Multi-beam phase difference measurement device and laser beam control system including the same

Abstract

To provide a multi-beam phase difference measurement device capable of making measurements more efficiently by simplifying the configuration of the measurement device for measuring the phase difference of multiple laser beams, and a laser beam control system including the same.SOLUTION: The multi-beam phase difference measurement device includes: a Sagnac interferometer 200 that includes a circular optical path in which a first laser beam, one of the multiple laser beam bundles split into two, orbits in a forward direction and a second laser beam, which is the other of the multiple laser beam bundles split into two, orbits in the opposite direction, using either the first or second laser beam that has passed through the circular optical path as a reference beam, and emits an interference laser beam bundle IL where the first and second laser beams interfere with each other; an interference intensity detector 400 for detecting the interference intensity of the image of the interference laser beam bundle; and an image transfer optical system 300 that is disposed between the Sagnac interferometer and the interference intensity detector, and transfers an image of the interference laser beam bundle to the interference intensity detector.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to a multi-beam phase difference measurement device and a laser beam control system including the same. [Background technology]

[0002] High-quality, high-power laser light is needed in fields such as laser processing, scientific research, nuclear fusion, space debris removal, and security. However, there is a limit to the output power that can be achieved from a single laser beam due to factors such as heat generation, optical breakdown, and nonlinear optical effects. For this reason, beam combining technology, which combines multiple laser beams to increase output power, is being researched.

[0003] When combining multiple laser beams, it is preferable to synchronize the phases of each laser, so a device or process is required to temporally modulate and / or demodulate (such as frequency shift or phase modulation) each laser beam.

[0004] For example, Patent Document 1 discloses generating an inner loop feedback signal using a sampled output beam associated with a laser amplifier and a sampled laser beam, and further discloses a nested loop system in which the inner loop feedback signal is output to a plurality of inner loop phase modulators corresponding to each laser amplifier, and the inner loop phase modulators perform phase modulation of the corresponding laser amplifier based on the inner loop feedback signal, thereby adjusting the phase of each laser amplifier.

[0005] Patent Document 2 also discloses that a detector detects an optical beat signal generated by interference between a frequency-shifted reference beam and an output signal from an amplifier arm equipped with a path length adjuster and an amplifier. Furthermore, a system is disclosed in which an OHD (optical heterodyne detection) beat signal containing information on the maximum amplitude corresponding to the difference from the minimum path length of the amplifier arm is generated based on the detected optical beat signal, and a path length adjuster adjusts the line length of the laser amplifier based on the OHD beat signal. This system includes multiple amplifier arms, and the same operation is performed for each amplifier arm.

[0006] Patent Document 3 also discloses the use of a spiral phase plate as an optical element that converts the phase distribution of a laser beam, which is reference light. A detection beam is generated by combining a reference beam and a detection beam that have passed through the spiral phase plate, and an angle θ indicating the azimuth angle is calculated based on the relationship between the light intensity distribution of the interference pattern of the detection beam and the azimuth angle of the center of intensity gravity of the interference pattern, and feedback control is performed on the phase of the detection beam. The configuration disclosed in Patent Document 3 enables phase difference control of the detection beam with a simpler configuration than Patent Documents 1 and 2. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 2016-502269 [Patent Document 2] U.S. Patent No. 7,884,997 [Patent Document 3] Patent Publication No. 2021-163805 Summary of the Invention [Problem to be solved by the invention]

[0008] Here, in the configurations disclosed in Patent Document 1 and Patent Document 2, when adjusting the phase of the laser amplifier, phase modulation or frequency shifting is required, which increases costs in terms of the number of devices and steps constituting the system.

[0009] Furthermore, in the configuration disclosed in Patent Document 3, the configuration of the phase detection device is simplified by using a spiral phase plate. However, when there are multiple laser beams, it is necessary to adjust the arrangement of the spiral phase plate in accordance with the reference light of each laser beam, making it extremely difficult to realize a configuration in which many laser beams are densely arranged.

[0010] The present disclosure has been made in consideration of the above circumstances, and aims to provide a multi-beam phase difference measurement device that can improve the efficiency of measurement by simplifying the configuration of a measurement device for measuring the phase difference between multiple laser beams, and a laser beam control system equipped with the same. [Means for solving the problem]

[0011] A multi-beam phase difference measurement device according to an embodiment of the present disclosure includes a circular optical path along which a first laser beam, which is one of the split laser beam bundles having a plurality of laser beams, circulates in a forward direction, and a second laser beam, which is the other of the split laser beam bundles different from the first laser beam, circulates in a reverse direction; the device includes a Sagnac interferometer that uses one of the first laser beam and the second laser beam that have passed through the circular optical path as a reference beam and emits an interference laser beam bundle in which the first laser beam and the second laser beam interfere with each other; an interference intensity detector that detects the interference intensity of an image of the interference laser beam bundle emitted from the Sagnac interferometer; and an image transfer optical system that is disposed between the Sagnac interferometer and the interference intensity detector and transfers the image of the interference laser beam bundle to the interference intensity detector.

[0012] A laser beam control system according to one embodiment of the present disclosure includes a multi-beam laser device that emits multiple laser beams, the above-mentioned multi-beam phase difference measurement device, a beam splitter that splits some of the multiple laser beams emitted from the multi-beam laser device into the laser beam bundle and guides them to the multi-beam phase difference measurement device, and a phase control device that adjusts the phases of the multiple laser beams based on the measurement results of the multi-beam phase difference measurement device. [Effects of the Invention]

[0013] According to the present disclosure, the configuration of a measurement device for measuring the phase difference between a plurality of laser beams can be simplified, thereby making the measurement more efficient. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a configuration diagram of a laser beam control system according to the present disclosure. [Figure 2] 1 is an example of a cross-sectional view of a laser beam bundle emitted from a multi-beam laser device provided in a laser beam control system of the present disclosure. [Figure 3] 1 is a configuration diagram of a multi-beam phase difference measurement device according to a first embodiment of the present disclosure. [Figure 4] 1 is an example diagram showing an interference pattern of an interfering laser beam bundle. [Figure 5] FIG. 3 is a waveform diagram showing the relationship between the output of each detector included in the interference intensity detector according to the first embodiment of the present disclosure and the phase difference. [Figure 6] FIG. 10 is a configuration diagram of a multi-beam phase difference measurement device according to a second embodiment of the present disclosure. [Figure 7] FIG. 10 is a waveform diagram showing the relationship between the output and the phase difference of each detector included in the interference intensity detector according to the second embodiment of the present disclosure. [Figure 8] 10 is an example of an image observed using a configuration equivalent to a multi-beam phase difference measurement device according to a second embodiment of the present disclosure. [Figure 9] FIG. 10 is a configuration diagram of a multi-beam phase difference measurement device according to a third embodiment of the present disclosure. [Figure 10]FIG. 1 is a diagram illustrating a first embodiment of an interference intensity detector according to the present disclosure. [Figure 11] FIG. 10 illustrates a second embodiment of an interference intensity detector of the present disclosure. [Figure 12] FIG. 10 is a diagram illustrating a third embodiment of an interference intensity detector of the present disclosure. [Figure 13] 13 is a diagram illustrating a phase distribution detected by the interference intensity detector of FIG. 12. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0015] (Configuration of laser beam control system) Fig. 1 is a configuration diagram of a laser beam control system 100 according to the present disclosure. Fig. 2 is an example of a cross-sectional view of a laser beam bundle 8 emitted from a multi-beam laser device 1 included in the laser beam control system 100 according to the present disclosure. As shown in FIG. 1, the laser beam control system 100 includes a multi-beam laser device 1 that emits multiple laser beams, a beam splitter 9 that splits the multiple laser beams 6 emitted from the multi-beam laser device 1, a multi-beam phase difference measurement device 10 that measures the phase difference between the multiple laser beams split by the beam splitter 9, and a phase control device 11 that adjusts the phases of the multiple laser beams 6 based on the measurement results of the multi-beam phase difference measurement device 10.

[0016] The multi-beam laser device 1 includes an oscillator 2 that generates a single laser beam, a splitter 3 that splits the single laser beam into multiple laser beams, a phase modulator 4 (4A to 4I) that modulates (shifts) the phase of the laser beam incident from the splitter 3 based on the output signal of a phase control device 11 described below, an amplifier 5 (5A to 5I) that amplifies the light intensity of the laser beam, and a polygon prism 7 that reflects or refracts the incident laser beam in a predetermined direction.

[0017] First, laser light emitted from oscillator 2 is split into multiple laser beams 6 (6A to 6I) by splitter 3, which then pass through phase modulator 4 and are amplified by amplifier 5. The multiple laser beams 6 emitted from amplifier 5 are each incident on polygon prism 7 at a predetermined position and a predetermined angle, and the multiple laser beams 6 are emitted from polygon prism 7 parallel to each other. A collection of multiple beams traveling in parallel in this manner is called laser beam bundle 8. The above-described configuration is an example, and may be modified as appropriate as long as it is capable of outputting a plurality of laser beams 6 and a laser beam bundle 8.

[0018] In the example of Figure 2, 61 laser beams 6 are bundled together, each of which has the same cross-sectional shape after being shaped by a polygon prism 7. When viewed in cross section, the laser beams are closely arranged to form a tiled laser beam bundle 8. Furthermore, by optimizing the incident surface of the polygon prism 7 to have a polygonal shape, for example a hexagonal shape, the cross section of each laser beam 6 can be shaped into a hexagon. The cross-sectional shape of each laser beam forming the laser beam bundle 8 is not limited to a hexagon, but may be any other polygonal shape such as a quadrangle.

[0019] 2 shows a case where the laser beam bundle 8 has a honeycomb structure, and the laser beam bundle 8 is surrounded by hexagonal rings whose outer diameters increase successively outward from the center of the most central laser beam (hereinafter referred to as the "central laser beam"). The circumscribing ring circumscribing the outer periphery of the central laser beam is designated as the first ring C11, and the circumscribing ring circumscribing the first ring C11 is designated as the second ring C12, the circumscribing ring circumscribing the second ring C12 is designated as the third ring C13, and the circumscribing ring circumscribing the third ring C13 is designated as the fourth ring C14. In this way, when the circumscribing rings are numbered sequentially from the innermost ring, the number of laser beams in the pth ring is 6p, and the total number of laser beams up to the sth ring is N B (s) is given by the following equation (1).

[0020]

number

[0021] Furthermore, when the size of the entire cross section of the laser beam bundle 8 is expressed as the diameter D2 of the circumscribing circle of the outermost ring, it is given by the following formula (2): In formula (2), D1 represents the diameter of the inscribing circle of each beam, and p represents the ring number.

[0022]

number

[0023] Returning to the explanation of Fig. 1, a portion of the power of the laser beam bundle 8 split by the beam splitter 9 is input to a multi-beam phase difference measurement device 10, which detects the phase difference between each laser beam 6 of the laser beam bundle 8. Then, the output data of the multi-beam phase difference measurement device 10 is guided to a phase control device 11, and the phases of all the laser beams 6 are matched by the phase modulators 4A to 4I installed on each beam, causing the laser beam bundle 8 to propagate as if it were a single laser beam.

[0024] The multi-beam phase difference measurement device 10 measures the phase difference between each laser beam of the incident laser beam bundle 8. The laser beam bundle 8 incident on the multi-beam phase difference measurement device 10 is not limited to a laser beam bundle that is closely arranged with no gaps when viewed cross-sectionally, but may also be a circular, hexagonal, or quadrangular laser beam bundle that is arranged in a tiled pattern with gaps.

[0025] (Configuration of multi-beam phase difference measurement device) FIG. 3 is a configuration diagram of a multi-beam phase difference measurement device according to the first embodiment of the present disclosure. The multi-beam phase difference measurement device 10 measures the phase difference between multiple laser beams and includes polarizers 12A and 12B, a Faraday rotator 13, a half-wave plate 14, a Sagnac interferometer 200, an image transfer optical system 300, and an interference intensity detector 400.

[0026] More specifically, the multi-beam phase difference measurement device 10 has a circular optical path along which a horizontally polarized laser beam (first laser beam), which is one of a plurality of split laser beam bundles, circulates in a forward direction, and a vertically polarized laser beam (second laser beam), which is another split laser beam bundle different from the horizontally polarized laser beam of the split laser beam bundles, circulates in a reverse direction. The device is also equipped with a Sagnac interferometer 200 that uses at least one of the horizontally polarized laser beam and the horizontally polarized laser beam that have passed through the circular optical path as a reference beam, and emits an interference laser beam bundle IL formed by the interference of the horizontally polarized laser beam and the horizontally polarized laser beam, an interference intensity detector 400 that detects the interference intensity of the image of the interference laser beam bundle IL emitted from the Sagnac interferometer, and an image transfer optical system 300 that is arranged between the Sagnac interferometer 200 and the interference intensity detector 400 and transfers the image of the interference laser beam bundle to the interference intensity detector 400.

[0027] The laser beam bundle 8 input into the multi-beam phase difference measurement device 10 first passes through a polarizer 12A, a Faraday rotator 13, a polarizer 12B, and a half-wave plate 14 before entering the Sagnac interferometer 200. Here, the two polarizers 12A and 12B and the Faraday rotator 13 prevent the laser beam that flows back from the Sagnac interferometer 200 from returning to the amplifier 5 located upstream in the laser beam's traveling direction. The half-wave plate 14 is an optical element that adjusts the polarization direction of the laser beam. By adjusting the rotation angle of the half-wave plate 14, the polarization direction of the laser beam that has passed through the half-wave plate 14 can be rotated by any angle relative to the polarization direction of the laser beam emitted from the polarizer 12B.

[0028] The Sagnac interferometer 200 includes a triangular circular optical path made up of a polarizing beam splitter 15 and two reflecting mirrors 17 and 20. The circular optical path also includes convex lenses 16, 18, 19, and 21. The Sagnac interferometer 200 also includes an aperture 33. The polarization components of the laser beam bundle 8 incident on the Sagnac interferometer 200 are separated into vertical and horizontal components by the polarizing beam splitter 15. Then, the laser beam (first laser beam) with the horizontally polarized component (p-polarized component) that passes through (transmits) the polarizing beam splitter 15 travels along an optical path in the forward direction (clockwise (CW) on the plane of FIG. 3). On the other hand, the laser beam (second laser beam) with the vertically polarized component (s-polarized component) that is reflected by the polarizing beam splitter 15 travels along an optical path in the reverse direction (counterclockwise (CCW) on the plane of FIG. 3).

[0029] 3, it is assumed that the optical axes of all light rays are on the same plane. The points where the optical path of the center of laser beam bundle 8 intersects with the reflecting surfaces of polarizing beam splitter 15 and two reflecting mirrors 17 and 20 are designated as points A1, B1, and C1, respectively. In this case, the circular optical path includes forward circular optical path 23 having a forward route of A1-B1-C1-A1, and backward circular optical path 24 having a reverse route of A1-C1-B1-A1, and the optical path lengths of both optical paths are completely equal.

[0030] A plurality of convex lenses 16, 18, 19, and 21 are arranged at predetermined positions within the circular optical path. The focal lengths of the convex lenses 16, 18, 19, and 21 are designated as f1, f2, f3, and f4, respectively. In the arrangement illustrated in FIG. 3, f1>f4 and f2=f3. The placement of each convex lens is determined based on its focal length: the distance between point A1 and point B1 is set to twice the focal length f1 of convex lens 16, and the distance from convex lens 16 to point A1 is equal to the distance from convex lens 16 to point B1.

[0031] In addition, the distance between point B1 and point C1 is set to twice the sum of the focal length f2 of convex lens 18 and the focal length f3 of convex lens 19, and each convex lens is positioned so that the distance from convex lens 18 to point B1 is focal length f2 and the distance from convex lens 19 to point C1 is focal length f3.

[0032] Furthermore, the distance between points A1 and C1 is set to twice the focal length f4 of the convex lens 21, and the convex lens 21 is positioned so that the distance from the convex lens 21 to point A1 is equal to the distance from the convex lens 21 to point C1.

[0033] As described above, the Sagnac interferometer 200 is configured using an optical system in which two pairs of afocal lenses are arranged in series, and each laser beam that has passed through either the forward or reverse optical path returns to the polarizing beam splitter 15. At this time, the image of the beam bundle incident on the Sagnac interferometer at the polarizing beam splitter 15 passes through a circular optical path and is again formed at the position of the polarizing beam splitter 15. Due to the relationship f1 > f4, the forward optical path forms a reduced image, and the reverse optical path forms an enlarged image. The two images formed at the position of the polarizing beam splitter 15 after passing through the two circular optical paths then reach the aperture 33. In the following description, the bundle of laser beams emerging from the aperture 33 will be referred to as an interference laser beam bundle IL.

[0034] The aperture 33 extracts a portion of the laser beam bundle emitted from the Sagnac interferometer 200 (specifically, the central region where the laser beams that have passed through each circular optical path overlap) and guides the interference laser beam bundle IL to the image transfer optical system 300.

[0035] The interference laser beam bundle IL is then transferred to an interference intensity detector 400 via a pair of afocal lenses 34 and 35 provided in the image transfer optical system 300 . The afocal lens pair 34, 35 transfers the interference image of the interference laser beam bundle IL at the polarizing beam splitter 15 to the light receiving surfaces of detectors 40A and 40B (described later) provided in the interference intensity detector 400. The detectors 40A and 40B have the same sensitivity.

[0036] (Method for detecting interference intensity) In the multi-beam phase difference measurement device 10, to measure the phase of each laser beam in the laser beam bundle 8, the central laser beam of the laser beam bundle expanded in the Sagnac interferometer 200 is used as the reference laser beam 90, and the interference pattern of all the laser beams reduced in the Sagnac interferometer 200 is observed to measure the phase difference. The conditions for this observation are described below.

[0037] In this example, first, the focal distance f2 and the focal distance f3 are the same distance. Furthermore, when the focal distance f1 is longer than the focal distance f4, the image of the laser beam bundle is enlarged in the reverse direction optical path 24. Here, the image magnification M ccw is given by the following equation (3):

[0038]

number

[0039] In addition, the image of the laser beam bundle is reduced in size in the forward circular optical path 23. Here, the image magnification M cw is given by the following formula (4): In formula (4), the variable M is a variable substituted for M=f1 / f4.

[0040]

number

[0041] From the above, according to the formulas (3) and (4), the ratio of the image of the reverse direction optical path 24 to the image of the forward direction optical path 23 is M squared.

[0042] A portion of the laser beam bundle that has passed through the forward optical path 23 and formed a reduced image at the position of point A1 passes through (transmits) the polarizing beam splitter 15 and is transferred to the interference intensity detector 400 via the aperture 33 and the afocal lens pair 34, 35. At the same time, a portion of the laser beam bundle that has passed through the reverse optical path 24 and formed an enlarged image at the position of point A1 is reflected by the polarizing beam splitter 15 and is transferred to the interference intensity detector 400 via the aperture 33 and the afocal lens pair 34, 35.

[0043] Here, the central laser beam of the laser beam bundle imaged at point A1 in a reduced size and the laser beam bundle imaged at point A1 in an enlarged size form an interference pattern, and this interference laser beam bundle IL passes through aperture 33 and is transferred to interference intensity detector 400. Also, when the image magnification is greater than the square root of the ratio of the diameter D2 of the circumscribing circle of the laser beam bundle to the diameter D1 of the inscribing circle of one laser beam, that is, M>(D2 / D1) 1 / 2 If so, the central laser beam of the expanded laser beam bundle and the entire reduced laser beam bundle will overlap, which will cause a change in optical interference intensity corresponding to the phase difference of each laser beam of the reduced laser beam bundle relative to the central laser beam of the expanded laser beam bundle.

[0044] Fig. 4 is an example diagram showing an interference pattern of the interference laser beam bundle IL. Fig. 4 shows an example of the interference pattern of the entire interference laser beam bundle IL, which is composed of 61 beams, obtained using the central laser beam as a reference beam. Fig. 4 shows that the interference intensity of each laser beam differs for each laser beam.

[0045] Furthermore, the interference intensity detected by detector 40A and detector 40B included in the interference intensity detector 400 takes a value between a maximum value and a minimum value depending on the difference between the phase of each laser beam and the phase of the central laser beam. Based on these interference intensities, the phase differences of all laser beams can be estimated with the central laser beam as a reference, and therefore, by correcting these phase differences, it is possible to match the phases of all laser beams.

[0046] 3, the interference intensity detector 400 includes a half-wave plate 36, a quarter-wave plate 37, a polarizing beam splitter 38, and detectors 40A and 40B. The horizontally polarized component and the vertically polarized component of the interference laser beam bundle IL incident on the interference intensity detector 400 are polarized and rotated in predetermined directions by the half-wave plate 36, which is located upstream of the detectors 40A and 40B in the traveling direction of the interference laser beam bundle IL, so that both components have horizontally polarized components and vertically polarized components, which interfere with each other.

[0047] A quarter-wave plate 37 is placed immediately after the half-wave plate 36, which imparts a phase difference of π / 2 between the horizontally and vertically polarized components. A polarizing beam splitter 38 is placed immediately after the quarter-wave plate 37, which separates the horizontally and vertically polarized components of the interference pattern and directs each component to detectors 40A and 40B, respectively.

[0048] Each of the detectors 40A and 40B detects an interference pattern of the interference laser beam bundle IL shown in Fig. 4. The configurations of the detectors 40A and 40B will be described later.

[0049] The calculator 41 extracts an output corresponding to each laser beam from the interference intensity distribution based on the interference patterns detected by the detectors 40A and 40B, and estimates the phase differences of all the laser beams with the central laser beam as the reference. In this way, the interference intensity detector 400 detects the phase differences of all the laser beams with the central laser beam as the reference.

[0050] Furthermore, based on the phase difference detected by the interference intensity detector 400, the phase control device 11 shown in FIG. 1 corrects the phase of each laser beam, thereby making it possible to match the phase of the laser beams other than the central laser beam with the phase of the central laser beam. The above is the configuration and functions of the multi-beam phase difference measurement apparatus 10 in this embodiment. The optical circuit path of the Sagnac interferometer 200 may be a quadrangular optical circuit path configured using three reflecting mirrors.

[0051] (Visibility of interference intensity changes) 3, an interference pattern of an interfering laser beam bundle is acquired using the central laser beam of the laser beam bundle expanded after passing through the backward optical path 24 as a reference laser beam 90 and the laser beam contracted after passing through the forward optical path 23 as a detected laser beam 91. Here, to maximize the visibility of the interference pattern to 100%, the light intensities of the reference laser beam 90 and the detected laser beam 91 must be equal in the interference intensity detector 400.

[0052] In order to equalize the light intensities of the reference laser beam 90 and the detected laser beam 91 in the interference intensity detector 400, the power P of the reference laser beam reflected by the polarizing beam splitter 15 located at the entrance (i.e., exit) of the Sagnac interferometer 200 and traveling along the reverse optical path 24 is ccw and the power P of the laser beam to be detected that passes through the polarizing beam splitter 15 and travels along the forward circular optical path 23. cw needs to be adjusted.

[0053] Here, when the image transfer in the Sagnac interferometer 200 is performed, taking into consideration the image magnification Mc (=M) of the reference laser beam expanded after passing through the backward optical path 24 and the image magnification Mc (=1 / M) of the test beam reduced after passing through the forward optical path 23, the light intensity I of the reference laser beam 90 in the interference intensity detector 400 can be calculated as follows: ccw The light intensity I of the detected laser beam 91 relative to cw The ratio is given by the following equation (5):

[0054]

number

[0055] Here, the power P of the reference laser beam traveling along the reverse optical path 24 of the Sagnac interferometer 200 is ccw and the power P of the laser beam to be detected traveling along the forward optical path 23. cw The ratio can be adjusted by the rotation angle of the half-wave plate 14. That is, regardless of the polarization direction of the light emitted from the polarizer 12B, by adjusting the rotation angle of the half-wave plate 14, it is possible to arbitrarily set the tilt angle α of the polarization direction of the laser beam that has passed through the half-wave plate 14 from the vertical direction.

[0056] When the rotation angle of the half-wave plate 14 is adjusted so that the laser beam is given an inclination angle α from the vertical direction of its polarization direction, the vertically polarized component (s-polarized component) of the laser beam that has passed through the half-wave plate 14 is reflected by the polarizing beam splitter 15 and travels along the reverse optical path 24, reaches the polarizing beam splitter 15 again, is reflected, and is guided to the interference intensity detector 400. At this time, the transmittance T ccw is the square of cos(α). On the other hand, the horizontally polarized component (p-polarized component) of the laser beam that has passed through the half-wave plate 14 and is transmitted through the polarizing beam splitter 15 travels along the forward circular optical path 23, passes through the polarizing beam splitter 15 again, and is guided to the interference intensity detector 400. At this time, the transmittance T cw is the square of sin(α). Therefore, the power P of the reference laser beam traveling on the reverse optical path 24 is ccw The power P of the detected laser beam traveling along the forward circular optical path 23 relative to cw The ratio of P cw / P ccw =[tan(α)] 2 This becomes:

[0057] That is, in the interference intensity detector 400, the condition under which the light intensities of the reference laser beam 90 and the detected laser beam 91 are equal is tan(α)=1 / M 2 Here, M is a variable that represents the ratio between the focal length f1 and the focal length f4, and is defined as M=f1 / f4.

[0058] It is desirable to make the difference between the light intensity of the reference laser beam 90 and the light intensity of the other laser beams small, since this generates a DC component in the interference intensity and leads to an increase in the estimation error of the phase difference.

[0059] (On the wavefront of an interfering laser beam bundle) In order to suppress errors in the detection and estimation of the phase difference between the laser beams by the interference intensity detector 400, it is necessary to suppress the defocus component of the interference laser beam bundle emitted from the Sagnac interferometer 200. In other words, it is necessary for the detected laser beam 91 and the reference laser beam 90 to be incident on the interference intensity detector 400 in a state where they are both parallel laser beams.

[0060] To this end, within the optical path of the circular optical path of the Sagnac interferometer 200, the convex lens 16 is positioned such that the distance between point A1 and point B1 is set to twice the focal length f1, and the distance from the convex lens 16 to point A1 is equal to the distance from the convex lens 16 to point B1.

[0061] Furthermore, the distance between points B1 and C1 is set to twice the sum of the focal length f2 of convex lens 18 and the focal length f3 of convex lens 19, and each convex lens is placed at a position where the distance from convex lens 18 to point B1 is focal length f2 and the distance from convex lens 19 to point C1 is focal length f3. Furthermore, convex lens 21 is placed at a position where the distance between points A1 and C1 is set to twice the focal length f4, and the distance from convex lens 21 to point A1 is equal to the distance from convex lens 21 to point C1.

[0062] Furthermore, in order to prevent the influence of spherical aberration on the interference pattern, the convex lens used in Sagnac interferometer 200 is an aspherical lens, and more preferably an aspherical lens with small wavefront distortion. Meanwhile, the configuration of Sagnac interferometer 200 in the present disclosure can avoid the influence of wavefront distortion of optical elements.

[0063] Here, the reference laser beam 90 and the detected laser beam 91, which overlap each other at a predetermined position on the interference intensity detector 400, pass through the same locations of each optical element, except for the polarizing beam splitter 15, on the optical path from the entrance of the Sagnac interferometer 200 to the interference intensity detector 400. In other words, the reference laser beam 90 and the detected laser beam 91 each undergo the same amount of phase change when passing through the inside of the Sagnac interferometer 200. Therefore, it is possible to suppress a change in the interference intensity between the reference laser beam 90 and the laser beam to be detected 91 caused by the wavefront distortion of each optical element, that is, an error in the estimated phase difference.

[0064] As described above, the configuration of the multi-beam phase difference measurement device 10 in this embodiment makes it possible to estimate with high accuracy the phase difference between the reference laser beam 90 and the detected laser beam 91. Furthermore, in this method, by using the polarizing beam splitter 15 in which distortion of the reflected wavefront and transmitted wavefront is negligible, it becomes possible to more reliably estimate with high accuracy the phase difference between the reference laser beam 90 and the detected laser beam 91.

[0065] (Requirements for Sagnac interferometer) The influence of vibrations of the reflecting mirrors 17 and 20 and the polarizing beam splitter 15 provided in the Sagnac interferometer 200 will be described below based on the configuration of the Sagnac interferometer 200.

[0066] (Reflector vibration) First, the influence of vibration of the reflecting mirrors 17 and 20 on the angle change in the Sagnac interferometer 200 of the present disclosure will be described. In the forward optical path 23, the focal point of the convex lens 16 is on the mirror surface of the reflecting mirror 17. Even if the tilt angle of the reflecting mirror 17 changes, the angle of the laser beam passing through the convex lens 18 and heading toward the convex lens 19 does not change. Similarly, in the backward optical path 24, the focal point of the convex lens 18 is on the mirror surface of the reflecting mirror 17. Therefore, even if the tilt angle of the reflecting mirror 17 changes, the angle of the laser beam passing through the convex lens 16 and heading toward the polarizing beam splitter 15 does not change. In this way, the vibration of the reflecting mirror 17 only imparts a slight lateral movement (movement perpendicular to the direction in which the light beam travels) to the laser beam traveling on the forward and backward optical paths.

[0067] Furthermore, in the forward optical path 23, the focal point of the convex lens 19 is on the mirror surface of the reflecting mirror 20. Therefore, even if the tilt angle of the reflecting mirror 20 changes, there is no change in the angle of the laser beam that passes through the convex lens 21 and heads toward the polarizing beam splitter 15. Similarly, in the reverse optical path 24, the focal point of the convex lens 21 is on the mirror surface of the reflecting mirror 20, so there is no change in the angle of the laser beam that passes through the convex lens 19 and heads toward the convex lens 18, even if the tilt angle of the reflecting mirror 20 changes. In this way, the vibration of the reflecting mirror 20 only imparts a slight lateral movement (movement perpendicular to the direction in which the light beam travels) to the laser beam traveling on the forward and reverse optical paths.

[0068] In this way, by defining the relative positions of the convex lenses 16, 18, 19, 21 and the reflecting mirrors 17, 20, the parallelism of the laser beams passing through the forward optical path 23 and the reverse optical path 24 is maintained. Therefore, as in the Sagnac interferometer 200 of the present disclosure, no intensity change appears in the interference pattern of the interfering laser beam bundle formed by the center laser beam of the detected laser beam bundle, which is a reduced laser beam bundle, and the reference laser beam bundle, which is an expanded laser beam bundle. Furthermore, since the lateral movement of the image of the reduced laser beam bundle caused by vibration of the reflecting mirror 17 and the reflecting mirror 20 is slight, the positional shift of the interference pattern image on the light receiving surfaces of the detectors 40A and 40B can be ignored.

[0069] (Vibration of polarizing beam splitters) Next, the influence of the polarization beam splitter 15 in the Sagnac interferometer 200 of the present disclosure on the change in angle due to vibration will be described. Even when the polarizing beam splitter 15 is tilted, there is no change in the angle of the transmitted light passing through the polarizing beam splitter 15. In other words, there is no effect on the laser beam passing through the forward circular optical path 23. On the other hand, when the polarizing beam splitter 15 is tilted by a tilt angle β, the reflected light from the polarizing beam splitter 15 is tilted by an angle 2β with respect to the optical axis. Then, when this tilted laser beam traveling along the reverse circular optical path 24 passes through two pairs of afocal lens optical systems (a pair of convex lenses 21 and 19, and a pair of convex lenses 18 and 16), it is tilted by an angle γ with respect to the optical axis. The value of angle γ is given by the following equation (6). Note that in equation (6), the variable M is a variable substituted for M=f1 / f4.

[0070]

number

[0071] Furthermore, when the laser beam traveling along the backward optical path 24 is reflected by the polarizing beam splitter 15 and travels toward the interference intensity detector 400, the traveling direction is tilted by an angle 2β due to the influence of the tilt angle β of the polarizing beam splitter 15. Therefore, the angle Δθ between the laser beam traveling along the forward optical path 23 and the laser beam traveling along the backward optical path 24 is BS is given by the following equation (7).

[0072]

number

[0073] Here, if the wavelength of the laser is λ, the spacing between the interference fringes in the two-beam interference is λ / Δθ BS If the spacing between the interference fringes is sufficiently larger than the size of the image on the interference intensity detector 400, it is possible to ignore errors when estimating the phase difference using the interference intensity detector 400. The spacing between the interference fringes when two beams of laser light with wavelength λ intersect at a common angle Δθ, that is, the period Λ over which the phase difference between the two beams is π (the optical path length difference is half the wavelength), is given by the following equation (8).

[0074]

number

[0075] Therefore, for example, when the wavelength is 1 μm, the image magnification is 3.5, and the tilt angle β of the polarizing beam splitter 15 is 10 μrad, the spacing between interference fringes is 7 cm. When the size of the image of the detected laser beam 91 is 1 cm, the change in interference intensity in the image of the detected laser beam 91 corresponds to a phase change of 0.14π rad. Therefore, the estimation error of the phase difference due to interference fringes caused by the tilt of the polarizing beam splitter 15 can be suppressed to ±7% of the wavelength. Therefore, by stabilizing the holding mechanism of the polarizing beam splitter 15 and suppressing the change in angle of the polarizing beam splitter 15, it is possible to suppress estimation errors in the phase difference in the detection results of the interference intensity detector 400.

[0076] Similarly, the effect of an angle change δ of the laser beam incident on the Sagnac interferometer 200 will be described. The angle Δθ between the laser beam on the forward circular path 23 and the laser beam on the reverse circular path 24 IB is Δθ IB≒ 2δ(M-1 / M). Under the same conditions as above, the error caused by interference fringes in phase estimation can be suppressed to ±7% of the wavelength. In this case, the angular change δ of the incident laser beam must be suppressed to 4.5 μrad.

[0077] Here, stabilization of the incident angle of the laser beam on the Sagnac interferometer 200 is important, but this is a common issue in all phase difference measurement methods using two-beam interference. Therefore, in this embodiment, a tolerance for angle change of the laser beam is defined, and the laser beam incident on the Sagnac interferometer 200 is focused to observe the spot position, and control is performed to correct the spot position deviation, thereby making it possible to maintain a stable angle. To achieve this, for example, a method is possible, such as utilizing light leaking from the reflecting mirror 17 to the back surface to stabilize the focused spot at point B1.

[0078] (Vibration of a convex lens) Next, we will explain the effect of vibration of the convex lenses 16, 18, 19, and 21. Because the convex lenses 16, 18, 19, and 21 are transmissive elements, their effect is smaller than that of reflecting mirrors, and it is sufficient to consider positional deviations in the laser beam propagation direction in the forward optical path 23 and the reverse optical path 24. Here, if the focal lengths of all the convex lenses 16, 18, 19, and 21 are equal, the effect on the interferometry due to positional deviations of the convex lenses 16, 18, 19, and 21 is canceled out.

[0079] Here, as with other optical elements, we consider the case where M=f1 / f4>1. If the movement distance of the convex lens 21 with the shortest focal length is Δx, then the angle change ε ​​corresponding to the movement of the focal point at the position of the reflecting mirror 20 is ε=Δx / f4. Therefore, the angle Δθ between the laser beam passing through the forward optical path 23 and the laser beam passing through the reverse optical path is L is Δθ L ≒ε(1-1 / M)=(Δx / f4)(1-1 / M). Note that the angle Δθ L can be made sufficiently small compared with the effect of the change in the angle of the reflecting mirror, and can be substantially ignored.

[0080] As described above, by strengthening the angle change of the laser beam incident on the Sagnac interferometer 200, the arrangement and angle change between the optical elements of the Sagnac interferometer 200, and the holding of the polarizing beam splitter 15, it is possible to stably maintain the function of the Sagnac interferometer 200.

[0081] (Regarding Phase Difference Estimation in the First Embodiment) The following describes phase difference estimation related to the multi-beam phase difference measurement apparatus 10 according to the first embodiment. Focusing on one beam element in the interference pattern in Fig. 4, the relationship between the output of each detector included in the interference intensity detector according to the first embodiment of the present disclosure and the phase difference Δφ is shown in the waveform diagram in Fig. 5. Note that the solid line in Fig. 5 is a waveform in which the optical power detected by detector 40A is normalized by its maximum value, and the dashed line is a waveform showing the normalized optical power of detector 40B.

[0082] As shown in FIG. 5, the normalized optical power PA observed by the detector 40A is given by PA=[1+sin(Δφ)] / 2. Similarly, the normalized optical power PB observed by the detector 40B is given by PB=[1-sin(Δφ)] / 2.

[0083] Furthermore, since PA-PB=sin(Δφ), the phase difference Δφ between the target laser beam 91 and the reference laser beam 90 can be calculated as Δφ=asin(PA-PB). This operation is then performed for each beam element of the interference pattern in FIG.

[0084] Each of the detectors 40A and 40B detects the interference intensity of the interference laser beam bundle IL emitted from the Sagnac interferometer 200. This makes it possible to measure the phase difference between a central laser beam (reference laser beam) of the interference laser beam bundle IL, which is used as a reference, and the laser beams of the interference laser beam bundle IL other than the central laser beam. The measurement result of the multi-beam phase difference measurement device 10 is output to the phase control device 11, which corrects the phase difference of each laser beam, thereby making it possible to match the phase of the entire interference laser beam bundle IL with the phase of the central laser beam.

[0085] In other words, it is possible to make the phase difference Δφ of all laser beams relative to the phase of the central laser beam always Δφ = 0, and when the laser beam bundle is viewed in cross section, multiple laser beams that are closely arranged in a tiled pattern can be coherently combined and propagated as if they were a single laser beam.

[0086] In the first embodiment, the multi-beam phase difference measurement device 10 assumes that the phase difference Δφ is in the range of −π / 2≦Δφ≦π / 2, and outputs an estimated value of the phase difference Δφ to the phase control device 11. As a result, the phase control device 11 performs a correction to bring the phase difference Δφ closer to zero.

[0087] In this case, the estimated value of the phase difference Δφ can be one of two cases: when the combination of normalized optical power PA and normalized optical power PB is a set of points C and C', or a set of points D and D', as shown in Fig. 5. If the combination of normalized optical power PA and normalized optical power PB is estimated to be a set of points C and C' but is actually a set of points D and D', the amount of correction required to make Δφ = 0 will be insufficient. However, by repeatedly performing feedback control, the phase difference Δφ will reach a region of 0 ≦ Δφ ≦ π / 2, and will eventually asymptotically reach Δφ = 0.

[0088] The same is true when the phase difference Δφ between the combination of normalized optical power PA and normalized optical power PB is −π<Δφ≦−π / 2; by repeatedly performing feedback control, the phase difference Δφ reaches the region of −π / 2≦Δφ≦0, and finally asymptotically reaches Δφ=0.

[0089] In this way, in the multi-beam phase difference measurement device 10 in the first embodiment, the steady-state phase difference control range can be set to -π / 2≦Δφ≦π / 2. Furthermore, since the phase difference near Δφ=0 can be uniquely estimated, oscillations in feedback control can be suppressed.

[0090] Below, we will explain an example where the output power of the laser amplifier fluctuates and the intensity ratio of the light intensity of each beam of the detected laser beam 91 to the light intensity of the reference laser beam 90 deviates from 1, i.e., the ratio of the light power detected by detector 40A and detector 40B deviates from 1. 5 shows the case where the optical intensities of the reference laser beam 90 and the detected laser beam 91 are equal, i.e., the optical powers detected by the detectors 40A and 40B are equal, but when the intensity ratio deviates from 1, the normalized optical power PA observed by the detector 40A is given by the following formula (9), where a is the optical power of the reference laser beam and b is the optical power of the detected laser beam.

[0091]

number

[0092] Furthermore, the normalized optical power PB observed by the detector 40B is given by the following formula (10): where a is the optical power of the reference laser beam, and b is the optical power of the laser beam to be detected.

[0093]

number

[0094] In the interference intensity detector 400 according to this embodiment, the normalized optical power PA and the normalized optical power PB can be measured simultaneously, and the sum of both measured values ​​is PA+PB=2(a+b). Therefore, when the outputs of detectors 40A and 40B are each normalized by PA+PB, the normalized optical power PnA is given by equation (11), and the normalized optical power PnB is given by equation (12). The difference between the normalized optical power PnA and the normalized optical power PnB is given by equation (13). Note that x in each equation is the ratio between the optical power of the reference laser beam 90 detected by detector 40A and the optical power of the target laser beam 91 detected by detector 40B, and x=b / a.

[0095]

number

[0096]

number

[0097]

number

[0098] Furthermore, from equation (13), the phase difference between the reference laser beam 90 and the target beam in the detected laser beam 91 is given by the following equation (14).

[0099]

number

[0100] Here, according to equation (13), when Δφ=0, PnA-PnB=0, and therefore no error occurs in estimating the phase difference. Therefore, by setting the operating point of the phase difference control to Δφ=0, even if the intensity ratio between the light intensity of the reference laser beam 90 and the light intensity of the detected laser beam 91 fluctuates, PnA-PnB=0 becomes the operating point, and feedback control can be stabilized.

[0101] (Second embodiment of multi-beam phase difference measurement device) 6 is a configuration diagram of a multi-beam phase difference measurement device according to a second embodiment of the present disclosure. The multi-beam phase difference measurement device 10 according to this embodiment differs in configuration from the multi-beam phase difference measurement device 10 according to the first embodiment in that the interference intensity detector 401 does not include the quarter-wave plate 37. Therefore, the interference intensity detector 401 according to this embodiment has a simpler configuration than the interference intensity detector 400 according to the first embodiment. Note that the multi-beam phase difference measurement device according to this embodiment has the same configuration as the multi-beam phase difference measurement device according to the first embodiment, except for the interference intensity detector 401.

[0102] The interference intensity detector 401 does not have a quarter-wave plate 37, but instead, by adjusting the angle of the fast axis (advance axis) of the half-wave plate 36, it is possible to arbitrarily set whether the interference intensity between the reference laser beam 90 and the detected laser beam 91, i.e., the interference intensity when the phase difference is zero, is maximum in detector 40A or detector 40B.

[0103] Here, in the following description, it is assumed that the interference intensity is maximum in detector 40B when the phase difference is zero. Fig. 7 is a waveform diagram showing the relationship between the output and the phase difference of each detector included in the interference intensity detector according to the second embodiment of the present disclosure. Here, the detected laser beam 91 passing through the forward optical path 23 of the Sagnac interferometer 200 is a component polarized horizontally relative to the plane of the paper, and the reference laser beam 90 passing through the reverse optical path 24 is a component polarized perpendicularly relative to the plane of the paper. The combined electric field of each polarization component changes depending on the phase difference Δφ between the reference laser beam 90 and the detected laser beam 91, and becomes clockwise or counterclockwise elliptically polarized or circularly polarized light.

[0104] At this time, by rotating the half-wave plate 36, the polarization of the reference laser beam 90 and the detected laser beam 91 is rotated by 45 degrees, and the normalized optical power PA obtained by normalizing the optical power observed by the detector 40A with the maximum value is given by PA=[1-cos(Δφ)] / 2. Similarly, the normalized optical power PB obtained by normalizing the optical power observed by the detector 40B with the maximum value is given by PB=[1+cos(Δφ)] / 2. The normalized optical power PA and the normalized optical power PB may be reversed depending on the setting angle of the fast axis of the half-wave plate 36 .

[0105] Furthermore, since PB-PA=cos(Δφ), the phase difference Δφ between the reference laser beam 90 and the detected laser beam 91 can be calculated as Δφ=acos(PB-PA).

[0106] This makes it possible to determine the phase differences of all laser beams with the central laser beam of the interference laser beam bundle IL as a reference, and furthermore, by correcting each phase difference, it becomes possible to maintain the phase of the entire laser beam bundle at a constant value. Here, when viewed cross-sectionally, multiple laser beams arranged closely together in a tiled pattern can be coherently combined, and all but the central beam can propagate as if they were a single laser beam.

[0107] As shown in FIG. 7 , the combination of the normalized optical power PB and the normalized optical power PA can be either a pair of points A and A′ or a pair of points B and B′, and the estimated value of the phase difference Δφ is not uniquely determined. Furthermore, if the intensity ratio between the optical intensities of the reference laser beam 90 and the detected laser beam 91 fluctuates from 1, the value of the intensity difference PB−PA does not converge even if feedback control for correcting the phase difference Δφ is continued. For this reason, when the phase of the detected laser beam 91 is controlled to match the phase of the reference laser beam 90, i.e., when control is performed with the goal of achieving a phase difference Δφ=0, there is a possibility that the phase difference Δφ will oscillate near Δφ=0. Therefore, in this embodiment, the operating point of the phase difference control is set to Δφ=π / 2, i.e., assuming that the phase difference Δφ is in the range of 0≦Δφ≦π, correction is performed to bring the estimated phase difference Δφ closer to π / 2.

[0108] If the combination of normalized optical power PA and normalized optical power PB is estimated to be the set of points A and A', but is actually the set of points B and B', the amount of correction required to achieve Δφ=π / 2 will be insufficient. However, by repeatedly performing feedback control, the phase difference Δφ will reach a region of 0≦Δφ≦π / 2, and will eventually asymptotically reach Δφ=π / 2.

[0109] The same is true when the phase difference Δφ between the combination of normalized optical power PB and normalized optical power PA is π<Δφ≦3π / 2; by repeatedly performing feedback control, the phase difference Δφ reaches the region of π / 2≦Δφ≦π, and finally asymptotically reaches Δφ=π / 2.

[0110] An example will be described below in which the output power of the laser amplifier fluctuates and the intensity ratio between the light intensity of the reference laser beam 90 and the light intensity of the laser beam 91 to be detected deviates from 1. 7 shows a case where the optical intensities of the reference laser beam 90 and the detected laser beam 91 are equal, but if the intensity ratio deviates from 1, the normalized optical power PA observed by the detector 40A is given by the following formula (15), where a is the optical power of the reference laser beam and b is the optical power of the detected laser beam.

[0111]

number

[0112] Furthermore, the normalized optical power PB observed by the detector 40B is given by the following formula (16): where a is the optical power of the reference laser beam, and b is the optical power of the laser beam to be detected.

[0113]

number

[0114] In the interference intensity detector 401 according to this embodiment, the normalized optical power PA and the normalized optical power PB can be measured simultaneously, and PA+PB=2(a+b). Therefore, when the outputs of detectors 40A and 40B are normalized by PA+PB, the normalized optical power PnA is given by equation (17), and the normalized optical power PnB of PB is given by equation (18). Furthermore, the difference between the normalized optical power PnA and the normalized optical power PnB is given by equation (19). Note that x in each equation is the intensity ratio between the optical power of the reference laser beam 90 detected by detector 40A and the optical power of the beam of interest among the detected laser beams 91 detected by detector 40B, and x=b / a.

[0115]

number

[0116]

number

[0117]

Mathematics

[0118] Here, from Equation (19), the phase difference between the reference laser beam 90 and the beam of interest among the detected laser beams 91 is given by the following Equation (20).

[0119]

Mathematics

[0120] At this time, the value of 2x 0.5 / (1 + x) does not exceed 1 and is defined as 0.5 < x < 2. That is, even if the intensity ratio between the light intensity of the reference laser beam 90 and the light intensity of the detected laser beam 91 changes up to twice, the change amount of the value of 2x 0.5 / (1 + x) is defined to be within 5.7% of the value before the change. Therefore, the estimation error of the phase difference when the normalized optical power PnB is maximum and the normalized optical power PnA is minimum causes an error of only 0.34 rad (1 / 19 wavelength) compared with the case of Δφ = 0. Also, it is represented by the coherent coupling efficiency cos(Δφ) between the reference laser beam 90 and the beam of interest among the detected laser beams 91. Even if the intensity ratio x between the light intensity of the reference laser beam 90 and the light intensity of the detected laser beam 91 varies within the range of 0.5 < x < 2, the coupling loss of the laser beam is 5.7% or less.

[0121] On the other hand, in the coupling of the laser beam, when performing feedback control to converge the phase difference of the detected laser beam 91 with respect to the reference laser beam 90 to 0, there is a possibility of continuously oscillating in the vicinity of Δφ = 0. However, according to equation (19), when Δφ=π / 2, PnB-PnA=0, and therefore no error occurs in estimating the phase difference. Therefore, by setting the operating point of the phase difference control to Δφ=π / 2, even if the intensity ratio between the light intensity of the reference laser beam 90 and the light intensity of the detected laser beam 91 fluctuates, PnB-PnA=0 becomes the operating point, and feedback control can be stabilized.

[0122] (Example of interference pattern observation) Next, an example of observing an interference pattern using a configuration equivalent to the multi-beam phase difference measurement device according to the second embodiment will be described. FIG. 8 shows an example of an observed image using a configuration equivalent to the multi-beam phase difference measurement device according to the second embodiment of the present disclosure. In this example, the interference pattern was observed using a binary phase element. Detectors 40A and 40B are CCD cameras, and the visibility of the interference pattern is maximized by adjusting the angle of the half-wave plate 14.

[0123] The binary phase element (not shown) is an optical element in which the surface of a glass substrate is divided into hexagonal regions (elements), a thin glass film is deposited only on selected elements, and the phase difference between the presence and absence of deposition is π. The binary phase element is also designed to be applicable to laser light with a wavelength of 532 nm.

[0124] In addition, the observation diagram shown in Figure 8 was obtained by adding a binary phase element (not shown) just before the polarizing beam splitter 15 in the laser beam propagation direction to the configuration shown in Figure 6 and passing a laser beam with a wavelength of 532 nm.

[0125] Figure 8(a) shows an observation diagram in which the half-wave plate 36 is adjusted so that the laser beam passing through the forward optical path 23 does not interfere with the laser beam passing through the reverse optical path 24, and only the laser beam passing through the forward optical path 23 is observed with the detector 40A, which is a CCD camera. The black line (edge) in Figure 8(a) indicates diffraction due to the edge step of the deposition. There is also a phase shift of π on both sides of the edge.

[0126] FIG. 8(b) shows an interference pattern observed by detector 40B, which is a CCD camera. The circular symbols in FIG. 8(b) correspond to the symbols on the curve of normalized optical power P B in FIG. 7 . Similarly, FIG. 8(c) shows an interference pattern observed by detector 40A, which is a CCD camera. The circular symbols in FIG. 8(c) correspond to the symbols on the curve of normalized optical power P A in FIG. 7 . In FIG. 8(b), compared to FIG. 8(a), the region of phase difference 0 (elements on which no glass thin film is deposited) has the maximum intensity, and the region of phase difference π (elements on which a glass thin film is deposited) has the minimum intensity (effectively 0 intensity). On the other hand, in FIG. 8(c), an interference pattern opposite to that in FIG. 8(b) is observed. As illustrated in FIGS. 8(b) and 8(c), the configuration according to this embodiment enables spatial phase distribution measurement, i.e., the phases of multiple laser beams in a laser beam bundle, to be easily measured.

[0127] Furthermore, when the first embodiment (see FIG. 3) is applied, it is clear that by adjusting the quarter-wave plate 37 immediately after the half-wave plate 36, images similar to those shown in FIGS. 8(a) and (b) can be obtained corresponding to phase differences of 0 and π.

[0128] (Third embodiment of multi-beam phase difference measurement device) 9 is a configuration diagram of a multi-beam phase difference measurement device according to a third embodiment of the present disclosure. The multi-beam phase difference measurement device 10 according to this embodiment differs from the multi-beam phase difference measurement device 10 according to the first embodiment in the configuration of the forward optical path 23 and the reverse optical path 24. The multi-beam phase difference measurement device according to this embodiment has the same configuration as the multi-beam phase difference measurement device according to the first embodiment, except for the zero-area Sagnac interferometer 201.

[0129] For example, in the configuration of the multi-beam phase difference measurement means of the first embodiment, the probability that the rotation of the laser device equipped with the phase difference measurement device shown in Fig. 3 occurs within the plane of the Sagnac interferometer and that the center of rotation is located inside the Sagnac interferometer is low. Therefore, the phase difference between the forward and reverse optical paths 23 and 24, which occurs due to the Sagnac effect, is small.

[0130] The Sagnac effect on the phase difference between the detection laser beam 91, which is a laser beam bundle reduced after passing through the forward optical path 23, and the reference laser beam 90, which is the central laser beam of the laser beam bundle expanded after passing through the reverse optical path 24, is constant throughout the entire laser beam bundle. That is, the phases of all laser beams except for the central laser beam of the laser beam bundle can be made equal to the phase of the central laser beam plus the phase change due to the Sagnac effect. Therefore, even if the Sagnac effect cannot be ignored, it does not substantially affect the coherent beam combination, and the power loss of the entire coherently combined laser beam bundle can be kept extremely small. In this embodiment, an aperture 32 is provided between the reflecting mirror 30 and the convex lens 29 to extract a portion of the laser beam passing through the reverse optical path 24 (specifically, the central region of the laser beam).

[0131] On the other hand, when the phase change due to the Sagnac effect becomes extremely large, the possibility that it may affect the phase difference measurement cannot be ignored. Therefore, the accuracy of the phase difference measurement is ensured by changing the configuration of the Sagnac interferometer 200 according to the first embodiment. The configuration of the circular optical path in the zero-area Sagnac interferometer 201 shown in Fig. 9 is based on a zero-area Sagnac interferometer configured so that laser beams traveling in different directions cancel each other out in the Sagnac effect, and the optical elements are arranged so that the area S1 of the triangle formed on the plane of Fig. 9 by points A2-B2-E2 on the circular optical path is equal to the area S2 of the triangle formed on the plane of Fig. 9 by points B2-C2-D2 on the circular optical path. In this way, by defining the arrangement of the optical elements, the Sagnac effect occurring in the zero-area Sagnac interferometer 201 is suppressed.

[0132] Here, the focal points of convex lenses 25 and 27 are on the surface of reflecting mirror 26, and the focal points of convex lenses 29 and 31 are on the surface of reflecting mirror 30, so that vibrations of reflecting mirror 26 and reflecting mirror 30 do not affect the detection results of the phase difference, as explained in the first embodiment. Furthermore, the influence of vibrations of polarizing beam splitter 15 is also the same as in the first embodiment.

[0133] On the other hand, when the installation angle of the reflecting mirror 28 changes by η due to vibration, the angle Δθ between the light emitted from the forward optical path 23 and the light emitted from the reverse optical path 24 changes. ZS is given by the following formula (21): where f5 to f8 are the focal lengths of the respective convex lenses.

[0134]

number

[0135] Therefore, in this embodiment, the angle Δθ given by the formula (21) ZS In order to prevent an increase in the polarization beam splitter 15, it is desirable to make the installation mechanism for the polarizing beam splitter 15 and the reflecting mirror 28 robust.

[0136] (First embodiment related to interference intensity detector) FIG. 10 is a diagram illustrating a first embodiment of an interference intensity detector according to the present disclosure. The interference intensity detector 400 detects the interference intensity of each laser beam in the interference laser beam bundle IL and estimates the phase difference of the other laser beams relative to the central laser beam in the interference laser beam bundle IL. Here, when the configuration of the interference intensity detector 400 corresponds to the first embodiment, the wave plate 50 corresponds to the half-wave plate 36 and the quarter-wave plate 37 arranged in series with each other. When the configuration of the interference intensity detector 400 corresponds to the second embodiment, the wave plate 50 corresponds to the half-wave plate 36.

[0137] The interference laser beam bundle IL that passes through the wave plate 50 is separated into individual laser beams via a lens array 51, and each laser beam is focused and guided to a polarization separation detector array 53 via a polarization-maintaining fiber bundle 52, which is a bundle of optical fibers with improved polarization-maintaining properties for the transmitted light. Each polarization separation detector in the polarization separation detector array 53 includes a fiber-type polarization separator 54 that separates the incident light into a vertically polarized component and a horizontally polarized component, a polarization-maintaining fiber 55 that guides the vertically polarized component, a polarization-maintaining fiber 56 that guides the horizontally polarized component, a photodiode 57A, and a photodiode 57B. Here, the fiber-type polarization separator 54 and the polarization-maintaining fibers 55 and 56 correspond to the polarizing beam splitter 38, and the photodiodes 57A and 57B correspond to the detectors 40A and 40B.

[0138] The photodiodes 57A and 57B receive the respective laser beams and output currents that depend on the phase difference of each laser beam relative to the central laser beam of the interference laser beam bundle IL to the calculator 41. Next, the calculator 41 converts the output currents of the photodiodes 57A and 57B of each channel of the polarization split detector array 53 into voltages, calculates the difference between the voltages, and calculates the phase difference of the laser beams corresponding to each channel.

[0139] The interference intensity detector 400 according to this embodiment can be made smaller. In addition, when a microcomputer capable of handling output for each channel is used as the calculator 41, the phase difference between each laser beam in the entire laser beam bundle can be measured at high speed.

[0140] (Second embodiment regarding interference intensity detector) FIG. 11 is a diagram illustrating a second embodiment of an interference intensity detector according to the present disclosure. In this embodiment, the polarization-maintaining fiber bundle 52 and the fiber-type polarization separator 54 in FIG.

[0141] In this embodiment, the central portion of each laser beam of the interference laser beam bundle IL is extracted by an aperture array 60, and then passes through a birefringent crystal 61. The laser beam passing through the birefringent crystal 61 is polarized and split into an ordinary ray component 63 that travels straight and an extraordinary ray component 62 (a polarized component perpendicular to the ordinary ray) that propagates in a direction different from the ordinary ray. Whether the ordinary ray or the extraordinary ray corresponds to horizontal or vertical polarization depends on the arrangement of the crystal axes of the birefringent crystal 61. Then, similar to the polarization splitter detector array 53 (53A to 53I) shown in FIG. 10 , the two split polarization components are converted into currents by two photodiodes built into a photodiode array 64. Then, a calculator 41 converts the output currents of each diode into voltages and calculates the difference between the voltages to calculate the phase difference of the laser beams corresponding to each channel.

[0142] It is also possible to replace the photodiode array 64 with an image recording element such as a CCD camera. In this case, the two polarization components corresponding to each laser beam of the interference laser beam bundle IL are measured based on the output image of the image recording element, and the phase difference of the laser beam corresponding to each channel is calculated by a calculator, just as in the case of the photodiode.

[0143] According to the interference intensity detector 400 of this embodiment, the assembly process of the interference intensity detector can be simplified.

[0144] (Regarding the third embodiment relating to the interference intensity detector) 12 is a diagram showing a third embodiment of the interference intensity detector of the present disclosure. In this embodiment, detectors 40A and 40B use image recording elements such as CCD cameras. This embodiment is preferably also applicable to cases where the interfering laser beam bundle IL includes a very large number of laser beams. This embodiment also differs from the other embodiments in that it includes a spatial light modulator 70 that imparts a four-way phase delay to all laser beams except the central beam of the laser beam bundle.

[0145] Fig. 13 is an example of a phase distribution provided by the spatial light modulator 70. As can be seen from Fig. 13, the laser beam bundle has a phase distribution as shown in Fig. 12 in all laser beam cross sections except for the central laser beam. That is, as shown in the detailed view of the phase distribution in Fig. 13, each laser beam except for the central beam is divided into four, and phase delays of π / 2, π, and 3π / 2 are given to each of the four laser beams based on one point (for example, the first quadrant in the detailed view of the phase distribution in Fig. 13). Note that no phase delay is given to the central laser beam of the laser beam bundle.

[0146] In the interference pattern of the laser beam bundle, i.e., the interference pattern between each laser beam and the central laser beam (reference laser beam), the interference intensity distribution corresponding to each laser beam is divided into four quadrants and has four intensities corresponding to phase delays of 0, π / 2, π, and 3π / 2. If the interference intensities in each quadrant are PQ1, PQ2, PQ3, and PQ4, respectively, the phase difference Δφ between the reference laser beam and each laser beam is given by Equation (22) based on the principle of phase-shifting interferometry.

[0147]

number

[0148] By performing image processing and calculations based on the interference pattern of the laser beam bundle measured using an image recording element such as a CCD camera and the interference intensity in each quadrant of the laser beam, the phase difference of each laser beam relative to the central laser beam (reference laser beam) of the laser beam bundle can be determined.

[0149] Some embodiments of the present disclosure provide the following advantages. In the multi-beam phase difference measurement device 10 disclosed herein, a laser beam bundle 8 having multiple laser beams is incident on a Sagnac interferometer 200. The laser beam bundle 8 is then separated into a laser beam bundle that passes through a forward-direction optical path 23, which is a circular optical path that circulates in the forward direction, and a laser beam bundle that passes through a reverse-direction optical path 24, which is a circular optical path that circulates in the reverse direction. After circulating along their respective optical paths, the separated laser beam bundles interfere with each other, using one of the laser beam bundles as a reference beam. In this way, by using the laser beams that have passed through the circular optical paths in different directions, an interference laser beam bundle can be generated that can measure changes in the interference intensity between the reference beam and the detected light, without the need for an external reference beam.

[0150] Furthermore, in the multi-beam phase difference measurement device 10 of the present disclosure, the Sagnac interferometer 200 may include a polarizing beam splitter 15 that outputs an image of the incident laser beam bundle 8 in each direction of the circular optical path, two pairs of afocal lens optical systems composed of four convex lenses 16, 18, 19, and 21, and multiple reflecting mirrors 17 and 20. With this configuration, by adjusting the focal lengths of the convex lenses 16, 18, 19, and 21 included in the Sagnac interferometer, the image of the laser beam bundle that has passed through the backward circular optical path 24 is enlarged, and the reference laser beam 90, which is the central laser beam of the enlarged laser beam bundle, can be used as the reference light. Furthermore, the image of the laser beam bundle that has passed through the forward circular optical path 23 is reduced, and the reduced laser beam bundle can be used as the detected laser beam 91. Then, the reference laser beam 90 and the laser beam to be detected 91 are superimposed on each other in the polarizing beam splitter 15, and the interference laser beam bundle IL is then imaged onto the interference intensity detector 400. This makes it possible to spatially resolve each of the multiple laser beams and measure the interference intensity without requiring an external reference light. Also, the influence of vibrations of the reflecting mirror and lens on the measurement of the interference intensity can be suppressed.

[0151] Furthermore, in the multi-beam phase difference measurement device 10 of the present disclosure, the Sagnac interferometer 200 positions the focal points of two corresponding pairs of afocal lens optical systems on the mirror surfaces of the multiple reflecting mirrors 17, 20 that make up the forward optical path 23 and the reverse optical path 24. This makes it possible to suppress the effect of the vibration of the reflecting mirrors 17, 20 on the interference pattern in the interference laser beam bundle IL, even if the reflecting mirrors 17, 20 vibrate.

[0152] Furthermore, in the multi-beam phase difference measurement device 10 of the present disclosure, when the laser beam bundle 8 incident on the Sagnac interferometer 200 is a laser beam bundle formed by a plurality of closely spaced laser beams that are tiled in cross section, the magnification ratio of the image of the Sagnac interferometer 200 can be made larger than the square root of the ratio of the diameter of the circumscribing circle of the laser beam bundle to the diameter of the inscribing circle of one laser beam located at the center of the laser beam bundle. This ensures that the reference laser beam 90, which is one laser beam located at the center of the enlarged image of the enlarged laser beam, and the entire reduced image, which is the image of the detected laser beam 91, which is the reduced laser beam bundle, overlap more reliably. Furthermore, the Sagnac interferometer can use the reference laser beam 90 as reference light to output an interference laser beam bundle IL in which the phase differences with each laser beam of the detected laser beam 91 are clarified.

[0153] The multi-beam phase difference measurement device 10 of the present disclosure may further include a half-wave plate 14 that is arranged upstream of the Sagnac interferometer 200 in the incident path of the laser beam bundle 8 that is incident on the Sagnac interferometer 200 and that functions to match the light intensities per unit area of ​​the enlarged and reduced images. This makes it possible to maximize the visibility of the interference pattern detected by the interference intensity detector 400.

[0154] Furthermore, in the multi-beam phase difference measurement device 10 of the present disclosure, the interference intensity detector 400 may include a half-wave plate 36, a quarter-wave plate 37, a polarizing beam splitter 38, and a calculator 41, and may output two interference intensity values, one for a horizontally polarized component and one for a vertically polarized component, corresponding to each laser beam of the interference laser beam bundle IL, and may use the difference between the outputs of the two polarization components normalized by the sum of the outputs of the two polarization components to uniquely calculate the phase difference of each laser beam relative to the laser beam located at the center of the interference laser beam bundle 8. This makes it possible to make the multi-beam phase difference measurement device 10 less susceptible to the influence of intensity fluctuations of each laser beam of the laser beam bundle 8 when used for feedback control so that the phase difference becomes zero.

[0155] Furthermore, in the multi-beam phase difference measurement device 10 of the present disclosure, the interference intensity detector 401 may include a half-wave plate 36, a polarizing beam splitter 38, and a calculator 41, and may output two interference intensity values, one for a horizontally polarized component and one for a vertically polarized component, corresponding to each laser beam of the interference laser beam bundle IL, and may use the difference between the outputs of the two polarization components normalized by the sum of the outputs of the two polarization components to uniquely calculate the phase difference of each laser beam relative to the laser beam located at the center of the interference laser beam bundle IL by the calculator 41. With this configuration, when the multi-beam phase difference measurement device 10 is used for feedback control to set the phase difference of each laser beam to π / 2, it can be made less susceptible to the influence of intensity fluctuations of each laser beam of the laser beam bundle. Furthermore, in the multi-beam phase difference measurement device 10 of the present disclosure, when one of the laser beam bundles passing through either the forward optical path 23 or the reverse optical path 24 is used as the reference light, the phases of all laser beams different from the laser beam that is the reference light can be made equal to a value obtained by adding a phase change due to the Sagnac effect to the phase of the laser beam that is the reference light.

[0156] Furthermore, in the multi-beam phase difference measurement device 10 of the present disclosure, the zero-area Sagnac interferometer allows the laser beam bundle passing through the forward optical path 23 and the laser beam bundle passing through the reverse optical path 24 to intersect within the zero-area Sagnac interferometer, and allows each of the foci of the two pairs of afocal lens optical systems to be positioned on each of the mirror surfaces of the multiple reflecting mirrors 26, 28, and 30. This makes it possible to suppress phase changes due to the Sagnac effect in the laser beam bundles passing through the forward optical path 23 and the reverse optical path 24. Furthermore, even if the reflecting mirrors 26 and 30 vibrate, the influence of the vibration of each reflecting mirror on the interference laser beam bundle IL can be suppressed.

[0157] Furthermore, in the multi-beam phase difference measurement device 10 of the present disclosure, the interference intensity detector 400 may further include a polarization-maintaining fiber bundle 52 that individually extracts each laser beam of the interference laser beam bundle IL after passing through the half-wave plate, and a polarization splitting detector array 53 that detects the two interference intensities of the horizontally polarized component and the vertically polarized component of each laser beam extracted by the polarization-maintaining fiber bundle. This allows the interference intensity detector 400 to be made smaller, and by using a microcomputer for each channel as the calculator 41, it is possible to perform high-speed beam phase difference measurement of the entire interference laser beam bundle IL.

[0158] Furthermore, in the multi-beam phase difference measurement device 10 of the present disclosure, the interference intensity detector 400 may further include a birefringent crystal 61 that individually separates the horizontally polarized component and the vertically polarized component of each laser beam of the interference laser beam bundle IL after passing through the wave plate 50 that includes the half-wave plate 36 and the quarter-wave plate 37, or the half-wave plate 36, and an image recording element such as a photodiode or a CCD camera that detects the two interference intensities of the extraordinary ray component 62 and the ordinary ray component 63 of each laser beam extracted by the birefringent crystal 61. This simplifies the assembly process of the interference intensity detector 400.

[0159] Furthermore, in the multi-beam phase difference measurement device 10 of the present disclosure, the interference intensity detector 400 may include an image recording element such as a CCD camera, and may also include a spatial light modulator 70 that is arranged upstream of the Sagnac interferometer 200 in the incidence path of the laser beam bundle 8 incident on the Sagnac interferometer 200, divides each of the laser beams incident on the Sagnac interferometer into four to give four types of phase differences, and spatially separates and generates four interference intensities for each laser beam on the image recording element such as a CCD camera. In this way, by using the principle of phase shift interferometry, it is possible to have a function of uniquely estimating the phase difference of each laser beam relative to the laser beam located at the center of the laser beam bundle 8, and to suppress the influence of intensity fluctuations of each laser beam of the laser beam bundle 8 on the estimated phase difference.

[0160] Although the present disclosure has been described above using embodiments, the technical scope of the present disclosure is not limited to the scope described in the above embodiments. Various modifications or improvements can be made to the above embodiments without departing from the gist of the present disclosure, and such modifications or improvements are also included in the technical scope of the present disclosure. Furthermore, the above embodiments may be combined as appropriate.

[0161] (Additional notes) The multi-beam phase difference measurement device and the laser beam control system including the same according to the present disclosure described above can be understood, for example, as follows. A multi-beam phase difference measurement device in a first aspect according to the present disclosure comprises a circular optical path along which a first laser beam, which is one of the split laser beam bundles having a plurality of laser beams, circulates in a forward direction, and a second laser beam, which is the other of the split laser beam bundles different from the first laser beam, circulates in a reverse direction, and is equipped with: a Sagnac interferometer (200, 201) that uses one of the first laser beam and the second laser beam that have passed through the circular optical path as a reference light, and emits an interference laser beam bundle (IL) in which the first laser beam and the second laser beam interfere with each other; interference intensity detectors (400, 401) that detect the interference intensity of an image of the interference laser beam bundle emitted from the Sagnac interferometer; and an image transfer optical system (300) that is arranged between the Sagnac interferometer and the interference intensity detector and transfers the image of the interference laser beam bundle to the interference intensity detector.

[0162] According to the multi-beam phase difference measurement device of the present disclosure, the Sagnac interferometer has a circular optical path along which a first laser beam, which is one of the split laser beam bundles having a plurality of laser beams, circulates in a forward direction, and a second laser beam, which is the other of the split laser beam bundles that is different from the first laser beam, circulates in a reverse direction, and either one of the first laser beam or the second laser beam that has passed through the circular optical path is used as a reference beam, and an interference laser beam bundle in which the first laser beam and the second laser beam interfere with each other is output to an interference intensity detector via an image transfer optical system. In addition, in the Sagnac interferometer, either the laser beam that passes through the circular optical path in a clockwise direction or the laser beam that passes through the circular optical path in a counterclockwise direction is used as the reference light. In this way, by using each laser beam that passes through the circular optical path in a different direction, an interference laser beam bundle can be generated that can measure changes in the interference intensity between the reference light and the light to be detected without requiring an external reference light. This simplifies the configuration of the measurement device for measuring the phase difference between multiple laser beams, thereby making the measurement more efficient. Furthermore, when one of the first laser beam and the second laser beam is used as the reference light, the phases of all laser beams different from the laser beam that is the reference light can be made equal to a value obtained by adding a phase change due to the Sagnac effect to the phase of the laser beam that is the reference light.

[0163] In a second aspect of the present disclosure, in the multi-beam phase difference measurement device of the first aspect, the Sagnac interferometer includes a beam splitter (15) that receives the laser beam bundle and outputs an image of the incident laser beam bundle in each circular direction of the circular optical path, two pairs of afocal lens optical systems that are configured by four lenses (16, 18, 19, 21), and a plurality of reflecting mirrors (17, 20), and the circular optical path passes through the afocal lens optical system in either a forward direction or a backward direction. The Sagnac interferometer magnifies the image of the laser beam that has passed through the circular optical path in one direction, and reduces the image of the laser beam that has passed through the circular optical path in the other direction.The Sagnac interferometer adjusts the focal lengths of the lenses so that a predetermined central area of ​​the magnified image, which is the image of the magnified laser beam, overlaps with the entire reduced image, which is the image of the reduced laser beam, and uses the laser beam in the predetermined central area of ​​the magnified image as a reference light (90) to emit an interference laser beam bundle that has an interference pattern throughout the entire laser beam bundle that corresponds to the phase difference with the reference light.

[0164] According to the multi-beam phase difference measurement device disclosed herein, the Sagnac interferometer includes a beam splitter that outputs an image of an incident laser beam bundle in each of the circular directions of the circular optical path, two pairs of afocal lens optical systems consisting of four lenses, and multiple reflecting mirrors. The circular optical path uses the afocal lens optical system to enlarge the image of the laser beam that has passed through the circular optical path in either the forward or reverse direction, and to reduce the image of the laser beam that has passed through the circular optical path in the other direction. With this configuration, by adjusting the focal lengths of the lenses included in the Sagnac interferometer, a predetermined central region of the enlarged image of the enlarged laser beam overlaps with the entire reduced image of the reduced laser beam. Using a laser beam in the predetermined central region of the enlarged image as a reference beam, an interference laser beam bundle having an interference pattern corresponding to the phase difference with the reference beam can be emitted across the entire laser beam bundle. This allows imaging of multiple laser beams and measurement of interference intensity without the need for an external reference beam. Furthermore, the influence of vibrations of the reflecting mirror and lens can be suppressed in measuring the interference intensity.

[0165] A multi-beam phase difference measurement device in a third aspect according to the present disclosure is characterized in that, in the second aspect, the Sagnac interferometer positions the focal points of the two pairs of afocal lens optical systems on the mirror surfaces of the plurality of reflecting mirrors that make up the circular optical path.

[0166] According to the multi-beam phase difference measurement device of the present disclosure, the Sagnac interferometer is characterized in that the focal points of two pairs of afocal lens optical systems are positioned on the mirror surfaces of the plurality of reflecting mirrors that make up the circular optical path, thereby making it possible to suppress the influence of vibration of each reflecting mirror on the interfering laser beam bundle even if each reflecting mirror vibrates.

[0167] A multi-beam phase difference measurement device in a fourth aspect of the present disclosure is characterized in that, in any of the second to third aspects, when the laser beam bundle incident on the Sagnac interferometer is a laser beam bundle (8) formed by a plurality of closely spaced laser beams that are tiled when viewed in cross section, the magnification ratio of the image of the Sagnac interferometer is greater than the square root of the ratio of the diameter of the circumscribing circle of the laser beam bundle to the diameter of the inscribing circle of one laser beam located at the center of the laser beam bundle.

[0168] According to the multi-beam phase difference measurement device disclosed herein, when the laser beam bundle incident on the Sagnac interferometer is a laser beam bundle formed by a plurality of closely spaced laser beams that form a tiled shape when viewed in cross section, the magnification ratio of the image of the Sagnac interferometer is greater than the square root of the ratio of the diameter of the circumscribing circle of one laser beam located at the center of the laser beam bundle to the diameter of the inscribing circle of the laser beam bundle. This ensures that a predetermined central region of the enlarged image, which is the image of the enlarged laser beam, overlaps more reliably with the entire reduced image, which is the image of the reduced laser beam, and it is possible to output an interference laser beam bundle having an interference intensity corresponding to the phase difference with each beam, using the laser beam in the predetermined central region of the enlarged image as a reference light.

[0169] A multi-beam phase difference measurement device in a fifth aspect of the present disclosure is any one of the second to fourth aspects, wherein the Sagnac interferometer has a polarizing beam splitter (15) at the entrance portion, and further includes a half-wave plate (14) that is arranged upstream of the Sagnac interferometer in the entrance path of the laser beam bundle that enters the polarizing beam splitter and that functions to match the light intensities per unit area of ​​the enlarged image and the reduced image.

[0170] According to the multi-beam phase difference measurement device of the present disclosure, the Sagnac interferometer has a polarizing beam splitter at its entrance portion, and further includes a half-wave plate that is arranged upstream of the Sagnac interferometer in the incidence path of the laser beam bundle incident on the polarizing beam splitter and that functions to match the light intensities per unit area of ​​the enlarged and reduced images, thereby maximizing the visibility of the interference pattern detected by the interference intensity detector.

[0171] A multi-beam phase difference measurement device in a sixth aspect according to the present disclosure is any of the first to fifth aspects, wherein the interference intensity detector comprises a half-wave plate (36) that rotates the polarization of each of the horizontally polarized component and the vertically polarized component of the interference laser beam bundle, a quarter-wave plate (37) that imparts a phase difference between the horizontally polarized component and the vertically polarized component, a polarizing beam splitter (38) that separates the interference laser beam bundle into a horizontally polarized component and a vertically polarized component, and a calculator (41) that calculates the phase difference of each laser beam of the interference laser beam bundle, and outputs two interference intensity values ​​of the horizontally polarized component and the vertically polarized component corresponding to each laser beam of the interference laser beam bundle, and uses the difference between the outputs of the two polarized components normalized by the sum of the outputs of the two polarized components to uniquely calculate the phase difference of each laser beam relative to a laser beam located at the center of the interference laser beam bundle by the calculator.

[0172] According to the multi-beam phase difference measurement device of the present disclosure, the interference intensity detector includes a half-wave plate, a quarter-wave plate, a polarizing beam splitter, and a calculator, and outputs two interference intensity values ​​of a horizontally polarized component and a vertically polarized component corresponding to each laser beam of the interference laser beam bundle, and the calculator uniquely calculates the phase difference of each laser beam relative to the laser beam located at the center of the interference laser beam bundle using the difference between the outputs of the two polarized components normalized by the sum of the outputs of the two polarized components. With this configuration, when the multi-beam phase difference measurement device is used for feedback control to make the phase difference of each laser beam zero, it can be made less susceptible to the influence of intensity fluctuations of each laser beam of the laser beam bundle.

[0173] A multi-beam phase difference measurement device in a seventh aspect according to the present disclosure is any of the first to fifth aspects, wherein the interference intensity detector comprises a half-wave plate (36) that rotates the polarization of each of the horizontally polarized component and the vertically polarized component of the interference laser beam bundle, a polarizing beam splitter (38) that separates the interference laser beam bundle into a horizontally polarized component and a vertically polarized component, and a calculator (41) that calculates the phase difference of each laser beam of the interference laser beam bundle, and outputs two interference intensity values ​​of the horizontally polarized component and the vertically polarized component corresponding to each laser beam of the interference laser beam bundle, and uses the difference between the outputs of the two polarized components normalized by the sum of the outputs of the two polarized components to uniquely calculate the phase difference of each laser beam relative to a laser beam located at the center of the interference laser beam bundle by the calculator.

[0174] According to the multi-beam phase difference measurement device of the present disclosure, the interference intensity detector includes a half-wave plate, a polarizing beam splitter, and a calculator, and outputs two interference intensity values ​​of a horizontally polarized component and a vertically polarized component corresponding to each laser beam of the interference laser beam bundle, and the calculator uniquely calculates the phase difference of each laser beam relative to the laser beam located at the center of the interference laser beam bundle using the difference between the outputs of the two polarized components normalized by the sum of the outputs of the two polarized components. With this configuration, when the multi-beam phase difference measurement device is used for feedback control to set the phase difference of each laser beam to π / 2, it can be made less susceptible to the influence of intensity fluctuations of each laser beam of the laser beam bundle.

[0175] The multi-beam phase difference measurement device in an eighth aspect of the present disclosure is a zero-area Sagnac interferometer (201) that is a modified version of the Sagnac interferometer in the first aspect, and further includes a beam splitter (15) that receives the laser beam bundle and emits an image of the incident laser beam bundle, two pairs of afocal lens optical systems composed of four lenses (25, 27, 29, 31), and a plurality of reflecting mirrors (26, 28, 30), and the zero-area Sagnac interferometer causes the first laser beam and the second laser beam to intersect with each other on the circular optical path, and positions the focal points of the two pairs of afocal lens optical systems on the mirror surfaces of two of the plurality of reflecting mirrors.

[0176] According to the multi-beam phase difference measurement device of the present disclosure, the zero-area Sagnac interferometer is characterized in that the first and second laser beams intersect each other in the circular optical path within the zero-area Sagnac interferometer, and the focal points of two pairs of afocal lens optical systems are positioned on the mirror surfaces of multiple reflecting mirrors, respectively. This makes it possible to suppress the effect of vibration of each reflecting mirror on the interfering laser beam bundle, even if each reflecting mirror vibrates.

[0177] In a ninth aspect of the multi-beam phase difference measurement device according to the present disclosure, in either the sixth or seventh aspect, the interference intensity detector further includes a polarization-maintaining fiber bundle (52) that individually extracts each laser beam of the interference laser beam bundle after passing through the half-wave plate, and a polarization separation detection unit (53) that detects two interference intensities of a horizontally polarized component and a vertically polarized component of each laser beam extracted by the polarization-maintaining fiber bundle.

[0178] According to the multi-beam phase difference measurement device of the present disclosure, the interference intensity detector further includes a polarization-maintaining fiber bundle that individually extracts each laser beam of the interference laser beam bundle after passing through the half-wave plate, and a polarization separation detection unit that detects the two interference intensities of the horizontally polarized component and the vertically polarized component of each laser beam extracted by the polarization-maintaining fiber bundle. This allows the interference intensity detector to be miniaturized, and by using a microcomputer for each channel as a computing unit, it is possible to perform high-speed phase difference measurement of the beams of the entire interference laser beam bundle.

[0179] In a tenth aspect of the multi-beam phase difference measurement device according to the present disclosure, in either the sixth or seventh aspect, the interference intensity detector further comprises a birefringent crystal (61) that individually separates the horizontally polarized component and the vertically polarized component of each laser beam of the interference laser beam bundle after passing through a wave plate (50) that has either both a half-wave plate (36) and a quarter-wave plate (37) or only a half-wave plate, and an image recording element such as a photodiode or a CCD camera that detects the two interference intensities of the horizontally polarized component and the vertically polarized component of each laser beam extracted by the birefringent crystal.

[0180] According to the multi-beam phase difference measurement device of the present disclosure, the interference intensity detector further includes a birefringent crystal that separates the horizontally polarized component and the vertically polarized component of each laser beam in the interfering laser beam bundle after passing through the half-wave plate, and an image recording element such as a photodiode or a CCD camera that detects the two interference intensities of the horizontally polarized component and the vertically polarized component of each laser beam extracted by the birefringent crystal, thereby simplifying the assembly process of the interference intensity detector.

[0181] In an eleventh aspect of the present disclosure, the multi-beam phase difference measurement device is either of the sixth or seventh aspects, wherein the interference intensity detector comprises an image recording element such as a CCD camera, and further comprises a spatial light modulator (70) that is arranged upstream of the Sagnac interferometer in the incident path of the laser beam bundle incident on the Sagnac interferometer, divides each of the laser beams incident on the Sagnac interferometer except for the central beam into four beams to give four types of phase differences, and generates four interference intensities for each of the laser beams on the image recording element such as the CCD camera.

[0182] According to the multi-beam phase difference measurement device of the present disclosure, the interference intensity detector includes an image recording element such as a CCD camera, and the device is also equipped with a spatial light modulator that is located upstream of the Sagnac interferometer in the incidence path of the laser beam bundle incident on the Sagnac interferometer, divides each of the laser beams incident on the Sagnac interferometer into four to give four types of phase differences, and generates four interference intensities for each of the laser beams except for the central beam on the image recording element such as the CCD camera.As a result, by using the principle of phase shift interferometry, the device has the function of uniquely estimating the phase difference of each laser beam relative to the laser beam located at the center of the laser beam bundle, and can suppress the influence of intensity fluctuations of each laser beam in the laser beam bundle on the estimated phase difference.

[0183] A laser beam control system in a twelfth aspect of the present disclosure includes a multi-beam phase difference measurement device in any of the first to eleventh aspects, a multi-beam laser device (1) that emits multiple laser beams (6A to 6I), a beam splitter (9) that splits a portion of the multiple laser beams emitted from the multi-beam laser device into the laser beam bundle and guides them to the multi-beam phase difference measurement device, and a phase control device (11) that adjusts the phases of the multiple laser beams based on the measurement results of the multi-beam phase difference measurement device. [Explanation of symbols]

[0184] 1. Multi-beam laser device 2. Oscillators 3 Turnout 4 Phase Modulator 5. Amplifier 7 Polygon Prism 8 laser beam bundle 9 Beam Splitter 10 Multi-beam phase difference measurement device 11 Phase control device 12A,12B Polarizer 13 Faraday rotator 14 Half-wave plate 15 Polarizing beam splitter 16,18,19,21,25,27,29,31 Convex lenses 17,20,26,28,30 Reflector 23 Forward light path 24 Reverse light path 33 Aperture 34,35 Afocal lens pair 36 Half-wave plate 37 1 / 4 wave plate 38 Polarizing Beam Splitter 40A, 40B detector 41 Arithmetic unit 50 wave plate 51 Lens Array 52 Polarization-maintaining fiber bundle 53 Polarization-separating detector array 61 Birefringent Crystals 70 Spatial Light Modulator 90 Reference laser beam 91 Detected laser beam 100 Laser Beam Control System 200 Sagnac interferometer 201 Zero-area Sagnac interferometer 300 Image transfer optical system 400,401 Interference Intensity Detector IL Interfering laser beam bundle

Claims

1. a Sagnac interferometer including a circular optical path along which a first laser beam, which is one of the split laser beam bundles having a plurality of laser beams, circulates in a forward direction, and a second laser beam, which is the other of the split laser beam bundles different from the first laser beam, circulates in a reverse direction, and which uses one of the first laser beam and the second laser beam that have passed through the circular optical path as a reference beam to emit an interference laser beam bundle in which the first laser beam and the second laser beam interfere with each other; an interference intensity detector that detects the interference intensity of the image of the interference laser beam bundle emitted from the Sagnac interferometer; an image transfer optical system disposed between the Sagnac interferometer and the interference intensity detector, the image of the interfering laser beam bundle being transferred to the interference intensity detector; A multi-beam phase difference measurement device comprising:

2. The Sagnac interferometer is a beam splitter that receives the laser beam bundle and outputs an image of the laser beam bundle in each circular direction of the circular optical path; two pairs of afocal lens optical systems each consisting of four lenses; Multiple reflectors and Equipped with the circular optical path enlarges an image of the laser beam that has passed through the circular optical path in either a forward rotation direction or a backward rotation direction by the afocal lens optical system, and reduces the image of the laser beam that has passed through the circular optical path in the other direction, 2. The multi-beam phase difference measurement device according to claim 1, wherein the Sagnac interferometer adjusts the focal lengths of the lenses so that a predetermined central region of an enlarged image, which is an image of the enlarged laser beam, overlaps with the entire reduced image, which is an image of the reduced laser beam, and uses the laser beam in the predetermined central region of the enlarged image as a reference light to emit an interference laser beam bundle having an interference pattern corresponding to the phase difference with the reference light across the entire laser beam bundle.

3. 3. The multi-beam phase difference measurement device according to claim 2, wherein the Sagnac interferometer positions the focal points of the two pairs of afocal lens optical systems on the mirror surfaces of the plurality of reflecting mirrors that form the circular optical path.

4. 3. The multi-beam phase difference measurement device according to claim 2, wherein when the laser beam bundle incident on the Sagnac interferometer is a laser beam bundle formed by a plurality of closely spaced laser beams that are tiled when viewed in cross section, the magnification ratio of the image of the Sagnac interferometer is greater than the square root of the ratio of the diameter of the circumscribing circle of the laser beam bundle to the diameter of the inscribing circle of one laser beam located at the center of the laser beam bundle.

5. 5. The multi-beam phase difference measurement device according to claim 2, wherein the Sagnac interferometer has a polarizing beam splitter at an entrance portion, and further comprises a half-wave plate that is arranged upstream of the Sagnac interferometer in an entrance path of the laser beam bundle that enters the polarizing beam splitter and that functions to match the light intensities per unit area of ​​the enlarged image and the reduced image.

6. The interference intensity detector a half-wave plate for rotating the polarization of each of the horizontally polarized component and the vertically polarized component of the interference laser beam bundle; a quarter wave plate that imparts a phase difference between the horizontally polarized component and the vertically polarized component; a polarizing beam splitter for separating the interference laser beam bundle into a horizontally polarized component and a vertically polarized component; a calculator for calculating a phase difference between each laser beam of the interference laser beam bundle; Preparation, 2. The multi-beam phase difference measurement device according to claim 1, wherein two interference intensity values ​​of a horizontally polarized component and a vertically polarized component are output corresponding to each laser beam of the interference laser beam bundle, and the calculator uniquely calculates a phase difference of each laser beam relative to a laser beam located at a center of the interference laser beam bundle using a difference between the outputs of the two polarized components normalized by the sum of the outputs of the two polarized components.

7. The interference intensity detector a half-wave plate for rotating the polarization of each of the horizontally polarized component and the vertically polarized component of the interference laser beam bundle; a polarizing beam splitter for separating the interference laser beam bundle into a horizontally polarized component and a vertically polarized component; a calculator for calculating a phase difference between each laser beam of the interference laser beam bundle; Preparation, 2. The multi-beam phase difference measurement device according to claim 1, wherein two interference intensity values ​​of a horizontally polarized component and a vertically polarized component are output corresponding to each laser beam of the interference laser beam bundle, and the calculator uniquely calculates a phase difference of each laser beam relative to a laser beam located at a center of the interference laser beam bundle using a difference between the outputs of the two polarized components normalized by the sum of the outputs of the two polarized components.

8. a zero-area Sagnac interferometer, which is a modified version of the Sagnac interferometer, a beam splitter onto which the laser beam bundle is incident and which emits an image of the incident laser beam bundle; two pairs of afocal lens optical systems each consisting of four lenses; Multiple reflectors and Furthermore, 2. The multi-beam phase difference measurement device according to claim 1, wherein the zero-area Sagnac interferometer is configured such that the first laser beam and the second laser beam intersect with each other on the circular optical path, and the focal points of the two pairs of afocal lens optical systems are positioned on the mirror surfaces of two of the plurality of reflecting mirrors.

9. The interference intensity detector a polarization-maintaining fiber bundle for individually extracting each laser beam of the interfering laser beam bundle after passing through the half-wave plate; a polarization separation detector that detects two interference intensities of the horizontally polarized component and the vertically polarized component of each laser beam extracted by the polarization-maintaining fiber bundle; The multi-beam phase difference measurement device according to claim 6 or 7, further comprising:

10. The interference intensity detector a birefringent crystal that individually separates the horizontally and vertically polarized components of each laser beam of the interfering laser beam bundle after passing through a wave plate comprising either both a half-wave plate and a quarter-wave plate, or only a half-wave plate; an image recording element such as a photodiode or a CCD camera that detects the interference intensity of two of the horizontally polarized component and the vertically polarized component of each laser beam extracted by the birefringent crystal; The multi-beam phase difference measurement device according to claim 6 or 7, further comprising:

11. The interference intensity detector includes an image recording element such as a CCD camera, a spatial light modulator that is arranged upstream of the Sagnac interferometer in an incident path of the laser beam bundle incident on the Sagnac interferometer, that divides all of the laser beams incident on the Sagnac interferometer except for a central beam into four beams, gives four types of phase differences, and generates four interference intensities for each of the laser beams on an image recording element such as the CCD camera; The multi-beam phase difference measurement device according to claim 1 , comprising:

12. a multi-beam laser device that emits a plurality of laser beams; The multi-beam phase difference measurement device according to claim 1 ; a beam splitter that splits a part of the plurality of laser beams emitted from the multi-beam laser device into the laser beam bundle and guides the split laser beam bundle to the multi-beam phase difference measurement device; a phase control device that adjusts the phases of the plurality of laser beams based on the measurement results of the multi-beam phase difference measurement device; A laser beam control system comprising:

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