Optical system for controlling light beam

Abstract

An optical system (100) comprises focusing optics (10) configured for generating a focused light beam (B) at a focusing region (Fr). The focusing optics (10) comprise a set of spherical surfaces (11s, 12s) configured for introducing respective spherical aberrations into the focused light beam (B). The spherical aberration is adjusted to maximize the length (Lr) of the focal region (Fr). The system comprises at least one rotatable element (22) configured for redirecting the focused beam (B) in a variable beam direction. The gradient index 10 plate (31) is configured to receive the light beam from the rotatable element (22) and linearize a correlation of a radial coordinate (R) of the focused light beam in a target plane (P) with a rotation angle (delta) of the rotatable element (22). Advantageously, the gradient index plate is flat and thus does not introduce additional aberrations that may interfere with the focusing function.

Description

[0001] Technical Field and Background

[0002] The present disclosure relates to an optical system for controlling the position of a light beam, for example for use as part of a laser marking system or a method of marking a target surface.

[0003] As background technology, US2010 / 0065537A1 describes a focusing optical system that focuses a laser beam generated by a laser source at a predetermined focal length, wherein the focusing optical system generates spherical aberration to increase the depth of focus while maintaining a small size of the focusing spot. The focusing optical system can be, for example, a single aspherical lens or a single diffraction focusing lens. Alternatively, the focusing optical system can be a composite optical system including at least two optical components. In addition, the composite optical system can include, for example, a first optical device having a focusing function and a second optical device having a spherical aberration generating function. The second optical device can be, for example, an aspherical phase plate or a diffraction phase plate. In addition, the focusing optical system can also include a laser beam deflection device, which is a polygonal mirror or a galvanometer mirror, wherein the first optical device is an f-θ lens. Therefore, a small light spot with a large depth of focus can be scanned at high speed on the focal plane.

[0004] For laser marking and other purposes, there remains a need to further simplify and improve the control of the beam position. Summary of the Invention

[0005] Aspects of the present disclosure relate to an optical system comprising focusing optics configured to produce a focused light beam. The system comprises beam steering optics having at least one rotatable element configured to receive the focused light beam and redirect the focused light beam toward a direction controlled by setting a rotation angle of the rotatable element. The system comprises a gradient refractive index (GRIN) plate configured to receive the redirected light beam from the rotatable element and redirect the focused light beam to a target plane. A focal region of the focused light beam overlaps the target plane at a radial coordinate. The GRIN plate is configured to linearize a dependence of the radial coordinate on the rotation angle.

[0006] It will be appreciated that the optical systems described herein may be particularly suitable for laser marking of products. For example, the linear dependence of the (radial) position of the beam focus and the rotation angle of a rotatable element (e.g., a mirror) can make the system easier to control and / or improve precision. The GRIN plates described herein can be easily optimized for a variety of focusing optical devices. The GRIN plate can receive a focused beam and redirect it to a well-controlled position while minimizing the effect on the focusing characteristics. In contrast, f-θ lenses are typically designed to receive and focus a collimated beam. In addition, compared to f-θ lenses, GRIN plates are flat and therefore do not introduce additional aberrations that may interfere with the focusing function. This is particularly important when using the preferred focusing optical devices described herein, which are specifically adjusted to introduce spherical aberration in the focused beam, so that a particularly long focusing area can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] These and other features, aspects, and advantages of the apparatus, systems, and methods of the present disclosure will become better understood with reference to the following description, appended claims, and accompanying drawings, in which:

[0008] Figures 1A to 1C shows a simulation of an optical system in which the radial coordinates of the focal region overlapping the target plane depend linearly on the rotation angle of the rotatable element;

[0009] Figure 2A Further details of the optical system are shown;

[0010] Figure 2B Shown are details of a gradient-index plate used to linearize the beam position, and simulated beams passing through the plate at different angles;

[0011] Figure 3A shows a graph comparing the linearity of beam position for different systems;

[0012] Figure 3B The deviation of the graph from the ideal linear correlation is shown;

[0013] Figure 4 shows focusing optics that introduce spherical aberration to produce an extended focus area;

[0014] Figure 5A shows the intensity distribution of a focused beam with spherical aberration along the axial and transverse coordinates;

[0015] Figure 5B shows the percentage of beam power passing through circular areas with different radii as a function of the axial coordinate;

[0016] Figure 6 Shown Figure 5ACross-sectional profiles of the focused beam at different axial positions shown in ;

[0017] Figure 7A and Figure 7B Shown is the control of an optical system for laser marking a product. DETAILED DESCRIPTION

[0018] The terms used to describe specific embodiments are not intended to limit the present invention. Unless the context clearly indicates otherwise, the singular forms "a", "an" and "the" used herein also include the plural forms. The term "and / or" includes any combination and all combinations of one or more of the associated listed items. It should be understood that the terms "include" and / or "comprise" specify the presence of the stated features, but do not exclude the presence or addition of one or more other features. It should also be understood that when a particular step of a method is referred to as being after another step, unless otherwise stated, the step may be performed directly after the other step, or one or more intermediate steps may be performed before performing the particular step. It should also be understood that when describing a connection between structures or components, unless otherwise stated, the connection may be established directly or through an intermediate structure or component.

[0019] The present invention will be described more fully below with reference to the accompanying drawings, in which embodiments of the invention are shown. For the sake of clarity, the absolute and relative sizes of systems, components, layers, and regions in the drawings may be exaggerated. The embodiments may be described with reference to schematic and / or cross-sectional views of possible idealized embodiments and intermediate structures of the invention. In the specification and drawings, like numbers refer to like elements throughout. Relative terms and their derivatives should be understood to refer to the orientation described or shown in the drawings in question. These relative terms are for convenience of description and do not require that the system be constructed or operated in a specific orientation unless otherwise specified.

[0020] Figures 1A to 1C A simulation of an embodiment of the optical system 100 is shown.

[0021] In some embodiments, the optical system 100 includes a focusing optical device 10. Preferably, the focusing optical device 10 includes at least one focusing element 12, which is configured to generate a focused light beam B. In one embodiment, the focusing optical device 10 is configured to receive a collimated light beam generated by a light source (not shown here) (e.g., a laser that generates a collimated (Gaussian) light beam). Alternatively, the light source can also generate a diverging or converging light beam received by the focusing optical device 10. Typically, the focusing optical device 10 includes at least one converging lens and / or a reflector. For example, the focusing element 12 is a positive lens with a positive focal length. More than one focusing element can also be used, such as two focusing elements 11, 12. For example, two positive lenses are shown in the figure, with the focal point located between the two positive lenses. Other or additional optical elements, such as a combination of positive and negative lenses and / or reflectors, can also be used to make the light beam received by the focusing optical device 10 ultimately converge.

[0022] In some embodiments, optical system 100 includes beam steering optics 20. Preferably, the beam steering optics include at least one rotatable element 22. In one embodiment, rotatable element 22 is configured to receive a focused beam B having an incident angle θ and redirect the focused beam B along a first beam direction B1. In another or additional embodiment, first beam direction B1 has a first beam angle α relative to central axis A. For example, central axis A is perpendicular to target plane P. In one embodiment, first beam angle α can be controlled by setting a rotation angle δ of rotatable element 22. For example, rotation can change the incident angle θ.

[0023] In some embodiments, the gradient index plate 31 is configured to receive the focused beam B along a first beam direction B1 and redirect the focused beam B along a second beam direction B2. Typically, the second beam direction B2 may be different from the first beam direction B1 (except at one or more specific locations where the beam direction may not be affected, such as the central axis A and / or a ring around the axis at a specific radius). In one embodiment, the second beam direction B2 has a second beam angle β relative to the central axis A. In another or additional embodiment, the focused beam B redirected along the second beam direction B2 has a focal region Fr that overlaps the target plane P at a radial coordinate R relative to the intersection of the central axis A and the target plane P. In a preferred embodiment, the gradient index plate 31 is configured to linearize the dependence of the radial coordinate R on the rotation angle δ, as described herein.

[0024] In some embodiments, the beam steering optics 20 include at least one rotatable mirror. In one embodiment, the beam steering optics are controlled by a galvanometer, such as a galvanometer mirror. Other or additional types of beam steering optics, such as a polygonal mirror, may also be used. For example, Figure 1A A rotatable element 22 formed of a rotatable mirror is shown, which is configured to reflect a focused light beam B toward a target plane P via a gradient refractive index plate 31. Preferably, the rotatable mirror has a mirror surface that coincides with one or more axes of rotation and / or receives the light beam coincident with the center of rotation. In this way, the origin of the reflected light beam can remain essentially independent of the rotation angle. In a preferred embodiment, for example, as shown in the figure, the rotatable element 22 is configured to receive the focused light beam B at an incident angle θ of 45 degrees. As a result, the mirror is able to reflect the light beam at a reflection angle of 45 degrees along a central axis A oriented perpendicular to the target plane P. In principle, this configuration can provide an optimal range. Of course, other configurations, such as those with different incident angles, are also conceivable.

[0025] As an alternative or in addition to the reflective optical device, it is also conceivable to use further rotatable elements to redirect the light beam, for example, including a rotatable transmissive element that affects the direction of the transmitted light beam. In addition, it is also possible to use more than one rotatable element configured to rotate about different axes, for example at least two rotatable elements. For example, Figures 1A to 1C The mirror 21 shown can be configured as an additional rotatable element for rotating the direction of the light beam in a direction transverse to the direction of the rotatable element 22. It should be noted that the use of a first rotatable element may significantly affect the position at which the light beam is received on the second rotatable element and the position at which the light beam will pass through the gradient index plate 31. In principle, this can be taken into account when designing the gradient index plate 31, for example, by designing different gradients in different directions. However, when a single rotatable element is used, which is preferably placed in the light beam path directly in front of the gradient index plate 31 and can be rotated in different directions, the design of the gradient index plate can be significantly simplified (e.g., radially symmetric).

[0026] Figure 2AFurther details of optical system 100 are shown. In some embodiments, such as shown, rotatable element 22 rotates by angle δ, causing the beam direction of the reflected light beam to change by an angle α = 2δ. In other or additional embodiments, such as shown, a reflected light beam along a first direction having a first beam angle α is received by gradient index plate 31, which may cause the transmitted light beam to be slightly redirected along a second beam direction B2 having a second beam angle β. As shown, gradient index plate 31 receives the light beam from rotatable element 22 at position R0, which depends on the first beam angle α. For example, this position can be expressed as R0 = A1·tan(α) = A1·tan(2·δ). In some embodiments, the distance A1 between the center of rotatable element 22 and gradient index plate 31 is relatively low (short), such as less than ten centimeters, preferably less than five centimeters, less than three centimeters, or even less than two centimeters. This can provide a relatively compact arrangement and / or allow a relatively small plate to receive light beams at different angles. In other or additional embodiments, distance A1 can be greater than five centimeters, or even greater than ten centimeters. Using a larger distance can be beneficial for manufacturability, for example, where a less steep gradient is required. At each position R0, the gradient index plate 31 is configured to redirect the light beam received from the first beam direction B1 along a corresponding second beam direction B2 that intersects the target plane P (preferably directly; alternatively, it can be redirected via another intermediate optical device). More specifically, the gradient index plate 31 has a gradient refractive index along its surface so that the position where the light beam intersects the target plane P (e.g., radial coordinate R) is linearly dependent on the rotation angle δ. For example, the ideal linear dependence can be expressed as R(δ)=C·δ, where "C" is a constant. For example, at Figure 3A , C≈97 mm / 10°, that is, C=9.7 mm for every one degree of rotation of the rotatable element 22 .

[0027] Figure 2B Details of a gradient index plate for linearizing the beam position R are shown, along with simulated beams of light passing through the plate at different angles. In some embodiments, the gradient index plate 31 has a gradient refractive index n along a surface of the plate that is disposed perpendicular to the central axis A. In one embodiment, the gradient refractive index n is radially symmetric about the central axis A. Typically, the refractive index at the center of the plate differs from the refractive index at the edge of the plate by, for example, at least 1% (1.01 times), at least 5% (1.05 times), up to 10% (1.1 times), or more. For example, the refractive index has a peak at the central axis A and decreases monotonically at a radial distance "r" from the center. In the simulation shown, the refractive index of the plate as a function of the radial distance from the center of the plate (central axis A) is expressed as the quadratic equation n(r) = n0 + n1·r + n2·r 2The constant n0 is set to 1.5, and the constants n1 and n2 are fitted to linearize the position dependence of the beam at a distance of 300 mm passing through the plate at a distance of 20 mm. This gives the constant n1 = 8.6476 × 10 -5 and n2=-2.1919×10 -3 The resulting refractive index "n" of a plate with a diameter of 16 mm (8 mm radius) thus varies between 1.5 at the center and 1.36 at the edge. Other gradients may of course be used, for example resulting in a smaller or larger proportionality constant "C" and / or to accommodate any other optical setup.

[0028] Figure 3A A graph comparing the linearity of the beam position R for different simulation systems is shown. The line indicated by "GRIN" corresponds to the linearity of the beam position R according to Figure 2B Figure 2 shows the beam position R of a system with a gradient refractive index (GRIN) plate. The line indicated by "f-θ" corresponds to a simulation of an f-θ lens. Similar to the GRIN plates described herein, f-θ lenses can in principle be used to linearize the dependence of the beam focus on the angle of incidence of the lens. Unlike the GRIN plates described herein, f-θ lenses are typically designed to receive a collimated beam and focus it, but this is not desired herein. Furthermore, the difference is that f-θ lenses are typically designed to maintain the focus at the target plane, while the focusing optics described herein do not need to have a relatively large depth of focus, which is generated before the GRIN plate. Yet another difference is that, compared to an f-θ lens, the GRIN plates described herein can reduce or completely avoid the introduction of unwanted additional aberrations in the beam, which might otherwise negatively affect the depth of focus generated by the focusing optics described herein. The GRIN plates can also be relatively light and / or compact, for example, and can be placed close to a rotating element. Finally, the GRIN plates can be relatively inexpensive and / or save material.

[0029] For comparison, the figure also illustrates what happens if no additional optical elements are placed between the rotatable element 22 and the target plane P. In this case, the position R can be expressed as Ap·tan(2δ), where Ap is the distance traversed by the light beam along the central axis A between the rotatable element 22 and the target plane P. Although it appears that the tangent function exhibits a similarity to a linear dependence for a small angular range up to α=2δ=20°, there are actually significant deviations, as shown in the following graph.

[0030] Figure 3BThe graph shows the deviation from the ideal linear correlation. As shown, for the GRIN plate and f-theta lens, the deviation from the linear fit is relatively small, while when no linearization element is used (denoted by "None"), the deviation is quite significant. The deviation, or error ε, is defined here as the difference between the corresponding correlation R(δ) and the linear fit C·δ, where the linear fit is a line through the origin with a directional coefficient C that minimizes the least-squares difference from the function R(δ) within the operating range δ = [δmin, δmax] (e.g., ±10 degrees). The total deviation can be quantified by the standard deviation of the value ε(δ) = |R(δ) - C·δ|. In this example, when no linearization element is used ("None"), the standard deviation from the linear fit is 1 mm; for the f-theta lens, it is 0.02 mm; and for the GRIN plate, it is 0.03 mm, representing a significant improvement.

[0031] In general, a GRIN plate can be said to linearize the dependence R(δ) of the radial coordinate R on the rotation angle δ when the standard deviation of the difference ε(δ) between this dependence relation R(δ) and a linear fit C·δ to this dependence is reduced by the GRIN plate compared to the case where no linearization element is used. For example, the linearization should be valid within the operating range (δmin, δmax) of the rotatable element (e.g., the maximum angle at which the light beam still passes through the GRIN plate). Preferably, the linearization results in a standard deviation of less than 1% of the operating range Rp = (Rδmin, Rδmax), more preferably less than 0.5%, less than 0.1%, less than 0.05%, or even less than 0.02%. For example, for the present GRIN plate, the standard deviation is 0.03 mm, which is only 0.015% of the operating range Rp (≈2×94 mm).

[0032] Figure 4 A focusing optic 10 is shown that introduces spherical aberration to produce a focused beam with an extended focal region Fr.

[0033] In some embodiments, the focusing optics 10 are configured to generate a focused beam B having a focal point located at a relatively long focal length Af (the path length distance between the last focusing element and the target plane P), e.g., at least twice greater than the working range Rp = |R(δmin), R(δmax)]. The relatively long focal length Af may correspond to a relatively long focal region and / or depth of focus. In other or additional embodiments, the focusing optics 10 include one or more optical elements configured to introduce aberrations (e.g., spherical aberration) into the beam. The inventors have discovered that by adjusting these aberrations, the depth of focus, e.g., the length Lr of the focal region Fr, can be significantly extended.

[0034] Preferably, the focusing optics 10 comprises at least one spherical element. For example, the focusing optics 10 comprises at least one spherical optical surface configured to introduce spherical aberration into the generated focused light beam B, wherein the spherical aberration is configured to maximize the length Lr of the focus region Fr, for example, as described below with reference to Figure 5A and Figure 5B More preferably, the focusing optical device 10 comprises at least two spherical optical surfaces on one or more optical elements. Most preferably, the focusing optical device 10 comprises at least a first optical element 11 and a second optical element 12, the first optical element 11 having at least one spherical surface 11s for introducing a first set of spherical aberrations into the light beam, the second optical element 12 having at least one spherical surface 12s for introducing a second set of spherical aberrations into the light beam, wherein the second optical element 12 is arranged at a distance from the first optical element 11, wherein the different sets of spherical aberrations are adjusted to interfere with each other to maximize the length Lr of the focal region Fr, for example, as described below. Figure 5A and Figure 5B defined. Alternatively, in addition to lenses and / or mirrors having one or more spherical optical surfaces, other optical elements can also be envisaged, such as metalens that can introduce spherical aberration. Preferably, but not necessarily, there should be a focus between the first optical element 11 and the second optical element 12. For example, the first optical element 11 is configured to focus the light beam at a position in front of the second optical element 12. This may cause the divergent light beam to be incident on the spherical surface of the second optical element 12 with a relatively large angular range, for example, enhancing spherical aberration. The required angular range can also be provided by other or additional settings (for example, including a combination of negative and positive lenses), for example, including a combination of negative and positive lenses. This may depend on the incident light beam and the lens strength. Alternatively, or in addition, the angular range can be variable, for example, set by a controller for determining spherical aberration, focus characteristics and / or other applications.

[0035] In one embodiment, for example as shown in the figure, the focusing optical device 10 includes a first optical element 11, and a spherical surface 11s of the first optical element 11 faces the collimated light beam received from the light source 5 (e.g., a laser). The spherical surface may introduce a first set of spherical aberrations in the light beam. In another or additional embodiment, for example as shown in the figure, the first optical element 11 has a flat optical surface opposite to the spherical surface. Alternatively, the flat optical surface can also be a second spherical surface. In another or additional embodiment, for example as shown in the figure, the focusing optical device 10 includes a second optical element 12, and the spherical surface 11s of the second optical element 12 faces the light beam received from the first optical element 11. Preferably, the first optical element 11 is a focusing element, and the second optical element 12 is located outside the focus of the first optical element 11. Therefore, the second optical element 12 can receive a divergent light beam from the first optical element 11. Alternatively, the first optical element 11 can also be a defocusing element, such as a negative lens or a mirror. Preferably, at least the second optical element 12 is a focusing element that, together with the first optical element 11, results in a converging non-diffracted light beam. Alternatively, in addition to a light beam having a relatively large focal area due to spherical aberration, other non-diffracted light beams, such as a Bessel light beam, may be used. For example, a Bessel light beam may be generated using an axicon and / or other optical devices.

[0036] As described above and shown in the figures, the GRIN plate 31 is flat. Due to its flatness (i.e., the absence of any curved optical surfaces), the GRIN plate does not introduce additional aberrations that could interfere with the focusing function. In particular, the flat GRIN plate does not interfere with the spherical aberration introduced into the light beam by at least one spherical optical surface of the focusing optics 10 (e.g., the spherical surfaces of optical elements 11 and / or 12). This allows the length Lr of the focal region Fr to be relatively unaffected, particularly when the angle at which the light beam impinges on the plate's surface is altered by the rotatable element 22. For example, if the GRIN plate were replaced with an f-theta lens having a curved surface, the curvature of the f-theta lens would interfere with the spherical aberration introduced by the prior focusing optics 10, which is specifically tuned to maximize the length Lr of the focal region Fr. The flatness of the GRIN plate also allows it to be placed at a relatively close distance A1 relative to the rotatable element 22 and receive the focused light beam at a relatively large angle of incidence in direction B1, where a curved optical surface would significantly affect the focusing characteristics of the light beam.

[0037] Figure 5A shows the intensity distribution of a focused beam with spherical aberration along the axial and transverse coordinates; Figure 5B The variation of the beam power percentage Ir / Itot passing through circular areas with different radii with the axial coordinate Z is shown; Figure 6 Shown in Figure 5A Cross-sectional profiles Ia, Ib, Ic of the focused light beam at different axial positions shown in FIG.

[0038] As described herein, the focusing optics are configured to focus the beam into a focal region Fr. The focal region Fr is understood to be the region of relatively narrow beam width, typically located near the focal point (waist) where the beam width is narrowest. For a Gaussian beam profile, the beam width is typically defined as (twice) the radius of the beam at which the relative intensity (irradiance) is 1 / e of its maximum value. 2 (13.5%). More generally, the width of a beam at any axial position along the optical axis of the beam can be defined as the diameter of a (circular) area that contains a certain percentage of the beam power or irradiance. For example, the D86 width is defined as the diameter of a circle centered at the centroid of the beam profile and containing 86% of the beam power. This percentage corresponds to the relative amount of power contained in a circular Gaussian beam profile integrated to 1 / e of its maximum value. 2 The latter definition applies more generally to beam profiles with multiple peaks, in particular, for example Figure 6 The beam profile shown has a set of concentric rings with different intensity peaks. For example, such concentric rings may represent spherical aberration and / or other aberrations of the focused beam, such as those intentionally introduced by focusing optics, as described herein.

[0039] In some embodiments, for example, Figure 5A As shown, the length Lr of the focal region Fr (also known as the depth of focus or focal depth) is defined as the length of the segment of the focused beam along the optical axis of the beam where the D86 width (diameter) is less than 400 μm (i.e., the radius is less than 200 μm). Preferably, this length is at least one centimeter, more preferably at least five centimeters, for example, up to ten centimeters or longer. For example, as shown for the current beam profile, the length Lr is approximately 100 mm, or 10 cm. Of course, other or additional definitions may be used, such as the length of the focal region where the D50 width is less than 200 μm (i.e., the radius is less than 100 μm). Preferably, this length is at least one centimeter, more preferably at least five centimeters. For example, as shown for the current beam profile, the length is approximately 8 cm. It will be appreciated that a beam with a relatively large (long) depth of focus can provide greater tolerance for the overlap of the focal region Fr with the target plane P. For example, the beam can be directed at different angles (α, β) relative to the target plane while maintaining a relatively small beam profile overlap, and / or the position of the target plane P can be varied within the focal region Fr. This can facilitate various applications such as laser marking, for example.

[0040] Figure 7A and Figure 7BControl of an optical system 100 for laser marking a product is shown. It will be appreciated that the optical system 100 described herein may be particularly suitable for laser marking a product. In particular, the linear dependence of the (radial) position of the beam focus and the rotation angle of the rotatable element 22 (e.g., a mirror) can make the system relatively easy to control and / or improve accuracy. It will also be appreciated that the relatively large depth of focus facilitates marking of various surfaces that may vary in height and / or shape relative to the target plane.

[0041] In one embodiment, the optical system 100 includes an actuator (not shown) configured to actuate the rotatable element 22, i.e., to determine the rotation angle δ. Typically, the actuator can be controlled by an input control signal Sc (e.g., an electrical signal). For example, this can be an analog control signal that implements a specific rotation in the actuator. Alternatively, or in addition, a digital control signal can be used. For example, a motor such as a stepper motor can be used that can only rotate in discrete steps with a fixed step size. Advantageously, by using a linearized GRIN plate, a fixed step size of the actuator rotation can be converted into a corresponding fixed step size of the focal region translation along the target plane. For example, it is possible to prevent the step size of the applied marking from deviating at the outer edges of the range.

[0042] In one embodiment, optical system 100 includes a controller 50 configured to receive an image to be marked on a product. For example, the image may comprise a line drawing defined by a set of X and Y coordinates. In another or additional embodiment, controller 50 is configured to control at least one rotatable element 22, for example, by sending a control signal Sc to an actuator of rotatable element 22 to set or change a rotation angle δ. In another or additional embodiment, controller 50 is configured to apply a marking M to the product by linearly rotating at least one rotatable element 22 by a corresponding rotation angle δ, where the rotation angle δ depends on the coordinates of the image (e.g., the X and Y coordinates of the line drawing).

[0043] In some embodiments, such as shown, controller 50 is configured to apply laser marking M as the product moves through marking region W. For example, marking region W is defined by the length Lr of focal region Fr and / or the rotational range δ = [δmin, δmax] of rotatable element 22, where gradient index plate 31 linearizes the beam position. In one embodiment, laser marking M is applied to the product as it moves through marking region W at a constant velocity V. For example, the product is moved by conveyor belt 40 or other transport mechanism. In another or additional embodiment, controller 50 is configured to impose a constant rotational speed (e.g., a continuously or intermittently increasing angle at a constant rate of change) on rotatable element 22 in addition to the variable rotation depending on the pattern to be applied, wherein the constant rotational speed is configured to cause focused beam B to track the velocity V of the product moving through marking region W. As will be appreciated, this allows for significantly faster and / or more efficient application of the marking compared to systems that require intermittently stopping the product's speed to apply the marking.

[0044] For the sake of clarity and brevity, features are described herein as being part of the same or separate embodiments. However, it should be understood that the scope of the invention may include embodiments having combinations of all or some of the features described. For example, while embodiments are shown for a laser marking system, the optical system can also be used for other or additional applications, such as laser writing or cutting. The various elements of the embodiments discussed and shown provide certain advantages, such as improved control over the position of the laser beam. Of course, it should be understood that any of the above-described embodiments or methods can be combined with one or more other embodiments or methods to provide further improvements in design and advantage finding and matching. It should be understood that the present disclosure provides unique advantages for laser marking and can generally be applied to any application where precise control of the laser beam is critical.

[0045] For clarity and brevity, features are described herein as being part of the same or separate embodiments. However, it should be understood that the scope of the present invention may include embodiments having combinations of all or some of the described features. For example, while the present teachings have particular advantages for optical systems that generate a focused beam and control the position of the focused region, those skilled in the art, with the benefit of this disclosure, may also envision alternative systems to achieve similar functionality and results. For example, an alternative optical system may be configured to control the position of a substantially collimated beam. For example, an alternative optical system may include a light source and / or one or more optical elements configured to generate such a collimated beam. Preferably, the collimated beam has a relatively small constant beam width, for example, having a substantially constant D86 beam width of less than one millimeter, preferably less than one-half millimeter, and more preferably less than 250 μm. Alternatively, or in addition, a non-diffracting beam, such as a Bessel beam, may be used. A relatively small but constant beam can also be used in various applications, such as laser marking of objects. It will be appreciated that alternative optical systems may include beam steering optics and gradient index plates that are the same or similar to those described herein. For example, a beam steering optical device includes at least one rotatable element configured to receive a collimated light beam having a certain angle of incidence and redirect the collimated light beam along a first beam direction, wherein the first beam direction has a first beam angle relative to a central axis, wherein the central axis is perpendicular to a target plane, wherein the first beam angle can be controlled by setting a rotation angle of the rotatable element, thereby changing the angle of incidence. For example, a gradient refractive index plate is configured to receive a collimated light beam along the first beam direction and redirect the collimated light beam along a second beam direction, wherein the second beam direction has a second beam angle relative to the central axis, wherein the collimated light beam redirected along the second beam direction has a substantially constant (small) beam width that overlaps the target plane at a radial coordinate relative to an intersection of the central axis and the target plane, wherein the gradient refractive index plate is configured to linearize the dependence of the radial coordinate on the rotation angle. Of course, it will be understood that any of the above-described embodiments can be combined with one or more other embodiments (including alternative optical systems) to provide further improvements in design and advantage discovery and matching. It will be appreciated that the present disclosure provides particular advantages for laser marking, and may generally be applied to any application requiring improved beam position control.

[0046] In interpreting the appended claims, it should be understood that the word "comprising" does not exclude the presence of other elements or activities than those listed in a given claim; the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements; any reference signs in a claim do not limit its scope; several "means" may be represented by the same or different items or structures or functions implemented; and unless expressly stated otherwise, any disclosed device or part thereof may be combined together or split into more parts. When one claim refers to another claim, this may indicate that synergistic advantages can be achieved by combining the respective features. However, just because certain measures are stated in mutually different claims does not mean that the combination of these measures cannot be fully utilized. Therefore, the present embodiment may include all working combinations of the claims, each of which may in principle refer to any preceding claim unless the context clearly excludes it.

Claims

1. An optical system (100), comprising: Focusing optics (10) comprising a set of focusing elements (11, 12) configured to generate a focused light beam (B), wherein the set of focusing elements (11, 12) comprises at least one spherical surface (11s, 12s) configured to introduce a corresponding set of spherical aberrations into the focused light beam (B), wherein the spherical aberrations are adjusted to maximize a length (Lr) of a focus region (Fr); A beam steering optical device (20) comprising at least one rotatable element (22) configured to receive a focused light beam (B) having an incident angle (θ) and redirect the focused light beam (B) along a first beam direction (B1), wherein the first beam direction (B1) has a first beam angle (α) relative to a central axis (A), wherein the central axis (A) is perpendicular to a target plane (P), wherein the first beam angle (α) can be controlled by setting a rotation angle (δ) of the rotatable element (22) to change the incident angle (θ); and A flat gradient refractive index plate (31) configured to receive the focused light beam (B) along the first beam direction (B1) and redirect the focused light beam (B) along a second beam direction (B2), wherein the second beam direction (B2) has a second beam angle (β) relative to the central axis (A), wherein the focused light beam (B) redirected along the second beam direction (B2) has a focal region (Fr) that overlaps the target plane (P) at a radial coordinate (R) relative to an intersection of the central axis (A) and the target plane (P), wherein the gradient refractive index plate (31) is configured to linearize a dependency of the radial coordinate (R) on the rotation angle (δ).

2. The optical system according to claim 1, wherein A focused light beam (B) along the first beam direction (B1) and having spherical aberration introduced by the at least one spherical surface (11s, 12s) of the set of focusing elements (11, 12) is received onto the flat gradient index plate (31) at a variable incident angle according to a controllable rotation angle (δ) of the rotatable element (22) placed directly in front of the flat gradient index plate (31), wherein the flatness of the gradient index plate (31) prevents the introduction of additional aberrations into the focused light beam (B) received onto the gradient index plate (31) at the variable incident angle, thereby maintaining a maximum length (Lr) of the focusing region (Fr) according to the adjustment of the spherical aberration.

3. The optical system according to any one of the preceding claims, wherein The rotatable element (22) is arranged in the path of the focused light beam (B) directly in front of the gradient index plate (31) and is rotatable about at least two different rotation axes.

4. An optical system according to any one of the preceding claims, wherein The axial distance (A1) between the center of the rotatable element (22) and the gradient index plate (31) is less than five centimeters.

5. The optical system according to any of the preceding claims, comprising at least one actuator configured to rotate the at least one rotatable element (22) by the rotation angle (δ) in discrete steps with a fixed step size.

6. An optical system according to any one of the preceding claims, wherein The gradient refractive index plate (31) comprises a gradient refractive index (n) having a refractive index that varies along a direction perpendicular to a central axis (A) of the plate, wherein the gradient refractive index (n) is radially symmetric around the central axis (A).

7. An optical system according to any one of the preceding claims, wherein The focusing optical device (10) comprises at least a first optical element (11) and a second optical element (12), wherein the first optical element has at least one spherical surface (11s) for introducing a first set of spherical aberrations into the light beam, and the second optical element has at least one spherical surface (12s) for introducing a second set of spherical aberrations into the light beam, wherein the second optical element (12) is arranged at a certain distance from the first optical element (11), wherein the different sets of spherical aberrations are adjusted to interfere with each other to maximize the length (Lr) of the focus region (Fr).

8. An optical system according to any one of the preceding claims, wherein The first length (Lr) of the focal region (Fr) is defined as the length of a segment of the focused light beam along the optical axis of the light beam where the D86 width is less than 400 μm, wherein the second length (Lr) of the focal region (Fr) is defined as the length of a segment of the focused light beam where the D50 width is less than 200 μm, wherein the first length and the second length are both at least five centimeters.

9. The optical system according to any one of the preceding claims, wherein: The focusing optics (10) are configured to receive a collimated light beam from a laser (5).

10. A laser marking system comprising: An optical system (100) according to any one of the preceding claims; as well as A controller (50) is configured to receive an image of a marking to be applied to a surface and control at least one rotatable element (22) to apply the marking.

11. The laser marking system according to claim 10, wherein: The controller (50) is configured to apply the marking by linearly rotating the rotatable element (22) by a corresponding rotation angle (δ) according to the corresponding coordinates of the feature in the received image.

12. The laser marking system according to claim 10 or 11, wherein: The controller (50) is configured to apply the marking as the product moves through the marking area (W), wherein the marking is applied to the product as the product moves through the marking area (W) at a constant speed (V), wherein the controller (50) is configured to add a constant rotational speed to the rotatable element (22) in addition to the variable rotation depending on the pattern to be applied, wherein the constant rotational speed is configured to cause the focused light beam (B) to track the product moving through the marking area (W) at the constant speed (V).

13. The laser marking system according to any one of claims 10 to 12, comprising a laser (5) configured to direct a collimated laser beam into the focusing optics (10), wherein The controller (50) is configured to control the light intensity of the focused light beam (B) intersecting the target plane (P).

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