Laser displacement meter and laser ultrasonic inspection apparatus using the same

A laser displacement meter includes: a laser array beam source unit including a plurality of lasers emitting beams with different wavelengths; a lens array unit including a plurality of lenses for focusing laser beams; a reflected beam lens array unit including a plurality of focusing lenses for focusing the beam reflected on the surface of the object; an optical filter array unit including a plurality of optical filters through which the reflected beam is selectively transmitted; and a photodetector array unit including a plurality of photodetectors for detecting the beam transmitted through the optical filters.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority of Japanese Patent Application No. 2017-143145 filed Jul. 25, 2017, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to ultrasonic technology, and particularly to a laser displacement meter and a laser ultrasonic inspection apparatus.

BACKGROUND ART

The laser ultrasonic inspection apparatus includes an ultrasonic excitation unit with a laser and an ultrasonic detection unit with a laser. The ultrasonic excitation by the laser is performed by irradiating test pieces with a pulsed laser beam (excitation laser). When the power density of the laser beam is low, thermal stress is generated due to the rapid heating-cooling process on a micro region of the surface, and the generated thermal stress serves as the source of distortion of the material, whereby ultrasonic signals are generated (a thermoelastic mode).

On the other hand, when the power density of the laser beam is high, a surface layer of the test piece is turned into plasma, and the pressure is applied to the test piece as a reaction to the expansion of plasma, whereby ultrasound is generated (an ablation mode).

The reception of ultrasonic waves by a laser is performed by measurement of the surface displacement induced by ultrasonic waves with a laser displacement meter. As the laser displacement meter, there is a method of using deflection of beam by surface displacement which is called a knife edge method. The knife edge method is inexpensive, and a speckle knife edge detector (hereinafter, referred to as SKED) capable of being applied to a rough surface is disclosed in JP-T-2015-505362. Therefore, a practical and inexpensive laser ultrasonic inspection apparatus can be realized using a knife-edge type detector.

An example of related art includes JP-T-2015-505362.

SUMMARY OF THE INVENTION

Technical Problem

For the measurement using laser ultrasonic testing, scanning of excitation laser and scanning of detection laser are necessary.

In the knife-edge type laser displacement meter, since the detection laser beam is irradiated at an angle to the surface of the test piece and the probe beam (reflected beam on the surface of the test piece) is received by a detection unit, the irradiation beam and the probe beam are not coaxial. Therefore, since the detection unit is also necessary to be scanned when the detection laser beam is scanned, it is difficult to realize.

Therefore, it is preferable to increase the measurement time by fixing and arraying the position of the irradiation beam and the position of the detection beam in order to simultaneously measure at multiple points.

However, when the pitch of the array of photodetectors is small, or when the surface of the test piece is rough and thus the reflected beam is scattered, crosstalk occurs in which a signal leaks to adjacent photodetectors and noise increases, whereby the SN ratio may deteriorate.

An object of the invention is to provide a practical and inexpensive laser displacement meter capable of reducing noise and the measurement time.

Solution to Problem

A preferred embodiment of the invention provides a laser displacement meter for detecting reflected beam from an object to measure a displacement occurring in the object, the laser displacement meter including: a laser array beam source unit including a plurality of lasers beams with different wavelengths; a lens array unit including a plurality of lenses for focusing laser beams; a lens array unit including a plurality of focusing lenses for focusing the beam reflected on the object; an optical filter array unit including a plurality of optical filters through which the reflected beam is selectively transmitted; and a photodetector array unit including a plurality of photodetectors for detecting the beam transmitted through the optical filters.

Advantageous Effects of the Invention

According to the invention, it is possible to obtain a practical and inexpensive laser displacement meter capable of reducing noise and the measurement time.

DESCRIPTION OF THE EMBODIMENTS

Embodiment of the invention will be described in detail below with reference to the drawings.

FIG. 1is a perspective view of a laser displacement meter13of a first embodiment, andFIG. 2is a side view of the laser displacement meter13.

The laser displacement meter13includes a plurality of laser beams15having at least two different wavelengths, a focusing lens array16including a plurality of lenses for focusing the respective laser beams, a probe beam focusing lens18including a plurality of focusing lenses for focusing multi-wavelength probe beam17reflected on a surface of a test piece, an optical filter array19including a plurality of optical filters through which each focused probe beam is transmitted, and a photodetector array20including a plurality of knife-edge type photodetectors12that detects the beam transmitted through the optical filter.

The multi-wavelength laser beam15is focused by the focusing lens array16, and is irradiated on the surface of a test piece6. The irradiated beam15is reflected on the surface of the test piece to become a probe beam17, and the probe beam is focused by the focusing lens18and is incident on the optical filter array19. Since the optical filter has a characteristic of transmitting only a beam of a specific wavelength range, the beam incident on each of the optical filters21is selectively transmitted, and is detected by each of the knife-edge type photodetectors12disposed behind the optical filters21.

In the knife-edge type photodetector12constituting the photodetector array20, deflection of the probe beam occurs due to the displacement of the surface of the test piece, and an electric signal is generated in proportion to the amount of deflection. By use of the optical filter array, a so-called detector-to-detector crosstalk, some of the beams incident on a certain photodetector is incident on another photodetector, is reduced, and thus a noise component due to the crosstalk can be suppressed.

The crosstalk will be described herein. The beams15of the respective wavelengths are reflected on respective measurement positions on the surface of the test piece6, and are incident on the respective knife-edge type photodetectors12of the photodetector array20. That is, the respective measurement positions are in one-to-one correspondence with the respective photodetectors12, and a certain photodetector is used to acquire only data at the corresponding measurement position. However, when the surface is rough, since the beam15is scattered, the beam reflected at a certain position is also incident on the photodetector other than the corresponding photodetector, and this beam becomes a noise component.

As a multi-wavelength laser beam source that emits the multi-wavelength laser beam15, a solid laser, a semiconductor laser, or the like can be applied. The solid laser can realize a multi-wavelength laser by performing wavelength conversion using a nonlinear crystal (BBO, LBO, KTP, LiNO3, or the like) for laser wavelength conversion.

The semiconductor laser can be widely varied in wavelength from a visible range to a near infrared range by a change of a semiconductor material, and can easily realize a multi-wavelength laser. As an optical filter of the optical filter array19, a bandpass type filter having a multilayer film structure is preferable, but a low-pass filter or a high-pass filter may be used. As the knife-edge type photodetector, a two-division photodiode, a position sensitive detector, and a speckle knife edge detector (SKED) disclosed in JP-T-2015-505362 are preferable.

Since detection sensitivity of the knife-edge type photodetector12has wavelength dependency, ways of correcting sensitivity of the respective photodetectors12are necessary. The ways of correcting the sensitivity include the following (1) to (3).

(1) A way of correcting the sensitivity by controlling a current supplied from a multi-wavelength laser power supply unit36illustrated inFIG. 7and adjusting an output of the respective multi-wavelength laser beams.

(2) A way of correcting the sensitivity by adjusting transmittance of the respective optical filters21.

(3) A way of correcting the sensitivity by adjusting an amplification degree of the electric signal amplifier38in the knife-edge type photodetector12illustrated inFIG. 7. By the correction of the sensitivity, it is possible to obtain a true signal intensity level from the respective detectors.

FIG. 3is a view illustrating a third embodiment in which a semiconductor laser is used as a multi-wavelength beam source of a laser displacement meter13. The laser displacement meter13according to the third embodiment includes: a multi-wavelength laser beam source22including a plurality of semiconductor lasers having at least two different wavelengths; a lens array including a plurality of collimator lenses23for collimating the respective laser beams; a lens array including anamorphic lenses24for forming a collimated beam into a circular beam shape; a lens array including irradiation beam focusing lenses16for focusing the circular collimated beam on the surface of the test piece; a probe beam focusing lens18including a plurality of focusing lenses for focusing a multi-wavelength probe beam17obtained by reflected on the surface of the test piece; an optical filter array19including a plurality of optical filters through which the probe beam of a specific wavelength out of the respective focused probe beams is selectively transmitted; and a photodetector array20including a plurality of knife-edge type photodetectors12for detecting the beam transmitted through the optical filter.

Examples of materials of the semiconductor laser include a GaN-based material, an AlGaAs/GaAs-based material, and a GaInAsP/InP-based material. The semiconductor laser can be reduced in size. Compared to a solid laser, since the semiconductor laser has many types of wavelengths, it is possible to provide lasers with different kinds of wavelengths as laser beam sources.

FIG. 4is a view illustrating a fourth embodiment in which an optical fiber is used in a laser displacement meter13. The laser displacement meter13according to the fourth embodiment includes: a multi-wavelength laser beam source22including a plurality of semiconductor lasers having at least two different wavelengths; a lens array including a plurality of collimator lenses23for collimating the respective laser beams; a lens array including anamorphic lenses24for forming a collimated beam into a circular beam shape; a lens array including semiconductor laser focusing lenses25for focusing the collimated beam to allow it to be incident on an optical fiber26; a lens array including collimator lenses23for collimating the beam emitted from the optical fiber26; a lens array including irradiation beam focusing lenses16for focusing the circular collimated beam on the surface of the test piece; a probe beam focusing lens18including a plurality of focusing lenses for focusing a multi-wavelength probe beam17obtained by reflection of the laser beam, which is irradiated onto the test piece, on the surface of the test piece; an optical filter array19including a plurality of optical filters through which only the respective focused probe beams are selectively transmitted; and a photodetector array20including a plurality of knife-edge type photodetectors12for detecting the beam transmitted through the optical filter.

According to the fourth embodiment, since the optical fiber is used, the measurement of the test piece located away from the laser beam source is facilitated.

A fifth embodiment in which a guide beam is used in a laser displacement meter13will be described with reference toFIG. 1. At least two or more wavelengths of the multi-wavelength laser beam are set to a visible range. In a case where the beam having the wavelength within the visible range is used as the guide beam, it is easy to find out where the beam of each wavelength is incident on the surface of the test piece6. For this reason, the user of the laser displacement meter13for measuring the displacement easily aligns the positions of the laser beams15.

A sixth embodiment in which a guide beam is used in a laser displacement meter13will be described with reference toFIG. 5. The laser displacement meter13according to the sixth embodiment includes: a plurality of laser beams15having at least two different wavelengths; guide beams27that are located at both ends of the plurality of laser beams15and have wavelengths in the visible range; a lens array16including a plurality of lenses for focusing the plurality of laser beams15; a probe beam focusing lens18including a plurality of focusing lenses for focusing a multi-wavelength probe beam17obtained by reflection on the surface of the test piece; an optical filter array19including a plurality of optical filters through which the probe beam of a specific wavelength out of the respective focused probe beams is selectively transmitted; and a photodetector array20including a plurality of knife-edge type photodetectors12for detecting the beam transmitted through the optical filter.

When the high-sensitivity wavelength range of the photodiode used for the knife-edge type photodetector12is out of the visible range, the wavelength of the laser beam15is preferably out of the visible range, and it is difficult to align the position of the laser beams15. In this case, the guide beams27having the wavelength in the visible range are added to both ends of the plurality of laser beams15, and thus the user of the laser displacement meter13easily align the positions of the laser beams15.

FIG. 6is a block diagram of a laser ultrasonic inspection apparatus40according to a seventh embodiment in which a laser displacement meter13is used. The laser ultrasonic inspection apparatus40includes an ultrasonic generation unit29, a detection unit31, an inter-unit synchronization control unit30, an A/D converter33, a signal processing unit34, and an imaging device35. The ultrasonic generation unit29includes an excitation laser1, a scanning mirror5for scanning the excitation laser1, and a control unit28necessary for synchronizing the excitation laser and a scanning mirror, the control unit28being provided in the ultrasonic generation unit.

The detection unit31is the laser displacement meter13according to the first to sixth embodiments which includes the multi-wavelength laser beam source32and the knife-edge type photodetector array20.

The synchronization control unit30synchronizes the ultrasonic generation unit29and the reception unit31. The synchronization control unit30is necessary to synchronize the ultrasonic generation time by the ultrasonic generation unit29and the signal acquisition start time by the detection unit31.

An output from the detection unit31is converted from an analog signal into a digital signal by the A/D converter33, the signal processing unit34processes the digital signal converted by the A/D converter33, and then a detection image is prepared by the imaging device35. Here, the ultrasonic generation unit29may be a piezoelectric type or an electrostatic capacitance type ultrasonic generation element in addition to a pulsed laser.