间辐射灵敏度差异较大,低照度条件下部分波段信噪较低的问题,引入了电子倍增CCD(EMCCD)作为感光器件。对该原理仪器在普通和电子倍增两种工作模式下的噪声进行了分析,推导了信噪比模型,并利用研制的原理样机进行了实验验证。在此基础上,提出了电子倍增模式下系统动态范围的评价方法,给出了以入瞳光谱辐亮度为判据的工作模式选择方法。结果表明,信噪比模型与实测结果吻合,电子倍增模式的合理选择有效提高了低照度条件下的信噪比,改善波段间辐射灵敏度的非一致性。     

输入信号和噪声对单模激光随机共振的影响

采用色抽运噪声和实虚部间关联的量子噪声驱动的单模激光损失模型,运用线性化近似方法计算了周期性信号加性输入时激光系统的输出光强信噪比,发现用信噪比与量子噪声实虚部间关联系数的关系曲线描述的随机共振现象.在抽运噪声自关联为短时关联情况下,当信号振幅增大和频率增快、抽运噪声色关联时间增大时,系统的随机共振加强;而噪声强度的增加会削弱系统的随机共振.在抽运噪声自关联为长时关联情况下,当信号振幅增大和量子噪声强度减弱时,系统的随机共振加强;而信号频率、抽运噪声强度、抽运噪声色关联时间的变化对系统随机共振的影响很小.     

不同紫外波段激发生物气溶胶荧光雷达系统性能分析

基于激光诱导生物荧光技术,分别采用紫外355nm和266nm激光作为激发光源,构建生物气溶胶荧光雷达监测系统模型.综合考虑不同激发波段,臭氧吸收以及太阳背景光等因素对激光雷达荧光探测效果的影响,对系统性能进行数值仿真分析.仿真结果表明,在四倍频266nm紫外波段的激光激发下,系统受地表臭氧的影响,白天的有效探测距离非常有限;在系统信噪比为10(SNR=10),臭氧浓度为50μg/L时,最大探测距离仅为300m;而夜间情况下,太阳背景光影响减弱,探测距离约为450m.三倍频355nm激发时,臭氧对系统的探测性能影响较小,夜间探测距离可达750m;白天太阳背景光对355nm的系统影响较大,在相同0.5mrad接收视场角下,其有效探测距离约为330m.为减少白天背景光的影响,将望远镜接收视场角压缩到0.3mrad,同时选用50nm带宽的滤光片,此时系统的探测距离为480m.由于355nm波段的激发荧光受白天太阳背景光的影响较大,在进行夜间探测时才可获得较好的效果;而266nm的激发波段可以很好的抑制背景光影响,能够实现对生物气溶胶的白天有效探测.     

样品温度对纳秒激光诱导土壤等离子体辐射特性的影响

利用脉宽8 ns,波长为532 nm的Nd:YAG单脉冲纳秒激光器,在一个标准大气压下入射到土壤中(样品土壤来自蚌埠学院校园),改变样品温度,获得了不同样品温度下激光诱导击穿光谱. 通过分析光谱,得到土壤中不同特征谱线的强度和信噪比. 分别利用Boltzmann斜线法和Stark展宽法计算并分析了等离子体电子温度和电子密度随样品温度的演化规律;同时讨论了提高样品温度和激光诱导土壤等离子体辐射增强的原因. 实验结果表明,随着样品温度的升高,等离子体的谱线强度、信噪比、电子温度和电子密度会逐渐增强,并且在温度为100 °C时达到最大.     

Stochastic resonance in time-delayed exponential monostable system driven by weak periodic signals

Based on the exponential monostable potential, we study an exponential monostable system with time-delayed feedback driven by weak periodic signals and additive Gaussian white noises. The small delay approximation is used to deduce the steady-state probability distribution and the effective potential function is derived. The system parameters l and b, time delay τ, feedback strength β can change the shapes of the potential function. The mean first-passage time (MFPT) is calculated, which plays an extremely important role in the research of particles escape. And the signal-to-noise ratio (SNR) of the system can be obtained by using the adiabatic approximation theory. The phenomenon of stochastic resonance is investigated under different system parameters and time-delayed feedback. The amplitude of SNR can be changed by adjusting the system parameters. When the feedback strength β is positive (or negative), the time delay τ can promote (or suppress) the stochastic resonance phenomenon. The SNR versus the noise intensity D presents the stochastic resonance phenomenon. In addition, the SNR increases non-monotonically with the increasing feedback strength β and the parameter b. Also, the analysis and numerical simulation results of SNR are in good agreement with the formula simulation.     

Delay induced steady-state transition and stochastic resonance for an ecological vegetation growth system subjected to multiplicative and additive noises

In this paper, our aim is to investigate the steady state properties and stochastic resonance (SR) phenomenon for an ecological vegetation growth system with time delay induced by the multiplicative and additive noises. Numerical results show that the SR phenomenon caused by time delay, different noise terms and a weak periodic signal occurs in the vegetation growth model under different values of system parameters. With regard to the stationary state properties of the vegetation system, the results indicate that the terms of different noises and time delay can all accelerate the shift from the substantial state to the barren one of the ecological system, restrain the development of the vegetation system and weaken the stability of the ecological system. On the other hand, the additive noise strength always enhances the SNR and the SR phenomenon, while the intensity of multiplicative noise often reduces the effect of the SR. In particular, time delay can play different roles in exciting the SR phenomenon in different cases.     

Signal-to-noise ratio of a dynamical saturating system: Switching from stochastic resonator to signal processor

We consider an isolated dynamical saturating system for processing a noisy sinusoidal signal, and evaluate its performance with the measure of the signal-to-noise ratio. The considered system is linear for small inputs, but exhibits saturation in its response for large inputs. This nonlinearity displays the nonlinear phenomenon of stochastic resonance for a large biased sinusoid in appropriate system parameter regions. Without the stochastic resonance phenomenon, this dynamical saturating system can achieve a signal-to-noise ratio gain exceeding unity for a noisy unbiased sinusoid. These numerical results manifest the nonlinearities and the signal-processing ability of this system acting as a stochastic resonator or a signal processor.