The article presents the results of designing, manufacturing, and studying the resonant properties of a square silicon membrane for use in a fiber-optic acoustic receiver. The dependences of resonant frequency on edge length (6-9 mm) and thickness of the membrane (30-50 μm) are obtained. The geometrical parameters of the membrane satisfying the values of resonant frequency (2-60 kHz), pressure (0.1-14 Pa), and deviation (10 nm) are determined: edge length is 8 mm and thickness is 40-50 μm (9.2-42.3 kHz). A series of square silicon membranes was fabricated by anisotropic wet etching. The amplitude-frequency characteristics of the membranes were experimentally measured using an adaptive holographic interferometer. For a square membrane of 8×8×0.044 m3, the experimentally measured resonant frequency was 10.1 kHz, which is consistent with the results of numerical simulation.
This paper presents the results of the analysis of acoustic emission signals registered by using fiber-optic sensors during the propagation of ultrasonic waves in a polymer composite material. Fiber-optical sensors for acoustic emission were constructed according to the scheme of an adaptive holographic interferometer. Unlike piezoelectric sensors, fiber-optic sensors are distributed type sensors. This imposes certain features on the detection of signals in plates in which fiberoptic sensors are embedded. It is established that the difference of the spectrum of acoustic emission signals registered in different directions of wave propagation. The local maximums of the spectrum are determined by the mode of wave propagation in the plate in different directions and the location of fiber-optic sensors.
The experimental setup for recording dynamic holograms in photorefractive crystal CdTe by IR radiation with several spectral components is developed and built. The results of the dynamic holograms' recording by radiation with two spectral components are presented and discussed.
We present laser microweight biosensor based on the adaptive holographic interferometer. The sensing element of the sensor is gold coated silicon microcantilever with dimensions of 210×40×5 μm3. Biomolecules of the interest being attached to the cantilever increase its mass which in its turn leads to decreasing the cantilever resonance frequency. The frequency shift of the detected signal is directly proportional to the total mass of adsorbed molecules. Cantilever oscillations are detected by the adaptive holographic interferometer based on two-wave mixing in a photorefractive CdTe crystal. For measuring the concentration of biological molecules dissolved in water the cantilever was installed in a cuvette which contains the solution. In the experiment, adsorbed molecules of BSA (Bovine Serum Albumin) with total mass of 3.1 ng were detected and weighted.
In this paper, we propose a theoretical model of vectorial two-wave mixing in optically active photorefractive cubic crystals of the point group 23 and 43m in orthogonal geometry. Proposed model takes into account polarization states of coupling waves, their dependence on the location in the crystal, orientation of wave vectors relative to the main crystallographic axes and optical activity of a crystal. Sets of the wave parameters which provide either a maximum efficiency of wave coupling or wave coupling efficiency insensitive to polarization state of signal wave are found with use of proposed model.
Laser Doppler Flowmetry and Laser Speckle Contrast Imaging are applied usually for assessment of parameters of the cutaneous blood flow during thermoregulation. Alternatively, this work explores the feasibility of blood pulsation imaging under incoherent green illumination for measuring the response of human body on local thermal impact. The proposed technique allows assessment of the cutaneous blood flow changes during thermoregulation simultaneously in different areas of the body. The experiment was conducted in two stages. The first stage is the introduction of tissue into a stressful state (cooling) and observation of its thermoregulation to the normal state. At the second stage, the tissue was heated up from the external heat source. It is found that, the blood pulsation increase in the preliminary cooled tissue is higher by 30-60% in comparison with that one obtained when the tissue was heated. It is shown that, the rate of blood pulsation amplitude increase of pre-cooled tissue is individual characteristic of a subject, while it is less depended on the subject's personal characteristicsin the case of tissue heating.
In this work, we present two-channel system for detecting acoustic waves which is based on adaptive holographic interferometer using multi-wave mixing in photorefractive crystal. Micromechanical silicon cantilevers with dimensions of 233×45×4 μm3 mounted on metal membrane are used as sensitive elements. For the first cantilever the detection threshold of acoustic pressure of the sensor on frequency of 6.9 kHz is 17 mPa. For the second cantilever the detection threshold of acoustic pressure of the sensor on frequency of 8 kHz is 49 mPa.
The orthogonal geometry of three-wave mixing in non-gyrotropic photorefractive crystal (PRC) with cubic symmetry is proposed and studied. In this geometry a single phase-modulated object wave and two elliptically polarized reference waves are mutually orthogonal and propagate in crystal along its principal axes [100], [010] and [001], respectively. It is shown that each of two holograms recorded by pair of object and reference waves operates independently as a polarization-selective element and produces a demodulation signal which is related with one of two polarization components of the object wave. The resulting demodulation signal is a superposition of two demodulation signals originated from two holograms which are combined incoherently. It is shown that orthogonal geometry of three-wave mixing opens a possibility to design a completely polarization-independent adaptive interferometer.
In this paper, we developed and investigated an adaptive fiber-optic hydrophone based on adaptive holographic interferometer which key element is dynamic hologram recorded in a photorefractive crystal of cubic symmetry in orthogonal geometry. Different modifications of primary fiber-optic receivers of acoustic pressure are designed and studied. The adaptive hydrophone provides high sensitivity to acoustic pressure (up to 8.6 mV/Pa) and high level of immunity to all kind of noisy instabilities of environment.
The paper presents the results of an analysis of the acoustic emission signals recorded both by piezoelectric and adaptive fiber-optical sensors. Fiber-optical sensors have been embedded into the polymer composite material during their manufacture. Fiber-optical sensors are designed on the base of adaptive interferometer that uses a dynamic hologram formed in the photorefractive crystal. It has been shown that the sensitivity of adaptive interferometer allows the detection of acoustic emission signals generated by the Hsu-Nielsen source. Peculiarities of material sound velocity determination with use of acoustic signals detected by distributed fiber-optical sensors are discussed.
We present the multichannel fiber-optical system for detection and reconstruction of weak acoustic field. Developed system is based on 32-channel adaptive holographic interferometer that employs multiplexing of dynamic holograms in a single photorefractive crystal of CdTe. The multichannel fiber-optical sensory system was tested for registration and spatial reconstruction of acoustic field created in a solid plate.
An adaptive fiber-optical interferometer, which is based on dynamic reflection hologram recorded in the photorefractive
crystal of cubic symmetry without applying any electrical field, is developed. Adaptive properties of dynamic hologram
enable the solution of an interferometer's working point uncontrollable drift problem caused by external factors.
Theoretical analysis has allowed us to find the optimal set of parameters for both interacting waves and crystal to reach
the maximal sensitivity of the measuring system. Use of semiconductor crystal CdTe with fast recording time of a
dynamic hologram makes it possible to achieve high cutoff frequencies at reasonably low light intensities. As a result
the measuring system is characterized by low energy consumption and ability of stable operation of long duration in
industrial environment.
An adaptive fiber-optical measuring system for vibrations monitoring, which is based on dynamic reflection hologram recorded in the photorefractive crystal of cubic symmetry without applying any electrical field is developed. Adaptive properties of dynamic hologram enable the solution of a interferometer's working point uncontrollable drift problem caused by external factors. Use of semiconductor crystal CdTe with fast recording time of a dynamic hologram makes it possible to achieve high cutoff frequencies at reasonably low light intensities. As a result the measuring system is characterized by low energy consumption and ability of stable operation of long duration in real environment. Theoretical analysis has allowed us to find the optimal set of parameters for both interacting waves and crystal to reach the maximal sensitivity of the measuring system.
Theoretical and experimental studying of the vectorial wave mixing in the photorefractive crystal in the reflection geometry without applying an external electric field is presented. We demonstrate that small phase modulation of the object wave is linearly transferred into the intensity modulation of the output beam without any auxiliary polarization element but just mixing light waves with different polarization states (e.g., elliptical and linear). Geometry in which anisotropic diffraction occurs was chosen. The linear phase demodulation occurs because the inherent π/2 phase difference between orthogonal components of elliptically polarized wave is transferred into the interference term of the transmitted object and diffracted reference waves.
An experimental and theoretical studying of adaptive processing a single-fiber multimode interferometer's signal by a reflection and transmission holograms formed in photorefractive BTO crystal without additional reference beam is presented.
An experimental studying of adaptive processing a single-fiber multimode interferometer's signal by means of a photorefractive crystal is presented. Opportunity of developing an adaptive processing system on the base of two photorefractive effects (polarization self-modulation and self-diffraction) is investigated and discussed. A simple mathematical model of a space-charge field forming in the photorefractive crystal under influence of an alternating optical field is considered. Frequency response of the self-diffraction and the polarization self-modulation effects in the photorefractive crystal is calculated on the base of this model.
A measuring system for long-term registration of fast processes in solid state objects under mechanical stresses based on using several single-fiber multimode interferometers (SFMMI) as sensors is proposed. Multi-channel adaptive correlation filter based on photorefractive crystal is used for simultaneous processing speckle signals from SFMMI. Mutual influence of the neighboring channels in crystal is analyzed, and it is shown that the simultaneous processing of several channels can be performed by using a single photorefractive crystal, even if optical fields from different sources are overlapped inside crystal. Such important property of proposed filter as its adaptability allows to separate fast processes of crack generating from slow processes of stress accumulating inside deformed object.
A problem of tomography reconstruction of functions of spatial distribution of vector physical fields by using two- dimensional measuring network based upon fiber-optics measuring lines having integral sensitivity is discussed. The problem solution was found for cases (1) output signal from measuring lines is formed under action of vector projection, (2) output signal is formed under action of the derivative of vector projection along measuring lines axis. (3) output signal is formed under action of square of vector projection.
Two fiber-optical methods for monitoring of stress processes in building structures under deformation are proposed. First method is based on using single-fiber double-mode interferometer as a sensor. It is shown that the single- fiber double-mode interferometer implanted into structure of a tested concrete unit allows to measure the unit structural deflection, absolute elongation and residual deformations in real time with accuracy not worse than 0.05 micrometers . It is also shown that the developed interferometric method allows to indicate a point of time of unit's crack initiation and also to estimate the crack opening displacement. Second method based on using single-fiber multimode as a sensor and photorefractive crystal allows to long-term monitor for dynamics of crack generation in real time.
Adaptive correlation filter recorded in photorefractive crystal as an effective instrument for processing signals of fiber-optical measuring network is considered in this paper. Some basic properties and parameters of the filter are experimentally and theoretically studied. We proposed a number of fiber-optical measuring techniques based on using this filter that can be used for different physical parameters monitoring.
A problem of tomography reconstruction of functions of spatial distribution of vector physical fields by using 2D measuring network based upon fiber-optics measuring lines having integral sensitivity is discussed. The problem solution was found for cases (1) output signal from measuring lines is formed under action of vector projection, (2) output signal is formed under action of the derivative of vector projection along measuring lines axis, (3) output signal is formed under action of square of vector projection.
A problem of tomography reconstruction of vector physical fields by using the sets of contour integrals from a vector projection and from projection of a vector derivative with respect to direction is investigated. A necessity of using nonrectilinear measuring lines for tomography reconstruction of vector field parameters distribution is shown. If the measuring line output signal is proportional to integral from projection of a researched vector it is possible to use the measuring line in the form of a narrow loop and the decision of a problem can be received by use of integral theorems. If the measuring line output signal is proportional to integral from projection of a vector derivative with respect to direction (e.g. problem of deformed two-dimensional object), it is possible to use the measuring line of step form and potential component of a vector field can be reconstructed. The method can be widely applied to research distribution of an electromagnetic, deforming and other vector fields inside elements of technical and technological objects constructions and also can become a basis of systems of vector physical fields monitoring.
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