Multi-beam phase difference measurement device and laser beam control system equipped with same

Abstract

Provided are a multi-beam phase difference measurement device with which efficiency can be improved by simplifying the configuration of a measurement device for measuring the phase difference of a plurality of laser beams, and a laser beam control system equipped with the multi-beam phase difference measurement device. This multi-beam phase difference measurement device is provided with: a Sagnac interferometer (200) which is provided with a circulation optical path in which a first laser beam, which is one of a branched plurality of laser beam fluxes, circulates in the forward direction, and in which a second laser beam, which is the other of the branched plurality of laser beam fluxes, circulates in the reverse direction, and which emits an interference laser beam flux IL in which the first laser beam and the second laser beam interfere with each other using either the first laser beam or the second laser beam that has passed through the circulation optical path as reference light; an interference intensity detector (400) which detects the interference intensity of an image of the interfering laser beam fluxes; and an image transfer optical system (300) which is disposed between the Sagnac interferometer and the interference intensity detector and transfers the image of the interfering laser beam fluxes to the interference intensity detector.

Description

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

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

[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 the lasers, 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 a nested loop system in which an inner loop feedback signal is generated using a sampled output beam associated with a laser amplifier and a sampled laser beam, and 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 discloses a system in which an optical beat signal generated by interference between a frequency-shifted reference beam and an output signal of an amplifier arm including a path length adjuster and an amplifier is detected by a detector. Based on the detected optical beat signal, 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, 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 similar operations are performed for each amplifier arm.

[0006] Patent Document 3 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 an 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 gravity of the intensity 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.

[0007] Patent Publication No. 2016-502269 U.S. Patent No. 7,884,997 JP 2021-163805 A

[0008] The configurations disclosed in Patent Documents 1 and 2 each require phase modulation or frequency shifting when adjusting the phase of the laser amplifier, which increases costs in terms of the number of devices and steps that make up the system.

[0009] The configuration disclosed in Patent Document 3 simplifies the configuration of the phase detection device by using a spiral phase plate. However, when there are multiple laser beams, it is necessary to adjust the position 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.

[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 light 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.

[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.

[0014] 12 is a configuration diagram of a laser beam control system according to the present disclosure. FIG. 13 is an example cross-sectional view of a laser beam bundle emitted from a multi-beam laser device provided in the laser beam control system according to the present disclosure. FIG. 14 is a configuration diagram of a multi-beam phase difference measurement device according to a first embodiment of the present disclosure. FIG. 15 is an example diagram showing an interference pattern of an interfering laser beam bundle. FIG. 16 is a waveform diagram showing the relationship between the output and phase difference of each detector provided in the interference intensity detector according to the first embodiment of the present disclosure. FIG. 17 is a configuration diagram of a multi-beam phase difference measurement device according to a second embodiment of the present disclosure. FIG. 18 is a waveform diagram showing the relationship between the output and phase difference of each detector provided in the interference intensity detector according to the second embodiment of the present disclosure. FIG. 19 is an example observation image using a configuration equivalent to the multi-beam phase difference measurement device according to the second embodiment of the present disclosure. FIG. 19 is an example observation image using a configuration equivalent to the multi-beam phase difference measurement device according to the second embodiment of the present disclosure. FIG. 19 is a configuration diagram of a multi-beam phase difference measurement device according to a third embodiment of the present disclosure. FIG. 19 is a diagram showing a first embodiment of an interference intensity detector according to the present disclosure. FIG. 19 is a diagram showing a second embodiment of an interference intensity detector according to the present disclosure. FIG. 19 is a diagram showing a third embodiment of an interference intensity detector according to the present disclosure. FIG. 19 is an example diagram of a phase distribution detected by the interference intensity detector of FIG.

[0015] (Configuration of the 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 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 differences 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 an oscillator 2 is split into multiple laser beams 6 (6A to 6I) by a splitter 3, which then pass through a phase modulator 4 and are amplified by an amplifier 5. The multiple laser beams 6 emitted from the amplifier 5 are each incident on a polygon prism 7 at a predetermined position and a predetermined angle, and the multiple laser beams 6 are emitted from the polygon prism 7 parallel to each other. A collection of multiple beams traveling in parallel in this manner is called a laser beam bundle 8. Note that the above-described configuration is one example, and may be modified as appropriate as long as it is capable of outputting multiple laser beams 6 and a laser beam bundle 8.

[0018] In the example of Figure 2, 61 laser beams 6 are bundled together, each having 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. Note that the cross-sectional shape of each laser beam forming the laser beam bundle 8 is not limited to a hexagon and may be another polygonal shape, such as a square.

[0019] 2 shows a case where the laser beam bundle 8 has a honeycomb structure, in which the laser beam bundle 8 is surrounded by hexagonal rings whose outer diameters increase successively outward from the center of the 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, 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 in order 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]

[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 ring located at the outermost periphery, it is given by the following mathematical formula (2): In mathematical formula (2), D1 represents the diameter of the inscribing circle of each beam, and p represents the ring number.

[0022]

[0023] Returning to the explanation of Figure 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. The multi-beam phase difference measurement device 10 detects the phase difference between each laser beam 6 in the laser beam bundle 8. The output data of the multi-beam phase difference measurement device 11 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. This causes 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 in cross section, but may also be a circular, hexagonal, or square laser beam bundle that is arranged in a tiled pattern with gaps.

[0025] 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. The multi-beam phase difference measurement device 10 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 apparatus 10 includes a Sagnac interferometer 200. The Sagnac interferometer 200 includes 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 of the split laser beam bundles different from the horizontally polarized laser beam, circulates in a reverse direction. The Sagnac interferometer 200 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 in which the horizontally polarized laser beam and the horizontally polarized laser beam interfere with each other. The multi-beam phase difference measurement device 10 comprises 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 200, 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 being incident on the Sagnac interferometer 200. The two polarizers 12A and 12B and the Faraday rotator 13 prevent the laser beam flowing backward from the Sagnac interferometer 200 from returning to the amplifier 5 located upstream in the laser beam 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 has a triangular circular optical path formed by a polarizing beam splitter 15 and two reflecting mirrors 17 and 20. Convex lenses 16, 18, 19, and 21 are arranged in the circular optical path. The Sagnac interferometer 200 has 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. The horizontally polarized component (p-polarized component) laser beam (first laser beam) 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 vertically polarized component (s-polarized component) laser beam (second laser beam) 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 at the center of the laser beam bundle 8 intersects with the reflecting surfaces of the polarizing beam splitter 15 and the two reflecting mirrors 17 and 20 are designated as points A1, B1, and C1, respectively. In this case, the circular optical path comprises a forward circular optical path 23 having a forward route of A1-B1-C1-A1, and a 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 arrangement 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 convex lens 16 is arranged at a position where the distance from point A1 to point B1 is equal to the distance from point B1 to point A1.

[0031] 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. 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.

[0032] The distance between point A1 and point C1 is set to twice the focal length f4 of the convex lens 21. The convex lens 21 is placed at a position where the distance from point A1 to point C1 is equal to the distance from point C1 to point A1.

[0033] The Sagnac interferometer 200 is configured using an optical system in which afocal lens pairs are arranged in two series stages. In the Sagnac interferometer 200, each laser beam that has passed through either the forward or reverse optical path returns to the polarizing beam splitter 15. The image of the beam bundle incident on the Sagnac interferometer 200 on the polarizing beam splitter 15 is formed again at the position of the polarizing beam splitter 15 via a circular optical path. Due to the relationship f1 > f4, the image of the beam bundle incident on the Sagnac interferometer 200 on the polarizing beam splitter 15 is a reduced image on the forward optical path and an enlarged image on the reverse optical path. The two images formed at the position of the polarizing beam splitter 15 after passing through the two circular optical paths reach the aperture 33. In the following description, the laser beam bundle emitted from the aperture 33 is 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 each laser beam that has passed through each circular optical path overlaps) and guides the interference laser beam bundle IL to the image transfer optical system 300.

[0035] The interference laser beam bundle IL is transferred to the interference intensity detector 400 via the afocal lens pair 34, 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 below) 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, the focal length f2 and the focal length f3 are the same. When the focal length f1 is longer than the focal length f4, the image of the laser beam bundle is enlarged in the reverse optical path 24. Image magnification M ｃｃｗ is given by the following equation (3).

[0038]

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

[0040]

[0041] From the above, according to the formulas (3) and (4), the ratio of the image on the reverse optical path 24 to the image on the forward 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 backward 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] The central laser beam of the laser beam bundle imaged at the position of point A1 in a reduced size and the laser beam bundle imaged at the position of point A1 in an enlarged size forms an interference pattern, and this interference laser beam bundle IL passes through the aperture 33 and is transferred to the interference intensity detector 400. When the image magnification is larger 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, i.e., 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 a diagram showing an example of an interference pattern of an interference laser beam bundle IL. Fig. 4 shows an example of an interference pattern of the entire interference laser beam bundle IL obtained when an interference laser beam bundle IL consisting of 61 beams is used with a 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 detectors 40A and 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, detectors 40A and 40B, and a calculator 41. 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 arranged 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 provides 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 guides each component to a detector 40A and a detector 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] 1 corrects the phase of each laser beam based on the phase difference detected by the interference intensity detector 400, thereby making it possible to match the phases of the laser beams other than the central laser beam with the phase of the central laser beam. The above is the configuration and function of the multi-beam phase difference measurement device 10 in this embodiment. The circular optical path of the Sagnac interferometer 200 may be a rectangular circular optical path formed using three reflecting mirrors.

[0051] 3, an interference pattern of an interfering laser beam bundle is acquired using a central laser beam of the laser beam bundle expanded after passing through the backward optical path 24 as a reference laser beam 90 and a laser beam contracted after passing through the forward optical path 23 as a detected laser beam 91. 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 It is necessary to adjust the

[0053] In the image transfer within the Sagnac interferometer 200, when the image magnification Mccw (=M) of the reference laser beam expanded by passing through the backward optical path 24 and the image magnification Mcw (=1 / M) of the test beam contracted by passing through the forward optical path 23 are taken into consideration, the light intensity I of the reference laser beam 90 within the interference intensity detector 400 is calculated as follows: ｃｃｗ The light intensity I of the laser beam 91 to be detected ｃｗ The ratio is given by the following equation (5).

[0054]

[0055] The power P of the reference laser beam traveling in the reverse optical path 24 of the Sagnac interferometer 200 ｃｃｗ and the power P of the laser beam to be detected traveling along the forward optical path 23. ｃｗ The ratio of the angle of rotation of the half-wave plate 14 can be adjusted by the rotation angle of the half-wave plate 14. 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 and is reflected by the polarizing beam splitter 15 travels along the reverse optical path 24, reaches the polarizing beam splitter 15 again, and is reflected and 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] 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 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] The difference between the light intensity of the reference laser beam 90 and the light intensity of the other laser beams generates a DC component in the interference intensity, which leads to an increase in the estimation error of the phase difference, and is therefore desirably small.

[0059] (Regarding the Wavefront of the 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 defocused component of the interfering laser beam bundle output from the Sagnac interferometer 200. The detected laser beam 91 and the reference laser beam 90 must be incident on the interference intensity detector 400 as parallel laser beams.

[0060] 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] 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 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. Convex lens 21 is placed at a position where the distance between point A1 and point C1 is set to twice the focal length f4, and where the distance from convex lens 21 to point A1 is equal to the distance from convex lens 21 to point C1.

[0062] To prevent the influence of spherical aberration on the interference pattern, the convex lens used in Sagnac interferometer 200 is an aspheric lens, and more preferably an aspheric lens with small wavefront distortion. In the configuration of Sagnac interferometer 200 in the present disclosure, the influence of wavefront distortion of optical elements can be avoided.

[0063] 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 changes in the interference intensity of the reference laser beam 90 and the detected laser beam 91, that are caused by wavefront distortions of each optical element, i.e., errors 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. 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 the 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] (Regarding Vibration of the Reflecting Mirror) The effect of vibration of the reflecting mirrors 17 and 20 on angular 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 located on the mirror surface of the reflecting mirror 17. Even if the tilt angle of the reflecting mirror 17 changes, there is no change in the angle of the laser beam passing through the convex lens 18 and proceeding toward the convex lens 19. Similarly, in the reverse optical path 24, the focal point of the convex lens 18 is located on the mirror surface of the reflecting mirror 17. Even if the tilt angle of the reflecting mirror 17 changes, there is no change in the angle of the laser beam passing through the convex lens 16 and proceeding toward the polarizing beam splitter 15. In this way, the vibration of the reflecting mirror 17 only imparts a slight lateral movement (movement perpendicular to the direction of travel of the light beam) to the laser beam traveling on the forward and reverse optical paths.

[0067] In the forward optical path 23, the focal point of the convex lens 19 is on the mirror surface of the reflecting mirror 20. 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 backward optical path 24, because the focal point of the convex lens 21 is on the mirror surface of the reflecting mirror 20, 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 backward optical paths.

[0068] In this way, by defining the relative positions of the convex lenses 16, 18, 19, and 21 and the reflecting mirrors 17 and 20, the parallelism of the laser beams passing through the forward optical path 23 and the reverse optical path 24 is maintained. As in the Sagnac interferometer 200 disclosed herein, no intensity change appears in the interference pattern of the interfering laser beam bundle formed by the central laser beam of the target laser beam bundle, which is a reduced laser beam bundle, and the reference laser beam bundle, which is an expanded laser beam bundle. 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 surface of the detectors 40A and 40B can be ignored.

[0069] (Regarding Vibration of Polarizing Beam Splitter) Next, the effect of vibration of the polarizing beam splitter 15 in the Sagnac interferometer 200 of the present disclosure on angular change will be described. Even when the polarizing beam splitter 15 is tilted, no angular change occurs in 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 rotation 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. When this tilted laser beam traveling along the reverse rotation 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 the angle γ is given by the following mathematical formula (6). In mathematical formula (6), the variable M is a variable substituted for M=f1 / f4.

[0070]

[0071] 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]

[0073] When the wavelength of the laser is λ, the spacing between the interference fringes in the two-beam interference is λ / Δθ ＢＳ and 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, i.e., 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]

[0075] Therefore, for example, if 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 will be 7 cm. If the size of the image of the laser beam 91 to be detected is 1 cm, the change in interference intensity in the image of the laser beam 91 to be detected corresponds to a phase change of 0.14π rad. Therefore, the estimation error of the phase difference due to the 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 the angle of the polarizing beam splitter 15, the estimation error of the phase difference in the detection result of the interference intensity detector 400 can be suppressed.

[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 optical path 23 and the laser beam on the reverse optical 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] It is important to stabilize the angle of incidence of the laser beam on the Sagnac interferometer 200, which is a common issue in all phase difference measurement methods using two-beam interference. In this embodiment, the allowable value for the angle change of the laser beam is specified, 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] (Regarding vibration of convex lenses) We will now explain the influence of vibration of the convex lenses 16, 18, 19, and 21. Because the convex lenses 16, 18, 19, and 21 are transmissive elements, their influence 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. When all of the convex lenses 16, 18, 19, and 21 have the same focal length, the influence on the interference measurement due to positional deviations of the convex lenses 16, 18, 19, and 21 is canceled out.

[0079] As with other optical elements, consider the case where M = f1 / f4 > 1. If the movement distance of the convex lens 21 with the shortest focal length is Δx, 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). Angle Δθ Ｌ can be made sufficiently small compared with the effect of the change in 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) Phase difference estimation related to the multi-beam phase difference measurement device 10 in the first embodiment will be described below. 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. 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] 5, the normalized optical power PA observed by detector 40A is given by PA=[1+sin(Δφ)] / 2. Similarly, the normalized optical power PB observed by detector 40B is given by PB=[1−sin(Δφ)] / 2.

[0083] Since PA-PB=sin(Δφ), the phase difference Δφ between the target beam in the detected laser beam 91 and the reference laser beam 90 can be found as Δφ=asin(PA-PB). This operation is 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 interfering 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 interfering laser beam bundle IL, which is used as a reference, and the laser beams of the interfering 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 interfering laser beam bundle IL with the phase of the central laser beam.

[0085] In other words, it is possible to always make the phase difference Δφ of all laser beams relative to the phase of the central laser beam 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] The estimated value of the phase difference Δφ can be one of two cases where the combination of the normalized optical power PA and the 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 . The combination of the normalized optical power PA and the normalized optical power PB is assumed to be a set of points C and C'. In reality, if the combination is a set of points D and D', the amount of correction required to achieve 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 the normalized optical power PA and the 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, the steady-state phase difference control range can be set to -π / 2≦Δφ≦π / 2 in the multi-beam phase difference measurement device 10 according to the first embodiment. Because 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 powers detected by the detectors 40A and 40B deviates from 1. Figure 5 shows a case where the light intensities of the reference laser beam 90 and the detected laser beam 91 are equal, i.e., the light powers detected by the detectors 40A and 40B are equal. When the intensity ratio deviates from 1, the normalized light power PA observed by the detector 40A is given by the following equation (9), where a is the light power of the reference laser beam and b is the light power of the detected laser beam.

[0091]

[0092] The normalized optical power P B observed by the detector 40B is given by the following equation (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]

[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 the two measured values ​​is PA+PB=2(a+b). When the outputs of detectors 40A and 40B are 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). In each equation, x is the ratio between the optical power of the reference laser beam 90 detected by detector 40A and the optical power of the beam of interest in the detected laser beam 91 detected by detector 40B, and x=b / a.

[0095]

[0096]

[0097]

[0098] 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]

[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 Apparatus) FIG. 6 is a configuration diagram of a multi-beam phase difference measurement apparatus according to a second embodiment of the present disclosure. The multi-beam phase difference measurement apparatus 10 according to this embodiment differs in configuration from the multi-beam phase difference measurement apparatus 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. The multi-beam phase difference measurement apparatus according to this embodiment has the same configuration as the multi-beam phase difference measurement apparatus according to the first embodiment, except for the interference intensity detector 401.

[0102] The interference intensity detector 401 does not include a quarter-wave plate 37, but 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 maximized in detector 40A or detector 40B.

[0103] In the following description, it is assumed that the interference intensity at detector 40B is maximum when the phase difference is zero. Figure 7 is a waveform diagram showing the relationship between the output and phase difference of each detector included in the interference intensity detector according to the second embodiment of the present disclosure. The detected laser beam 91 passing through the forward optical path 23 of the Sagnac interferometer 200 is a horizontally polarized component with respect to the plane of the paper, while the reference laser beam 90 passing through the reverse optical path 24 is a vertically polarized component with respect to the plane of the paper. The combined electric field of the polarization components 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.

[0104] 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 respect to 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 respect to 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=cos(PB-PA).

[0106] This allows the phase differences of all laser beams to be calculated using the central laser beam of the interference laser beam bundle IL as a reference. By correcting each phase difference, it becomes possible to maintain a constant phase for the entire laser beam bundle. 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 , there are two combinations of normalized optical powers P B and P A: a pair of points A and A′, and a pair of points B and B′. Therefore, the estimated value of the phase difference Δφ is not uniquely determined. If the intensity ratio between the optical intensities of the reference laser beam 90 and the detected laser beam 91 varies from 1, the value of the intensity difference P B − P A does not converge even if feedback control for correcting the phase difference Δφ is continued. Therefore, when the phase of the detected laser beam 91 is made to coincide with the phase of the reference laser beam 90, i.e., when control is performed with a target phase difference Δφ = 0, there is a possibility that the phase difference Δφ will oscillate near Δφ = 0. In this embodiment, assuming that 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, the estimated phase difference Δφ is corrected to approach π / 2.

[0108] It is assumed that the combination of normalized optical power PA and normalized optical power PB is the set of points A and A'. In reality, if it is 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] Below, we will explain an example where the output power of the laser amplifier fluctuates and the intensity ratio between the optical intensity of the reference laser beam 90 and the optical intensity of the detected laser beam 91 deviates from 1. Fig. 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 equation (15), where a is the optical power of the reference laser beam and b is the optical power of the detected laser beam.

[0111]

[0112] The normalized optical power P B observed by the detector 40B is given by the following equation (16): where a is the optical power of the reference laser beam and b is the optical power of the detected laser beam.

[0113]

[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). 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 for PB is given by equation (18). The difference between the normalized optical power PnA and the normalized optical power PnB is given by equation (19). In each equation, x 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 in the detected laser beam 91 detected by detector 40B, and x=b / a.

[0115]

[0116]

[0117]

[0118] From equation (19), the phase difference between the reference laser beam 90 and the target beam among the laser beams to be detected 91 is given by the following equation (20).

[0119]

[0120] 2x ０．５ The value of / (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 by up to two times, 2x ０．５ The change in the value of / (1 + x) is specified to be within 5.7% of the value before the change. Therefore, the estimation error of the phase difference at which the normalized optical power PnB is maximum and the normalized optical power PnA is minimum is only 0.34 rad (1 / 19 wavelength) compared to when Δφ = 0. The coherent coupling efficiency between the reference laser beam 90 and the target laser beam 91 is expressed as cos(Δφ), and even if the intensity ratio x between the optical intensity of the reference laser beam 90 and the optical intensity of the target laser beam 91 varies in the range of 0.5 < x < 2, the laser beam coupling loss is 5.7% or less.

[0121] On the other hand, when performing feedback control to converge the phase difference of the reference laser beam 90 and the detected laser beam 91 to 0 in laser beam combination, there is a possibility that oscillation will continue near Δφ=0. 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 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 Observation of Interference Pattern) An example of observation of 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 image observed 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. The 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 designed to be applicable to laser light with a wavelength of 532 nm.

[0124] The observation diagram shown in Figure 8 was obtained by adding a binary phase element (not shown) immediately before the polarizing beam splitter 15 in the laser beam propagation direction to the configuration shown in Figure 6 and allowing a laser beam with a wavelength of 532 nm to pass through.

[0125] 8A is an observation diagram in which only the laser beam passing through the forward optical path 23 is observed with a detector 40A, which is a CCD camera, after adjusting the half-wave plate 36 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. The black line (edge) in FIG. 8A indicates diffraction due to the edge step of the vapor deposition. There is also a phase shift of π on both sides of the edge.

[0126] FIG. 8B shows an interference pattern observed by detector 40B, which is a CCD camera. The circular symbols in FIG. 8B correspond to the symbols on the curve of normalized optical power P B in FIG. 7 . Similarly, FIG. 8C shows an interference pattern observed by detector 40A, which is a CCD camera. The circular symbols in FIG. 8C correspond to the symbols on the curve of normalized optical power P A in FIG. 7 . In FIG. 8B , compared to FIG. 8A , 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. 8C , an interference pattern opposite to that in FIG. 8B is observed. As illustrated in FIGS. 8B and 8C , the configuration according to this embodiment allows for easy spatial phase distribution measurement, i.e., measurement of the phases of multiple laser beams in a laser beam bundle.

[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 in FIGS. 8A and 8B can be obtained corresponding to phase differences of 0 and π.

[0128] (Third Embodiment of Multi-Beam Phase Difference Measurement Apparatus) Fig. 9 is a configuration diagram of a multi-beam phase difference measurement apparatus according to a third embodiment of the present disclosure. The multi-beam phase difference measurement apparatus 10 according to this embodiment differs from the multi-beam phase difference measurement apparatus 10 according to the first embodiment in the configuration of each of the forward light path 23 and the reverse light path 24. The multi-beam phase difference measurement apparatus according to this embodiment has the same configuration as the multi-beam phase difference measurement apparatus 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 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. The phase difference between the forward optical path 23 and the reverse optical path 24, which occurs due to the Sagnac effect, is small.

[0130] The Sagnac effect on the phase difference between the detected laser beam 91, which is a laser beam bundle reduced by passing through the forward optical path 23, and the reference laser beam 90, which is a central laser beam of the laser beam bundle expanded by passing through the reverse optical path 24, is constant throughout the entire laser beam bundle. That is, the phases of all laser beams except 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. Even if the Sagnac effect is not negligible, 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 installed 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 of affecting the phase difference measurement cannot be ignored. By changing the configuration of the Sagnac interferometer 200 according to the first embodiment, the accuracy of the phase difference measurement can be ensured. The configuration of the circular optical path in the zero-area Sagnac interferometer 201 shown in FIG. 9 conforms to a zero-area Sagnac interferometer configured so that laser beams traveling in different directions cancel each other out in the Sagnac effect. Each optical element is positioned so that the area S1 of the triangle formed on the plane of FIG. 9 by points A2, B2, and 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, and D2 on the circular optical path. By defining the position of each optical element in this way, the Sagnac effect occurring within the zero-area Sagnac interferometer 201 is suppressed.

[0132] 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. 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 backward optical path 24 ＺＳ is given by the following formula (21): where f5 to f8 are the focal lengths of the respective convex lenses.

[0134]

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

[0136] (First Embodiment of Interference Intensity Detector) FIG. 10 is a diagram showing a first embodiment of an interference intensity detector of the present disclosure. The interference intensity detector 400 detects the interference intensity of each laser beam in the interfering laser beam bundle IL and estimates the phase difference of the other laser beams relative to the central laser beam in the interfering laser beam bundle IL. 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 and focused by a lens array 51, and then guided to a polarization separation detector array (polarization separation detection unit) 53 via a polarization-maintaining fiber bundle 52, which is a bundle of optical fibers with enhanced 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, and photodiodes 57A and 57B. The fiber-type polarization separator 54 and the polarization-maintaining fibers 55 and 56 correspond to the polarization beam splitter 38. The photodiodes 57A and 57B correspond to 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. 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. 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 of Interference Intensity Detector) Fig. 11 is a diagram showing a second embodiment of an interference intensity detector according to the present disclosure. This embodiment has a configuration in which the polarization-maintaining fiber bundle 52 and the fiber-type polarization separator 54 in Fig. 10 are replaced with a birefringent crystal 61.

[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 separated into an ordinary ray component 63 that propagates in a straight line 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. Similar to the polarization splitting detector array 53 (53A to 53I) shown in FIG. 10 , the two separated polarization components are converted into currents by two photodiodes built into a photodiode array 64. A calculator 41 converts the output current of each diode into a voltage and calculates the difference between the voltages to calculate the phase difference of the laser beam 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. Based on the output image of the image recording element, two polarization components corresponding to each laser beam of the interference laser beam bundle IL are measured, 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] (Third Embodiment of Interference Intensity Detector) Fig. 12 is a diagram showing a third embodiment of an interference intensity detector according to the present disclosure. In this embodiment, detectors 40A and 40B use image recording elements such as CCD cameras. This embodiment is preferably also applicable when the interfering laser beam bundle IL has a very large number of laser beams. This embodiment 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 for the central beam of the laser beam bundle.

[0145] Fig. 13 is an example of the 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 at all laser beam cross sections except for the central laser beam. 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 with one reference point (for example, the first quadrant in the detailed view of the phase distribution in Fig. 13). 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. When 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]

[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 of the present disclosure, a laser beam bundle 8 having multiple laser beams is incident on the Sagnac interferometer 200. The laser beam bundle 8 is 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 that can measure changes in the interference intensity between the reference beam and the detected light can be generated without requiring an external reference beam.

[0150] In the multi-beam phase difference measurement device 10 disclosed herein, 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, the focal lengths of the convex lenses 16, 18, 19, and 21 included in the Sagnac interferometer are adjusted to enlarge the image of the laser beam bundle that has passed through the backward circular optical path 24, 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. 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. The polarizing beam splitter 15 superimposes the reference laser beam 90 and the detected laser beam 91, and then the interference laser beam bundle IL is imaged on the interference intensity detector 400. This allows us to spatially resolve each of the multiple laser beams and measure the interference intensity without the need for an external reference beam, and also reduces the effects of vibrations of the reflecting mirror and lens on the interference intensity measurement.

[0151] 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] 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 clearly defined.

[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 serves 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] 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 is zero.

[0155] 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 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 using the difference between the outputs of the two polarized components normalized by the sum of the outputs of the two polarized components 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. In the multi-beam phase difference measurement device 10 disclosed herein, 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 that are 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] 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 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] 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 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 measure the phase difference of the beams of the entire interference laser beam bundle IL at high speed.

[0158] 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] 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 the CCD camera. In this way, 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 a laser beam located at the center of the laser beam bundle 8, and can suppress the influence of intensity fluctuations of each laser beam in 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 above-described multi-beam phase difference measurement device according to the present disclosure and the laser beam control system including the same can be understood, for example, as follows. A multi-beam phase difference measurement device in a first aspect according to 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, and includes 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, an interference intensity detector (400, 401) 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 (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 disclosed herein, the Sagnac interferometer includes a circular optical path along which a first laser beam, which is one of the split laser beam bundles having multiple 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. Using either the first or second laser beam that has passed through the circular optical path as a reference beam, an interference laser beam bundle in which the first and second laser beams interfere with each other is output to an interference intensity detector via an image transfer optical system. In the Sagnac interferometer, either the laser beam that has passed through the circular optical path in a clockwise direction or the laser beam that has passed through the circular optical path in a counterclockwise direction is used as the reference beam. In this way, using each laser beam that has passed through the circular optical path in a different direction, an interference laser beam bundle that can measure changes in the interference intensity between the reference beam and the detected light can be generated without the need for an external reference beam. This simplifies the configuration of the measurement device for measuring the phase difference between multiple laser beams, thereby improving measurement efficiency. When one of the first laser beam and the second laser beam is used as the reference light, the phases of all laser beams that are 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, the multi-beam phase difference measurement device of the first aspect is characterized in that 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 enlarges 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 enlarged image, which is the image of the enlarged 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 enlarged image as a reference light (90), and emits 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 enlarges the image of the laser beam that has passed through the circular optical path in either the forward or reverse direction using 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. With this configuration, by adjusting the focal lengths of the lenses included in the Sagnac interferometer, a predetermined central region of the enlarged image, which is the image of the enlarged laser beam, overlaps with the entire reduced image, which is the 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] In a third aspect of the present disclosure, in the multi-beam phase difference measurement device of 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 positions the focal points of two pairs of afocal lens optical systems on the corresponding mirror surfaces of the multiple reflecting mirrors that make up the circular optical path, thereby suppressing the effect of vibration of each reflecting mirror on the interference laser beam bundle even if each reflecting mirror vibrates.

[0167] In a fourth aspect of the multi-beam phase difference measurement device according to the present disclosure, 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 are tiled 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] In a sixth aspect of the multi-beam phase difference measurement device according to the present disclosure, in any one of the first to fifth aspects, the interference intensity detector includes 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] In a seventh aspect of the present disclosure, in any one of the first to fifth aspects, the multi-beam phase difference measurement device 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 interfering 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 interfering 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 according to 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 is configured so 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.

[0176] According to the multi-beam phase difference measurement device of the present disclosure, the zero-area Sagnac interferometer has the first and second laser beams intersecting each other in the circular optical path, and the two pairs of afocal lens optical systems each having a focal point positioned on each of the mirror surfaces of a plurality of reflecting mirrors, thereby suppressing 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 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 measurement of the phase difference of the beams of the entire interference laser beam bundle.

[0179] In a tenth 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 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 of 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 a 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 provided with a spatial light modulator that is arranged 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. 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 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 (11) that adjusts the phases of the multiple laser beams based on the measurement results of the multi-beam phase difference measurement device.

[0184] REFERENCE SIGNS LIST 1 Multi-beam laser device 2 Oscillator 3 Splitter 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 lens 17, 20, 26, 28, 30 Reflector 23 Forward loop optical path 24 Backward loop optical path 32, 33 Aperture 34, 35 Afocal lens pair 36 Half-wave plate 37 Quarter-wave plate 38 Polarizing beam splitter 40A, 40B Detector 41 Calculator 50 Wave plate 51 Lens array 52 Polarization-maintaining fiber bundle 53 Polarization separation detector array (polarization separation detection section) 61 Birefringent crystal 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 Interference laser beam bundle

Claims

1. A multi-beam phase difference measurement device comprising: a circular optical path along which a first laser beam, which is one of the split laser beam bundles having multiple 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; 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 located between the Sagnac interferometer and the interference intensity detector and transfers the image of the interference laser beam bundle to the interference intensity detector.

2. The Sagnac interferometer comprises a beam splitter into which the laser beam bundle is incident and which emits an image of the incident laser beam bundle in each of the circular directions of the circular optical path, two pairs of afocal lens optical systems composed of four lenses, and a plurality of reflecting mirrors, wherein 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 reduces the image of the laser beam that has passed through the circular optical path in the other direction, and wherein the Sagnac interferometer is configured so that, by adjusting the focal lengths of the lenses, a predetermined central region of the enlarged image, which is the image of the enlarged 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 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 over the entire laser beam bundle.

3. A multi-beam phase difference measurement device as described in claim 2, wherein the Sagnac interferometer positions the focal points of the two pairs of afocal lens optical systems on the respective mirror surfaces of the plurality of reflecting mirrors that make up the circular optical path.

4. A multi-beam phase difference measurement device as described in 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 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 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. A multi-beam phase difference measurement device according to any one of claims 2 to 4, wherein the Sagnac interferometer has a polarizing beam splitter at the entrance, and further comprises a half-wave plate that is arranged upstream of the Sagnac interferometer in the incident 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 image and the reduced image.

6. A multi-beam phase difference measurement device as claimed in claim 1, wherein the interference intensity detector comprises: a half-wave plate 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 that imparts a phase difference between the horizontally polarized component and the vertically polarized component; a polarizing beam splitter that separates the interference laser beam bundle into a horizontally polarized component and a vertically polarized component; and a calculator that calculates the phase difference of each laser beam of the interference laser beam bundle, wherein the multi-beam phase difference measurement device 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 the laser beam located at the center of the interference laser beam bundle.

7. A multi-beam phase difference measurement device as claimed in claim 1, wherein the interference intensity detector comprises a half-wave plate which 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 which separates the interference laser beam bundle into a horizontally polarized component and a vertically polarized component, and a calculator which 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 the laser beam located at the center of the interference laser beam bundle.

8. A multi-beam phase difference measurement device as described in claim 1, which is a zero-area type Sagnac interferometer that is a modified version of the Sagnac interferometer, further comprising: a beam splitter 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; and a plurality of reflecting mirrors, wherein the zero-area type Sagnac interferometer is configured so 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. A multi-beam phase difference measurement device according to claim 6 or claim 7, wherein the interference intensity detector further comprises: 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 two interference intensities of the horizontally polarized component and the vertically polarized component of each laser beam extracted by the polarization-maintaining fiber bundle.

10. A multi-beam phase difference measurement device according to claim 6 or claim 7, wherein the interference intensity detector further comprises: a birefringent crystal 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 that has either both a half-wave plate and a quarter-wave plate, 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.

11. A multi-beam phase difference measurement device according to claim 1, wherein the interference intensity detector comprises an image recording element such as a CCD camera, and further comprises a spatial light modulator which is arranged upstream of the Sagnac interferometer in the incident path of the laser beam bundle incident on the Sagnac interferometer, and which divides all 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.

12. A laser beam control system comprising: a multi-beam laser device that emits multiple laser beams; a multi-beam phase difference measurement device as defined in claim 1; a beam splitter that splits a portion of the multiple laser beams emitted from said multi-beam laser device into said laser beam bundle and directs it to said multi-beam phase difference measurement device; and a phase control device that adjusts the phases of each of the multiple laser beams based on the measurement results of said multi-beam phase difference measurement device.

Images