Annular laser system

Abstract

An annular laser system (1) is provided with: an annular laser combining device (3) that spatially and coaxially combines annular laser beams emitted by annular laser devices (2A, 2B, 2C) coaxially; and a focusing optical system (4) that focuses the beam combined by the annular laser combining device (3). The annular laser devices (2A, 2B, 2C) emit annular laser beams having different inner diameters and outer diameters at different divergence angles. The divergence angles are angles at which aberration at the focal point is reduced on condition that the focusing optical system (4) has a spherical surface not including higher-order aspheric coefficients, has a parabolic surface, or has a hyperbolic surface.

Description

Circular Laser System

[0001] The present disclosure relates to a ring laser system.

[0002] Circular laser light can be generated in various ways and is used in industrial fields such as laser processing. Since the power obtainable from a single laser beam is usually limited, beam combining technology is known, which combines multiple laser beams to obtain greater power. For example, spatial combining, which is one form of beam combining technology, does not improve brightness because the beams are incoherent and do not overlap. However, spatial combining can increase the power by the number of laser beams, so it is used as a simple power increase method.

[0003] Spatial compounding of beams such as Gaussian beams is achieved by arranging the beams in a line or in a square or triangular lattice pattern. On the other hand, spatial compounding of annular beams with a circular cross section can be achieved by arranging the beams in a lattice pattern as described above, or by arranging them in the radial direction of the ring. In this case, the outer and inner diameters of the rings of the multiple annular laser beams to be spatially compounded are different from each other.

[0004] For example, Patent Document 1 describes a laser beam combining device that spatially combines a plurality of circular laser beams. The laser beam combining device described in Patent Document 1 includes a plurality of shaping optical units that emit circular laser beams with different amounts of change in outer dimension per unit propagation distance, and the shaping optical units are arranged so that the emitted circular laser beams are concentric.

[0005] International Publication No. 2015 / 06417

[0006] In conventional circular laser systems, if the divergence angles of the circular laser beams are non-uniform, the focusing optical system cannot properly correct the aberrations of the spatially combined beams. For example, due to spherical aberration, a circular laser beam with a large diameter may be focused at a point shorter than the focal length, and the spatially combined beam may not be focused at a single point in the propagation direction.

[0007] The present disclosure is intended to solve the above-mentioned problems, and aims to provide a circular laser system that can reduce the effects of aberrations in a beam formed by spatially combining multiple circular laser beams.

[0008] The circular laser system according to the present disclosure comprises a plurality of circular laser devices, each of which emits a circular laser beam having a hollow intensity distribution in a cross section perpendicular to the beam propagation direction; a circular laser combining device that spatially combines the plurality of circular laser beams emitted by the plurality of circular laser devices on a coaxial line; and a focusing optical system that focuses the beam combined by the circular laser combining device, wherein the plurality of circular laser devices emit circular laser beams having different inner and outer diameters, each at a different divergence angle, and the divergence angle is an angle at which aberrations at the focusing point of the focusing optical system are reduced, provided that the focusing optical system is a sphere that does not contain a high-order aspherical coefficient, or a parabolic surface, or a hyperbolic surface.

[0009] According to the present disclosure, the plurality of circular laser devices are configured to have divergence angles of the circular laser beams that reduce aberrations at the focusing point of the focusing optical system, provided that the focusing optical system is a spherical surface that does not include a high-order aspherical coefficient, or a parabolic surface, or a hyperbolic surface. This allows the circular laser system according to the present disclosure to reduce the influence of aberrations on the beam that is spatially combined from the plurality of circular laser beams.

[0010] 1 is a block diagram showing a configuration of a conventional circular laser system according to a first embodiment of the present invention;

[0011] First Embodiment First, a conventional circular laser system and its problems will be described with reference to FIG. 1. Note that in the following description, the polarization distribution of the circular laser will not be considered. FIG. 1 is a block diagram showing the configuration of a conventional circular laser system 100. In FIG. 1, the circular laser system 100 combines circular laser beams emitted by circular laser devices 101A, 101B, and 101C, respectively, and focuses and irradiates the combined beam. In addition to the circular laser devices 101A, 101B, and 101C, the circular laser system 100 includes a circular laser combining device 102 and a focusing optical system 103.

[0012] The circular laser devices 101A, 101B, and 101C generate circular laser beams. The circular laser beams have a hollow intensity distribution in a cross section perpendicular to the beam propagation direction. To generate the circular laser beams, the circular laser devices 101A, 101B, and 101C include, for example, an oscillator consisting of an unstable resonator, an axicon optical system for converting a Gaussian beam into a circular beam, or a combiner for combining the TEM01 mode and TEM10 mode of a Hermite-Gaussian beam with a phase difference.

[0013] The circular laser devices 101A, 101B, and 101C emit circular laser beams having different inner and outer diameters. For example, as shown in FIG. 1, the circular laser beam emitted from the circular laser device 101A has the largest inner and outer diameters, and the circular laser beam emitted from the circular laser device 101C has the smallest inner and outer diameters. Furthermore, the circular laser beam emitted from the circular laser device 101B has smaller inner and outer diameters than the circular laser beam emitted from the circular laser device 101A, but larger than the circular laser beam emitted from the circular laser device 101B.

[0014] For example, if the annular laser device includes an unstable resonator, the inner and outer diameters of the annular laser beam are determined by the size of the mirrors that make up the unstable resonator. If the annular laser device includes an axicon optical system, a circular laser beam can be generated by passing through the axicon optical system, and the inner and outer diameters of the circular laser beam can be controlled by changing the apex angle or its installation interval. Furthermore, if the annular laser beam is generated by synthesizing Hermite-Gaussian beams, the inner and outer diameters of the circular laser beam can be determined by adjusting the focal length of the collimating lens, etc.

[0015] In the conventional ring laser system 100, the ring laser devices 101A, 101B, and 101C output a ring laser beam as a substantially parallel beam, as shown in FIG. 1 . For example, if the ring laser device includes an unstable resonator, a substantially parallel ring laser beam can be emitted by adjusting the radius of curvature of the mirrors that make up the unstable resonator or an optical system such as a lens arranged downstream of the oscillator that makes up the unstable resonator. If the ring laser device includes an axicon optical system, a substantially parallel ring laser beam can be emitted by configuring the device with two axicon optical elements and setting their apex angles to approximately the same. Furthermore, in the synthesis of Hermite-Gaussian beams, a substantially parallel ring laser beam can be emitted by adjusting an optical system such as a lens.

[0016] The circular laser combining device 102 spatially combines circular laser beams having different inner and outer diameters emitted from the circular laser devices 101A, 101B, and 101C on a coaxial line with the centers of the rings aligned. For example, the circular laser combining device 102 can combine multiple circular laser beams by using a perforated mirror. When combining a first circular laser beam and a second circular laser beam, the perforated mirror is tilted 45° with respect to the beam propagation direction of the first circular laser beam and positioned so that the first circular laser beam travels straight through the hole in the mirror. The first circular laser beam and the second circular laser beam can be combined on a coaxial line by reflecting the second circular laser beam, which has a larger diameter than the first circular laser beam, in the direction through which the first circular laser beam passed.

[0017] The focusing optical system 103 is composed of a refractive or reflective optical system, and when approximately parallel light is input, it outputs focused light. For example, the focusing optical system 103 may be a single lens or a combination of multiple lenses. When irradiating the combined beam at a long distance, such as in spatial transmission, or when focusing the combined beam to a small spot on the order of μm, the beam is expanded and then focused using a beam expander or the like. Note that each optical element constituting the focusing optical system 103 has a spherical shape.

[0018] In the conventional annular laser system 100, if the divergence angle of the annular laser beam is non-uniform, the focusing optical system 103 cannot properly correct the aberration of the spatially combined beam. Therefore, due to spherical aberration at the focusing point of the combined beam, the annular laser beam with a large diameter is focused at a point shorter than the focal length, and is not focused at one point in the beam propagation direction as shown in FIG.

[0019] In contrast, the circular laser system according to the first embodiment emits a plurality of circular laser beams at different divergence angles that reduce aberrations at the focusing point, and spatially combines and outputs the beams, provided that the focusing optical system is a sphere that does not include a high-order aspherical coefficient, or a paraboloid, or a hyperboloid. Because the circular laser beams are emitted at divergence angles that reduce aberrations at the focusing point, the circular laser system 1 can reduce aberrations in the beam that is spatially combined from the plurality of circular laser beams.

[0020] 2 is a block diagram showing the configuration of the ring laser system 1 according to the first embodiment. In FIG. 2, the ring laser system 1 combines the ring laser beams emitted by the ring laser devices 2A, 2B, and 2C, and condenses and irradiates the combined beam. In addition to the ring laser devices 2A, 2B, and 2C, the ring laser system 1 includes a ring laser combining device 3 and a condensing optical system 4.

[0021] The circular laser beams generated by the circular laser devices 2A, 2B, and 2C are hollow beams with an intensity distribution in a cross section perpendicular to the beam propagation direction, similar to Fig. 1. In order to generate the circular laser beams, the circular laser devices 2A, 2B, and 2C are equipped with, for example, an oscillator consisting of an unstable resonator, an axicon optical system for converting a Gaussian beam into a circular beam, or a combiner for combining the TEM01 mode and TEM10 mode of a Hermite-Gaussian beam with a phase difference.

[0022] The circular laser devices 2A, 2B, and 2C emit circular laser beams whose inner and outer diameters differ from one another. For example, as shown in FIG. 2, the circular laser beam emitted from the circular laser device 2A has the largest inner and outer diameters. The circular laser beam emitted from the circular laser device 2C has the smallest inner and outer diameters. The circular laser beam emitted from the circular laser device 2B has smaller inner and outer diameters than the circular laser beam emitted from the circular laser device 2A, but larger than the circular laser beam emitted from the circular laser device 2B.

[0023] For example, if the ring laser devices 2A, 2B, and 2C include an unstable resonator, the inner and outer diameters of the ring laser beam are determined by the size of the mirrors that make up the unstable resonator. If the ring laser devices 2A, 2B, and 2C include an axicon optical system, a ring laser beam can be generated by passing through the axicon optical system, and the inner and outer diameters of the ring laser beam can be controlled by changing the apex angle or the installation interval. If the ring laser devices 2A, 2B, and 2C generate a ring laser beam by synthesizing Hermite-Gaussian beams, the inner and outer diameters of the ring laser beam can be determined by adjusting the focal length of the collimator lens, etc.

[0024] The circular laser combining device 3 spatially combines circular laser beams having different inner and outer diameters emitted from the circular laser devices 2A, 2B, and 2C on a coaxial line with the centers of the rings aligned. For example, the circular laser combining device 3 can combine multiple circular laser beams by using a perforated mirror. When combining a first circular laser beam and a second circular laser beam, the perforated mirror is tilted 45° with respect to the beam propagation direction of the first circular laser beam and positioned so that the first circular laser beam travels straight through the hole in the mirror. By reflecting the second circular laser beam, which has a larger diameter than the first circular laser beam, in the direction through which the first circular laser beam passed, the first circular laser beam and the second circular laser beam can be combined on a coaxial line.

[0025] The focusing optical system 4 is composed of a refractive or reflective optical system, and when approximately parallel light is input, it outputs focused light. The focusing optical system 4 may be a single lens or a combination of multiple lenses. When irradiating the combined beam at a long distance, such as in spatial transmission, or when focusing the combined beam to a small spot on the order of μm, the beam is expanded using a beam expander or the like before being focused. Each optical element constituting the focusing optical system 4 is spherical in shape.

[0026] Like the ring laser system 100, the ring laser devices 2A, 2B, and 2C generate a ring laser beam of approximately parallel light, but each emits it at a different small divergence angle. The divergence angle is selected to be a value that geometrically reduces the focal point depending on the focusing optical system 4. That is, in the ring laser system 1, the shape of the focusing optical system 4 is not determined in accordance with the beam input to the focusing optical system 4, but the divergence angle of the ring laser beam emitted by the ring laser device is determined so that aberration at the focal point of the focusing optical system 4 is corrected.

[0027] The annular laser combining device 3 combines three annular laser beams having different diameters emitted by the annular laser devices 2A, 2B, and 2C coaxially to combine them. The annular laser beams pass through axially symmetric annular portions of the optical elements that make up the focusing optical system 4. That is, in the radial aberration components generated in the focusing optical system 4, aberrations generated in a minute diameter range corresponding to the width of each annular laser beam are completely associated with each annular laser beam.

[0028] The annular laser devices 2A, 2B, and 2C are configured so that the divergence angles can be set independently. For example, the reduction of aberrations by the divergence angles set for the annular laser devices 2A, 2B, and 2C is remarkable for aberration components that do not depend on the angular component θ, with respect to the radial component ρ and the angular component θ, when expanded in Zernike polynomials, and is particularly useful for correcting defocus or spherical aberration. In other words, the aberrations reduced by the divergence angles are, for example, axially symmetric aberrations such as spherical aberrations.

[0029] If the axially symmetric aberrations can be reduced to an allowable limit by the divergence angle, there is no need to correct the axially symmetric aberrations with high-order aspherical coefficients in the converging optical system 4. Since the aberrations at the converging point do not include axially symmetric aberrations, the converging optical system 4 does not need to be an optical system with a complex structure that can be corrected with high-order aspherical coefficients. This makes it possible to adopt a converging optical system 4 with a simple structure that can correct aberrations other than axially symmetric aberrations.

[0030] When the focusing optical system 4 is a Cassegrain-type reflective optical system with a parabolic mirror as the primary mirror, the parabolic surface eliminates spherical aberration, which is an axially symmetric aberration. However, a parabolic surface is more difficult to manufacture than a spherical or ellipsoidal surface. In contrast, in the annular laser system 1, the divergence angle of the annular laser beams emitted by the annular laser devices 2A, 2B, and 2C is determined so that the spherical aberration at the focusing point is reduced, assuming that the focusing optical system 4 is a parabolic surface. This allows the focusing optical system 4 to function as if it were a parabolic surface, even though it is a spherical reflecting mirror.

[0031] Furthermore, even if the focusing optical system 4 is a spherical reflective optical system that does not include a high-order aspherical coefficient or a hyperbolic reflective optical system, the divergence angle of the circular laser beam is determined so as to reduce aberration at the focusing point, on the condition that the focusing optical system 4 is a spherical reflective mirror that does not include a high-order aspherical coefficient or a hyperbolic reflective optical system, in the circular laser system 1. This allows the focusing optical system 4 to function as if it were a sphere or a hyperbolic mirror that does not include a high-order aspherical coefficient.

[0032] The focusing optical system 4 may correct aberrations remaining in the beam spatially combined by the annular laser combining device 3 with high-order aspherical coefficients. In the annular laser system 1, even if the aberrations cannot be completely reduced by the divergence angle, the focusing optical system 4 corrects the aberrations remaining in the beam with high-order aspherical coefficients. For example, the focusing optical system 4 corrects only the axially symmetric aberrations remaining in the beam with high-order aspherical coefficients. This is effective when there are many axially symmetric aberration components remaining in the spatially combined beam, i.e., aberration components that do not depend on the angle component θ.

[0033] For example, if the ring laser devices 2A, 2B, and 2C are equipped with unstable resonators, a ring laser beam of approximately parallel light can be emitted by adjusting the radius of curvature of the mirrors that make up the unstable resonators or an optical system such as a lens arranged downstream of the oscillator that makes up the unstable resonators. If the ring laser devices 2A, 2B, and 2C are equipped with axicon optical systems, a ring laser beam of approximately parallel light can be emitted by configuring them with two axicon optical elements and setting their apex angles to approximately the same. Furthermore, in the synthesis of Hermite-Gaussian beams, a ring laser beam of approximately parallel light can be emitted by adjusting an optical system such as a lens.

[0034] A specific example of the first embodiment will be described below, using specific numerical values. Here, as an example of a configuration for generating a substantially parallel circular laser beam, a configuration using two refractive axicon lenses will be described. The axicon lenses are arranged so that their apex angles face each other, and the beam is incident on the flat side of the axicon lens. The wavelength of the laser beam is 1070 nm, and the refractive index is 1.5066. The beam incident on the axicon lens is a substantially parallel Gaussian beam with a radius ω = 2.5 mm. This can be obtained, for example, by collimating the output from a single-mode fiber laser with NA = 0.1 using a lens with a focal length of 25 mm.

[0035] The difference between the inner and outer diameters of the annular beam generated by the axicon lens is 2.5 mm, which is the same as the radius of the Gaussian beam incident on the axicon lens. The apex angle of the front-stage axicon lens is α1, and the apex angle of the rear-stage axicon lens is α2. For simplicity of explanation, if the apex angles α1 and α2 are small enough so that sin θ ≒ θ holds, the incident Gaussian beam will be converged by the front-stage axicon lens at an angle of (n-1)α1, where n is the refractive index, and will then become a diverging annular beam.

[0036] If the distance between the axicon lenses is L, the inner diameter of the annular beam is, according to the geometrical optics relationship, ((n-1)α1(L+ωα1)-ω) / (1-(n-1)α1α2). For example, if the apex angle α1 is 8.2°, the diameter is 5 mm when L=100 mm, 7.5 mm when L=133.6 mm, and 10 mm when L=167.2 mm. Since the difference between the inner and outer diameters is 2.5 mm, the outer diameter is 7.5 mm when L=100 mm, 10 mm when L=133.6 mm, and 12.5 mm when L=167.2 mm. If α2=α1=8.2°, the annular beam that has passed through the two axicon lenses becomes approximately parallel light, regardless of the value of L.

[0037] If α2 and α1 are slightly shifted, the beam will converge or diverge according to the relationship (n-1) / n×(α2-α1). In optical design, it is common to use an optical design simulator with advanced optimization functions. Therefore, for example, the downstream focusing optical system can be configured with a simple spherical lens without aberration correction, and then α2 and L (i.e., the divergence angle of each of the multiple circular laser beams) can be optimized as optimization parameters to improve the focused diameter and wavefront at the focusing point, thereby achieving the functionality of the first embodiment.

[0038] Of course, the optimization parameters and the optimization order are not limited to these. In this example, the focusing optical system 4 is a plano-convex spherical lens with a curvature radius of 25.3 mm and a thickness of 5 mm, placed 100 mm away from the axicon lens. When three approximately parallel circular laser beams with α1 = α2 = 8.2° and L = 100 mm, 133.6 mm, and 167.2 mm are incident, the axially symmetric aberration components Z4 of the fringe Zernike coefficients are 1.98λ, 6.89λ, and 18.2λ, respectively, and Z9 are -1.65λ, -4.28λ, and -9.62λ, respectively, resulting in large aberrations.

[0039] On the other hand, if the apex angle for the annular beam with L = 100 mm is α2 = 8.5°, the apex angle for the annular beam with L = 133.6 mm is α2 = 9.0°, and the apex angle for the annular beam with L = 167.2 mm is α2 = 9.8°, Z4 is 0.26λ, 0.35λ, and 0.46λ, respectively, and Z9 is -1.05λ, -1.84λ, and -2.86λ, respectively, which is a significant improvement. Here, it is assumed that the three laser beams are coaxially combined through a perforated mirror or the like. In this way, the annular laser system 1 is suitable for correcting axially symmetric aberrations.

[0040] As described above, the ring laser system 1 according to the first embodiment includes the ring laser devices 2A, 2B, and 2C, each emitting a ring laser beam having a hollow intensity distribution in a cross section perpendicular to the beam propagation direction, the ring laser combining device 3, which spatially combines the ring laser beams emitted by the ring laser devices 2A, 2B, and 2C on the same axis, and the focusing optical system 4, which focuses the beam combined by the ring laser combining device 3. The ring laser devices 2A, 2B, and 2C emit ring laser beams having different inner and outer diameters, respectively, at different divergence angles, and the divergence angle is an angle at which aberrations at the focusing point of the focusing optical system 4 are reduced, provided that the focusing optical system 4 is a spherical surface that does not include a high-order aspherical coefficient, a parabolic surface, or a hyperbolic surface. On the condition that the focusing optical system 4 is a sphere that does not include a high-order aspherical coefficient, or a paraboloid, or a hyperboloid, the circular laser beams are emitted from the circular laser devices 2A, 2B, and 2C at a divergence angle that reduces aberration at the focusing point of the focusing optical system 4, and therefore the beam obtained by spatially combining these on the same axis has reduced aberration at the focusing point of the focusing optical system 4. This allows the circular laser system 1 to reduce the influence of aberration on the beam obtained by spatially combining multiple circular laser beams.

[0041] In the circular laser system 1 according to the first embodiment, the focusing optical system 4 corrects, with a high-order aspherical coefficient, the aberration remaining in the beam spatially combined by the circular laser combining device 3. This allows the circular laser system 1 to further reduce the aberration of the beam formed by spatially combining multiple circular laser beams.

[0042] In the circular laser system 1 according to the first embodiment, the divergence angle is selected so that the focal point is geometrically small in accordance with the focusing optical system 4. This allows the circular laser system 1 to reduce the influence of aberrations on the beam formed by spatially combining multiple circular laser beams.

[0043] In the circular laser system 1 according to the first embodiment, the remaining aberrations do not include axially symmetric aberrations, which eliminates the need for correction with high-order aspherical coefficients in the focusing optical system 4, and allows the focusing optical system 4 to have a simple structure.

[0044] Embodiment 2. A circular laser system according to embodiment 2 will be described with reference to FIG. 2. The circular laser system 1 according to embodiment 2 generates circular laser beams having multiple wavelengths. The circular laser beams having multiple wavelengths are incoherently overlapped on the same axis and can be generated by a dispersive element such as a diffraction grating or a prism. The circular laser devices 2A, 2B, and 2C are wavelength-combining laser devices that spatially combine circular laser beams having multiple wavelengths on the same axis.

[0045] In the ring laser system 1 according to the second embodiment, the optical section that sets the width of the outer diameter and the inner diameter of the ring laser beams emitted by the ring laser devices 2A, 2B, and 2C, the optical section that sets the divergence angle, the ring laser combining device 3, and the focusing optical system 4 are all composed of reflective optical systems. Even if the ring laser beams emitted from the ring laser devices 2A, 2B, and 2C have multiple wavelengths, the focusing optical system 4, which is entirely composed of reflective optical systems, can focus these combined beams without chromatic aberration, similar to a monochromatic beam. The ring laser system 1 according to the second embodiment can obtain output beam power suitable for various applications by increasing the power of the output beam by spatially combining ring laser beams of multiple wavelengths and by increasing the power due to the number of wavelengths.

[0046] Although the case where the circular laser synthesizing device 3 synthesizes beams emitted from the circular laser devices 2A, 2B, and 2C has been shown, the circular laser system 1 may also synthesize beams emitted from four or more circular laser devices.

[0047] Any of the components of the embodiments may be modified or omitted.

[0048] The circular laser system according to the present disclosure can be used for, for example, laser processing.

[0049] 1,100 Circular laser system, 2A, 2B, 2C, 101A, 101B, 101C Circular laser device, 3,102 Circular laser synthesis device, 4,103 Focusing optical system.

Claims

1. A ring laser system comprising: a plurality of ring laser devices, each of which emits a ring laser beam having a hollow intensity distribution in a cross section perpendicular to the beam propagation direction; a ring laser combining device that spatially combines the plurality of ring laser beams emitted by the plurality of ring laser devices on a coaxial line; and a focusing optical system that focuses the beam combined by the ring laser combining device, wherein the plurality of ring laser devices emit the ring laser beams, each of which has a different inner diameter and outer diameter, at a different divergence angle, and the divergence angle is an angle at which aberration at the focusing point of the focusing optical system is reduced, provided that the focusing optical system is a sphere that does not contain a high-order aspherical coefficient, or a parabolic surface, or a hyperbolic surface.

2. The circular laser system according to claim 1, characterized in that the focusing optical system corrects the aberration remaining in the beam spatially combined by the circular laser combining device with a high-order aspheric coefficient.

3. The circular laser system according to claim 1, characterized in that the divergence angle is selected so that the focal point becomes geometrically small in accordance with the focusing optical system.

4. The circular laser system according to claim 1, characterized in that the remaining aberrations do not include axially symmetric aberrations.

5. The circular laser system according to claim 2, characterized in that the aberration at the focal point is axially symmetric.

6. The multiple circular laser devices are wavelength combining laser devices that spatially combine the circular laser beams of multiple wavelengths on the same axis, and the optical part that sets the width between the outer diameter and the inner diameter of the circular laser beams emitted by the multiple circular laser devices, the optical part that sets the divergence angle, the circular laser combining device, and the focusing optical system are reflective optical systems. A circular laser system as described in any one of claims 1 to 5, characterized in that:

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