OTDR fiber-optical chemical sensor system for detection and location of hydrocarbon leakage

A distributed sensing system for apolar hydrocarbons is presented which is built from a polymer-clad silica fiber adapted to an optical time domain reflectometer (OTDR) set-up. OTDR measurements allow locating and detecting chemicals by measuring the time delay between short light pulses entering the fiber and discrete changes in the backscatter signals that are caused by local extraction of hydrocarbons into the fiber cladding. The light guiding properties of the fiber are affected by interaction of the extracted chemicals with the evanescent wave light field extending into the fiber cladding. Distributed sensing of pure liquid hydrocarbons (HC) and aqueous HC solutions with a commercially available mini-OTDR adapted to sensing fibers of up to 1km length could be demonstrated. A pulsed laser diode emitting at the 850 nm telecommunication wavelength was applied in the mini-OTDR to locate the HCs by analyzing the step drop (light loss) in the backscatter signal, which is induced by local refractive index (RI) increase in the silicone cladding due to the extracted HC. The prototype instrument can be applied for monitoring hydrocarbon leakage in large technical installations, such as tanks, chemical pipelines or chemical waste disposal containments.     

Signal processing and calibration of continuous-wave focused CO(2) Doppler lidars for atmospheric backscatter measurement

Two continuous-wave (CW) focused CO(2) Doppler lidars (9.1 and 10.6 μm) were developed for airborne in situ aerosol backscatter measurements. The complex path of reliably calibrating these systems, with different signal processors, for accurate derivation of atmospheric backscatter coefficients is documented. Lidar calibration for absolute backscatter measurement for both lidars is based on range response over the lidar sample volume, not solely at focus. Both lidars were calibrated with a new technique using well-characterized aerosols as radiometric standard targets and related to conventional hard-target calibration. A digital signal processor (DSP), a surface acoustic wave spectrum analyzer, and manually tuned spectrum analyzer signal analyzers were used. The DSP signals were analyzed with an innovative method of correcting for systematic noise fluctuation; the noise statistics exhibit the chi-square distribution predicted by theory. System parametric studies and detailed calibration improved the accuracy of conversion from the measured signal-to-noise ratio to absolute backscatter. The minimum backscatter sensitivity is ~3 × 10(-12) m(-1) sr(-1) at 9.1 μm and ~9 × 10(-12) m(-1) sr(-1) at 10.6 μm. Sample measurements are shown for a flight over the remote Pacific Ocean in 1990 as part of the NASA Global Backscatter Experiment (GLOBE) survey missions, the first time to our knowledge that 9.1-10.6-μm lidar intercomparisons were made. Measurements at 9.1 μm, a potential wavelength for space-based lidar remote-sensing applications, are to our knowledge the first based on the rare isotope (12)C (18)O(2) gas.     

Optical time-domain reflectometry of bent plastic optical fibers

Optical time-domain reflectometry (OTDR) signals of step-index plastic optical fibers (POF's) and graded-index POF's were measured with a laser diode and an avalanche photodiode. When bent step-index and graded-index POF's were used, the OTDR signal behavior differed. The OTDR signal of the bent graded-index POF's had a step that corresponds to a curvature loss, but the step-index POF's had a spike signal at a bend, which indicated the occurrence of backscattering. The peak intensity was proportional to the square of the curvature. The refractive-index variation of the bent step-index POF's was measured, and the dependence of the peak intensity on the curvature was shown to agree with that predicted by the scattering from the refractive-index perturbation.     

Spectral recovery by analytic continuation in crossing-based spectrum analysis

By the use of analytic continuation, the correct spectrum of an undersampled analog input signal f(a) (t) of a true bandwidth B is recovered from an aliased Fourier spectrum that is computed directly from a data set consisting of sinusoid-crossing locations {t(i)}, where the signal f(a) (t) intersects with a reference sinusoid r(t) with a frequency of W 相似文献   

宽带声学多普勒流速剖面仪的中频正交采样算法

宽带声学多普勒流速剖面仪(Broadband Acoustic Doppler Current Profile,BBADCP)发射由伪随机编码调制的正弦信号,信号带宽在频域上产生展宽,成为带通信号。宽带回波信号的处理多采用正交解调的方法,但此方法对采样率的要求较高,硬件较难实现。利用数字中频正交采样法来实现,大大降低了采样频率,并且针对BBADCP解模糊速度的要求,进行了宽带回波信号的仿真试验,结果证明插值的数字中频正交采样法适合于BBADCP的信号处理。     

Signal spectrum analysis and period estimation by using delayedsignal sampling

The paper describes a signal power spectrum analyzer and a signal period estimator whose bandwidth is not limited by the mean sampling time. The procedure relies on the evaluation of the input signal autocorrelation function in different delayed time instants, located at either equispaced or random time instants. To do this, a recursive random sampling process in the time domain was used in order to avoid any bandwidth limitation due to the sampling strategy in the evaluation of each autocorrelation function. The signal power spectrum as well as its period, provided that an approximate value of the fundamental frequency is known, can finally be evaluated. Some theoretical background and experimental work are reported in the paper for validating the performance of the method     

Performance degradation of a Michelson interferometer due to random sampling errors

The performance of a standard Michelson interferometer is degraded by disturbances that cause the interferogram signal to be sampled at nonconstant time intervals. A formula that shows how the power spectrum of the random disturbances interacts with the signal to contaminate different regions of the measured spectrum is derived for the spectral noise. The sampling noise does not look conventionally noiselike because it is correlated over large regions of the measured spectrum, and adjustment of the unbalanced background interferogram to match the size of the balanced background interferogram minimizes the sampling-noise amplitude.     

Particle backscatter and extinction profiling with the spaceborne high-spectral-resolution Doppler lidar ALADIN: methodology and simulations

The European Space Agency will launch the Atmospheric Laser Doppler Instrument (ALADIN) for global wind profile observations in the near future. The potential of ALADIN to measure the optical properties of aerosol and cirrus, as well, is investigated based on simulations. A comprehensive data analysis scheme is developed that includes (a) the correction of Doppler-shifted particle backscatter interference in the molecular backscatter channels (cross-talk effect), (b) a procedure that allows us to check the quality of the cross-talk correction, and (c) the procedures for the independent retrieval of profiles of the volume extinction and backscatter coefficients of particles considering the height-dependent ALADIN signal resolution. The error analysis shows that the particle backscatter and extinction coefficients, and the corresponding extinction-to-backscatter ratio (lidar ratio), can be obtained with an overall (systematic+statistical) error of 10%-15%, 15%-30%, and 20%-35%, respectively, in tropospheric aerosol and dust layers with extinction values from 50 to 200 Mm(-1); 700-shot averaging (50 km horizontal resolution) is required. Vertical signal resolution is 500 m in the lower troposphere and 1000 m in the free troposphere. In cirrus characterized by extinction coefficients of 200 Mm(-1) and an optical depth of >0.2, backscatter coefficients, optical depth, and column lidar ratios can be obtained with 25%-35% relative uncertainty and a horizontal resolution of 10 km (140 shots). In the stratosphere, only the backscatter coefficient of aerosol layers and polar stratospheric clouds can be retrieved with an acceptable uncertainty of 15%-30%. Vertical resolution is 2000 m.     

一种半导体激光云高仪时序控制和信号处理方案

半导体激光云高仪APD探测器获取的大气气溶胶后向散射信号信噪比较低,难以直接用于反演云高、垂直能见度等大气参数.针对后向散射信号的这种特点,提出了一种半导体激光云高仪时序控制和信号处理实现方案,在完成后向散射信号的高速采集、分段累积平均的同时实现整个系统发射、接收、维护、数据存储和上传等复杂时序控制.经对比实验验证,该...     

ZFFT与Chirp-Z变换细化选带的频谱分析对比

在细化选带频谱分析中，复调制细化方法（ZFFT）和线性调频Z变换方法（Chirp—Z变换）是常用的两种方法。通过理论分析和仿真计算，对两者在算法、特点和误差方面进行对比分析表明：对于单频率和谱线干涉不严重的多频率谐波成分，使用FFT后进行校正，或者使用CZT细化分析，均能得到高精度的频率、幅值和相位，不必使用ZFFT；对于发生严重干涉现象的密集多频率谐波成分，ZFFT通过增大细化倍数后重采样，把干涉的各频率成分分离后进行校正可获得高精度的信号参数，但CZT只是把细化分析频带局部放大，无法消除干涉影响，提高频率分辨率也无法分离出信号的真实频率成分。通过增大采样点数，减少干涉产生的误差，CZT可以获得较高精度的信号参数，但却大大增加了运算时间。     

Combined Raman-elastic backscatter lidar method for the measurement of backscatter ratios

A variation of the conventional combined Raman-elastic backscatter lidar method, the 1-2-3 lidar method, is described and analyzed. This method adds a second transmitter wavelength to the conventional combined Raman-elastic backscatter lidar. This transmitter wavelength is identical to that of the Raman receiver. One can generate the transmitted beam at this wavelength by Raman shifting the laser radiation in molecular nitrogen or oxygen. Measuring a second elastic lidar signal at the Raman-shifted wavelength makes it possible to eliminate differential transmission effects that can cause systematic errors in conventional combined Raman-elastic backscatter lidar.     

Fiber Bragg grating location by a side-scatter technique based on cladding-mode coupling

At wavelengths slightly shorter than the normal resonance for a fiber Bragg grating, radiation can be coupled into cladding and radiation modes that become visible external to the fiber. We describe experimental measurements on a fiber Bragg grating under broadband illumination, showing the axial variation and angular distribution of side-scattered radiation. The scattering signal can locate the grating position subject to an offset in the backscatter direction. The spectrum of scattered radiation is consistent with a published theoretical model of mode coupling by a grating in a finite cladding fiber.     

基于稳健全息阶比谱提取微弱故障特征

提出了一种稳健全息阶比谱方法,用于提取变速器变速过程的微弱故障特征。首先分析了转速波动对传统阶比方法分析精度的影响,构造了基于转速调整的改进阶比方法,对水平与垂直方向加速过程信号的改进阶比谱进行全息分析,构成稳健全息阶比谱,并与单测点改进阶比谱结果对比。试验表明,改进阶比谱能有效抑制转速波动对等角度采样的影响,提高了阶比谱分析精度,有效增强了全息阶比谱的稳健性;相对于任一方向单测点阶比谱,稳健全息阶比谱既能分析变速过程信号,又能融合多点信息,其量化参数能更准确评价诊断对象的技术状态,证明稳健全息阶比谱是一种全面、可靠提取变速过程微弱故障特征的有效方法。     

另一类幅度非均匀取样信号的数字频谱及补偿算法研究

研究了高速并行数字化系统中由ADC增益的不一致性造成的幅度非均匀取样信号的特点,推导出了这类幅度非均匀采样信号的数字频谱表达式,详细分析了输入正弦信号时的数字频谱,得到了系统的信噪比表达式。提出了一种利用IFFT运算对输出信号的频谱进行重构的补偿方法,计算机仿真验证了该方法的有效性。     

Perfect reconstruction of digital spectrum from nonuniformlysampled signals

In this paper, we consider the problem of reconstructing the digital spectrum from a set of nonuniformly spaced time samples of a signal. Specifically, we deal with the situation where the timing offsets of each sampling instance are known and have a periodic structure. Examples include the random equivalent time sampling system of a digital scope, and an ultra high speed waveform digitizing system with interleaved A/D converters. An algorithm which allows one to reconstruct the digital spectrum perfectly is derived, and a numerical example is presented. The algorithm is noniterative and precise, and should prove to be useful in many applications     

FFT幅相联合的快速高精度频率估计方法

快速傅里叶变换（Fast Fourier Transform,FFT）常用于信号频率估计,采用填零的方法可降低幅度谱频率搜索间隔的量化误差,但是会使频率估计的计算量成倍增加。本文提出了一种FFT幅相联合的快速高精度频率估计算法,首先利用信号采样的频谱序列和尾首样本差确定幅度谱及峰值位置,然后由频谱序列在幅度谱峰值位置和信号采样的尾首样本差来确定频率搜索间隔的量化误差校正值。因此,所提方法同时利用了幅度谱峰值的位置信息与相位信息。分析结果表明,与仅基于幅度谱搜索的FFT算法相比,所提方法的计算复杂度更低,且定位精度更高。     

机器人六维腕力传感器标定方法和标定装置的研究

通过对机器人六维腕力传感器系统的各种标定方法的分析讨论,说明了应采用阶跃响应的方法来对它的动态性能进行标定。从矩形脉冲频谱对系统传递函数影响的角度,说明了为减少矩形脉冲频谱对传递函数标定曲线的干扰,应使阶跃激励开始时刻前的采样点数为零。新设计的实验方法和实验装置便于测出标定中使用的激励信号,通过数据采样系统的“软触发”功能,准确抓取到阶跃开始时刻,减少了干扰。同时,在该实验台上加载所使用的定滑轮数量较少,也有效地减小了标定误差。     

Development of a parallel demodulation system used for extrinsic Fabry-Perot interferometer and fiber Bragg grating sensors

A parallel demodulation system for extrinsic Fabry-Perot interferometer (EFPI) and fiber Bragg grating (FBG) sensors is presented, which is based on a Michelson interferometer and combines the methods of low-coherence interference and a Fourier-transform spectrum. The parallel demodulation theory is modeled with Fourier-transform spectrum technology, and a signal separation method with an EFPI and FBG is proposed. The design of an optical path difference scanning and sampling method without a reference light is described. Experiments show that the parallel demodulation system has good spectrum demodulation and low-coherence interference demodulation performance. It can realize simultaneous strain and temperature measurements while keeping the whole system configuration less complex.