Patent Publication Number: US-11644545-B2

Title: Distance measuring device, distance measuring method, and three-dimensional shape measuring apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a distance measuring device, a distance measuring method, and a three-dimensional shape measuring apparatus. 
     BACKGROUND ART 
     As a technique relating to an optical measuring instrument, for example, PTL 1 describes “A rod-shaped shaft (support member)  36  fixedly and integrally provided in a housing portion  34 , a reflection mirror  37  held by the shaft  36  in a movable state with the shaft  36  as a center, and a holding member  35  for holding the reflection mirror  37  at a predetermined position before and after a pivot are disposed in the housing portion  34 . The shaft  36 , the reflection mirror  37 , and the holding member  35  constitute a direction control unit for changing a traveling direction of a light for measurement output from a light source  38  to a predetermined direction (for example, 90 degrees).”, “The holding member  35  holds the reflection mirror  37  at a first position where the light for measurement from the light source  38  is not reflected before the reflection mirror  37  pivots about the shaft  36 . In addition, the holding member  35  functions to hold the reflection mirror  37  at the second position, after the reflection mirror  37  is rotated about the shaft  36  by a predetermined angle (for example, 45 degrees) to move to a second position where the traveling direction of the light for measurement from the light source  38  is changed to a predetermined direction (90 degrees).”, and “As a result, the reflection mirror  37  does not change the traveling direction of the light for measurement from the light source  38  at the first position, and operates to change the traveling direction of the light for measurement from the light source  38  to a predetermined direction at the second position.”. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP-A-2007-271601 
     SUMMARY OF INVENTION 
     Technical Problem 
     In a case of measuring a shape of a three-dimensional object by irradiating light, by changing a direction of irradiation, measurement can be performed while suppressing a movement of a measurement unit that emits light. 
     In the technique described in PTL 1, the direction of irradiation is changed by moving a mirror installed in the housing portion. In order to move the mirror in the housing portion, miniaturizing of the housing portion is limited. In this case, especially when measuring a narrow portion, the measurement may be limited. 
     The present invention has been made in view of such a situation, and it is an object to be able to measure a distance of a narrow portion. 
     Solution to Problem 
     Although the present application includes a plurality of means to solve at least a portion of the problem, if an example is given, it is as follows. In order to solve the above problems, a distance measuring device according to an aspect of the present invention includes a light emitting unit that outputs a measurement light, a first polarization state control unit that controls a polarization state of the measurement light output from the light emitting unit, a second polarization state control unit that controls the polarization state of the measurement light of which apolarization state is controlled by the first polarization state control unit, and an optical path switching element that selects an emission direction of the measurement light of which a polarization state is controlled by the second polarization state control unit, in which the second polarization state control unit controls the polarization state of the measurement light so that the measurement lights are emitted from the optical path switching element in a plurality of emission directions, and the optical path switching element receives a reflected light obtained by reflecting the emitted measurement light by an object. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to measure a distance of a narrow portion. 
     Problems, configurations, and effects other than those described above will be apparent from the description of the embodiments below. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic view illustrating a configuration example of a distance measuring device according to an embodiment of the present invention. 
         FIGS.  2 A and  2 B  are a diagram for describing an operation of a polarization beam splitter as an example of an optical path switching element. 
         FIG.  3    is a diagram for describing a relationship among a first polarization state control unit, a second polarization state control unit, and the optical path switching element. 
         FIG.  4    is a diagram for describing a relationship among the first polarization state control unit, the second polarization state control unit, and the optical path switching element. 
         FIGS.  5 A and  5 B  are a diagram for describing an operation by a combination of a birefringence plate as an example of the optical path switching element and a mirror. 
         FIG.  6    is a diagram illustrating a first configuration example of a distance measuring control unit. 
         FIG.  7    is a graph for describing an example of a method of determining a reflection position on a surface of an object T from a reflection intensity profile. 
         FIG.  8    is a diagram illustrating a second configuration example of the distance measuring control unit. 
         FIG.  9    is a diagram illustrating a third configuration example of the distance measuring control unit. 
         FIG.  10    is a diagram for describing a relationship among the first polarization state control unit, the second polarization state control unit, and the optical path switching element. 
         FIG.  11    is a schematic view illustrating a first configuration example of a three-dimensional shape measuring apparatus adopting the distance measuring device. 
         FIG.  12    is a schematic view illustrating a second configuration example of the three-dimensional shape measuring apparatus adopting the distance measuring device. 
         FIG.  13    is a diagram illustrating an example of functional blocks of the three-dimensional shape measuring apparatus. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a plurality of embodiments according to the present invention will be described based on the drawings. In all the drawings for describing each embodiment, the same reference numeral is attached to the same member in principle, and the repetitive description thereof will not be repeated. In addition, in the following embodiments, it goes without saying that the constituent elements (including element steps and the like) are not necessarily essential except in a case of being particularly clearly stated and in a case of being considered to be obviously essential in principle. In addition, when saying “formed of A”, “containing of A”, “having A”, and “including A”, except in a case of being particularly stated that only that element is specified, it goes without saying that it does not exclude other elements. Similarly, in the following embodiments, when referring to the shapes, positional relationships, and the like of components, except in a case of being particularly clearly stated and in a case of being obviously considered not to be in principle, substantially similar or similar shapes and the like are included. 
     &lt;Configuration Example of Distance Measuring Device According to One Embodiment of Present Invention&gt; 
       FIG.  1    is a schematic view illustrating a configuration example of a distance measuring device  10  according to an embodiment of the present invention. The distance measuring device  10  is configured to connect a distance measuring control unit  110  and a measurement probe  160  via a connection cable  150 . 
     The distance measuring control unit  110  generates measurement light and outputs the measurement light to the measurement probe  160  via the connection cable  150 . The connection cable  150  is made of, for example, an optical fiber, and guides the measurement light output from the distance measuring control unit  110  to the measurement probe  160 . The measurement probe  160  irradiates an object T with the measurement light, receives the measurement light (reflected light) reflected by the object T, and outputs the received reflected light to the distance measuring control unit  110  via the connection cable  150 . 
     The measurement probe  160  includes a lens unit  161 , a rotation unit  162 , an optical path switching element  163 , a measurement probe tip end unit  164 , a first polarization state control unit  165 , and a second polarization state control unit  166 . 
     The lens unit  161  focuses the measurement light guided from the distance measuring control unit  110  via the connection cable  150  and guides the measurement light to the first polarization state control unit  165 . The rotation unit  162  has a rotation drive unit such as a motor. The rotation unit  162  rotationally drives a motor or the like according to control from a distance calculation unit (not illustrated) built in a control device  210  ( FIG.  6   ), and simultaneously rotates the second polarization state control unit  166  and the optical path switching element  163  about a rotation axis parallel to the measurement light output from the lens unit  161 . 
     The optical path switching element  163  is made of, for example, a polarization beam splitter  180  ( FIGS.  2 A and  2 B ) or a combination of a birefringence plate  181  and a mirror  182  ( FIGS.  5 A and  5 B ). The optical path switching element  163  selectively emits the measurement light of which a circularly polarized light direction is controlled by a linear polarization switching element  302  ( FIG.  6   ) and the first polarization state control unit  165  according to the direction of the polarization. Specifically, the optical path switching element  163  emits the light toward at least one of a first direction  300   a  (z-axis direction) that is the same as the traveling direction of the measurement light output from the lens unit  161  and a second direction  300   b  (direction parallel to xy plane) substantially orthogonal to the first direction  300   a.    
     The measurement probe tip end unit  164  locks the second polarization state control unit  166  and the optical path switching element  163 , and passes the light emitted from the optical path switching element  163  in the first direction  300   a  or the second direction  300   b . The measurement probe tip end unit  164  is, for example, a tubular shape having an opening portion in the first direction  300   a , is made of a material that transmits light, and locks the optical path switching element  163  to at least a portion of the inner wall. The measurement probe tip end unit  164  rotates about the rotation axis parallel to the measurement light output from the lens unit  161 , and the second polarization state control unit  166  and the optical path switching element  163  rotate as the measurement probe tip end unit  164  rotates. The measurement probe tip end unit  164  points from a rotation position of the rotation unit  162  to a position where the optical path switching element  163  is locked, and rotates with the control of the rotation unit  162 . 
     The structure of the measurement probe tip end unit  164  is not limited to the example described above. For example, the measurement probe tip end unit  164  may lock the second polarization state control unit  166  and the optical path switching element  163  to one or more columns, and the optical path switching element  163  may be rotated as the column is driven. In addition, the measurement probe tip end unit  164  may be formed of, for example, a transparent two-layer cylinder, the second polarization state control unit  166  and the optical path switching element  163  may be locked to the inner layer side cylinder, and the second polarization state control unit  166  and the optical path switching element  163  may be rotated. In addition, the second polarization state control unit  166  and the optical path switching element  163  may be adhered directly and locked to the measurement probe tip end unit  164 , or the second polarization state control unit  166  and the optical path switching element  163  may be individually locked to the measurement probe tip end unit  164 . 
     The first polarization state control unit  165  is formed of, for example, a quarter wavelength plate, a liquid crystal element, or a fiber type polarization control element. The first polarization state control unit  165  is fixed inside the measurement probe  160  and does not rotate. The first polarization state control unit  165  controls the polarization of the measurement light output from the distance measuring control unit  110 , and changes, for example, the circularly polarized light direction of the measurement light. The detailed operation of the first polarization state control unit  165  will be described later. 
     The second polarization state control unit  166  is formed of, for example, a quarter wavelength plate. As described above, the second polarization state control unit  166  is rotated by the rotation unit  162  about the optical path switching element  163  and the first direction  300   a  as the rotation axis. The second polarization state control unit  166  outputs the polarization state of the measurement light controlled by the first polarization state control unit  165  to the optical path switching element  163  while maintaining a constant state with respect to the optical path switching element  163 . 
     In the distance measuring device  10 , the measurement light output from the distance measuring control unit  110  reaches the first polarization state control unit  165  via the connection cable  150  and the lens unit  161 , and the polarization is controlled by the first polarization state control unit  165 . The measurement light of which polarization is controlled by the first polarization state control unit  165  is again controlled in polarization by the second polarization state control unit  166 , and reaches the optical path switching element  163 , and is emitted in the first direction  300   a  or the second direction  300   b  according to the polarization. 
     The light emitted from the optical path switching element  163  reaches the object T, and the light reflected or scattered by the object T travels in a path opposite to that at the time of emission, that is, in the order of the optical path switching element  163 , the second polarization state control unit  166 , the first polarization state control unit  165 , the lens unit  161 , and the connection cable  150 , to reach the distance measuring control unit  110 . The distance measuring control unit  110  converts the reached measurement light into a predetermined electric signal and outputs the electric signal to the control device  210 . In the control device  210 , a built-in distance calculation unit calculates the distance to the object T based on a predetermined electric signal input from the distance measuring control unit  110 . 
     In a case where the object T has, for example, a cylindrical shape, the depth to the bottom of the cylindrical shape can be measured by using the measurement light emitted in the first direction  300   a . In addition, for example, the shape of the side surface of the cylindrical shape can be measured by using the measurement light emitted in the second direction  300   b.    
     Next, the operation of the optical path switching element  163  will be described.  FIGS.  2 A and  2 B  are a diagram for describing the operation in a case where the polarization beam splitter  180  is used as the optical path switching element  163 .  FIG.  2 A  illustrates a state where the measurement light as linearly polarized light oscillates in a horizontal direction of the drawing.  FIG.  2 B  illustrates a state where the measurement light as linearly polarized light oscillates in the depth direction (direction perpendicular to drawing) of the drawing. 
     In the state illustrated in  FIG.  2 A , the incident measurement light passes through the reflection surface of the polarization beam splitter  180 , and travels in the same first direction  300   a  as the incident direction. The light reflected by the object T travels the same path backward and reaches the distance measuring control unit  110 . 
     In addition, in the state illustrated in  FIG.  2 B , the incident measurement light is reflected by the reflection surface of the polarization beam splitter  180 , and travels in the second direction  300   b  substantially orthogonal to the incident direction of the measurement light. In the light traveling in the second direction  300   b , the light reflected by the object T travels the same path backward in the same manner as the light traveling in the first direction  300   a , and reaches the distance measuring control unit  110 . 
     When the polarization state of the measurement light is controlled by a polarization stabilization unit  301  to be described later, the linear polarization switching element  302 , the first polarization state control unit  165 , and the second polarization state control unit  166  so as to maintain a predetermined angle with respect to the optical path switching element  163  utilizing this property, it is possible to maintain the traveling direction of the measurement light in the first direction  300   a  or the second direction  300   b.    
     That is, by controlling the polarization of the measurement light by the polarization stabilization unit  301 , the linear polarization switching element  302 , the first polarization state control unit  165 , and the second polarization state control unit  166 , the traveling direction of the measurement light can be switched to the first direction  300   a  or the second direction  300   b.    
     Next,  FIGS.  3  and  4    are diagrams for describing a relationship among the first polarization state control unit  165 , the second polarization state control unit  166 , and the optical path switching element  163 . However, in  FIGS.  3  and  4   , a quarter wavelength plate  305  is adopted as the first polarization state control unit  165 , a quarter wavelength plate  311  is adopted as the second polarization state control unit  166 , and the polarization beam splitter  180  is adopted as the optical path switching element  163 . 
     In a case where the optical axis of the quarter wavelength plate  305  is used as a reference, as illustrated in  FIG.  3   , when the angle α in an oscillation direction of the linearly polarized light in the measurement light incident on the quarter wavelength plate  305  is t/4, the measurement light emitted from the quarter wavelength plate  305  has a circular polarization state (left circularly polarized light)  307   a  that rotates clockwise when viewed in the traveling direction. 
     In addition, as illustrated in  FIG.  4   , when the angle α in the oscillation direction of the linearly polarized light in the measurement light incident on the quarter wavelength plate  305  is 3π/4, the measurement light emitted from the quarter wavelength plate  305  has a circular polarization state (right circularly polarized light)  307   b  that rotates counterclockwise when viewed in the traveling direction. 
     Normally, the quarter wavelength plate has a property of emitting linearly polarized light that oscillates in a direction according to the rotation direction of circularly polarized light when the light of circularly polarized light is incident. Therefore, as illustrated in  FIG.  3   , when the left circularly polarized light  307   a  is incident on the quarter wavelength plate  311 , the linearly polarized light having an angle of π/4 with respect to the optical axis of the quarter wavelength plate  311  is emitted. In addition, as illustrated in  FIG.  4   , when the right circularly polarized light  307   b  is incident on the quarter wavelength plate  311 , the linearly polarized light having an angle of 3π/4 with respect to the optical axis of the quarter wavelength plate  311  is emitted. 
     The polarization beam splitter  180  disposed on the rear portion of the quarter wavelength plate  311  transmits the linearly polarized light in the oscillation direction parallel to an incident surface  309 , that is, emits the linearly polarized light in the first direction  300   a . In addition, the polarization beam splitter  180  reflects the linearly polarized light in the oscillation direction forming an angle of π/2 with respect to the incident surface  309 , that is, emits the linearly polarized light in the second direction  300   b.    
     The quarter wavelength plate B 311  and the polarization beam splitter  180  are rotated by the rotation unit  162 . Therefore, if the quarter wavelength plate B 311  and the polarization beam splitter  180  are rotated in a state where the quarter wavelength plate B 311  and the polarization beam splitter  180  are locked to the measurement probe tip end unit  164 , respectively, so that the angle between the optical axis of quarter wavelength plate B 311  and the incident surface of the polarization beam splitter  180  is π/4, the distance measurement by the measurement light traveling in the first direction  300   a  and the distance measurement by the measurement light traveling in the second direction  300   b  can be performed. 
     Incidentally, the switching of the oscillation direction of the linearly polarized light in the measurement light incident on the quarter wavelength plate  305  is performed by the linear polarization switching element  302  ( FIG.  6   ). In a case where no voltage is applied to the linear polarization switching element  302 , as illustrated in  FIG.  3   , the linear polarization switching element  302  adjusts the oscillation direction of the linearly polarized light in the measurement light incident on the quarter wavelength plate  305  to a first measurement light oscillation direction  306   a . In addition, in a case where a voltage is applied to the linear polarization switching element  302 , as illustrated in  FIG.  4   , the linear polarization switching element  302  adjusts the oscillation direction of the linearly polarized light in the measurement light incident on the quarter wavelength plate  305  is a second measurement light oscillation direction  306   b . That is, by electrically switching and controlling the linear polarization switching element  302 , it is possible to switch from the measurement probe tip end unit  164  to the first direction  300   a  or the second direction  300   b  to emit the measurement light. 
     For example, as illustrated in  FIG.  3   , in a case of irradiating in the first direction  300   a  with the measurement light, the voltage application to the linear polarization switching element  302  may be stopped so that the oscillation direction of the linearly polarized light in the measurement light is the first measurement light oscillation direction  306   a  having an inclination of π/4 with respect to a main axis  308  of the quarter wavelength plate A. As a result, the measurement light is converted into the left circular polarization state by the quarter wavelength plate A 305 , and thereafter converted into linearly polarized light oscillating parallel to the incident surface  309  of the polarization beam splitter  180  by the quarter wavelength plate B 311  and emitted in the first direction  300   a.    
     In addition, for example, as illustrated in  FIG.  4   , in a case of irradiating in the second direction  300   b  with the measurement light, the voltage application to the linear polarization switching element  302  may be performed so that the oscillation direction of the linearly polarized light in the measurement light is the second measurement light oscillation direction  306   b  having an inclination of 3π/4 with respect to the main axis  308  of the quarter wavelength plate A. As a result, the measurement light is converted into the right circular polarization state by the quarter wavelength plate A 305 , and thereafter converted into linearly polarized light oscillating perpendicular to the incident surface  309  of the polarization beam splitter  180  by the quarter wavelength plate B 311  and emitted in the second direction  300   b.    
     Although the example in which the quarter wavelength plate  305  is adopted as the first polarization state control unit  165  is described so far, the liquid crystal element may be adopted as the first polarization state control unit  165 . In that case, the first polarization state control unit  165  can change the polarization direction of the measurement light to be output by controlling the voltage applied to the liquid crystal element as the first polarization state control unit  165  and controlling an optical rotation of the liquid crystal element. 
     In addition, a fiber type polarization control element may be adopted as the first polarization state control unit  165 . In that case, if twisting or compression is applied to the fiber type polarization control element as the first polarization state control unit  165 , the polarization direction of the measurement light output from the first polarization state control unit  165  can be controlled by induction of birefringence. 
     Next,  FIGS.  5 A and  5 B  are a diagram for describing the operation in a case of the combination of the birefringence plate  181  and the mirror  182  is used for the optical path switching element  163 .  FIG.  5 A  illustrates a state where the measurement light as the linearly polarized light oscillates in the depth direction of the drawing (direction perpendicular to drawing).  FIG.  5 B  illustrates a state where the measurement light as the linearly polarized light oscillates in the horizontal direction of drawing. 
     A birefringence plate  181  has the property of shifting the optical path according to the polarization state of the measurement light. Therefore, the birefringence plate  181  forming the optical path switching element  163  is installed to straighten the measurement light of the linearly polarized light oscillating in the depth direction of the drawing as illustrated in  FIG.  5 A , and to shift the optical path of the measurement light of the linearly polarized light oscillating in the horizontal direction of the drawing as illustrated in  FIG.  5 B . Furthermore, the mirror  182  forming the optical path switching element  163  is disposed on the optical path shifted by the birefringence plate  181  to change an emission direction of the shifted measurement light. 
     As a result, as in the case where the polarization beam splitter  180  is used for the optical path switching element  163  ( FIGS.  2 A and  2 B ), it is possible to selectively emit in the first direction  300   a  having the same optical axis as the measurement light emitted from the lens unit  161  or in the second direction  300   b  different in the optical axis from the first direction  300   a  with the light. 
     However, it is required to note that the relationship between the polarization direction and the light emission direction is reversed between the case where the polarization beam splitter  180  is used as the optical path switching element  163  ( FIGS.  2 A and  2 B ) and the case where the combination of the birefringence plate  181  and the mirror  182  is used as the optical path switching element  163  ( FIGS.  5 A and  5 B ). 
     As described above, the measurement probe  160  can emit in different directions with the measurement light from the optical path switching element  163  of the measurement probe tip end unit  164 . Therefore, for example, as compared with a case where the emission direction of the measurement light is selectively changed by providing the mirror inside the measurement probe tip end unit  164  and driving the mirror, the measurement probe tip end unit  164  can be miniaturized as much as the mirror is unnecessary. 
     Next, a configuration example of the distance measuring control unit  110  will be described. 
     &lt;First Configuration Example of Distance Measuring Control Unit  110 &gt; 
       FIG.  6    illustrates a first configuration example of the distance measuring control unit  110 . The first configuration example of the distance measuring control unit  110  measures the distance to the object T using a frequency modulated continuous waves (FMCW) or a swept-source optical coherence tomography (SS-OCT) (or wavelength sweep OCT) as a distance measuring method. Although the basic principles of FMCW and SS-OCT are common, FMCW is mainly used for long distance measurement using a light source with a long coherence length, and SS-OCT is mainly used for the measurement of fine structures using a light source with a short coherence length. 
     The first configuration example of the distance measuring control unit  110  is connected to the control device  210  in addition to the measurement probe  160 . The control device  210  is provided with a distance calculation unit (not illustrated) that calculates a distance to the object T using a predetermined electric signal input from the distance measuring control unit  110 . The distance calculation unit may be provided in the distance measuring control unit  110 . The control device  210  allows a display device  220  to display the calculation result of the distance to the object T. In addition, the control device  210  may be connected to be able to directly communicate with the measurement probe  160  without via the distance measuring control unit  110 . 
     The first configuration example of the distance measuring control unit  110  includes a light emitting unit  101 , an oscillating unit  102 , optical fiber couplers  103 ,  104 ,  106 , and  114 , an optical fiber  105 , light receiving units  107  and  109 , an optical circulator  108 , a reference mirror  112 , the optical switches  113   a  and  113   b , a distance measuring control mechanism control unit  111 , a polarization stabilization unit  301 , and a linear polarization switching element  302 . 
     The oscillating unit  102  injects a triangular wave current to the light emitting unit  101  based on a sweep waveform signal from the distance measuring control mechanism control unit  111 , and modulates the drive current. The light emitting unit  101  generates frequency modulated (FM) light temporally frequency-swept at a constant modulation speed by the modulated drive current, and outputs the generated light to the polarization stabilization unit  301 . 
     The light emitting unit  101  may be configured as a semiconductor laser device with an external resonator, and the resonant wavelength of the light emitting unit  101  may be changed by a triangular wave control signal from the oscillating unit  102 . Also in this case, the light emitting unit  101  can generate FM light temporally frequency-swept. 
     The polarization stabilization unit  301  normally stabilizes the polarization state of the FM light input from the light emitting unit  101  as a linear polarization state in a constant direction and outputs the FM light to the linear polarization switching element  302 . The linear polarization switching element  302  outputs the direction of the linearly polarized light of the FM light input from the polarization stabilization unit  301  as it is to the rear portion or rotates the direction by π/2 and outputs the direction to the rear portion by applying a voltage to the built-in liquid crystal element. 
     Since the polarization stabilization unit  301  and the linear polarization switching element  302  are used to output the linearly polarized light having a desired oscillation direction, a combination of a general polarization state analyzer and a polarization state generator can be adopted. 
     The light (linearly polarized light) output from the linear polarization switching element  302  is incident on the optical fiber coupler  103 . The optical fiber coupler  103  splits the incident light into two, and outputs one to the optical fiber coupler  104  of the reference optical system. The optical fiber couplers  103 ,  104 , and  114  may be, for example, beam splitters. 
     The light incident on the optical fiber coupler  104  is split into two, and after a predetermined optical path difference is provided for one of these, the light is multiplexed by the optical fiber coupler  106  and received by the light receiving unit  107 . The light receiving unit  107  functions as a Mach-Zehnder interferometer, and detects a constant reference beat signal proportional to the optical path difference. 
     The other of the light split into two by the optical fiber coupler  103  is guided to the optical fiber coupler  114  by the optical circulator  108 , and split into two by the optical fiber coupler  114 . One is reflected by the reference mirror  112  to be a reference light, and the other is output to the measurement probe  160  via the optical switches  113   a  and  113   b  to be irradiated the object T. The operation of the optical switches  113   a  and  113   b  will be described later. The reflected light (measurement light) reflected by the object T is returned to the distance measuring control unit  110  via the connection cable  150 . 
     The measurement light returned to the distance measuring control unit  110  passes through the optical switches  113   a  and  113   b , is multiplexed with the reference light reflected by the reference mirror  112  by the optical fiber coupler  114 , and is guided to the light receiving unit  109  by the optical circulator  108 . The light receiving unit  109  detects a measurement beat signal generated by the interference between the reference light and the measurement light. 
     The distance measuring control mechanism control unit  111  performs A/D conversion of the measurement beat signal detected by the light receiving unit  109  using the reference beat signal detected by the light receiving unit  107  as a sampling clock. Alternatively, the distance measuring control mechanism control unit  111  samples the reference beat signal and the measurement beat signal at a constant sampling clock. 
     More specifically, the distance measuring control mechanism control unit  111  performs Hilbert transform of the reference beat signal to generate a signal of which a phase is shifted by π/2, a local phase of the signal is obtained from the reference signal before and after Hilbert transform, and the phase is interpolated to determine the timing at which the reference signal has a constant phase. Furthermore, the distance measuring control mechanism control unit  111  interpolates and samples the measurement beat signal in accordance with the timing to resample the measurement signal based on the reference signal. 
     The distance measuring control mechanism control unit  111  can obtain similar result even if the measurement signal is sampled and A/D converted using the reference beat signal as a sampling clock by the built-in AD/DA converter. 
     Regarding the analysis of the beat signal, although a time difference Δt exists in the arrival timing of the measurement light and the reference light to the light receiving unit  109 , since the frequency of the light source changes during this time, the measurement beat signal of the beat frequency fb equal to the frequency difference due to this change is detected. Assuming that the frequency sweep width is Δv and the time required for modulation by Δv is T, there is a relationship of the following equation (1) 
     
       
         
           
             
               
                 
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     Since the distance L to the object T is half of the distance traveled by light during Δt, it can be calculated as in the following equation (2) using the light velocity c in the atmosphere. 
     
       
         
           
             
               
                 
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                   ) 
                 
               
             
           
         
       
     
     When the first fourier transform (FFT) is performed on the measurement signal obtained by the distance measuring control mechanism control unit  111 , and the peak position and the size thereof are obtained, the reflection position and the reflection light amount of the object T can be known. For example, in the OCT device, since it is desired to visualize the scattering position and the size of scattering of a translucent body such as a living body, the amplitude spectrum of the FFT can be used as it is. In the present embodiment, in order to obtain the position of the surface of the object T accurately, interpolation as illustrated in  FIG.  7    is performed to increase the distance detection resolution. 
       FIG.  7    is a graph for describing an example of a method of determining the reflection position on the surface of the object T from the reflection intensity profile. In  FIG.  7   , the horizontal axis indicates the frequency of the FFT, and the vertical axis indicates the reflection intensity. As illustrated in  FIG.  7   , discrete data is obtained near the peak of the reflection intensity. The interval between points, that is, the distance resolution, is c/2Δv. In a case of SS-OCT, the normal wavelength is 1300 nm, the sweep width is 100 nm, and the frequency sweep width Δv is 17.8 THz, so that the distance resolution c/2Δv is 8.4 m. 
     In addition, in a case of FMCW, the normal wavelength is 1500 nm, the sweep width is 2 nm, and the frequency sweep width Δv is 267 GHz, so that the distance resolution c/2Δv is 0.56 mm. On the other hand, when a function such as a quadratic function or a Gaussian function is fitted using three or more points near the peak as illustrated in  FIG.  7    and interpolation is performed using values near the peak of the fitted function, it is possible to increase the resolution to approximately 1/10. 
     Next, the optical switches  113   a  and  113   b  forming the distance measuring control unit  110  will be described. The optical switches  113   a  and  113   b  are controlled to be switched by the distance measuring control mechanism control unit  111 . In order to obtain a beat signal due to interference between the reference light and the measurement light, the difference between the optical path length from the optical fiber coupler  114  to the reference mirror  112  and the optical path length from the optical fiber coupler  114  to the object T is required to be the coherence length of the light emitting unit  101  or less. In order to keep this condition, the optical switch  113   a  and the optical switch  113   b  are simultaneously switched according to the distance from the optical fiber coupler  114  to the object T, and the length of the optical fiber between the switches is changed. 
     In addition, in a case where the difference between the optical path length from the optical fiber coupler  114  to the reference mirror  112  and the optical path length from the optical fiber coupler  114  to the object T is too long, that is, in a case where the coherence length is long, the beat frequency is too high to be detected by the light receiving unit  109 . Therefore, the optical switch  113   a  and the optical switch  113   b  are simultaneously switched to change the length of the optical fiber between the switches so that the beat frequency is a frequency detectable by the light receiving unit  109 . 
     In the first configuration example illustrated in  FIG.  6   , the optical switches  113   a  and  113   b  switch two optical fibers having different lengths, and three or more optical fibers having different lengths may be switched according to the range of the measurement target. In addition, the timing at which the optical switches  113   a  and  113   b  switch the optical fibers may be constant, or may be changed according to the conditions such as the distance from the optical path switching element  163  of the object T. For example, the optical switch  113   a  and the optical switch  113   b  may be switched every one rotation in synchronization with the rotation of the optical path switching element  163 . 
     In addition, although it is described that the optical fiber is used for the optical path, once light is propagated in free space using an optical fiber collimator or the like, the light may be switched by the mirror or the like, or the mirror may be moved to change the optical path length. 
     In addition, an optical switch similar to the optical switches  113   a  and  113   b  may be provided in the optical path between the optical fiber coupler  114  used for branching and the reference mirror  112 , and the length of the optical fiber may be switched. 
     In the first configuration example illustrated in  FIG.  6   , the optical path from the optical fiber coupler  114  to the optical switch  113   b  is installed in the distance measuring control unit  110 . However, these optical paths may be installed in the measurement probe  160  instead of the distance measuring control unit  110 . 
     In addition, a distance measuring method performed using the distance measuring control unit  110  is not limited to the above-described example. For example, it is possible to use a method of irradiating the object T with pulse or burst light and measuring the time until the pulse or burst is received as in a time of flight (TOF) method, and a method of irradiating the object T with light continuously modulated in intensity and measuring the phase of the received signal as in a Phase⋅Shift method or an optical comb distance measuring method. In addition, a distance may be measured by measuring defocus, or a white confocal method, an astigmatism method, a knife edge method, or a conoscopic holographic method may be adopted. 
     &lt;Second Configuration Example of Distance Measuring Control Unit  110 &gt; 
     Next,  FIG.  8    illustrates a second configuration example of the distance measuring control unit  110 . The second configuration example of the distance measuring control unit  110  measures the distance to the object T using a spectral domain-optical coherence tomography (SD-OCT) (or frequency domain OCT) as a distance measuring method. 
     The second configuration example of the distance measuring control unit  110  includes the optical circulator  108 , the optical fiber coupler  114 , the reference mirror  112 , the distance measuring control mechanism control unit  111 , a broadband light emitting unit  115 , a spectroscope  116 , the polarization stabilization unit  301 , and the linear polarization switching element  302 . Among the components of the second configuration example of the distance measuring control unit  110 , the components common to the components of the first configuration example ( FIG.  6   ) are given the same reference numerals, and the description thereof will be appropriately omitted. 
     The broadband light emitting unit  115  generates broadband light as measurement light according to the control from the distance measuring control mechanism control unit  111 . The generated measurement light reaches the optical circulator  108  via the polarization stabilization unit  301  and the linear polarization switching element  302 , is guided to the optical fiber coupler  114  by the optical circulator  108 , and is split into two by the optical fiber coupler  114 . One of the split measurement lights is emitted to the object T via the measurement probe  160 . In addition, the other of the split measurement lights is reflected by the reference mirror  112  to be a reference light. The measurement light reflected by the object T returns to the distance measuring control unit  110  via the measurement probe  160 , is multiplexed with the reference light by the optical fiber coupler  114 , is guided to the spectroscope  116  by the optical circulator  108 , and the spectrum is analyzed. 
     This spectrum illustrates oscillations of frequency proportional to the difference in optical path length between the object T and the reference mirror  112 . Therefore, the distance measuring control mechanism control unit  111  in the second configuration example realizes the distance measurement of the object T by analyzing this frequency. 
     &lt;Third Configuration Example of Distance Measuring Control Unit  110 &gt; 
     Next,  FIG.  9    illustrates a third configuration example of the distance measuring control unit  110 . In the third configuration example of the distance measuring control unit  110 , measurement light can be simultaneously emitted in both directions from the measurement probe tip end unit  164  of the measurement probe  160  without switching between the first direction  300   a  and the second direction  300   b.    
     Similar to the first configuration example ( FIG.  6   ), the third configuration example of the distance measuring control unit  110  measures the distance to the object T using the spectral domain-optical coherence tomography (SD-OCT) (or frequency domain OCT) as the distance measuring method. Among the components of the third configuration example of the distance measuring control unit  110 , the components common to the components of the first configuration example ( FIG.  6   ) are given the same reference numerals, and the description thereof will be appropriately omitted. 
     In the third configuration example of the distance measuring control unit  110 , the linear polarization switching element  302  is omitted from the first configuration example ( FIG.  6   ), optical fiber couplers  314  and  315  between the optical fiber coupler  103  and the optical circulator  108 , and the optical fibers  316   a  and  316   b  are added, and a light receiving unit  117  and an optical fiber coupler  118  are added to the rear portion of the optical circulator  108 . 
     The optical fiber coupler  314  splits light, which is output from the polarization stabilization unit  301  and input via the optical fiber coupler  103 , in which the linear polarization state in a constant direction is stabilized, split into two. The optical fiber  316   a  guides one of the light split into two by the optical fiber coupler  314  to the optical fiber coupler  315  while maintaining the linear polarization state. The optical fiber  316   b  is adjusted so that the difference between the length thereof and the length of the optical fiber  316   a  is longer than the coherence length of the measurement light. As a result, the interference between the left circularly polarized light  307   a  and the right circularly polarized light  307   b  ( FIG.  10   ) simultaneously output from the first polarization state control unit  165  can be prevented. The optical fiber  316   b  is physically connected to the optical fiber coupler  315  in a state of being twisted by π/2 in comparison with the optical fiber  316   a . As a result, light of which linear polarization states are orthogonal to each other is incident on the optical fiber coupler  315  via the optical fibers  316   a  and  316   b . The optical fiber coupler  315  multiplexes the lights of which linear polarization states are orthogonal to each other and outputs the multiplexed light to the optical circulator  108 . 
     The optical fiber coupler  118  splits the light in which the reference light and the measurement light are multiplexed, which are incident from the optical circulator  108 , into two, outputs one to the light receiving unit  109 , and outputs the other to the light receiving unit  117 . 
     The light receiving unit  109  corresponds to the reflected light from the first direction  300   a  ( FIG.  1   ), and detects a constant reference beat signal proportional to the optical path difference. The light receiving unit  117  corresponds to the reflected light from the second direction  300   b  ( FIG.  1   ), and detects a constant reference beat signal proportional to the optical path difference. 
     Next,  FIG.  10    is a diagram for describing the relationship among the first polarization state control unit  165 , the second polarization state control unit  166 , and the optical path switching element  163  in the third configuration example of the distance measuring control unit  110 . However, in  FIG.  10   , the quarter wavelength plate  305  is adopted as the first polarization state control unit  165 , the quarter wavelength plate  311  is adopted as the second polarization state control unit  166 , and the polarization beam splitter  180  is adopted as the optical path switching element  163 . 
     In a case where the optical axis of the quarter wavelength plate  305  is used as a reference, as illustrated in  FIG.  10   , when the measurement light of which linear polarization states are orthogonal to each other is incident on the quarter wavelength plate  305 , the circular polarization state (left circularly polarized light)  307   a  that rotates clockwise when the traveling direction is viewed from the quarter wavelength plate  305  and the circular polarization state (right circularly polarized light)  307   b  that rotates counterclockwise when the traveling direction is viewed are simultaneously emitted. 
     Next, in the quarter wavelength plate  311 , the linearly polarized light having an angle of π/4 with respect to the optical axis of the quarter wavelength plate  311  is emitted corresponding to the incident left circularly polarized light  307   a . The linearly polarized light having an angle of 3π/ 4  with respect to the optical axis of the quarter wavelength plate  311  is emitted corresponding to the incident right circularly polarized light  307   b.    
     The polarization beam splitter  180  disposed on the rear portion of the quarter wavelength plate  311  transmits the linearly polarized light in the oscillation direction parallel to the incident surface  309 , that is, emits with the linearly polarized light in the first direction  300   a . In addition, the polarization beam splitter  180  reflects the linearly polarized light in the oscillation direction having an angle of π/2 with respect to the incident surface  309 , which is simultaneously incident, that is, emits with the linearly polarized light in the second direction  300   b.    
     Therefore, in the third configuration example of the distance measuring control unit  110 , the measurement light can be simultaneously emitted in both directions from the measurement probe tip end unit  164  of the measurement probe  160  without switching between the first direction  300   a  and the second direction  300   b . The distance measuring control mechanism control unit  111  can calculate the distances to the object T in both the first direction  300   a  and the second direction  300   b  substantially simultaneously. 
     &lt;Another Configuration Example of Distance Measuring Control Unit  110 &gt; 
     The first configuration example ( FIG.  6   ) and the third configuration example ( FIG.  9   ) of the distance measuring control unit  110  described above adopt FMCW or SS-OCT as the distance measuring method, and the second configuration example ( FIG.  8   ) of the distance measuring control unit  110  adopts SD-OCT as the distance measuring method. Examples of another distance measuring method that may be adopted by the distance measuring control unit  110  include a white confocal method. 
     In the configuration example of the distance measuring control unit  110  adopting the white confocal method, the reference mirror  112  and the optical fiber coupler  114  are omitted from the second configuration example ( FIG.  8   ), and instead, in the lens unit  161  of the measurement probe  160 , a configuration in which chromatic aberration occurs intentionally is adopted, and the measurement probe  160  of which the focal position is different depending on the wavelength of measurement light is used. 
     In the case of this configuration example, when the light reflected or scattered on the object T is collected again by the lens unit  161  and returned to the distance measuring control unit  110 , only wavelengths that are in focus at the distance to the object T are captured. That is, when the light is detected by the spectroscope  116  and the wavelength at which the spectrum reaches the peak is calculated by the distance measuring control mechanism control unit  111 , distance measurement of the object T can be realized. According to this configuration example, the detected spectrum data itself can be obtained as the data illustrated in  FIG.  7    without performing the FFT on the measurement light. 
     &lt;First Configuration Example of Three-Dimensional Shape Measuring Apparatus Adopting Distance Measuring Device  10 &gt; 
     Next,  FIG.  11    is a schematic view illustrating a first configuration example of a three-dimensional shape measuring apparatus  20  adopting the distance measuring device  10 . The three-dimensional shape measuring apparatus  20  measures the three-dimensional shape of the object T. 
     The first configuration example of the three-dimensional shape measuring apparatus  20  includes a moving mechanism  250  ( FIG.  13   ) including an xz-axis moving mechanism  251  and a y-axis moving mechanism  252 . 
     The distance measuring device  10  having the measurement probe  160  is installed in the xz-axis moving mechanism  251 . The xz-axis moving mechanism  251  can move in the x-axis direction (horizontal direction in the drawing) and the z-axis direction (vertical direction in the drawing). The measurement probe tip end unit  164  of the measurement probe  160  also moves along with the movement of the xz-axis moving mechanism  251 . The y-axis moving mechanism  252  is a gate-shaped structure, and can move in the y-axis direction (depth direction in the drawing). The y-axis moving mechanism  252  supports the xz-axis moving mechanism  251 , and the measurement probe tip end unit  164  of the measurement probe  160  supported by the xz-axis moving mechanism  251  also moves along with the movement of the y-axis moving mechanism  252 . Therefore, according to the xz-axis moving mechanism  251  and the y-axis moving mechanism  252 , the posture of the object T in three degrees of freedom can be controlled. 
     The configuration of the moving mechanism is not limited to the configuration m described above, and any configuration that can move the measurement probe tip end unit  164  in three axial directions may be used. For example, a configuration that moves the measurement probe tip end unit  164  in three axial directions may be used by installing only the measurement probe  160  in the xz-axis moving mechanism  251  without installing the distance measuring control unit  110  in the xz-axis moving mechanism  251 . 
     The three-dimensional shape measuring apparatus  20  has a normal axis configuration used in a three-dimensional measuring apparatus, and it is possible to realize highly functional non-contact shape measurement by installing the measurement probe  160  of the distance measuring device  10  instead of the probe of the three-dimensional measuring apparatus. 
     In addition, in a normal three-axis processing machine, the Z-axis is provided on the tool side, and the x-axis and y-axis are provided on the object T side in many cases, and the configuration is different from that of the three-dimensional shape measuring apparatus  20  in  FIG.  11   . However, when the measurement probe  160  is installed in the three-axis processing machine, on-machine measurement on the processing machine can be realized. 
     In addition, when the measurement probe  160  is installed in a multi-degree of freedom robot and the measurement probe tip end unit  164  is moved, a three-dimensional shape measuring apparatus  20  capable of measurement with a high degree of freedom can be realized. 
     &lt;Second Configuration Example of Three-Dimensional Shape Measuring Apparatus Adopting Distance Measuring Device  10 &gt; 
     Next,  FIG.  12    is a schematic view illustrating a second configuration example of the three-dimensional shape measuring apparatus  20  adopting the distance measuring device  10 . In the second configuration example, a rotation mechanism  256  is added to the first configuration example ( FIG.  11   ). That is, the second configuration example of the three-dimensional shape measuring apparatus  20  includes the moving mechanism  250  ( FIG.  13   ) including the xz-axis moving mechanism  251 , the y-axis moving mechanism  252 , and the rotation mechanism  256 . 
     The rotation mechanism  256  is locked by the rotation axis  253  supported by the structure  254 , and rotates about the rotation axis  253  parallel to the xy plane. In addition, the rotation mechanism  256  rotates about the rotation shaft which is a rotation shaft (not illustrated) orthogonal to the rotation axis  253  and parallel to the z-axis. 
     A sample stage  255  is installed on the rotation mechanism  256 , and the sample stage  255  rotates as the rotation mechanism  256  rotates. As a result, the object T placed on the sample stage  255  rotationally moves. Therefore, according to the rotation mechanism  256 , the posture of the object T in two degrees of freedom can be controlled. 
     That is, the second configuration example of the three-dimensional shape measuring apparatus  20  not only can control the relative position three degrees of freedom between the measurement probe  160  and the object T using the xz-axis moving mechanism  251  and the y-axis moving mechanism  252 , but also can control the relative position two degrees of freedom using the rotation mechanism  256 , so that a total of five degrees of freedom can be controlled. As a result, it is possible to measure every part of the object T from all directions. 
     By installing the measurement probe  160  in a normal five-axis processing machine, it is possible to realize on-machine measurement on the processing machine. In addition, since the number and the configuration of the degrees of freedom differ depending on the processing machine, the configuration of the three-dimensional shape measuring apparatus  20  is not limited to the first configuration example illustrated in  FIG.  11    and the second configuration example illustrated in  FIG.  12   . 
     Next,  FIG.  13    is a diagram illustrating an example of functional blocks of the three-dimensional shape measuring apparatus  20 . The three-dimensional shape measuring apparatus  20  is provided with a calculation unit  260 , the distance measuring control unit  110 , the measurement probe  160 , a display unit  280 , and the moving mechanism  250 . The distance measuring control unit  110  and the measurement probe  160  correspond to the distance measuring device  10 . 
     The calculation unit  260  generally controls entire three-dimensional shape measuring processing using a calculation device such as a central processing unit (CPU). The display unit  280  includes a display device that displays the measurement result. 
     The calculation unit  260  includes the distance calculation unit  261 , a shape calculation unit  262 , and a moving mechanism control unit  263 . The distance calculation unit  261  analyzes the measurement beat signal and the reference beat signal received by the distance measuring control unit  110 , and converts the signals into a distance. In addition, the distance calculation unit  261  controls the measurement probe  160  to control the rotation angle of the measurement probe tip end unit  164 . 
     The shape calculation unit  262  measures the shape of the object T using data notified from the distance calculation unit  261 . The data notified from the distance calculation unit  261  includes data in the detection direction of the measurement light. The shape calculation unit  262  allows the display unit  280  to display the measured shape of the object T. 
     The moving mechanism control unit  263  controls the moving mechanism  250  to control the relative position between the measurement probe  160  and the object T. The position and posture of the object T controlled by the moving mechanism control unit  263  are notified to the distance calculation unit  261 . The calculation unit  260  may be installed in the distance measuring control unit  110  or the measurement probe  160 . 
     Hereinbefore, although each embodiment and modification according to the present invention are described, the present invention is not limited to an example of the embodiment described above, and includes various modifications. For example, the example of the embodiment described above is described in detail in order to make the present invention easy to understand, and the present invention is not limited to one provided with all the configurations described here. In addition, a portion of the configuration of the example of the embodiment can be replaced with a configuration of another example. In addition, the configuration of another example can be added to the configuration of the example of the embodiment. In addition, another configuration can be added, deleted, or replaced to a portion of the configuration of the example of each embodiment. In addition, each of the configurations, functions, processing units, processing means described above may be realized by hardware, for example, by designing a portion or all of these with an integrated circuit. In addition, control lines and information lines in the drawings indicate what is considered to be necessary for the description, and do not necessarily indicate all. It may be considered that substantially all configurations are connected to each other. 
     In addition, the configuration of the inspection apparatus described above can also be classified into more components according to processing content. In addition, one component can be classified to perform more processing. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10 : distance measuring device 
               20 : three-dimensional shape measuring apparatus 
               101 : light emitting unit 
               102 : oscillating unit 
               103 ,  104 : optical fiber coupler 
               105 : optical fiber 
               106 : optical fiber coupler 
               107 : light receiving unit 
               108 : optical circulator 
               109 : light receiving unit 
               110 : distance measuring control unit 
               111 : distance measuring control mechanism control unit 
               112 : reference mirror 
               113   a ,  113   b : optical switch 
               114 : optical fiber coupler 
               115 : broadband light emitting unit 
               116 : spectroscope 
               117 : light receiving unit 
               118 : optical fiber coupler 
               150 : connection cable 
               160 : measurement probe 
               161 : lens unit 
               162 : rotation unit 
               163 : optical path switching element 
               164 : measurement probe tip end unit 
               165 : first polarization state control unit 
               166 : second polarization state control unit 
               180 : polarization beam splitter 
               181 : birefringence plate 
               182 : mirror 
               210 : control device 
               220 : display device 
               250 : moving mechanism 
               251 : xz-axis moving mechanism 
               252 : y-axis moving mechanism 
               253 : rotation axis 
               254 : structure 
               255 : sample stage 
               256 : rotation mechanism 
               260 : calculation unit 
               261 : distance calculation unit 
               262 : shape calculation unit 
               263 : moving mechanism control unit 
               280 : display unit 
               300   a : first direction 
               300   b : second direction 
               301 : polarization stabilization unit 
               302 : linear polarization switching element 
               305 : quarter wavelength plate 
               306   a : first measurement light oscillation direction 
               306   b : second measurement light oscillation direction 
               307   a : left circularly polarized light 
               307   b : right circularly polarized light 
               311 : quarter wavelength plate 
               314 : optical fiber coupler 
               315 : optical fiber coupler 
               316   a ,  316   b : optical fiber