Abstract:
A pulse laser oscillator ( 11 ) outputs a first laser beam, a beam splitter splitting the first laser beam into split beams, optical paths ( 12, 13, 14, 15, 16 ) propagating light of split beams split, respectively, taking different times for light propagation thereof, a condenser superimposing light of split beams propagated through the optical paths, respectively, on an identical spot of a measuring material ( 100 ), for irradiation therewith, a laser interferometer ( 30 ) irradiating the measuring material ( 100 ) with light of a second laser beam, having light intensity variations resulted from interferences between reference light and light of the second laser beam reflected or scattered, as bases to detect ultrasonic waves energized by light of the first laser beam and transmitted in the measuring material ( 100 ), a waveform analyzer ( 32 ) calculating a metallic microstructure or a material property of the measuring material ( 100 ) based on ultrasonic waves.

Description:
TECHNICAL FIELD 
     Embodiments described herein generally relate to a measuring apparatus and a measuring method for metallic microstructures or material properties and those using ultrasonic waves energized by a pulse laser beam. 
     BACKGROUND ART 
     There has been a wide-spread use of ultrasonic wave for measurements of metallic microstructures or material properties. For instance, in the PTL 1 below, there is disclosed a grain size measuring apparatus for measuring grain sizes in a steel plate, using the principle that energized ultrasonic waves, transmitted through a steel plate, have different attenuation characteristics depending on grain sizes in the steel plate. 
     It is known that grain sizes, attenuated ultrasonic waves, and ultrasonic frequencies generally follow a scatter law to be defined (by expression 1), such that: 
     
       
         
           
             
               
                 
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     Here, denoted by a is an attenuation rate (dB/mm) of an ultrasonic wave, D is a grain size (mm), f is a frequency (MHz) of the ultrasonic wave, and n is a coefficient representing a scatter mode, typically within 1 to 4 or near. 
     In other words for ultrasonic waves scattered at crystal grains, the attenuation due to the scattering is promoted as the frequency becomes high. This tendency has an increased significance as the grain size increases. Accordingly, resultant differences in the attenuation rate can be based on to measure qualities of a metallic material, including the grain size, for instance. 
     For use to energize ultrasonic waves in a material to be measured, there are available known methods including a method employing a piezoelectric vibrator (as a first method), a method employing electromagnetic forces (as a second method), and a method employing a pulse laser beam (as a third method). Among them, the first method needs to closely attach the piezoelectric vibrator to the material to be measured, with an intervening medium (liquid) having a matched acoustic characteristic. Moreover, energized ultrasonic waves need to have frequencies typically about a few MHz or less. The second method permits ultrasonic waves to be energized in a non-contact manner, with a limited spacing (a stand-off distance) typically about a few mm from the material to be measured. Besides, this measuring material has to be a magnetic body. Namely, the second method is inapplicable to inspections such as that of a carbon steel (having a hot austenite structure) in a hot processing, that is a non-magnetic body, or of a stainless steel that also is a non-magnetic body. 
     In comparison with them, the third method is wide-spread because of advantages permitting non-contact measurements, large standoff distances (several 100 mm), and measurements of non-magnetic bodies. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP2006-84392 A 
     SUMMARY 
     Technical Problem 
     However, in the measurement of grain size, for instance, if energized ultrasonic waves have excessively high frequencies, the attenuation due to scattering at crystal grains develops at excessively high rates. As a result, energized ultrasonic waves become faint before reaching a detection point, where detected ultrasonic waves have worsened signal-to-noise ratios causing a debased measurement precision. On the other hand, if energized ultrasonic waves have excessively low frequencies, they undergo significant attenuation due to their diffusion irrelevant to the grain size, still constituting a difficulty in the measurement 
     Accordingly, for measurements of metallic microstructures or material properties, energized ultrasonic waves should have frequencies selected as necessary. 
     In the meanwhile, according to the third method, energized ultrasonic waves have a frequency distribution typically depending on equipment-specific factors including the pulse width of a pulse laser beam in use. They are subject to the structure of a pulse laser oscillator; and hard to change. In a real sense, it was difficult to energize ultrasonic waves of adequate frequencies in accordance with an object of measurement. 
     For instance, there is a Q-switched solid-state pulse laser oscillator, stable in the performance and wide-spread for industrial use, which can output a pulse laser beam with a pulse width, typically in a range of several nanoseconds or more. When a material to be measured is irradiated with such a pulse laser beam, there appear ultrasonic waves energized as pulse ultrasonic waves of a half wave length, having their frequency components distributed over a range of 10 to 100 (MHz), with principal peaks within a range of 20 to 50 (MHz) or near. Detected waveforms can be processed to take out specific frequency components, using one of techniques for frequency analyses, such as a Fourier transform or a wavelet transform, as well known. However, those techniques tend to have a lower frequency resolution, as a shorter time is taken for recording phenomena being the targets to be analyzed. Pulse ultrasonic waves of the half wave length are recorded within a very short time, and provide lowered frequency resolutions. Therefore, it was difficult to avoid mixing frequency components else. 
     Moreover, in particular for high frequency components, their amplitudes as intensities are small, so they provide low signal-to-noise ratios. For such reasons, resultant precision was occasionally insufficient for measurements of metallic microstructures or material properties, in particular for measurements of grain sizes in a metallic material, as an issue. 
     Further, in order for pulse laser beams to be output with a pulse width of 1 nanosecond or less, there is a short pulse laser oscillator available in the market, which however is adapted to output a pulse laser beam with a very small light energy per pulse. In a use of a pulse laser beam from the short pulse laser oscillator, energized ultrasonic waves were faint, failing to provide detection signals with sufficient intensities, giving debased signal-to-noise ratios. Resultant precision was occasionally insufficient for measurements of metallic microstructures or material properties, in particular for measurements of grain sizes in a metallic material, as an issue. 
     Embodiments herein have been devised in view of the problem described. It is an object of embodiments herein to provide a measuring apparatus and a measuring method for metallic microstructures or material properties. They are to be adapted for use of a pulse laser oscillator having a typical pulse width, to energize ultrasonic vibrations sustainable over durations of one and half wavelengths or more, involving desirable frequency components more than ever. This adaptation allows for measurements of metallic microstructures or material properties, in particular for more highly precise measurements to be made of grain sizes in a metallic material within a grain size range of several micrometers or less. 
     Solution to Problem 
     To achieve the object, according to embodiments herein, there is provided a measuring apparatus for metallic microstructures or material properties including in a first aspect thereof a pulse laser oscillator, a beam splitter, optical paths, a condenser, a laser interferometer, and a waveform analyzer. The pulse laser oscillator is made up to output a first laser beam. The beam splitter is made up to split the output first laser beam into split beams. The optical paths are made up to propagate light of split beams split by the beam splitter, respectively, taking different times for light propagation thereof. The condenser is made up to superimpose light of split beams propagated through the optical paths, respectively, on an identical spot of a measuring material, for irradiation therewith. The laser interferometer is made up to irradiate the measuring material with light of a second laser beam, and have light intensity variations resulted from interferences between reference light and light of the second laser beam reflected or scattered from the measuring material, as bases to detect ultrasonic waves energized by light of the first laser beam and transmitted in the measuring material. The waveform analyzer is made up to calculate a metallic microstructure or a material property of the measuring material based on ultrasonic waves detected by the laser interferometer. 
     According to embodiments herein, the measuring apparatus for metallic microstructures or material properties further includes in a second aspect thereof an optical path length changer made up to change a difference in optical path length at one or more of the optical paths in the first aspect 
     According to embodiments herein, the measuring apparatus for metallic microstructures or material properties includes in a third aspect thereof a high refractive index material provided on an optical path at one or more of the optical paths in the first aspect 
     According to embodiments herein, the measuring apparatus for metallic microstructures or material properties has a fourth aspect, wherein among the optical paths in the first aspect, a first optical path and a second optical path requiring a longer light propagation time than that have a length difference in between, different from a length difference between the second optical path and a third optical path requiring a longer light propagation time than this. 
     According to embodiments herein, there is provided a measuring method for material properties including in a first aspect thereof splitting a first laser beam into split beams, propagating light of the split beams through optical paths having different light propagation times, respectively, irradiating an identical spot of a measuring material with light of split beams propagated through the optical paths, respectively, irradiating the measuring material with light of a second laser beam, having light intensity variations resulted from interferences between reference light and light of the second laser beam reflected or scattered from the measuring material, as bases to detect ultrasonic waves energized by light of the first laser beam and transmitted in the measuring material, and analyzing detected waveforms of the ultrasonic waves, calculating a metallic microstructure or a material property of the measuring material. 
     Advantageous Effects 
     As will be seen from the foregoing, according to embodiments herein, there is an adaptation achieved for use of a pulse laser oscillator having a typical pulse width, to energize ultrasonic vibrations sustainable over durations of one and half wavelengths or more, involving desirable frequency components more than ever, thereby allowing for measurements of metallic microstructures or material properties, in particular for more highly precise measurements to be made of grain sizes in a metallic material within a grain size range of several micrometers or less. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing a configuration of a measuring apparatus for metallic microstructures or material properties according to an example 1 of embodiments herein. 
         FIG. 2  is a diagram showing a configuration of a laser interferometer in the measuring apparatus for metallic microstructures or material properties according to the example 1 of embodiments. 
         FIG. 3  includes part (A) as a graph showing an example of a sequence of pulse laser beams derived from a pulse laser oscillator in the measuring apparatus for metallic microstructures or material properties according to the example 1 of embodiments, and part (B) as a graph showing an example of a sequence of ultrasonic waves energized by the measuring apparatus for metallic microstructures or material properties according to the example 1 of embodiments. 
         FIG. 4  is a diagram showing a configuration of a measuring apparatus for metallic microstructures or material properties according to an example 2 of embodiments herein. 
         FIG. 5  is a diagram showing a configuration of a measuring apparatus for metallic microstructures or material properties according to an example 3 of embodiments herein. 
         FIG. 6  gives illustrations showing examples of a half mirror  25  in the measuring apparatus  3  for metallic microstructures or material properties according to the example 3 of embodiments. 
         FIG. 7  is a diagram showing a configuration of a measuring apparatus for metallic microstructures or material properties according to an example 4 of embodiments herein. 
         FIG. 8  is a diagram showing a configuration of a laser interferometer  33  in the measuring apparatus for metallic microstructures or material properties according to the example 4 of embodiments. 
         FIG. 9  is a diagram showing a configuration of a measuring apparatus for metallic microstructures or material properties according to an example 5 of embodiments herein. 
         FIG. 10  is a diagram showing a configuration of a measuring apparatus for metallic microstructures or material properties according to an example 6 of embodiments herein. 
         FIG. 11  is a diagram showing a configuration of a measuring apparatus for metallic microstructures or material properties according to an example 7 of embodiments herein. 
         FIG. 12  includes part (A) as a graph showing an example of a sequence of pulse laser beams derived from a pulse laser oscillator in the measuring apparatus for metallic microstructures or material properties according to the example 7 of embodiments, and part (B) as a graph showing an example of a sequence of ultrasonic waves energized by the measuring apparatus  7  for metallic microstructures or material properties according to the example 7 of embodiments. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     There will be described examples of embodiments herein, with reference to the drawings. 
     EXAMPLE 1 
     Description is now made of a measuring apparatus for metallic microstructures or material properties taken as an example for energizing ultrasonic waves to measure grain sizes in a metallic material, according to an example 1 of embodiments herein. 
     &lt;&lt;Configuration of the Measuring Apparatus for Metallic Microstructures or Material Properties&gt;&gt; 
       FIG. 1  shows in a diagram a configuration of the measuring apparatus for metallic microstructures or material properties according to the example 1 of embodiments. 
     As shown in  FIG. 1 , the measuring apparatus  1  for metallic microstructures or material properties according to the example 1 of embodiments includes a pulse laser oscillator  11 , a half wave plate  12 , a first polarizing beam splitter  13 , a combination of reflecting mirrors  14  and  15 , a second polarizing beam splitter  16 , a condensing lens  17 , a laser interferometer  30 , an oscilloscope  31 , and a waveform analyzing computer  32 . 
     The pulse laser oscillator  11  is provided with a Q-switched solid-state pulse laser light source employing an Nd:YAG (neodymium-doped yttrium-aluminum-garnet), and adapted to output a pulse laser beam  201  with a pulse width within degrees ranging a few nanoseconds to a dozen nanoseconds. It is noted that the pulse laser light source employed may be, for instance, a semiconductor-excited solid-state pulse laser light source, a pulse gas laser light source, a fiber laser light source, a semiconductor laser light source, or a flash lamp, or any one of those light sources combined with a laser amplifier or the like. Further, embodiments herein are applicable not simply in the case using a Q-switched solid-state pulse laser, of which output is linearly polarized, but even in cases in which the state of polarization is different as will be described later on. 
     The half wave plate  12  is an optical element for rotating the direction of polarization of linearly polarized light The half wave plate  12  is operable to revolve about an optical axis of the pulse laser beam  201  output from the pulse laser oscillator  11 , causing the direction of polarization to be rotated by twice the revolved angle. Here, it is fixed at an angle to have a ratio of 1:1 set up between an amount of light of a first split beam  202  and an amount of light of a second split beam  203 , as they are established as splits by the first polarizing beam splitter  13  to be described below 
     The first polarizing beam splitter  13  is an optical element adapted to be permeable for a set of horizontal polarized (i.e. parallel-to-paper polarized) components of the pulse laser beam  201  to transmit as the first split beam  202 , and reflective for a set of vertical polarized (i.e. normal-to-paper polarized) components of the pulse laser beam  201  to go as the second split beam  203  along an optical path oriented in a perpendicular direction relative to the above-noted optical axis. 
     The reflecting mirrors  14  and  15  are arranged in positions for reflecting the second split beam  203  having been reflected at the first polarizing beam splitter  13 , to make it incident to the second polarizing beam splitter  16  to be described below. 
     The reflecting mirrors  14  and  15  are arranged to delay the time elapsed for the second split beam  203  to travel after being split to reach a measuring material  100 , so that ultrasonic waves energized when the measuring material  100  is irradiated with light of the first split beam  202  overlap in part with ultrasonic waves energized when the measuring material  100  is irradiated with light of the second split beam  203 . It is noted that the measuring material  100 , not limited to a metallic material, may be a non-metallic material such as a glass, ceramics, or rigid plastic. 
     The second polarizing beam splitter  16  works to combine the first split beam  202  and the second split beam  203  with each other. 
     The condensing lens  17  condenses light of the first split beam  202  and the second split beam  203  combined by the second polarizing beam splitter  16 , on an identical spot on the measuring material  100 . This creates plasma at a surface of the measuring material  100 , having ultrasonic waves energized in the measuring material  100 . Then, energized ultrasonic waves travel inside the measuring material  100 , reaching an opposing surface, where they appear in the form of micro vibrations. The measuring material  100  being irradiated with light of the first split beam  202  and light of the second split beam  203  thus has micro vibrations produced thereon, which can be detected in the form of electric signals by the laser interferometer  30 . 
     The oscilloscope  31  is operable to display waveforms in accordance with electric signals detected by the laser interferometer  30 . 
     The waveform analyzing computer  32  is adapted for operations to calculate grain sizes in the measuring material  100  based on electric signals detected by the laser interferometer  30 . For instance, the waveform analyzing computer  32  is operable on waveforms detected by the laser interferometer  30 , for extracting therefrom waveforms of reflection echoes in a repetition of longitudinal ultrasonic waves, followed by applying thereto a continuous wavelet transform, to determine vibration powers by frequencies. Then, the waveform analyzing computer  32  is operable on vibration powers of longitudinal echoes, for their fitting using logarithmic functions, to determine attenuation rates α by frequencies. These values provide a basis for use of the relationship shown (in the expression 1), to calculate grain sizes. 
       FIG. 2  shows in a diagram a configuration of the laser interferometer  30  in the measuring apparatus  1  for metallic microstructures or material properties according to the example 1 of embodiments. 
     As shown in  FIG. 2 , the laser interferometer  30  includes a narrow line-width laser light source  101 , a beam splitter  102 , a combination of condensing lens  103 ,  104 , and  106 , a photorefractive crystal  105 , and a photodiode  107 . 
     The narrow line-width laser light source  101  is a light source operable to output a narrow line-width laser beam  211  with a high-wave number stability and a favorable coherency. 
     The beam splitter  102  works to split a laser beam  211  output from the narrow line-width laser light source  101 , into two, having either transmitting as a detecting beam  211  toward the condensing lens  103 , the other being refracted to go as a pump beam  213  to be incident to the photorefractive crystal  105 . 
     The condensing lens  103  condenses light of the detecting beam  211  on a surface of the measuring material  100  opposing the surface irradiated with light of the first split beam  202  and light o the second split beam  203 . 
     The condensing lens  104  condenses light of the detecting beam  211  reflected by the measuring material  100 , to make it strike into the photorefractive crystal  105 . 
     The photorefractive crystal  105  is made up to work when irradiated with rays of light, as a crystal responsive to their being light or dark by having charges moved in accordance therewith, inducing changes in refraction indices. 
     There are rays of light of the detecting beam  211  and rays of light of the pump beam  213  incoming to the photorefractive crystal  105 , where they intersect each other, creating interference fringes. Then, the photorefractive crystal  105  has distributions of refraction indices produced therein in fringe shapes according to light and dark contrasts of the interference fringes. They each act as a diffractive gating, where part of light of the pump beam  213  is diffracted toward light of the detecting beam  212 . In such a situation, the measuring material  100  is vibrated at a high speed. This provides the detecting beam  212  with a varied optical path, causing rays of light of the detecting beam  212  to be deviated in phases relative to distributions of refraction indices, with resultant variations in amounts of light diffracted from light of the pump beam  213  to light of the detecting beam  212 . 
     The photodiode  107  is operable to convert fractions of light it has received into electric signals, affording to detect, as electric signals, variations in amounts of light diffracted from light of a pump beam  213  to light of a detecting beam  212  that represent a high-speed vibration on a measuring material. 
     &lt;&lt;Operations of the Measuring Apparatus for Metallic Microstructures or Material Properties&gt;&gt; 
     Description is now made of operations of the measuring apparatus  1  for metallic microstructures or material properties according to the example 1 of embodiments, with reference to  FIG. 1  and  FIG. 2 . 
     The pulse laser oscillator  11  outputs a pulse laser beam  201 , which is split by the first polarizing beam splitter  13  into a first split beam  202  and a second split beam  203 . The first split beam  202 , transmitted across the first polarizing beam splitter  13 , transmits across the second polarizing beam splitter  16 , too, before it arrives at a measuring material  100 . 
     On the other hand, the second split beam  203 , reflected by the first polarizing beam splitter  13 , is further reflected by the reflecting mirrors  14  and  15  and still by the second polarizing beam splitter  16 , before it arrives at the measuring material  100 . 
     The first split beam  202  and the second split beam  203  thus travel different optical paths after being output from the pulse laser oscillator  11  until their arrivals at the measuring material  100 , so that the second split beam  203  is delayed from the first split beam  202  to reach the measuring material  100 . 
       FIG. 3(A)  shows in a graph an example of a sequence of pulse laser beams derived from the pulse laser oscillator  11  in the measuring apparatus  1  for metallic microstructures or material properties according to the example 1 of embodiments.  FIG. 3(B)  shows in a graph an example of a sequence of ultrasonic waves energized by the measuring apparatus  1  for metallic microstructures or material properties according to the example 1 of embodiments. 
       FIG. 3(A)  illustrates intensities of light of a first split beam  202  having reached a measuring material  100 , being maximized at a point of time t 1 , and intensities of light of a second split beam  203  having reached the measuring material  100 , being maximized at a point of time t 3 . 
     Then,  FIG. 3(B)  illustrates amplitudes of an ultrasonic wave energized at the point of time t 1  with light of the first split beam  202  having reached the measuring material  100 , being maximized at a point of time t 2 , and amplitudes of an ultrasonic wave energized at the point of time t 3  with light of the second split beam  202  having reached the measuring material  100 , being maximized at a point of time t 4 . 
     Such being the case, there is a first split beam  202  combined with a second split beam  203  delayed therefrom to reach a measuring material  100 , so that ultrasonic waves energized when the measuring material  100  is irradiated with light of the first split beam  202  overlap in part with ultrasonic waves energized when the measuring material  100  is irradiated with light of the second split beam  203 . Accordingly, there can be energized ultrasonic waves much involving specific frequency components. 
     Here, the first split beam  202  and the second split beam  203  have their arrival times at a surface of the measuring material  100 , with a difference Δt in between, which is representative (by expression 2), letting f be a frequency of energized ultrasonic waves, such that:
 
Δ t=k/f    (expression 2),
 
where k is a correction factor in consideration of characteristics at rising and falling edges of a pulse laser beam, within degrees ranging 0.5 to 2.
 
     The first split beam  202  and the second split beam  203  have their optical path lengths, with a difference ΔL in between as necessary to establish the arrival time difference Δt, which is representative (by expression 3), such that:
 
Δ L=c   0   ·Δt    (expression 3),
 
where c 0  is the speed of light in the air, approximately 3×10 8  m/s.
 
     Therefore, the reflecting mirrors  14  and  15  may well be arranged in position to make the optical path length difference between the first split beam  202  and the second split beam  203  equal to ΔL (i.e. in position substantially at a distance of ΔL/2). 
     In the meanwhile, if the above-noted Δt is excessively long relative to the pulse width of the pulse laser beam, ultrasonic waves energized with light of the first split beam  202  and ultrasonic waves energized with light of the second split beam  203  individually travel, respectively, failing to overlap, thus missing the effect of having energized ultrasonic waves much involve desirable frequency components. 
     Accordingly, between the times elapsed for the first split beam  202  and the second split beam  203  to arrive at a surface of the measuring material  100 , the difference Δt may well conform to a relationship to a pulse width t p  of the pulse laser beam that is defined (by expression 4), such that:
 
Δ t&lt;a·t   p    (expression 4),
 
where a is a constant that may well be set generally within degrees about 5 for ensured overlaps between ultrasonic waves energized with light of the first split beam  202  and ultrasonic waves energized with light of the second split beam  203 .
 
     Such being the case, given an optical path length difference ΔL calculated using (expression 2) through (expression 4), the reflecting mirrors  14  and  15  can be arranged in position to have a resultant optical path length difference equal to the ΔL (i.e. in position substantially at a distance of ΔL/2), allowing for ultrasonic waves to be energized in a measuring material  100 , much involving specific frequency components. As will be seen from the foregoing, according to the example 1 of embodiments, the measuring apparatus  1  for metallic microstructures or material properties employs a first split beam  202  combined with a second split beam  203  travelling an optical path requiring a longer light travel time than that, with a resultant difference between their arrival times at a surface of a measuring material  100 , permitting energized ultrasonic waves to much involve specific frequency components in accordance therewith. This allows for measurements of metallic microstructures or material properties, in particular for well precise measurements to be made of grain sizes in the metallic material  100 . 
     Moreover, according to the example 1 of embodiments, the measuring apparatus  1  for metallic microstructures or material properties includes a combination of reflecting mirrors  14  and  15  arranged to delay the time elapsed for the second split beam to travel after being split to reach the measuring material  100 , so that ultrasonic waves energized when the measuring material  100  is irradiated with light of the first split beam overlap in part with ultrasonic waves energized when the measuring material  100  is irradiated with light of the second split beam. However, this configuration is not restrictive. There may well be a configuration including prisms or retro-reflectors substituting for the reflecting mirrors  14  and  15 . 
     Further, according to the example 1 of embodiments, the measuring apparatus  1  for metallic microstructures or material properties includes a laser interferometer  30  of a two-wave mixing system provided with a photorefractive crystal  105 . However, this configuration is not restrictive. There may well be a configuration including, instead of the laser interferometer  30 , a Fabry-Perot interferometer adapted to measure high-frequency vibrations even on a rough surface of a measuring material  100 , or a Michelson interferometer adapted to measure high-frequency vibrations on a mirror-finished surface of a measuring material  100   
     In the example 1 of embodiments described, the measuring apparatus  1  for metallic microstructures or material properties is taken as an example adapted to measure grain sizes in a measuring material  100  that has material properties else. It is noted that this example is not restrictive. There may well be an adaptation to measure a tensile strength, yield strength, or formability of the measuring material  100 , a crystalline orientation in the measuring material, or the like. 
     EXAMPLE 2 
     In the example 1 of embodiments described, the measuring apparatus  1  for metallic microstructures or material properties is taken as an example employing a pair of beam splitters to provide a difference in length between an optical path for a first split beam  202  and an optical path for a second split beam  203 , whereas this example is not restrictive. 
     Description is now made of a measuring apparatus for metallic microstructures or material properties taken as an example employing a single beam splitter to provide a difference in length between an optical path for a first split beam  202  and an optical path for a second split beam  203 , according to an example 2 of embodiments herein. 
       FIG. 4  shows in a diagram a configuration of a measuring apparatus for metallic microstructures or material properties according to the example 2 of embodiments. 
     As shown in  FIG. 4 , the measuring apparatus  2  for metallic microstructures or material properties according to the example 2 of embodiments includes a pulse laser oscillator  11 , a half wave plate  12 , a polarizing beam splitter  22 , a condensing lens  17 , a combination of reflecting mirrors  18  and  19 , a combination of quarter wave plates  20  and  21 , a laser interferometer  30 , an oscilloscope  31 , and a waveform analyzing computer  32 . 
     Among those constituent components, the combination of reflecting mirrors  18  and  19 , the combination of quarter wave plates  20  and  21 , and the polarizing beam splitter  22  will be described. For each of the other components not to be described, refer to description of a component designated by an identical reference sign, as it is identical in configuration, in the measuring apparatus  1  for metallic microstructures or material properties according to the example 1 of embodiments. 
     The quarter wave plate  20  is an optical element for changing a linear polarization to a circular polarization, and a circular polarization to a linear polarization. The quarter wave plate  20  has properties acting on linear polarized light, to rotate the direction of polarization by 90 degrees when the light is twice transmitted across the plate  20 . The quarter wave plate  21  has a similar configuration to the quarter wave plate  20 . 
     There is a first split beam  202  reflected by the first polarizing beam splitter  13 , and circularly polarized by the quarter wave plate  20 . The reflecting mirror  18  reflects this first split beam  202  toward the first polarizing beam splitter  13 . 
     There is a second split beam  203  transmitted across the first polarizing beam splitter  13 , and circularly polarized by the quarter wave plate  21 . The reflecting mirror  19  reflects this second split beam  203  toward the first polarizing beam splitter  13 . 
     The polarizing beam splitter  22  is adapted to be permeable for a set of horizontal polarized (i.e. parallel-to-paper polarized) components of a pulse laser beam  201  to transmit as a second split beam  203 , and reflective for a set of vertical polarized (i.e. normal-to-paper polarized) components of the pulse laser beam  201  to go as a first split beam  202  along an optical path oriented in a perpendicular direction relative to the afore-mentioned optical axis. The polarizing beam splitter  22  is further adapted to combine a first split beam  202  reflected by the reflecting mirror  18  and a second split beam  203  reflected by the reflecting mirror  19 , with each other. 
     Therefore, in the measuring apparatus  2  for metallic microstructures or material properties according to the example 2 of embodiments, the reflecting mirror  19  or  18  is arranged in position to make the optical path length difference between the first split beam  202  and the second split beam  203  equal to ΔL (i.e. in position substantially at a distance of ΔL/2), like the measuring apparatus  1  for metallic microstructures or material properties according to the example 1 of embodiments. 
     As will be seen from the foregoing, according to the example 2 of embodiments, the measuring apparatus  1  for metallic microstructures or material properties employs a single beam splitter to provide a difference between an optical path for a first split beam  202  and an optical path for a second split beam  203 . The optical path difference serves to provide a difference between arrival times at a surface of a measuring material  100 , which permits energized ultrasonic waves to much involve specific frequency components in accordance therewith. This allows for measurements of metallic microstructures or material properties, in particular for well precise measurements to be made of qualities of a metallic material  100 , including grain sizes among others. 
     EXAMPLE 3 
     In the example 1 of embodiments described, the measuring apparatus  1  for metallic microstructures or material properties is taken as an example including a pulse laser oscillator  11  for outputting a pulse laser beam of linear polarized light, to provide a difference in length between an optical path for a first split beam  202  and an optical path for a second split beam  203 , the split beams being split from the pulse laser beam of linear polarized light. It however is noted that the pulse laser beam, in no way limited to a linear polarization, may well be non-polarized. 
       FIG. 5  shows in a diagram a configuration of a measuring apparatus for metallic microstructures or material properties according to an example 3 of embodiments herein. 
     As shown in  FIG. 5 , the measuring apparatus  3  for metallic microstructures or material properties according to the example 3 of embodiments includes a pulse laser oscillator  24 , a half mirror  25 , a combination of condensing lens  17  and  27 , a combination of reflecting mirrors  14 ,  15 , and  26 , a laser interferometer  30 , an oscilloscope  31 , and a waveform analyzing computer  32 . 
     Among those constituent components, the pulse laser oscillator  24 , the half mirror  25 , the reflecting mirror  26 , and the condensing lens  27  will be described. For each of the other components not to be described, refer to description of a component designated by an identical reference sign, as it is identical in configuration, in the measuring apparatus  1  for metallic microstructures or material properties according to the example 1 of embodiments. 
     The pulse laser oscillator  24  is adapted to output a pulse laser beam  201  that is a non-polarized laser beam having a pulse width within degrees ranging a few nanoseconds to a dozen nanoseconds. 
     The half mirror  25  is a mirror reflector for splitting the pulse laser beam  201  output from the pulse laser oscillator  24  into a first split beam  202  and a second split beam  203 . 
       FIG. 6  shows in illustrations in part (A) and part (B) thereof examples of the half mirror  25  in the measuring apparatus  3  for metallic microstructures or material properties according to the example 3 of embodiments. 
     As shown in  FIG. 6(A) , the half mirror  25  is formed as a combination of a permeable plate  301  adapted to transmit light of a pulse laser beam  201  output from the pulse laser oscillator  24 , and a set of reflective elements  302  fixed thereon. The reflective elements  302  are made to occupy a total area, whereby the permeable plate  301  has a total area retained to be 1:1 thereto. 
     The half mirror  25  may well have a configuration illustrated in  FIG. 6(B) . 
       FIG. 6(B)  shows an example in which the half mirror  25  is provided as a combination of a reflecting mirror  303  adapted to reflect light of a pulse laser beam  201  output from the pulse laser oscillator  24 , and a set of holes  304  formed therein. The holes  304  are formed to occupy a total area, whereby the reflecting mirror  303  has a total area retained to be 1:1 thereto. 
     There is a second split beam  203  reflected by the reflecting mirror  15 . This split beam  203  is reflected by the reflecting mirror  26  toward the condensing lens  27 . 
     A measuring material  100  is set. On a surface of the measuring material  100 , there is a spot on which light of the first split beam  202  is condensed. For the second split beam  203  reflected by the reflecting mirror  26 , the condensing lens  27  is arranged to condense its light on substantially the same spot as that spot, for irradiation therewith. 
     Therefore, in the measuring apparatus  3  for metallic microstructures or material properties according to the example 3 of embodiments, the reflecting mirrors  14 ,  15 , and  26  are arranged as necessary to make the optical path length difference between the first split beam  202  and the second split beam  203  equal to ΔL (i.e. in a position or positions substantially at a distance of ΔL/2), like the measuring apparatus  1  for metallic microstructures or material properties according to the example 1 of embodiments. 
     As will be seen from the foregoing, according to the example 3 of embodiments, the measuring apparatus  3  for metallic microstructures or material properties is operable, even in a situation using light of a pulse laser beam that is non-polarized for irradiating a measuring material  100 , to make use of a combination of a first split beam  202  and a second split beam  203  travelling an optical path requiring a longer light travel time than that This combination serves to provide a difference between arrival times at a surface of the measuring material  100 , which permits energized ultrasonic waves to much involve specific frequency components in accordance therewith. This allows for measurements of metallic microstructures or material properties, in particular for well precise measurements to be made of qualities of a metallic material  100 , including grain sizes among others. 
     According to the example 3 of embodiments, the measuring apparatus  3  for metallic microstructures or material properties has a configuration including a half mirror  25 . However, there may well be a configuration including, instead of the half mirror  25 , a non-polarized beam splitter adapted to split a non-polarized laser beam. 
     EXAMPLE 4 
     In the example 1 of embodiments described, the measuring apparatus  1  for metallic microstructures or material properties is taken as an example including a laser interferometer  30  adapted to use light of a laser beam  211  for irradiating a surface of a measuring material  100  opposing a surface irradiated with light of a first split beam  202  and light of a second split beam  203 . The laser interferometer  30  is further adapted to have light intensity variations resulted from interferences between reference light and light of the laser beam  211  reflected from the measuring material  100 , as bases to detect ultrasonic waves energized by light of the first split beam  202  and light of the second split beam  203  and transmitted in the measuring material. It however is noted that this example is not restrictive. 
     Description is now made of a measuring apparatus  4  for metallic microstructures or material properties taken as an example including a laser interferometer adapted to use light of a laser beam  211  for irradiating an identical surface of a measuring material  100  irradiated with light of a first split beam  202  and light of a second split beam  203 , and have light intensity variations resulted from interferences between reference light and light of the laser beam  211  reflected from the measuring material  100 , as bases to detect ultrasonic waves energized by light of the first split beam  202  and light of the second split beam  203  and transmitted in the measuring material, according to an example 4 of embodiments herein. 
       FIG. 7  shows in a diagram a configuration of a measuring apparatus for metallic microstructures or material properties according to the example 4 of embodiments. 
     As shown in  FIG. 7 , the measuring apparatus  4  for metallic microstructures or material properties according to the example 4 of embodiments includes a pulse laser oscillator  11 , a half wave plate  12 , a first polarizing beam splitter  13 , a combination of reflecting mirrors  14  and  15 , a second polarizing beam splitter  16 , a condensing lens  17 , a laser interferometer  33 , an oscilloscope  31 , and a waveform analyzing computer  32 . 
     Among those constituent components, the laser interferometer  33  will be described. For each of the other components not to be described, refer to description of a component designated by an identical reference sign, as it is identical in configuration, in the measuring apparatus  1  for metallic microstructures or material properties according to the example 1 of embodiments. 
       FIG. 8  shows in a diagram a configuration of the laser interferometer  33  in the measuring apparatus  4  for metallic microstructures or material properties according to the example 4 of embodiments. 
     As shown in  FIG. 8 , the laser interferometer  33  includes a narrow line-width laser light source  101 , a beam splitter  102 , a combination of condensing lens  103 ,  104 , and  106 , a photorefractive crystal  105 , a photodiode  107 , and a wavelength selecting filter  108 . Among those constituent components, the wavelength selecting filter  108  will be described. For each of the other components not to be described, refer to description of a component designated by an identical reference sign, as it is identical in configuration, in the measuring apparatus  1  for metallic microstructures or material properties according to the example 1 of embodiments. 
     As shown in  FIG. 8 , the laser interferometer  33  is made up to use light of a detecting beam  211  split from a laser beam  211  to irradiate a surface of a measuring material  100  irradiated with light of a first split beam  202  and light of a second split beam  203 , and condense light of the detecting beam  211  reflected on the surface of the measuring material  100 , to provide as incident light to the photorefractive crystal  105 . 
     In this situation, the wavelength selecting filter  108 , installed on an optical path of light of the detecting beam  211 , works to keep light of the first split beam  202  and the second split beam  203  reflected on the surface of the measuring material  100 , from striking into the photorefractive crystal  105 . 
     As will be seen from the foregoing, according to the example 4 of embodiments, the measuring apparatus  4  for metallic microstructures or material properties is operable to use light of a laser beam  211  to irradiate a surface of a measuring material  100  irradiated with light of a first split beam  202  and light of a second split beam  203 , thereby having light intensity variations resulted from interferences between reference light and light of the laser beam  211  reflected from the measuring material  100 , as bases to detect ultrasonic waves energized by light of the first split beam  202  and light of the second split beam  203  and transmitted in the measuring material  100 . 
     EXAMPLE 5 
     In the example 1 of embodiments described, the measuring apparatus  1  for metallic microstructures or material properties is taken as an example including a combination of reflecting mirrors  14  and  15  fixed in position so that ultrasonic waves energized with light of a first split beam  202  and ultrasonic waves energized with light of a second split beam  203  overlap with each other. However, there may well be employed a combination of reflecting mirrors  14  and  15  arranged to be movable. 
     Description is now made of a measuring apparatus  1  for metallic microstructures or material properties taken as an example including a combination of reflecting mirrors  14  and  15  arranged to be movable so that ultrasonic waves energized with light of a first split beam  202  and ultrasonic waves energized with light of a second split beam  203  overlap with each other, according to an example 5 of embodiments herein. 
       FIG. 9  shows in a diagram a configuration of a measuring apparatus for metallic microstructures or material properties according to the example 5 of embodiments. 
     As shown in  FIG. 9 , the measuring apparatus  5  for metallic microstructures or material properties according to the example 5 of embodiments includes a pulse laser oscillator  11 , a half wave plate  12 , a first polarizing beam splitter  13 , a combination of reflecting mirrors  14  and  15 , a second polarizing beam splitter  16 , a condensing lens  17 , a laser interferometer  33 , an oscilloscope  31 , and a waveform analyzing computer  32 . It further includes an optical path length calculator  41 , a driver  42 , a motor  43 , a revolving shaft  44 , and a mirror cabinet  45 . 
     Among those constituent components, the optical path length calculator  41 , the driver  42 , the motor  43 , the revolving shaft  44 , and the mirror cabinet  45  will be described. For each of the other components not to be described, refer to description of a component designated by an identical reference sign, as it is identical in configuration, in the measuring apparatus  1  for metallic microstructures or material properties according to the example 1 of embodiments. Here, the optical path length calculator  41 , the driver  42 , the motor  43 , the revolving shaft  44 , and the mirror cabinet  45  constitute a section, which is referred to as an optical path length changer. 
     The optical path length calculator  41  serves when a frequency of an ultrasonic wave to be energized is input in accordance with the user&#39;s operation, to calculate an optical path length for a second split beam  203 , as necessary, to energize ultrasonic waves of the input frequency. 
     The driver  42  is adapted to generate a drive signal to establish the optical path length for the second split beam  203 , as it is calculated by the optical path length calculator  41 . 
     The motor  43  follows the drive signal generated by the driver  42  to make the revolving shaft  44  revolve, thereby causing the mirror cabinet  45  to move in either direction X 1  or X 2 , carrying the combination of reflecting mirrors  14  and  15  accommodated therein. 
     As will be seen from the foregoing, according to the example 5 of embodiments, the measuring apparatus  5  for metallic microstructures or material properties is operable to make use of a combination of a first split beam  202  and a second split beam  203  travelling an optical path requiring a longer light travel time than that, in the manner of rendering variable the difference between arrival times at a surface of a measuring material  100 . This permits energized ultrasonic waves to much involve specific frequency components, as necessary, along with the context of measurement, thus affording to change ultrasonic frequency components available for the measurement, in an adequate manner complying with the context of measurement. This allows for measurements of metallic microstructures or material properties, in particular for well precise measurements to be made of grain sizes in the metallic material  100 . 
     According to the example 5 of embodiments, the measuring apparatus  5  for metallic microstructures or material properties employs a motor to be driven to displace a mirror cabinet  45  in either direction X 1  or X 2 . However, this is not restrictive. There may well be use of a hydraulic or pneumatic cylinder to move a mirror cabinet  45  in either direction X 1  or X 2 . 
     EXAMPLE 6 
     In the example 1 of embodiments described, the measuring apparatus  1  for metallic microstructures or material properties is taken as an example including a combination of reflecting mirrors  14  and  15  arranged in position so that ultrasonic waves energized with light of a first split beam  202  and ultrasonic waves energized with light of a second split beam  203  overlap with each other. However, there may well be a high refractive index material installed in an optical path of a second split beam, to make the second split beam travel a shorter optical path to arrive at a measuring material  100 . 
     Description is now made of a measuring apparatus for metallic microstructures or material properties taken as an example including a high refractive index material installed along an optical path of a second split beam, according to an example 6 of embodiments herein. 
       FIG. 10  shows in a diagram a configuration of a measuring apparatus for metallic microstructures or material properties according to the example 6 of embodiments. 
     As shown in  FIG. 10 , the measuring apparatus  6  for metallic microstructures or material properties according to the example 6 of embodiments includes a pulse laser oscillator  11 , a half wave plate  12 , a first polarizing beam splitter  13 , a combination of reflecting mirrors  14  and  15 , a second polarizing beam splitter  16 , a condensing lens  17 , a laser interferometer  30 , an oscilloscope  31 , a waveform analyzing computer  32 , and a combination of high refractive index materials  51  and  52 . 
     Among those constituent components, the combination of high refractive index materials  51  and  52  will be described. For each of the other components not to be described, refer to description of a component designated by an identical reference sign, as it is identical in configuration, in the measuring apparatus  1  for metallic microstructures or material properties according to the example 1 of embodiments. 
     The high refractive index material  51  is installed along a course of an optical path of a second split beam  203  extending between the first polarizing beam splitter  13  and the reflecting mirror  14 . The high refractive index material  52  is installed along a course of the optical path of the second split beam  203  extending between the reflecting mirror  15  and the second polarizing beam splitter  16 . 
     The high refractive index materials  51  and  52  are each formed by using a medium that has a higher refractive index than the air, so that the optical path of the second split beam  203  has a contracted length. 
     In the meanwhile, in the high refractive index materials  51  and  52 , that is, in the medium having a higher refractive index than the air, light has a slower speed c 1  than in the air, which is representative (by expression 5), such that:
 
 c   1   =c   0   /n    (expression 5),
 
where n is a refractive index of the high refractive index materials  51  and  52 . Therefore, according to the example 6 of embodiments, the measuring apparatus  6  for metallic microstructures or material properties has a length difference ΔL′ between an optical path of a first split beam  202  and the optical path of the second split beam  203  which is representative (by expression 6), such that:
 
Δ L′=c   1   ·Δt=c   0   ·t/n    (expression 6).
 
     Accordingly, the optical path of the second split beam  203  can be cut down in length, for instance, by approximately 31% when the high refractive index materials  51  and  52  are made of a quartz glass that has a refractive index n equal to 1.45. 
     It is noted that the high refractive index materials  51  and  52  each have an input end face and an output end face for the second split beam  203  to strike in and out. It is preferable to apply an AR (anti-reflective) coat to each end, to suppress losses in light amount due to reflection at the end faces. 
     As will be seen from the foregoing, according to the example 6 of embodiments, the measuring apparatus  6  for metallic microstructures or material properties includes a high refractive index material set installed on an optical path of a second split beam  203 . Accordingly, even in a situation suffering from constraints, such as those due to an installation space of equipment, in which the optical path of the second split beam  203  might otherwise have failed to save a sufficient length as the high refractive index material set is unused, it is still possible for the measuring apparatus  6  to employ a first split beam  202  combined with the second split beam  203  travelling the optical path requiring a longer light travel time than that, with a resultant difference between their arrival times at a surface of a measuring material  100 , permitting energized ultrasonic waves to much involve specific frequency components in accordance therewith. This allows for measurements of metallic microstructures or material properties, in particular for well precise measurements to be made of grain sizes in a metallic material. 
     EXAMPLE 7 
     In the example 3 of embodiments described, the measuring apparatus  1  for metallic microstructures or material properties is taken as an example including a pulse laser oscillator  24  adapted to output a pulse laser beam as a non-polarized laser beam, and having a first split beam  202  and a second split beam  203  split from the pulse laser beam, providing a length difference between their optical paths. 
     Description is now made of a measuring apparatus  1  for metallic microstructures or material properties taken as an example including a pulse laser oscillator  24  adapted to output a pulse laser beam as a non-polarized laser beam, and having a first split beam  202 , a second split beam  203 , and a third split beam split from the pulse laser beam, providing length differences among their optical paths, according to an example 7 of embodiments herein. 
       FIG. 11  shows in a diagram a configuration of a measuring apparatus for metallic microstructures or material properties according to the example 7 of embodiments. 
     As shown in  FIG. 11 , the measuring apparatus  7  for metallic microstructures or material properties according to the example 7 of embodiments includes a pulse laser oscillator  24 , a combination of condensing lens  17 ,  27 , and  64 , a combination of reflecting mirrors  14 ,  15 ,  26 ,  62 , and  63 , a combination of non-polarizing beam splitters  60  and  61 , a laser interferometer  30 , an oscilloscope  31 , and a waveform analyzing computer  32 . 
     Among those constituent components, the condensing lens  64 , the reflecting mirrors  62  and  63 , and the non-polarizing beam splitters  60  and  61  will be described. For each of the other components not to be described, refer to description of a component designated by an identical reference sign, as it is identical in configuration, in the measuring apparatus  3  for metallic microstructures or material properties according to the example 3 of embodiments. 
     There is a pulse laser beam  201  output from the pulse laser oscillator  24 , and split at the non-polarizing beam splitter  60  into a first split beam  202  and a fourth split beam  205 . The first split beam  202  is transmitted across the non-polarizing beam splitter  60 . The fourth split beam  205  is reflected by the non-polarizing beam splitter  60  toward the non-polarizing beam splitter  61 . The non-polarizing beam splitter  60  is made up to give the first split beam  202  an amount of light to be 1:2 in the ratio to an amount of light it gives the fourth split beam  205 . 
     The fourth split beam  205 , split at the non-polarizing beam splitter  60 , enters the non-polarizing beam splitter  61 , where it is split into a second split beam  203  and a third split beam  204 . The second split beam  203  is reflected by the non-polarizing beam splitter  61  toward the reflecting mirror  62 . The third split beam  204  is transmitted across the non-polarizing beam splitter  61 . Here, the non-polarizing beam splitter  61  is made up to give the second split beam  203  an amount of light to be 1:1 in the ratio to an amount of light it gives the third split beam  204 . 
     The second split beam  203 , split at the non-polarizing beam splitter  61 , is reflected by the reflecting mirrors  62  and  63  toward the condensing lens  64 . 
     A measuring material  100  is set. On a surface of the measuring material  100 , there is a spot on which light of the first split beam  202  is condensed. For the second split beam  203  reflected by the reflecting mirrors  62  and  63 , the condensing lens  64  is arranged to condense its light on substantially the same spot as that spot, for irradiation therewith. 
     The non-polarizing beam splitters  60  and  61 , the reflecting mirrors  26 ,  62 , and  63 , and the condensing lens  64  are arranged in positions to establish an optical path for the second split beam  203 , such that ultrasonic waves energized when the measuring material  100  is irradiated with light of the first split beam  202  overlap in part with ultrasonic waves energized when the measuring material  100  is irradiated with light of the second split beam  203 . 
     Further, the non-polarizing beam splitters  60  and  61 , the reflecting mirrors  14 ,  15 , and  26 , and the condensing lens  27  are arranged in positions to establish an optical path for the third split beam  204 , such that ultrasonic waves energized when the measuring material  100  is irradiated with light of the second split beam  203  overlap in part with ultrasonic waves energized when the measuring material  100  is irradiated with light of the third split beam  204 . 
     As will be seen from the foregoing, according to the example 7 of embodiments, the measuring apparatus  7  for metallic microstructures or material properties is operable to employ, among others derived from an output pulse laser beam, a combination of a first split beam  202  and a second split beam  203  travelling an optical path requiring a longer light travel time than that, with a resultant difference between their arrival times on a surface of a measuring material  100 , and a combination of the second split beam  203  and a third split beam  204  travelling an optical path requiring a longer light travel time than that, with a resultant difference between their arrival times on the surface of the measuring material  100 , in the manner of rendering the differences different from each other. This permits a single shot of pulse laser beam to energize two kinds of ultrasonic waves with different frequency components, allowing for measurements of metallic microstructures or material properties, in particular for well precise measurements to be made of grain sizes in a metallic material. 
       FIG. 12(A)  as a graph showing an example of a sequence of pulse laser beams derived from a pulse laser oscillator in the measuring apparatus for metallic microstructures or material properties according to the example 7 of embodiments.  FIG. 12(B)  shows in a graph an example of a sequence of ultrasonic waves energized by the measuring apparatus  7  for metallic microstructures or material properties according to the example 7 of embodiments. 
       FIG. 12(A)  illustrates intensities of light of a first split beam  202  having reached a measuring material  100 , being maximized at a point of time t 11 , intensities of light of a second split beam  203  having reached the measuring material  100 , being maximized at a point of time t 13 , and intensities of light of a third split beam  204  having reached the measuring material  100 , being maximized at a point of time t 15 . 
     Then,  FIG. 12(B)  illustrates amplitudes of an ultrasonic wave energized at the point of time t 11  with light of the first split beam  202  having reached the measuring material  100 , being maximized at a point of time t 12 , amplitudes of an ultrasonic wave energized at the point of time t 13  with light of the second split beam  203  having reached the measuring material  100 , being maximized at a point of time t 14 , and amplitudes of an ultrasonic wave energized at the point of time t 15  with light of the second split beam  204  having reached the measuring material  100 , being maximized at a point of time t 16 . 
     Such being the case, there is a first split beam  202  combined with a second split beam  203  delayed therefrom to reach a measuring material  100 , so that ultrasonic waves energized when the measuring material  100  is irradiated with light of the first split beam  202  overlap in part with ultrasonic waves energized when the measuring material  100  is irradiated with light of the second split beam  203 . 
     Further, the second split beam  203  is combined with a third split beam  204  delayed therefrom to reach the measuring material  100 , so that ultrasonic waves energized when the measuring material  100  is irradiated with light of the second split beam  203  overlap in part with ultrasonic waves energized when the measuring material  100  is irradiated with light of the third split beam  204 . 
     Accordingly, a single shot of pulse laser beam can serve to energize two kinds of ultrasonic waves with different frequency components, allowing for measurements of metallic microstructures or material properties, in particular for well precise measurements to be made of grain sizes in a metallic material. 
     It is noted that according to the example 7 of embodiments, the measuring apparatus  7  for metallic microstructures or material properties may well have a set of high refractive index materials installed on an optical path of a second split beam  203  and an optical path of a third split beam  204 , like the measuring apparatus  6  for metallic microstructures or material properties according to the example 6 of embodiments. 
     Industrial Applicability 
     Embodiments herein are applicable to quality measurements of rolling materials in hot rolling mills. 
     Reference Signs List 
     
         
           1 ,  2 ,  3 ,  4 ,  5 ,  6 , or  7  . . . a measuring apparatus for metallic microstructures or material properties 
           11  or  24  . . . a pulse laser oscillator 
           12  . . . a half wave plate 
           13  . . . a first polarizing beam splitter 
           14 ,  15 ,  18 ,  19 ,  26 ,  62 , or  63  . . . a reflecting mirror 
           16  . . . a second polarizing beam splitter 
           17 ,  27 , or  64  . . . a condensing lens 
           20  or  21  . . . a quarter wave plate 
           22  . . . a polarizing beam splitter 
           25  . . . a half mirror 
           30  or  33  . . . a laser interferometer 
           31  . . . an oscilloscope 
           32  . . . a waveform analyzing computer 
           41  . . . an optical path length calculator 
           42  . . . a driver 
           43  . . . a motor 
           44  . . . a revolving shaft 
           45  . . . a mirror cabinet 
           51  or  52  . . . a high refractive index material, 
           60  or  61  . . . a non-polarizing beam splitter 
           60  . . . a non-polarizing beam splitter 
           100  . . . a measuring material 
           101  . . . a narrow line-width laser light source 
           102  . . . a beam splitter 
           103 ,  104 , or  106  . . . a condensing lens 
           105  . . . a photorefractive crystal 
           107  . . . a photodiode 
           108  . . . a wavelength selecting filter