4.4 μm中红外激光外差光谱探测技术研究

在国内首次报道以窄线宽的4.4 μm外腔量子级联激光(QCL)作为本地振荡光源,黑体作为辐射光源的激光外差光谱实验装置的建立和测量工作。激光外差光谱是一种高灵敏度的光谱探测技术,并可以用于发展一种小型的光谱探测系统,进行地基或星载的地球大气或天文观测。介绍了激光外差的基本理论、装置的建立和实验测量工作。此激光外差光谱实验装置采用4.4 μm外腔量子级联激光,出光功率高达180 mW,在4.38～4.52 μm间连续可调,具有很宽的光谱调谐范围,能实现CO₂,CO和N₂O等大气重要分子的同时测量。通过开展不同压力下CO₂气体的激光外差光谱测量,对激光外差光谱实验装置的性能进行了评估。目前该激光外差系统的信噪比达到86,低于散粒噪声极限条件的理论计算值287,系统的光谱分辨率约为0.007 8 cm-1,能满足较窄线宽条件下的高分辨率激光外差光谱的测量。分析结果表明,中红外激光外差光谱系统具有很高的瞬态信噪比及光谱分辨率,在高精度测量大气温室气体的柱浓度和温室气体垂直廓线分布方面具有广阔的应用前景。     

回转件弯扭测量系统及其不确定度分析

为了准确诊测轴系振动特性,基于激光多普勒的频移原理,提出一种能同时测量旋转轴系弯曲振动、扭转振动和轴系旋转速度的方法。设计了能分离弯扭振动的多普勒外差干涉光路,结合光学差拍及参考光技术,构建了测量系统数学模型。对影响激光多普勒弯扭振动测量的主要因素进行了分析,讨论了各参量对测量结果的影响,并给出了相应的测量不确定度。在置信水平为95%时,瞬时转速的不确定为0.079 r/min,弯曲振动速度分量的不确定为0.001 4 mm/s,其精度能满足旋转状态下轴系振动的综合测量要求。     

多光束激光外差高精度测量玻璃厚度的方法

利用基于激光外差测量技术和激光多普勒技术的非接触式多光束激光外差测量方法,得到了光电探测器输出电流的谐波表达式.讨论了该测量方法用于玻璃厚度超精密测量的可行性及理论依据,并利用Matlab软件对不同情况进行了仿真实验.结果表明：该方法在不同入射角时测量平板玻璃厚度最大的误差为0.3%,明显比其他测量方法精度高. 关键词： 外差探测 多光束激光外差 激光多普勒技术 非接触式测量     

激光外差干涉快速超精密测量模型研究

为了精确地描述激光外差干涉在快速超精密测量中的位移测量．建立了激光外差干涉快速超精密测量模型。传统外差干涉测量模型采用舍去高阶误差的方法,便于分析与快速计算,但存快速纳米精度测量中,高阶误差已经影响到测量精度．根据多普勒频移公式,通过分析激光外差干涉的测量原理．在已有的激光干涉测量模型上增加了u^2/c的积分项．相当于将传统测量模型进行了高阶误差补偿。通过理论分析可知,当最高测量速度为1m/s,运行位移为3m时,该测量模型能够减小约18nm的测量误差,解决了传统测量模型存存的残余误差累计问题,从而为激光外差干涉在快速超精密测量领域的应用提供了一种理论依据。     

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.     

10～500 kHz水听器的激光外差干涉法原级校准

阐述了水听器在10~500 kHz频率范围的激光外差干涉法原级校准的原理和测量装置。利用激光外差干涉系统非接触测量水中反光薄膜在声波调制下引起的多普勒频移,经过零点信号解调算法得到水质点振速,准确复现水下声压量值,之后将水听器置于水中测量点完成对其自由场电压灵敏度的原级校准。对各误差因素进行了分析,详细给出了系统测量不确定度的评定方法,在所测频段的不确定度为0.6 dB (k=2)。将激光外差干涉法水听器校准结果与自由场互易法结果进行了对比,在整个测量频率范围内结果等效,且前者不确定度更小。为建立新的水声计量基准和实现水声量值的扁平化传递奠定了基础。     

Surface plasmon quantum cascade lasers as terahertz local oscillators

We characterize a heterodyne receiver based on a surface-plasmon waveguide quantum cascade laser (QCL) emitting at 2.84 THz as a local oscillator, and an NbN hot electron bolometer as a mixer. We find that the envelope of the far-field pattern of the QCL is diffraction-limited and superimposed onto interference fringes, which are similar to those found in narrow double-metal waveguide QCLs. Compared to the latter, a more directional beam allows for better coupling of the radiation power to the mixer. We obtain a receiver noise temperature of 1050 K when the mixer is at 2 K, which, to our knowledge, is the highest sensitivity reported at frequencies beyond 2.5 THz.     

Measurement of properties of a smooth-walled spline-profile feed horn around 840 GHz

For astrophysical applications at submillimeter wavelengths, efficient coupling of the feed horn pattern in the focal plane to the radiation pattern of the telescope is paramount. To date, at 840 GHz with high band width, corrugated feed horns have been used with very good results. The goal of developing instruments for KOSMA above 1 THz where state-of-the-art heterodyne systems are still governed by receiver noise rather than by radiative background, requires fabrication of corrugated structures of the order of λ/3 or less. This is non-trivial, and here we present the performance of a smooth-walled spline-profile feed horn, which is considerably easier to manufacture compared to a corrugated feed horn. The performance is assessed via radiation patterns in H, E and 45° D-Plane in co- and cross-polarization.     

A microprocessor stabilized submillimeter laser system

A mechanically and thermally stable submillimeter laser has been constructed and has an amplitude and frequency stability suitable for use as a local oscillator in a heterodyne receiver for astronomical investigations. The microprocessor stabilization reduces the receiver noise for different kinds of Schottky barrier diodes used. A short-time amplitude stability (10 minutes) of about ±0.5% and a long time amplitude stability (1 hour) of about ±1.2% have been recorded. Integration times up to one hour with a spectral resolution better than 200 KHz are possible with such a stabilized system.     

A Novel Laser Heterodyne Technique for the Measurement of Electron Density in Gas Discharges: Application to a TEA CO2 Laser Plasma

We describe a novel intracavity laser heterodyne technique for the measurement of electron density in gas discharges. The method produces time-resolved data from a single pulse and we have used it to monitor electron densities in a TEA laser discharge with a typical CO2 laser gas mixture. Good agreement was found between the measured electron density and its value calculated from current density and drift velocity. A maximum value of 5 × 1013 cm-3 was observed.     

Common-path heterodyne laser-induced thermal acoustics for seedless laser velocimetry

We demonstrate the use of a novel technique for the detection of heterodyne laser-induced thermal acoustic signals that allows the construction of a highly stable seedless laser velocimeter. A common-path configuration is combined with quadrature detection to provide flow direction, to greatly improve robustness to misalignment and vibration, and to give reliable velocity measurement at low-flow velocities. Comparison with Pitot tube measurements in the free stream of a wind tunnel shows root-mean-square errors of 0.67 m/s over the 0-55-m/s velocity range.     

Effect of bevelling on the thickness vibration of high-frequency Pb(ZrTi)O3 ceramic transducers

The effects of bevelling on the thickness vibration of Pb(ZrTi)O₃ piezoelectric-ceramic disc transducers for generating high-frequency ultrasonic power were studied by measuring electrical characteristics and displacement distributions. A laser heterodyne system was employed for the contact-free measurement of vibration. It was shown for a wide range of diameter-to-thickness ratios that bevelling circular edges was effective in reducing the coupled radial vibrations and enhancing the uniform thickness vibration. The vibration characteristics required for generating high-frequency ultrasound were substantially improved by slight bevelling. These results were accounted for by energy trapping of thickness vibration and by the consequent decrease in the equivalent mass of the transducers.     

Frequency and phase-lock control of a 3 THz quantum cascade laser

We have locked the frequency of a 3 THz quantum cascade laser (QCL) to that of a far-infrared gas laser with a tunable microwave offset frequency. The locked QCL line shape is essentially Gaussian, with linewidths of 65 and 141 kHz at the -3 and -10 dB levels, respectively. The lock condition can be maintained indefinitely, without requiring temperature or bias current regulation of the QCL other than that provided by the lock error signal. The result demonstrates that a terahertz QCL can be frequency controlled with 1-part-in-10(8) accuracy, which is a factor of 100 better than that needed for a local oscillator in a heterodyne receiver for atmospheric and astronomic spectroscopy.     

Laser Doppler velocimeter for remote measurement of polluted water and aerosols discharges

A continuous wave Doppler velocimeter has been developed for remote estimation of the volume of polluted water and gaseous discharges using an actively stabilized ring dye laser. Doppler shifted backscattered signals induced by fluids in motion using heterodyne detection technique has been applied for measurement of Doppler shifts. Polluted water and gaseous discharge flow rates have been measured remotely for simulation purposes. The system is capable of measuring Doppler shifts as low as 0.25 MHz corresponding to 0.1 m/s velocities of the fluids.     

A scheme for CO2 laser scattering to measure the ion temperature in a Tokamak plasma

A simple optical scheme is proposed for scattering of pulsed high-power CO₂ laser radiation from a plasma, and to achieve heterodyne detection to receive the very low scattered radiation intensity and resolve the narrow spectrum. Ion temperature measurement with microsecond time resolution in fusion oriented Tokamak plasmas seems feasible.     

用塞曼双频激光器作光源的一种新颖长扫描表面的仪器设计

本文报道一套光学结构用于同步辐射反射镜表面轮廓高精度大面积的测量。采用了光学外差测量与接收技术,以伺服聚焦双折射透镜组分离“o”光与“e”光的聚焦区域,并再共轴组合比较其相位差,从而获得表面起伏的误差值。线性运动的测量工作台使镜面可测量范围从数微米到一米,而最小起伏量及曲率可达很高的测量精度。     

多普勒振镜正弦调制多光束激光外差测量玻璃厚度的方法

本文提出了一种多普勒振镜正弦调制多光束激光外差测量玻璃厚度的新方法. 基于激光外差技术和多普勒效应, 通过做简谐振动的多普勒振镜对不同时刻入射光的频率进行正弦调制, 把待测厚度信息加载到外差信号的频率差中, 通过快速傅里叶变换对外差信号解调后可以同时得到多个待测玻璃厚度值, 经加权平均处理可以提高待测厚度的测量精度. 利用这种新方法, 通过MATLAB仿真测量了不同玻璃厚度值, 结果表明：该测量结果的最大相对误差小于0.008%.     

Time-resolved electron transport studies on InGaAs/GaAs-QWIPs

Due to the short internal response time, quantum-well infrared photodetectors (QWIPs) are interesting for high-speed applications such as heterodyne spectroscopy or laser pulse monitoring. We studied the photocurrent transients of InGaAs/GaAs-QWIPs after irradiation with infrared laser pulses of 250 fs duration. The excitation wavelength of about 9 μm matches the peak wavelength of the QWIP structure. The photocurrent transient consists of two different dynamical components, representing the fast photoionization in the quantum-wells and the slow injection current that compensates the remaining space charge. The investigations of the different components as a function of temperature and bias voltage were performed on a nanosecond time-scale. The experimental separation of the two photocurrent contributions allows us to determine the photoconductive gain. The Fourier transform of the photocurrent transient was compared with other experimental methods including heterodyne detection and microwave rectification. The quantitative agreement between these different measurement techniques is excellent.     

Heterodyne detection of CO2 laser radiation scattered from a plasma arc jet

Observations have been made of CO₂ laser radiation scattered incoherently from an argon arc jet plasma. The heterodyne detection technique employed is discussed.     

Heterodyne spectroscopy for astrophysical applications using tunable laser sidebands at 10 microns

A heterodyne system was assembled from a microwave tunable sideband laser, a blackbody source, a gas cell, a CdHgTe photomixer and a filterbank. A measurement of line profiles of CH₃F is presented and compared with absorption measurements. System sensitivity is sufficient for high resolution measurement of fine structure lines from astronomical objects, though falling short by a factor of 12, when compared with the most sensitive astronomical heterodyne spectrometers. The usefulness of such a system for astrophysical applications is discussed.