An optical system with a ring-shaped light spot

Abstract

The present invention belongs to the field of laser processing, and discloses an optical system of an annular light spot, including a collimation unit, a phase modulation unit and a focusing unit arranged in sequence along the optical path, the phase modulation unit including at least one spiral phase plate located in the optical path; the laser beam can be added with a spiral phase factor to achieve phase modulation through the spiral phase plate located in the optical path, so that the energy distribution of the laser beam becomes annular distribution. Furthermore, the spiral phase plate can be set on a rotating axis, and the state of being located inside and outside the optical path can be switched by rotating around the rotating axis, so that a variable annular light spot can be achieved by switching each spiral phase plate. The present invention improves the components of the optical system and the coordination mode between the components, etc., and can obtain an annular light spot with energy distribution concentrated at the edge, larger diameter and longer focal depth. Furthermore, the size of the light spot is adjustable, and it is suitable for various laser processing fields such as laser cutting.

Description

Technical Field

[0001] The invention belongs to the field of laser processing, and more specifically, relates to an optical system of a ring-shaped light spot. Background Art

[0002] For a light spot with ordinary Gaussian energy distribution, most of the energy is concentrated in the center, while for a light spot with annular energy distribution, the energy is distributed around the spot, with low energy in the center. This kind of energy distribution light spot has great advantages in laser cutting, marking, cladding and micro-machining.

[0003] When using a light spot with Gaussian energy distribution for laser processing, ablation and other phenomena may occur if the center energy is too high. Due to insufficient energy at the edge, when performing deep processing such as laser cutting, the deeper the processing depth, the more insufficient the energy at the edge, resulting in an uneven cutting section. The use of a light spot with an annular energy distribution can effectively solve these problems. Compared with the Gaussian distribution, the annular distribution is more evenly distributed in energy, and the energy of the annular light spot is distributed at the edge. When performing laser cutting of thick plates, the problem of uneven cross-section caused by insufficient edge energy can be effectively avoided. The main method of generating annular beams in the prior art is computational holography. According to the principle of computational holography, a spatial light modulator is used to generate annular beams. By designing a holographic pattern, annular beams with arbitrary spatial distribution can be generated. This method has a certain damage threshold, and due to the loss of energy and the diffraction of the light beam, the conversion efficiency of the annular beam is not high. Summary of the invention

[0004] In view of the above defects or improvement needs of the prior art, the purpose of the present invention is to provide an optical system of annular spot, especially an optical system of variable annular spot, wherein by improving the components of the optical system and the cooperation mode between the components, an annular spot with energy distribution concentrated at the edge, larger diameter and longer focal depth can be obtained; and by further setting a rotating axis to adjust the number and topological charge of the inserted optical path, the diameter of the spot can be changed, and the size of the spot can be adjusted, which is suitable for various laser processing fields such as laser cutting. The laser beam is collimated by the collimation unit and incident from one end (such as the plane end) of the spiral phase plate in the phase modulation unit. The outgoing beam is added with a spiral phase factor of the corresponding topological charge to become an annular beam, and finally passes through the focusing unit to obtain an annular spot with annular energy distribution. The present invention can generate an annular spot through a spiral phase plate, which is of great significance in laser processing technology, and can further autonomously adjust the number of spiral phase plates to achieve variable topological charge of the annular focusing spot, which can be flexibly adjusted according to needs when processing different materials.

[0005] To achieve the above object, according to one aspect of the present invention, an optical system of an annular spot is provided, characterized in that it comprises a collimating unit, a phase modulation unit and a focusing unit arranged in sequence along the optical path, wherein:

[0006] The collimation unit is used to collimate the input laser to obtain a collimated laser beam;

[0007] The phase modulation unit includes at least one spiral phase plate located in the optical path; any one of the spiral phase plates includes two end faces, one of which is a planar structure, and the other end face is an opposite face having a spiral-shaped step; and for any one of the spiral phase plates, the central axis of the spiral phase plate coincides with the beam center of the collimated laser beam, and the laser beam can be incident from the planar end face of the corresponding spiral phase plate or from the step end face of the corresponding spiral phase plate, and a spiral phase factor is added to the spiral phase plate to perform phase modulation, so that the energy distribution of the laser beam becomes a ring distribution; the phase modulation unit as a whole can modulate the phase of the collimated laser beam through the action of the spiral phase plate, thereby emitting a laser beam with a changed phase;

[0008] The focusing unit is used to focus the laser beam whose phase is changed, so as to obtain an annular light spot.

[0009] According to another aspect of the present invention, there is provided an optical system with a variable annular spot, characterized in that it comprises a collimating unit, a phase modulation unit and a focusing unit arranged in sequence along the optical path, wherein:

[0010] The collimation unit is used to collimate the input laser to obtain a collimated laser beam;

[0011] The phase modulation unit includes a plurality of spiral phase plates located on a rotation axis, and the spiral phase plates can switch between being located inside and outside the optical path by rotating around the rotation axis; any one of the spiral phase plates includes two end faces, one of which is a planar structure, and the other is an opposite face having a spiral-shaped step; and for any one of the spiral phase plates, when it is rotated to a state of being located inside the optical path, the central axis of the spiral phase plate coincides with the beam center of the collimated laser beam, and the laser beam can be incident from the planar end face of the corresponding spiral phase plate or from the step end face of the corresponding spiral phase plate, and a spiral phase factor is added to the spiral phase plate for phase modulation, so that the energy distribution of the laser beam becomes a ring distribution; the phase modulation unit as a whole can modulate the phase of the collimated laser beam through the action of the spiral phase plate, thereby emitting a laser beam with a changed phase;

[0012] The focusing unit is used to focus the phase-changed laser beam to obtain an annular light spot;

[0013] The variable annular spot optical system can realize a variable annular spot by switching the state of each spiral phase plate being located inside the optical path and outside the optical path.

[0014] As a further preferred embodiment of the present invention, a laser is further arranged before the collimation unit, and the laser emitted by the laser serves as the input laser of the collimation unit. The laser emitted by the laser has a Gaussian distribution of light intensity.

[0015] As a further preferred embodiment of the present invention, there are multiple spiral phase plates, and for any spiral phase plate with a topological charge of l, the phase change around the optical axis is 2π×l.

[0016] As a further preference of the present invention, the phase modulation unit includes five spiral phase plates located on the rotating axis, and the topological charges l of the five spiral phase plates are n, n, 2n, 4n, and 8n, respectively, where n is a pre-set non-zero integer; for any spiral phase plate with a topological charge of l, the phase change around the optical axis is 2π×l.

[0017] As a further preferred embodiment of the present invention, the collimating unit includes a lens group for use with the collimating unit.

[0018] As a further preferred embodiment of the present invention, the focusing unit includes a lens group for use with the focusing unit.

[0019] Through the above technical scheme conceived by the present invention, compared with the prior art, in order to solve the problem that the focused light spot with Gaussian energy distribution has concentrated energy but insufficient edge energy in laser processing, the present invention uses a spiral phase plate as an optical element to generate an annular focused light spot for the first time. The present invention applies the conventional optical element in the prior art, the spiral phase plate, to laser processing for the first time to generate related applications of annular light spots; and further, the topological charge of the final output laser beam can be adjusted by controlling the number of spiral phase plates inserted into the optical path. The present invention uses a spiral phase plate to generate annular light beams, which is easy to implement. The step height of the spiral phase plate is usually in the micrometer range, and the initial light beam is expanded by a beam expansion system with basically no divergence. Therefore, the spiral phase plate has basically no attenuation on the light intensity of the light beam, but only changes the phase of the light beam. For light sources with good monochromaticity, the generated annular light beam has higher purity.

[0020] The present invention uses spiral phase plates to construct phase modulation units. These spiral phase plates are consistent with the prior art and are transparent plates with a fixed refractive index, one side of which is a planar structure and the opposite side has a spiral structure (similar to a rotating step); the thickness of the spiral phase plate changes with the change of the azimuth angle. After the laser beam passes through the spiral phase plate, the phase of the output beam changes accordingly due to the different optical paths at different azimuth angles. The output beam is added with a spiral phase factor exp(ilθ), where l is the topological charge of the spiral phase plate and i represents the complex number symbol, thereby becoming a ring beam.

[0021] When the refractive index, thickness parameters of the spiral phase plate and the wavelength of the incident light are determined, the topological charge of the obtained annular light beam is also determined and cannot be flexibly adjusted; therefore, the present invention further proposes a more preferred optical system with variable annular spot size. In other words, since a spiral phase plate can only produce an annular spot with one topological charge after the refractive index and thickness parameters are determined, in view of this, the present invention can flexibly switch the state of a spiral phase plate being located inside or outside the optical path by designing a rotating axis. In this way, based on the principle of topological charge superposition of annular light beams, multiple spiral phase plates are used to generate annular focusing spots with different topological charges, thereby achieving variable annular spot size.

[0022] The spiral phase plate in the phase modulation unit needs to keep the light beam incident from the plane end (or step end) of the spiral phase plate; when the spiral phase plate in the phase modulation unit is inserted into the optical path (i.e., rotated to a state located in the optical path), the center of the spiral phase plate must be aligned with the center of the laser beam. The Gaussian distributed laser beam is incident from the plane end face of each spiral phase plate or the step end face of each spiral phase plate, and the center of the beam is aligned with the center of the spiral phase plate. The phase of the outgoing beam is changed, and a spiral phase factor is added, and the energy distribution becomes annular distribution, and finally an annular light spot is obtained after focusing.

[0023] The size of the annular spot obtained by the optical system of the variable annular spot is affected by the topological charge. The larger the topological charge, the larger the area of ​​the energy depression of the annular spot, and the larger the spot diameter. The present invention changes the topological charge by arranging a plurality of spiral phase plates located on the rotating axis, and the spiral phase plates inserted into the system can be superimposed to achieve the variable and adjustable annular spot. In other words, based on the present invention, the spot size can be adjusted by adjusting the number of spiral phase plates located in the optical path. The phase modulation unit in the variable annular spot optical system is composed of a plurality of spiral phase plates fixed on a rotating axis, and the spiral phase plate can be inserted into the optical path or removed from the optical path by rotating on the rotating axis (i.e., switching the state of being located inside the optical path and being located outside the optical path). When any spiral phase plate is inserted into the optical path, the center of the spiral phase plate can be aligned with the collimation unit and the focusing unit in the system.

[0024] The topological charge of an annular beam satisfies the superposition principle, that is, the size of the topological charge of an annular beam satisfies the sum relationship. For example, two annular beams with exactly the same topological charge can be superimposed into an annular beam with twice the topological charge. For another example, if two annular beams with the same topological charge value but opposite signs are superimposed, a beam with zero topological charge (that is, a plane light field) is formed. Therefore, by superimposing multiple spiral phase plates, the topological charge l of the annular beam can be changed, thereby changing the size of the annular spot obtained by the final focus.

[0025] Specifically, the present invention has the following characteristics:

[0026] (1) Under the same conditions, the beam is added with a spiral phase factor, and the focused annular spot has a larger radius and a longer focal depth than the ordinary focused spot. In addition, the high edge energy can effectively avoid adverse effects such as ablation during laser processing, and the material is heated more evenly, resulting in better processing quality;

[0027] (2) The annular spot generated by the spiral phase plate can be regarded as pure phase modulation, the change in amplitude can be ignored, the energy conversion rate is high, the loss is reduced, and it is suitable for high-power laser processing;

[0028] (3) The annular light spot produced by the spiral phase plate has higher purity, stronger stability, and is not easily affected by optical effects such as diffraction;

[0029] (4) According to the actual work needs, the number of spiral phase plates in the system can be adjusted autonomously. By superimposing multiple spiral phase plates, the topological charge can be changed, thereby changing the size of the annular focusing spot. The size of the annular spot can be continuously adjusted to meet the cutting requirements of different plate thicknesses.

[0030] (5) The system has good anti-adjustment characteristics. Changes in spot size, beam offset, and focus offset will not affect the light field of the annular energy distribution;

[0031] (6) When used in laser additive manufacturing or laser surface treatment, the topological charge of the light spot is changed to achieve adjustable light spot width without affecting the uniformity of its light field distribution, thereby achieving a consistent uniformity effect.

[0032] In summary, using the optical system of the present invention, the laser beam is collimated into parallel light after passing through the collimation unit, and then a ring beam is obtained through phase modulation, and finally a focusing unit is passed to focus into a high-energy ring spot, which can be used in multiple fields such as laser cutting, marking, cladding, and micro-processing. The present invention can generate a ring spot through a spiral phase plate, which is of great significance in laser processing technology, and can independently adjust the number of spiral phase plates to achieve a variable topological charge of the ring-shaped focusing spot, which can be flexibly adjusted according to one's own needs when processing different materials; it can effectively solve the current problems of uneven cross-section of laser thick plate cutting caused by excessive central energy of the Gaussian beam and insufficient edge energy, and ablation during processing, and the spot size is adjustable, the method of use is flexible, and the energy utilization rate is high, which is suitable for laser high-power processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 Schematic diagram of the variable annular spot optical system in Example 1 of the present invention.

[0034] Figure 2 It is a schematic diagram of the structure of the spiral phase plate in the variable annular spot optical system of the present invention.

[0035] Figure 3 Schematic diagram of a collimated Gaussian beam in Example 1 of the present invention.

[0036] Figure 4 This is a phase diagram of a spiral phase plate with a topological charge of 1 in the phase modulation unit of Example 1 of the present invention.

[0037] Figure 5 This is a phase diagram of a spiral phase plate with a topological charge of 2 in the phase modulation unit of Example 1 of the present invention.

[0038] Figure 6 It is the focused light spot obtained by only passing through the collimating and focusing unit when the light beam does not pass through any spiral phase plate in Example 1 of the present invention.

[0039] Figure 7 This is a cross-sectional light field distribution diagram on the focusing plane after passing through a spiral phase plate with a topological charge of 1 in Example 1 of the present invention.

[0040] Figure 8 This is a cross-sectional light field distribution diagram on the focusing plane after passing through two spiral phase plates with topological charges of 1 and 1 respectively in Example 1 of the present invention.

[0041] Fig. 9 This is a cross-sectional light field distribution diagram on the focusing plane after passing through three spiral phase plates with topological charges of 1, 1 and 2 respectively in Example 1 of the present invention.

[0042] Fig.10Schematic diagram of the variable annular spot optical system in Example 2 of the present invention.

[0043] Figure 1 , Figure 2 , Fig.10 The meanings of the various figures in the figure are as follows: 1 is the incident laser, 2 is the collimation unit, 3 is the switching shaft, 4 is the spiral phase plate, 5 is the focusing unit, 6 is the focusing plane, 7 is the scanning galvanometer, 8 is the motor connected to the scanning galvanometer 7, 9 is another scanning galvanometer, and 10 is the motor connected to the scanning galvanometer 9. DETAILED DESCRIPTION

[0044] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0045] Embodiment 1:

[0046] The schematic diagram of the variable annular spot optical system provided by the first embodiment of the present invention is as follows Figure 1 As shown in the figure, the whole system is divided into a collimation unit, a phase modulation unit and a focusing unit. The laser beam is collimated into parallel light by the lens group of the collimation unit, and then incident from the plane end of the spiral phase plate in the phase modulation unit, and the output becomes a ring beam, whose topological charge is equal to the topological charge of the spiral phase plate. Finally, it is focused by the lens group in the focusing unit to obtain a light spot with an annular energy distribution.

[0047] like Figure 2 The figure shows a schematic diagram of the structure of the spiral phase plate in the variable annular spot optical system of the present invention. The spiral phase plate is an optical element in the prior art. In the prior art, the spiral phase plate is made into a step shape for ease of processing. The increased thickness of the step of the spiral phase plate can be directly calculated by referring to the prior art according to the topological charge of the spiral phase plate and the refractive index of the material (such as Wu Mingbo, "Research on the Measurement of Topological Charge of Vortex Beams", etc.); the thickness of the spiral phase plate is generally in the micron range and can be ignored, so the influence of the phase plate on the light intensity can be ignored.

[0048] like Figure 3 The figure shows a schematic diagram of a collimated Gaussian beam according to the first embodiment of the present invention, wherein the axis of the Gaussian beam is in a straight line with the axes of the collimating lens and the focusing lens.

[0049] like Figure 1As shown, the present invention fixes a plurality of spiral phase plates on a rotating shaft, and can autonomously adjust the number of phase plates inserted into the optical path system by rotation. That is, the spiral phase plates in the optical path can be flexibly switched according to actual needs, for example:

[0050] like Figure 4 The figure shows a phase simulation diagram of a spiral phase plate with a topological charge of 1 in a phase modulation unit of the first embodiment of the present invention. After the light beam passes through the phase plate, a corresponding phase factor will be added.

[0051] like Figure 5 The figure shows a phase simulation diagram of a spiral phase plate with a topological charge of 2 in a phase modulation unit of the first embodiment of the present invention. After the light beam passes through the phase plate, a corresponding phase factor will be added.

[0052] like Figure 6 The figure shows the first embodiment of the present invention. When the light beam does not pass through any spiral phase plate, the focused light spot is obtained only through the collimating and focusing unit. It can be seen that the light field distribution of the focused light spot is Gaussian distribution, and the spot radius is about 0.1 mm.

[0053] like Figure 7 The figure shows the cross-sectional light field distribution diagram of the first embodiment of the present invention after passing through a spiral phase plate with a topological charge of 1 (n=1 at this time; of course, n can also be preset to be equal to other integers, including positive integers and negative integers). It can be seen that after the phase modulation of the spiral phase plate, a focused light spot with a ring-shaped energy distribution with low energy in the center and high energy at the edge is obtained, with a topological charge of 1, a phase change of 2π, and a spot radius of about 0.2 mm.

[0054] like Figure 8 The figure shows the cross-sectional light field distribution diagram at the location of the focusing plane 6 after passing through two spiral phase plates with topological charges of 1 and 1, respectively, in the first embodiment of the present invention. By superimposing the topological charges of the spiral phase plates, the topological charges of the annular light spot can be superimposed accordingly, and the topological charge of the annular light spot finally obtained is 2 (equal to 1 plus 1, the topological charge satisfies the superposition principle), the phase change is 4π, and the spot radius is about 0.35 mm.

[0055] like Fig. 9 The first embodiment of the present invention is shown in the cross-sectional light field distribution diagram at the location of the focusing plane 6 after passing through three spiral phase plates with topological charges of 1, 1 and 2 respectively. The obtained annular spot topological charge is 4 (equal to 1 plus 1 plus 2, the topological charge satisfies the superposition principle), the phase change is 8π, and the spot radius is about 0.5mm. Two annular beams with the same topological charge can be superimposed to obtain an annular beam with twice the topological charge. The topological charges of several spiral phase plates in the phase modulation unit of the present invention should follow this rule.

[0056] Embodiment 2:

[0057] The schematic diagram of the variable annular spot system provided by the second embodiment of the present invention is as follows: Fig.10 As shown, after the laser beam passes through the collimation unit and the phase modulation unit, it passes through two scanning galvanometers and finally passes through the focusing objective lens. The principle is the same as that of Example 1. By scanning with two galvanometers, the focal position can be changed for marking.

[0058] The optical path and the light field energy distribution on the focusing plane of Example 2 are consistent with those of Example 1. Since the annular light spot generated by the spiral phase plate has high stability and can still maintain the characteristics of annular distribution when deviating from the focus position, the annular energy output can be better maintained during the high-speed moving marking process of the galvanometer scanning. Compared with the application of Example 1 in the vertical space, Example 2 is mainly applied in the horizontal space.

[0059] In general, the optical system of the variable annular spot in the present invention can obtain an annular energy distribution spot with low light intensity loss, adjustable spot size, low center energy, and high edge energy, which can be used in multiple laser processing fields such as laser cutting, marking, and cladding. It effectively solves the current negative problems such as uneven cross-section of laser thick plate cutting and ablation during processing caused by excessive center energy and insufficient edge energy of Gaussian beam, and the spot size is adjustable, the use method is flexible, and the energy utilization rate is high, which is suitable for laser high-power processing.

[0060] For the planar end face and the step end face of the spiral phase plate, when they are in the optical path, the laser incident direction needs to be consistent. For example, the first spiral phase plate laser can be incident from the step surface, and all subsequent spiral phase plates must be placed in the same direction as the first spiral phase plate, so that the laser is incident from the step end face of the corresponding spiral phase plate.

[0061] The above embodiment is a detailed introduction to the optical system of variable annular spot based on the optimal technical solution of the present invention. Of course, around the core concept of the present invention, which is to use a conventional optical element such as a spiral phase plate to generate an annular focused light spot, the present invention can also realize an optical system of fixed annular spot. The optical system of fixed annular spot also includes a collimation unit, a phase modulation unit and a focusing unit arranged in sequence along the optical path, and the collimation unit and the focusing unit can completely refer to the optical system of variable annular spot. Only the phase modulation unit needs to be adjusted. At this time, the phase modulation unit will not be set with a rotation axis, but a predetermined number of spiral phase plates will be fixed in the optical path (that is, the predetermined number of spiral phase plates can always be located in the optical path). These spiral phase plates are at least one spiral phase plate, and of course, they can also be multiple spiral phase plates.

[0062] In addition, the collimation unit and the focusing unit in the optical system of the present invention use a combined lens group to reduce aberrations. The topological charge of each spiral phase plate can be preset, including each spiral phase plate located on the rotation axis. Figure 1 For example, Figure 1 The five spiral phase plates shown have topological charges l of n, n, 2n, 4n, and 8n, respectively, where n is 1; for any spiral phase plate with a topological charge of l, the phase change around the optical axis is 2π×l. With reference to the prior art, the thickness of the spiral phase plate can be calculated based on the material refractive index of the spiral phase plate and its topological charge, and the height of each step can be calculated by averaging each step, and the corresponding manufacturing and processing can be performed to prepare the spiral phase plate by oneself. The higher the processing accuracy, the better, and the height of each step is generally in the micrometer range; of course, it can also be purchased. The collimating unit and the focusing unit in the optical system of the present invention can be conventional collimating and focusing lenses.

[0063] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. An optical system with a variable annular spot, characterized in that: It includes a collimation unit, a phase modulation unit and a focusing unit which are sequentially arranged along the optical path, wherein: The collimation unit is used to collimate the input laser to obtain a collimated laser beam; The phase modulation unit includes a plurality of spiral phase plates located on a rotation axis, and the spiral phase plates can switch between being located inside and outside the optical path by rotating around the rotation axis; any one of the spiral phase plates includes two end faces, one of which is a planar structure, and the other is an opposite face having a spiral-shaped step; and for any one of the spiral phase plates, when it is rotated to a state of being located inside the optical path, the central axis of the spiral phase plate coincides with the beam center of the collimated laser beam, and the laser beam can be incident from the planar end face of the corresponding spiral phase plate or from the step end face of the corresponding spiral phase plate, and a spiral phase factor is added to the spiral phase plate for phase modulation, so that the energy distribution of the laser beam becomes a ring distribution; the phase modulation unit as a whole can modulate the phase of the collimated laser beam through the action of the spiral phase plate, thereby emitting a laser beam with a changed phase; The focusing unit is used to focus the phase-changed laser beam to obtain an annular light spot; The variable annular spot optical system can realize a variable annular spot by switching the state of each spiral phase plate being located inside the optical path and outside the optical path.

2. The optical system according to claim 1, characterized in that: A laser is also arranged before the collimation unit. The laser emitted by the laser serves as the input laser of the collimation unit. The laser emitted by the laser has a Gaussian distribution of light intensity.

3. The optical system according to claim 1, wherein: There are multiple spiral phase plates. For any spiral phase plate with a topological charge of l, the phase change around the optical axis is 2π×l.

4. The optical system according to claim 1, wherein: The phase modulation unit includes five spiral phase plates located on the rotating axis, and the topological charges l of the five spiral phase plates are n, n, 2n, 4n, and 8n respectively, where n is a pre-set non-zero integer; for any spiral phase plate with a topological charge of l, the phase change around the optical axis is 2π×l.

5. The optical system according to claim 1, characterized in that: The collimating unit includes a lens group for use with the collimating unit.

6. The optical system according to claim 1, wherein: The focusing unit includes a lens group for use with the lens group.

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