WO1998038476A1 - Procede et dispositif permettant de mesurer les images de vibrations optiques photorefractives - Google Patents

Procede et dispositif permettant de mesurer les images de vibrations optiques photorefractives Download PDF

Info

Publication number
WO1998038476A1
WO1998038476A1 PCT/US1998/003862 US9803862W WO9838476A1 WO 1998038476 A1 WO1998038476 A1 WO 1998038476A1 US 9803862 W US9803862 W US 9803862W WO 9838476 A1 WO9838476 A1 WO 9838476A1
Authority
WO
WIPO (PCT)
Prior art keywords
wavefront
vibration
image
modulated
vibrating
Prior art date
Application number
PCT/US1998/003862
Other languages
English (en)
Inventor
Kenneth L. Telschow
Vance A. Deason
Thomas C. Hale
Original Assignee
Lockheed Martin Idaho Technologies Company
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Lockheed Martin Idaho Technologies Company filed Critical Lockheed Martin Idaho Technologies Company
Priority to AU65382/98A priority Critical patent/AU6538298A/en
Publication of WO1998038476A1 publication Critical patent/WO1998038476A1/fr

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01H—MEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES
    • G01H9/00—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means
    • G01H9/002—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means for representing acoustic field distribution
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02041—Interferometers characterised by particular imaging or detection techniques
    • G01B9/02047—Interferometers characterised by particular imaging or detection techniques using digital holographic imaging, e.g. lensless phase imaging without hologram in the reference path

Definitions

  • This invention relates generally to apparatus and methods for characterizing a vibrating image of an object of interest, and more particularly to vibration detection assemblies, methods for detecting vibration of objects of interest, and for performing spectral vibration analysis, such as a device and method which utilize the photorefractive effect in selected materials for the synchronous detection of the dynamic displacement of a vibrating specimen in a manner that produces a full -field image of the vibration displacement without scanning of a small probe over the specimen.
  • An interferometer is an instrument that is well recognized in the art in which light from a source is split into two or more beams which are subsequently reunited after traveling over different paths and which display individual interference patterns.
  • the above-noted techniques typically use pulsed time domain techniques and wide bandwidths, and are usually employed to obtain real-time surface motion under various conditions.
  • Frequency domain continuous measurements are useful in particular applications, such as when performing structural analysis, because frequency domain measurements record the randomly or continuously excited vibrational spectrum of the entire object being analyzed. While the above-described techniques have provided some degree of success, there exist several shortcomings needing resolution. For example, for the case of time domain-based analysis techniques, a significant signal-to-noise ratio improvement can be gained by employing a reduced bandwidth of the measurement as compared to the time domain methods, but at the expense of measurement speed.
  • a speckle reflection is a light phenomenon which accompanies the scattering of coherent light from a surface. Such a phenomenon may occur, for example, when a light source such as a laser hits a roughened or inhomogeneous medium and generates a random- intensity distribution of light that gives the surface or medium a granular appearance. Speckle reflection can be corrected by limiting detection to a single speckle, or by using self-beating interferometers such as a Fabry-Perot interferometer.
  • Some prior art techniques use a photorefractive effect in optically nonlinear materials to detect vibration. Such a method provides an active mechanism for compensation using the spatial and temporal characteristics of photorefractivity. Accordingly, several optical frequency domain measurement methods have been proposed using photorefractive, two- and four-wave mixing, in selected materials. These techniques provide a response that is substantially a nonlinear function of the specimen vibration displacement amplitude (typically, a Bessel function having order zero) and often do not provide a measure of the vibration phase.
  • a major shortcoming of the prior art techniques is their inability to image more than one surface point at a time, wherein only point vibration detection is possible. Furthermore, prior art techniques require a significant amount of computer post processing of scanned point measurements data to produce an image of the vibration displacement of the specimen surface.
  • the object of the present invention is to provide a vastly improved vibration detection assembly and method particularly suited for use with diffusely reflecting surfaces and having a greatly enhanced sensitivity, linear output for small vibration amplitudes (proportional to Bessel function of order one), while simultaneously providing an image of the vibration amplitude over the surface of the specimen while enabling surface imaging.
  • Fig. 1 is a simplified diagrammatic view of a preferred embodiment of the invention illustrating an apparatus for optical lock-in vibration imaging using photorefractive two-wave mixing.
  • Fig. 2 is a greatly simplified diagrammatic view representing the first through sixth (a-f) vibrational mode shapes for a rigidly clamped disc.
  • Fig. 3 is a simplified image representation of the vibration imaging measurement results showing the first through sixth (a-f) vibrational mode shapes corresponding to Fig. 2 views a-f of the diffuse reflecting clamped circular plate of Fig. 1.
  • Fig. 4 is an intensity profile taken through the center in a horizontal direction of the second mode shape image, Fig. 3b, and demonstrating direct displacement amplitude and phase detection.
  • Fig. 5 is a simplified diagrammatic view of an alternative embodiment of the invention illustrating an apparatus for optical lock-in vibration imaging using photorefractive four-wave mixing.
  • An apparatus and method are disclosed for performing noncontacting measurements that characterize a vibrating image of an object of interest.
  • the measurements characterize the surface displacement of a vibrating object in a manner that provides an image showing the vibration displacement of all points on the surface with a single measurement.
  • the method includes utilizing a source of coherent light that is split into two beams, object and reference. The object beam is reflected off the vibrating surface, where it is phase modulated by the vibrating surface at the mechanical vibration frequency and phase.
  • the reference beam is optically phase modulated by suitable electrical, acoustical, magnetic or other optical means, at a fixed modulation depth and frequency that is substantially a fixed difference from the vibration frequency that is to be measured.
  • the modulated object and reference beams are interfered inside a photorefractive material producing a three-dimensional interference pattern.
  • This interference volume hologram induces a space charge electric field distribution that produces, through electro-optic effects, an index of refraction volume hologram or Bragg grating in direct correspondence with the specimen vibration displacement amplitude and phase.
  • the index of refraction volume grating varies at the fixed difference frequency between the object frequency and the modulated reference frequency.
  • the index of refraction volume grating is produced by interfering beams whose frequencies may vary, while the difference between the frequencies is maintained fixed, the grating is always of the same spatial and temporal form within the photorefractive material for all object vibration frequencies above the photorefractive response cutoff frequency.
  • Either four-wave or two-wave coupling methods can be used to read the induced grating, producing an output beam whose intensity is a function of the specimen vibration mechanical displacement and mechanical phase.
  • Anisotropic self-diffraction, with or without polarization rotation, within the photorefractive material is one of the possible mechanisms that can be used to provide high optical throughput and selectivity for the vibration signal component of the output beam.
  • This vibration component of the output beam consists of a product of the zero and first order Bessel functions of the object vibration displacement amplitude all multiplied by the cosine of the phase difference between the object and reference beam modulations. This produces an output that is linear in the vibration displacement amplitude for amplitudes that are small compared with the optical wavelength. In this manner, a vibration spectrum detector is produced that can record an image of the vibration motion simultaneously over the entire surface of the object without the need for scanning of a point measurement beam.
  • the object beam comprises an object wavefront and the reference beam comprises a reference wavefront. Accordingly, the object wavefront and the reference wavefront cooperate to enable production of an image of a vibrating medium extending in at least two dimensions. According to one implementation detailed below, a beam expander enables generation of a sufficiently expanded object wavefront from the object beam.
  • FIG. 10 A preferred embodiment of the invention is illustrated in the accompanying drawings particularly showing a vibration detection assembly for detecting vibration in an object of interest generally designated with the numeral 10 in Figure 1.
  • Such assembly 10 comprises a vibration imaging device, or vibration spectrum analyzer.
  • such invention comprises a two-wave mixing configuration for reading out a vibration-induced phase grating image, which may include the use of anisotropic self- diffraction, with or without polarization rotation, to improve the signal to noise ratio. Improved sensitivity is provided for diffusely reflecting surfaces, providing a greatly enhanced optical throughput compared to the four-wave method.
  • the object of interest 12 may be all manner of gas, liquid or solid and the vibration to be detected includes the acoustic or ultrasonic motion of the gas, liquid or solid.
  • implementation of the apparatus comprises a method for detecting vibration in an object of interest according to another aspect of this invention.
  • the invention comprises a method for characterizing a vibrating image of an object of interest. More particularly, the invention comprises a method for measuring and providing an image of periodic physical variations in an object of interest.
  • the present invention according to vibration detection apparatus 10 is configured to analyze vibration of object of interest 12. More particularly, a vibrating medium such as a surface 14 on object of interest 12 vibrates at a given frequency when driven by an externally applied vibration force.
  • One technique for vibrating a specimen such as object of interest 12 is to apply an excitation via a shaker such as a wide bandwidth piezoelectric transducer that is placed in contact with the specimen, or object of interest, to induce vibrational modes of the object of interest 12.
  • An emitter of coherent light 16 in one configuration a laser such as an Argon laser, emits light having a substantially fixed wavelength and a predetermined luminous output. For example, a luminous output of 514 nanometers and 200 mW can be emitted by laser 16.
  • Light emitter, or laser, 16 generates light that is transmitted along a first beam path generally indicated by reference numeral 20.
  • a beam splitter 18 constructed of any conventional design well known in the art is positioned and oriented along first beam path 20 so as to generate object and reference beams 22 and 24, respectively.
  • Object beam 22, which travels along first beam path 20, has two courses.
  • Such first beam path 20 includes a first course 26 and a second course 28.
  • Reference beam 24, which travels along a second beam path 30, also has two courses.
  • Such second beam path 30 includes a third course 32 and a fourth course 34.
  • Emitter 16 is configured to produce two or more wavefronts, one wavefront comprising object beam 22 and another wavefront comprising reference beam 24. It is understood that object beam 22 comprises an object wavefront that is modulated by a vibrating medium, or surface 14.
  • first course 26 a beam splitter 18 and a beam expander 36 are interposed along first course 26.
  • Beam splitter 18 is formed from a conventional design presently understood in the art. Beam splitter 18 is located to split a beam of light emitted from light emitter 16 so as to form first beam path 20 and second beam path 30.
  • First course 26 extends between light emitter 16 and vibrating specimen 12, whereas third course 32 extends between light emitter 16 and beam splitter 18.
  • first beam path 20 and second beam path 30 are separated by beam splitter 18, wherein third course 32 is reflected via beam splitter 18 to form fourth course 34.
  • first beam path 20 provides for an object beam 22
  • second beam path 30 provides for a reference beam 24
  • a third beam path 35 provides for output beam 37 produced from a two- wave mixing process of this invention that is discriminated via anisotropic self-diffraction, with polarization rotation, which produces a rotation in the linear polarization of the diffracted reference beam with respect to that of the transmitted object beam.
  • the anisotropic self-diffraction can be implemented without polarization rotation.
  • Beam expander 36 is interposed along first course 26 and is operative to expand object beam 22 prior to being directed onto surface 14 of the vibrating specimen of the object of interest 12.
  • Object of interest 12 reflects the expanded object beam 22 extending along first course 26 from a diffusely reflecting surface of the specimen ' undergoing continuous vibration to second course 28.
  • Beam expander 36 is formed from a conventional design presently understood in the art.
  • object of interest 12 is located in a predetermined location to reflect the object beam 22, traveling along the first course 26, in the direction of second course 28.
  • the expanded object beam 22 travels along the second course 26, reflects off of vibrating surface 14, and travels along the second course 28.
  • a collection lens 40 and a polarizer 42 which cooperate to focus and discriminate image light scattered from the specimen into a sensing media comprising a photorefractive substance, in one form a photorefractive crystal 44.
  • the sensing media is configured to receive in combination the object beam 22 and reference beam 24.
  • Object beam 22 and reference beam 24 are received in the form of a reflected object wavefront and a modulated beam, such as a modulated reference wavefront.
  • collection lens 40 comprises an optical lens configured to focus object beam 22, along second course 28, onto a desired location of photorefractive crystal 44. Accordingly, collection lens 40 is positioned to image light scattered from surface 14 of specimen 12 in the photorefractive crystal 44. Collection lens 40 is formed from a conventional design presently understood in the art.
  • polarizer 42 comprises an input high extinction ratio polarizer configured to select, or discriminate, one component of the object beam 22 reflected from surface 14 of specimen 12.
  • Polarizer 42 is formed from a conventional design presently understood in the art.
  • Polarizer 42 is configured to select one component of the object beam 22 reflected from specimen 12.
  • object beam 22 Upon being reflected off of surface 14 of vibrating object of interest 12, object beam 22 has been impressed with information defining the given vibration displacement amplitude and vibration phase of object 12.
  • object beam 22 is combined to interfere with reference beam 24 within photorefractive substance 44, here a bismuth silicon oxide (BSO) crystal, via a two-wave anisotropic self-diffraction, with or without polarization rotation, approach.
  • photorefractive substance 44 here a bismuth silicon oxide (BSO) crystal
  • object and reference beams are mutually coherent so as to interfere within the photorefractive material.
  • An electro-optic phase modulator 38 operates on reference beam 24 to produce a modulated reference beam.
  • the phase modulated reference beam and the reflected object beam 22 interfere within and pass through the photorefractive substance 44 to create a space charge field having a magnitude which is directly proportional to the vibration displacement amplitude and a cosine function of the vibration phase, for small amplitudes.
  • the space charge field produces an index of refraction grating by the electro-optic effect which contains information of the vibration state of the surface.
  • photorefractive substance 44 has a given response time wherein the induced grating within substance 44 passes reflected object beam 22 and reference beam 24.
  • Object beam 22 and reference beam 24 interfere within photorefractive substance 44 to create a space charge field and resulting induced grating which develop within the response time of the photorefractive substance.
  • Object beam 22 is reflected off of vibrating object of interest 12, having a vibration displacement amplitude and a vibration phase.
  • the photorefractive substance 44 passes the reflected object beam 22 and the reference beam 24 such that their interference therein creates a space charge field induced grating having a diffraction efficiency which is directly proportional to the vibration displacement amplitude and a cosine function of the vibration phase, for small amplitudes.
  • photorefractive substance 44 comprises a sensing media having a detection resolution within a limited bandwidth, wherein reference beam 24 is modulated at a frequency such that a difference frequency between object beam 22 and reference beam 24 is within a response range of the sensing media.
  • the sensing media produces an image of a vibrating surface 14 having an output intensity that is substantially linear with small physical variations of vibrating surface 14.
  • calibration measurements have been implemented using a piezoelectric translation mirror.
  • the excited vibrational modes of the specimen determine the frequency-dependent displacement amplitude-of the surface, which is transferred into phase modulation ⁇ of the object beam.
  • the reference beam is phase modulated by an electro-optic modulator 38 at a fixed modulation depth ⁇ 2 .
  • Modulated beams are then combined and interfere inside a bismuth silicon oxide (BSO) photorefractive crystal 44 at an external angle between the beams of about 45 degrees.
  • BSO bismuth silicon oxide
  • a two-wave mixing configuration was used for read-out of the photorefractive index grating produced within crystal 44.
  • a polarizer 46 and an imaging lens 48 Prior to detecting such forward diffracted beam, discriminate and focus such forward diffracted beam.
  • measurement device 100 comprises a charge coupled device (CCD) or other imaging device operable to image the forward diffracted beam.
  • Polarizer 46 and imaging lens 48 are constructed in the same manner as polarizer 42 and collection lens 40, respectively.
  • polarizer 46 comprises an output high extinction ratio polarizer configured to select, or discriminate (using the anisotropic self-diffraction, with or without polarization rotation, property), the diffracted wavefront at the output of the crystal 44 from the forward transmitted object wave.
  • Polarizer 42 is formed from a conventional design presently understood in the art. Polarizer 42 is configured to select one component of the diffracted wavefront.
  • imaging lens 48 comprises an optical lens configured to focus the diffracted wavefront passed by photorefractive substance 44 onto CCD camera 50 where it is detected. Accordingly, collection lens 40 is positioned to image light diffracted from substance 44 onto detector 50. Collection lens 40 is formed from a conventional design presently understood in the art.
  • an electro-optic modulator 38 is interposed along fourth course 34.
  • Modulator 38 is configured to modulate reference beam 24.
  • EOM electro-optic modulator
  • Electro-optic phase modulator 38 is located to modulate the reference beam at a substantially fixed modulation depth, as will be discussed in greater detail below.
  • Reference beam 24, which has been acted upon by the electro-optic phase modulator 38, emerges as a modulated reference beam which is directed at an angle to photorefractive substance, or crystal, 44.
  • Photorefractive substance 44 is positioned at a given location along both the second course 28 of first beam path 20 and the fourth course 34 of the second beam path 30, where the first and second beam paths 20 and 30 come together.
  • photorefractive substance 44 comprises a crystal of bismuth silicon oxide capable of generating a photorefractive grating at a predetermined Bragg angle.
  • Other photorefractive substances can also be employed with equal success, according to other aspects of the invention.
  • Modulated reference beam 24 is interfered with object beam 22, inside of a bismuth silicon oxide crystal 44 at an external angle of about 45 degrees. This optical interference creates a space charge field which develops, within the response time of same, an index of refraction grating.
  • the index of refraction grating produces through two-wave mixing output beam 37 extending along third beam path 35.
  • the resulting output beam 37 is directed at a photodetector 50 in the form of a CCD camera where it is detected and characterized via graphical representation as a graphic image 54 on display 52.
  • CCD camera 50 produces a signal coupled via a communication line 56 to deliver a graphic image 54 for viewing by a user on a graphics display 52.
  • graphic image can be delivered to a computer containing image processing software capable of selectively acquiring and storing desired images.
  • a two-wave mixing optical lock-in method is implemented via vibration detection apparatus 10 wherein anisotropic self-diffraction is employed so as to provide improved optical throughput and selectivity.
  • the anisotropic self-diffraction can be implemented with polarization rotation.
  • the anisotropic self-diffraction can be implemented without polarization rotation. In order to achieve maximum sensitivity, a significant component of the directly transmitted object beam 22 must be discriminated against.
  • Two-wave mixing comprises use of an optically active photorefractive material or substance 44, such as bismuth silicon oxide (BSO) crystal, which enables discrimination via anisotropic self-diffraction.
  • Anisotropic self-diffraction which is understood in the optics art, produces a rotation in the linear polarization of the diffracted reference beam 24 with respect to that of the transmitted object beam 22.
  • the thickness of photorefractive crystal (PRC) 44 can be adjusted to enhance sensitivity by discriminating against the transmitted object beam.
  • the thickness of crystal 44 is adjusted in order to achieve a 90-degree polarization shift between diffracted reference beam 24 and transmitted object beam 22.
  • Such polarization shift enables elimination of the directly transmitted object beam 22 via the use of high extinction ratio polarizers 42 and 46.
  • the diffracted reference beam is imparted with an intensity that is dependent on the vibration displacement and is temporally modulated at the frequency difference between the mixing waves.
  • Such temporal modulation is analogous to the four-wave mixing case disclosed in Applicant's co-pending Provisional Patent Application Serial No. 60/039,171 co-pending U.S. Patent Application No. 08/787,073.
  • the two-wave mixing case requires that only the output intensity of output beam 37 need be measured in order to obtain both the vibration amplitude and the phase. Hence, direct imaging can be carried out at intensity levels suitable for industrial applications.
  • the photodetector signal can be processed with a conventional electrical lock-in method that is set to a bandwidth of about 1 Hz.
  • a lock-in model can be developed using one-dimensional plane wave coupled mode analysis. Such details are understood in the art and have been described in Applicant's Provisional Application Serial No. 60/039,171, filed on February 26, 1997, incorporated herein by reference.
  • the photorefractive crystal 44 acts as a mixing and low-pass filtering element that provides the benefits of lock-in detection.
  • the space charge field responds only to slowly varying phase modulations occurring within the material response time constant, allowing only terms existing around the difference frequency to have a significant effect on the quantity.
  • the space charge field modulates the local refractive index through the linear electro-optic effect.
  • Such effect creates a diffraction grating within the crystal that contains the low- frequency phase information that is desired.
  • Several methods can be used to read out the diffraction grating, including the four-wave mixing technique disclosed in Applicant's pending U.S. Patent Application Serial No.
  • One suitable experimental setup for vibration detection apparatus 10 of Figure 1 using self-diffraction entails light emitter 16 comprising an Argon laser source at 514 nm, 500 mW. Such laser source is split into two legs, object beam 22 and reference beam 24. Also according to such setup, object of interest 12 comprises a clamped circular plate constructed as described below with reference to Figures 2a-d and 3a-f. Object beam 22 is expanded and reflected from a diffusely reflecting surface 14 of the specimen, or clamped circular plate, 12 while it is forced to undergo continuous vibration. In order to produce a diffuse reflection, the surface of such plate was roughened by sandblasting. A similar technique and polished specimen was utilized with the four-wave measurement apparatus and method of the related art technique.
  • the clamped plate was excited into vibration by a wideband piezoelectric transducer placed in contact with an outer rim of the circular plate. Reflected light scattered from the specimen was then imaged onto a plane in the photorefractive crystal 44 via collection lens 40.
  • modulated reference beam was also expanded and imaged within the photorefractive crystal 44 to produce a volume holographic grating having an external angle between the object and reference beams 22 and 24 of about 45 degrees.
  • Photorefractive crystal 44 was sized 10mm by 10mm by 2.25 mm, with the crystal being cut along the ⁇ 001> and ⁇ 110> directions.
  • FIG. 1 The above-described experimental setup used a rigidly clamped stainless steel circular disc for object of interest 12 of Figure 1.
  • a circular disk has well-known theoretical mode shapes (see Figs. 2a-f) which can be used to show that vibrational spectral measurement can be carried out via a swept network mode.
  • the stainless steel circular disc had a diameter of 19.1 mm and a thickness of 0.79 mm.
  • Vibrational modes of the plate were excited by a wide bandwidth piezoelectric transducer placed in contact with the outer rim of the specimen, producing the first six modes shown in Figures 3a-f.
  • Figures 2a-f depict the corresponding resonance frequencies for these shapes as calculated using an analytical plate wave model, assuming the elastic modulus parameters for stainless steel.
  • Figures 2a-f illustrate a diagrammatic representation of the first (2a) through sixth (2f) vibrational mode shapes for a rigidly clamped disc. Regions of relative phase are shown along with nodal lines and circles. Plus (+) and minus (-) signs denote regions of positive phase relative to regions of negative phase.
  • FIG. 3a-f illustrate exemplary diagrammatic vibration imaging measurements obtained via two-wave mixing with the apparatus 10 of Figure 1 showing the first (Fig. 3a) through sixth (Fig. 3f) vibrational mode shapes of a diffuse reflecting clamped circular plate.
  • Figure 3 shows images of the first six modes for a clamped plate, as obtained via either the four-wave (described below) or two-wave mixing methods of this invention.
  • the presence of nodal lines and circles are clearly defined.
  • the relative phase of vibration displacements are readily distinguishable.
  • frequency, ⁇ is set equal to 2 Hz, between the object beam frequency ( ⁇ ,) and the reference beam frequency ⁇ u ⁇ ) at the respective excitation frequency.
  • Figure 4 illustrates an intensity profile taken through the center of the second mode shape image of Figure 3b, demonstrating a direct displacement amplitude and phase detection.
  • the vibration image of the second resonant mode of the clamped disc is shown as a horizontal intensity profile that demonstrates the ability of the technique to gauge displacement and phase directly.
  • Profile data is shown after background noise was subtracted and smoothing was performed to remove speckle effects.
  • The-above-described photorefractive optical lock-in vibration spectral measurement apparatus and method can use optical synchronous or lock-in detection, including conventional electrical lock-in detection for narrow bandwidth high-sensitivity measurements.
  • Sub-nanometer level detectability has been demonstrated using a point vibration detection method with a 1 Hz bandwidth.
  • Two readout methods are possible, the earlier four-wave mixing technique which was described in Applicant's co-pending U.S. Patent Application Serial No. 08/787,073, and the above-described two-wave mixing technique.
  • the two-wave and four-wave mixing techniques are capable of producing an output intensity that is directly proportional to the amplitude of the vibration being measured for small amplitudes. Furthermore, it can provide the capability for mechanical phase measurement if synchronous excitation is used.
  • output can be coupled to a conventional electrical lock-in amplifier.
  • Imaging has been shown for diffusely scattering surfaces (via the two-wave technique described above), showing detectable displacement in the 5-40 nanometer range. Such method is capable of flat frequency response over a wide range, from the cut-off of the photorefractive effect to the high GHz region, and is applicable to rough surfaces. It is possible to combine the high sensitivity of the point detection method with the full -field information capability of the imaging method through the addition of a sensor array and multi-channel lock-in electronics.
  • the photorefractive substance 44 has a given cut-off frequency
  • the modulated reference beam 24 has a frequency difference which is less than about the cut-off frequency of photorefractive substance 44.
  • the measurement device, or photodetector, 50 is positioned so as to measure both the vibration amplitude and the phase of the object of interest 12.
  • object beam 22, traveling along second course 28, has a given frequency
  • the modulated reference beam 24, traveling along fourth course 34 has a frequency which is offset by an amount of about 25 Hz from the given frequency of the reflected object beam.
  • photorefractive substance 44 comprises a material such as a bismuth silicon oxide
  • the cut-off frequency of such bismuth silicon oxide is about 70 Hz
  • the response time is about 0.015 seconds.
  • the excited vibrational modes of the object of interest 12 determine the frequency-dependent displacement amplitude of the sample surface 14 which, in turn, is transferred into the phase modulation of the object beam 22 which travels along second course 28.
  • reference beam 24 is phase modulated by the electro-optic phase modulator 38 at a fixed modulation depth.
  • the modulated reference beam and the reflected object beam are then combined and interfere inside a photorefractive substance, such as bismuth silicon oxide 44, at an external angle of about 45 degrees.
  • the interference generates a space charge field distribution that mirrors the interference pattern.
  • a two-wave mixing configuration is used for demodulation of the photorefractive interference grating produced within the photorefractive substance 44.
  • the reference beam 24 is directed into the photorefractive substance 44 where it interferes with the expanded, reflected, focused (via lines 40) and polarized (via polarizer 42) object beam 22.
  • the resulting output beam 37 moves along the third beam path 35 and is then analyzed at polarizer 46 and focused by imaging lens 48 and presented at the photodetector of CCD camera 50.
  • the present invention can be employed in different modes.
  • the photorefractive substance 44 acts as a mixing and low pass filtering element.
  • the measured signal intensity which can be calculated for each of these modes, is a time-dependent photorefractive first order response theory in the single grating approximation.
  • the signal excitation and reference modulations are not synchronized, resulting in a strong dependence of the results on the path dependent phase shift ((p, - ⁇ 2 ) .
  • This can be reduced by shifting the measurement to a fixed offset frequency ( ⁇ ⁇ 1/r) .
  • the signal excitation and reference modulation are maintained coherent at a fixed frequency difference ( ⁇ /2/7) .
  • This second mode ensures that the AC, or beat component of the signal beam intensity is always present at the photodetector 50.
  • both the vibration amplitude, and phase of the vibrating object of interest 12 can be directly measured by the light intensity at the photodetector 50. It should be recognized that this method discriminates against static or time varying phase shifts outside the given bandwidth and can be extended to all vibration frequencies above from the reciprocal of the photorefractive response time (67 Hz for BSO).
  • the first preferred embodiment depicted in Figure 1 describes an apparatus and method for imaging vibrations across an object surface by collecting laser light that is scattered from the surface.
  • an optical lock-in detection technique is achieved that provides high noise immunity through narrow bandwidth signal processing.
  • Such an apparatus and method achieves sub-nanometer surface displacement sensitivity in a manner that allows spectral analysis of vibraHon modes of structures.
  • the novelty of this apparatus and method includes using photorefractive materials for optical processing that replaces conventional interferometry apparatus. Additionally, novelty includes the employment of synchronous optical detection in a manner that provides an image of the vibration displacement amplitudes over the surface of the vibrating specimen.
  • the novelty includes an implementation that utilizes the optical interference inside the photorefractive material to provide an output beam intensity that is substantially directly proportional to the vibration displacement amplitude over the specimen surface.
  • the image plane surface comprises a charge-coupled device camera.
  • FIG. 5 A second preferred embodiment of the invention is illustrated in Figure 5 particularly showing another vibration detection assembly for detecting vibration in an object of interest generally designated with the numeral 110.
  • Such invention comprises a four- wave mixing configuration for reading out a vibration-induced phase grating image, which may include the use of anisotropic self- diffraction, with or without polarization rotation, to improve the signal to noise ratio.
  • vibration detection apparatus 110 is configured to analyze vibration of object of interest 12 similar to apparatus 10 (of Fig. 1) but by implementing a four-wave vibration detection technique.
  • Surface 14 on object of interest 12 forms a vibrating medium that vibrates at a given frequency when driven by a vibration force.
  • Light emitter 16, constructed as described with reference .to Figure 1, emits light having a substantially fixed wavelength and a predetermined luminous output. For example, a luminous output of 514 nanometers and 200 mW can be emitted by laser 16.
  • light output from light emitter, or laser, 16 generates light that is transmitted along a first beam path generally indicated by reference numeral 120. Such light is then split into two beams, with a beam splitter 118, comprising an object beam 122 and a reference beam 124.
  • Object beam 122 comprises an object wavefront.
  • reference beam 124 comprises a reference wavefront.
  • Beam splitter 118 constructed similar to beam splitter 18 (of Fig. 1), is positioned and oriented along a first beam path 120 so as to generate object and reference beams 122 and 124, respectively.
  • Object beam 122 which travels along first beam path 120, has two courses.
  • Such first beam path 120 includes a first course 62 and a second course 64.
  • Reference beam 124 which travels along a second beam path 130, also has two courses.
  • Such second beam path 130 includes a third course 66 and a fourth course 68.
  • object beam 122 is phase modulated by vibrating surface 14, and reference beam 124 is phase modulated by a suitable modulator 38.
  • modulator 38 comprises an electro-optic modulator.
  • Object beam 122, reflected from vibrating specimen 12 (object of interest) is recombined within a photorefractive substance 44 along with reference beam 124 to produce an interference pattern.
  • Such interference pattern is produced in a manner similar to that produced in the two-wave approach described above with reference to Figure 1.
  • Photorefractive substance 44 Positioned at a given location along both the second course 64 of the first beam path 120, and the fourth course 68 of the second beam path 130, is photorefractive substance 44.
  • Photorefractive substance 44 forms a sensing media that is similar to that used in the embodiment depicted in Figure 1, but having a photorefractive grating at a predetermined Bragg angle.
  • Modulated reference beam 124 passes through photorefractive substance 44 at a given external angle to form a conjugate signal beam 70 which is directed along a third beam path 71.
  • Third beam path 71 has a first course 72 and a second course 74.
  • a mirror 60 is positioned in a given location to reflect the light from the interference pattern in a counter-propagating path along a fifth course 69.
  • a second beam splitter 218 is positioned between the first and second courses 72 and 74, the second course being directed at a photodetector, such as an imaging camera, 50 which is located at the termination of second course 74.
  • the resulting interference pattern has an interference intensity distribution that produces a corresponding space charge distribution that creates an index of refraction grating through electro-optic effects.
  • photorefractive substance 44 Due to response time, photorefractive substance 44 has a given cutoff frequency, and the modulated reference beam 124 has a frequency difference which is less than about the cutoff frequency of the photorefractive substance 44.
  • Imaging device 50 is configured to measure both the vibration amplitude and the phase of the object of interest 12.
  • the fourth and fifth courses 68 and 69 are counter- propagating, with fourth course 68 scattering from the grating of the photorefractive substance 44 to form conjugate signal beam 70.
  • reflected object beam 122 traveling along second course 64 has a given frequency
  • reference beam 124 is modulated to have a frequency which is offset by an amount of about 25 Hz from the given frequency of the reflected object beam.
  • the cutoff frequency is about 70 Hz and the response time is about 0.015 seconds.
  • Modulated reference beam 124 and reflected object beam 122 are combined and interfere inside photorefractive substance 44, here bismuth silicon oxide, at an external angle of about 55 degrees.
  • photorefractive substance 44 here bismuth silicon oxide
  • demodulation of the photorefractive interference grating is produced within photorefractive substance 44.
  • Reference beam 124 interacts with the grating and produces an output beam that is retro-reflected back into photorefractive substance 44 via mirror 60, along the same path.
  • the retro-reflected beam extends along fifth course 69 to become a probe beam that reads the vibration information by diffracting off the grating and traveling along the original object beam path, in an opposite direction corresponding to third beam path 71.
  • Fifth course 69 is configured to match the Bragg angle of the photorefractive grating produced in photorefractive substance 44.
  • the resulting scattered wave or conjugate signal beam 70 moves along third beam path 71 and is then sampled at second beam splitter 218 and deflected toward imaging device 50.
  • Second beam splitter 218 comprises any suitable beam splitter, including polarized or unpolarized, and is used to split off the signal beam for recording with such suitable imaging device, or camera; for example, a CCD camera.
  • the imaging device comprising CCD camera 50 is coupled with display 52, both directly and by way of image processing apparatus 58.
  • image processing apparatus 58 is configured to enable post-processing of an image that is collected via camera 50 so as to enhance identification of distinguishing visual characteristics representative of vibration modes occurring on surface 14. Such enhanced signals are then output to display 52. Alternatively, such signals can be output to a computer, printer and/or other peripheral device for evaluation, record-keeping, and analysis by a user. For the case where camera 50 directly outputs a signal to display 52, such signal can be used by an observer to visually identify vibration modes as they are detected from a diffusely reflecting surface 14 of object 12.
  • camera 50 comprises a detector configured to detect the image of a vibrating surface wherein the reflected object beam 122 and the modulated reference beam 124 are combined in association with the sensing media, or photorefractive substance 44.
  • Object beam 122 and reference beam 124 interfere and produce simultaneous vibration measurements distributed over object 12 so as to produce an image of the vibration.
  • Display 52 and/or image processing apparatus 58 further aid in visual identification of the detected image of vibration.
  • object beam 122 comprises one wavefront and reference beam 124 comprises another wavefront. The one wavefront and another wavefront are combined in association with the sensing media, or photorefractive material 44, to interfere and produce simultaneous vibration, measurements distributed over object 12.
  • Such interference produces an image of the vibrating medium, or surface 14, having an output intensity that is substantially linear with small physical variations within the vibrating medium.
  • Camera 50 then aids in detecting such image.
  • a single beam or wave can be reflected off of an object, with the reflected wave being split by a beam splitter so as to form a first wave and a second wave.
  • the first wave can be modulated via a modulator so as to produce a reference wavefront, or beam, which is input into a sensing media.
  • the second wave is split off in the beam splitter so as to be directly input into the sensing media.
  • the reference wavefront, or beam, and the object wavefront, or beam are each derived from a single, common wave.
  • Such an implementation can be realized in a two-wave or a four-wave configuration.
  • any of a number of modulation schemes can be utilized with this invention relative to a reference image. Accordingly, an image can be derived directly from an object as a reference image. Alternative ways of modulating such a reference image include phase, amplitude, polarization, spatially or temporally modulating such reference image. Furthermore, the rate of change of interference patterns within a sensing media can be maintained within the response capability of the sensing media, irrespective of the object image and reference image modulations. Such characteristic allows the sweeping of such modulations throughout a large spectral range so as to provide imaging spectral analysis of an object's vibrations without exceeding the response capabilities of a sensing media, or medium. According to such implementations, the intensity of the output image at every point is substantially linearly related to the vibration amplitude at the corresponding point of the vibrating medium, for small motions, or low amplitudes.
  • periodic physical variations in an object can be measured and imaged, including but not limited to sonic or ultrasonic vibrations, where the rate of detected signal modulation is maintained within a fixed bandwidth by suitable modulation control of the sensing wavefronts or beams.
  • the above-described implementations each include two or more separate waves that are sufficiently mutually coherent to produce interference.
  • the interfering separate waves are superimposed or mixed within a suitable sensing media.
  • An imaging device such as an object beam collection lens, is used to create an image of the object on or within the sensing media in order to provide a means for imaging the sensed data.
  • At least one of the beams, or wavefronts is modulated by interacting with an object that is undergoing some oscillatory change in physical condition.
  • a reference beam is synchronously, or asynchronously, modulated in a similar manner.
  • such reference beam is imparted with a fixed offset.
  • the sensing media then undergoes a detectable modification in its properties by responding to the interference pattern that is generated by mixing the object and reference beams.
  • the interference pattern then generates an image of the vibrating medium.
  • the difference in modulation rate between the reference and object beams is controlled and fixed such that the- .beams produce an interference pattern whose spatial distribution in the sensing media is varying at a rate within the response time of the media.
  • the sensing media always records a signal within its bandwidth, independent of the actual modulation rate of the object or reference beams.
  • the sensing media contains an image of the entire visible object
  • the resulting data consists of a full field of two-dimensional image of the distribution of the physical variation in the object that is being measured. Accordingly, the sensing media produces an output beam indicative of the vibration of the object. In certain implementations, an image is produced inside the sensing media.

Landscapes

  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Remote Sensing (AREA)
  • Investigating Or Analysing Materials By Optical Means (AREA)

Abstract

L'invention concerne un procédé et un appareil permettant de caractériser l'image vibrante (54) d'un objet (12). Ce procédé utilise également deux ou plusieurs fronts d'onde, au moins un de ces fronts d'onde (26) étant modulé par son interaction avec l'élément vibrant (14) de l'objet (12). L'autre front d'onde (30) est modulé de manière à ce que la fréquence différence entre les premier (26) et second (30) fronts d'onde se situe dans la plage de réponse du dispositif de mesure (44). Ces deux fronts d'onde (26, 30) modulés sont associés à ce dispositif de mesure (44) pour interférer l'un avec l'autre, et produire simultanément des mesures de vibration qui sont ensuite réparties dans l'objet (12), de manière à produire une image (54) de l'élément vibrant (14). Cette image (54) présente une intensité de sortie sensiblement linéaire par rapport aux faibles variations physiques à l'intérieur dudit élément vibrant (14). Dans un mode de réalisation, l'appareil comprend un analyseur (10) de spectre de vibrations présentant un émetteur (16), un modulateur (38), un dispositif de mesure (44) et un détecteur (50), ces composants étant configurés de manière à pouvoir mettre en oeuvre ledit procédé.
PCT/US1998/003862 1997-02-26 1998-02-25 Procede et dispositif permettant de mesurer les images de vibrations optiques photorefractives WO1998038476A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU65382/98A AU6538298A (en) 1997-02-26 1998-02-25 Imaging photorefractive optical vibration measurement method and device

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US3917197P 1997-02-26 1997-02-26
US60/039,171 1997-02-26

Publications (1)

Publication Number Publication Date
WO1998038476A1 true WO1998038476A1 (fr) 1998-09-03

Family

ID=21904045

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/US1998/003862 WO1998038476A1 (fr) 1997-02-26 1998-02-25 Procede et dispositif permettant de mesurer les images de vibrations optiques photorefractives

Country Status (2)

Country Link
AU (1) AU6538298A (fr)
WO (1) WO1998038476A1 (fr)

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1012536A1 (fr) * 1998-07-08 2000-06-28 Bechtel BWXT Idaho, LLC Appareil et procede de mesure et de visualisation des ondes progressives
WO2001073373A1 (fr) * 2000-03-24 2001-10-04 Optonor As Procede de mesure de vibration et interferometre
US7684103B2 (en) 2006-05-03 2010-03-23 Gentex Corporation Controllably dissolving spacing member and associated electrochromic device and method for manufacturing the same
US8885242B2 (en) 2012-01-17 2014-11-11 Gentex Corporation Variable transmission electrochromic window and associated aircraft window system
US8928966B1 (en) 2012-08-08 2015-01-06 Gentex Corporation Electrochromic polyelectrolyte gel medium having improved creep resistance and associated electrochromic device
EP3504524A4 (fr) * 2016-08-29 2020-04-29 Elbit Systems Land and C4I Ltd. Détection optique de vibrations
CN111323375A (zh) * 2018-12-17 2020-06-23 波音公司 用于确定物体的结构特性的方法和系统
US11371878B2 (en) 2016-08-29 2022-06-28 Elbit Systems Land And C4I Ltd. Optical detection of vibrations

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4836658A (en) * 1987-04-21 1989-06-06 The General Electric Company, P.L.C. Optical devices for forming image and reference beams with a polarizing beamsplitter and a spatial light modulator
US5037203A (en) * 1985-10-10 1991-08-06 Rockwell International Corporation Asymmetric ring laser gyroscope and method for detecting rotation with acentric photorefractive crystal
US5131748A (en) * 1991-06-10 1992-07-21 Monchalin Jean Pierre Broadband optical detection of transient motion from a scattering surface by two-wave mixing in a photorefractive crystal
US5335548A (en) * 1992-06-19 1994-08-09 The United States Of America As Represented By The Department Of Energy Non-linear optical crystal vibration sensing device
US5680212A (en) * 1996-04-15 1997-10-21 National Research Council Of Canada Sensitive and fast response optical detection of transient motion from a scattering surface by two-wave mixing

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5037203A (en) * 1985-10-10 1991-08-06 Rockwell International Corporation Asymmetric ring laser gyroscope and method for detecting rotation with acentric photorefractive crystal
US4836658A (en) * 1987-04-21 1989-06-06 The General Electric Company, P.L.C. Optical devices for forming image and reference beams with a polarizing beamsplitter and a spatial light modulator
US5131748A (en) * 1991-06-10 1992-07-21 Monchalin Jean Pierre Broadband optical detection of transient motion from a scattering surface by two-wave mixing in a photorefractive crystal
US5335548A (en) * 1992-06-19 1994-08-09 The United States Of America As Represented By The Department Of Energy Non-linear optical crystal vibration sensing device
US5680212A (en) * 1996-04-15 1997-10-21 National Research Council Of Canada Sensitive and fast response optical detection of transient motion from a scattering surface by two-wave mixing

Cited By (12)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1012536A1 (fr) * 1998-07-08 2000-06-28 Bechtel BWXT Idaho, LLC Appareil et procede de mesure et de visualisation des ondes progressives
EP1012536A4 (fr) * 1998-07-08 2002-01-02 Bechtel Bwxt Idaho Llc Appareil et procede de mesure et de visualisation des ondes progressives
WO2001073373A1 (fr) * 2000-03-24 2001-10-04 Optonor As Procede de mesure de vibration et interferometre
JP2003529063A (ja) * 2000-03-24 2003-09-30 オプトノール アーエス 振動測定方法及び干渉計
US6970251B2 (en) 2000-03-24 2005-11-29 Optonor As Method for vibration measurement and interferometer
US7684103B2 (en) 2006-05-03 2010-03-23 Gentex Corporation Controllably dissolving spacing member and associated electrochromic device and method for manufacturing the same
US8885242B2 (en) 2012-01-17 2014-11-11 Gentex Corporation Variable transmission electrochromic window and associated aircraft window system
US8928966B1 (en) 2012-08-08 2015-01-06 Gentex Corporation Electrochromic polyelectrolyte gel medium having improved creep resistance and associated electrochromic device
EP3504524A4 (fr) * 2016-08-29 2020-04-29 Elbit Systems Land and C4I Ltd. Détection optique de vibrations
US11371878B2 (en) 2016-08-29 2022-06-28 Elbit Systems Land And C4I Ltd. Optical detection of vibrations
CN111323375A (zh) * 2018-12-17 2020-06-23 波音公司 用于确定物体的结构特性的方法和系统
CN111323375B (zh) * 2018-12-17 2024-03-26 波音公司 用于确定物体的结构特性的方法和系统

Also Published As

Publication number Publication date
AU6538298A (en) 1998-09-18

Similar Documents

Publication Publication Date Title
US6134006A (en) Imaging photorefractive optical vibration measurement method and device
CA2302775C (fr) Appareil et procede de mesure et de visualisation des ondes progressives
US5827971A (en) Optical vibration detection spectral analysis assembly and method for detecting vibration in an object of interest
US4968144A (en) Single beam AC interferometer
JPH0769153B2 (ja) 被検体の非破壊的解析方法及び装置
JPH10503592A (ja) 物体の振動状態を測定する方法
US8179535B2 (en) Remote sensing of underwater acoustic fields
Tiziani et al. From speckle pattern photography to digital holographic interferometry
Telschow et al. Direct imaging of traveling Lamb waves in plates using photorefractive dynamic holography
US5477324A (en) Method and apparatus for detecting surface wave vector dynamics using three beams of coherent light
WO1998038476A1 (fr) Procede et dispositif permettant de mesurer les images de vibrations optiques photorefractives
US6401540B1 (en) Method and apparatus for detecting internal structures of bulk objects using acoustic imaging
US6486962B1 (en) Method and apparatus for assessing material properties of sheet-like materials
Reibold et al. Laser interferometric measurement and computerized evaluation of ultrasonic displacements
Joyeux et al. Real time measurement of angström order transverse displacement or vibrations, by use of laser speckle
Stoffregen et al. Scanning laser Doppler vibration analysis system
Pedersen et al. Detection of non-sinusoidal periodic vibrations using phase-modulated TV-holography
Lal et al. Multiple-beam LDV system for buried landmine detection
Halliwell et al. Vibration measurement techniques using laser technology: laser vibrometry and TV holography
US4270388A (en) Method and apparatus employing zero order Raman-Nath diffraction information to visualize longitudinal character of an acoustic wave field
Köhler Stroboscopic visualization of ultrasonic fields on solids by sampling
Telschow et al. Imaging of lamb waves in plates for quantitative determination of anisotropy using photorefractive dynamic holography
WO2000014477A1 (fr) Procede et appareil d'evaluation des proprietes materielles de materiaux en feuilles
Telschow et al. Material property measurement of metallic parts using the INEEL laser ultrasonic camera
Martarelli et al. 3.1 Single-Point Vibrometer

Legal Events

Date Code Title Description
AK Designated states

Kind code of ref document: A1

Designated state(s): AL AM AT AU AZ BA BB BG BR BY CA CH CN CU CZ DE DK EE ES FI GB GE GH GM GW HU ID IL IS JP KE KG KP KR KZ LC LK LR LS LT LU LV MD MG MK MN MW MX NO NZ PL PT RO RU SD SE SG SI SK SL TJ TM TR TT UA US UZ VN YU ZW

AL Designated countries for regional patents

Kind code of ref document: A1

Designated state(s): GH GM KE LS MW SD SZ UG ZW AM AZ BY KG KZ MD RU TJ TM AT BE CH DE DK ES FI FR GB GR IE IT LU MC NL PT SE BF BJ CF CG CI CM GA GN ML MR NE SN TD TG

DFPE Request for preliminary examination filed prior to expiration of 19th month from priority date (pct application filed before 20040101)
121 Ep: the epo has been informed by wipo that ep was designated in this application
REG Reference to national code

Ref country code: DE

Ref legal event code: 8642

NENP Non-entry into the national phase

Ref country code: JP

Ref document number: 1998537895

Format of ref document f/p: F

122 Ep: pct application non-entry in european phase