Spectroscopic Measurement of Air Temperature

Optical dimensional measurements have to be corrected for the refractive index of air. The refractive index is conventionally calculated from parameters of ambient air using either Edlén or Ciddor equations or their modified versions. However, these equations require an accurate knowledge of ambient conditions and especially the temperature of air. For example, to reach an uncertainty of 10^?7 in dimensions, the air temperature has to be known at ~100 mK level. This does not necessarily cause problems in a stable laboratory environment. However, if measurements are done outdoors or in an industrial environment, variations in temperature can be very rapid and local temperature gradients can cause significant error if not taken into account. Moreover, if the required distance is long, the temperature over the whole measurement path can be impractical or impossible to determine at sufficient temporal or spatial resolution by conventional temperature measurement techniques. The developed method based on molecular spectroscopy of oxygen allows both lateral spatial and temporal overlap of the temperature measurement with the actual distance measurement. Temperature measurement using spectroscopy is based on a line intensity ratio measurement of two oxygen absorption lines, previously applied for measurements of high temperatures in flames. The oxygen absorption band at 762 nm is a convenient choice for two-line thermometry since the line strengths are practical for short- and long-distance measurements and suitable distributed feedback lasers are commercially available. Measurements done on a 67 m path at ambient conditions demonstrate that the RMS noise of 22mK, or 7.5 × 10^?5, near 293 K using 60 s measurement time can be achieved, which is to our knowledge the best reported resolution.     

基于Edlén公式空气折射率测量系统的校准研究

为了检验对激光干涉仪测量精度有很大影响的空气折射率测量系统的测试性能,提出一种可用于基于Edlén公式的空气折射率测量系统的校准方法,并且设计了专门的校准装置,该装置主要由温度、气压以及湿度测量系统组成。实验证明：取包含因子k=2时,温度在(5~40)℃区间,测量不确定度U优于0.02 ℃;气压在(66~105)kPa区间,U=18 Pa;湿度在(10~90)%RH区间,U=2.0%RH,等效于整个系统对应的空气折射率测量的相对不确定度优于1×10^-7     

High-accuracy spectrometer for measurement of regular spectral transmittance

A high-accuracy spectrometer has been developed for measuring regular spectral transmittance. The spectrometer is an automated, single-beam instrument that is based on a grating monochromator, reflecting optics, and an averaging sphere detector unit with a silicon photodiode. The uncertainties related to wavelength calibration, detector nonlinearity, system instability, beam displacement, polarization, stray light, interreflections, and beam uniformity are determined for the visible spectral range from 380 to 780 nm. A total uncertainty of 3 × 10(-4) (1σ) is estimated for transmittance measurements of homogeneous neutral-density filters. The uncertainty of the wavelength scale is 0.06 nm. As a specific application, calibration of V(λ)-correction filters is studied. To verify the accuracy of the transmittance measurements, a comparison of the measured and predicted transmittances of a sample of high-purity fused silica is made, revealing agreement at the 5 × 10(-4) level.     

Active suppression of air refractive index fluctuation using a Fabry-Perot cavity and a piezoelectric volume actuator

Air refractive index fluctuation (Δn(air)) is one of the largest uncertainty sources in precision interferometry systems that require a resolution of nanometer order or less. We introduce a method for the active suppression of Δn(air) inside a normal air-environment chamber using a Fabry-Perot cavity and a piezoelectric volume actuator. The temporal air refractive index (n(air)) at a local point is maintained constant with an expanded uncertainty of ~4.2 × 10(-9) (k = 2), a sufficiently low uncertainty for precise measurements unaffected by Δn(air) to be made inside a chamber.     

The Influence of the Atmosphere on the Wavelength of the He-Ne Laser and the Solution of Corrections of the Laser Interferometer

A simple approximation of the wavelength of single frequency He-Ne laser as a function of temperature, pressure and relative humidity is presented. This approximation is suitable for the simple application in the portable laser interferometer. Comparison with the Owens and Edlén formulae is also presented and it is shown that the linear correction limits its accuracy.     

Thermal lens micro optical systems

This paper describes two types of miniaturized thermal lens optical systems that use optical fibers, SELFOC microlenses and light sources. The first system consists of a compact diode pumped solid-state laser (532 nm) as an excitation light source, a laser diode (635 nm) as a probe light source, an acoustoptic modulator as an excitation light modulator, fiber-based and conventional optics, and a detection system that combines a pinhole, an interference filter, and a photodiode. The second system consists of two laser diodes as the excitation (658 nm) and probe (780 nm) light sources, fiber-based optics, and the same detection system as the first one. The performance of the two systems was evaluated by the limit of detection (LOD) using standard solutions of sunset yellow (SY) and nickel(II) phthalocyaninetetrasulfonic acid tetrasodium salt (NiP). The LODs of the first system for SY and second system for NiP were calculated to be 3.7 x 10(-8) (1.7 x 10(-6) AU) and 7.7 x 10(-9) M (3.4 x10(-6) AU), respectively. These results were consistent with the expected values obtained from photothermal parameters.     

Near-infrared excited Raman optical activity

Measurements of near-infrared scattered circular polarization Raman optical activity (SCP-ROA) are presented using laser excitation at 780 nm for samples of S-(-)-alpha-pinene and L-alanyl-L-alanine. These are the first measurements of ROA outside the blue-to-green visible region between 488 and 532 nm. Comparison of Raman and ROA intensities measured with excitation at 532 and 780 nm demonstrate that the expected frequency to the fourth-power dependence for Raman scattering and the corresponding fifth-power dependence for ROA are observed. It can be concluded that, to within this frequency dependence, the same level of efficiency of Raman and ROA measurements using commercial instrumentation with 532 nm excitation is maintained with the change to near-infrared excitation at 780 nm.     

Improved frequency stability of an external cavity diode laser by eliminating temperature and pressure effects

The construction of a passively stabilized external cavity diode laser operating at 780 nm is reported. The sensitivity of laser frequency to changes in air pressure was studied and subsequently eliminated. The relative frequency stability obtained was 4 × 10(-9) for an integration time of 4000 s.     

光干涉法高精度材料线膨胀系数测量

中国计量科学研究院研制的高精度材料线膨胀系数测量装置,满足温度范围为5～40 ℃、被测件长度在20～1 000 mm之间的线膨胀系数测量。采用激光干涉法测量被测件长度变化量,用高精度温度传感器测量温度值。设计了热平衡式干涉镜,利用空气折射率修正和零位误差补偿技术,保证在5～40 ℃变温范围内激光干涉仪的测量精度。以500 mm标准量块作为测量对象,线膨胀系数测量结果与德国物理技术研究院(PTB)测量结果的相对偏差为0.2%。材料线膨胀系数测量不确定度达到3×10^-8K^-1。     

Precision ranger for measuring large mechanical components

We present a precision laser ranger system for the measurement of large manufactured components and structures. The system was developed based on a beat-wave interferometry principle. The light source of this system is a frequency-stabilized laser with a frequency stability of 1 x 10(-7) (in open air) or 10(-8) (in the laboratory). The laser operates in two longitudinal modes, and the two modes are generated in common resonator; therefore the two beams are naturally coaxial. The precision ranger system does not need a long guide or any heavy machinery. In this system an adaptive filter and a wavelet-transform program are used to improve the measurement accuracy. The system described here has a measuring range of 0-20 m and a measuring uncertainty of 30 microm/10 m.     

基于偏振激光干涉技术的波长计

提出了一种基于偏振激光干涉技术的波长计,该波长计能在微米行程上实现10-6的测量精度,位移行程的缩减有效地减小了环境温度、波长漂移、空气折射率对测量精度的影响。给出了两种未知激光源（DL100-Toptica和Nd:YAG）的波长测量值及不确定度值,同时还给出了该波长计与商用波长计的比对结果。     

多波长测量技术

给出了3种多波长测量技术的应用实例,采用多波长二极管激光器干涉仪测量表面轮廓,3个二极管激光器分别发射780nm、823nm和825nm3种近红外光波,可以得到2个近似15μm和29μm的合成波长,激光二极管的注入电流用1MHz左右的3种不同频率调制,以获得干涉仪的相位调制,这3个干涉信号用1个光电检测器和锁相放大器检测,采用同样的干涉仪,仅用2种波长进行了圆度偏差测量,论述了相移干涉测量技术,其试件的长度表示为光源波长的倍数,如果只使用1种波长,则要求确切知道整数干涉级;而使用多种波长,如532nm、633nm和780nm时,可得到几个独立的长度,此时整数衍射级不同,可以通过“精确小数法”计算得到,PTB精密干涉仪长度测量不确定度很小,其单值性范围约0.6mm,因此其近似的长度估算足以满足超精密长度测量的要求。     

Measurement of air refractive index fluctuation based on interferometry with two different reference cavity lengths

A measurement method based on interferometry with two different reference cavity lengths is presented and applied in air refractive index measurement in which the two cavity lengths and a laser wavelength are combined to generate two wavelength equivalents of cavity. Corresponding calculation equations are derived, and the optical path configuration is designed, which is inspired by the traditional synthetic wavelength method. Theoretical analyses indicate that the measurement uncertainty of the determined index of refraction is about 2.3×10^-8, which is mainly affected by the length precision of the long vacuum cavity and the ellipticity of polarization components of the dual-frequency laser, and the range of nonambiguity is 3.0×10^-5, which is decided by the length difference of the two cavities. Experiment results show that the accuracy of air refractive index measurement is better than 5.0×10^-8 when the laboratory conditions changes slowly. The merit of the presented method is that the classical refractometry can be also used without evacuation of the gas cavity during the experiment. Furthermore, the application of the traditional synthetic wavelength method may be extended by using the wavelength equivalents of cavity, any value of which can be easily acquired by changing cavity length rather than using actual wavelengths whose number is limited.     

Optical properties of randomly distributed soot: improved polarimetric and intensity scattering functions

Reference scattering curves for polarization and intensity produced by aggregates and agglomerates of ethylene and kerosene soot are obtained for scattering angles in the 10-170 degrees range. The polarization measurements were obtained with the Propriétés Optiques des Grains Astronomiques et Atmosphériques instrument for particles that levitate in microgravity during parabolic flights and on the ground by an air draught technique. The intensity measurements were obtained also on the ground with a Laboratoire de Metéorologie Physique nephelometer. The maximum polarization is of the order of 80% at a scattering angle of 80 degrees at lambda = 632.8 nm and approximately 75% at an angle of 90 degrees at lambda = 543.5 nm. The polarization increases by approximately 10% when the size of the agglomerate increases from 10 microm to a few hundred micrometers. The intensity curve exhibits a strong increase at small scattering angles. These reference curves will be used in the near future for the detection of stratospheric soot by remote-sensing measurement techniques.     

计算电容屏蔽电极位移测量的不确定度评估

计算电容是电学阻抗单位复现的基准装置,是电容、电感和交流电阻的溯源依据.它是电磁计量领域内除量子电压、量子电阻基准之外准确度最高的装置.计算电容装置实现微小电容值(0.2至0.6 pF)精确测量的关键在于对其屏蔽电极位移的高准确度测量.从计算电容屏蔽电极位移测量的基本原理出发,系统分析了屏蔽电极位移测量结果的影响因素,...     

Prism-pair interferometry by homodyne interferometers with a common light source for high-accuracy measurement of the absolute refractive index of glasses

A prism-pair interferometer comprising two homodyne interferometers with a common light source was developed for high-precision measurements of the refractive index of optical glasses with an uncertainty of the order of 10(-6). The two interferometers measure changes in the optical path length in the glass sample and in air, respectively. Uncertainties in the absolute wavelength of the common light source are cancelled out by calculating a ratio between the results from the interferometers. Uncertainties in phase measurement are suppressed by a quadrature detection system. The combined standard uncertainty of the developed system is evaluated as 1.1×10(-6).     

Testing the radiometric accuracy of Fourier transform infrared transmittance measurements

We have investigated the ordinate scale accuracy of ambient temperature transmittance measurements made with a Fourier transform infrared (FT-IR) spectrophotometer over the wavelength range of 2-10 mum. Two approaches are used: (1) measurements of Si wafers whose index of refraction are well known from 2 to 5 mum, in which case the FT-IR result is compared with calculated values; (2) comparison of FT-IR and laser transmittance measurements at 3.39 and 10.6 mum on nominally neutral-density filters that are free of etaloning effects. Various schemes are employed to estimate and reduce systematic error sources in both the FT-IR and laser measurements, and quantitative uncertainty analyses are performed.     

Femtosecond laser electronic excitation tagging for quantitative velocity imaging in air

Time-accurate velocity measurements in unseeded air are made by tagging nitrogen with a femtosecond-duration laser pulse and monitoring the displacement of the molecules with a time-delayed, fast-gated camera. Centimeter-long lines are written through the focal region of a ～1?mJ, 810?nm laser and are produced by nonlinear excitation and dissociation of nitrogen. Negligible heating is associated with this interaction. The emission arises from recombining nitrogen atoms and lasts for tens of microseconds in natural air. It falls into the 560 to 660?nm spectral region and consists of multiple spectral lines associated with first positive nitrogen transitions. The feasibility of this concept is demonstrated with lines written across a free jet, yielding instantaneous and averaged velocity profiles. The use of high-intensity femtosecond pulses for flow tagging allows the accurate determination of velocity profiles with a single laser system and camera.     

喷嘴式空气流量测量不确定度分析与评定

为了解可直接测量参数对喷嘴式空气流量测量结果准确性及可靠程度的影响,找出对测试结果影响较大的环节以进行提高测量准确度的改进,建立喷嘴式空气流量测量数学模型,分别对测试喷嘴处空气比容和测试喷嘴前后气体静压差引入的不确定度进行分析,对喷嘴式空气流量测量进行合成标准不确定度评定和扩展不确定度评定。结果表明：测试空气质量流量为0．119kg／s时,合成标准不确定度为2．396×10kg／s,扩展不确定度为1．433×10^-3kg／s,测试相对误差为1．20％,其中测试喷嘴前后气体静压差引入的不确定度较大,提出选用量程为0～500Pa的差压变送器较为适宜。     

Radio frequency controlled synthetic wavelength sweep for absolute distance measurement by optical interferometry

We present a new technique applied to the variable optical synthetic wavelength generation in optical interferometry. It consists of a chain of optical injection locking among three lasers: first a distributed-feedback laser is used as a master to injection lock an intensity-modulated laser that is directly modulated around 15 GHz by a radio frequency generator on a sideband. A second distributed-feedback laser is injection locked on another sideband of the intensity-modulated laser. The variable synthetic wavelength for absolute distance measurement is simply generated by sweeping the radio frequency over a range of several hundred megahertz, which corresponds to the locking range of the two slave lasers. In this condition, the uncertainty of the variable synthetic wavelength is equivalent to the radio frequency uncertainty. This latter has a relative accuracy of 10(-7) or better, resulting in a resolution of +/-25 microm for distances exceeding tens of meters. The radio frequency generator produces a linear frequency sweep of 1 ms duration (i.e., exactly equal to one absolute distance measurement acquisition time), with frequency steps of about 1 MHz. Finally, results of absolute distance measurements for ranges up to 10 m are presented.