Transduction mechanisms of the Fabry-Perot polymer film sensing concept for wideband ultrasound detection

The transduction mechanisms of a wideband (30 MHz) contact ultrasound sensor based upon the use of a thin polymer film acting as a Fabry-Perot interferometer have been investigated. Polyethylene terepthalate (PET) sensing elements, illuminated by the free-space collimated output of a wavelength-tunable DBR laser diode, have been used to study the sensor transfer function, sensitivity, the effect of water absorption, and frequency response characteristics. Acoustic performance was evaluated by comparing the sensor output with that of a calibrated PVDF membrane hydrophone using laser-generated acoustic transients as a source of broadband ultrasound. An ultrasonic acoustic phase sensitivity of 0.1 rad/MPa, a linear operating range to 5 MPa, and a noise-equivalent-pressure of 20 kPa over a 25 MHz measurement bandwidth were obtained using a water-backed 50 mum PET sensing film. A model of frequency response that incorporates the effect of an adhesive layer between the sensor film and backing material has been developed and validated for different sensing film thicknesses, backing configurations, and adhesive layer thicknesses.     

Embedded fiber-optic Fabry-Perot ultrasound sensor

A fiber-optic ultrasound sensor is presented. The sensor consists of a continuous length of single-mode optical fiber with a built-in Fabry-Perot interferometer. The acoustic pressure produces changes in the index of refraction along the interferometer cavity through the strain-optic effect, thus modulating the reflected power of the light propagating in the fiber. The dielectric internal mirrors that form the interferometer are fabricated by joining a fiber coating with a TiO(2) film at one end to an uncoated fiber by electric arc fusion splicing. Experimental results have been obtained for sensors embedded in plastic and graphite composite materials, using ultrasound waves in the range from 100 kHz to 5 MHz. Values for the optical phase shift amplitude as large as 0.5 rad were obtained at an acoustic frequency of 200 kHz for a 1.1-cm-long interferometer embedded in plastic.     

Two-dimensional ultrasound receive array using an angle-tuned Fabry-Perot polymer film sensor for transducer field characterization and transmission ultrasound imaging

A 2-D optical ultrasound receive array has been investigated. The transduction mechanism is based upon the detection of acoustically induced changes in the optical thickness of a thin polymer film acting as a Fabry-Perot sensing interferometer (FPI). By illuminating the sensor with a large-area laser beam and mechanically scanning a photodiode across the reflected output beam, while using a novel angle-tuned phase bias control system to optimally set the FPI working point, a notional 2-D ultrasound array was synthesized. To demonstrate the concept, 1-D and 2-D ultrasound field distributions produced by planar 3.5-MHz and focused 5-MHz PZT ultrasound transducers were mapped. The system was also evaluated by performing transmission ultrasound imaging of a spatially calibrated target. The "array" aperture, defined by the dimensions of the incident optical field, was elliptical, of dimensions 16 x 12 mm and spatially sampled in steps of 0.1 mm or 0.2 mm. Element sizes, defined by the photodiode aperture, of 0.8, 0.4, and 0.2 mm were variously used for these experiments. Two types of sensor were evaluated. One was a discrete 75-microm-thick polyethylene terephthalate FPI bonded to a polymer backing stub which had a wideband peak noise-equivalent pressure of 6.5 kPa and an acoustic bandwidth 12 MHz. The other was a 40-microm Parylene film FPI which was directly vacuum-deposited onto a glass backing stub and had an NEP of 8 kPa and an acoustic bandwidth of 17.5 MHz. It is considered that this approach offers an alternative to piezoelectric ultrasound arrays for transducer field characterization, transmission medical and industrial ultrasound imaging, biomedical photoacoustic imaging, and ultrasonic nondestructive testing.     

A Remote,Non-destructive Laser Ultrasonic Material Evaluation System with Simplified Optical Fibre Interferometer Detection

An optical non-contact ultrasonic testing system is presented. It uses broadband surface acoustic wave impulses generated with a cylindrical lens focused laser line source. The detection of the ultrasound is achieved by a simple and yet effective design of optical fibre interferometer which provides good sensitivity and manoeuvrability. The effectiveness of the fibre interferometer is demonstrated by accurately measuring the broadband surface waves on various common metal structures. The measurement system’s ability for surface material evaluation is also demonstrated from fitting experimental surface wave dispersion curves with theoretical simulations. This measurement system can test small localised areas and miniature samples that were previously difficult to examine.     

Fiber Fabry-Perot sensors for detection of partial discharges in power transformers

A diaphragm-based interferometric fiberoptic sensor that uses a low-coherence light source was designed and tested for on-line detection of the acoustic waves generated by partial discharges inside high-voltage power transformers. The sensor uses a fused-silica diaphragm and a single-mode optical fiber encapsulated in a fused-silica glass tube to form an extrinsic Fabry-Perot interferometer, which is interrogated by low-coherence light. Test results indicate that these fiber optic acoustic sensors are capable of faithfully detecting acoustic signals propagating inside transformer oil with high sensitivity and wide bandwidth.     

光纤非本征法布里-玻罗传感器兰姆波检测灵敏度分析

基于兰姆波的结构工况检测技术在评估复合材料和金属结构的安全性和耐久性方面发挥着重要的作用。作为对传统的压电换能器（PZT）的一种很好的替代,光纤传感器在传感方面的应用正被广泛地挖掘出来,包括兰姆波检测。本文从理论上建立了超声兰姆波作用下光纤非本征法布里．玻罗（EFPI）传感器参数与其输出性能之间的关系。数值结果显示了传感器的性能与其相对于声源的方向角以及传感器的计量长度与超声波长的比值相关。所得出的结论对于EFPI传感器精确地探测兰姆波提供了理论依据。     

Stabilization of an embedded fiber optic Fabry-Perot sensor forultrasound detection

A method is proposed to stabilize an intrinsic fiber optic Fabry-Perot interferometric sensor embedded in a solid material for the purpose of detecting ultrasound. Without stabilization the interferometer drifts out of quadrature due to the presence of low-frequency dynamic strains. Stabilization is achieved by using active homodyne stabilization which tunes the laser frequency to maintain quadrature. A control loop shifts the laser frequency by 10 GHz at rates less than 25 Hz in order to compensate for induced drifts. The stabilization procedure was tested for a sensor embedded in an epoxy plate. Ultrasonic pulses, generated by a 5 MHz piezoelectric transducer, were detected with the embedded fiber sensor stabilized in the presence of applied low frequency strains. Improvements in sensitivity which result from stabilization are demonstrated. Additionally, a simulated acoustic emission signal, generated by a lead pencil break (Hsu-Neilson source), was detected with the sensor stabilized in the presence of dynamic strains     

Velocity dispersion of acoustic waves in cancellous bone

Measurement of ultrasonic attenuation and velocity in cancellous bone are being applied to aid diagnosis of women with high fracture risk due to osteoporosis. However, velocity dispersion in cancellous bone has received little attention up to now. The overall goal of this research was to characterize the velocity dispersion of human cancellous bone based on a spectral analysis of ultrasound transmitted through the bone specimens. We have followed a systematic approach, beginning with the investigation of a test material, moving on to the investigation of bone specimens. Particular attention is given to diffraction effect, a potential source of artifacts. Parametric images of phase velocity (measured at the center frequency of the pulse spectrum), slope of attenuation coefficient (dB/cm/MHz) and velocity dispersion were obtained by scanning 15 bone specimens. We have demonstrated that the diffraction effect is negligible in the useful frequency bandwidth, and that the ultrasonic parameters reflect intrinsic acoustic properties of bone tissue. The measured attenuation showed approximately linear behavior over the frequency range 200 to 600 kHz. Velocity dispersion of cancellous bone in the frequency range 200 to 600 kHz was unexpectedly found to be either negative or positive and not correlated with the slope of attenuation coefficient. There was a highly significant correlation between the slope of attenuation coefficient and phase velocity at the center frequency of the spectrum. This behavior contrasts with other biological or nonbiological materials where the local form of the Kramers-Kronig relationship provides accurate prediction of velocity dispersion from the experimental frequency dependent-attenuation for unbounded waves.     

Toward virtual biopsy through an all fiber optic ultrasonic miniaturized transducer: a proposal

The present generation of devices based on opto-acoustic and acousto-optic conversion lets us foresee the possibility of realizing complete miniaturized transmitting-receiving transducers, able to generate and detect wideband ultrasounds by laser light. In the present paper, a miniaturized ultrasonic transducer entirely based on fiber optic technology is proposed. Such a device springs from the conjunction between our research, which has produced a highly efficient fiber optic opto-acoustic source, with the results obtained by other researchers concerning the realization of an ultrasonic receiver based on optical interferometry. Making use of the thermo-elastic effect for ultrasound generation, a source of ultrasound can be obtained by coupling a fiber optic to pulsed laser, if a film capable of absorbing laser light is placed onto fiber end. Starting from these remarks, we propose an efficient opto-acoustic source, able to generate pressure pulses with amplitude of the order of 10(4) Pa and bandwidth extending up to 40 MHz and beyond by using graphite materials as absorbing film. This solution makes use of a low-power pulsed laser as optical source possible. An ultrasonic receiving element was realized placing a Fabry-Perot cavity over the tip of a fiber optic. The cavity thickness modulation induced by ultrasonic beam is detected by an interferometer optical technique. We have realized a prototype of a receiving device that exhibits a sensitivity comparable with that of piezoelectric devices (10-100 nV/Pa) and an almost flat bandwidth extending up to 20 MHz or more. The extreme miniaturization of the resulting ultrasonic transducer, together with its wide ultrasonic frequency bandwidth, is the first step toward ultrasonic tissue biopsy. In this paper, before discussing the problem of constructing a complete ultrasonic transducer composed by a transmitter and receiver, the results carried out in these fields during the last decade are reviewed.     

Experimental validation for the diffraction effect in the ultrasonic field of piston transducers and its influence on absorption and dispersion measurements

The aim of this work was to study the diffraction effects in the ultrasonic field of piston source transducers and their importance for accurate measurements of attenuation and dispersion in viscoelastic materials. In laboratory measurements, the diffraction phenomena are mainly due to the beam spread of the ultrasonic wave propagating in viscoelastic materials. This effect is essentially related to the estimated attenuation and dispersion in the material. In this work, a frequency domain system identification approach, using the maximum likelihood estimator (MLE), was applied to the measured data in order to determine a function for correcting the diffraction losses in both normal and oblique incidences for a large frequency band (300 kHz to 3 MHz). The effective radius of the used transmitter was determined by the inverse problem when ultrasonic beam propagation was investigated in a water medium. Using the estimated radius, the propagation through viscoelastic materials was established, and the acoustic parameters of these materials were estimated. Attention was paid to the determination of the attenuation and dispersion in the materials. These quantities were compared to those obtained without diffraction correction in order to see the influence of introducing the diffraction correction into the propagation model     

Ultrasonic attenuation and backscatter from flowing whole blood are dependent on shear rate and hematocrit between 10 and 50 MHz

Ultrasonic backscatter has recently been used extensively to investigate erythrocyte aggregation, which is an inherent hematological phenomenon in the blood circulation system. The size of rouleaux can be estimated by measuring certain parameters of signals backscattered from flowing blood. However, most measurements of backscatter from blood use a constant value for the attenuation coefficient to compensate for the loss of ultrasound energy. This correction may be inaccurate because the attenuation varies with the blood properties, which prompted us to explore the effects of hemodynamic properties on ultrasonic attenuation and backscatter to better understand the blood rheological behaviors. Experiments were performed on porcine whole blood in a Couette flow apparatus. Ultrasonic attenuation and the backscattering coefficient of blood were measured at various frequencies (from 10 to 50 MHz), hematocrits (from 0 to 60%), and shear rates (from 0.1 to 200 s?1). The results indicated that the attenuation and backscattering coefficients of blood are highly variable, depending in a complex manner on shear rate, hematocrit, and the measurement ultrasound frequency. The attenuation of blood decreased rapidly with increasing shear rates, eventually reaching a steady state asymptotically, and increased linearly with the hematocrit from 10 to 50 MHz at various shear rates, and also with the ultrasound frequency. The effect of erythrocyte aggregation means that the change in ultrasonic attenuation in blood with shear rate may be attributed to the absorption mechanism, which is enhanced by the increased blood viscosity at lower shear rates. Compensating the measured backscattering coefficients of blood for the shear-rate-dependent attenuation coefficient increased the accuracy of erythrocyte aggregation assessments. Together, the experimental results suggest that the shear-rate-dependent attenuation coefficient should be considered in future developments of ultrasonic technologies for characterizing blood rheology when the ultrasound frequency is higher than 20 MHz.     

A photoacoustic imager with light illumination through an infrared-transparent silicon CMUT array

A novel hardware design and preliminary experimental results for photoacoustic imaging are reported in this paper. This imaging system makes use of an infrared-transparent capacitive micromachined ultrasonic transducer (CMUT) chip for ultrasound reception and illuminates the image target through the CMUT array. The cascaded arrangement between the light source and transducer array allows for a more compact imager head and results in more uniform illumination. Taking advantage of the low optical absorption coefficient of silicon in the near infrared spectrum as well as the broad acoustic bandwidth that CMUTs provide, an infrared-transparent CMUT array has been developed for ultrasound reception. The center frequency of the polysilicon-membrane CMUT devices used in this photoacoustic system is 3.5 MHz, with a fractional bandwidth of 118% in reception mode. The silicon substrate of the CMUT array has been thinned to 100 μm and an antireflection dielectric layer is coated on the back side to improve the infrared-transmission rate. Initial results show that the transmission rate of a 1.06-μm Nd:Yag laser through this CMUT chip is 12%. This transmission rate can be improved if the thickness of silicon substrate and the thin-film dielectrics in the CMUT structure are properly tailored. Imaging of a metal wire phantom using this cascaded photoacoustic imager is demonstrated.     

Characterization of Grain Size and Yield Strength in AISI 301 Stainless Steel Using Ultrasonic Attenuation Measurements

AISI 301 stainless steel samples were annealed over the temperature range of 800?C1200°C for 60?minutes to produce different grain sizes. These samples were characterized by ultrasonic immersion technique, tensile test, and optical microscopy. The attenuation of ultrasonic waves measured at a frequency of 20?MHz showed a good correlation with average grain size, hardness, and yield strength of AISI 301 stainless steel samples. A?new equation was derived for calculation of yield strength on the basis of ultrasonic wave attenuation and Hall-Petch relation.     

A study of the water sorption kinetic of Poly(HEMA) hydrogels bySLAM and FT-IR measurements

The relative changes in the ultrasonic attenuation measured by scanning laser acoustic microscopy (SLAM) during water sorption in poly-hydroxy-ethil-methacrilate[Poly(HEMA)] are analyzed. Acoustical images performed at 10 and 30 MHz of Poly(HEMA) during water absorption show an ultrasonic attenuation increase followed by a decrease. This phenomenon is analyzed in terms of reflections, scattering, and absorption of the acoustic waves. However, the contribution of scattering and reflection may be neglected and the time dependence of acoustic attenuation is attributed to absorption of the acoustic waves occuring in accordance with the glass transition, although induced by a solvent and not by temperature. Fourier transform infrared spectrometry (FT-IR) measurements indicate that bound water is responsible for the glass-rubber transition. On the other hand, the longitudinal velocity changes are only affected by the variations of density and mechanical properties of the polymer during water sorption. The traditional weight gain data are compared with the attenuation changes observed by ultrasonic measurements     

High-temperature and broadband immersion ultrasonic probes

Immersion ultrasonic probes for measurements and imaging at high temperature are presented. The probes consist of sol-gel-sprayed thick films as piezoelectric ultrasonic transducers (UTs) directly deposited onto steel buffer rods. They operate in pulse-echo mode at temperatures up to 500/spl deg/C. The operating ultrasonic frequency is between 5 MHz and 20 MHz, controlled by the film thickness. The ultrasonic thickness measurement of a steel plate with the probe fully immersed in molten zinc at 450/spl deg/C was demonstrated using ultrasonic plane waves. For imaging purposes, the probing end of the steel buffer rod was machined into a semispherical concave shape to form an ultrasonic lens and achieve high spatial resolution with focused ultrasound in liquids. Ultrasonic surface and subsurface imaging using a mechanical raster scan of the focused probe in silicone oil at 200/spl deg/C was also carried out. The importance of the signal-to-noise ratio (SNR) in the pulse-echo measurement is discussed.     

An Active Fibre-optic Interferometric Sensor Based on a Low Finesse Fibre-optic Fabry-Perot

Abstract

We demonstrate a novel active fibre-optic interferometric sensor based on a temperature-stabilized low-finesse fibre-optic Fabry-Perot. In this apparatus, information on the optical phase was extracted from the frequency shift of the He-Ne laser of which the cavity length was thermally tuned to compensate change in the interference signal. It is shown that an improvement in the stability of the interference signal by two orders of magnitudes can be realized. The tracking achievable between the Fabry-Perot interferometer and the laser was better than 1 part in 10⁸. That is, the relative frequency stability of the laser was ? 1·0 × 10^?8 over an interval of tens of minutes. Detection of a change of 5 nm in the optical path length of the fibre-optic Fabry-Perot was also demonstrated. The dynamic range of this interferometer is over 40 dB and the linearity characteristics are excellent. These performance characteristics were limited by the long-term thermal drift of the apparatus, not the low reflectivity of the fibre ends of the Fabry-Perot interferometer.     

Noncontact detection of ultrasonic waves using fiber optic Sagnac interferometer

This paper describes a fiber optic sensor suitable for noncontact detection of ultrasonic waves. This sensor is based on the fiber optic Sagnac interferometer, which has a path-matched configuration and does not require active stabilization. Quadrature phase bias between two interfering laser beams in the Sagnac loop is applied by controlling the birefringence using a fiber polarization controller. A stable quadrature phase bias can be confirmed by observing the interferometer output according to the change of phase bias. Additional signal processing is not needed for the detection of ultrasonic waves using the Sagnac interferometer. Ultrasonic oscillations produced by conventional ultrasonic piezoelectric transducers were successfully detected, and the performance of this interferometer was investigated by a power spectrum analysis of the output signal. Based on the validation of the fiber optic Sagnac interferometer, noncontact detection of laser-generated surface waves was performed. The configured Sagnac interferometer is very effective for the detection of small displacement with high frequency, such as ultrasonic waves used in conventional nondestructive testing (NDT)     

Sagnac-type fiber-optic array sensor for detection of bulk ultrasonic waves

In this paper, we describe a fiber optic array sensor suitable for detection of bulk ultrasonic waves. This sensor is based on an intrinsic fiber optic Sagnac interferometer. The fiber array is formed by multiple folding of a continuous length of an optical fiber into flat coils. Depending on the orientation of the fiber array with respect to the ultrasonic wave, the proposed sensor can act as a conventional in-phase detector or as a narrowband detector. In the narrowband mode, the center frequency of detection can be tuned by adjusting the spacing of the fiber array elements to be equal to the ultrasonic wavelength of interest. This feature distinguishes this array sensor from conventional hydrophones in which a receiver is typically much smaller than the acoustical wavelength. It is shown that the array sensor provides an enhanced signal-to-noise ratio (SNR) compared with a single element detection scheme. Results are presented for detection of ultrasonic waves in water arising from both piezoelectric and laser ultrasonic sources. Potential areas of application of this sensor include process monitoring, smart structures, bio-medical ultrasound, and chemical sensing.     

A nondestructive integrity test for membrane filters based on air-coupled ultrasonic spectroscopy

This work describes the application of an ultrasonic air-coupled characterization technique to membrane filters. Coefficient of transmission of sound at normal incidence through each membrane in the frequency range 0.55 MHz-2.4 MHz was measured. For all cases, at least one thickness resonance was observed. From these measurements density, velocity, and attenuation of ultrasonic longitudinal waves are calculated and compared to available filtration data such as water flux measurements and bubble point data, both provided by manufacturers. Results show that velocity of ultrasonic waves in membrane filters depends on the membrane grade and can be correlated to filtration properties; attenuation per wavelength is independent of membrane grade but sensitive to moisture content. Advantages of this technique over other conventional membrane tests are pointed out.     

A Fiber Optic Ultrasonic System to Monitor the Cure of Epoxy

A fiber optic ultrasonic system is described which monitors the cure of an epoxy resin. Ultrasound is generated using a high-power optical fiber to deliver high-energy pulses of light to the prepared surface of an aluminum mold that contains the curing epoxy resin. The generated ultrasound is detected using a local fiber optic ultrasound sensor embedded in the curing epoxy resin. The system was used to measure the ultrasonic signal velocity and ultrasonic attenuation throughout the cure of a neat epoxy resin at room temperature. Similar measurements also were performed using a piezoelectric transducer for ultrasound generation and an embedded fiber optic sensor for detection, which provided verification of the results using the complete fiber optic system. The complete fiber optic system demonstrated adequate sensitivity throughout the entire cure to measure the ultrasonic signal velocity and ultrasonic attenuation.