A laser device and a method for coherently coupling and outputting multiple laser beams.

Abstract

To enable highly efficient and stable coherent coupling of multiple beams with a simple configuration. [Solution] A laser device comprising: a light source that outputs multiple laser beams and a reference laser beam; a phase adjuster that adjusts the phase of each of the multiple laser beams; an extraction unit that extracts a portion of the light from each of the multiple laser beams as multiple detected beams; a multiplexer that combines the multiple detected beams and the reference laser beam to output multiple combined beams; a photodetector that detects the light intensity obtained by interference between the multiple detected beams and the reference laser beam at the detection position of the combined beams; and a calculator that controls the amount of phase adjustment of the phase adjuster.

Description

[Technical Field]

[0001] The present invention relates to a laser device that coherently couples and outputs multiple laser beams, and to a method for coherently coupleing and outputting multiple laser beams. [Background technology]

[0002] High-power lasers are used in fields such as tracking and removing space debris, laser nuclear fusion, high-intensity field science, new material creation research, and laser processing. However, in high-power laser devices, heat generation and beam quality degradation and optical destruction due to nonlinear optical effects are factors that limit output. For this reason, beam coupling technology is being researched and developed to increase output by preparing multiple relatively small, high-beam-quality laser devices and coherently coupling their output beams (aligning the phases of the laser light waves) (see, for example, Patent Documents 1 and 2, and Non-Patent Document 1). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Patent No. 7468861 [Patent Document 2] U.S. Patent No. 9134538 [Non-patent literature]

[0004] [Non-Patent Document 1] P. Zhou, Z. Liu, X. Wang, Y. Ma, H. Ma, X. [Overview of the project] [Problems that the invention aims to solve]

[0005] However, these coherent beam coupling techniques sometimes resulted in complex and large-scale laser beam phase control systems, difficulties in speeding up phase control, and reduced efficiency and stability of the coherent beam coupling output.

[0006] Therefore, the present invention has been made in view of these points, and aims to enable highly efficient and stable coherent coupling of multiple beams with a simple configuration. [Means for solving the problem]

[0007] In a first embodiment of the present invention, a light source that outputs a plurality of laser beams of the same wavelength and a reference laser beam of the same wavelength as the plurality of laser beams; a plurality of phase adjusters that adjust the phase of each of the plurality of laser beams; a multi-beam laser output unit that outputs each of the plurality of laser beams whose phases have been adjusted by the plurality of phase adjusters so as not to overlap with each other; an extraction unit that extracts a portion of the light from each of the plurality of laser beams that are output in parallel so as not to overlap with each other as a plurality of detected beams; and a combined wave output unit that combines each of the plurality of detected beams extracted by the extraction unit with the reference laser beam to output a plurality of combined beams so as not to overlap with each other. The present invention provides a laser apparatus comprising: a device; an array optical system that extracts laser light from the region where the reference laser beam and each of the detected beams overlap in a cross-section of the multiplexed beam behind the multiplexer, and spatially separates the extracted multiplexed beams; one or more photodetectors having a light-receiving surface for receiving the multiple multiplexed beams at detection positions of the multiple multiplexed beams, and detecting the light intensity obtained by interference between the detected beam and the reference laser beam in the multiple multiplexed beams; and a calculator that controls the amount of phase adjustment of the phase adjuster so that the phases of the multiple laser beams match the phase of the reference laser beam based on the detection results of the photodetectors.

[0008] The multi-beam laser output unit outputs the laser beams parallel to each other with an angular error smaller than the diffraction-limited beam divergence angle, and the multiplexer may combine each of the plurality of detected beams and the reference laser beam parallel to each other with an angular error smaller than the diffraction-limited beam divergence angle.

[0009] The laser device may further include an optical system that reduces and images the entire set of multiple combined beams extracted by the array optical system while keeping them spatially separated.

[0010] The calculator may determine the amount of adjustment for the phase of each of the multiple laser beams by changing the phase of each of the multiple laser beams so that the sum of the light intensities of the combined beam detected by the photodetector is kept near the maximum value within a predetermined error.

[0011] The system further comprises an optical system that reduces and images the multiple multiple beams into groups at different detection positions, the light-receiving surfaces of the multiple photodetectors receive the multiple multiple beams that the optical system has reduced and imaged into groups, and the computing unit controls the amount of phase adjustment of the multiple phase adjusters that adjust the phase of the multiple laser beams belonging to the group for each group.

[0012] The calculator may change the phase of each of the multiple laser beams belonging to each group and determine the amount of adjustment for the phase of each of the multiple laser beams belonging to each group so that the light intensity detected by the photodetector corresponding to each group is maintained near the maximum value.

[0013] The light source may output one of the plurality of laser beams as the reference laser beam, and the multiplexer may combine the plurality of detected beams extracted by the extraction unit and the reference laser beam in parallel with an angular error smaller than the diffraction-limited beam divergence total angle.

[0014] The arithmetic unit may use a stochastic parallel gradient descent method to change the phases of the multiple laser beams, determine an adjustment amount including the sign of the phases of the multiple laser beams so as to increase the sum of the light intensities of the multiple laser beams detected by the photodetector, and continue to control the adjustment amount including the sign of the multiple laser beams so as to keep the sum of the light intensities detected by the photodetector near the maximum value within a predetermined error, and keep the phases of the multiple laser beams in line with the phase of the reference beam.

[0015] The photodetector may have a plurality of photodetectors that detect the light intensity of each of the plurality of multiplexed beams, and the calculator may divide the plurality of photodetection results corresponding to the plurality of multiplexed beams into a plurality of groups, and control the amount of phase adjustment of the plurality of phase adjusters that adjust the phase of the plurality of laser beams belonging to each of the divided groups based on the amount of variation in light intensity detected by the photodetector.

[0016] The photodetector identifies and detects the light intensity of each of the multiple multiplexed beams as an image, and the computing unit divides the multiple light detection results corresponding to the multiple multiplexed beams into multiple groups, measures the amount of change in the sum of light intensity for each of the divided groups, controls the amount of phase adjustment of the multiple phase adjusters that adjust the phase of the multiple laser beams belonging to the group based on the amount of change in the sum of light intensity measured for each group, and may change the grouping based on the amount of change in the sum of light intensity measured for each group so that the light intensity for each group is maximized and the change in light intensity for each group is less than or equal to a target value.

[0017] In a second aspect of the present invention, there are steps of outputting a plurality of laser beams having the same wavelength and a reference laser beam having the same wavelength as the plurality of laser beams, outputting the plurality of laser beams with adjusted phases so that they do not overlap with each other, extracting a part of the light of each of the plurality of laser beams with adjusted phases as a plurality of detected beams respectively, combining each of the extracted plurality of laser beams with the reference laser beam and outputting a plurality of combined beams so that they do not overlap with each other, in the cross-section of the combined beam, extracting the laser light in the region where the reference laser beam overlaps with each of the detected beams respectively, and spatially separating the plurality of extracted combined beams, receiving the plurality of combined beams at the detection positions of the plurality of combined beams, and detecting the light intensity obtained by the interference between the detected beam and the reference laser beam in the plurality of combined beams, and based on the detection result of the light intensity, controlling the adjustment amount of the phases of the plurality of laser beams, and adjusting the phases of the plurality of laser beams so that the phases of each of the plurality of laser beams coincide with the phase of the reference beam, thereby providing a method for coherently combining and outputting a plurality of laser beams.

Advantages of the Invention

[0018] According to the present invention, there is an effect that a plurality of beams can be coherently combined with high efficiency and stability with a simple configuration.

Brief Description of the Drawings

[0019] [Figure 1] Shows a configuration example of a conventional laser device 1. [Figure 2] Shows a configuration example of the coupling phase control unit 30 of the laser device 1 shown in FIG. 1. [Figure 3] Shows a simulation result representing the convergence speed of the 36-beam coupling operation when SPGD is used in a conventional laser device 1. [Figure 4] Shows a configuration example of the laser device 2 according to the present embodiment. [Figure 5] This embodiment shows an example of the configuration of the coupled phase control unit 50. [Figure 6] A first modified example of the coupled phase control unit 50 according to this embodiment is shown. [Figure 7] This embodiment shows an example of the configuration of the third optical system 54 and photodetector 55 in a first modified example. [Figure 8] The simulation results showing the convergence speed of the 36-beam coupling operation when SPGD control is used in the laser device 2 according to this embodiment are shown. [Figure 9] The simulation results of the stability of the 36-beam coupling operation when using SPGD in the laser device 2 according to this embodiment are shown. [Figure 10] A second modified example of the coupled phase control unit 50 according to this embodiment is shown. [Figure 11] A third modified example of the photodetector 55 according to this embodiment is shown. [Figure 12] A modified example of the array optical system 66 according to this embodiment is shown. [Modes for carrying out the invention]

[0020] <Example of a conventional laser device 1 configuration> Figure 1 shows an example of the configuration of a conventional laser device 1. Laser device 1 outputs multiple laser beams (hereinafter, laser beams may be abbreviated as beams) with their phases aligned. This process of aligning the phases of multiple beams and outputting them is called coherent beam coupling. Here, the beam formed by coherently coupling multiple beams will be abbreviated as a coupled output beam.

[0021] When coherently coupling multiple laser beams, there are two methods: one in which each laser beam is propagated along the same optical path, and another in which each beam is arranged in a tile-like pattern and propagated parallel to each other. Generally, the former is called Filled aperture coherent beam combining, and the latter is called Tiled aperture coherent beam combining. In this embodiment, the former will be abbreviated as FACBC, and the latter as TACBC.

[0022] In relation to the above TACBC, it is known that an optical vortex beam is used as a reference beam in phase difference measurement for coherent coupling of multiple beams (for example, Patent Document 1 above). This phase difference measurement method is expected to improve the accuracy of phase difference measurement between the beam to be detected and the reference beam. However, since this method uses the same number of detectors as the beams to be coupled, it has the disadvantage that the phase difference measurement and control system becomes complex when the number of beams increases.

[0023] Furthermore, in relation to the above FACBC, a method for coherently coupling multiple laser oscillators using multiple high-reflectivity mirrors, a laser gain medium, a diffractive optical element, an output mirror, and a single photodetector for measuring the power of the coupled beam has been disclosed (for example, Patent Document 2 mentioned above).

[0024] The technique used in Patent Document 2 achieves coherent coupling by controlling the resonator lengths of multiple oscillators to maximize the output of a single photodetector based on the measurement results of that photodetector. Here, stochastic parallel gradient descent is used as the algorithm for controlling the resonator lengths (for example, Non-Patent Document 1 mentioned above). In this embodiment, this algorithm will be abbreviated as SPGD.

[0025] The conventional laser device 1 shown in Figure 1 outputs multiple coherent beams using an SPGD. The laser device 1 comprises a multi-beam laser output unit 10, an extraction unit 20, and a coupling phase control unit 30.

[0026] The multi-beam laser output unit 10 outputs multiple amplified beams in parallel so that they do not overlap. Figure 1 shows an example in which the multi-beam laser output unit 10 outputs N amplified beams from B-1 to BN (where N is an integer of 2 or more). The laser device 1 then aligns the phases of the multiple amplified beams and outputs a combined output beam. The multi-beam laser output unit 10 includes a light source 11, a phase adjuster 12, a preamplifier 13, and a main amplifier 14.

[0027] The light source 11 outputs multiple laser beams having the same wavelength and the same time-space coherence. Figure 1 shows an example in which the light source 11 includes a laser oscillator 15 and a beam splitter 16. The laser oscillator 15 outputs seed light for the laser beam. The laser beam output by the laser oscillator 15 may be continuous light, or it may be pulsed light.

[0028] The beam splitter 16 splits the optical path through which the beam output from a single laser oscillator 15 passes into multiple paths, and outputs beams from each of the multiple branched optical paths. The beam splitter 16 includes, for example, a beam splitter, mirrors, etc. (not shown), and divides the input beam into multiple beams. Figure 1 shows an example in which the beam splitter 16 divides the beam output from the laser oscillator 15 into N beams. Note that if the laser oscillator 15 is a device that outputs laser light through an optical fiber, the beam splitter 16 may be an optical fiber type splitter.

[0029] The phase adjuster 12 adjusts the phase of each of the multiple laser beams output by the light source 11. The phase adjuster 12 is provided in each of the multiple optical paths through which the multiple beams output by the light source 11 pass. Figure 1 shows an example in which N phase adjusters 12 are provided. The phase adjuster 12 may include a phase modulator that adjusts the length of the optical path through which the beam passes, or it may include an electro-optic phase modulator instead. It is also possible to omit the phase adjuster for one beam.

[0030] The preamplifier 13 and main amplifier 14 amplify the beam whose phase has been adjusted by the phase adjuster 12. Figure 1 shows an example in which N preamplifiers 13 and N main amplifiers 14 are provided. Each of the main amplifiers 14 outputs the amplified beam as an amplified beam. Depending on the light intensity of the multiple beams output by the light source 11, additional amplifiers may be provided. If the light intensity of the multiple beams output by the light source 11 is sufficiently strong, either the preamplifier 13 or the main amplifier 14 may be omitted. In a virtual plane located immediately before the extraction unit 20 and perpendicular to the optical axis of the amplified beam, the optical length of each amplified beam, i.e., the optical length from the laser oscillator 15 to the virtual plane, is adjusted to match with an error sufficiently smaller than the coherence length.

[0031] The extraction unit 20 extracts portions of light from multiple laser beams, whose phases are adjusted by multiple phase adjusters 12 and which are output in parallel so as not to overlap, as multiple detected beams. The extraction unit 20 has, for example, a beam splitter. Figure 1 shows an example in which the extraction unit 20 is composed of one beam splitter, and the N detected beams extracted from each of the N amplified beams are designated as B1-1 to B1-N.

[0032] The coupled phase control unit 30 supplies a phase control signal to the phase tuner 12 to adjust the phase of the phase tuner 12 based on the light intensity obtained from the interference of multiple detected beams. The coupled phase control unit 30 generates the phase control signal so that the light intensity obtained from the interference of multiple detected beams is maximized. Such a coupled phase control unit 30 will be described next.

[0033] Figure 2 shows an example of the configuration of the coupled phase control unit 30 of the conventional laser device 1 shown in Figure 1. The coupled phase control unit 30 includes an optical system 31, a photodetector 32, a calculator 33, and a phase control signal generator 34.

[0034] The optical system 31 focuses multiple beams to be detected. The optical system 31 includes, for example, a lens 35 and a pinhole 36. The multiple beams to be detected are focused on the focal plane of the lens 35, and the intensity distribution of the focusing pattern changes according to the phase relationship of the multiple beams to be detected. The intensity near the center of the focusing pattern is maximum when the phases of the multiple beams to be detected coincide. Therefore, if a pinhole 36 of an appropriate size is placed centered on the focal position of the lens 35, the light intensity detected by the photodetector 32 after passing through the pinhole 36 will be maximum when the phases of the multiple beams to be detected coincide.

[0035] The photodetector 32 has a light-receiving surface larger than the aperture size of the pinhole 36 and is located immediately after the pinhole 36. The photodetector 32 has a photodetector such as a photodiode.

[0036] For example, if the phases of all of the multiple beams to be detected (the phases of N beams) coincide, the spot size of the focusing pattern in the focal plane becomes the minimum, and the light intensity passing through the pinhole 36, i.e., the light intensity detected by the photodetector 32, becomes the maximum. Then, the phases of the multiple amplified beams output by the multi-beam laser output unit 10 coincide, and the laser device 1 outputs multiple coherently coupled beams. In this case, for example, if N coherently coupled beams are focused, the peak intensity at the focusing point can be increased by up to N times compared to the case where the phases of the multiple beams are random.

[0037] On the other hand, if the phases of all the multiple beams to be detected are random, the focusing pattern size near the focal point and focal point will be about the same as in the case of a single beam, resulting in random speckle. The average interference intensity (average intensity of the speckle) will decrease compared to when multiple beams are coherently coupled, so the light intensity passing through the pinhole 36 (the light intensity detected by the photodetector 32) will be minimized. Furthermore, the more beams to be detected whose phases coincide, the smaller the focusing pattern size in the focal plane tends to be, and the higher the peak intensity tends to be.

[0038] Furthermore, if the phases of m beams (where m is less than N) among the multiple detected beams coincide, the intensity of the interfering light will not reach its maximum, but it will reach a maximum value, and the phase control of the multiple beams may stall while maintaining that maximum value.

[0039] The arithmetic unit 33 acquires the detection signal from the photodetector 32 and calculates the amount of phase adjustment for the phase adjuster 12 corresponding to the detection signal. Based on the calculated amount of phase adjustment, the arithmetic unit 33 controls the phase control signal generator 34. The phase control signal generator 34 generates a phase control signal to adjust the phase of the phase adjuster 12 in accordance with the control signal received from the arithmetic unit 33 and transmits the phase control signal to the multiple phase adjusters 12.

[0040] The arithmetic unit 33 controls the phase control signal generator 34 by executing a program via a personal computer, microcontroller, or FPGA (Field Programmable Gate Array). The arithmetic unit 33 has a memory circuit that stores parameters, programs, etc., used for calculations.

[0041] <Control operation by arithmetic unit 33> The arithmetic unit 33 controls the phase adjustment amount of the multiple phase adjusters 12 based on the SPGD so that the detection signal of the photodetector 32 is maximized. The basic principle of the SPGD is well known, so it is briefly summarized below.

[0042] As illustrated in Figures 1 and 2, when there are N beams to be detected, the signal intensity P detected by the photodetector 32 is determined by the control parameter u for each beam to be detected. i Using (where i represents the i-th detected beam, i=1,2,···,N), P(u1,u2,···,u N ) is expressed as. Here u i This corresponds to the phase control signal (for example, the signal voltage) supplied to the phase adjuster 12 (12-1, 12-2, ..., 12-N).

[0043] If δui is the change in the control parameter for the i-th beam, then Pp(u1+δu1,u2+δu2,···,u N +δu N ) and Pm(u1-δu1,u2-δu2,···,u N -δu N ) takes on different values, and the difference between Pp and Pm is expressed as ΔP as shown in the following equation.

number

[0044] To maximize the interference intensity (the light intensity detected by the photodetector 32), control parameters that increase the interference intensity should be applied, and this operation should be repeated (in this embodiment, this may be called a repeating operation or repeating phase control). Therefore, when moving from the j-th repeating operation to the next step ((j+1)th operation), the control parameters are updated sequentially according to the following equation.

number

[0045] Here, γ is called the update gain. To maximize the interference intensity, γ is set to > 0, and feedback control for each beam phase is repeated many times based on equation (2). Even after the interference intensity asymptotically approaches the maximum value, if the iterative operation based on equation (2) is continued, the interference intensity will remain near the maximum value with slight increases and decreases.

[0046] <Simulation results of control operation> Figure 3 shows the simulation results of the control operation when using SPGD in the conventional laser device 1 described above. The multi-beam laser output unit 10 outputs 36 amplified beams having the same output intensity. Here, the amplified beams and the detected beams are assumed to be in a two-dimensional arrangement such as 6 rows and 6 columns. The horizontal axis of Figure 3 represents the number of iterations of phase control, and the vertical axis represents the power of the combined output beam normalized by the average power of one beam. In other words, when the phases of all amplified beams are matched, the sum of the combined output beams will be approximately 36 at its maximum value.

[0047] In the simulation, the initial phase of each of the 36 beams was set randomly, and it was assumed that there were no phase disturbances in each beam path from the beam splitter 46 to the main amplifier 14 during the time period in which iterative phase control was performed (in other words, the iterative phase control speed was sufficiently faster than the phase disturbance). Note that in the conventional method shown in Figure 3, the path to the steady state varies with each simulation trial, so the curves C1 to C4 in Figure 3 are plots of the average of 20 simulation results for different update gains (γ = 0.002, 0.005, 0.1, 0.2).

[0048] As is clear from Figure 3, when the update gain γ in equation (Equation 2) is large, the power of the coupled output beam rises quickly but stalls at a low maximum. In this case, the phases of some of the amplified beams can be matched, but not all of them. The stalled maximum tends to increase as the update gain γ decreases. In other words, it can be seen that by reducing the update gain γ at the expense of the rise time of the coupled output beam power, the phases of more amplified beams can be matched.

[0049] Then, when the update gain γ is 0.02, the combined output beam reaches its maximum power of 36. In other words, it was found that the phases of 36 amplified beams can be matched. It was also found that the number of iterations required to obtain this maximum power is approximately 400, which is consistent with the value shown in Figure 4 of the aforementioned Non-Patent Document 1.

[0050] As described above, SPGD is characterized by its simplicity, using only one photodetector 32. SPGD is based on an iterative operation of changing the phase of each amplified beam to maximize the intensity of the coupled beam, but during the phase-changing process, there is a concern that the light intensity detected by the photodetector 32 may stagnate at one of several maximum values. Therefore, in order to match the phases of all amplified beams, the update gain had to be reduced and the time spent on phase control increased.

[0051] For example, Non-Patent Literature 1 shows that the number of iterations required to bring the phase of each of multiple beams to the optimal value is proportional to the number of beams to be combined, and that 50 beams require more than 500 iterations. Therefore, the laser device 2 according to this embodiment uses such a simple beam phase adjustment method of SPGD, and enables high-speed and stable coherent coupling and output of multiple beams with a simple configuration. Such a laser device 2 will be described next.

[0052] <Example configuration of laser device 2> Figure 4 shows an example of the configuration of the laser device 2 according to this embodiment. Similar to the laser device 1, the laser device 2 outputs multiple laser beams coherently coupled together. In the laser device 2 according to this embodiment, components that are substantially the same as those in the operation of the laser device 1 described in Figures 1 to 3 are given the same reference numerals, and redundant explanations are omitted. The laser device 2 comprises a multi-beam laser output unit 40, an extraction unit 20, and a coupling phase control unit 50.

[0053] The multi-beam laser output unit 40 outputs multiple amplified beams and a reference beam. The multi-beam laser output unit 40 outputs multiple laser beams whose phases have been adjusted by multiple phase adjusters 12, so that each beam does not overlap with the others. Figure 4 shows an example in which the multi-beam laser output unit 40 outputs N amplified beams (N is an integer of 2 or more) from B-1 to BN and a reference beam. The laser device 2 then matches the phases of the multiple amplified beams and outputs a combined output beam. The multi-beam laser output unit 40 includes a light source 41, phase adjusters 12, a preamplifier 43, and a main amplifier 14.

[0054] The light source 41 outputs multiple laser beams having the same wavelength and the same time-space coherence, and a high-beam-quality reference laser beam having the same wavelength and time coherence as the multiple laser beams. Figure 4 shows an example in which the light source 41 includes a laser oscillator 15 and a beam splitter 46. In Figure 4, an example is shown in which the light source 41 has a laser oscillator 15 and a beam splitter 46 that splits the seed laser light output by the laser oscillator 15 into multiple laser beams and a reference laser beam.

[0055] The beam splitter 46 splits the optical path through which the beam output from one laser oscillator 15 passes into multiple paths, and outputs beams from each of the multiple branched optical paths. The beam splitter 46 includes, for example, a beam splitter, mirrors, etc. (not shown), and divides the input beam into multiple beams. Figure 4 shows an example in which the beam splitter 46 divides the beam output from the laser oscillator 15 into (N+1) beams. Note that if the laser oscillator 15 is a device that outputs laser light through an optical fiber, the beam splitter 46 may be an optical fiber type splitter.

[0056] The preamplifier 43 amplifies the multiple laser beams output by the light source 41 and the reference beam. Depending on the light intensity of the reference beam output by the light source 41, the main amplifier 14 and / or other optical amplifiers may be provided to amplify the reference beam. Alternatively, if the light intensity of the reference beam output by the light source 41 is sufficiently strong, the preamplifier 43 may not be necessary. The amplification of the multiple laser beams is the same as that of the preamplifier 13 and main amplifier 14 in conventional laser devices 1, so the explanation is omitted here.

[0057] The multi-beam laser output unit 40 described above arranges multiple laser beams, whose phases are adjusted by multiple phase adjusters 12, in space so that they do not overlap with each other, and outputs the laser beams parallel to each other with an angular error smaller than the diffraction-limited beam divergence angle. The multi-beam laser output unit 40 according to this embodiment differs from the multi-beam laser output unit 10 in Figure 1 in that it outputs a reference beam.

[0058] Furthermore, the coupled phase control unit 50 according to this embodiment supplies a phase control signal to the phase adjuster 12 for adjusting the phase of the phase adjuster 12 based on the interference between each of the multiple detected beams extracted by the extraction unit 20 and the reference beam. Such a coupled phase control unit 50 will be described next.

[0059] <Example of configuration of coupling phase control unit 50> Figure 5 shows an example of the configuration of the coupled phase control unit 50 according to this embodiment. Similar to Figure 2, Figure 5 shows an example where there are N amplified beams and N detected beams. The coupled phase control unit 50 includes a first optical system 51, a multiplexer 53, a second optical system 52, a third optical system 54, a photodetector 55, a calculator 56, and a phase control signal generator 57.

[0060] The first optical system 51 reduces the detected beam and guides the reduced multiple detected beams to the multiplexer 53. The first optical system 51 includes, for example, a first lens 61, a second lens 62 paired with the first lens 61, and a filter 63. Figure 5 shows an example where the first lens 61 and the second lens 62 are convex lenses, and the distance between them is equal to the sum of their respective focal lengths. Multiple detected beams propagating parallel to each other are incident on the first lens 61 parallel to its optical axis, focused, and then output as multiple parallel beams by the second lens 62. The focal length (f2) of the second lens 62 is shorter than the focal length (f1) of the first lens 61, and the ratio of the size of the output beam to the size of the detected beam incident on the first optical system 51 is f2 / f1. The ratio of focal lengths f2 / f1 is the beam reduction ratio, and by appropriately reducing the beam reduction ratio, the size of the optical elements downstream of the multiplexer 53 can be reduced.

[0061] In this embodiment, the beam output by the first optical system 51 may be referred to as "multiple reduced-detection beams." Furthermore, since the phase relationships of each reduced-detection beam are preserved from the phase relationships of each detection beam incident on the first optical system 51, the beam output by the first optical system 51 may also be referred to as "multiple detection beams."

[0062] Multiple beams to be detected are reduced in size by passing through the first optical system 51. The first optical system 51 converts, for example, the beams to be detected from B1-1 to B1-N into reduced beams to be detected from B2-1 to B2-N. The overall size of the reduced beams to be detected is smaller than the size of the third optical system 54, i.e., the aperture of the fifth lens 67.

[0063] The filter 63 adjusts the light intensity of multiple reduced beams to be detected. The filter 63 is, for example, a reflector and / or ND filter having a predetermined attenuation amount. The first optical system 51 only needs to be able to direct multiple beams to be detected into the third optical system 54 via the multiplexer 53, and may have other types of lenses, or further lenses, mirrors, etc. Also, if the aperture of the third optical system 54 is larger than the total size of the multiple beams to be detected, the first optical system 51 may have only the filter 63.

[0064] The second optical system 52 guides the reference beam to the multiplexer 53. The second optical system 52 includes, for example, a third lens 64 and a fourth lens 65. The third lens 64 magnifies the reference beam incident from the multi-beam laser output unit 40, generating a magnified parallel reference beam. Figure 5 shows an example where the third lens 64 and the fourth lens 65 are convex lenses. The second optical system 52 only needs to be able to magnify the reference beam output by the multi-beam laser output unit 40 and guide it to the multiplexer 53, and may have other types of lenses, or further lenses, mirrors, etc.

[0065] The multiplexer 53 combines each of the multiple detected beams extracted by the extraction unit 20 with the reference laser beam to output multiple multiplexed beams that do not overlap with each other. The multiplexer 53 propagates the enlarged reference beam in parallel with the multiple reduced detected beams. This is called multiplexing. In other words, it combines each of the multiple detected beams extracted by the extraction unit 20 with the reference laser beam in parallel with each other with an angular error smaller than the diffraction-limited beam divergence angle to output multiple multiplexed beams. The multiplexer 53 is, for example, one or more multiplexing mirrors. The multiplexing mirrors may also be half mirrors. Because each of the reduced detected beams is superimposed with the reference beam by the multiplexer 53, each reduced detected beam interferes with the reference beam.

[0066] For example, the detected beam B2-1 interferes with the reference beam to form a single combined beam B3-1, the detected beam B2-2 interferes with the reference beam to form a single combined beam B3-2, ..., the detected beam B2-N interferes with the reference beam to form a single combined beam B3-N.

[0067] Furthermore, the optical lengths of each beam output from the light source 11 to the multiplexer 53 as multiple reduced-detection beams, and the optical length of the reference beam output from the light source 11 to the multiplexer 53, are assumed to coincide with an error sufficiently smaller than the coherence length. In other words, the multi-beam laser output unit 10, extraction unit 20, first optical system 51, second optical system 52, and multiplexer 53 are arranged such that the optical lengths of the multiple reduced-detection beams from the output end of the light source 11 beam (the output end of one laser oscillator 15 in this example) to the multiplexer 53, and the optical length of the reference beam from the output end of the light source 11 (the output end of one laser oscillator 15 in this example) to the multiplexer 53, coincide with an error sufficiently smaller than the coherence length.

[0068] Furthermore, it is desirable that the attenuation amount of filter 63 be set so that the light intensity per unit area of ​​each reduced-detection beam and the light intensity per unit area of ​​the reference beam are approximately the same. This allows for efficient interference between multiple reduced-detection beams and the reference beam (for example, the visibility of the interference can be brought close to 100%).

[0069] The third optical system 54 guides multiple multiplexed beams to the photodetector 55. The third optical system 54 includes an array optical system 66, a fifth lens 67, and a sixth lens 68. The array optical system 66 extracts laser light from the region where the reference laser beam and each of the beams to be detected overlap in the cross-section of the multiplexed beam behind the multiplexer 53, and spatially separates the extracted multiple multiplexed beams.

[0070] The array optical system 66 is, for example, an aperture array having multiple through-holes that allows laser light (composite beam) from the region where the reference laser beam and the beam to be detected overlap in the cross-section of the combined beam behind the multiplexer 53 to pass through. The array optical system 66 has, for example, N through-holes. The through-holes have a shape approximately equal to the region where the reduced beam to be detected and the reference beam overlap in a plane perpendicular to the optical axis of the combined beam. For example, the diameter of the through-holes is approximately equal to the beam diameter of the reduced beam to be detected. It is desirable that the array optical system 66 allows only laser light (composite beam) from the region where the reference laser beam and the beam to be detected overlap to pass through.

[0071] Figure 5 shows an example where the fifth lens 67 and the sixth lens 68 are convex lenses, and the distance between them is equal to the sum of their respective focal lengths. Multiple combined beams, combined by the multiplexer 53, are incident on the fifth lens 67, and the incident multiple combined beams are focused onto the surface plane of the fifth lens 67. In the conventional laser device 1, a photodetector 32 was placed at the focal point where the multiple beams were focused, but in the laser device 2 of this embodiment, a photodetector 55 is placed at a position different from the focal point.

[0072] Similar to the first optical system 51, the sixth lens 68 and the fifth lens 67 form a pair and output multiple multiplexed beams as multiple reduced parallel beams. In other words, the fifth lens 67 and the sixth lens 68 are optical systems that reduce and image each multiplexed beam while keeping them spatially separated within the entire set of multiple multiplexed beams extracted by the array optical system 66. Therefore, each of the multiple multiplexed beams is incident on the photodetector 55 in a spatially arranged state so as not to overlap with each other. As described above, the third optical system 54 reduces and images the entire set of multiple multiplexed beams at the detection position of the multiple multiplexed beams.

[0073] The third optical system 54 only needs to be able to guide the multiple multiplexed beams to the photodetector 55 while keeping adjacent multiplexed beams separated to the extent that they do not interfere with each other, and may have other types of lenses, or may have lenses, mirrors, etc. As described above, the first optical system 51 and the third optical system 54 guide the multiple multiplexed beams (B3-1 to B3-N) to the photodetector 55 in such a way that the distance between two adjacent multiplexed beams is greater than or equal to the beam diameter of each multiplexed beam at the detection position of the multiple multiplexed beams, that is, each of the multiple multiplexed beams B3 does not overlap with any of the other multiplexed beams. As a result, while the conventional laser device 1 measures the interference intensity between all the beams to be detected in light intensity detection, the laser device 2 in this disclosure measures the interference intensity between each beam to be detected and the reference beam, which is a major difference.

[0074] The photodetector 55 has a light-receiving surface that receives multiple multiple beams that have been reduced and combined at the detection position of multiple multiple beams, and detects the light intensity obtained when the detected beam and the reference laser beam interfere in the multiple multiple multiple beams. The photodetector 55 includes, for example, a photodetector such as a photodiode. The photodetector 55 has a light-receiving surface that receives multiple multiple multiple beams separated by the array optical system 66 while keeping them separated. The photodetector 55 outputs the sum of the interference intensities between each of the detected beams and the reference beam among the multiple multiple multiple beams it has received as the detection result.

[0075] Furthermore, if the overall size of the reduced-to-detect beam is small, a photodetector 55 with a light-receiving surface large enough to detect the entire reduced-to-detect beam at once may be available. In this case, the fifth lens 67 and the sixth lens 68 can be omitted, and the photodetector 55 can be placed in close proximity immediately after the array optical system 66. In this way, if the array optical system 66 is an aperture array, placing the array optical system 66 and the photodetector 55 in close proximity can prevent the diffracted light from the aperture edge of the array optical system 66 corresponding to each reduced-to-detect beam from interfering with the adjacent reduced-to-detect beam.

[0076] The arithmetic unit 56 controls the amount of phase adjustment of the phase adjuster 12 so that the phases of the multiple laser beams match the phase of the reference laser beam, based on the detection results of the photodetector 55. The arithmetic unit 56 controls the phase control signal generator 57 to transmit a phase control signal from the phase control signal generator 57. For example, the arithmetic unit 56 transmits a control signal to the phase control signal generator 57 to transmit a phase control signal corresponding to the amount of phase adjustment to all phase adjusters 12. The phase control signal generator 57 generates a phase control signal to adjust the phase of the phase adjuster 12 in response to the control signal received from the arithmetic unit 56 and transmits it to all phase adjusters 12.

[0077] It goes without saying that when the amount of phase adjustment of the phase adjuster 12 changes, the phases of the amplified beam, the detected beam, and the reduced detected beam change according to the changed amount of phase adjustment. Therefore, in this embodiment, the act of the arithmetic unit 56 changing the amount of phase adjustment of the phase adjuster 12 may be expressed as "changing the phase of the laser beam," "changing the phase of the amplified beam," "changing the phase of the detected beam," "changing the phase of the reduced detected beam," etc.

[0078] The arithmetic unit 56 controls the phase control signal generator 57 by executing a program according to this embodiment via a personal computer, microcontroller, or FPGA. The arithmetic unit 56 has a memory circuit for storing parameters, programs, etc., used for calculations.

[0079] When the arithmetic unit 56 changes the phase of all N reduced-detection beams, for example, the phase difference between reduced-detection beam B2-1 and the reference beam changes, so the light intensity of the combined beam B3-1 increases or decreases according to the changed phase. However, reduced-detection beam B2-1 does not interfere with the other detected beams B2-2, B2-3, ..., B2-N. In this way, the light intensity of each combined beam depends only on the phase difference between each reduced-detection beam, i.e., each amplified beam and the reference beam, and the light intensity of each combined beam is maximum when the phases of each amplified beam and the reference beam coincide (when the phase difference is zero).

[0080] This allows the arithmetic unit 56 to determine the amount of adjustment for the phase of each of the multiple laser beams so that the sum of the light intensities of the combined beam detected by the photodetector 55 is kept near the maximum value within a predetermined error. Here, the predetermined error is preferably 10% or less, more preferably 5% or less, and even more preferably 3% or less.

[0081] For example, the arithmetic unit 56 can asymptotically match the phases of multiple laser beams to the phase of a reference beam by repeatedly controlling the amount of phase adjustment of the corresponding phase adjuster 12 for each multiplexed beam so that the detection result of the photodetector 55 is maximized. In this way, the arithmetic unit 56 can quickly output a coherently coupled beam by matching the phases of multiple amplified beams with simple control operations.

[0082] Furthermore, the arithmetic unit 56, similar to the conventional laser device 1, uses stochastic parallel gradient descent (SPGD) to change the phases of multiple laser beams and determines adjustment amounts, including the sign of the phases of the multiple laser beams, so that the sum of the light intensities of the multiple combined beams detected by the photodetector 55 increases. Here, the arithmetic unit 56 ensures that the light intensity detected by the photodetector 55 reaches its maximum value and is maintained near the maximum value as the number of SPGD iterations increases. For example, by continuously controlling the adjustment amounts, including the sign of the multiple laser beams, the arithmetic unit 56 maintains the sum of the light intensities detected by the photodetector 55 near the maximum value within a predetermined error, and keeps the phases of the multiple laser beams aligned with the phase of the reference beam.

[0083] In the laser device 2 according to this embodiment, as the amount of phase adjustment of multiple beams is repeatedly controlled, the output value of the photodetector 55 tends to increase monotonically more easily than in the conventional laser device 1. Therefore, even when using an SPGD in the same way as in the conventional laser device 1, the phase of the amplified beam can be matched to the phase of the reference beam at a faster rate.

[0084] Furthermore, in the laser device 2, the output value of the photodetector 55 for iterative control of the phase adjustment amount of multiple beams has only one maximum value, and that maximum value is equal to the maximum value. Therefore, the laser device 2 can avoid the sum of the light intensities of the combined beams stagnating at a maximum value smaller than the maximum value, and can match the phase of the amplified beam to the phase of the reference beam.

[0085] Although the laser device 2 according to this embodiment has been described as having one photodetector 55, it is not limited to this. The laser device 2 may have multiple photodetectors 55. Such a laser device 2 will be described next.

[0086] <First modified example of the coupling phase control unit 50> Figure 6 shows a first modified example of the coupled phase control unit 50 according to this embodiment. The laser device 2 may also use the coupled phase control unit 50 of the first modified example. The coupled phase control unit 50 has a plurality of photodetectors 55. Figure 6 illustrates two of the plurality of photodetectors 55, 55-1 and 55-2.

[0087] The third optical system 54 reduces and images multiple multiplex beams into groups at different detection positions. The third optical system 54 has multiple fifth lenses 67 and multiple sixth lenses 68. The paired fifth lenses 67 and sixth lenses 68 perform the same roles as the fifth lens 67 and sixth lens 68 described in Figure 5, reducing the multiple multiplex beams into groups and guiding them to the light-receiving surfaces of the corresponding photodetectors 55-1 and 55-2. The light-receiving surfaces of the multiple photodetectors 55 receive the multiple multiplex beams that the third optical system 54 has reduced and imaged into groups.

[0088] For example, the fifth lens 67-1 and the sixth lens 68-1 form a pair, reducing some of the multiple multiplexed beams (the beams of the first group) and outputting them as multiple beams that propagate parallel to the direction of propagation. The photodetector 55-1 then detects the sum of the light intensities obtained by the interference between the detected beam and the reference beam contained in each of the multiple multiplexed beams included in the first group.

[0089] Furthermore, for example, the fifth lens 67-2 is paired with the sixth lens 68-2 and reduces the beams not included in the first group (the beams of the second group) from among the multiple multiplexed beams and outputs them as multiple beams propagating parallel to the direction of propagation. The photodetector 55-2 then detects the sum of the light intensities obtained by the interference between the detected beam and the reference beam contained in each of the multiple multiplexed beams included in the second group.

[0090] <Example configuration of the third optical system 54 and photodetector 55 in the first modified example> Figure 7 is a side view showing an example configuration of the third optical system 54 and photodetector 55 in a first modified example according to this embodiment. The upper part of Figure 7 illustrates two pairs of fifth and sixth lenses and two photodetectors, similar to Figure 6, but there are four combinations of fifth lenses, sixth lenses and photodetectors (the number of groups is four). Nine multiplexed beams are incident on one photodetector 55. In other words, Figure 7 shows an example where the laser device 2 has 36 amplified beams.

[0091] The lower part of Figure 7 shows an example in which nine multiplexed beams are incident on each of the light-receiving surfaces of four photodetectors 55. In other words, the lower part of Figure 7 is a plane perpendicular to the optical axis of the multiplexed beams, showing a plan view of the light-receiving surfaces of the four photodetectors 55. The nine multiplexed beams are arranged in a 3x3 array, and the 36 multiplexed beams (and amplified beams) are arranged in a 6x6 array (two-dimensional array). In this embodiment, the pattern of multiple multiplexed beams received by one of the multiple photodetectors 55 shown in Figure 7 is called a divided interference pattern array. The entirety of multiple divided interference pattern arrays is called an interference pattern array.

[0092] Of the multiple multiplexed beams, two adjacent beams are separated by more than the diameter of each beam, so they do not interfere with each other. In other words, there are no interference fringes in the interference pattern of each multiplexed beam; the intensity is uniform. The interference intensity (light intensity) of each multiplexed beam takes its maximum value when the phases of the detected beam and the reference beam coincide, and becomes zero when the phases are shifted by π (= 180 degrees, i.e., half a wavelength). Therefore, the interference intensity of each multiplexed beam depends only on the phase difference between the detected beam and the reference beam of each multiplexed beam, and takes a positive continuous value between zero and the maximum value.

[0093] The arithmetic unit 56 controls the amount of phase adjustment of the multiple phase adjusters 12, which adjust the phase of multiple laser beams belonging to a group, for each group. For example, the arithmetic unit 56 changes the phase of each of the multiple laser beams belonging to the first group so that the light intensity detected by the photodetector 55-1 corresponding to the first group reaches its maximum value, that is, it determines the amount of phase adjustment for each of the multiple laser beams belonging to the first group.

[0094] The arithmetic unit 56 performs similar operations on the other groups to determine the amount of phase adjustment for the multiple beams belonging to the other groups. In this way, the arithmetic unit 56 changes the phase of each of the multiple laser beams belonging to each group to determine the amount of phase adjustment for each of the multiple laser beams belonging to each group so that the light intensity detected by the photodetector 55 corresponding to each group is maintained near the maximum value within a predetermined error.

[0095] The arithmetic unit 56 may execute the control operations of multiple groups in parallel, or it may execute the control operations of multiple groups sequentially. The arithmetic unit 56 controls the amount of phase adjustment of the corresponding phase tuner 12 for each detected beam so that the detection result of each photodetector 55 reaches its maximum value. The arithmetic unit 56 may also use SPGD for each group.

[0096] In SPGD, the number of iterations of phase control required to reach the maximum interference intensity is proportional to the number of beams. Therefore, by dividing the reduced detection beams into multiple groups and performing iterative control as in this example, the phase control operation of the entire laser device 2 can be executed at a higher speed.

[0097] As described above, the problem of staying at the maximum value in SPGD described with the conventional laser device 1 does not occur. Also, for each divided group, by making the sum of the interference intensities maximum, the phases of the respective detected beams (amplified beams) become equal and coincide with the phase of the reference beam. Next, SPGD applied to such a divided interference pattern array will be described.

[0098] <SPGD Applied to Divided Interference Pattern Array> Let the number of combined beams belonging to each group of the divided interference pattern array be M, and the signal intensity of the photodetector 55 corresponding to the interference intensity of the i-th combined beam be q i If the signal intensity corresponding to the sum of the interference intensities of each of the M combined beams is Q, the signal intensity Q can be expressed by the following equation.

Equation

[0099] In each group, let the change amount of the control parameter u i for the detected beam included in the i-th combined beam be δu i Then, Qp(u1 + δu1, u2 + δu2, ···, u M + δu M ) and Qm(u1 - δu1, u2 - δu2, ···, u M - δu M ) take different values, and the difference between Qp and Qm is represented as ΔQ. <00003八十七>

Equation

[0100] To maximize the sum Q of interference intensities, equation (2) can be transformed as follows. When moving from the j-th iteration to the next step (the j+1th iteration), the control parameters are updated sequentially according to the following equation.

number

[0101] <Simulation results of control operation> Figure 8 shows the simulation results of the control operation when using SPGD in the laser device 2 according to this embodiment. In Figure 8, similar to the simulation results shown in Figure 3, it is assumed that the multi-beam laser output unit 10 outputs 36 amplified beams having the same output intensity. In the simulation, the initial phase of each of the 36 beams was set randomly, and it was assumed that there were no phase disturbances in each beam optical path from the beam splitter 46 to the main amplifier 14 during the time period in which iterative phase control was performed.

[0102] In Figure 8, the horizontal axis represents the number of iterations of phase control, and the vertical axis represents the power of the combined output beam normalized by the average power of one beam. In this case, if the vertical axis reaches 36, it means that the phases of all 36 beams are perfectly aligned. In Figure 8, the solid line labeled C5 shows the result of applying SPGD to 36 beams divided into 6 groups of 6 beams each. Note that curve C5 is a plot of the result of one trial when the update gain γ is 0.7, as the path to the maximum value in each simulation trial was stable.

[0103] Furthermore, the dashed curve labeled C1 is the simulation result (reproduced from Figure 3) of applying SPGD when the update gain γ is 0.02 in the conventional laser device 1. Curve C1 is the result of plotting the average value of 20 trials because the path to the maximum value changes significantly with each simulation trial. Comparing the two curves C5 and C1, it can be seen that with the laser device 2 according to this embodiment, the number of iterations required for the power of the combined output beam to reach the maximum value of 36 is about 75, which is about 1 / 5.3 of the number of iterations compared to the conventional method. This iteration ratio is close to the ratio of the number of beams received by the photodetector 55, which is 6 (=36 / 6), i.e., the number of beam divisions, so it can be seen that the time required for coherent coupling can be shortened according to the number of beam divisions.

[0104] Figure 9 shows the simulation results of the control operation stability when using SPGD in the laser device 2 according to this embodiment. In the simulation shown in Figure 9, it was assumed that the phase of each of the 36 beams having the same power starts from a random initial value and is subjected to sinusoidal disturbances over time, and was given by the following equation.

number

[0105] δφ is the amplitude of the phase disturbance, t is the time, and T is the period of the phase disturbance, with both t and T being the time required for one step of iterative control. θ is the phase of the sinusoidal phase disturbance, and the subscript k represents the k-th detected beam. δφ was randomly assigned between zero and π (=180 degrees), T was randomly assigned between 670 and 1120 steps with an inter-beam average of 840 steps, and θ was randomly assigned between zero and 2π.

[0106] In Figure 9, the curve labeled C6 shows how the power of the combined output beam changes with each step of the iterative phase control when 36 beams are divided into 6 groups of 6 beams each and subjected to SPGD control. The results of three trials are plotted, and in each trial, the maximum value is reached after approximately 100 iterations, and then a stable output is maintained for 900 steps.

[0107] In contrast, the curve labeled C7 shows the simulation results when SPGD is applied to the conventional laser device 1 under the same disturbance. Curve C7 shows different behavior in each of the three trials, and it can be seen that the power of the combined output beam not only remains below the maximum value but also fluctuates significantly. As described above, it can be seen that the laser device 2 according to this embodiment can coherently combine and output multiple beams at a faster speed and more stably compared to the conventional laser device 1.

[0108] For the case of a phase disturbance similar to that shown in curve C6 of Figure 9, simulations were performed by changing the number of groups into which multiple multiplexed beams were divided. Table 1 summarizes the relationship between the average value of the time change of coherent coupling efficiency and the number of group divisions, based on the results of simulations in which 36 multiplexed beams were divided into 6 groups of 6 beams each, 4 groups of 9 beams each, 3 groups of 12 beams each, and 2 groups of 18 beams each. For comparison, the simulation results using conventional laser device 1 are also shown as "no reference beam". [Table 1]

[0109] The average coupling efficiency shown in Table 1 is defined as the average value of the coupled output beam power between 200 and 1000 iterations, divided by the maximum coupled power of the 36 beams. The coefficient of variation is the standard deviation of the coupling output beam power variation between 200 and 1000 iterations divided by the mean value. Both the coupling efficiency and the coefficient of variation are expressed as percentages.

[0110] Table 1 shows that, compared to the coupling loss (approximately 20%) and power variation coefficient (9%) obtained by SPGD control using conventional laser device 1, the SPGD control using laser device 2 according to this embodiment achieves both high efficiency and high stability. Furthermore, it can be seen that increasing the number of group divisions improves the average coupling efficiency and the variation coefficient.

[0111] <Second modified example of coupling phase control unit 50> In the laser device 2 according to this embodiment, an example has been described in which the light source 41 outputs a reference laser beam, but the invention is not limited to this. One of the multiple amplified beams output and amplified by the light source 41 may be used as the reference beam. Such a laser device 2 will be described next.

[0112] When using one of the multiple amplified beams originating from the light source 41 as the reference beam, the overall configuration of the laser device 2 is similar to the configuration example shown in Figure 1, in which the multi-beam laser output unit 40 (light source 41) does not output a reference laser beam. Furthermore, the reference laser beam is not input to the coupled phase control unit 50. Then, within the coupled phase control unit 50, one of the detected beams is extracted as the reference laser beam. Such a coupled phase control unit 50 will be described next.

[0113] Figure 10 shows a second modified example of the coupled phase control unit 50 according to this embodiment. In the coupled phase control unit 50 of the second modified example, components whose operation is substantially the same as those of the coupled phase control unit 50 of the first modified example described in Figure 6 are given the same reference numerals, and redundant explanations are omitted. Also, in Figure 10, the arithmetic unit 56 and the phase control signal generator 57 are omitted from the description. The coupled phase control unit 50 of the second modified example further includes a reference laser beam selector / injector 70.

[0114] The reference laser beam selector / injector 70 extracts a reference laser beam from among the multiple laser beams extracted by the extraction unit 20. The reference laser beam selector / injector 70 includes a beam splitter 71, an aperture 72, a seventh lens 73, an eighth lens 74, and a first reflector 75.

[0115] The beam splitter 71 splits the multiple reduced beams to be detected output from the first optical system 51. The light that passes through the beam splitter 71 is guided to the photodetector 55 as multiple reduced beams to be detected. The first optical system 51 further includes a second reflector 76 and an optical length adjustment plate 77.

[0116] The second reflector 76 reflects the multiple reduced-detection beams that have passed through the beam splitter 71 toward the photodetector 55. The optical length adjustment plate 77 is an optical element for adjusting the optical length of the multiple reduced-detection beams, as will be described later. The optical length adjustment plate 77 may be provided between the second reflector 76 and the multiplexer 53 as shown in Figure 10, or between the beam splitter 71 and the second reflector 76. Similarly, the filter 63 may be provided between the second reflector 76 and the multiplexer 53 as shown in Figure 10, or between the beam splitter 71 and the second reflector 76.

[0117] Light reflected by the beam splitter 71 is directed towards the aperture 72. The aperture 72 extracts one of the multiple reduced-detection beams reflected by the beam splitter 71. Figure 10 shows an example where the aperture 72 extracts the reduced-detection beam B1-4 as the reference beam and uses it as the reference beam. The seventh lens 73 and the eighth lens 74 enlarge the beam diameter of the reference beam extracted by the aperture 72. Figure 10 shows an example where the seventh lens 73 and the eighth lens 74 are convex lenses. The first reflector 75 reflects the enlarged reference beam towards the multiplexer 53.

[0118] The multiplexer 53 combines each of the multiple reduced-detection beams that have passed through the beam splitter 71 with the reference beam, which is one of the multiple reduced-detection beams reflected by the beam splitter 71, so that they propagate in parallel with each other, and guides them to one or more photodetectors 55. In this way, the multiplexer 53 combines the multiple detection beams extracted by the extraction unit 20 and the reference laser beam by superimposing them in parallel with an angular error smaller than the diffraction-limited beam divergence angle.

[0119] The operation of one or more photodetectors 55 that receive the combined beam is the same as described above, so a redundant explanation is omitted. As described above, the coupled phase control unit 50 of the second modified example uses one of the multiple amplified beams as a reference laser beam and matches the phases of the multiple amplified beams to the phase of that one amplified beam. This configuration of the coupled phase control unit 50 of the second modified example functions as a Mach-Zehnder interferometer.

[0120] Furthermore, it is desirable to set the interference visibility of the Mach-Zehnder interferometer to 100%. Therefore, the transmittance of the filter 63 is adjusted so that the beam intensity passing through the right-handed optical path on the plane of the paper (the optical path of the reference laser beam reflected by the beam splitter 71) and the beam intensity passing through the left-handed optical path on the plane of the paper (the optical paths of the multiple reduced undetected beams that passed through the beam splitter 71) are approximately equal. In addition, it is desirable that the difference in optical length between the right-handed and left-handed optical paths be sufficiently shorter than the coherence length. To this end, the optical length of the optical length adjustment plate 77 plays a role in canceling out the difference between the increase in optical length due to the lens pair (73, 74) inserted in the right-handed optical path and the increase in optical length due to the filter 63 inserted in the left-handed optical path. The optical length adjustment plate 77 is, for example, a plate-shaped member of transparent glass material having a thickness corresponding to the amount of optical length adjustment.

[0121] According to the coupled phase control unit 50 of the second modified example described above, the coupled phase control unit 50 of the second modified example eliminates the need for optical systems and the like from the light source for outputting the reference laser beam to the coupled phase control unit 50. Furthermore, since the phase of one amplified beam, which is also used as the reference laser beam, is used as the reference, a phase adjuster 12 for adjusting the phase of the one amplified beam is not required. In addition, a simple fixed phase shifter or the like may be used as the phase adjuster 12 for the one amplified beam.

[0122] In the laser device 2 of this embodiment described above, an example has been explained in which the photodetector 55 receives multiple multiplex beams and detects the sum of the light intensities of the received multiple multiplex beams, but the invention is not limited to this. Alternatively, the photodetector 55 may have multiple photodetectors and detect the light intensity of each of the multiple multiplex beams. The photodetector 55 may have, for example, an image detector such as a charge-coupled device (CCD) or CMOS. The coupled phase control unit 50 of such a laser device 2 will be described next.

[0123] <Third modified example of coupling phase control unit 50> Figure 11 shows a third modified example of the coupled phase control unit 50 according to this embodiment. In the coupled phase control unit 50 of the third modified example, components that are substantially the same as those in the coupled phase control unit 50 described in Figures 5, 6, and 10 are given the same reference numerals, and redundant explanations are omitted. In other words, the laser device 2 may have the configuration of the third modified example shown in Figure 11 instead of the configuration from the array optical system 66 to the photodetector 55 of the coupled phase control unit 50 described in Figures 5, 6, and 10.

[0124] As described above, the array optical system 66 allows multiple multiplex beams to pass through multiple through-holes. In Figure 11, the multiple multiplex beams are assumed to be arranged in a two-dimensional array of 6 rows and 6 columns. The photodetector 55 receives these multiple multiplex beams arranged in this two-dimensional array, identifies them as an image, and detects them.

[0125] The right-hand figure in Figure 11 shows an example of an image detected by the photodetector 55 in the third modified example. The image detected by the photodetector 55 is a collection of interference patterns created by each of the multiple multiplexed beams. Figure 11 illustrates that the interference pattern array is 6 x 6, corresponding to the multiple multiplexed beams being arranged in a 6 x 6 two-dimensional array. Each pixel region corresponding to a multiplexed beam detects the interference intensity between the beam under detection and the reference laser beam, and its detection output (charge amount, etc.) is maximized when the phase of the beam under detection matches the phase of the reference laser beam.

[0126] The arithmetic unit 56 may control the amount of phase adjustment of the phase tuner 12 corresponding to each multiplexed beam so that the detection output of the pixel region corresponding to each multiplexed beam is maximized. Alternatively, the arithmetic unit 56 may calculate the sum of the detection outputs of the pixel regions corresponding to a group of multiplexed beams and, as described above, control the amount of phase adjustment of the phase tuner 12 of each beam belonging to the group using SPGD.

[0127] For example, the arithmetic unit 56 calculates the sum of the light intensities of 36 multiplexed beams arranged in a 6x6 grid, and controls the phase adjustment amount of 36 (35 if the reference laser beam is treated as a single amplified beam) phase adjusters 12 using SPGD. Alternatively, the arithmetic unit 56 may divide the multiple multiplexed beams into multiple groups, calculate the sum of the light intensities for each group, and control the phase adjustment amount of the phase adjusters 12 for the beams belonging to each group. In this case, when grouping the multiplexed beams, one can select from a predetermined grouping method, or the grouping can be optimized automatically. Note that the control operation by the arithmetic unit 56 has already been explained and will be omitted here.

[0128] The diagram on the right in Figure 11 shows the case where the number of divisions is set to 4, as indicated by the dashed auxiliary lines. It illustrates an example where 36 multiplexed beams are divided into 4 groups of 9 beams arranged in a 3x3 grid. The arithmetic unit 56 then controls the amount of phase adjustment of the multiple phase adjusters 12, which adjust the phase of the multiple laser beams belonging to each divided group.

[0129] As explained in Figure 9 and Table 1, the light intensity of the combined beam can fluctuate more due to the environment in which the laser device 2 is installed, the aging of the laser device 2, etc. Simulation results show that SPGD control can be performed more stably by dividing the beam into a larger number of groups to address such fluctuations. In addition, the magnitude and speed of phase disturbance may differ for each beam due to differences in the characteristics and degree of aging of each preamplifier and main amplifier in the laser device 2. Therefore, it is desirable for the arithmetic unit 56 to optimize the group division on-site based on the amount of fluctuation in the light intensity of each combined beam detected by the photodetector 55.

[0130] In this case, the arithmetic unit 56 may perform SPGD control and then determine the number of group divisions according to the result. As described above, the light intensity of the multiplexed beam may fluctuate, so for example, if SPGD control is performed with a predetermined number of divisions (1 as an example), the sum of the light intensities of the multiplexed beams with respect to the number of SPGD iterations will fluctuate in accordance with the amount of fluctuation in light intensity. Therefore, the arithmetic unit 56 determines the number of group divisions so that the fluctuation of the sum of the light intensities of the multiplexed beams with respect to the number of SPGD iterations is less than or equal to a target value. At this time, the number of beams may differ for each group. Furthermore, instead of this group division method, the arithmetic unit 56 may optimize the combination of beams to be grouped so that the fluctuation of the sum of the light intensities of the multiplexed beams with respect to the number of SPGD iterations is less than or equal to a target value. To optimize group division in the field, methods such as machine learning can be used.

[0131] The arithmetic unit 56, for example, performs SPGD control while changing the group division, and calculates and stores the amount of variation in the total light intensity of the combined beam for all groups. When the amount of variation in the total light intensity of the combined beam for all groups is minimized and below the target value, the arithmetic unit 56 determines that the group division is optimal for the field and continues SPGD control under the optimal group division. In this way, the arithmetic unit 56 can change the group division based on the amount of variation in the total light intensity measured for each group, so that the light intensity for each group is maximized and the variation in the light intensity for each group is below the target value.

[0132] As described above, the laser device 2 according to this embodiment can divide multiple multiplexed beams into an appropriate number of groups or group them into appropriate combinations through SPGD trials. Then, the laser device 2 can output multiple coherent beams with high efficiency and stability through optimized SPGD.

[0133] <Modified form of array optical system 66> In the laser apparatus 2 according to this embodiment described above, an example was given in which the array optical system 66 is an aperture array having multiple through holes, but it is not limited to this. The array optical system 66 only needs to be able to extract light from the spatial region where each of the multiple reduced-detection beams overlaps with the reference beam. A modified example of such an array optical system 66 will be described next.

[0134] Figure 12 shows a modified example of the array optical system 66 according to this embodiment. In the coupled phase control unit 50 shown in Figure 12, components whose operation is substantially the same as those described in Figure 5 are given the same reference numerals, and redundant explanations are omitted. The modified array optical system 66 is an optical fiber array in which multiple optical fibers are arranged in an array.

[0135] Multiple optical fibers each extract light from at least a portion of the spatial region where each of the multiple reduced beams to be detected overlaps with the reference beam. The multiple optical fibers then guide the extracted light to the photodetector 55. Even with such an array optical system 66, it is possible to extract laser light from the region where the reference laser beam and each of the beams to be detected overlap in the cross-section of the combined beam behind the multiplexer 53, and guide the extracted multiple combined beams to the detection position of the photodetector 55 in a spatially separated state.

[0136] According to the laser device 2 of this embodiment described above, the phases of multiple laser beams can be matched to the phase of a reference laser beam, and the multiple beams can be coherently coupled and output.

[0137] Although the present invention has been described above using embodiments, the technical scope of the present invention is not limited to the scope described in the above embodiments, and various modifications and changes are possible within the scope of its gist. For example, all or part of the apparatus can be configured by functionally or physically distributing and integrating in any unit. Furthermore, new embodiments resulting from any combination of multiple embodiments are also included in the embodiments of the present invention. The effects of the new embodiments resulting from the combinations are combined with the effects of the original embodiments. [Explanation of Symbols]

[0138] 1. Laser device 2. Laser device 10 Multi-beam laser output section 11 Light source 12 Phase Controller 13 Preamplifier 14 Main amplifier 15. Laser Oscillator 16 Beam Splitter 20 Extraction part 30 Coupled Phase Control Unit 31 Optical system 32 Photodetectors 33 Arithmetic unit 34 Phase control signal generator 35 lens 36 pinholes 40 Multi-beam laser output section 41 Light source 43 Preamplifier 46 Beam Splitter 51 1st optical system 52 Second optical system 53 Multiplexer 54 Third optical system 55 Photodetector 56 Arithmetic unit 57 Phase control signal generator 61 First Lens 62 Second Lens 63 filters 64 Third Lens 65 Fourth lens 66 Array Optical Systems 67 Fifth lens 68 Lens No. 68 70 Reference Laser Beam Selection / Injector 71 Beam Splitter 72 Aperture 73. Lens No. 7 74. Lens No. 8 75 1st reflector 76 Second reflector 77 Optical length adjustment plate 100 First Laser Device 200 Second Laser Device

Claims

1. A light source that outputs multiple laser beams of the same wavelength and a reference laser beam of the same wavelength as the multiple laser beams, Multiple phase adjusters for adjusting the phase of each of the multiple laser beams, A multi-beam laser output unit that outputs multiple laser beams whose phases have been adjusted by multiple phase adjusters, such that each of them does not overlap with one another. An extraction unit that extracts portions of the light from multiple laser beams, which are output parallel to each other so as not to overlap, as multiple detected beams, A multiplexer that combines each of the multiple detected beams extracted by the extraction unit with the reference laser beam to output multiple multiple beams that do not overlap with each other, An array optical system that extracts laser light from the region where the reference laser beam and each of the detected beams overlap in a cross-section of the multiplexed beam behind the multiplexer, and spatially separates the extracted multiplexed beams, One or more photodetectors are provided at the detection positions of the multiple multiple beams, each having a light-receiving surface for receiving the multiple multiple beams, and for detecting the light intensity obtained when the detected beam and the reference laser beam interfere in the multiple multiple beams. A calculator that controls the amount of phase adjustment of the phase adjuster so that the phases of the multiple laser beams match the phase of the reference laser beam, based on the detection results of the photodetector. Equipped with, Laser device.

2. The multi-beam laser output unit outputs the laser beams parallel to each other with an angular error smaller than the diffraction-limited beam divergence total angle. The multiplexer combines each of the multiple detected beams and the reference laser beam parallel to each other with an angular error smaller than the diffraction-limited beam divergence total angle. The laser apparatus according to claim 1.

3. The optical system further comprises an optical system that reduces and images the entire set of multiple multiplex beams extracted by the array optical system while keeping them spatially separated. The laser apparatus according to claim 1.

4. The calculator determines the amount of adjustment for the phase of each of the multiple laser beams so that the sum of the light intensities of the combined beams detected by the photodetector is maintained near the maximum value within a predetermined error by changing the phase of each of the multiple laser beams. The laser apparatus according to claim 1.

5. The optical system further comprises reducing and imaging multiple multiple beams at different detection positions for each of multiple groups, The light-receiving surfaces of the multiple photodetectors receive the multiple multiple beams that the optical system has reduced and imaged for each group, The calculator controls, for each group, the amount of phase adjustment of the multiple phase adjusters that adjust the phase of the multiple laser beams belonging to the group. The laser apparatus according to claim 1.

6. The calculator changes the phase of each of the multiple laser beams belonging to each group, and determines the amount of phase adjustment for each of the multiple laser beams belonging to each group, such that the light intensity detected by the photodetector corresponding to each group is maintained near the maximum value. The laser apparatus according to claim 5.

7. The light source outputs one of the plurality of laser beams as the reference laser beam. In the above-mentioned multiplexer, the plurality of detected beams extracted by the extraction unit and the reference laser beam are superimposed in parallel with an angular error smaller than the diffraction-limited beam divergence total angle, The laser apparatus according to claim 1.

8. The aforementioned arithmetic unit, Using a stochastic parallel gradient descent method, while changing the phases of the multiple laser beams, an adjustment amount including the sign of the phases of the multiple laser beams is determined such that the sum of the light intensities of the multiple laser beams detected by the photodetector increases. By continuously controlling the adjustment amount, including the sign of the multiple laser beams, the sum of the light intensities detected by the photodetector is kept near the maximum value within a predetermined error, and the phases of the multiple laser beams are kept aligned with the phase of the reference beam. The laser apparatus according to claim 1.

9. The photodetector has multiple photodetectors and detects the light intensity of each of the multiple multiple beams, In the aforementioned computing unit, the multiple photodetection results corresponding to the multiple multiplexed beams are divided into multiple groups, and the amount of phase adjustment of the multiple phase adjusters that adjust the phase of the multiple laser beams belonging to the group is controlled based on the amount of variation in light intensity detected by the photodetector for each of the divided groups. The laser apparatus according to claim 1.

10. The photodetector identifies and detects the light intensity of each of the multiple multiplexed beams as an image. The aforementioned arithmetic unit, Multiple photodetection results corresponding to multiple multiplexed beams are divided into multiple groups, and the amount of variation in the sum of light intensity is measured for each of the divided groups. Based on the amount of variation in the sum of the light intensities measured for each group, the amount of phase adjustment of the multiple phase adjusters that adjust the phases of the multiple laser beams belonging to the group is controlled. Based on the amount of variation in the sum of the light intensities measured for each group, the grouping is changed so that the light intensity for each group is maximized and the variation in light intensity for each group is less than or equal to a target value. The laser apparatus according to claim 1.

11. The steps include outputting multiple laser beams of the same wavelength and a reference laser beam of the same wavelength as the multiple laser beams, The steps include: outputting each of the multiple phase-adjusted laser beams so that they do not overlap with each other; The steps include: extracting a portion of the light from multiple phase-adjusted laser beams as multiple detected beams; The steps include combining each of the extracted laser beams with the reference laser beam to output multiple combined beams that do not overlap with each other, The steps include extracting laser light from the region where the reference laser beam and each of the detected beams overlap in a cross-section of the multiplexed beam, and spatially separating the extracted multiplexed beams, The steps include: detecting multiple multiple beams at detection positions, receiving light from multiple multiple beams, and detecting the light intensity obtained when the detected beam and the reference laser beam interfere in the multiple multiple beams; Based on the detection result of the light intensity, the steps include controlling the amount of phase adjustment of the multiple laser beams to adjust the phase of each of the multiple laser beams so that the phase of each of the multiple laser beams matches the phase of the reference beam, Having, A method of coherently combining and outputting multiple laser beams.

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