Measurement of in-plane and out-of-plane ultrasonic displacements by optical heterodyne interferometry

An heterodyne optical probe, which permits one to measure out-of-plane and in-plane displacements at the surface of a specimen excited by ultrasound is presented. The principles at the basis of the two modes of operation are explained and the sensitivities for in-plane and out-of-plane detection are analyzed. The optical layout of the probe and the schematic of its demodulation circuitry are presented. Its accuracy is tested with Rayleigh surface waves. Examples of application to laser-generated Rayleigh and Lamb waves are also presented.     

Measurement of gas density by heterodyne interferometry

A method for measuring the density of a gas by heterodyne interferometry is investigated. The molar refractions are calculated for the more common commercial gases. The experimental results are compared with the density calculated from the virial equation of state. This article is the first of a series continued from the preceding issue of the journal and taken from the Proceedings of the Third Joint Scientific-Technical Conference on Optical Methods of Flow Research, held at the Moscow Power Institute in June, 1995. Translated from Izmeriel'naya Tekhnika, No. 2, pp. 30–33, February, 1996.     

Measurement of in-plane displacement by wavelength-modulated heterodyne speckle interferometry

The use of wavelength-modulated light incorporated into an optical-path-difference speckle interferometer is demonstrated as a heterodyne technique for measuring the in-plane displacement of a rough object. The in-plane displacement can be determined from the measured phase variation of the heterodyne speckle signal. We also improved the optical configuration to create a high-contrast interference pattern. Experimental results reveal that the proposed method can detect displacement up to a long range of 220 μm and displacement variation down to the nanometer range. Moreover, the sensitivity can reach up to 0.8°/nm. The performance of the system is discussed.     

Real-time heterodyne speckle pattern interferometry using the correlation image sensor

A real-time method for heterodyne speckle pattern interferometry using the correlation image sensor (CIS) is proposed. The CIS demodulates the interference phase of heterodyned speckle wavefronts pixelwise at an ordinary video frame rate. The proposed method neither suffers loss of spatial resolution nor requires a high frame rate. Interferometers for out-of-plane and in-plane deformation are developed with a 200 × 200 pixel CIS camera. Experimental results confirm that the proposed method realizes real-time imaging of a rough-surfaced object under deformation. The average standard deviations of demodulated phase-difference images for the out-of-plane and in-plane interferometers are 0.33 and 0.13 rad, respectively.     

Measurement error caused by speckle noise in heterodyne and quasi-heterodyne holographic interferometry

In heterodyne and quasi-heterodyne holographic interferometry, the measuring accuracy is limited by speckle noise. It is important to know the relation between measurement accuracy and speckle noise in both theory and application. This problem has been discussed in the past under the assumption that the speckle noise produces only a small measuring error. However, this assumption is not reasonable in practice. The effect of speckle noise on the measurement accuracy is analyzed by the use of a general statistical method. The results obtained reveal the general relation between the measuring accuracy and system parameters, and thus constitute important guidelines to the application of these techniques.     

High-sensitivity small-angle sensor based on surface plasmon resonance technology and heterodyne interferometry

A high-sensitivity small-angle sensor based on surface plasmon resonance technology and heterodyne interferometry is proposed that uses a new technique with two right-angle prisms. Interestingly, the technique provides a novel method for designing small-angle sensors with high sensitivity and high resolution. Its theoretical resolution can reach 1.2x10(-7) rad over the measurement range of -0.15 degrees < or =theta< or =0.15 degrees . The method has some merits, e.g., a simple optical setup, easy operation, high resolution, high sensitivity, and rapid measurement. Its feasibility is demonstrated.     

Real-time heterodyne imaging interferometry: focal-plane amplitude and phase demodulation using a three-phase correlation image sensor

A method of real-time heterodyne imaging interferometry using a three-phase correlation image sensor (3PCIS) is proposed. It simultaneously demodulates the amplitude and phase images of an incident interference pattern at an ordinary frame rate with good accuracy, thus overcoming the trade-off among measurement time, spatial resolution, and demodulation accuracy suffered in conventional interferometry. An experimental system is constructed with a 64x64 3PCIS camera operated at 30 frames/s and a two-frequency He-Ne laser with a beat frequency of 25 kHz. The results obtained for a scanning mirror and heated silicone oil confirm the proposed method.     

Measurement of thermally induced vibrations of microelectronic devices by use of a heterodyne electronic speckle pattern interferometry imaging technique

An imaging technique to measure modulated surface displacements on microelectronic devices is presented. A device is supplied by a sinusoidal current that creates a modulated variation of temperature. To measure the induced normal surface displacement, we use an electronic speckle pattern interferometry setup in which we introduce a secondary modulation using an electro-optic modulator. To extract the displacement information, we then analyze the term atthe blinking frequency, which is equal tothe difference between the frequency of the surface displacement and the frequency of the secondary modulation. As the photodetector is a visible CC D camera, weapply heterodyne detection byusing a multichannel lock-in scheme. We have experimented with this new technique on a membrane to measure the amplitude of modulated surface displacement induced by the Joule effect.     

Intrinsic fiber-optic ultrasonic sensor array using multiplexed two-wave mixing interferometry

An intrinsic multiplexed laser interferometer is presented that allows for the simultaneous detection of acoustic waves by an array of fiber-optic sensors. The phase-modulated signals from each sensor are demodulated by use of an adaptive two-wave mixing setup. The light from each sensing fiber in the array is mixed with a reference beam in a single photorefractive crystal (PRC), and the output beams from the PRC are imaged onto separate photodetectors to create a multiplexed two-wave mixing (MTWM) system. The sensing fibers are embedded in graphite-epoxy composite panels, and detection of both acoustic emission and ultrasonic signals in these materials is demonstrated. The intrinsic MTWM system is an effective tool for the simultaneous demodulation of signals from a large fiber sensor array. Also, the adaptive nature of the MTWM setup obviates the need for active stabilization against ambient noise.     

Analytical modeling of the periodic nonlinearity in heterodyne interferometry

The periodic nonlinearity that arises from nonideal laser sources and imperfections of optical components limits the accuracy of displacement measurements in heterodyne interferometry at the nanometer level. An analytical approach to investigating the nonlinearity is presented. Frequency mixing, polarization mixing, polarization-frequency mixing, and ghost reflections are all included in this investigation. A general form for the measurement signal, including that of the distortions, is given. The analytical approach is also applicable to homodyne interferometry.     

Optical heterodyne detection of pulsed ultrasonic pressures

An optical heterodyne interferometer was used for measuring short ultrasonic pulses in fluids. The optical phase shift of the probe beam, proportional to the acoustic pressure integrated along the light path, was demodulated quantitatively by electronic processing. The detection sensitivities of 10(-3) (Pa/ radicalHz) in water and 10(-4 ) (Pa/ radicalHz) in air were deduced. The experimental results obtained in water as well as in air show that this optical heterodyne technique offers a very sensitive way of measuring ultrasonic pressures in the low megahertz range even for very low pressures.     

Measurement of liquid-film thickness by laser interferometry

Here the variation of a liquid-film thickness at small Reynolds numbers is discussed. The film thickness measurement by laser interferometry corresponds to the liquid flowing on the inner surface of a small-bore glass tube. An adequate theoretical background for the techniques used in this experiment is discussed to demonstrate the capability of the experimental technique. An advantage of this method is that it shows the shape of the thin film on the inner surface of the vertical tube at a point in a horizontal cross section. The results obtained from this experiment show that the flow of liquid films on vertical surfaces is inherently unstable and three dimensional even at a Reynolds number smaller than 1.     

Comparative study of autodyne and heterodyne laser interferometry for imaging

For given laser output power, object under investigation, and photodiode noise level, we have theoretically compared the signal-to-noise ratios of a heterodyne scanning imager based on a Michelson interferometer and of an autodyne setup based on the laser optical feedback imaging (LOFI) technique. In both cases, the image is obtained point by point. In the heterodyne configuration, the beating between the reference beam and the signal beam is realized outside the laser cavity (i.e., directly on the detector), while in the autodyne configuration, the wave beating takes place inside the laser cavity and therefore is indirectly detected. In the autodyne configuration, where the laser relaxation oscillations play a leading role, we have compared one-dimensional scans obtained by numerical simulations with different lasers' dynamical parameters. Finally, we have determined the best laser for LOFI applications and the experimental conditions for which the LOFI detection setup (autodyne interferometer) is competitive compared to a heterodyne interferometer.     

光学倍频影响激光外差干涉测量精度的机理

为了同时提高激光外差干涉测量的分辨力和精度,必须深入分析光学倍频对激光外差干涉测量精度的影响机理。在此基础上,建立了光学倍频相位测量模型,从理论上证明,光学倍频能够实现对激光外差干涉信号的细分,提高测量分辨力。光学倍频改变非线性误差的相位而使非线性误差减小,但同时改变了激光干涉多普勒频移的v/c平方项,使得残余累计误差增大。仿真结果表明,光学N倍频使非线性误差减小到原来的1/N,但使残余累计误差增大N2倍。因此,光学倍频仅适用于低速测量的场合。     

Measurement of aptamer-protein interactions with back-scattering interferometry

We report the quantitative measurement of aptamer-protein interactions using backscattering interferometry (BSI) and show that BSI can determine when distinct binding regions are accessed. As a model system, we utilized two DNA aptamers (Tasset and Bock) that bind to distinct sites of a target protein (human α-thrombin). This is the first time BSI has been used to study a multivalent system in free solution wherein more than one ligand binds to a single target. We measured aptamer equilibrum dissociation constants (K(d)) of 3.84 nM (Tasset-thrombin) and 5.96 nM (Bock-thrombin), in close agreement with the literature. Unexpectedly, we observed allosteric effects such that the binding of the first aptamer resulted in a significant change in the binding affinity of the second aptamer. For example, the K(d) of Bock aptamer binding to preformed Tasset-thrombin complexes was 7-fold lower (indicating higher affinity) compared to binding to thrombin alone. Preliminary modeling efforts suggest evidence for allosteric linkage between the two exosites.     

Alternative method for measuring the full-field refractive index of a gradient-index lens with normal incidence heterodyne interferometry

A linearly/circularly polarized heterodyne light beam coming from a heterodyne light source with an electro-optic modulator in turn enters a modified Twyman-Green interferometer to measure the surface plane of a GRIN lens. Two groups of periodic sinusoidal segments recorded by a fast complementary metal-oxide semiconductor camera are modified, and their associated phases are derived with the unique technique. The data are substituted into the special equations derived from the Fresnel equations, and the refractive index can be obtained. When the processes are applied to other pixels, the full-field refractive-index distribution can be obtained similarly. Its validity is demonstrated.     

Vibration maps of capacitive micromachined ultrasonic transducers by laser interferometry

A 1.8-mm × 1.8-mm capacitive micromachined ultrasonic transducer (CMUT) element is experimentally characterized by means of optical measurements. Optical displacement measurements provide information on the resonant behavior of the single membranes and also allow us to investigate the dispersion in the frequency spectrum of adjacent membranes. In addition, higher order mode shapes are observed, showing that either symmetrical or asymmetrical modes are excited in CMUT membranes. Laser interferometry vibration maps, combined with quantitative displacement measurements, provide information about the quality and repeatability of the fabrication process, which is a basic requirement for 2D array fabrication for ultrasound imaging