Patent Publication Number: US-2023152163-A1

Title: Phase difference measuring device, beam output apparatus and phase difference measuring method

Description:
TECHNICAL FIELD 
     The present invention relates to a phase difference measuring device, a beam output apparatus and a phase difference measuring method, and can be suitably used as a phase difference measuring device, a beam output apparatus and a phase difference measuring method for a laser beam, for example. 
     BACKGROUND 
     High-quality and high-power laser light is required in fields such as laser processing, scientific research, nuclear fusion, space debris removal, and security. However, due to factors such as heat generation, optical damage, and non-linear optical effects, there is a limit to an output that can be achieved as a single laser beam. Therefore, the technology for combining a plurality of laser beams is being studied to increase the output power. 
     Herein, when combining a plurality of laser beams, it is preferable to match a phase of each laser beam. Therefore, a device, a process, or the like for temporal modulation and/or demodulation (frequency shift, phase modulation, or the like) of each laser beam is required. 
     In connection with the above, Patent Literature 1 (US 2009/0134310 A1) discloses a laser system that combines a plurality of laser beams. In this laser system, a frequency shift is applied to each laser beam in order to calculate a phase of each laser beam. 
     Patent Literature 2 (Japanese Patent Publication No. 2014-216418) discloses a phase-locked laser device that combines a plurality of laser beams. In this phase-locked laser device, a frequency shift and a phase modulation are applied to each laser beam in order to calculate a phase of each laser beam. 
     In addition, in order to respond to a change in beam intensity, it is necessary to observe an interference pattern by use of a plurality of sensors and calculate. When a pointing of a laser beam changes, an appearance of interference fringes changes (for example: the interference fringes are tilted, a spacing between the interference fringes changes, or the like). In order to respond to such changes, it is necessary to observe the interference pattern by use of a plurality of sensors. 
     In connection to the above, Patent Literature 3 (Japanese Patent No. 6071202) discloses a multi-beam coupling device that couples a plurality of beams. In this multi-beam coupling device, a spatial interference pattern is used in order to control a phase of each laser beam. Since a phase is controlled based on a one-dimensional interference pattern, phase measurement is necessary at two or more positions for each beam. 
     In addition, Non-Patent Literature 1 (K. Sueda, G. Miyaji, N. Miyanaga and M. Nakatsuka, “Laguerre-Gaussian beam generated with a multilevel spiral phase plate for high intensity laser pulses”, OPTICS EXPRESS, Optical Society of America. Jul. 26, 2004, pp. 3548-3553.) discloses a phase distribution of a 16-step spiral phase plate, a prototype example, an interference pattern generated by a beam with 16-step spiral phase plate and a reference beam. 
     CITED LIST 
     Patent Literature 
     
         
         [Patent Literature 1] US 2009/0134310 A1 
         [Patent Literature 2] Japanese Patent Publication No. 2014-216418 
         [Patent Literature 3] Japanese Patent No. 6071202 
       
    
     Non-Patent Literature 
     
         
         [Non-Patent Literature 1] K. Sueda, G. Miyaji, N. Miyanaga and M. Nakatsuka, “Laguerre-Gaussian beam generated with a multilevel spiral phase plate for high intensity laser pulses”, OPTICS EXPRESS, Optical Society of America, Jul. 26, 2004, pp. 3548-3553. 
       
    
     SUMMARY 
     Phase difference between laser beams will be detected or measured in order to generate a high-power and high-quality laser beam. Other objectives and new features will be clarified by disclosures of the present description and attached drawings. 
     A phase difference measuring device according to an embodiment is provided with a phase conversion device and a detection device. The phase conversion device converts a first laser beam that passes therethrough so that the first laser beam includes a phase distribution of one cycle in an azimuth direction in a cross section of the first laser beam that has passed therethrough included in an arbitrary virtual plane perpendicular to an optical axis of the first laser beam. The detection device detects an azimuth angle of an intensity centroid of an interference pattern and detects an inter-beam phase difference of a second laser beam with respect to the first laser beam based on the azimuth angle. The interference pattern is generated by a shaped detection beam obtained by cutting out a part of a cross section of a detection beam into a shape of a circle with a point, where the cross section of the detection beam and an optical axis of the phase conversion device intersect, as a center thereof. The cross section of the detection beam is perpendicular to an optical axis of the detection beam. The detection beam is obtained by combining a first partial intensity laser beam and a second partial intensity laser beam on a same optical path. The first partial intensity laser beam has at least a part of an intensity component of the first laser beam that has passed through the phase conversion device. The second partial intensity laser beam has at least a part of an intensity component of a second laser beam derived from a laser beam as seed light from which the first laser beam derives. 
     A beam output apparatus according to an embodiment is provided with a first beam splitter, a phase conversion device, a detection device, and a phase controller. The first beam splitter splits a laser beam that is to be seed light into a first laser beam and a second laser beam. The phase conversion device converts the first laser beam that passes therethrough, so that the first laser beam includes a phase distribution of one cycle in an azimuth direction in a cross section of the first laser beam that has passed therethrough included in an arbitrary virtual plane perpendicular to an optical axis of the first laser beam. The detection device detects an azimuth angle of an intensity centroid of an interference pattern and detects an inter-beam phase difference of the second laser beam with respect to the first laser beam based on the azimuth angle. The interference pattern is generated by a shaped detection beam obtained by cutting out a part of a cross section of the detection beam into a shape of a circle with a point, where the cross section of the detection beam and an optical axis of the phase conversion device intersect, as a center thereof. The cross section of the detection beam is perpendicular to an optical axis of a detection beam. The detection beam is obtained by combining a first partial intensity laser beam and a second partial intensity laser beam on a same optical path. The first partial intensity laser beam has at least a part of an intensity component of the first laser beam that has passed through the phase conversion device. The second partial intensity laser beam has at least a part of an intensity component of the second laser beam. The phase controller controls a phase of the second laser beam based on the phase difference. 
     A phase difference measuring method according to an embodiment includes: converting a first laser beam that passes through a phase conversion device, so that the first laser beam includes a phase distribution of one cycle in an azimuth direction in a cross section of the first laser beam that has passed therethrough included in an arbitrary virtual plane perpendicular to an optical axis of the first laser beam; detecting an azimuth angle of an intensity centroid of an interference pattern; and detecting an inter-beam phase difference of a second laser beam with respect to the first laser beam based on the azimuth angle. The interference pattern is generated by a shaped detection beam obtained by cutting out a part of a cross section of the detection beam into a shape of a circle with a point, where the cross section of the detection beam and an optical axis of the phase conversion device intersect, as a center thereof. The cross section of the detection beam is perpendicular to an optical axis of a detection beam. The detection beam is obtained by combining a first partial intensity laser beam and a second partial intensity laser beam on a same optical path. The first partial intensity laser beam has at least a part of an intensity component of the first laser beam that is converted. The second partial intensity laser beam has at least a part of an intensity component of the second laser beam derived from a laser beam as seed light from which the first laser beam derives. 
     According to an embodiment, a high-power and high-quality laser beam can be generated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram that shows a configuration example of a beam output apparatus according to an embodiment. 
         FIG.  2 A  is a diagram for describing an operation principle of a phase difference measuring device according to an embodiment. 
         FIG.  2 B  is a diagram that shows an example of a phase distribution in a cross-section perpendicular to an optical axis direction of a reference beam that has passed through a spiral phase plate according to an embodiment. 
         FIG.  3    is a diagram that shows a configuration example of an aperture according to an embodiment. 
         FIG.  4 A  is a diagram for describing a calculation of a phase difference from an interference pattern obtained by a phase difference measuring device according to an embodiment. 
         FIG.  4 B  is a diagram for describing a calculation of a phase difference from an interference pattern obtained by a phase difference measuring device according to an embodiment. 
         FIG.  4 C  is a diagram for describing a calculation of a phase difference from an interference pattern obtained by a phase difference measuring device according to an embodiment. 
         FIG.  5    is a graph that shows an example of a relationship of a measured value of an azimuth angle of an interference pattern intensity centroid obtained by a phase difference measuring device according to an embodiment and a phase difference measurement error, both with respect to an actual inter-beam phase difference. 
         FIG.  6    is a diagram that shows an example of a measurement result of an angle that represents an azimuth angle of an interference pattern intensity centroid obtained by a phase difference measuring device according to an embodiment. 
         FIG.  7    is a graph that shows an example of measurement error of a phase difference obtained by a phase difference measurement device according to an embodiment. 
         FIG.  8    is a diagram that shows an example of a hologram that can generate a spiral phase distribution. 
         FIG.  9 A  is a perspective view that shows a configuration example of a sixteen-step spiral phase plate according to an embodiment. 
         FIG.  9 B  is a diagram that shows an example of a phase distribution in a cross section that is perpendicular to an optical axis direction of a reference beam that has passed through the multilevel spiral phase plate shown in  FIG.  9 A . 
         FIG.  9 C  is a diagram that shows an example of an interference pattern generated by a reference beam that has passed through the multilevel spiral phase plate shown in  FIG.  9 A  and a test beam. 
         FIG.  9 D  is a graph that shows an example of a relationship of a measured value of an azimuth angle of an interference pattern intensity centroid obtained by a phase difference measuring device that uses the multilevel spiral phase plate shown in  FIG.  9 A  and a phase difference measurement error, both with respect to an actual phase difference between a reference beam that has passed through the multilevel spiral phase plate and a test beam. 
         FIG.  10    is a diagram that shows a configuration example of a beam output apparatus according to an embodiment. 
         FIG.  11    is a diagram that partially shows a configuration example of a beam output apparatus according to an embodiment. 
         FIG.  12    is a diagram that shows a configuration example of a beam output apparatus according to an embodiment. 
         FIG.  13    is a diagram that shows a configuration example of a beam output apparatus according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment for implementing a phase difference measuring device, abeam output apparatus and a phase difference measuring method according to the present invention will be described below with reference to attached drawings. Hereinafter, a laser light beam may be referred to as a laser beam or a beam. 
     First Embodiment 
     Components of the beam output apparatus  1  according to an embodiment will be described with reference to  FIG.  1   .  FIG.  1    is a diagram that shows a configuration example of a beam output apparatus  1  according to an embodiment. 
     The beam output apparatus  1  in  FIG.  1    is provided with a laser oscillator  2 , a beam splitter  3 , a reference beam generation section  4 , a test beam generation section  5  and a phase difference measuring device  6 . 
     Components of the reference beam generation section  4  in  FIG.  1    will be described. The reference beam generation section  4  is provided with a beam expander  44  and a reflector  45 . 
     Components of the test beam generation section  5  in  FIG.  1    will be described. The test beam generation section  5  is provided with a controller  51 , a phase controller  53 , a beam expander  54 , an amplifier  55 , and a reflector  57 . 
     Components of the phase difference measuring device  6  in  FIG.  1    will be described. The phase difference measuring device  6  is provided with a spiral phase plate  61 , a beam splitter  62 , and a detection device  60 . The detection device  60  is provided with an aperture  63 , a four-quadrant detector  64 , and a processor  65 . 
     Connection relationship between components of the beam output apparatus  1  in  FIG.  1    will be described. The laser oscillator  2  generates a laser beam L 0  that is to be seed light. The beam splitter  3  is arranged downstream of the laser oscillator  2  and splits the generated laser beam L 0  into a first laser beam and a second laser beam. The first laser beam and the second laser beam may be outputted to different directions. 
     The reference beam generation section  4  is arranged downstream of the beam splitter  3  and incidents the first laser beam to output a reference beam L 1 . In other words, the reference beam generation section  4  is arranged at a position where the first laser beam is incident. 
     In the reference beam generation section  4 , the beam expander  44  may be arranged upstream to the reflector  45 . In other words, the beam expander  44  enlarges a cross-sectional area of the first laser beam and outputs it. In addition, the reflector  45  reflects the first laser beam of which the cross-sectional area is enlarged to output as the reference beam L 1 . It should be noted that the order of enlarging the cross-sectional area and reflecting may be changed. In other words, the beam expander  44  may be arranged upstream to the reflector  45  and may be arranged downstream of the reflector  45 . 
     The test beam generation section  5  is arranged downstream of the beam splitter  3  and incidents the second laser beam to output a test beam L 2 . In other words, the test beam generation section  5  is arranged at a position where the second laser beam outputted from the beam splitter  3  is incident. 
     In the test beam generation section  5 , the phase controller  53  incidents the second laser beam and shifts a phase thereof. At that time, the phase controller  53  performs a phase shift of the second laser beam based on a phase control signal CS that is electrically provided from the controller  51 . The beam expander  54  receives the second laser beam of which the phase is shifted, enlarges a cross-sectional area thereof, and outputs it. In other words, the beam expander  54  is arranged downstream of the phase controller  53 . The amplifier  55  receives the second laser beam of which the cross-sectional area is enlarged, amplifies an optical intensity thereof, and outputs it. In other words, the amplifier  55  is arranged downstream of the beam expander  54 . The reflector  57  reflects the second laser beam of which the optical intensity is amplified and outputs it as the test beam L 2 . In other words, the reflector  57  is arranged downstream of the amplifier  55 . The controller  51  is electrically provided with a detector signal DS from the processor  65  of the phase difference measuring device  6  that will be described later, and outputs the phase control signal CS based on the detector signal DS. In other words, although the controller  51  is electrically connected upstream to the phase controller  53 , the controller  51  may not be optically connected between other components of the test beam generation section  5 . It should be noted that an optical arrangement of the components of the test beam generation section  5  may be arbitrarily changed within a technically consistent range. 
     The beam splitter  62  has a low reflection coating on one side and an anti-reflection coating on the other side, and guides a test beam L 22 , that is a slight part of an intensity component (power) of the test beam L 2  being reflected thereby, and a reference beam L 12 , that is a most part of an intensity component (power) of the reference beam L 1  being transmitted therethrough, to the detection device  60 , in parallel to each other. The phase difference measuring device  6  receives the reference beam L 1  and the test beam L 2  and outputs the detector signal DS that shows a result of measuring an inter-beam phase difference between those two beams L 1  and L 2 . In other words, the phase difference measuring device  6  is arranged downstream of the reference beam generation section  4  and is arranged downstream of the test beam generation section  5  as well. It should be noted that, from the reference beam L 1  and the test beam L 2  that are inputted, the phase difference measuring device  6  outputs a most part of the test beam L 2  and a slight part of the reference beam L 1 , that were not used for the phase difference measurement, as a combined beam L 3 . It should be noted that the combined beam L 3  may be outputted to be combined to another beam, and this combining may be performed by another optical system that is not illustrated. 
     In the phase difference measuring device  6 , the spiral phase plate  61  receives and passes the reference beam L 1  and converts a phase distribution of the passing reference beam L 1 . The beam splitter  62  transmits, on one hand, a most part of the reference beam L 1  of which the phase distribution is converted as the reference beam L 12 , and reflects a slight part as a residual reflected beam L 11 . In addition, the beam splitter  62  reflects, on the other hand, a slight part of the test beam L 2  as the test beam L 22  and outputs a most part of the test beam L 2  as a transmitted beam L 21 . In addition, the test beam L 22  and the reference beam L 12  are outputted to the detection device  60  as a detection beam L 4 . In other words, the beam splitter  62  is arranged at a position where the reference beam L 1  and the test beam L 2  intersect. It should be noted that a transmittance and a reflectance of the beam splitter  62  are not limited to the above example. 
     In the detection device  60 , the aperture  63  receives the detection beam L 4  and passes a part thereof. The four-quadrant detector  64  is provided with quadrant sensors that detect an optical intensity of the part of the detection beam L 4  that has passed through the aperture  63  and generates optical intensity signals PS 1  to PS 4  that electrically represent an optical intensity distribution thereof. In other words, the four-quadrant detector  64  is a sensor device arranged downstream of the aperture  63 . The processor  65  is electrically supplied with the optical intensity signals PS 1  to PS 4 , calculates the phase difference based on them, and generates the detector signal DS that electrically represents the calculated phase difference. In other words, the processor  65  is electrically connected to downstream of the four-quadrant detector  64 . The processor  65  may be provided with a computer provided with a microcomputer chip that performs predetermined processes or a Central Processing Unit (CPU) that executes predetermined programs, a storage device that stores the programs and various data, and various interfaces that perform inputs and outputs of data from or to outside. It should be noted that an analog microcomputer is more advantageous for increasing control speed. 
     An operation of the beam output apparatus  1  in  FIG.  1   , that is, the phase difference measuring method according to the present embodiment, will be described. At first, the laser oscillator  2  generates the laser beam L 0  that is to be the seed light. This laser beam L 0  may be a continuous wave or a pulsed wave. 
     Next, the beam splitter  3  splits the laser beam L 0  into a first laser beam and a second laser beam. Herein, the beam splitter  3  may be a half mirror that transmits a part of the laser beam L 0  as the first laser beam and reflects another part of the laser beam L 0  as the second laser beam. In addition, ratios of transmission and reflection do not need to be the same. 
     Next, the reference beam generation section  4  receives the first laser beam and generates the reference beam L 1 . At first, the beam expander  44  enlarges the cross-sectional area of the first laser beam and outputs it as a parallel beam. The beam expander  44  may be provided with a combination of a plurality of lenses, for example. Next, the reflector  45  reflects the first laser beam, of which the cross-sectional area is enlarged, to output as the reference beam L 1 . It should be noted that the order of the enlarging the cross-sectional area and the reflecting may be changed. 
     In parallel to the generation of the reference beam L 1 , the test beam generation section  5  receives the second laser beam and generates the test beam L 2 . At first, the controller  51  generates the phase control signal CS based on the detector signal DS. Next, the phase controller  53  controls the phase of the second laser beam based on the phase control signal CS. For example, the phase controller  53  adjusts the phase of the second laser beam that passes through the phase controller  53  based on the phase control signal CS. Next, the beam expander  54  enlarges the cross-sectional area of the second laser beam of which the phase is shifted and outputs it as a parallel beam. Next, the amplifier  55  amplifies the second laser beam of which the cross-sectional area is enlarged. Next, the reflector  57  reflects the amplified second laser beam and outputs it as the test beam L 2  to the beam splitter  62 . It should be noted that the order of shifting the phase, enlarging the cross-sectional area, amplifying, and reflecting of the second laser beam may be changed within a technically consistent range. 
     Next, the phase difference measuring device  6  measures an inter-beam phase difference of the test beam L 2  with respect to the reference beam L 1 .  FIG.  2 A  is a diagram for describing an operation principle of the phase difference measuring device  6  according to an embodiment. At first, the spiral phase plate  61  converts the phase distribution of the reference beam L 1  that passes therethrough. The spiral phase plate  61  converts an incident beam of which the phase is spatially uniform to a transmitted beam having a spatial phase distribution. For example, the spiral phase plate  61  shown in  FIG.  2 A  has a configuration in that a material is transparent and a thickness gradually increases in an azimuth direction with the optical axis A 1  thereof as a center of rotation. By this spiral phase plate  61 , the converted phase distribution of the reference beam L 1  is configured to totally include a range of phase of one cycle, for example a range of phase from zero radian to 2π radians, along the azimuth direction in an arbitrary cross-section perpendicular to the optical axis A 1  of the reference beam L 1 . Preferably, the spiral phase plate  61  may be configured so that, when going around in a predetermined direction along the azimuth angle in an arbitrary cross-section perpendicular to the optical axis A 2  of the reference beam L 1 , a distribution density of an arbitrary phase included in a phase range of one cycle is uniform. For example, the spiral phase plate  61  may be configured so that the phase becomes equal to the azimuth angle in a desired cross-section of the reference beam L 1  perpendicular to the optical axis A 2 . 
       FIG.  2 B  is a diagram that shows an example of a phase distribution in a cross-section perpendicular to the axis A 1  of the reference beam L 1  that has passed through a spiral phase plate  61  according to an embodiment. In the example of  FIG.  2 B , a phase difference between a phase at an arbitrary point included in an arbitrary cross-section perpendicular to the optical axis A 1  shown in  FIG.  2 A  of the reference beam L 1  that has passed through the spiral phase plate  61  according to the present embodiment and a reference phase at an arbitrary reference point included in an x-axis matches an azimuth angle from the x-axis to the arbitrary point with the optical axis A 1  of the reference beam L 1  as a center of rotation. In  FIG.  2 B , a more advanced phase is represented in a darker color, a more lagging phase is represented in a lighter color, the darkest color represents zero radian phase, and the lightest color represents 2π radians phase. A phase distributed in such a way will be referred to as a spiral phase distribution for convenience. 
     The beam splitter  62  combines the reference beam L 1  that has passed through the spiral phase plate  61  and the test beam L 2 , and generates the combined beam L 3  and the detection beam L 4  for detecting the phase difference. In the combined beam L 3 , optical axes of the transmitted beam L 21  and the residual reflected beam L 11  are parallel to each other, and therefore, it is necessary to adjust an intersection angle between the test beam L 2  and the reference beam L 1  and the arrangement angle of the beam splitter  62  so that the optical axes of the test beam L 22  and the reference beam L 12  are parallel to each other in the detection beam L 4  for detecting the phase difference. Herein, the beam splitter  62  may transmit a most part (for example: more than 99%) of the reference beam L 1  as the reference beam L 12  and reflect a slight part (for example: less than 1%) thereof as the residual reflected beam L 11 . 
     The beam splitter  62  may transmit a most part (for example: more than 99%) of the test beam L 2  as the transmitted beam L 21  and reflect a slight part (for example: less than 1%) as the test beam L 22 . 
     It should be noted that, from a point of view of combining the combined beam L 3  with another combined beam to generate a strong laser beam, it is not necessary, originally, to include the residual reflected beam L 11  in the combined beam L 3 . However, since a ratio of the residual reflected beam L 11  included in the combined beam L 3  is very small (for example: 1/10,000), there is no concern that a quality of the combined beam L 3  is substantially deteriorated. 
     A group of the reference beam L 12  and the test beam L 22  will be referred to as detection beam L 4 . The detection device  60  detects an azimuth angle of an intensity centroid in an interference pattern that the reference beam L 12  and the test beam L 22 , that are included in the detection beam L 4 , generate. At first, the aperture  63  masks a part of the detection beam L 4  that passes therethrough.  FIG.  3    is a diagram that shows a configuration example of the aperture  63  according to an embodiment. In the example of  FIG.  3   , a part, that is included inside a circle having a radius R 1  with the optical axis A 4  shown in  FIG.  2 A  as a center, of the detection beam L 4 , and a part, that is included outside a circle having a radius R 2  with the optical axis A 4  as a center, of the detection beam L 4 , are masked by the aperture  63 . 
     It should be noted that, a phase of a part of the detection beam L 4  that is included in a region closed to the optical axis A 4  may be uncertain. In addition, an optical intensity of a part of the detection beam L 4  that is included in a region far from the optical axis A 4  may be non-uniform. Therefore, by masking those regions by the aperture  63 , it is expected that an accuracy of detection of the azimuth angle of the intensity centroid, that will be described below, improves. A part of the detection beam L 4  that was not masked by the aperture  63  will be referred to as a shaped detection beam L 5  for convenience. 
     The four-quadrant detector  64  receives the shaped detection beam L 5  and detects an optical intensity thereof. Herein, an interference pattern generated by the reference beam L 12  and the test beam L 22  is drawn in the shaped detection beam L 5  received by the four-quadrant detector  64 .  FIG.  4 A  is a diagram that shows an example of a reference interference pattern generated by a reference beam L 12  and a test beam L 22  according to an embodiment. In  FIG.  4 A , in the interference pattern, a part of which an optical intensity is higher is represented by a lighter color, and a part of which an optical intensity is lower is represented by a darker color. An outer circumference of the interference pattern in  FIG.  4 A  is a circle and this circle corresponds to a circle that has a radius R 2  of the aperture  63 . In addition, a center of the interference pattern in  FIG.  4 A  corresponds to an optical axis of the shaped detection beam L 5 . The optical intensity of the interference pattern in  FIG.  4 A  is more strongly distributed in a right direction when viewed from the center with respect to the paper surface. The centroid of the optical intensity distribution in the interference pattern will be referred to as an optical intensity centroid of the interference pattern or an interference pattern intensity centroid  71 , for convenience. In addition, in a cartesian coordinate system x-y, an azimuth angle θ of the interference pattern intensity centroid  71  is defined to take a positive value counterclockwise from the x-axis. In the example of  FIG.  4 A , it is shown that the angle θ is zero radian, and this shows that the inter-beam phase difference between the reference beam L 12  and the test beam L 22 , that is, the inter-beam phase difference between the reference beam L 1  and the test beam L 2  on the beam splitter  62 , is zero. 
     The four-quadrant detector  64  has a quadrant optical intensity sensor. The quadrant optical intensity sensor detects respective optical intensities of the shaped detection beam L 5  that is split into four parts. That is, an arbitrary cartesian coordinate system x-y is defined on a virtual plane perpendicular to the optical axis of the shaped detection beam L 5  with an origin that matches this optical axis, and then four quadrants according to this cartesian coordinate system x-y is considered. A first quadrant is a region of which the azimuth angle in  FIG.  2 B  is from zero radian to π/2 radians, a second quadrant is a region from π/2 radians to n radians, a third quadrant is a region from n radians to 3π/2 radians, and a fourth quadrant is a region from 3π/2 radians to 2π radians. 
     The quadrant optical intensity sensor that the four-quadrant detector  64  has respectively detects optical intensities of the four parts of the shaped detection beam L 5  that respectively correspond to those four quadrants. The optical intensity detected in the first quadrant will be referred to as P 1 , the optical intensity detected in the second quadrant will be referred to as P 2 , the optical intensity detected in the third quadrant will be referred to as P 3 , and the optical intensity detected in the fourth quadrant will be referred to as P 4 . The four-quadrant detector  64  generates optical intensity signals PS 1  to PS 4  that electrically represent the detected four optical intensities P 1  to P 4 , respectively, and outputs them to the processor  65 . 
     The processor  65  is supplied by the four-quadrant detector  64  with the optical intensity signals PS 1 , PS 2 , PS 3  and PS 4  that electrically represent optical intensities, and generates the detector signal DS. As an example, the processor  65  can calculate the angle θ that represents the azimuth angle  72 , from a coordinate X on the x-axis and a coordinate Y on the y-axis of the interference pattern intensity centroid  71 , based on the following formula. 
         X =( P   1   −P   2   −P   3   +P   4 )/( P   1   +P   2   +P   3   +P   4 ) 
         Y =( P   1   +P   2   −P   3   −P   4 )/( P   1   +P   2   +P   3   +P   4 ) 
       cos θ= X /( X   2   +Y   2 ) 1/2  
 
       sin θ= Y /( X   2   +Y   2 ) 1/2  
 
     An example of a relationship between an optical intensity distribution of an interference pattern and an azimuth angle  72  of an interference pattern intensity centroid  71  will be shown with reference to  FIG.  4 B  and  FIG.  4 C .  FIG.  4 B  and  FIG.  4 C  are diagrams that show examples of relationships between an optical intensity distribution of an interference pattern obtained by the phase difference measuring device  6  according to an embodiment and an azimuth angle  72  of an interference pattern intensity centroid  71 . In the example of  FIG.  4 B , the interference pattern intensity centroid  71  exists on a bisector of an intersection angle of the x-axis and y-axis in the second quadrant, and an angle θ that represents the azimuth angle  72  thereof is 3π/4 radians. In the example of  FIG.  4 C , the interference pattern intensity centroid  71  exists on a bisector of an intersection angle of the x-axis and y-axis in the third quadrant, and an angle θ that represents the azimuth angle  72  thereof is 5π/4 radians. 
     The processor  65  generates, after calculating the angle θ that represent the azimuth angle  72 , the detector signal DS that represents the angle θ, and outputs it to the controller  51  of the test beam generation section  5 . 
     Next, the test beam generation section  5  performs, based on the detector signal DS, a feedback control of the phase of the test beam L 2 . At first, the controller  51  receives the detector signal DS that is outputted from the processor  65  of the phase difference measuring device  6 . The controller  51  generates, based on the angle θ that the detector signal DS represents, the phase control signal CS for controlling the phase controller  53 , and outputs it to the phase controller  53 . Next, the phase controller  53  receives the phase control signal CS, and controls, based on a value that the phase control signal CS represents, the test beam L 2  so that the phase difference with respect to the reference beam L 1  becomes zero with a desired accuracy. 
     It will be described that the measurement of the inter-beam phase difference between the reference beam L 1  and the test beam L 2 , by the phase difference measuring device  6  according the present embodiment, has an excellent accuracy, with reference to  FIG.  5    to  FIG.  7   .  FIG.  5    is a graph that shows an example of a relationship of a measured value of an azimuth angle  72  of an interference pattern intensity centroid  71  obtained by a phase difference measuring device  6  according to an embodiment and a phase difference measurement error, both with respect to an actual inter-beam phase difference. In the graph of  FIG.  5   , the horizontal axis represents the actual phase difference of the test beam L 2  with respect to the reference beam L 1 , that is, the inter-beam phase difference, the left-side vertical axis represents the measured value of the azimuth angle  72  of the interference pattern intensity centroid  71  obtained by the phase difference measuring device  6  according to the present embodiment, and the right-side vertical axis represents a difference value between them, that is, a measurement error of the phase difference. Of the two graphs included in the graph of  FIG.  5   , the solid line graph corresponds to the left-side vertical axis, and the dotted line graph corresponds to the right-side vertical axis. As it can be read from  FIG.  5   , the measurement error of the inter-beam phase difference according to the present embodiment is included within a range of an order of ±1.5×10 5 ×π radians. As described above, in the example of  FIG.  5   , the measurement of the inter-beam phase difference between the reference beam L 1  and the test beam L 2  by the phase difference measuring device  6  according to the present embodiment has an excellent accuracy. 
       FIG.  6    is a diagram that shows an example of a measurement result of an angle θ that represents an azimuth angle  72  of an interference pattern intensity centroid  71  obtained by a phase difference measuring device  6  according to an embodiment. The horizontal axis and the vertical axis of  FIG.  6    correspond to each axis of the cartesian coordinate system x-y in a plane perpendicular to an optical axis of the detection beam L 4 , respectively. In the example of  FIG.  6   , in each of cases where an inter-beam phase difference Δϕ between the reference beam L 1  and the test beam L 2  is π/2 radians, −π/6 radians and −5π/6 radians, measurements of the angle θ that represent the azimuth angle  72  has been performed by changing an optical intensity ratio of the test beam L 22  with respect to the reference beam L 12  from 0.44 to 4. Consequently, it has been obtained a result in that, in each of phase differences Δϕ, a position of the interference pattern intensity centroid  71  just moves on a straight line D 1  (in case of π/2 radians), a straight line D 2  (in case of −π/6 radians) or a straight line D 3  (in case of −5π/6 radians) that correspond to each phase differences Δϕ, and that the angles θ does not change. As described above, a measurement of a phase difference between a reference beam L 1  and a test beam L 2  by the phase difference measurement device  6  according to the present embodiment is not affected by a difference of ratio of optical intensities of the reference beam L 1  and the test beam L 2 . 
       FIG.  7    is a graph that shows an example of measurement error of a phase difference obtained by a phase difference measurement device  6  according to an embodiment. In  FIG.  7   , the horizontal axis represents the inter-beam phase difference between the reference beam L 1  and the test beam L 2 , and the vertical axis represents a measurement error of the phase.  FIG.  7    includes a first graph G 1 , a second graph G 2 , a third graph G 3 , and a fourth graph G 4 . The first graph G 1  represents a measurement error of the angle θ that corresponds to the inter-beam phase difference in a case where an optical axis direction (pointing) of the test beam L 22  is shifted of DLD/4 in a vertical direction (y-axis direction) with respect to the reference beam L 12 , that is, in a case where a parallelism of the test beam L 22  with respect to the reference beam L 12  is shifted of DLD/4 in the vertical direction. Herein, DLD is a Diffraction Limited Divergence (angle) and is herein defined as the below formula with respect to a flat top intensity circular beam. 
         DLD= 2.44×λ/ D  
 
     Herein, λ represents a wavelength of the shaped detection beam L 5  (in fact, a wavelength of each beam that derives from the laser oscillator  2 ), and D represents a beam diameter of the shaped detection beam L 5 . In addition, the second graph G 2  represents a measurement error of the angle θ that corresponds to the inter-beam phase difference in a case where the parallelism of the test beam L 22  with respect to the reference beam L 12  is shifted of DLD/4 in a horizontal direction (x-axis direction). Similarly, the third graph G 3  represents a measurement error of the angle θ that corresponds to the inter-beam phase difference in a case where the parallelism of the test beam L 22  with respect to the reference beam L 12  is shifted of DLD/5 in a horizontal direction. In addition, the fourth graph G 4  represents a measurement error of the angle θ that corresponds to the inter-beam phase difference in a case where the parallelism of the test beam L 22  with respect to the reference beam L 12  is shifted of DLD/5 in a vertical direction. 
     When focusing on the first graph G 1  and the second graph G 2  in  FIG.  7   , an absolute value of the measurement error is less than 0.05π radians. In addition, when focusing on the third graph G 3  and the fourth graph G 4  in  FIG.  7   , an absolute value of the measurement error is less than 0.03π radians. As described above, a measurement of the inter-beam phase difference between the reference beam L 1  and the test beam L 2  by the phase difference measuring device  6  according to the present embodiment can suppress an effect due to a shift of the optical axes between the reference beam L 12  and the test beam L 22 , that is, a shift of intersection angle between the reference beam L 1  and the test beam L 2 , in other words a parallelism between the reference beam component L 11  and the test beam component L 21  in the combined beam L 3 , to be relatively small. 
     It should be noted that, when the shift in optical axis direction of the test beam L 22  with respect to the reference beam L 12  exceeds DLD/4, the parallelism between the reference beam component L 1 I and the test beam component L 21  in the combined beam L 3  is shifted, and a measurement and correction of a beam pointing are separately necessary for any method of beam combining. Therefore, herein, it is not necessary to consider a case where the shift in the optical axis of the test beam L 22  with respect to the reference beam L 12  exceeds DLD/4. 
     It should be noted that the configuration of the beam output apparatus  1  in  FIG.  1    is merely an example, and a part of the above-described components may be omitted or replaced to another component, another component may be added, or a positional relationship between components may be changed, as long as the above-described operation can be performed. 
     As a variation example of the beam output apparatus  1  shown in  FIG.  1   , a hologram corresponding to the spiral phase plate  61  may be used instead of the spiral plate  61 . This hologram may be a phase modulation type hologram or an amplitude modulation type hologram. Since such a technology is disclosed in a non-patent literature 1 (K. Sueda, G. Miyaji, N. Miyanaga and M. Nakatsuka, “Laguerre-Gaussian beam generated with a multilevel spiral phase plate for high intensity laser pulses”, OPTICS EXPRESS. Optical Society of America, Jul. 26, 2004, pp. 3548-3553), further detailed description will be omitted.  FIG.  8    is a diagram that shows an example of a hologram  7  that can generate a spiral phase distribution. 
     As another variation example of the beam output apparatus  1  shown in  FIG.  1   , a case where the spiral phase plate  61  is replaced with a multilevel spiral phase plate  610  will be described.  FIG.  9 A  is a perspective view that shows a configuration example of a multilevel spiral phase plate  610  according to an embodiment. The multilevel spiral phase plate  610  shown in  FIG.  9 A  has a function of giving a spiral step-like phase distribution, wherein each step has a fan shape of which bottom surface has a central angle of π/8 radians, and has a shape in which a total of sixteen prisms, that have thicknesses different from each other, are gathered so that each bottom surface is arranged on a same plane and a central angle of each bottom surface is in contact with each other. In addition, in the multilevel spiral phase plate  610  shown in  FIG.  9 A , a difference between a thickness of each prism with the fan-shape bottom surface and the thickness of the prism with the fan-shape bottom surface of which the thickness is the thinnest is a multiple of π/8 radians, and is distributed within a range from 0 radian to 15π/8 radians. Furthermore, each prism with the fan-shape bottom surface is arranged in an order of thickness thereof to rotate the central angle thereof. In other words, the multilevel spiral phase plate  610  shown in  FIG.  9 A  is obtained by approximating the spirally curbed surface of the spiral phase plate  61  shown in  FIG.  2 A  with a group of a plurality of parallel planes. It is expected that the multilevel spiral phase plate  610  that is configured as above can be manufactured cheaper and easier compared to the spiral phase plate  61  that has a spirally curbed surface. Since the multilevel spiral phase plate  610  shown in  FIG.  9 A  has a total of sixteen different thicknesses, it may be referred to as a sixteen-step spiral phase plate. Although the number of the steps in the example of  FIG.  9 A  is sixteen, it should be noted that this value is merely an example and does not limit the present embodiment. 
       FIG.  9 B  is a diagram that shows an example of a phase distribution in a cross section that is perpendicular to an optical axis direction of a reference beam L 1  that has passed through the multilevel spiral phase plate  610  shown in  FIG.  9 A . While in the phase distribution shown in  FIG.  2 B , that is obtained in the case where the spiral phase plate  61  is used, the phase continuously changes in a direction of rotation around the optical axis, in the phase distribution shown in  FIG.  9 B , the phase in the phase distribution also changes stepwise at positions where the thickness of the multilevel spiral phase plate  610  changes stepwise. A laser beam that has such a phase distribution will be referred to as a laser beam having a multilevel spiral wavefront, for convenience. 
       FIG.  9 C  is a diagram that shows an example of an interference pattern generated by a reference beam that has passed through the multilevel spiral phase plate  610  shown in  FIG.  9 A  and a test beam. While the interference intensity continuously changed in a direction of rotation around an optical axis in the optical intensity distribution of the interference pattern shown in  FIG.  4 B , that is obtained in case where the spiral phase plate  61  is used, the interference intensity also changes stepwise at positions where the thickness of the multilevel spiral phase plate  610  changes stepwise in the optical intensity distribution of the interference pattern shown in  FIG.  9 C . 
       FIG.  9 D  is a graph that shows an example of a relationship of a measured value of an azimuth angle  72  of an interference pattern intensity centroid  71  obtained by a phase difference measuring device  6  that uses the multilevel spiral phase plate  610  shown in  FIG.  9 A  and a phase difference measurement error, both with respect to an actual phase difference of the test beam with respect to the reference beam that has passed through the multilevel spiral phase plate  610 . The graph in  FIG.  9 D  is different in the phase difference measurement error, that is represented by the dotted line, in comparison with the graph shown in  FIG.  5   . That is, the measurement error shown in  FIG.  9 D  is included in the range from 0 radian to an order of 0.007n radians. Although this range is wider than the measurement error shown in  FIG.  5    in the case where the spiral phase plate  61  is used, it can be considered that this range is sufficiently small as a phase difference measurement error. As described above, the measurement of the phase difference between the reference beam L 1  and the test beam L 2  can be performed with a sufficiently high accuracy even if the spiral phase plate  61  included in the beam output apparatus  1  shown in  FIG.  1    is replaced with the multilevel spiral phase plate  610 . 
     Attention should be paid in that the spiral phase plate  61  shown in  FIG.  1   , the hologram  7  shown in  FIG.  8    and the multilevel spiral phase plate  610  shown in  FIG.  9 A  are all a phase conversion device that converts a laser beam that passes therethrough so that a cross section of this laser beam, that is included in an arbitrary virtual plane perpendicular to an optical axis of this laser beam that has passed therethrough, includes a phase distribution of one cycle. 
     Second Embodiment 
     A beam output apparatus  1  according to an embodiment will be described with reference to  FIG.  10   .  FIG.  10    is a diagram that shows a configuration example of a beam output apparatus  1  according to an embodiment. 
     The beam output apparatus  1  according to the present embodiment is approximatively equivalent to an apparatus in that four beam output apparatuses  1  shown in  FIG.  1    are prepared and integrated. In other words, the beam output apparatus  1  according to the present embodiment generates a laser beam L 0  to be seed light and splits it into a reference beam L 1  and a test beam L 2 , and in addition, splits the test beam L 2  into four test beams L 2 A, L 2 B, L 2 C and L 2 D. A cross-sectional area of the reference beam L 1  is enlarged by a beam expander  44 , and spiral phase plates  61 A,  61 B,  61 C and  61 D are arranged in the enlarged reference beam to equivalently generate four reference beams so as to match optical paths of the test beams L 2 A, L 2 B, L 2 C and L 2 D that are reflected by the beam splitter  62 . Herein, four spiral phase plates  61 A,  61 B,  61 C and  61 D may be appropriately arranged so that four reference beams that have respectively passed through the four spiral phase plates  61 A,  61 B,  61 C and  61 D have a same phase. Furthermore, the beam output apparatus  1  according to the present embodiment measures four phase differences between four reference beams L 1  and four test beams L 2 , performs a feedback control of the phases of four test beams L 2 A, L 2 B, L 2 C and L 2 D based on this measurement result via four detector signals DSA, DSB, DSC and DSD and four phase control signals CSA, CSB, CSC and CSD, and outputs a combined beam L 3  constituted of components of four test beams L 2 A, L 2 B, L 2 C and L 2 D that are respectively transmitted through the beam splitter  62 . 
     Components of the beam output apparatus  1  according to the present embodiment will be described. The beam output apparatus  1  in  FIG.  10    is provided with a laser oscillator  2 , a beam splitter  3 , a reference beam generation section  4 , a test beam generation section  5  and a phase difference measurement device  6 . 
     The reference beam generation section  4  according to the present embodiment is provided with an optical path length adjustment section  42 , a reflector  43  and the beam expander  44 . Although the optical path length adjustment section  42  in  FIG.  10    is provided with four mirrors  42 A,  42 B,  42 C and  42 D, this value is merely an example and does not limit the present embodiment. The beam expander  44  may be configured similarly to the beam expander  44  shown in  FIG.  1   . 
     The test beam generation section  5  according to the present embodiment is provided with a controller  51 , a beam splitter  52 , four phase controllers  53 A,  53 B,  53 C and  53 D, four beam expanders  54 A,  54 B,  54 C and  54 D, and four amplifiers  55 A,  55 B,  55 C and  55 D. When four beam expanders  54 A,  54 B,  54 C and  54 D are not distinguished, they will be simply referred to as beam expanders  54 . When four amplifiers  55 A,  55 B,  55 C and  55 D are not distinguished, they will be simply referred to as amplifiers  55 . 
     The phase difference measurement device  6  according to the present embodiment is provided with four spiral phase plates  61 A,  61 B,  61 C and  61 D that have a same material and a same thickness; a beam splitter  62 ; four apertures  63 A,  63 B,  63 C and  63 D; four four-quadrant detectors  64 A,  64 B,  64 C and  64 D; and four processors  65 A,  65 B,  65 C and  65 D. When four spiral phase plates  61 A,  61 B,  61 C and  61 D are not distinguished, they will be simply referred to as spiral phase plates  61 . When four apertures  63 A,  63 B,  63 C and  63 D are not distinguished, they will be simply referred to as apertures  63 . When four four-quadrant detectors  64 A,  64 B,  64 C and  64 D are not distinguished, they will be simply referred to as four-quadrant detectors  64 . When four processors  65 A,  65 B,  65 C and  65 D are not distinguished, they will be simply referred to as processors  65 . 
     It will be described that the optical path length adjustment section  42  can adjust an optical path length of the reference beam L 1 . The optical path length adjustment section  42  receives the reference beam L 1  outputted from the beam splitter  3 , reflects it by use of four mirrors  42 A,  42 B,  42 C and  42 D in this order, and outputs it to the reflector  43 . At that time, for example, by fixing the mirrors  42 A and  42 D, and by appropriately moving the positions of the mirrors  42 B and  42 C, the length of the optical path through which the reference beam L 1  passes via the mirrors  42 A  42 B,  42 C and  42 D can be adjusted, and a difference of optical path length between the reference beam L 1 , that is included in the detection beam L 4  that arrives to the four-quadrant detector  64 , and the test beam L 2  can be brought closer to zero. 
     Specifically, when the laser beam L 0  is a pulsed wave, it is preferable that this optical path length difference is shorter than a distance obtained by multiplying 1/10 of a pulse time width by the speed of light. It should be noted that, when the laser beam L 0  is a continuous wave, it is preferable that this optical path length difference is shorter than a fraction of a coherence length and it is more desirable that this optical path length difference is shorter than 1/10 of the coherence length. 
     The beam splitter  52  is a splitting device that splits the test beam L 2  into a plurality of test beams L 2 A, L 2 B, L 2 C and L 2 D. The beam splitter  52  is provided with mirrors  52 A,  52 C,  52 D,  52 F,  52 G,  52 H,  52 I,  52 K,  52 L and  52 M, and half mirrors  52 B,  52 E and  52 J. In the example of  FIG.  10   , a part of the test beam L 2  outputted from the beam splitter  3  is reflected or transmitted by the mirror  52 A, the half mirror  52 B, the mirrors  52 C,  52 D, the half mirror  52 E, the mirrors  52 F and  52 G in this order and arrives to the phase controller  53 A as the test beam L 2 A. In addition, a part of the test beam L 2  outputted from the beam splitter  3  is reflected or transmitted by the mirror  52 A, the half mirror  52 B, the mirrors  52 C.  52 D, the half mirror  52 E, and the mirror  52 H in this order and arrives to the phase controller  53 B as the test beam L 2 B. In addition, a part of the test beam L 2  outputted from the beam splitter  3  is reflected or transmitted by the mirror  52 A, the half mirror  52 B, the mirror  52 I, the half mirror  523 , and the mirrors  52 L and  52 M in this order and arrives to the phase controller  53 C as the test beam L 2 C. In addition, a part of the test beam L 2  outputted from the beam splitter  3  is reflected or transmitted by the mirror  52 A, the half mirror  52 B, the mirror  52 I, the half mirror  52 J, and the mirror  52 K in this order and arrives to the phase controller  53 D as the test beam L 2 D. 
     The test beam L 2 A arrives to the four-quadrant detector  64 A via the phase controller  53 A, the beam expander  54 A, the amplifier  55 A, and the beam splitter  62 . The test beam L 2 B arrives to the four-quadrant detector  64 B via the phase controller  53 B, the beam expander  54 B, the amplifier  55 B, and the beam splitter  62 . The test beam L 2 C arrives to the four-quadrant detector  64 C via the phase controller  53 C, the beam expander  54 C, the amplifier  55 C, and the beam splitter  62 . The test beam L 2 D arrives to the four-quadrant detector  64 D via the phase controller  53 D, the beam expander  54 D, the amplifier  55 D, and the beam splitter  62 . 
     A part of the reference beam L 1  outputted from the beam expander  44  passes through the spiral phase plate  61 A, the beam splitter  62 , and the aperture  63 A and arrives to the four-quadrant detector  64 A. Another part of the reference beam L 1  outputted from the beam expander  44  passes through the spiral phase plate  61 B, the beam splitter  62 , and the aperture  63 B and arrives to the four-quadrant detector  64 B. A still another part of the reference beam L 1  outputted from the beam expander  44  passes through the spiral phase plate  61 C, the beam splitter  62 , and the aperture  63 C and arrives to the four-quadrant detector  64 C. A still another part of the reference beam L 1  outputted from the beam expander  44  passes through the spiral phase plate  61 D, the beam splitter  62 , and the aperture  63 D, and arrives to the four-quadrant detector  64 D. 
     Since the four-quadrant detectors  64 , the processors  65 , and the controllers  51  are same as the case in  FIG.  1   , further detailed descriptions thereof will be omitted. 
     A variation example of the beam output apparatus  1  shown in  FIG.  10    will be described with reference to  FIG.  11   .  FIG.  11    is a diagram that partially shows a configuration example of the beam output apparatus  1  according to an embodiment. 
     The beam output apparatus  1  in  FIG.  11    is obtained by adding following changes to the beam output apparatus  1  in  FIG.  10   . That is, two beam splitters  62 A and  62 B are used instead of the beam splitter  62  in  FIG.  10   . At first, the arrangement of the beam expander  54  and the amplifier  55  is adjusted so that the test beams L 2 A to L 2 D becomes parallel to each other. Next, the arrangement of the beam expander  44  and the parallelism of the beam splitters  62 A and  62 B are adjusted so that the optical axes of the reference beam L 1  and the test beams L 2  become parallel. Furthermore, the test beams L 2  outputted from the amplifiers  55  are made incident to the four-quadrant detectors  64  via two beam splitters  62 A and  62 B. 
     In the configuration example of  FIG.  11   , the test beams L 2  can be significantly dimmed because the number of the beam splitters is increased compared to the case of  FIG.  10   . In other words, if a ratio for dimming an optical intensity of the test beams L 2  outputted from the amplifiers  55  to an optical intensity that the sensor of the four-quadrant detector  64  can withstand is larger, the optical intensity of the combined beam L 3  can be made higher. 
     In addition, in the configuration example of  FIG.  11   , the component of the reference beam L 1  that is not transmitted by the beam splitter  62 B can be outputted to a direction different from the combined beam L 3 . Therefore, since the reference beam L 1  is not mixed in the combined beam L 3  at all, in can be expected that the quality of the combined laser beam is further improved. 
     Third Embodiment 
     A beam output apparatus  10  according to an embodiment will be described with reference to  FIG.  12   .  FIG.  12    is a diagram that shows a configuration example of a beam output apparatus  10  according to an embodiment that combines two beams on a same optical path. 
     The beam output apparatus  10  according to the present embodiment generates a high-power and high-quality laser beam by splitting the laser beam L 0  to be seed light by the beam splitter  3  into a first beam L 10  and a second beam L 20 , amplifying both the first beam L 10  and the second beam L 20  and combining them, measuring the inter-beam phase difference between the first beam L 10  and the second beam L 20 , and performing a feedback control of the phase of the second beam L 20  to match the phase of the first beam L 10 . It should be noted that the target of the matching may be another phase obtained by adding a predetermined phase difference to the phase of the first beam L 10 . 
     Components of the beam output apparatus  10  according to the present embodiment will be described. The beam output apparatus  10  of  FIG.  12    is provided with a laser oscillator  2 , abeam splitter  3 , a mirror  31 , and abeam combination unit  11 . The beam combination unit  11  is provided with a first beam generation section  14 , a second beam generation section  15 , a mirror  111 , a polarizing beam splitter  112 , and a phase difference measuring device  16 . 
     The first beam generation section  14  is provided with a half-wave plate  41 , an optical path length adjustment section  42 , a beam expander  54 A, and an amplifier  55 A. 
     The second beam generation section  15  is provided with a controller  51 , a phase controller  53 , a beam expander  54 B, and an amplifier  55 B. 
     The phase difference measuring device  16  is provided with abeam splitter  161 , a half-wave plate  162 , a polarizing beam splitter  163 , a beam splitter  164 , a power monitor  165 , a polarizing beam splitter  166 , an optical path length adjustment section  167 , a mirror  168 , a half-wave plate  169 , a mirror  170 , and a phase difference measuring device  6 . Since the configuration of the phase difference measuring device  6  is the same as in the case of  FIG.  1   , further detailed description thereof will be omitted. 
     Operations of the components of the beam output apparatus  10  according to the present embodiment will be described. Herein, since components with symbols that are same as the beam output apparatus  1  shown in  FIG.  1    and/or the beam output apparatus  1  shown in  FIG.  10    operate in a same way, detailed description thereof may be omitted. 
     The laser oscillator  2  outputs the laser beam L 0  that is to be seed light. The beam splitter  3  splits the laser beam L 0  into the first beam L 10  and the second beam L 20 . 
     The first beam generation section  14  receives, amplifies, and outputs the first beam L 10 . Herein, at first, the half-wave plate  41  rotates the linear polarization direction of the first beam L 10  of π/2 radians. Next, when the laser beam L 0  is a pulse wave, the optical path length adjustment section  42  adjusts the optical path length difference between the first beam L 10  and the second beam L 20  at the polarizing beam splitter  112  to be a distance shorter than a distance obtained by multiplying 1/10 of the pulse time width by the speed of light. Alternatively, when the laser beam L 0  is a continuous wave, the optical path length adjustment section  42  adjusts the optical path length difference between the first beam L 10  and the second beam L 20  at the polarizing beam splitter  112  to be a distance shorter than a fraction of the coherence length. Next, the beam expander  54 A enlarges the cross-sectional area of the first beam L 10 . Next, the amplifier  55 A amplifies the optical intensity of the first beam L 10 . 
     The second beam generation section  15  receives the second beam L 20  via the mirror  31 , amplifies it and outputs it. Herein, at first, the controller  51  generates the phase control signal CS based on the detector signal DS supplied from the phase difference measuring device  6 . Next, the phase controller  53  adjusts the phase of the second beam L 20  based on the phase control signal CS. Next, the beam expander  54 B enlarges the cross-sectional area of the second beam L 20 . Next, the amplifier  55 B amplifies the optical intensity of the second beam L 20 . 
     Herein, attention should be paid in that the linear polarization directions are perpendicular between the first beam L 10  outputted from the first beam generation section  14  and the second beam L 20  outputted from the second beam generation section  15 . Herein, it will be described an example with a supposition in that the first beam L 10  is s-polarized with respect to the polarizing beam splitter  112  and the second beam L 20  is p-polarized, and a supposition in that the intensities of the first beam L 10  and the second beam L 20  are the same. 
     The polarized beam splitter  112  combines the first beam L 10 , that is outputted from the first beam generation section  14  and reflected by the mirror  111 , and the second beam L 20 , that is outputted from the second beam generation section  15 , and outputs the combined beam L 30 . At that time, the combined beam L 30  includes a component derived from the s-polarized first beam L 10  and a component derived from the p-polarized second beam L 20 . That is, when the phases of the s-polarized beam and the p-polarized beam are the same, the combined beam is linear-polarized with an inclination of 45 degrees, and is elliptically polarized or circularly polarized in other cases. 
     The phase difference measuring device  16  receives the combined beam L 30  outputted from the polarizing beam splitter  112 , measures the phase difference between the first beam L 10  and the second beam L 20  both included the combined beam L 30 , outputs the detector signal DS for performing the feedback control to the phase of the second beam L 20  based on the result of this measurement, and gives, to the combined beam L 30 , linear polarization and polarization angle that are appropriate to the polarizing beam splitter  163 , as a result of the phase feedback control. 
     Herein, at first, the beam splitter  161  splits the combined beam L 30  into a combined beam L 31  and a detection beam L 40 . Next, the half-wave plate  162  rotates respective polarization directions of the s-polarized component and the p-polarized component of the combined beam L 30  of 45 degrees. As a result, the linear polarization direction, or a main axis direction of the ellipse in case of elliptical polarization, of the combined beam L 31  of which the polarization has been rotated, matches the p-polarization of the polarizing beam splitter  163 , and the output beam L 32  is obtained. At that time, when the combined beam L 31 , of which the polarization has been rotated, is elliptically polarized, the component, that is s-polarized with respect to the polarizing beam splitter  163 , of the polarized component of the combined beam L 31 , of which the polarization has been rotated, is outputted to the dump port as the dump beam L 33 . Next, the beam splitter  164  takes out a part of the output beam L 32  as a monitor beam L 34  and the power monitor  165  monitors the optical intensity of the output beam L 35 . It should be noted that, when the beam splitter  164  splits, a most part (for example: more than 99%) of the output beam L 32  may be transmitted as the output beam L 35 , and a slight part (for example: less than 1%) of the output beam L 32  may be split as the monitor beam L 34 . At that time, when the phases of the s-polarized beam and the p-polarized beam are the same just after the polarizing beam splitter  112 , the power received by the power monitor  165  becomes maximal. 
     Next, the polarizing beam splitter  166  splits the detection beam L 40 , that is split by the beam splitter  161 , into a s-polarized first detection beam L 41  (a part of the first beam L 10  included in the combined beam L 30 , obtained as a detection beam) and a p-polarization second detection beam L 42  (a part of the second beam L 20  included in the combined beam L 30 , obtained as a detection beam). Next, the half-wave plate  169  rotates the linear polarization direction of the s-polarized first detection beam L 41  of 90 degrees, and outputs the p-polarized first detection beam L 41 . Next, the first detection beam L 41  is inputted to the four-quadrant detector  64  of the phase difference measuring device  6 , via the mirror  170 , the spiral phase plate  61 , the beam splitter  62 , and the aperture  63 . On the other hand, the second detection beam L 42  is also inputted to the four-quadrant detector  64  of the phase difference measuring device  6 , via the optical path length adjustment section  167 , the mirror  168 , the beam splitter  62 , and the aperture  63 . Herein, attention should be paid in that the linear polarization directions of the first detection beam L 41  and the second detection beam L 42  at the four-quadrant detector  64  are the same, and therefore a visibility of the interference pattern generated by the first detection beam L 41  and the second detection beam L 42  becomes higher. It should be noted that the beam splitter  62  in the embodiment shown in  FIG.  12    may be a half mirror. 
     Since subsequent operations of the four-quadrant detector  64 , the processor  65  and the controller  51  are the same as in the case of each of above-described embodiments, further detailed description will be omitted. 
     As described above, since the beam output apparatus  10  according to the present embodiment can split the laser beam L 0  that is to be seed light into two laser beams, amplify each of the split laser beams by two amplifiers  55 A and  55 B, and use a most part thereof as the output beam L 35  on an identical optical path, it is expected that a high-power laser beam, that is a combination of two to the power of n (n is an integer) beams on an identical optical path, can be generated by a deduction of a plurality of combinations. Herein, attention should be paid in that the tile-like combination shown in  FIG.  10    and the filled-aperture combination shown in  FIG.  12    are of different embodiments. 
     A variation example of the beam output apparatus  10  according to the present embodiment will be described with reference to  FIG.  13   .  FIG.  13    is a diagram that shows a configuration example of a beam output apparatus  100  according to an embodiment. 
     The beam output apparatus  100  in  FIG.  13    is provided with two beam output apparatuses  10  in  FIG.  12   , measures a phase difference between two combined beams L 35 A and L 35 B outputted from those two beam output devices  140  and  150 , performs a feedback control of a phase of one combined beam L 35 B, and can combine two combined beams L 35 A and L 35 B with matched phases. 
     Components of the beam output apparatus  100  in  FIG.  13    will be described. The beam output apparatus  100  is provided with a laser oscillator  2 , a beam splitter  30 , a mirror  310 , a first beam output device  140 , a second beam output device  150 , a mirror  111 , a polarizing beam splitter  112 , and a phase difference measuring device  160  for measuring a phase difference between a first combined beam L 35 A and a second combined beam L 35 B. 
     The first beam output device  140  is provided with abeam splitter  3 , a mirror  31 , and a beam combination unit  11 A. 
     The second beam output device  150  is provided with a controller  51  for controlling the phase of the second combined beam L 35 B, a phase controller  53 , a beam splitter  3 , a mirror  31 , and a beam combination unit  11 B. 
     An operation of the beam output apparatus  100  in  FIG.  13    will be described. The beam output apparatus  100  in  FIG.  13    operates in a same way as the beam output apparatus  10  in  FIG.  12   . At first, the laser oscillator  2 , the beam splitter  30 , and the mirror  310  in  FIG.  13    correspond to the laser oscillator  2 , the beam splitter  3 , and the mirror  31  in  FIG.  12   , respectively. Next, each of the first beam output device  140  and the second beam output device  150  in  FIG.  13    is provided with the beam combination unit  11  in  FIG.  12   . In the example of  FIG.  13   , the configuration that is provided to the first beam output device  140  and corresponds to the beam combination unit  11  in  FIG.  12    is shown as a beam combination unit  11 A, and the configuration that is provided to the second beam output device  150  and corresponds to the beam combination unit  11  in  FIG.  12    is shown as a beam combination unit  11 B. Next, the mirror  111 , the polarizing beam splitter  112 , and the phase difference measuring device  160  in  FIG.  13    correspond to the mirror  111 , the polarizing beam splitter  112 , and the phase difference measuring device  16  in  FIG.  12   , respectively. In addition, the beam splitter  3  and the mirror  31  in  FIG.  13    correspond to the beam splitter  3  and the mirror  31  in  FIG.  12   , and the controller  51  and the phase controller  53  in  FIG.  13    have same functions as the controller  51  and the phase controller  53  in  FIG.  12   , respectively. 
     More specifically, at first, each of the two beam combination units  11 A and  11 B operates in a same way as the beam combination unit  11  in  FIG.  12   . That is, each unit splits an inputted laser beam into two beams by use of the beam splitter  3 , amplifies each of those two laser beams, measures the phase difference between the two amplified laser beams, performs a feedback control of the phase of one of the laser beams based on the result of this measurement, and outputs a beam that is a combination of the two laser beams with matched phases. 
     Next, the polarizing beam splitter  112  and the phase difference measurement device  160  operate in same ways as the polarizing beam splitter  112  and the phase difference measurement device  16  in  FIG.  12   . That is, the polarizing beam splitter  112  in  FIG.  13    outputs a third combined beam that is a combination of a first combined beam, that is outputted from the beam combination unit  1 A and arrives via the mirror  111 , and a second combined beam that is outputted from the beam combination unit  11 B. In addition, the phase difference measuring device  160  measures a phase difference between at least a part of the first combined beam and at least a part of the second combined beam that are included in the third combined beam, and generates a detector signal for performing a feedback control of the phase of the second combined beam based on the result of this measurement. The controller  51  generates the phase control signal based on this detector signal, and the phase controller  53  controls the phase of the laser beam that arrives via the mirror  310  based on this phase control signal. 
     As described above, by making the configuration of the beam output apparatus  10  in  FIG.  12    a recursive nesting structure (tree structure), the beam output apparatus  100  in  FIG.  13    can split the laser beam L 0  into four beams to amplify each of them separately, match the phases thereof, and combine. Furthermore, by making the configuration of the beam output apparatus  100  in  FIG.  13    a recursive nesting structure, the laser beam L 0  can be split into eight beams, each of which is separately amplified, matched in phases, and combined. Similarly, by repeating the recursive nesting structure, the laser beam L 0  can be split into two to power of an arbitrary number, each of which is separately amplified, matched in phases, and combined, and therefore a laser beam with higher power and higher quality can be generated. 
     Although the invention made by the inventors has been specifically described above based on the embodiments, it is needless to say that the present invention is not limited to the above-described embodiments and can be variously modified within a range of not departing from the gist thereof. In addition, each feature described in the above-described embodiments may be freely combined within a range of a technical consistence. 
     The phase difference measuring device  6  according to each embodiment is understood for example as below. 
     (1) A phase difference measuring device  6  according to a first aspect is provided with a phase conversion device  61 ,  7 ,  610  and a detection device  60 . 
     There is an effect in that the phase difference measuring device  6  according to the first aspect can measure an inter-beam phase difference of a test beam with respect to a reference beam and adjust it, by use of the phase conversion device  61 ,  7 ,  610  and the detection device  60 . 
     The phase conversion device  61 ,  7 ,  610  has a function of converting a first laser beam L 1  that passes therethrough so that the first laser beam L 1  includes a phase distribution of one cycle in an azimuth direction in a cross section of the first laser beam L 1  that has passed therethrough included in an arbitrary virtual plane perpendicular to an optical axis of the first laser beam L 1 . As a device that converts the phase distribution in such a way, there is an optical device such that a phase of a transmitted beam is different according to a part of the laser beam that passes therethrough. As such an optical device, there are a spiral phase plate  61 , a hologram  7  corresponding to the spiral phase plate  61 , a multilevel spiral phase plate  610 , or the like. In other words, the phase conversion device  61 ,  7 ,  610  represents a superordinate concept of the spiral phase plate  61 , the hologram  7 , and the multilevel spiral phase plate  610 . 
     The detection device  60  has a function of detecting an azimuth angle  72  of an intensity centroid  71  of an interference pattern and detecting an inter-beam phase difference of a second laser beam L 2  with respect to the first laser beam L 1  based on the azimuth angle  72 . The interference pattern is generated by a shaped detection beam L 5  obtained by cutting out a part of a cross section of a detection beam L 4  into a shape of a circle with a point, where the cross section of the detection beam L 4  and an optical axis of the phase conversion device  61 ,  7 ,  610  intersect, as a center thereof. The cross section of the detection beam is perpendicular to an optical axis A 4  of the detection beam L 4 . The detection beam L 4  is obtained by combining a reference beam L 12  that is a first partial intensity laser beam and a test beam L 22  that is a second partial intensity laser beam on a same optical path. The first partial intensity laser beam has at least a part of an intensity component of the first laser beam L 1  that has passed through the phase conversion device  61 ,  7 ,  610 . The second partial intensity laser beam has at least a part of an intensity component of the second laser beam L 2  derived from a laser beam L 0  as seed light from which the first laser beam L 1  derives. As a device having such a function, there is a four-quadrant detector  64  provided with four sensors, that detect optical intensities P 1 , P 2 , P 3  and P 4  that are respectively included in four quadrants of the interference pattern, or the like. 
     (2) A phase difference measuring device  6  according to a second aspect is the phase difference measuring device  6  according to the first aspect, and is further provided with a sensor device  64  and a processor  65 . As a result, there is an effect in that, since the optical intensities P 1 , P 2 , P 3  and P 4  included in a plurality of areas included in the interference pattern can be respectively detected, the azimuth angle  72  of the interference pattern intensity centroid  71  can be calculated based on the detection result, and the inter-beam phase difference can be calculated based on the azimuth angle  72 , a detection of the inter-beam phase difference can be realized by fewer sensors and with higher accuracy. Such optical intensities P 1 , P 2 , P 3  and P 4  may be the optical intensities P 1 , P 2 , P 3  and P 4  of the four quadrants of the interference pattern. 
     (3) a phase difference measuring device  6  according to a third aspect is the phase difference measuring device  6  according to the second aspect, and is further provided with a four-quadrant detector  64 . As a result, there is an effect in that the detection of the azimuth angle can be realized by four sensors with high accuracy. 
     (4) The phase difference measuring device  6  according to a fourth aspect is the phase difference measuring device  6  according any one of the first to third aspects, and is further provided with the spiral phase plate  61 . As a result, there is an effect in that, once the optical intensities P 1 , P 2 , P 3  and P 4  respectively included in four quadrants of the interference pattern are detected, the desired inter-beam phase difference to measure can be corresponded to a specific phase in the phase distribution of one cycle included in the cross section of the reference beam. 
     (5) A phase difference measuring device  6  according to a fifth aspect is the phase difference measuring device  6  according to any one of the first to third aspects, and is further provided with the hologram  7  of the spiral phase plate  61 . As a result, there is an effect in that, once the optical intensities P 1 , P 2 , P 3  and P 4  respectively included in four quadrants of the interference pattern are detected, the desired inter-beam phase difference to measure can be corresponded to a specific phase in the phase distribution of one cycle included in the cross section of the reference beam, in an easier way than the case of using the spiral phase plate  61  itself. 
     (6) A phase difference measuring device  6  according to a sixth aspect is the phase difference measuring device  6  according to any one of the first to third aspects, and is further provided with the multilevel spiral phase plate  610 . As a result, there is an effect in that, once the optical intensities P 1 , P 2 , P 3  and P 4  respectively included in four quadrants of the interference pattern are detected, the desired inter-beam phase difference to measure can be corresponded to a specific phase in the phase distribution of one cycle included in the cross section of the reference beam, in an easier way than the case of using the spiral phase plate  61  itself. 
     (7) A beam output apparatus  1 ,  10 ,  100  according to a first aspect is provided with a first beam splitter  3 ,  30 , a phase conversion device  61 ,  7 ,  610 , a detection device  60 , and a phase controller  53 . 
     There is an effect in that the beam output apparatus  1 ,  10 ,  100  can generate a high-power and high-quality laser beam by use of the first beam splitter  3 ,  30 , the phase conversion device  61 ,  7 ,  610 , the detection device  60 , and the phase controller  53 . 
     The first beam splitter  3 ,  30  has a function of splitting a laser beam L 0  that is to be seed light into a first laser beam L 1  and a second laser beam L 2 . 
     The phase conversion device  61 ,  7 ,  610  has a function of converting a first laser beam L 1  that passes therethrough so that the first laser beam L 1  includes a phase distribution of one cycle in an azimuth direction in a cross section of the first laser beam L 1  that has passed therethrough included in an arbitrary virtual plane perpendicular to an optical axis of the first laser beam L 1 . As a device that converts the phase distribution in such a way, there is an optical device such that a phase is different according to a part of the laser beam that passes therethrough. As such an optical device, there are a spiral phase plate  61 , a hologram  7  corresponding to the spiral phase plate  61 , a multilevel spiral phase plate  610 , or the like. 
     The detection device  60  has a function of detecting an azimuth angle  72  of an intensity centroid  71  of an interference pattern and detecting an inter-beam phase difference of a second laser beam L 2  with respect to the first laser beam L 1  based on the azimuth angle  72 . The interference pattern is generated by a shaped detection beam L 5  obtained by cutting out a part of a cross section of a detection beam L 4  into a shape of a circle with a point, where the cross section of the detection beam L 4  and an optical axis of the phase conversion device  61 ,  7 ,  610  intersect, as a center thereof. The cross section of the detection beam is perpendicular to an optical axis A 4  of the detection beam L 4 . The detection beam L 4  is obtained by combining a reference beam L 12  that is a first partial intensity laser beam and a test beam L 22  that is a second partial intensity laser beam on a same optical path. The first partial intensity laser beam has at least a part of an intensity component of the first laser beam L 1  that has passed through the phase conversion device  61 ,  7 ,  610 . The second partial intensity laser beam has at least a part of an intensity component of the second laser beam L 2 . As a device having such a function, there is a four-quadrant detector  64  provided with four sensors, that detect optical intensities P 1 , P 2 , P 3  and P 4  that are respectively included in four quadrants of the interference pattern, or the like. 
     The phase controller  53  has a function of controlling the phase of the second laser beam L 2  based on the measured value of the azimuth angle  72 . As a result, when combining the first laser beam L 1  as a phase reference and the second laser beam L 2 , the inter-beam phase difference between the both beams can be adjusted. As a device having such a function, there is a phase controller that can adjust the phase of the laser beam that passes therethrough. 
     (8) A beam output apparatus  1 ,  10 ,  100  according to a second aspect is the beam output apparatus  1 ,  10 ,  100  according to the first aspect and is further provided with a sensor device  64  and a processor  65 . As a result, there is an effect in that, since the optical intensities P 1 , P 2 , P 3  and P 4  included in a plurality of areas included in the interference pattern can be respectively detected, the azimuth angle  72  of the interference pattern intensity centroid  71  can be calculated based on the detection result, and the inter-beam phase difference can be calculated based on the azimuth angle  72 , a detection of the inter-beam phase difference can be realized by fewer sensors and with higher accuracy. Such optical intensities P 1 , P 2 , P 3  and P 4  may be the optical intensities P 1 , P 2 , P 3  and P 4  of the four quadrants of the interference pattern. In addition, as a device having such a function, there is a four-quadrant detector  64  or the like. 
     (9) A beam output apparatus  1 ,  10 ,  100  according to a third aspect is the beam output apparatus  1 ,  10 ,  100  according to the second aspect, and is further provided with a four-quadrant detector  64 . As a result, there is an effect in that the detection of the azimuth angle  72  can be realized by four sensors with high accuracy. 
     (10) A beam output apparatus  1 ,  10 ,  100  according to a fourth aspect is the beam output apparatus  1 ,  10 ,  100  according to any one of the first to third aspects, and is further provided with the spiral phase plate  61 . As a result, there is an effect in that, once the optical intensities P 1 , P 2 , P 3  and P 4  respectively included in four quadrants of the interference pattern are detected, the desired inter-beam phase difference to measure can be corresponded to a specific phase in the phase distribution of one cycle included in the cross section of the reference beam. 
     (11) A beam output apparatus  1 ,  10 ,  100  according to a fifth aspect is the beam output apparatus  1 ,  10 ,  100  according to any one of the first to third aspects, and is further provided with the hologram  7  of the spiral phase plate  61 . As a result, there is an effect in that, once the optical intensities P 1 , P 2 , P 3  and P 4  respectively included in four quadrants of the interference pattern are detected, the desired inter-beam phase difference to measure can be corresponded to a specific phase in the phase distribution of one cycle included in the cross section of the reference beam, in an easier way than the case of using the spiral phase plate  61  itself. 
     (12) A beam output apparatus  1 ,  10 ,  100  according to a sixth aspect is the beam output apparatus  1 ,  10 ,  100  according to any one of the first to third aspects, and is further provided with the multilevel spiral phase plate  610 . As a result, there is an effect in that, once the optical intensities P 1 , P 2 , P 3  and P 4  respectively included in four quadrants of the interference pattern are detected, the desired inter-beam phase difference to measure can be corresponded to a specific phase in the phase distribution of one cycle included in the cross section of the reference beam, in an easier way than the case of using the spiral phase plate  61  itself. 
     (13) A beam output apparatus  1 ,  10 ,  100  according to a seventh aspect is the beam output apparatus  1 ,  10 ,  100  according to any one of the first to sixth aspects and is further provided with an amplifier  55 . The amplifier  55  amplifies the second laser beam L 2 . As a result, there is an effect in that a laser beam with a high quality and a higher power can be generated. 
     (14) A beam output apparatus  1 ,  10 ,  100  according to an eighth aspect is the beam output apparatus  1 ,  10 ,  100  according to the first to seventh aspects and is further provided with a splitting device  52  and a plurality of amplifiers  55 . The splitting device  52  has a function of splitting the second laser beam L 2  into a plurality of test beams L 2 A, L 2 B, L 2 C and L 2 D that are a plurality of second partial intensity laser beams, each of which has a part of intensity component of the second laser beam L 2 . As a device that has such a function, there is a combination  52  of mirrors and half mirrors, or the like. The plurality of amplifiers  55  respectively amplify the plurality of test beams (the second partial intensity laser beams) L 2 A, L 2 B, L 2 C and L 2 D. The phase conversion device  61 A,  61 B,  61 C and  61 D according to the eighth aspect includes a plurality of phase conversion devices  61 A,  61 B,  61 C and  61 D that are arranged so that a plurality of first partial cross section laser beams, that are obtained by cutting out the first laser beam L 1  by parts of beam cross sections, respectively pass therethrough. Each of the plurality of phase conversion devices  61 A,  61 B,  61 C and  61 D has a function of converting each of the plurality of first partial cross section laser beams that passes therethrough, so that the each first partial cross section laser beam includes a phase distribution of one cycle in an azimuth direction in a cross section of the each first partial cross section laser beam that has passed therethrough included in an arbitrary virtual plane perpendicular to an optical axis of the each first partial cross section laser beam. The detection device  60  according to the eighth aspect has a function of detecting an azimuth angle  72  of an intensity centroid  71  of each of a plurality of interference patterns by a plurality of shaped detection beams obtained by cutting out a part of a cross section, that is perpendicular to an optical axis of each of a plurality of detection beams, of the plurality of detection beams obtained by combining the plurality of first partial cross section laser beams and the plurality of test beams (second partial intensity laser beams), respectively on same optical paths into a shape of a circle with a point, where the cross section of the each detection beam and an optical axis of the each phase conversion devices  61 A,  61 B,  61 C and  61 D intersect, as a center thereof, and detecting an inter-beam phase difference of each of the plurality of test beams (second partial intensity laser beams) L 2 A, L 2 B, L 2 C and L 2 D with respect to the plurality of first partial cross section laser beams based on the azimuth angle  72 . The phase controllers  53 A,  53 B,  53 C and  53 D according to the eighth aspect has a function of controlling the phase of each of the test beams (second partial intensity laser beams) L 2 A, L 2 B, L 2 C and L 2 D based on each of the inter-beam phase differences. As a result, there is an effect in that a high-quality and high-power laser beam can be generated since the inter-beam phase difference of each of the plurality of test beams (second partial intensity laser beams) L 2 A, L 2 B, L 2 C and L 2 D with respect to the plurality of first partial cross section laser beams can be controlled simultaneously. 
     (15) A beam output apparatus  1 ,  10 ,  100  according to a ninth aspect is the beam output apparatus  1 ,  10 ,  100  according to the first to eighth aspects and is provided with a half-wave plate  41 , an amplifier  55 A that is different from the amplifier  55  according to the seventh or eighth aspect, a polarizing beam splitter  112 , a second beam splitter  161 , and a third beam splitter  166 . When a laser beam L 0  outputted by the laser oscillator  2  is linear-polarized, it is necessary to make the polarization direction of the first beam L 10  be s-polarized with respect to the polarizing beam splitter  112  and the polarization direction of the second beam L 20  be p-polarized, in order to split the laser beam L 0  by a beam splitter  3 , that is a half mirror, into two beams to respectively amplify and then combine them by a polarization composition with the polarizing beam splitter  112 . For this reason, the half-wave plate  41  rotates a linear polarization direction of at least one of the first laser beam L 10  and the second laser beam L 20 . Although in  FIG.  12    the half-wave plate  41  is shown only in the optical path of the first beam L 10  for convenience, the present aspect is not limited to this example. The amplifier  55 A, that is different from the amplifier  55  according to the seventh or eighth aspect, amplifies the second laser beam L 20 . The polarizing beam splitter  112  combines the first laser beam L 10  and the second laser beam L 20 , of which the linear polarization directions are different from each other, to generate a third laser beam L 30 . The second beam splitter  161  splits the third laser beam L 30  into a combined laser beam L 31  and a detection laser beam L 40 . The third beam splitter  166  splits the detection laser beam L 40  into a first detection laser beam L 41  and a second laser beam L 42 . The detection device  60  according to the ninth aspect detects the azimuth angle  72  of the intensity centroid  71  of the interference pattern generated by the first detection laser beam L 41  and the second detection laser beam L 42 . As a result, there is an effect in that a laser beam with a high quality and a higher power can be generated, since the beam that was amplified as a reference beam for generating an interference pattern can be used and two beams can be combined on a same optical path. 
     (16) A phase difference measuring method according to a first aspect is provided with a first step of converting a first laser beam L 1  that has passed through a phase conversion device  61 ,  7 ,  610  to include a phase distribution of one cycle in an azimuth direction in a cross section of the first laser beam L 1  that has passed therethrough included in an arbitrary virtual plane perpendicular to an optical axis of the first laser beam L 1 , and a second step of detecting an azimuth angle  72  of an intensity centroid  71  of an interference pattern and detecting an inter-beam phase difference of a second laser beam L 2  with respect to the first laser beam L 1  based on the azimuth angle  72 . The interference pattern is generated by a shaped detection beam L 5  obtained by cutting out a part of a cross section of the detection beam L 4  into a shape of a circle with a point, where the cross section of the detection beam L 4  and an optical axis of the phase conversion device  61 ,  7 ,  610  intersect, as a center thereof. The cross section of the detection beam L 4  is perpendicular to an optical axis A 4  of a detection beam L 4 . The detection beam L 4  is obtained by combining a reference beam L 12  that is a first partial intensity laser beam and a test beam L 22  that is a second partial intensity laser beam on a same optical path. The first partial intensity laser beam has at least a part of an intensity component of the first laser beam L 1  that is converted. The second partial intensity laser beam has at least a part of an intensity component of the second laser beam L 2  derived from a laser beam L 0  as seed light from which the first laser beam L 1  derives. 
     The phase difference measuring method according to the first aspect has an effect in that a high-power and high-quality laser beam can be generated by the first step and the second step. 
     The first step has a function of converting a first laser beam L 1  that passes therethrough so that the first laser beam L 1  that has passed therethrough includes a phase distribution of one cycle in an azimuth direction in a cross section of the first laser beam L 1  included in an arbitrary virtual plane perpendicular to an optical axis of the first laser beam L 1  that has passed therethrough. Such a function may be realized by use of an optical device such that a phase is different according to a part of a laser beam that passes therethrough. As such an optical device, there are a spiral phase plate  61 , a hologram  7  corresponding to the spiral phase plate  61 , a multilevel spiral phase plate  610 , or the like. 
     The second step has a function of detecting an azimuth angle  72  of an intensity centroid  71  of an interference pattern generated by a shaped detection beam L 5  obtained by cutting out a part of a cross section, that is perpendicular to an optical axis A 4  of a detection beam L 4 , of the detection beam L 4  obtained by combining a reference beam L 12  that is a first partial intensity laser beam, that has at least a part of an intensity component of the converted first laser beam L 1 , and a test beam L 22  that is a second partial intensity laser beam, that has at least a part of an intensity component of a second laser beam L 2  derived from a laser beam L 0  as seed light from which the first laser beam L 1  derives, on a same optical path, into a shape of a circle with a point, where the cross section of the detection beam L 4  and an optical axis of the phase conversion device  61 ,  7 ,  610  intersect, as a center thereof, and detecting an inter-beam phase difference of the second laser beam L 2  with respect to the first laser beam L 1  based on the azimuth angle  72 . As a device having such a function, there is a four-quadrant detector  64  provided with four sensors, that detect optical intensities P 1 , P 2 , P 3  and P 4  that are respectively included in four quadrants of the interference pattern, or the like. 
     The present application claims priority of Japanese Patent Application No. 2020-061991, filed on Mar. 31, 2020, the whole disclosure of which is incorporated herein by reference.