Compulsorily Phase Locked Interferometer Using Orthogonally Polarized Light Source of a Modulated LD with High Extinction Ratio

This paper describes a compulsorily phase locked differential interferometer using an orthogonally polarized light source of a modulated LD with high extinction ratio to reduce non-linearity of the interferometer caused by polarization cross-talk. The current modulated LD is used as a light source to make the interferometer compact and for the scanning phase of the interferometer. The interferometer is operated compulsorily at the maximum inclination point of the fringe intensity curve by fringe scanning and an electric system. A Wollaston prism of high extinction ratio (50 dB) is used to combine the polarizing beams and to make the polarization cross-talk very small. In one light source the polarized output beams are on the same propagation axis; in the other they have a small crossing angle (2.5 mrad ∼ 10 mrad) to completely exclude non-linearity of the interferometer causded by polarization cross-talk. Using jets of a gas mixture of nitrogen and ethylene, this interferometer was demonstrated to be useful in detecting the photothermal effect of a photothermal velocimeter under phase fluctuation in a turbulent flow.     

Interferometer study of the turbulent flow by noise-signal sounding

We consider the interferometer method by which a turbulent water flow is remotely studied by sounding with a noise ultrasound signal. It is shown that the processing of signals received by two spatially separated receivers makes it possible to extract the signal phase difference caused by the concentration fluctuations of cavity air bubbles on the propagation paths. Spectral analysis of the random phase difference permitted diagnostics of the propagation medium. In particular, it is shown that the output signal of the interferometer carries information on the spatial spectrum of parameter fluctuations of the medium, and on the velocity of inhomogeneities if the interferometer base is parallel to the turbulent flow. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 2, pp. 95–103, February 2007.     

Self-referenced Doppler optical coherence tomography

Doppler optical coherence tomography (DOCT) allows simultaneous micrometer-scale resolution cross-sectional imaging of tissue structure and blood flow. We demonstrate a fiber-optic polarization-diversity-based differential phase contrast DOCT system as a method to perform self-referenced velocimetry in highly scattering media. Using this strategy, we reduced common-mode interferometer noise to <1 Hz and improved Doppler estimates in a scattering flow phantom by a factor of 5.     

Phase-referenced Doppler optical coherence tomography in scattering media

We present a fiber-based, low-coherence interferometer that significantly reduces phase noise by incorporating a second, narrowband, continuous-wave light source as a phase reference. By incorporating this interferometer into a Doppler OCT system, we demonstrate significant velocity noise reduction in reflective and scattering samples using processing techniques amenable to real-time implementation. We also demonstrate 90% suppression of velocity noise in a flow phantom.     

便携式泰曼格林型干涉仪装调方法研究

 为满足光学系统对高精度检测的要求,依据一种新型便携式干涉仪的设计方案,研究了装调方法。该型干涉仪基于改进的泰曼-格林光路结构,其设计测试精度达到λ/10（P-V,λ=630 nm）,具有参考镜离轴、体积小巧、便携、精度较高、成本低及便于批量装备等新特点。针对该新型干涉仪的结构特点,研究了以两个互相垂直的光轴为基准对整体结构进行装调的新方法,并用装调完善的干涉仪进行了实测试验和比对试验。试验结果表明,该新型干涉仪器对已知面型精度为λ/10（P-V,λ=630 nm）的标准球面镜进行检测,其精度可达到0.09 λ;在相同的测试条件下进行比对实验时,该新型干涉仪对普通标准球面镜的检测结果为0.053 λ（RMS,λ=630 nm）,ZYGO干涉仪的检测结果为0.051 λ（RMS,λ=632.8 nm）,两者测量能力较为接近。采用新装调方法进行装调的该新型便携式干涉仪的实际检测精度达到了设计要求,新装调方法可以满足该新型干涉仪的指标要求。     

Closed-loop phase-shifting interferometry with a laser diode

A closed-loop phase-shifting Fizeau-type interferometer at lambda=633 nm was constructed with the direct frequency modulation of laser diodes. The interferometer is servo controlled fully in the phase domain where optical phases are detected by a two-frequency optical heterodyne method. Stepwise phase shifting was accomplished with good stabilization against external disturbances (vibration, air flow, etc.) and laser frequency fluctuations.     

Zygo interferometer for measuring refractive index of liquids and its application for heavy water analysis

The Zygo interferometer for measuring refractive index of liquids such as heavy water is presented. The accuracy of measurement in the refractive index of liquids is found to be ±0.0002 in the Zygo interferometer. An application of Zygo interferometer for heavy water analysis is also presented. The interferometer is found to be useful for determining the percentage purity of heavy water with an accuracy of ±5% in the purity range of 0–100%.     

A polarization interferometer with simplified double-refracting prisms

A polarization interferometer is described which has prisms made of Iceland spar and of glass. The refractive index of glass is approximately equal to that of Iceland spar for the ordinary beam. Interferograms are presented showing the supersonic flow around a sphere and the flow behind a shock wave diffracting around the edge of a nozzle.     

Modelling and experimental analysis of the performance of a laser Doppler vibrometer used to measure vibrations through combustive flows

The problem of measuring surface vibrations by a laser Doppler vibrometer operating with the measuring beam across a combustive flow is discussed, in order to assess the possibility of experimentally determining the dynamic behaviour of a burner under operating conditions. The instrument performance is analysed in terms of interfering and modifying inputs through the development of a model of the interferometer and by experimental validation of its predictions. Experiments are carried out on an unconfined CH₄ flame of a Bunsen burner and a metal surface under a known forced vibration. The laser vibrometer output is influenced by the presence of the flame: major effects are optical path length variations of the measuring arm of the interferometer and beam movements. All effects occur at the typical flame flickering frequency and cause a distorted vibration spectra and a reduced signal-to-noise ratio. Beam wandering and defocusing are documented by image acquisition.     

Collimation testing of a CO2 laser beam with a shearing interferometer

A shearing interferometer based on a Michelson interferometer is proposed for collimation testing of a CO₂ laser beam. The shear and tilt between the interfering wavefronts is obtained by replacing one or both mirrors of the Michelson interferometer with double mirrors arranged to act as 90 ° or near 90 ° prisms in retroreflecting mode. Detailed analysis for two experimental configurations of this system are presented.     

Determination of shock wave velocity from photographs obtained using a polarization interferometer

A polarization shear interferometer is described in which one of the two images of the flow being studied is produced outside the interference field and serves as a shadow pattern. This image is recorded again using an additional light source — a pulsed laser. In this way one can determine the shock wave velocity and establish, on the basis of gas-dynamic relationships, the proper numeration of the interference fringes on the interferogram.     

A holographic interferometer and fringe analyzer, and their use for the study of supersonic flow

In previous papers the author has discussed the uses of interterometry and holography in aerodynamics, and the design of interferometers using laser sources. The present paper gives a practical example of an application of these techniques to a particular wind tunnel and a particular aerodynamic project.

The nature of the wind tunnel and the type of flow leads naturally to the use of holography with non-diffuse light, and to a particular interferometer design, an aperture-expanding interferometer in which the light is reflected back through the tunnel. It leads also to the use of a sensitivity doubling technique in the analyzer.

The optical and engineering design of the interferometer, analyzer, and of automatic systems for model rotation and camera operation are described. Some results are presented which show that the practical problems have been overcome and that good accuracy is attainable.     

A high efficiency polarizing interferometer for the far infrared fourier-transform spectroscopy

Conclusion The new polarizing interferometer was constructed, in which all the radiation entering the interferometer can be utilized. From the experimental results in the atmosphere and for the water-vapour, it is seen that this spectrometer can work satisfactorily from 5 cm^–1 to 1/2d cm^–1. Comparison of the spectrun by this interferometer with one by the standard Michelson interferometer shows that the gain of luminosity is almost as expected. Grid with substrate causes the interference and produces the spectral response in the higher frequency region. Another configuration of two grids and two plane mirrors is proposed for the polarizing interferometer of high luminosity and of very simple design.     

A simple high-sensitivity interferometric position sensor for test mass control on an advanced LIGO interferometer

A small Michelson interferometer has been configured as a tracking mirror displacement sensor in order to achieve both large dynamic range (2.1 mm) and excellent sensitivity across a broad frequency range (6Hz–3 kHz). The interferometer is illuminated by a simple LED, uses broadband, non-polarising beamsplitters and contains no lensing optics. A DC-coupled balanced detector provides an error signal that is used to position the tracking mirror of the Michelson interferometer so as to maintain an interferometer operating position close to the centre of a particular fringe. The total interferometric sensor provides a small, simple and cost-effective means of achieving high-resolution displacement measurements.     

Zone-plate interferometry at 13 nm wavelength

We report on interferometry using a two-zone-plate common-path interferometer operating at a wavelength of 13 nm. The interferometer was set up with a laser-driven high-harmonic source emitting radiation with the high degree of spatial and temporal coherence necessary for interferometry. The interferometer is suited for investigations of the coherence properties of the light source employed, as well as for simultaneous measurements of the real and imaginary part of the complex index of refraction in the 100 eV regime. This is demonstrated in a proof of principle experiment with a piece of Zr-foil as the phase-shifting and absorbing sample. PACS 41.50.+h; 42.65.Ky; 42.87.Bg; 07.60.Ly     

Experimental Confirmation of an Analytical Model of the Sound Propagation in a Rectangular Duct in the Presence of Impedance Transitions and Development of an Impedance Eduction Method Based on it

Acoustical Physics - The authors have carried out a systematic study of the accuracy in the impedance eduction of liners in “interferometer with flow” rigs in the absence of grazing...     

Design and fabrication of a variable frequency grating and its application as a lateral-shear interferometer having a variable shear

The design and fabrication of holographic gratings of frequency variable from 50 to 750 lines/mm over length of 75 mm have been described. An application of these variable frequency gratings as a lateral-shear interferometer has also been suggested. The variable shear interferometer is found to be useful for testing of lenses with a maximum sensitivity in the detection of wavefront aberrations. Comparison of single-frequency grating having uniform pitch of 100 lines/mm with a grating of frequency variable from 59 to 220 lines/mm using an average pitch of 100 lines/mm over a small area as a lateral-shear interferometer have been made. Our experimental observations show that a small area of average pitch of 100 lines/mm on the grating having a variable frequency of 59–220 lines/mm over a distance of 75 mm behaves as a lateral-shear interferometer in the same way as any area of pitch of 100 lines/mm on the grating of uniform pitch of 100 lines/mm.     

Laser optical fibre heterodyne interferometer with frequency indicating of the phase shift of a light signal in an optical waveguide

Self-sustained oscillations in a laser optical fibre heterodyne interferometer are discussed; oscillations were produced by closing into a loop the single-mode optical fibres of the interferometer. Such oscillatory systems are shown to feature high-frequency stability. It is proposed to use this oscillating interferometer as a high-sensitivity sensor of external factors affecting the optical waveguide.     

“Noninverted” interference fringes upon light reflection from a Fabry-Perot interferometer with an asymmetric diffraction mirror

This paper describes the calculation, fabrication, and experimental investigation of a reflection two-mirror interferometer whose front mirror is asymmetric in reflection and made as a thin-layer diffraction metal structure (Al, Ag) in combination with a dielectric coating (MgF₂, ZnS). In reflection, this interferometer creates in the zero diffraction order a transmission-like pattern similar to that produced by an ordinary Fabry-Perot interferometer operating in transmitted light.     

Real-time phase-resolved functional optical coherence tomography by use of optical Hilbert transformation

We have developed a novel real-time phase-resolved functional optical coherence tomography system that uses optical Hilbert transformation. When we use a resonant scanner in the reference arm of the interferometer, with an axial scanning speed of 4 kHz, the frame rate of both structural and Doppler blood-flow imaging with a size of 100 by 100 pixels is 10 Hz. The system has high sensitivity and a larger dynamic range for measuring the Doppler frequency shift that is due to moving red blood cells. Real-time images of in vivo blood flow in human skin obtained with this interferometer are presented.