Probe transit effect in interferometry of fast moving samples

When a fast moving transparent sample (with a speed close to c) is probed with a laser pulse, some artifacts can occur from data analysis. These artifacts are connected to the transit time of the probe through the sample and can mask the presence of a steep gradient of refractive index in the sample. We study this problem in the case of interferometry. In fact, the problem can affect the femtosecond interferometry of the media under ultrafast ionization by a propagating laser pulse. Two algorithms are introduced: the first based on the phase difference and the second based on visibility. Both algorithms allow for the reconstruction, under suitable assumptions, of an approximated refractive index profile from the distorted fringes.     

Direct measurement of the group refractive index of air with interferometry between adjacent femtosecond pulses

The group refractive index of air in laboratory conditions is measured directly between adjacent femtosecond laser pulses by a new interferometry technique. Measurement of the repetition rate of the mode-locked pulse train that gives the maximum amplitude of the interference-signal envelope enables us to determine the group refractive index of air within a standard deviation of 2 x 10(-7). This simple method without vacuum reference is attractive for measuring the group refractive index needed for precise distance measurements in open fields.     

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。     

高精度激光干涉测量中环境误差因素的综合补偿

分析了高精度激光干涉测量中环境误差因素对测量的影响,针对不同情况,提出了两种对空气折射率进行实时误差补偿的方法。     

Real-time differential refractometry without interferometry at a sensitivity level of 10(-6)

We present a refractometer based on the principle of total internal reflection that can sensitively record, in real time, the refractive index of fluids over a wide range of refractive indices. The device uses a divergent laser beam and a linear diode array, and has no mechanical or optical moving parts, enabling us to achieve the measurement of a refractive index at a sensitivity level of 10(-6). Our refractometer does not rely on interferometry, thus enabling the device to be compact, portable, and inexpensive. To the best of our knowledge, this is the first time a noninterferometric device that performs real-time differential refractometry with a sensitivity of better than 10(-5) has been demonstrated in the literature. We show that our experimental results agree very well with Fresnel theory. We establish a theoretical limit on the sensitivity of this class of refractometers.     

Separation of measurement of the refractive index and the geometrical thickness by use of a wavelength-scanning interferometer with a confocal microscope

An improved system for the separate measurement of the refractive index and the geometrical thickness that constitutes a hybrid configuration of a confocal microscope and a wavelength-scanning heterodyne interferometer with a laser diode is presented. The optical path difference can be measured in less than 1 s, which is 10 times quicker than with the low-coherence interferometry previously used, and with a resolution of 10 mum with a fixed reference mirror. Separate measurement of the refractive index and the geometrical thickness of glass plates was demonstrated by use of the arrangement in place of the low-coherence interferometer used previously.     

Design and test of a simple high-temperature laser microrefractometer

The design of a laser microrefractometer that is suitable for temperature-dependent measurements is described. The refractive index of methylene iodide is measured in the temperature range of 22-92 °C for laser wavelengths covering almost the entire visible range of the spectrum: 442, 488, 515, 543, 594, and 633 nm. A detailed analysis of the temperature-related experimental error is made.     

Spatially resolved determination of atomic particle densities and line shapes within an arc plasma by tomographic resonance interferometry

The particle density of ground-state chromium atoms within one cross section of an arc plasma was measured spatially resolved, and the spatial distribution of the line shape of the chromium resonance line at 427.48 nm was partly determined. The measurements were performed with a newly developed setup that combines the methods of resonance interferometry and refractive tomography. The wavelength of a dye laser was scanned over the investigated transition, and the refractive index was measured spatially and spectrally resolved by use of tomography. For each spatial point the particle density and the local line shape were calculated from the measured spectral refractivity distribution by the method of resonance interferometry. We describe the physical principles, the optical arrangement, and the numerical apparatus, and we discuss the results and further possibilities.     

Optical properties of vinyon copolymer fibre

Birefringence for vinyon fibres have been investigated. The principal refractive index parallel and perpendicular to the stretch direction were found. Double-beam and multiple-beam interferometry methods were applied. The double-beam technique was applied to the mean birefringence and the mean refractive indices for plane-polarized light parallel and perpendicular to the fibre axis. The diffraction of a He-Ne laser beam was used to measure the dimensional parameters, transverse sectional shape, and area of the fibre. The fibre area was used to calculate the principal refractive indices. The Becke-line method was used to measure the refractive indices of the outer layer of the fibre (skin) for polarized light parallel and perpendicular to the fibre axis. The results were found to be in good agreement with published data.     

Integrated-optical wavelength sensor with self-compensation of thermally induced phase shifts by use of a LiNbO3 unbalanced Mach-Zehnder interferometer

We demonstrate an integrated-optical unbalanced Mach-Zehnder interferometer in lithium niobate for detecting wavelength shifts of light sources, such as laser diodes and superluminescentdiodes at lambda = 844 nm. The output signal can be used to stabilize the light source. Because of the temperature dependence of the effective refractive index and the thermal expansion of the substrate, the device acts also as a temperature sensor. The temperature sensitivity of the interferometer was compensated for by the combination of proton exchanged- and annealed proton exchanged-channel waveguides by approximately two orders of magnitude. The thermo-optic coefficients of the extraordinary effective refractive index in integrated optical channel waveguides in LiNbO8 have been measured with high accuracy over a temperature range from 10 degrees C to 40 degrees C.     

Reflectance spectra and refractive index of a Nd:YAG laser-oxidized Si surface

The reflectance spectra and refractive index of Nd:YAG laser-oxidized SiO₂ layers with thicknesses from 15 to 75 nm have been investigated with respect to the laser beam energy density and substrate temperature. Thickness and refractive index of films have been determined from reflectance measurements at normal light incidence in the spectral range 300–800 nm. It was found that the oxide-growth conditions at higher substrate temperatures and laser powers greater than 3.36 J cm⁻² provides a better film quality in terms of both optical thickness and refractive index. However, the refractive indices of the films are smaller in the whole spectral range studied as compared to that of conventional thermally grown SiO₂. This might be due to the porous structure formed during the laser-assisted oxidation. The results suggest the need of post-oxidation annealing to improve the refractive indices of the films, suitable for Si-device applications.     

Photothermal measurements with a Jamin interferometer

We present a numerical study of the behavior of the signal in a photothermal experiment combining the mirage effect and a Jamin interferometer. Our analysis is limited to a square-pulse excitation by a weakly absorbed Gaussian pump laser beam with a large radius. We investigate the influence of three parameters: the time of illumination of the sample, the time of extinction of the pump beam, and the height of the probe-beam arm of the Jamin interferometer above the sample. We show that the path difference, which induces the variation of intensity at the output of the Jamin interferometer, is caused by both probe-beam deflection and temperature variation of the refractive index of air above the sample. The first effect is dominant for short times, and interferometry is a sensitive tool to monitor it.     

测量薄膜折射率的光栅衍射干涉方法

根据光的干涉理论,讨论了条纹周期数对测量薄膜折射率不确定性的影响,推导出干涉条纹错位量与薄膜折射率和厚度的关系式。由此提出采用光栅衍射干涉测量薄膜折射率的方法和实验方案。实验表明:该方法的干涉条纹测量精度达λ/10~λ/20,薄膜折射率测量精度可达 0.01 以上。     

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.     

Measuring mixing dynamics of transparent fluids with electronic speckle pattern interferometry

Electronic speckle pattern interferometry (ESPI) combined with phase-shifting techniques is used for studying the mixing dynamics of transparent fluids. The potentials of the technique for studying fluid mixing are illustrated for simple examples of water flow and moving water droplets in water. With ESPI we could actually follow water droplets moving in water and water flowing in water on a television monitor at the video rate. A He-Ne laser (lambda(0) = 632.8 nm) was used as the light source, and phase stepping was applied, giving an interferometric sensitivity better than lambda(0)/10. The observed phase changes are due to changes in the refractive index caused by small temperature differences between the droplets and the surrounding water. Temperature differences of less than 0.1 K are detectable for droplets of a diameter of 4 mm.     

Fabrication of a thermally actuated tunable grating and its application as a CO2 laser beam profile analyzer

We propose a thermally actuated tunable grating for measuring the beam profile of a CO(2) laser. The grooves of a transmissive grating are filled with a liquid whose refractive index depends on temperature. A visible laser as a probe and a CO(2) laser as a heat source are illuminated on the grating. The CO(2) laser is absorbed, and depending on its beam profile, a temperature profile is induced on the grating. The refractive index of the heated liquid is changed, resulting in a change of efficiency of the grating for the probe laser. By using the 1st orders of diffraction in a 4f imaging system, the beam profile of the CO(2) laser is imaged onto a CCD camera by the probe laser.     

Interferogram Evaluation of Axially Symmetric Phase Objects

Some methods for calculating the refractive index distribution from the interferograms of axially symmetric phase objects are presented. The ideal interferometry condition is assumed, neglecting the curvature of light rays in passing through the phase object. The methods presented are compared to each other for their accuracy and required computer time. Examples of the temperature fields of axially symmetric burner flames are given.     

Refractive-index measurement based on the effects of total internal reflection and the uses of heterodyne interferometry

A new method for measuring the refractive index is presented. First, the phase difference between s and p polarizations that is due to the total internal reflection is measured by heterodyne interferometry. Then, substituting this phase difference into the Fresnel equations, we can obtain the refractive index of the test medium.     

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     

Determination of the refractive index and the chiral parameter of a chiral solution based on chiral reflection equations and heterodyne interferometry

This study develops a method for determining the chiral parameter and the refractive index of an isotropic chiral medium using chiral reflection equations and critical angle phenomena. Linearly polarized light propagates back and forth in a parallelogram prism between two parallel compartments with chiral solutions. A beam splitter then divides the light that emerges from the prism into a reflected light beam and a transmitted light beam. The two beams pass through a compensator and an analyzer, respectively, to cause phase compensation and interference of s and p polarizations. The phase difference between the two interference signals are initially optimized by a suitable optical arrangement and subsequently measured by heterodyne interferometry. Additionally, the refractive index of the solution is determined from the critical angle that occurred at the discontinuity of the phase difference between the two interference signals. These results are substituted into derived equations to calculate the chiral parameter. The approach has the merits of both common-path interferometry and heterodyne interferometry.