Patent Publication Number: US-11656073-B2

Title: Optical deflector parameter measurement device, method, and program

Description:
This patent application is a national phase filing under section 371 of PCT/JP2019/046962, filed Dec. 2, 2019, which claims the priority of Japanese patent application no. 2018-235207, filed Dec. 17, 2018, each of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a light deflector parameter measurement device, a method, and a program that measure the parameters of a light deflector that changes the emission angle of output light from a diffraction grating according to the wavelength of the output light of a wavelength sweeping light source that is incident onto the diffraction grating. 
     BACKGROUND 
     Non-Patent Literature 1 describes a light deflector that causes a light beam output from a wavelength sweeping light source to be incident onto a diffraction grating and changes the emission angle of the light beam emitted from the diffraction grating (deflects light) according to a diffraction grating equation. 
       FIG.  21    shows an example of the light deflector. A light deflector  100  includes a fiber collimator  101  and a diffraction grating  102 . The output light of a wavelength sweeping light source  103  is made into a light beam through the fiber collimator  101  and incident onto the diffraction grating  102  at an incident angle α. The light beam diffracted by the diffraction grating  102  is emitted at an emission angle β according to a diffraction grating equation. At this time, the emission angle β is represented by the following expression.
 
Expression 1
 
β=sin −1 (sin α− Nm λ( t ))  (1)
 
     Here, N represents the number of the ruled lines of the diffraction grating  102 , m represents a diffraction order, and t represents time. The output light of the wavelength sweeping light source  103  is represented by a time function such as λ(t) since a wavelength λ fluctuates with time. 
     It appears from expression (1) that the emission angle β of the diffraction grating  102  fluctuates (deflects) with time as the wavelength λ fluctuates with time. As described above, there is the light deflector that deflects a light beam with the diffraction grating  102  using the time change of the wavelength λ of the wavelength sweeping light source  103 . 
     In the light deflector  100  shown in  FIG.  21   , it is necessary to exactly determine the incident angle α of the output light beam of the wavelength sweeping light source  103  onto the diffraction grating  102  in order to exactly find the emission angle β with respect to the time t as is clear from expression (1). As a specific method, it is presumed that the accuracy of the incident angle α is improved with a graduated stage or the like. However, due to the deviation of a jig, the contraction of adhesive, or the like used to fix the fiber collimator  101 , there is a case that the incident angle α is deviated from its default. The same applies to the jig or the adhesive used to fix the diffraction grating  102 . 
     When a position deviation or an angle deviation occurs after the installation of an optical component (the fiber collimator  101 ) and the diffraction grating  102  that emit the light beam of the wavelength sweeping light source, the incident angle α of the light beam onto the diffraction grating  102  is deviated from its assumed angle. Therefore, the emission angle β of the light beam from the diffraction grating  102  is deviated from its assumed angle. As a result, the relationship between the time t and the emission angle β is deviated from its assumed relationship. 
     CITATION LIST 
     Non Patent Literature 
     
         
         [NPL 1] Toni Kodaira, Shogo Yagi, Kazuo Fujiura, Jiro Mori, and Takeshi Watanabe, “Hatyôsôin Gizyutu wo Ôyôsita Hikari Sôin Hôsiki Iti Keisoku Sisutemu (Light Sweeping Position Measurement System with Application of Wavelength Sweeping Technology)”, Optical and electro-optical engineering contact, Japan Optomechatronics Association, vol. 55, No. 8, pp. 18-27, issued on Aug. 20, 2017. 
       
    
     SUMMARY 
     Technical Problem 
     Embodiments of the present invention have been made in order to solve the above problem and has an object of providing a light deflector parameter measurement device, a method, and a program that can exactly measure the incident angle of the output light beam of the wavelength sweeping light source of a light deflector on a diffraction grating. 
     Means for Solving the Problem 
     Embodiments of the present invention provide a light deflector parameter measurement device that measures a parameter of a light deflector that changes an emission angle of output light from a diffraction grating according to a wavelength of the output light of a wavelength sweeping light source that is incident onto the diffraction grating, the device including: a first photodetector that receives the output light from the light deflector; a stage that moves the first photodetector to a plurality of positions along a direction of a first axis perpendicular to a groove direction of the diffraction grating of the light deflector; a first wavelength calculation unit that calculates the wavelength of the output light of the wavelength sweeping light source of the light deflector for each time; a second wavelength calculation unit that calculates a wavelength of the light received from the light deflector by the first photodetector positioned by the stage on the basis of an output signal of the first photodetector and the wavelength calculated by the first wavelength calculation unit; and a parameter calculation unit that calculates an incident angle α of the output light beam of the wavelength sweeping light source onto the diffraction grating and an angle θ G , which is formed by a second axis that is perpendicular to the groove direction of the diffraction grating and perpendicular to the first axis and a line perpendicular to a surface of the diffraction grating, by performing fitting so that coordinates of the first photodetector that are obtained for each position of the first photodetector positioned by the stage and the wavelength calculated by the second wavelength calculation unit conform to a prescribed relational expression. 
     In addition, a configuration example of the light deflector parameter measurement device according to embodiments of the present invention further includes a deflection origin calculation unit that calculates coordinates of a deflection origin of the light deflector, wherein the relational expression is an expression in which the coordinates of the first photodetector, the wavelength of the output light from the light deflector, the incident angle α, the angle θ G , the coordinates of the deflection origin of the light deflector, the number of ruled lines of the diffraction grating, and a diffraction order are associated with each other, the stage moves, before moving the first photodetector along the direction of the first axis to find the incident angle α and the angle θ G , the first photodetector to move along the first axis and the second axis so that a plurality of the coordinates of the first photodetector can be acquired with respect to one deflection angle of the light deflector for each different deflection angle, and the deflection origin calculation unit finds linear approximations obtained from the plurality of the coordinates of the first photodetector with respect to the same deflection angle of the light deflector for each deflection angle and calculates coordinates of an intersecting point of the linear approximations as the coordinates of the deflection origin of the light deflector. 
     In addition, a configuration example of the light deflector parameter measurement device according to embodiments of the present invention further includes a wavelength acquisition optical system for acquiring the wavelength of the output light of the wavelength sweeping light source, wherein the wavelength acquisition optical system includes a coupler that distributes the light from the wavelength sweeping light source, an interferometer that receives one output light of the coupler and outputs an interference signal by causing light that propagates through two optical paths having a different optical length to interfere with each other, a wavelength filter that causes light having a specific wavelength of the other output light of the coupler to pass therethrough, and a second photodetector that outputs a signal obtained by photoelectrically converting the light that passes through the wavelength filter as a filter signal, and the first wavelength calculation unit calculates the wavelength of the output light of the wavelength sweeping light source for each time on the basis of the interference signal, the filter signal, and a prescribed maximum transmission wavelength of the wavelength filter. 
     In addition, in a configuration example of the light deflector parameter measurement device according to embodiments of the present invention, the first wavelength calculation unit is constituted by a phase calculation unit that calculates a phase of the interference signal, a relative wavenumber calculation unit that calculates a relative wavenumber of light from the phase of the interference signal, a peak time acquisition unit that acquires a peak time of the filter signal, an absolute wavenumber calculation unit that calculates an absolute wavenumber from the maximum transmission wavelength of the wavelength filter and the peak time, and a wavelength calculation processing unit that calculates the wavelength of the output light of the wavelength sweeping light source from the absolute wavenumber. 
     In addition, embodiments of the present invention provide a light deflector parameter measurement method for measuring a parameter of a light deflector that changes an emission angle of output light from a diffraction grating according to a wavelength of the output light of a wavelength sweeping light source that is incident onto the diffraction grating. The method includes a first step of moving a photodetector that receives the output light from the light deflector to a plurality of positions along a direction of a first axis perpendicular to a groove direction of the diffraction grating of the light deflector by a stage; a second step of calculating the wavelength of the output light of the wavelength sweeping light source of the light deflector for each time; a third step of calculating a wavelength of the light received from the light deflector by the photodetector positioned by the stage on the basis of an output signal of the photodetector and the wavelength calculated in the second step; and a fourth step of calculating an incident angle α of the output light beam of the wavelength sweeping light source onto the diffraction grating and an angle θ G , which is formed by a second axis that is perpendicular to the groove direction of the diffraction grating and perpendicular to the first axis and a line perpendicular to a surface of the diffraction grating, by performing fitting so that coordinates of the photodetector that are obtained for each position of the photodetector positioned by the stage and the wavelength calculated in the third step conform to a prescribed relational expression. 
     In addition, in a configuration example of the light deflector parameter measurement method according to embodiments of the present invention, the relational expression is an expression in which the coordinates of the photodetector, the wavelength of the output light from the light deflector, the incident angle α, the angle θ G , the coordinates of the deflection origin of the light deflector, the number of ruled lines of the diffraction grating, and a diffraction order are associated with each other. The method further includes a fifth step of moving, before the first step, the photodetector along the first axis and the second axis so that a plurality of the coordinates of the photodetector can be acquired with respect to one deflection angle of the light deflector for each different deflection angle; and a sixth step of finding a linear approximation obtained from the plurality of the coordinates of the photodetector with respect to the same deflection angle of the light deflector for each deflection angle and calculating coordinates of an intersecting point of the linear approximations as the coordinates of the deflection origin of the light deflector. 
     In addition, a light deflector parameter measurement program causes a computer to perform the above respective steps. 
     Effects of Embodiments of the Invention 
     According to embodiments of the present invention, it is possible to exactly measure the incident angle of an output light beam of the wavelength sweeping light source on a diffraction grating in a light deflector and exactly find the relationship between time and the emission angle of the diffraction grating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram showing the configuration of a light deflector parameter measurement device according to an embodiment of the present invention. 
         FIG.  2    is a block diagram showing the configuration of the signal processing device of the light deflector parameter measurement device in an embodiment of the present invention. 
         FIG.  3    is a flowchart for describing the operation of the signal processing device of the light deflector parameter measurement device in an embodiment of the present invention. 
         FIG.  4    is a diagram for describing the principle of acquiring the coordinates of the deflection origin of a light deflector in an embodiment of the present invention. 
         FIG.  5    is a diagram for describing a method for controlling the position of a photodetector in an embodiment of the present invention. 
         FIG.  6    is a diagram showing the waveform of the output signal of the photodetector in an embodiment of the present invention. 
         FIGS.  7 A and  7 B  are diagrams showing a measurement result example of the coordinates of the deflection origin (the origin of the world coordinate system) of the light deflector in an embodiment of the present invention. 
         FIG.  8    is a diagram showing the configuration of a wavelength acquisition optical system in an embodiment of the present invention. 
         FIG.  9    is a diagram showing the configuration of the first wavelength calculation unit of the signal processing device in an embodiment of the present invention. 
         FIG.  10    is a flowchart for describing the operation of the first wavelength calculation unit of the signal processing device in an embodiment of the present invention. 
         FIG.  11    is a diagram showing the configuration of the phase calculation unit of the wavelength calculation unit in an embodiment of the present invention. 
         FIG.  12    is a flowchart for describing the operation of the phase calculation unit of  FIG.  11   . 
         FIG.  13    is a diagram showing another configuration of the phase calculation unit of the wavelength calculation unit in an embodiment of the present invention. 
         FIG.  14    is a flowchart for describing the operation of the phase calculation unit of  FIG.  13   . 
         FIGS.  15 A to  15 C  are diagrams for describing the relationship between an interference signal phase and a wavenumber. 
         FIG.  16    is a diagram for describing a method for finding an incident angle of the output light beam of the wavelength sweeping light source on a diffraction grating and a method for finding an angle formed by the surface of the diffraction grating and an L-axis. 
         FIG.  17    is a diagram showing the waveform of the output signal of the photodetector and the wavelength of the output light of the wavelength sweeping light source in an embodiment of the present invention. 
         FIGS.  18 A and  18 B  are diagrams for describing a method for finding the wavelength of the output light of the wavelength sweeping light source from the output signal of the photodetector in an embodiment of the present invention. 
         FIG.  19    is a diagram showing results obtained when the incident angle of the output light beam of the wavelength sweeping light source onto the diffraction grating and an angle formed by the surface of the diffraction grating and the L-axis using a Levenberg-Marquardt method in an embodiment of the present invention. 
         FIG.  20    is a block diagram showing the configuration of a computer that realizes the signal processing device according to an embodiment of the present invention. 
         FIG.  21    is a diagram showing the configuration of a conventional light deflector. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Embodiments 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings.  FIG.  1    is a diagram showing the configuration of a light deflector parameter measurement device according to an embodiment of the present invention. The light deflector parameter measurement device includes a photodetector (hereinafter called a PD)  1  that receives output light from a light deflector wo, a biaxial translation automatic stage  2  that causes the PD  1  to move, a signal capturing device  3  that captures the output signal of the PD  1 , a stage driver  4  that drives the biaxial translation automatic stage  2 , a signal processing device  5  that performs the control of the whole light deflector parameter measurement device and the calculation of parameters, and a wavelength acquisition optical system  6  that is used to acquire the wavelength of the output light of a wavelength sweeping light source  103 . 
       FIG.  2    is a block diagram showing the configuration of the signal processing device  5  of the light deflector parameter measurement device according to the present embodiment. The signal processing device  5  is constituted by a stage control unit  50  that controls the biaxial translation automatic stage  2  through the stage driver  4 , a deflection origin calculation unit  51  that calculates the coordinates of the deflection origin of the light deflector  100 , a wavelength calculation unit  52  (first wavelength calculation unit) that calculates the wavelength of the output light of the wavelength sweeping light source  103  of the light deflector  100  for each time, a wavelength calculation unit  53  (second wavelength calculation unit) that calculates the wavelength of light received from the light deflector  100  by the PD  1  positioned by the biaxial translation automatic stage  2 , and a parameter calculation unit  54  that calculates an incident angle α of the output light beam of the wavelength sweeping light source  103  on a diffraction grating  102  and an angle θ G  formed by an L-axis that will be described later and a line perpendicular to the surface of the diffraction grating  102 . 
       FIG.  3    is a flowchart for describing the operation of the signal processing device  5 . The stage control unit  50  of the signal processing device  5  performs the movement of the PD  1  so that a plurality of the coordinates of the PD  1  can be acquired with respect to one deflection angle β of the light deflector  100  for each different deflection angle through the stage driver  4  (step S 1  of  FIG.  3   ). 
     The deflection origin calculation unit  51  of the signal processing device  5  finds linear approximations acquired from a plurality of the coordinates of the PD  1  with respect to the same deflection angle β of the light deflector  100  for each deflection angle β, and calculates the coordinates of the intersecting point of the linear approximations as the coordinates of the deflection origin of the light deflector  100  (step S 2  of  FIG.  3   ). The processing of steps S 1  and S 2  described above is performed in advance before the calculation of the parameters (an incident angle α and an angle θ G ) of the light deflector. 
     Next, the stage control unit  50  of the signal processing device  5  causes the PD  1  to move to a plurality of positions along the direction of a first axis perpendicular to the groove direction of the diffraction grating  102  of the light deflector  100  (S 3  of  FIG.  3   ). 
     The wavelength calculation unit  52  of the signal processing device  5  calculates the wavelength of the output light of the wavelength sweeping light source  103  of the light deflector  100  for each time (step S 4  of  FIG.  3   ). The calculation of the wavelength of the output light of the wavelength sweeping light source  103  is performed at all times. 
     The wavelength calculation unit  53  of the signal processing device  5  calculates the wavelength of light received from the light deflector  100  by the PD  1  positioned by the biaxial translation automatic stage  2  on the basis of the output signal of the PD  1  and the wavelengths calculated by the wavelength calculation unit  52  (step S 5  of  FIG.  3   ). 
     The parameter calculation unit  54  of the signal processing device  5  calculates an incident angle α of the output light beam of the wavelength sweeping light source  103  onto the diffraction grating  102  and an angle θ G  formed by the L-axis that will be described later and the line perpendicular to the surface of the diffraction grating  102  according to a method that will be described below (step S 6  of  FIG.  3   ). Here, the parameter calculation unit  54  performs fitting so that the coordinates of the PD  1  acquired for each position of the PD  1  positioned by the biaxial translation automatic stage  2  and the wavelengths calculated by the wavelength calculation unit  53  conform to a prescribed relational expression. 
     Hereinafter, the light deflector parameter measurement device of the present embodiment will be described in further detail. As the wavelength sweeping light source  103  and the light deflector  100  are described above with reference to  FIG.  21   , a represents the incident angle of a light beam from a fiber collimator  101  onto the diffraction grating  102 , β represents the emission angle of the light beam from the diffraction grating  102  (the deflection angle of the light deflector  100 ), and γ represents the emission angle of the light beam from the light deflector  100  to the PD  1 . 
       FIG.  1    shows the origin and the axes of a world coordinate system, which are defined as follows. First, the origin O of the world coordinate system represents the deflection origin of the light deflector  100 . The x-axis (the first axis, i.e., the vertical direction of  FIG.  1   ) of the world coordinate system is an axis that is perpendicular to the groove direction (a direction orthogonal to the space of  FIG.  1   ) of the diffraction grating  102  of the light deflector  100 , and that passes through the origin O of the world coordinate system. The L-axis (the second axis, i.e., the horizontal direction of  FIG.  1   ) of the world coordinate system is an axis that is perpendicular to the groove direction of the diffraction grating  102 , passes through the origin O of the world coordinate system, and is perpendicular to the x-axis. 
     A plane containing both the x-axis and the L-axis is determined according to the above conditions, but the x-axis and the L-axis are not uniquely determined. A reason why the x-axis and the L-axis are not uniquely determined will be described later. Note that the light axis of the light beam emitted from the fiber collimator  101  of the light deflector  100  is perpendicular to the groove direction of the diffraction grating  102 . At this time, the light deflector  100  deflects inside an x-L plane. 
     On the other hand, the axes of the stage coordinate system of the biaxial translation automatic stage  2  that causes the PD  1  to move are named as an x′-axis (first axis) and an L′-axis (second axis). It is assumed that the x-axis is parallel to the x′-axis, and that the L-axis is parallel to the L′-axis. That is, when the x′-axis and the L′-axis are determined, the x-axis and the L-axis are determined as axes that pass through the origin O of the world coordinate system, and that are parallel to the x′-axis and the L′-axis, respectively. Even if a user arranges the biaxial translation automatic stage  2  of the x′-axis and the L′-axis in any way, the x-axis and the L-axis are determined according to the arrangement under such a definition. Therefore, the user can freely determine the x-axis and the L-axis to facilitate measurement. 
     Meanwhile, there is almost no possibility that an origin O′ of a two-dimensional coordinate system (an x′-L′ coordinate system, i.e., a stage coordinate system) represented by the x′-axis and the L′-axis and the origin O of a two-dimensional coordinate system (an x-L coordinate system, i.e., the world coordinate system) represented by the x-axis and the L-axis exactly match each other. In  FIG.  1   , the coordinates of the deflection origin of the light deflector  100  (the deviation amount between the origin O and the origin O′) are represented by (L 0 , x 0 ). 
     Note that when the parameters of the light deflector wo are actually measured, the biaxial translation automatic stage  2  is installed so that a surface through which the deflection beam of the light deflector wo passes and a surface along which the PD  1  moves become parallel to each other. At this time, the x-axis of the world coordinate system is parallel to the x′-axis of the biaxial translation automatic stage  2  and is determined as an axis that passes through the origin O of the world coordinate system. That is, the x-axis is uniquely determined when the biaxial translation automatic stage  2  is installed, and the x-axis is changed according to a way in which the biaxial translation automatic stage  2  is installed. 
     The operations of the signal capturing device  3  that captures the output of the PD  1 , the stage driver  4  that causes the biaxial translation automatic stage  2  to operate, and the signal processing device  5  that processes a captured signal will be described later. 
     When the coordinate system of  FIG.  1    is used, the following relationships are established.
 
Expressions 2, 3, and 4
 
 x=L  tan(θ G +sin −1 (sin α− Nm λ( t )))  (2)
 
 x=x′+x   0   (3)
 
 L=L′+L   0   (4)
 
     As described above, λ(t) represents the wavelength of the output light of the wavelength sweeping light source  103 , N represents the number of the ruled lines of the diffraction grating  102 , m represents a diffraction order, and t represents time. Further, θ G  represents an angle (diffraction grating angle) formed by the surface of the diffraction grating  102  and the L-axis. 
     Where the angles α and θ G  that represent the parameters of the deflector are found, it is only necessary to determine the angles α and θ G  so as to establish expression (2) if N m  position coordinates (L i , x i ) (i=1 to N m ) and wavelengths λ i  acquirable at the respective positions of the PD  1  can be acquired when the PD  1  is caused to move N m  times. As a method for this, a Newton-Raphson method, a steepest descent method, a Levenberg-Marquardt method, or the like can be used. A method for finding the angles α and θ G  using (L i , x i , λ i ) and expression (2) will be described later. 
     From here, a method for measuring the coordinates (L 0 , x 0 ) of the deflection origin of the light deflector  100  (the deviation amount between the origin O of the world coordinate system and the origin O′ of the stage coordinate system) and a method for measuring the wavelength λ(t) of the output light of the wavelength sweeping light source  103  will be described as preparations for acquiring the angles α and θ G  using expression (2). 
     First, the method for acquiring the coordinates (L 0 , x 0 ) of the deflection origin of the light deflector  100  will be described.  FIG.  4    shows the principle of acquiring the coordinates (L 0 , x 0 ) of the deflection origin of the light deflector  100 . It is assumed that the position coordinates of the PD  1  that receives a light beam from the diffraction grating  102  when the deflection angle of the light deflector  100  (the emission angle of the diffraction grating  102 ) is β j  are (L′ j,i , x′ j,i ). Here, j represents a difference in angle, and i represents a difference in position of the PD  1  that receives light at the same deflection angle β j . 
     As shown in  FIG.  4   , the movement of the PD  1  by the biaxial translation automatic stage (not shown) and the acquisition of a plurality of the position coordinates (L′ j,i , x′ j,i ) of the PD  1  with respect to one deflection angle β j  are performed for each different deflection angle β j . When the position coordinates (L′ j,i , x′ j,i ) with respect to the same deflection angle β j  are linearly approximated by a least-squares method or the like using the position coordinates (L′ j,i , x′ j,i ), lines having different deflection angles β j  cross each other at the deflection origin (the origin O of the world coordinate system). Note that the position coordinates (L′ j,i , x′ j,i ) of the PD  1  can be acquired from the stage control unit  50 . 
     Since an error occurs in the calculation of actual lines, the lines having different deflection angles β j  hardly cross each other at one point. However, in such a case, the coordinates of a deflection origin that becomes optimum in terms of a least-squares method can be calculated using a generalized inverse matrix by Moore Penrose. When it is assumed that the coordinates of the deflection origin (the origin O of the world coordinate system) calculated here are (L′ 0 , x′ 0 ), (L′ 0 , x′ 0 ) are the coordinates of the stage coordinate system and are vectors representing the deviation between the origin O′ of the stage coordinate system and the origin O of the world coordinate system. Therefore, the coordinates (L 0 , x 0 ) of the deflection origin of the light deflector  100  can be calculated as in the following expression.
 
Expression 5
 
( L   0   ,x   0 )=( L′   0   ,x′   0 )  (5)
 
     Next, a method for moving the PD  1  so that the light deflector  100  has the same deflection angle β j  will be described.  FIG.  5    is a diagram for describing a method for controlling the position of the PD  1 . 
     The signal capturing device  3  acquires an output signal S p (t) of the PD  1  in synchronization with the trigger signal of the wavelength sweeping light source  103  (the synchronization signal of wavelength sweeping). 
       FIG.  6    shows the waveform of the output signal S p (t) of the PD  1 . The stage control unit  50  of the signal processing device  5  acquires a deviation e n  between the peak time of the output signal S p (t) of the PD  1  that is acquired by the signal capturing device  3  and a target time corresponding to the deflection angle β j  of the light deflector  100 . Then, the stage control unit  50  calculates a change u n  in operation amount on the x′-axis by PI control computation from the deviation e n . A calculation expression for calculating the change u n  is as follows. 
     
       
         
           
             
               
                 
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     In expression (6), n represents the number of feedback times, K p  represents a gain, and T i  represents a time constant. The stage control unit  50  obtains the latest operation amount x′ n+1  under x′ n+1 =x′ n +u n . x′n represents an immediately-preceding operation amount. The stage control unit  50  outputs the operation amount x′n, to the stage driver  4 . According to the operation amount x′ n+1 , the stage driver  4  controls the biaxial translation automatic stage  2  and causes the PD  1  to move to a position corresponding to the operation amount x′ n+1  along the x′-axis. 
     The acquisition of the output signal S p (t) of the PD  1 , the acquisition of the deviation e n , and the PI control of the position of the PD  1  described above are repeatedly performed until the deviation e n  becomes a prescribed value Sδe or less with respect to one position coordinates (L′ j,i , x′ j,i ) of one deflection angle β j . 
       FIGS.  7 A and  7 B  show a measurement result example of the coordinates (L′ 0 , x′ 0 ) of the deflection origin (the origin O of the world coordinate system).  FIG.  7 A  shows the whole diagram (corresponding to  FIG.  4   ).  FIG.  7 B  shows the vicinity of the intersecting point (the position of Q of  FIG.  7 A ) of linear approximations obtained when a plurality of position coordinates (L′ j,i , x′ j,i ) with respect to the same deflection angle β j  are linearly approximated by a least-squares method. The horizontal axis and the vertical axis of  FIGS.  7 A and  7 B  are L′ and x′, respectively. 
       FIGS.  7 A and  7 B  show five linear approximations F 1  to F 5  obtained for five different deflection angles β j . Further, measurement data on the right side of  FIG.  7 A  shows a plurality of position coordinates (L′ j,i , x′ j,i ) for each deflection angle β j . Note that in the example of  FIGS.  7 A and  7 B , the coordinates x′ are acquired at five time points for each coordinates L′, and the measurement data of position coordinates (L′ j,i , x′ j,i ) is acquired for 55 points in total while the coordinates L′ are changed 10 times. The following expression is obtained from the five linear approximations F 1  to F 5 . 
     
       
         
           
             
               
                 
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                   b 
                 
               
               
                 
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                   ) 
                 
               
             
           
         
       
     
     Here, A represents a matrix, and b represents a column vector. The number of the columns of the matrix A is two, and the number of the rows thereof corresponds to the number of lines. Since there are the five linear approximations in the example of  FIGS.  7 A and  7 B , the number of the rows of the matrix A is five in the case of this example. The number of the elements of the column vector b corresponds to the number of linear approximations. Since there are the five linear approximations in the example of  FIGS.  7 A and  7 B , the number of the elements of the column vector b is five in the case of this example. As shown in  FIG.  7 B , the linear approximations F 1  to F 5  do not cross each other at one point. 
     The deflection origin calculation unit  51  of the signal processing device  5  finds the coordinates (L′ 0 , x′ 0 ) of the stage coordinate system as shown in the following expression using a Moore-Penrose generalized inverse matrix (A T A) −1 A T  from the linear approximations F 1  to F 5  and inverts the positive and negative codes of the coordinates (L′ 0 , x′ 0 ) of the stage coordinate system to calculate the coordinates (L 0 , x 0 ) of the deflection origin of the light deflector  100  (the deviation amount between the origin O of the world coordinate system and the origin O′ of the stage coordinate system). 
     
       
         
           
             
               
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   8 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     [ 
                     
                       
                         
                           
                             L 
                             0 
                             ′ 
                           
                         
                       
                       
                         
                           
                             x 
                             0 
                             ′ 
                           
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             A 
                             T 
                           
                           ⁢ 
                           A 
                         
                         ) 
                       
                       
                         - 
                         1 
                       
                     
                     ⁢ 
                     
                       A 
                       T 
                     
                     ⁢ 
                     b 
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     In the example of  FIGS.  7 A and  7 B , the specific calculation result of the coordinates (L 0 , x 0 ) of the deflection origin of the light deflector  100  is as follows. 
     
       
         
           
             
               
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   9 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     [ 
                     
                       
                         
                           
                             L 
                             0 
                           
                         
                       
                       
                         
                           
                             x 
                             0 
                           
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       - 
                       
                         [ 
                         
                           
                             
                               
                                 L 
                                 0 
                                 ′ 
                               
                             
                           
                           
                             
                               
                                 x 
                                 0 
                                 ′ 
                               
                             
                           
                         
                         ] 
                       
                     
                     = 
                     
                       [ 
                       
                         
                           
                             
                               3 
                               ⁢ 
                               
                                 1 
                                 . 
                                 1 
                               
                               ⁢ 
                               4 
                               ⁢ 
                               0 
                               ⁢ 
                               4 
                             
                           
                         
                         
                           
                             
                               
                                 - 
                                 1 
                               
                               ⁢ 
                               
                                 1 
                                 . 
                                 1 
                               
                               ⁢ 
                               2 
                               ⁢ 
                               9 
                               ⁢ 
                               5 
                             
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     Next, the method for acquiring the wavelength λ(t) of the output light of the wavelength sweeping light source  103  will be described. When the wavelength λ(t) of the output light of the wavelength sweeping light source  103  is known in advance, processing to calculate the parameters α and θ G  of the light deflector using expression (2) can be performed. However, when the wavelength λ(t) is unknown or when the wavelength λ(t) fluctuates with a change in environment such as temperature and humidity, the wavelength λ(t) has to be acquired. 
       FIG.  8    shows the configuration of the wavelength acquisition optical system  6 . The wavelength acquisition optical system  6  is constituted by a coupler  60 , a wavelength filter  61 , a photodetector (PD)  64 , a Mach-Zehnder interferometer  62 , and a balanced photodetector (BPD)  63 . The Mach-Zehnder interferometer  62  is constituted by couplers  620  and  621 , circulators  622  and  623 , prisms  624  and  625 , and mirrors  626  and  627 . 
     It is possible to add the wavelength acquisition optical system  6  to the measurement system shown in  FIGS.  1  and  4   . In this case, it is only necessary to insert a coupler (not shown) between the wavelength sweeping light source  103  and the light deflector  100  of  FIG.  1    and input one and the other light out of light bifurcated by the coupler to the light deflector  100  and the wavelength acquisition optical system  6 , respectively. 
     The coupler  60  of the wavelength acquisition optical system  6  distributes the light from the wavelength sweeping light source  103 . Out of the light, one light from the coupler  60  is incident on the wavelength filter  61 , and the other light is incident on the Mach-Zehnder interferometer  62 . 
     The wavelength filter  61  is a band-pass filter that causes light having a specific wavelength to selectively pass therethrough. The PD  64  photoelectrically converts the light having passed through the wavelength filter  61  and outputs an electric signal (filter signal) S f (t). 
     The Mach-Zehnder interferometer  62  has two optical paths having an optical path length difference of 2z. The BPD  63  outputs an interference signal s(t) by finding the difference between two electric signals obtained by photoelectrically converting the light interfered by the coupler  621  after having propagated through the two optical paths of the Mach-Zehnder interferometer  62 . 
     The signal capturing device  3  acquires the interference signal s(t) from the BPD  63  and the filter signal S f (t) from the wavelength filter  61  in synchronization with the trigger signal of the wavelength sweeping light source  103  (the synchronization signal of wavelength sweeping). The wavelength calculation unit  52  of the signal processing device  5  calculates the wavelength λ(t) of the output light of the wavelength sweeping light source  103  on the basis of the interference signal s(t), the filter signal S f (t), and a prescribed maximum transmission wavelength λ f  of the wavelength filter  61 . 
       FIG.  9    shows the configuration of the wavelength calculation unit  52 . The wavelength calculation unit  52  is constituted by a phase calculation unit  520 , a relative wavenumber calculation unit  521 , a peak time acquisition unit  522 , an absolute wavenumber calculation unit  523 , and a wavelength calculation processing unit  524 .  FIG.  10    is a flowchart for describing the operation of the wavelength calculation unit  52 . 
     First, the phase calculation unit  520  calculates a phase θ′(t) of the interference signal s(t) (step S 100  of  FIG.  10   ). 
     The relative wavenumber calculation unit  521  calculates a relative wavenumber k′(t) of light from the phase θ′(t) (step S 101  of  FIG.  10   ). 
     The peak time acquisition unit  522  acquires a peak time t f  of the filter signal s f (t) (step S 102  of  FIG.  10   ). 
     The absolute wavenumber calculation unit  523  calculates an absolute wavenumber k(t) from the maximum transmission wavelength λf of a wavelength filter and the peak time t f  (step S 103  of  FIG.  10   ). 
     Then, the wavelength calculation processing unit  524  calculates the wavelength λ(t) from the wavenumber k(t) (step S 104  of  FIG.  10   ). 
     First, the phase calculation unit  520  that calculates the phase θ′(t) from the interference signal s(t) obtained from the Mach-Zehnder interferometer  62  will be described.  FIG.  11    is a diagram showing the configuration of the phase calculation unit  520  in a case in which Fourier transform is used. In this case, the phase calculation unit  520  is constituted by a Fourier transform unit  5200 , a negative frequency component zeroing unit  5201 , a Fourier inverse transform unit  5202 , and a deflection angle calculation unit  5203 .  FIG.  12    is a flowchart for describing the operation of the phase calculation unit  520  in a case in which Fourier transform is used. 
     The Fourier transform unit  5200  performs the Fourier transform of the input interference signal s(t) to find a frequency spectrum S(v) (step S 200  of  FIG.  12   ). 
     The negative frequency component zeroing unit  5201  finds a frequency spectrum S + (v) resulting from the zeroing of a negative frequency component of the frequency spectrum S(v) (step S 201  of  FIG.  12   ). 
     The Fourier inverse transform unit  5202  finds a complex signal s + (t) resulting from the Fourier inverse transform of the frequency spectrum S + (v) (step S 202  of  FIG.  12   ). 
     Then, the deflection angle calculation unit  5203  calculates the deflection angle of the complex signal s + (t) (an angle with respect to a real axis) as the phase θ′(t) of the interference signal s(t) (step S 203  of  FIG.  12   ). 
       FIG.  13    is a diagram showing the configuration of the phase calculation unit  520  in a case in which Hilbert transform is used. In this case, the phase calculation unit  520  is constituted by a Hilbert transform unit  5204 , a complex number generation unit  5205 , and a deflection angle calculation unit  5206 .  FIG.  14    is a flowchart for describing the operation of the phase calculation unit  520  in a case in which Hilbert transform is used. 
     The Hilbert transform unit  5204  performs the Hilbert transform of the input interference signal s(t) to find a complex signal s i (v) (step S 300  of  FIG.  14   ). 
     The complex number generation unit  5205  finds a complex signal s + (t)(=s(t)+js i (t)) in which the interference signal s(t) and the complex signal si(v) are added together (step S 301  of  FIG.  14   ). Here, j represents an imaginary unit. 
     The deflection angle calculation unit  5206  calculates the deflection angle of the complex signal s + (t) as the phase θ′(t) of the interference signal s(t) (step S 302  of  FIG.  14   ). 
     Here, the deflection angle calculation units  5203  and  5206  will be described in further detail although they get off the main subject of embodiments of the present invention. The deflection angle of a complex number is actually found for each discrete time t n . Here, n represents an identifier for identifying time, and it is assumed that the time increases for each increment of n. Further, it is assumed that to represents an equal interval. 
     When the found deflection angle is represented by θ′ init (t n ), the deflection angle θ′ init (t n ) generally falls within the range of 0 to 2 π or −π to π. Note that the range of the deflection angle θ′ init (t n ) depends on a processing system that calculates a deflection angle. The convergence of a phase within the width of 2 π like this is called phase wrapping processing. 
     However, since the relationship θ′(t)=2zk(t) is established as shown in expression (10) that will be described later, it is necessary to calculate the phase θ′(t) beyond the range of 0 to 2 π or −π to π. Therefore, the following calculation (I) to (VI) is performed. Such calculation is called phase connection or phase unwrapping. It is assumed that n of (I) to (VI) is 0 to N−1.
         (I) Initialize a parameter p so that p=0   (II) θ′(t n )=θ init (t n ) when n=0   (III) θ′(t n )=θ′ init (t n )+2 πp when n≠0 and |θ′ init (t n )−θ′ init (t n−1 )|&lt;π   (IV) θ′(t n )=θ′ init (t n )+2 πp after the value of the parameter p is incremented by one when n≠0 and θ′ init (t n )−θ′ init (t n−1 )≥π   (V) θ′(t n )=θ′ init (t n )+2 πp after the value of the parameter p is decremented by one when n≠0 and θ′ init (t n )−θ′ init (t n−1 )≤π   (VI) The processing of (II) to (VI) is repeatedly performed while n is changed from 0 to N−1.       

     Just for reference, a method for calculating the phase θ′(t) from the interference signal s(t) obtained from the Mach-Zehnder interferometer  62  is based on the following principle. The interference signal s(t) is represented by the following expression.
 
Expression 10
 
 s ( t )= A  cos(2 zk ( t ))= A  cos(θ( t ))(=
 
 A  cos(2 z ( k ′( t )+ k   c ))= A  cos((θ′( t )+θ c )))  (10)
 
     Here, A represents an amplitude, 2z represents the optical path length difference between the two arms of the Mach-Zehnder interferometer  62  shown in  FIG.  8   , and k(t) represents the wavenumber of the light from the wavelength sweeping light source  103 . Further, k(t)=k′(t)+k c  and θ(t)=θ′(t)+θ c  are established. θ c  represents the difference between an observation value θ′(t) of a phase and a true value θ(t) of the phase, and k c  represents the difference between a wavenumber k′(t) calculated from the observation value θ′(t) of the phase and a true value k(t) of the wavenumber. 
     A reason for the occurrence of a difference like k c  and θ c  is that θ′(t) before phase connection is a wrapped value θ′ init (t n ) (a value within the range of 0 to 2π or −π to π) when θ′(t) is found from the complex number s + (t) in the above description. 
     That is, since θ′ init (t n ) is first calculated rather than the true value θ(t) of a deflection angle from the complex number s + (t) and then phase connection processing is performed on the basis of a phase at a certain time (time at n=0 in the processing of (I) to (VI)), the difference θ c  occurs. Due to the occurrence of the difference θ c  a difference k c  between a true value and an observation value occurs as for the wavenumber k′(t) calculated from the phase θ′(t). 
     Note that the amplitude A of expression (10) generally fluctuates with time (that is, a time function) but is a constant for simplification here. 
     Expression (10) can be transformed as follows. 
     
       
         
           
             
               
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   11 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     s 
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       A 
                       ⁢ 
                       
                         cos 
                         ⁡ 
                         
                           ( 
                           
                             θ 
                             ⁡ 
                             
                               ( 
                               t 
                               ) 
                             
                           
                           ) 
                         
                       
                     
                     = 
                     
                       
                         A 
                         2 
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             e 
                             
                               j 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 θ 
                                 ⁡ 
                                 
                                   ( 
                                   t 
                                   ) 
                                 
                               
                             
                           
                           + 
                           
                             e 
                             
                               
                                 - 
                                 j 
                               
                               ⁢ 
                               
                                 θ 
                                 ⁡ 
                                 
                                   ( 
                                   t 
                                   ) 
                                 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     The following expression is obtained when n e −jθ(t)  is removed from expression (11). 
     
       
         
           
             
               
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   12 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       s 
                       + 
                     
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       A 
                       2 
                     
                     ⁢ 
                     
                       e 
                       
                         j 
                         ⁢ 
                         
                           θ 
                           ⁡ 
                           
                             ( 
                             t 
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     According to expression (12), the deflection angle of the complex number s + (t) is θ(t). Therefore, θ( t ) can be obtained when the deflection angle of the complex number s + (t) is found.
 
Expression 13
 
θ( t )=arg( s   + ( t ))  (13)
 
     The complex number s + (t) is one obtained when the negative frequency component of the interference signal s(t) is zeroed, but can be found using Fourier transform or Hilbert transform like the configuration shown in  FIG.  11    or  FIG.  13   . 
     Note that an example using the Mach-Zehnder interferometer is shown in the above description. However, it is only necessary to obtain the interference of light having passed through two optical paths having an optical path length difference, and a Michelson interferometer may be used. When a Fabry-Perot interferometer is used, one light is output. Therefore, a single-input photodetector (PD) is used as a light receiving element instead of a BPD, and a DC cut filter or high-pass filter is put between the PD and the signal capturing device  3  according to the circumstances. 
     Next, the relative wavenumber calculation unit  521  that calculates the relative wavenumber k′(t) of light from the phase θ′(t) of the interference signal s(t) will be described. Prior to the description, a reason for calculating a relative wavenumber k′(t) of light rather than the absolute wavenumber k(t) from the phase θ′(t) will be described. 
     From expression (10), 2zk(t)=θ(t) can be seen. Therefore, it seems from expression (10) that the wavenumber k(t) is possibly found under the calculation of k(t)=θ(t)/2z. However, the wavenumber k(t) is not actually found according to the calculation method. This is because, since expression (10) is established even if the phase θ(t) is set at 0 for each 271, there is a many-to-one relationship between the phase θ( t ) and the wavenumber k(t) and thus the phase θ′(t) and the wavenumber k(t) found by the configurations of  FIGS.  10  and  13    do not necessarily correspond to each other. That is, the phase θ(t) is not properly found by the configurations of  FIGS.  10  and  13   . 
     Accordingly, in the present embodiment, the relative wavenumber calculation unit  521  finds the relative wavenumber k′(t) based on t=t 0  rather than the wavenumber k(t) for the wavelength λ(t) of the output light of the wavelength sweeping light source  103  according to the following expression. 
     
       
         
           
             
               
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   14 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       k 
                       ′ 
                     
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         θ 
                         ′ 
                       
                       ⁡ 
                       
                         ( 
                         t 
                         ) 
                       
                     
                     
                       2 
                       ⁢ 
                       z 
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
     Next, the absolute wavenumber calculation unit  523  that calculates the absolute wavenumber k(t) that is the wavenumber k f  at a time t f  at which light passes through the wavelength filter  61  from the relative wavenumber k′(t) will be described. The peak time acquisition unit  522  that acquires the peak time t f  from the filter signal s f (t) will be described later. 
       FIG.  15 A  shows the time change of the phase θ′(t).  FIG.  15 B  shows the time changes of the relative wavenumber k′(t) and the absolute wavenumber k(t).  FIG.  15 C  shows the time change of the filter signal sf(t). θ′(t 0 ) and k′(t 0 ) do not necessarily become 0 but become values within the range of 0 to 2 π or −π to π. When a time at which the peak of the filter signal s f (t) appears is t f  and the wavelength of light passing through the wavelength filter  61  in the greatest amount is λ f , the wavenumber k f =k(t f ) corresponding to the peak time tf and the peak wavelength λ f  is as follows. 
     
       
         
           
             
               
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   15 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     k 
                     f 
                   
                   = 
                   
                     
                       k 
                       ⁡ 
                       
                         ( 
                         
                           t 
                           f 
                         
                         ) 
                       
                     
                     = 
                     
                       
                         2 
                         ⁢ 
                         π 
                       
                       
                         λ 
                         f 
                       
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
     In addition, when the wavenumber corresponding to k′=0 is k 0  as shown in  FIG.  15 B , k 0  can be calculated as follows. 
     
       
         
           
             
               
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   16 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     k 
                     0 
                   
                   = 
                   
                     
                       
                         k 
                         f 
                       
                       - 
                       
                         
                           k 
                           ′ 
                         
                         ⁡ 
                         
                           ( 
                           
                             t 
                             f 
                           
                           ) 
                         
                       
                     
                     = 
                     
                       
                         
                           2 
                           ⁢ 
                           π 
                         
                         
                           λ 
                           f 
                         
                       
                       - 
                       
                         
                           k 
                           ′ 
                         
                         ⁡ 
                         
                           ( 
                           
                             t 
                             f 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
     
     The absolute wavenumber calculation unit  523  calculates the absolute wavenumber k(t) according to the following expression using the wavenumber k 0  obtained by expression (16). 
     
       
         
           
             
               
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   17 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     k 
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           k 
                           ′ 
                         
                         ⁡ 
                         
                           ( 
                           t 
                           ) 
                         
                       
                       + 
                       
                         k 
                         0 
                       
                     
                     = 
                     
                       
                         
                           k 
                           ′ 
                         
                         ⁡ 
                         
                           ( 
                           t 
                           ) 
                         
                       
                       + 
                       
                         
                           2 
                           ⁢ 
                           π 
                         
                         
                           λ 
                           f 
                         
                       
                       - 
                       
                         
                           k 
                           ′ 
                         
                         ⁡ 
                         
                           ( 
                           
                             t 
                             f 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   17 
                   ) 
                 
               
             
           
         
       
     
     Finally, the wavelength calculation processing unit  524  calculates the wavelength λ(t) of the output light of the wavelength sweeping light source  103  from the wavenumber k(t) according to the following expression. 
     
       
         
           
             
               
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   18 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     λ 
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       2 
                       ⁢ 
                       π 
                     
                     
                       k 
                       ⁡ 
                       
                         ( 
                         t 
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   18 
                   ) 
                 
               
             
           
         
       
     
     The peak time acquisition unit  522  that has not been described above will be described. As a method for deriving the peak time t f , there is a method in which the peak of the filter signal s f (t) is searched for to acquire the peak time t f . However, since the time t is a discrete value in the present embodiment, the filter signal s f (t) also becomes discrete data. Therefore, when the peak of the filter signal s f (t) is searched for, the peak time t f  can be obtained only with the accuracy of a sampling cycle. 
     In order to more precisely measure the peak time t f , it is presumed that fitting using a function is performed on the data of a prescribed number in the vicinity of the peak of the filter signal s f (t) to acquire the peak time t f . For example, when fitting is performed using a quadratic function s=a 2 t 2 +a 1 t+a 0 , it is presumed that the peak time acquisition unit  522  calculates the peak time t f  as t f =−a 1 /(2a 2 ). 
     Further, the peak time acquisition unit  522  may search for the peak of the filter signal s f (t) through interpolation between the data of the filter signal s f (t) to obtain the peak time t d . As a method for interpolating the filter signal s f (t), a method such as a method in which the Fourier transform of the filter signal s f (t) is performed to find a frequency spectrum, zero is padded as the high frequency component of the frequency spectrum (zero padding), and the Fourier inverse transform of the frequency spectrum is performed to obtain the interpolated filter signal s f (t) and spline interpolation is presumed. 
     Further, if the function fitting of the vicinity of the peak of the filter signal s f (t) is performed to obtain the peak time t f  after the above interpolation, accuracy is more improved. 
     The peak time t f  of the filter signal s f (t) obtained by the interpolation or the function fitting described above does not generally match the sampling time of the signal capturing device  3  of  FIG.  8   . 
     However, the data of the relative wavenumber k′(t) of light exists in a discrete manner only at its sampling time. Therefore, even if the absolute wavenumber calculation unit  523  in  FIG.  9    makes an attempt to obtain k′(t f ) to find the wavenumber k 0 , the peak time t f  does not necessarily match the discrete time t of the data of the wavenumber k′(t). In this case, the peak time acquisition unit  522  may only calculate the wavenumber k′(t f ) through interpolation using wavenumbers k′(t) prior to and subsequent to the peak time t f . For example, it is presumed that linear interpolation is performed using two wavenumbers k′(t) adjacent to the peak time t f  of the filter signal s f (t). 
     The method for finding the coordinates (L 0 , x 0 ) of the deflection origin of the light deflector  100  (the deviation amount between the origin O of the world coordinate system and the origin O′ of the stage coordinate system) and the method for finding the wavelength λ(t) of the output light of the wavelength sweeping light source  103  are described above. Next, a method for finding the incident angle α of an output light beam from the wavelength sweeping light source  103  onto the diffraction grating  102  and a method for finding the angle θ G  formed by a line perpendicular to the surface of the diffraction grating  102  and the L-axis will be described. 
     Although  FIG.  16    is almost the same diagram as  FIG.  1   ,  FIG.  16    shows a state in which the position of the PD  1  is moved parallel to an x′ coordinate axis. Like this, only coordinates x′ are changed with coordinates L′ fixed to obtain each output signal S p (t) of the PD  1 . 
       FIG.  17    is a diagram showing output signals S p (t a ), S p (t b ), S p (t c ), and S p (t d ) of the PD  1  and wavelengths λ(t a ), λ(t b ), λ(t c ), and λ(t d ) of the output light of the wavelength sweeping light source  103  obtained from the output signals S p (t a ), S p (t b ), S p (t c ), and S p (t d ) when the PD  1  is positioned at positions x′ a , x′ b , x′ c , and x′ d  shown in  FIG.  16    at times t a , t b , t c , and t d . 
       FIGS.  18 A and  18 B  are diagrams for describing a method for finding the wavelength λ of the output light of the wavelength sweeping light source  103  from the output signal S p (t) when the PD  1  is positioned at the coordinates (L i , x i ).  FIG.  18 A  shows the wavelength λ(t) calculated by the wavelength calculation unit  52  of the signal processing device  5 .  FIG.  18 B  shows the output signal S p (t) of the PD  1 . 
     The wavelength calculation unit  53  of the signal processing device  5  acquires the output signal S p (t) of the PD  1  when the PD  1  is positioned at the coordinates (L i , x i ), searches for the peak position of the output signal S p (t), and sets the peak time of the peak position as t i . Then, the wavelength calculation unit  53  finds the wavelength λ(t i ) where t=t i  from the wavelength λ(t) calculated by the wavelength calculation unit  52 , and sets λ(t i ) as the wavelength λ i  of the output light of the wavelength sweeping light source  103  when the PD  1  is positioned at the coordinates (L i , x i ). 
     As a method for obtaining the peak time t i  of the output signal S p (t) of the PD  1 , a method for obtaining the peak time t f  of the above filter signal s f (t) can be used. Since the method is the same as the above, its description will be omitted. 
     In this manner, the PD  1  is moved parallel to the x′ coordinate axis by the biaxial translation automatic stage  2  controlled by the stage control unit  50  to acquire the coordinates (L i , x i ) of the PD  1  and the measurement data (L′ i , x i , λ i ) (i=1 to N m ) of the wavelength λ i  of the output light of the wavelength sweeping light source  103  at each of the N m  positions. Note that the value of L′ i  of each measurement data (L′ i , x′ i , λ i ) (i=1 to N m ) is the same since PD  1  is moved with the coordinates L′ fixed as described above. Further, the coordinates (L i , x i ) of the PD  1  can be obtained from the stage control unit  50 . 
     Meanwhile, the following relational expression can be established when expressions (2) to (4) are integrated with each other.
 
Expression 19
 
 x′+x   0 =( L′+L   0 )tan(θ G +sin −1 (sin α− Nm λ( t )))  (19)
 
     The parameter calculation unit  54  of the signal processing device  5  calculates the incident angle α of the output light beam of the wavelength sweeping light source  103  onto the diffraction grating  102  and the angle θ G  formed by the line perpendicular to the surface of the diffraction grating  102  and the L-axis from expression (19) on the basis of the acquired data (L′ i , x′ i , λ i ) (i=1 to N m ) and the coordinates (L 0 , x 0 ) of the deflection origin of the light deflector  100  that is calculated in advance by the deflection origin calculation unit  51 . Note that it goes without saying that x′ i  of the measurement data is substituted into x′ of expression (19), L′ i  of the measurement data is substituted into L′ of expression (19), and λ i  of the measurement data is substituted into λ(t) of expression (19). 
     Since there is generally a measurement error or the like, it is presumed that the incident angle α and the angle θ G  with which expression (19) is completely established hardly exist. Therefore, the parameter calculation unit  54  of the signal processing device  5  calculates the incident angle α of the output light beam of the wavelength sweeping light source  103  onto the diffraction grating  102  and the angle θ G  formed by the line perpendicular to the surface of the diffraction grating  102  and the L-axis by function fitting in which the function best fitted to the acquired measurement data (L′ i , x′ i , λ i ) (i=1 to N m ), the coordinates (L 0 , x 0 ) of the deflection origin of the light deflector  100 , the number N of the ruled lines of the diffraction grating  102 , and the diffraction order m is found. As an example of such function fitting, a Newton-Raphson method, a steepest descent method, a Levenberg-Marquardt method, or the like can be used. 
       FIG.  19    shows results obtained when α and θ G  are calculated using the Levenberg-Marquardt method. In  FIG.  19   , a horizontal axis shows a wavelength, a vertical axis shows coordinates x′,  140  shows measurement data, and  141  shows a fitting result. Here, it is assumed that the number N of the ruled lines of the diffraction grating  102  is 1201, the diffraction order m is 1, x 0  is −11.1295 mm, and L 0  is 31.1404 mm. 
     The example of  FIG.  19    shows the fitting results obtained when L i =510 mm(L′ i =478.8596 mm), and shows the fact that the measurement data of (L′ i , x′ i , λ i ) and the value of calculated by substituting the calculated α, θ G , and (L′ i , λ i ) into expression (19) almost match each other, and that fitting goes well. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 L(cm) 
                 θ G (deg) 
                 α(deg) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 11 
                 64.0474 
                 43.46563 
               
               
                 31 
                 63.94833 
                 43.54618 
               
               
                 51 
                 64.03323 
                 43.49067 
               
               
                 Average 
                 64.00965 
                 43.50082 
               
               
                   
               
            
           
         
       
     
     Further, Table 1 shows results obtained when α and θ G  are each calculated using the Levenberg-Marquardt method with respect to the group of the measurement data of (L′ i , x′ i , λ i ) in a case in which L i  is changed to no mm, 310 mm, or 510 mm. The values of a and θ G  show slight differences. In order to further increase accuracy in calculating α and θ G , it may be possible to find α and θ G  with respect to a plurality of the groups of the data of L i  and average the same. In the table, the average values of α and θ G  of the above three groups are also shown. 
     In the manner described above, the present embodiment makes it possible to exactly measure the incident angle α of the output light beam of the wavelength sweeping light source  103  onto the diffraction grating  102  and exactly find the relationship between the time t and the emission angle β of the diffraction grating  102 . 
     The signal processing device  5  described in the present embodiment can be realized by a computer including a CPU (Central Processing Unit), a storage device, and an interface and a program that controls the hardware resources.  FIG.  20    shows a configuration example of the computer. The computer includes a CPU  200 , a storage device  201 , and an interface device (hereinafter abbreviated as an I/F)  202 . The signal capturing device  3 , the stage driver  4 , or the like is connected to the I/F  202 . In such a computer, a light deflector parameter measurement program for realizing the light deflector parameter measurement method of embodiments of the present invention is stored in the storage device  201 . The CPU  200  performs the processing described in the present embodiment according to the program stored in the storage device  201 . Note that it is also possible to provide the program via a network. 
     INDUSTRIAL APPLICABILITY 
     Embodiments of the present invention can be applied to a technology to measure the parameters of a light deflector. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  64  Photodetector 
           2  Biaxial translation automatic stage 
           3  Signal capturing device 
           4  Stage driver 
           5  Signal processing device 
           50  Stage control unit 
           51  Deflection origin calculation unit 
           52 ,  53  Wavelength calculation unit 
           54  Parameter calculation unit 
           60  Coupler 
           61  Wavelength filter 
           62  Mach-Zehnder interferometer  62   
           63  Balanced photodetector 
           100  Light deflector 
           101  Fiber collimator 
           102  Diffraction grating 
           103  Wavelength sweeping light source 
           520  Phase calculation unit 
           521  Relative wavenumber calculation unit 
           522  Peak time acquisition unit 
           523  Absolute wavenumber calculation unit 
           524  Wavelength calculation processing unit 
           5200  Fourier transform unit 
           5201  Negative frequency component zeroing unit 
           5202  Fourier inverse transform unit 
           5203  Deflection calculation unit 
           5204  Hilbert transform unit 
           5205  Complex number generation unit 
           5206  Deflection angle calculation unit