JP5095289B2 - Interference fringe stabilization device and non-destructive inspection device using the same - Google Patents

Description

  The present invention relates to an interference fringe stabilization device and a nondestructive inspection device using the same.

  Conventionally, as this type of nondestructive inspection device, a laser ultrasonic remote sensing device capable of inspecting the presence / absence or depth of an internal defect of an inspection object such as a concrete structure or a metal structure has been proposed ( For example, see Patent Document 1).

  Here, as shown in FIG. 7, the nondestructive inspection apparatus disclosed in Patent Document 1 includes a detection laser 11 that outputs signal light composed of continuous output laser light, and an elastic wave ( The ultrasonic wave excitation laser 21 that outputs excitation light composed of pulsed laser light for exciting (ultrasonic waves), and the detection laser 11 disposed on the optical path between the detection laser 11 and the inspection object Ob. A polarization beam splitter 104 that branches a part of the output signal light as reference light, and a reference light that is output from the detection laser 11 and reflected by the inspection object Ob and frequency-modulated, and the reference beam branched by the polarization beam splitter 104. A photorefractive crystal 109 constituting an interferometer that is incident so that light intersects at a predetermined angle, and a beam splitting beam emitted from the photorefractive crystal 109. 112 includes two photodetectors 113a and 113b that detect the interference light branched off at 112, and an oscilloscope 114 that records the outputs of the photodetectors 113a and 113b. The oscilloscope 114 provides the outputs of the photodetectors 113a and 113b. On the other hand, an internal defect of the inspection object Ob is detected by performing an appropriate frequency analysis.

  In the non-destructive inspection apparatus described above, a dynamic hologram is formed inside the photorefractive crystal 109 by the reference light and the frequency-modulated signal light, and the reference light is diffracted by the dynamic hologram. Interference light is emitted from the photorefractive crystal 109.

  The non-destructive inspection apparatus disclosed in Patent Document 1 includes a signal light monitor 116 that detects the intensity of signal light emitted from the photorefractive crystal 109 in a direction along the incident direction of the reference light, and a signal light monitor. Based on the output of 116, the density variable filter 102 disposed on the optical path between the detection laser 11 and the polarization beam splitter 104 so that the intensity ratio between the signal light and the reference light is substantially constant. Feedback control means for feedback control of the transmittance, and it is possible to detect internal defects even when the movable mirror 107 is driven to scan the signal light on the surface of the inspection object Ob.

By the way, in the above-described nondestructive inspection apparatus, a high electric field is applied to the photorefractive crystal 109 in order to increase the detection sensitivity by increasing the amount of the reference light diffracted by the dynamic hologram (that is, improving the diffraction efficiency). At the same time, it is conceivable to improve the diffraction efficiency in the photorefractive crystal 109 by providing a phase shift mirror for phase shifting the reference light incident on the photorefractive crystal 109 to shift the phase of the reference light.
Japanese Patent Laying-Open No. 2005-147813 (paragraphs [0022]-[0050], FIG. 1-5)

  However, in the nondestructive inspection apparatus disclosed in Patent Document 1, when the inspection object Ob is located far away from the nondestructive inspection apparatus, the nondestructive inspection apparatus and the inspection object Ob are independent. Even when the inspection object Ob is not irradiated with the excitation light from the elastic wave excitation laser 21, the interference fringe vibrates (moves in a direction in which bright portions and dark portions are alternately arranged). ) The contrast of the interference fringe is deteriorated, and the detection accuracy and detection sensitivity of the internal defect are lowered.

  Further, when the above-described phase shift mirror is provided in the non-destructive inspection apparatus disclosed in Patent Document 1, the displacement amount due to relative vibration between the non-destructive inspection apparatus and the inspection object Ob as compared with the displacement of the phase shift mirror. Therefore, the diffraction efficiency of the photorefractive crystal 109 could not be improved even if the reference light was phase-shifted.

  Further, when the reference light incident on the photorefractive crystal 109 is phase-shifted to improve the diffraction efficiency, a linear phase shift cannot be given to the reference light due to vibration of the nondestructive inspection apparatus.

  The present invention has been made in view of the above reasons, and its purpose is to stabilize interference fringes that can stabilize interference fringes between reference light and signal light that is reflected and frequency-modulated by an inspection object. An object of the present invention is to provide an apparatus and a nondestructive inspection apparatus capable of improving the detection accuracy of an internal defect of an inspection object.

  According to a first aspect of the present invention, there is provided an interference fringe detecting means for detecting an interference fringe between a correction signal light branched from a frequency-modulated signal light reflected from an inspection object and a correction reference light branched from a reference light. A phase shift from a stable state of the interference fringes detected by the interference fringe detection means, and a wave front control mirror disposed on the optical path of the reference light before branching the correction reference light and controlling the wave front of the reference light. And a control means for controlling the position of the wavefront control mirror so as to suppress it.

  According to this invention, the interference fringe detecting means for detecting the interference fringes between the correction signal light branched from the reference light reflected from the inspection object and frequency-modulated, and the reference light branched from the reference light; Suppresses the phase shift from the stable state of the interference fringes detected by the interference fringe detection means and the wave front control mirror that is arranged on the optical path of the reference light before branching the correction reference light and controls the wave front of the reference light Control means for controlling the position of the wavefront control mirror so that the control means suppresses the phase shift from the stable state of the interference fringes detected by the interference fringe detection means. Since the position of the mirror for controlling is controlled, it is possible to prevent the correction reference light and the correction signal light from independently vibrating due to relative vibration between the interference fringe stabilization device and the inspection object. Reference light for correction and correction signal Can suppress the phase shift of the interference fringes between, is reflected by the inspection object and the reference beam makes it possible to stabilize the interference fringes of the frequency-modulated optical signal.

  According to a second aspect of the present invention, there is provided a detection laser that outputs laser light, an elastic wave excitation laser that outputs excitation light for exciting an elastic wave to an inspection object, and a detection laser and an inspection object. A beam branching means for branching a part of the laser light output from the detection laser as a reference light, and a signal light which is output from the detection laser and reflected by the inspection object and frequency-modulated. A vibration detecting unit having a phase conjugate element that interferes with the reference beam branched by the beam branching unit and detecting the vibration of the inspection object using the interference light emitted from the phase conjugate element; A defect detection means for detecting an internal defect of an inspection object based on the output, and the interference fringe stabilization device according to claim 1 are provided.

  According to this invention, since the interference fringe stabilization device according to claim 1 is provided, the reference light and the signal light are uniquely generated due to relative vibration between the nondestructive inspection device and the inspection object. It is possible to prevent vibration and suppress the phase shift of the interference fringes between the reference light and the signal light, and it is possible to improve the detection accuracy of the internal defect of the inspection object existing in the distance.

  According to a third aspect of the present invention, in the second aspect of the present invention, the vibration detecting means is a phase shift mirror that shifts the phase of the reference light, the reference light that is phase shifted by the phase shift mirror, and the frequency reflected by the inspection object. A photorefractive crystal, which is the phase conjugate element that enters the modulated signal light so as to intersect at a predetermined angle, a pair of electrodes for applying an electric field to the photorefractive crystal, and the photorefractive crystal. And a photodetector for detecting interference light.

  According to this invention, the vibration detection means includes a phase shift mirror that shifts the phase of the reference light, and a pair of electrodes for applying an electric field to the photorefractive crystal that is the phase conjugate element. The diffraction efficiency in the photorefractive crystal that is the phase conjugate element can be improved, and the detection sensitivity of internal defects in the defect detection means can be improved.

  The invention of claim 4 is characterized in that in the invention of claim 2 or claim 3, it comprises signal light scanning means for scanning the surface of the inspection object with signal light.

  According to this invention, an internal defect can be inspected over a wide range of the inspection object.

  In the first aspect of the invention, the phase shift of the interference fringes caused by the relative vibration between the interference fringe stabilization device and the inspection object can be suppressed, and the reference light and the signal light that is reflected and frequency-modulated by the inspection object The interference fringes can be stabilized.

  In the invention of claim 2, the phase shift of the interference fringes caused by the relative vibration between the nondestructive inspection apparatus and the inspection object can be suppressed, and the detection accuracy of the internal defect of the inspection object existing far away can be improved. There is an effect of becoming.

  The nondestructive inspection apparatus (laser ultrasonic remote sensing apparatus) of the present embodiment has a configuration as shown in FIG. 1 and is elastic to the detection laser 11 that outputs a laser beam (laser beam) and the inspection object Ob. An elastic wave excitation laser 21 that outputs excitation light for exciting waves (ultrasonic waves) and an optical path disposed between the detection laser 11 and the inspection object Ob and output from the detection laser 11 First beam branching means 12 for branching the reference light from the laser light, signal light output from the detection laser 11 and reflected by the inspection object Ob and frequency-modulated, and branched by the first beam branching means 12. A vibration detecting means 30 that has a phase conjugate element 33 that interferes with the reference light and detects the vibration of the inspection object Ob using the interference light emitted from the phase conjugate element 33; The defect detection means 70 for detecting the internal defect De of the inspection object Ob based on the above, the correction reference light branched from the reference light, and the correction light branched from the frequency-modulated signal light reflected from the inspection object Ob Controls the interference fringe stabilization device 40 for stabilizing the interference fringes with the signal light, the signal light scanning means 50 for scanning the signal light from the detection laser 11 on the surface of the inspection object Ob, and the signal light scanning means 50. And a scanning control device 60 comprising a microcomputer or the like. In addition, the arrow in FIG. 1 has shown the advancing path | route of the laser beam output from the laser 11 for a detection. Further, the nondestructive inspection apparatus of this embodiment is installed on an optical surface plate (not shown) with an air suspension.

  In this embodiment, a concrete structure is assumed as the inspection object Ob, a cavity, a crack, a jumper, or the like is assumed as the internal defect De of the inspection object Ob, and a laser having a wavelength of 532 nm is used as the detection laser 11. Nd: YAG laser (second harmonic) that outputs light continuously is adopted, and Nd: YAG laser (fundamental wave) that outputs pulse laser light with a pulse width of 10 ns is adopted as an elastic wave excitation laser. However, the inspection object Ob and the lasers 11 and 12 are not particularly limited.

  The first beam branching unit 12 described above is configured by a beam splitter, and the signal light scanning unit 50 is configured by a scanning mirror (scanning mirror) 51 and a driving device (scanning device) 52 that drives the scanning mirror 51. Has been.

  In the nondestructive inspection apparatus of this embodiment, two half mirrors 13 and 14 are disposed on the optical path between the first beam branching means 12 and the scanning mirror 51. Here, the half mirror 13 on the side close to the beam branching unit 12 reflects the signal light from the detection laser 11 and transmits the excitation light from the elastic wave excitation laser 21. The half mirror 14 on the side far from the first beam branching unit 12 reflects the signal light and the excitation light, and transmits the signal light that is reflected by the inspection object Ob and frequency-modulated. Further, the nondestructive inspection apparatus of the present embodiment has a second beam composed of a beam splitter that branches the correction signal light from the signal light transmitted through the half mirror 14 on the side opposite to the scanning mirror 51 side in the half mirror 14. A branching means 15 is arranged. A mirror 16 that reflects the correction signal light toward the interference fringe stabilization device 40 is disposed between the second beam branching means 15 and the interference fringe stabilization device 40.

The vibration detection means 30 includes a phase shift mirror 32 that shifts the phase of the reference light, the reference light phase-shifted by the phase shift mirror 32, and the signal light that is reflected by the inspection object Ob and frequency-modulated, at a predetermined angle. A phase conjugate element 33 made of a photorefractive crystal (eg, Bi ₁₂ SiO ₂₀ crystal) incident so as to intersect with each other, a pair of electrodes 34 and 34 for applying an electric field to the phase conjugate element 33, and a pair of electrodes A power supply circuit (not shown) for applying a voltage between 34 and 34 and a photodetector 35 formed of a photomultiplier tube for detecting interference light emitted from the phase conjugate element 33 are provided. The phase conjugate element 33 is not limited to a photorefractive crystal, and may be any non-linear optical element in which a phase conjugate phenomenon occurs and two-wave mixing occurs.

Here, when a photorefractive crystal made of Bi ₁₂ SiO ₂₀ crystal is used as the phase conjugate element 33, as shown in FIG. 2, the electric field strength E is increased and the intersection of the incident signal light and the reference light is made. The smaller the angle, the higher the gain (two-wave mixing gain). The phase conjugate element 33 increases the intensity of the interference light by shifting (delaying) the phase of the reference light by the phase shift mirror 32. In short, in the nondestructive inspection apparatus of this embodiment, the vibration detection means 30 has a pair of electrodes for applying an electric field to the phase shift mirror 32 that shifts the phase of the reference light and the photorefractive crystal that is the phase conjugate element 33. 34 and 34, the diffraction efficiency of the photorefractive crystal as the phase conjugate element 33 can be improved, and the detection sensitivity of the internal defect in the defect detection means 70 can be improved.

  The defect detection means 70 records the output of the photodetector 35 and performs an inspection based on the result of the frequency analysis performed by the FFT operation unit of the oscilloscope and the oscilloscope having an FFT operation unit that performs frequency analysis by FFT (Fast Fourier Transform). The signal processing unit includes a microcomputer that evaluates the presence / absence and depth of an internal defect of the object Ob. Here, the defect detection means 70 performs frequency analysis of the vibration spectrum of the surface of the inspection object Ob corresponding to the depth and size of the internal defect De of the inspection object Ob, and evaluates the presence / absence and depth of the internal defect De. The function of detecting the internal defect De by applying the principle of the vibration spectrum method, and the presence / absence and depth of the internal defect De are evaluated based on the time difference from the irradiation of the laser beam to the inspection object Ob until the multiple reflection echo is detected. It has a function of detecting the internal defect De by applying the principle of the reflection echo method.

  Here, in the case of applying the vibration spectrum method, when the vibration spectrum of the surface of the inspection object Ob is obtained, a characteristic peak does not exist in the frequency spectrum in a portion where there is no internal defect De, but there is an internal defect De. In this case, since a characteristic peak (peak of the vibration frequency of the primary mode) appears in the frequency spectrum according to the depth of the internal defect De, the vibration frequency of the primary mode and the depth of the internal defect De appear in the signal processing unit. If the relationship table is stored in advance, the signal processor can evaluate the presence and depth of the internal defect De. Further, when the multiple reflection echo method is applied, the frequency spectrum of the longitudinal multiple reflection echo differs depending on the depth of the internal defect De of the inspection object Ob. The depth of the internal defect De can be determined based on the velocity of the longitudinal wave in the object Ob.

  The interference fringe stabilization device 40 detects interference fringes between the correction signal light branched from the signal light reflected by the inspection object Ob and frequency-modulated and the correction reference light branched from the reference light. Means 41, a wavefront control mirror 42 that is disposed on the optical path of the reference light before branching the correction reference light and controls the wavefront of the reference light, and a stable state of interference fringes detected by the interference fringe detection means 41 Control means 43 for controlling the position of the wavefront control mirror 42 so as to suppress the phase shift from the. Here, the wavefront control mirror 42 is disposed on the optical path between the first beam branching means 12 and the phase shift mirror 32 of the vibration detecting means 30. However, between the wavefront control mirror 42 and the vibration detecting means 30, the third beam branching means 17 comprising a beam splitter for branching the reference light for correction from the reference light is arranged, and the third beam branching is performed. Between the means 17 and the interference fringe stabilization device 40, a mirror 18 that reflects the correction reference light toward the interference fringe stabilization device 40 is disposed. Here, the above-described vibration detection means 30 is arranged between the third beam branching means 17 and the phase shift mirror 32 and reflects the reference light from the third beam branching means 17 side to the phase shift mirror 32 side. A mirror 31 is provided.

  The interference fringe detector 41 includes a plurality of channels (for example, eight) of light receiving units 141a arranged on a single plane, a plurality of channels of light receiving elements 141, and outputs (analog voltage signals) of the light receiving units 141a of the light receiving elements 141. ), And a signal processing circuit unit 142 having an amplifier that amplifies the output after the offset voltage is removed. Here, as the light receiving element 141, a linear array element in which eight light receiving portions 141a are arranged in a one-dimensional array on one plane is used, and the size of the light receiving surface (effective photocathode) of the light receiving portion 141a ( (Channel size) is set to 2.0 mm × 2.5 mm, and the pitch (channel pitch) of the light receiving surfaces of the light receiving portion 141a is set to 2.8 mm. Each light receiving portion 141a of the light receiving element 141 is configured by a photomultiplier tube having a light receiving sensitivity wavelength range of 300 to 880 nm. A CCD camera is used as the light receiving element 141, and the pixel of the CCD camera is the light receiving portion 141a. You may make it comprise.

  The control unit 43 controls the drive circuit unit 45 based on the output of the actuator 44 composed of a piezo translator that drives the wavefront control mirror 42, the drive circuit unit 45 that drives the actuator 44, and the signal processing circuit unit 142. And a control unit 46 composed of a microcomputer or the like. The control unit 46 converts the output of the signal processing circuit unit 142 from analog to digital, and the control signal to be supplied to the drive circuit unit 45 to digital- A D / A converter 46b for analog conversion, an interface 46c for connecting a support device 48 composed of a personal computer equipped with appropriate programs for performing various settings of the control unit 46 (servo setting of actuator, return to origin), and the like are provided. Yes. The control unit 43 performs various settings of the control unit 46 by appropriately operating an operation unit (for example, a keyboard, a pointing device, etc.) of the support device 48, and the driving state and settings are displayed on the display unit 50 including the display of the support device 48. The state and the light receiving state of the light receiving element 141 can be displayed. The interference fringe stabilization apparatus 40 includes a power supply circuit 47 that steps down a DC voltage obtained by AC / DC conversion of an output of an AC power supply such as a commercial power supply and supplies the voltage to the control unit 46. In addition, if the control unit 46, the detection laser 11, the vibration excitation laser 21, the vibration detection means 30, the drive device 60, and the like are collectively controlled in the support device 48 described above, non-destructive operation is possible. Automation of inspection equipment becomes easy.

The control unit 46 includes three light receiving units 141a among the eight light receiving units 141a of the light receiving element 141 (hereinafter, for convenience of explanation, the light receiving unit 141a _{1 of} the channel ₁ , the light receiving unit 141a _{2 of} the channel ₂ , and the channel 3) suppressing a phase shift from the stable state of the interference fringe based on an output of the called light receiving portion 141a ₃₎ (i.e., the interference fringes are stabilized) so to control the position of the wavefront controlling mirror 42 (for wavefront control The mirror 42 is displaced).

  Here, the principle of determining the displacement amount Δd of the wavefront control mirror 42 in the control unit 46 will be described, and then a specific operation will be described.

Now, the interference fringes formed on the surface of the light receiving element 141 in the state without the influence of disturbance such as relative vibration between the inspection object Ob and the non-destructive inspection apparatus are interference patterns as shown in FIG. If the light intensity distribution (interference intensity distribution) shown in FIG. 4A is present, the phase shift amount of interference fringes (the displacement amount in the horizontal axis direction in FIG. 4A) is φ. , The light receiving portions 141a ₁ , 141a ₂ , 141a _{3 of the} respective channels ₁ , ₂ , ₃ are positions indicated by Ch1, Ch2, and Ch3 in FIGS. 3A and 4A (where Ch3 is the same as Ch1) If the output from the light receiving portions 141a ₁ , 141a ₂ , 141a _{3 of} the channels ₁ , ₂ and ₃ is the signals U1, U2 and U3, respectively,
U1 = 0.5U _max (−sinφ + 1)
U2 = 0.5U _max = 0.5U _max (sinφ + 1)
U3 = 0.5U _max (−cosφ + 1)
It becomes. That is, U1 and U2 are different in phase by 180 °. Here, if φ is the wavelength of the laser beam output from the detection laser 11,
φ = (2π / λ) λ
It is represented by

Further, the signal contrast K for judging the quality of the contrast of the interference fringes using the signal U2 from the light receiving portion 141a ₂ of the signals U1 and channel 2 from the light receiving portion 141a ₁ of the channel 1,
K = (U2-U1) / (U2 + U1) = sinφ
In other words, when Δφ = 0 (that is, when the interference fringes are in a stable state), K = 0. Further, in the present embodiment, as indicated by a one-dot chain line in FIG. 3B, the phase shift amount from the interference fringes in FIG. 3A is equal to or less than Δφ = π / 10 (corresponding to Δλ = λ / 20). If so, the interference fringes are regarded as stable. _{Specifically, U1 = 0.34U max, U2 =} 0.65U max becomes, since the K = 0.31, a signal for determining whether the interference fringes with a contrast value K is stable When the threshold value Ks of the contrast K is set to 0.31 (corresponding to Δλ = λ / 20) and the signal contrast K is larger than Ks, the phase control of the reference light is performed. Therefore, for example, when the phase shift amount Δφ of the interference fringes is larger than π / 10 as shown in FIGS. 3C and 3D and FIGS. 4C and 4D, the phase control of the reference light is performed. Do.

Further, when the displacement amount of the wavefront control mirror 42 is Δd and the incident angle of the reference light to the wavefront control mirror 42 is θ,
Δλ = 2Δdcosθ
It becomes. Here, when the phase is greatly shifted from FIG. 3A as in the interference fringes shown in FIG. 3C, the amount of displacement of the wavefront control mirror for stabilizing the interference fringes is Δd _stab , and the signal contrast Is K,
Δd _stab = λsin ⁻¹ K / 4πcosθ
It becomes. Further, when U3> 0.5, such as when the phase is shifted by 180 ° from FIG. 3A as in the interference fringes shown in FIG. 3D, the wavefront is shifted and corrected by Δλ = λ. If the correction displacement amount of the wavefront control mirror 42 is Δd _corr ,
Δd _corr = λ / 4cosθ
When the displacement amount of the wavefront control mirror d for correcting the interference fringes under all conditions that are not in a stable state to be in a stable state is Δd,
Δd = Δd _stab + Δd _corr
Therefore, the control unit 46 can stabilize the interference fringes by controlling the position of the wavelength control mirror 42 so that the displacement amount of the wavefront control mirror 42 becomes Δd.

  Next, the operation of the control unit 46 will be specifically described.

When a pulse laser beam is output from the elastic wave excitation laser 21 (S1) and a synchronization pulse is input from the support device 48 (S2), the control unit 46 passes through each of the channels 1, 2, and 3 through the A / D converter 46a. The signals from the light receiving portions 141a ₁ , 141a ₂ , 141a ₃ are read (S3), and the signal contrast K = (U2−U1) / (U2 + U1) is calculated using the signals U1 and U2 (S4). After S4, the signal contrast K is compared with a preset threshold value Ks (= 0.3) (S5). When the condition | K | ≦ Ks is satisfied, the value of U3 is read and U3 <0.5 If this is the case (S6), it can be considered that the interference fringes in a stable state as shown in FIG. 3A are formed, so it is determined that the interference fringes are stabilized, and the vibration detection means 30 A vibration signal is recorded in the light detection unit 35 (S7).

On the other hand, in S5, if | K |> Ks, it can be considered that interference fringes as shown in FIGS. 3B, 3C, or 3D are formed. A digital value U _stab corresponding to the displacement Δd _stab of the wavefront control mirror 42 for stabilization is obtained (S8), and then the digital corresponding to the displacement Δd of the wavefront control mirror 42 for stabilizing the interference fringes. The value U _piezo = U _stab + U _corr is calculated, the digital value U _piezo is converted into an analog voltage signal by the D / A converter 46b, and given to the drive circuit unit 45, thereby displacing the wavefront control mirror 42 by Δd. (S9), the process returns to S2. In S6, if U3 ≧ 0.5, it can be considered that an interference fringe as shown in FIG. 3D is formed, and therefore the digital value U _corr corresponding to the above-described corrected displacement d _corr is shown. Is obtained (S10), S9 is performed thereafter, and the process returns to S2.

As can be seen from the above description of the operation, the control unit 46 repeatedly obtains the digital value U _piezo until the interference fringes are stabilized, and controls the position of the wavefront control mirror 42 each time the digital value U _piezo is obtained. . The frequency for repeatedly controlling the position of the wavefront control mirror 42 (the frequency of the above-mentioned synchronization pulse) is preferably set to a frequency that is about 1/10 of the lowest frequency of the vibration frequency of the inspection object Ob. For example, when the inspection object Ob is a concrete structure, if the minimum frequency is 1 kHz, the frequency for repeatedly controlling the position of the wavefront control mirror 42 is preferably set to about 100 kHz. When the inspection object Ob is a metal structure, it is desirable that the frequency for repeatedly controlling the position of the wavefront control mirror 42 is about 100 kHz if the minimum frequency is 1 MHz. In the present embodiment, the maximum displacement amount of the wavefront control mirror 42 is set to 5 μm.

  Here, in FIG. 6A, the inspection object Ob is placed on the optical surface plate of the nondestructive inspection apparatus of the present embodiment, and light is received when there is no relative vibration between the inspection object Ob and the nondestructive inspection apparatus. The interference fringes formed on the surface of the element 141 are shown. In FIG. 5B, the inspection object Ob is arranged away from the optical surface plate of the nondestructive inspection apparatus, and the relative relationship between the inspection object Ob and the nondestructive inspection apparatus is shown. The figure shows the interference fringes formed on the surface of the light receiving element 141 when the operation of the interference fringe stabilization device 40 is stopped under the condition that there is vibration (the frequency of the relative vibration is 20 Hz). (C) shows the interference fringes formed on the surface of the light receiving element 141 when the interference fringes are stabilized by the interference fringe stabilization device 40 in a state where there is relative vibration between the inspection object Ob and the nondestructive inspection device. . FIG. 6 shows that the interference fringes can be stabilized by the interference fringe stabilization device 40.

  In the interference fringe stabilization device 40 in the present embodiment described above, interference between the correction signal light branched from the signal light reflected by the inspection object Ob and frequency-modulated and the correction reference light branched from the reference light. Detected by interference fringe detection means 41 for detecting fringes, a wavefront control mirror 42 that is arranged on the optical path of the reference light before branching the reference light for correction, and controls the wavefront of the reference light. Control means 43 for controlling the position of the wavefront control mirror 42 so as to suppress the phase shift from the stable state of the interference fringes to be detected, and is detected by the interference fringe detection means 41 by the control means 43 Since the position of the wavefront control mirror 42 is controlled so that the phase shift from the stable state of the interference fringes is suppressed, it is caused by the relative vibration between the interference fringe stabilization device 40 and the inspection object Ob. The original reference light and the correction signal light can be prevented from vibrating independently, the phase shift of the interference fringes between the correction reference light and the correction signal light can be suppressed, and reflected by the reference light and the inspection object Ob. It is possible to stabilize interference fringes with frequency-modulated signal light.

  In addition, since the nondestructive inspection apparatus of the present embodiment includes the above-described interference fringe stabilization apparatus 40, the reference light and the signal are caused by relative vibration between the nondestructive inspection apparatus and the inspection object Ob. It is possible to prevent the light from vibrating independently, to suppress the phase shift of the interference fringes between the reference light and the signal light, and to improve the detection accuracy of the internal defect De of the inspection object Ob existing in the distance.

  Note that the speckle signal light for correction is speckled due to the surface state of the inspection object Ob, such as a concrete structure, or the condensing state of the laser light output from the detection laser 11 on the surface of the inspection object Ob. When there are many patterns or speckle patterns affect interference fringes, insert a device that reduces high-order wavefront patterns such as a spatial filter on the optical axis of the signal light before branching the correction signal light. Therefore, it is desirable to reduce the influence of the speckle pattern on the interference fringes.

The nondestructive inspection apparatus in embodiment is shown, (a) is a schematic block diagram, (b) is a principal part block diagram. It is characteristic explanatory drawing of the photorefractive crystal in the same as the above. It is operation | movement explanatory drawing same as the above. It is operation | movement explanatory drawing same as the above. It is operation | movement explanatory drawing same as the above. It is operation | movement explanatory drawing same as the above. It is a schematic block diagram of the nondestructive inspection apparatus which shows a prior art example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Detection laser 12 Beam branching means 21 Elastic wave excitation laser 30 Vibration detection means 32 Phase shift mirror 33 Phase conjugate element (photorefractive crystal)
34 Electrode 35 Photodetector 40 Interference fringe stabilization device 41 Interference fringe detection means 42 Wavefront control mirror 43 Control means 50 Signal light scanning means 70 Defect detection means Ob Inspection object De Internal defect

Claims (4)

  Interference fringe detection means for detecting an interference fringe between the correction signal light branched from the frequency-modulated signal light reflected by the inspection object and the correction reference light branched from the reference light, and the correction reference light is branched For controlling the wavefront so as to suppress the phase shift from the stable state of the interference fringes detected by the interference fringe detection means and the wavefront control mirror disposed on the optical path of the reference light before the control An interference fringe stabilization apparatus comprising: control means for controlling the position of the mirror.   It is disposed on the optical path between the detection laser that outputs laser light, the elastic wave excitation laser that outputs excitation light for exciting the elastic wave in the inspection object, and the detection laser and the inspection object. Beam branching means for branching a part of the laser beam outputted from the detection laser as reference light, and the signal light outputted from the detection laser and reflected by the inspection object and frequency-modulated and branched by the beam branching means A vibration detection means having a phase conjugate element that interferes with the reference light and detecting the vibration of the inspection object using the interference light emitted from the phase conjugate element, and the inspection object based on the output of the vibration detection means A non-destructive inspection apparatus comprising: a defect detection means for detecting an internal defect of the interference fringe and the interference fringe stabilization apparatus according to claim 1.   The vibration detection unit is configured so that the phase shift mirror that shifts the phase of the reference light, the reference light phase-shifted by the phase shift mirror, and the signal light that is reflected by the inspection object and frequency-modulated intersect at a predetermined angle. A photorefractive crystal that is the phase conjugate element that enters the light source, a pair of electrodes for applying an electric field to the photorefractive crystal, and a photodetector that detects interference light emitted from the photorefractive crystal. The nondestructive inspection apparatus according to claim 2.   4. The nondestructive inspection apparatus according to claim 2, further comprising signal light scanning means for scanning the signal light on the surface of the inspection object.