Optical system for controlling a light beam

Abstract

The optical system (100) comprises a focusing optic (10) configured to generate a focused beam (B) at a focal region (Fr). The focusing optic (10) comprises a set of spherical surfaces (11s, 12s) configured to introduce respective spherical aberrations into the focused beam (B). The spherical aberrations are adjusted to maximize the length (Lr) of the focal region (Fr). The system comprises at least one rotatable element (22) configured to redirect the focused beam (B) along a variable beam direction. A gradient index plate (31) is configured to receive the beam from the rotatable element (22) and linearize the dependence of the radial coordinate (R) of the focused beam at the target plane (P) as a function of the rotation angle (δ) of the rotatable element (22). Preferably, the gradient index plate is flat and therefore does not introduce additional aberrations that may interfere with the focusing function.

Description

[Technical Field]

[0001] The present disclosure relates to an optical system for controlling the position of a light beam used, for example, as part of a laser marking system or method for marking a target surface. [Background technology]

[0002] By way of background, U.S. Patent Application Publication No. 2010 / 0065537 describes a focusing optical system that focuses a laser beam generated by a laser source at a predetermined focal length. The focusing optical system introduces spherical aberration to increase the depth of focus while maintaining a small focused light spot size. The focusing optical system may be, for example, a single aspherical lens or a single diffractive focusing lens. Alternatively, the focusing optical system may be a compound optical system including at least two optical components. The compound optical system may also include, for example, a first optical means having a focusing function and a second optical means having a spherical aberration-generating function. The second optical means may be, for example, an aspherical phase plate or a diffractive phase plate. The focusing optical system may also include a laser beam deflection means, such as a polygon mirror or a galvanometer mirror, and the first optical means is an f-theta lens. This allows a small spot with a large depth of focus to be scanned across the focal plane at high speed.

[0003] There remains a need for further simplification and improved accuracy in controlling the position of a light beam for laser marking and other purposes. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] US Patent Application Publication No. 2010 / 0065537 Summary of the Invention

[0005] [Problem to be solved by the invention] [Means for solving the problem] Aspects of the present disclosure relate to an optical system including a focusing optical system configured to generate a focused beam. The system includes a beam steering optical system with at least one rotatable element configured to receive the focused beam and redirect the focused beam in a direction controlled by setting a rotation angle of the rotatable element. The system includes a gradient index (GRIN) plate configured to receive the redirected beam from the rotatable element and redirect the focused beam toward a target plane. The focused beam has a focal region that overlaps with the target plane in a radial coordinate. The GRIN plate is configured to linearize the dependence of the radial coordinate as a function of the rotation angle.

[0006] As will be appreciated, the optical systems described herein may be particularly well-suited for laser marking of products. For example, the linear dependence of the (radial) position of the beam focus as a function of the rotation angle of a rotatable element, e.g., a mirror, may make the system easier to control and / or improve accuracy. The GRIN plates described herein may be easily optimized for a variety of focusing optics. GRIN plates can receive a focused beam and redirect the beam to a well-controlled position while minimizing impact on the focusing characteristics. In contrast, f-theta lenses are typically designed to receive a collimated beam and cause the collimated beam to focus. Furthermore, in contrast to f-theta 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 optics described herein, which are specifically tuned to introduce spherical aberration into the focused beam so that a particularly long focal range can be achieved. [Brief explanation of the drawings]

[0007] These and other features, aspects, and advantages of the presently disclosed devices, systems, and methods will become better understood from the following description, appended claims, and accompanying drawings. [Figure 1A] 1 shows a simulation of an optical system in which the radial coordinate of a focal region overlapping a target plane depends linearly on the rotation angle of a rotatable element. [Figure 1B] 1 shows a simulation of an optical system in which the radial coordinate of a focal region overlapping a target plane depends linearly on the rotation angle of a rotatable element. [Figure 1C] 1 shows a simulation of an optical system in which the radial coordinate of a focal region overlapping a target plane depends linearly on the rotation angle of a rotatable element. [Figure 2A] 1 shows further details of the optical system. [Figure 2B] 1 shows details of a gradient index plate for linearizing beam position and simulated light beams traversing such a plate at different angles. [Figure 3A] 10 shows a graph comparing the beam position linearity of different systems. [Figure 3B] The deviation of the graph from the ideal linear dependence is shown. [Figure 4] 1 shows a focusing optical system that introduces spherical aberration to produce an extended focal region. [Figure 5A] 1 shows the intensity profile along the axial and lateral coordinates of a focused beam with spherical aberration. [Figure 5B] 1 shows the percentage of beam power passing through circular regions with different radii as a function of axial coordinate. [Figure 6] 5B shows cross-sectional profiles of the focused beam at different axial positions shown in FIG. 5A. [Figure 7A] 1 shows the control of an optical system for laser marking of a product. [Figure 7B] 1 shows the control of an optical system for laser marking of a product. DETAILED DESCRIPTION OF THE INVENTION

[0008] The terms used to describe particular embodiments do not limit the invention. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms unless the context clearly dictates otherwise. The term "and / or" includes any and all combinations of one or more of the associated listed items. It can be understood that the terms "comprises" and / or "comprising" specify the presence of the stated features but do not exclude the presence or addition of one or more other features. When a particular step of a method is recited after another step, it can be further understood that, unless otherwise specified, it can follow directly from the other step, or that one or more intermediate steps can be performed before performing the particular step. Similarly, when a connection between structures or components is described, it can be understood that this connection can be established directly or through an intermediate structure or component, unless otherwise specified.

[0009] The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. In the drawings, absolute and relative sizes of systems, components, layers, and regions may be exaggerated for clarity. Embodiments may be described with reference to schematic illustrations and / or cross-sectional views of potentially 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 construed to refer to the orientation as described at the time or as shown in the drawings being discussed. These relative terms are for convenience of description and do not require that the system be constructed or operated in a particular orientation, unless specifically specified.

[0010] 1A-1C show a simulation of one embodiment of an optical system 100. FIG.

[0011] In some embodiments, the optical system 100 includes a focusing optical system 10. Preferably, the focusing optical system 10 includes at least one focusing element 12 configured to generate a converging beam B. In one embodiment, the focusing optical system 10 is configured to receive a collimated light beam generated by a light source (not shown here), for example, a laser generating a collimated (Gaussian) beam. Alternatively, the light source may also generate a diverging or converging beam that is received by the focusing optical system 10. Typically, the focusing optical system 10 includes at least one converging lens and / or mirror. For example, the focusing element 12 is a positive lens with a positive focal length. Multiple focusing elements, for example, two focusing elements 11 and 12, can also be used. For example, the figure shows two positive lenses with a focal point between them. Other or additional optical elements, for example, a combination of positive and negative lenses and / or mirrors, can also be used to result in a net convergence of the light beam received by the focusing optical system 10.

[0012] In some embodiments, optical system 100 includes beam steering optics 20. Preferably, beam steering optics includes at least one rotatable element 22. In one embodiment, rotatable element 22 is configured to receive focused beam B at incident angle θ and redirect focused beam B along a first beam direction B1. In another or further embodiment, first beam direction B1 has a first beam angle α with respect to central axis A. For example, central axis A is perpendicular to target plane P. In one embodiment, first beam angle α is controllable by setting a rotation angle δ of rotatable element 22. For example, rotation can change incident angle θ.

[0013] 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. In general, the second beam direction B2 may differ from the first beam direction B1 (except at one or more specific locations, such as the central axis A and / or a ring of a specific radius around the axis where the beam direction may not be affected). In one embodiment, the second beam direction B2 has a second beam angle β with respect to the central axis A. In another or further embodiment, the focused beam B redirected along the second beam direction B2 has a focal region Fr that overlaps with 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, as described herein, the gradient index plate 31 is configured to linearize the dependence of the radial coordinate R as a function of the rotation angle δ.

[0014] In some embodiments, the beam steering optics 20 comprises at least one rotatable mirror. In one embodiment, the beam steering optics is galvanometer-controlled, e.g., a galvanometric mirror. Other or additional types of beam steering optics, such as a polygonal mirror, can also be used. For example, FIG. 1A shows a rotatable element 22 formed by a rotatable mirror configured to reflect a focused beam B toward a target plane P via a gradient index plate 31. Preferably, the rotatable mirror has a mirror plane that coincides with one or more rotation axes, and / or the light beam is received coincident with the center of rotation. In this way, the origin of the reflected beam can be essentially maintained regardless of the rotation angle. In a preferred embodiment, for example, as shown, the rotatable element 22 is configured to receive the focused beam B at a 45-degree incident angle θ, so that the mirror can reflect the beam at a 45-degree reflection angle along a central axis A that is oriented perpendicular to the target plane P. In principle, such a configuration can provide an optimal range. Of course, other configurations, for example, having different incident angles, can be envisioned.

[0015] Instead of or in addition to the reflective optics, additional rotatable elements for redirecting the beam can also be envisioned, e.g., comprising a rotatable transmissive element that affects the direction of the transmitted beam. Also, more than one rotatable element can be used, e.g., at least two rotatable elements configured to rotate about different axes. For example, the mirror 21 shown in FIGS. 1A-1C can be configured as an additional rotatable element for rotating the direction of the beam transverse to the direction of the rotatable element 22. Note that the use of a first rotatable element can significantly affect the position at which the beam is received on the second rotatable element and also the position at which the beam traverses the gradient index plate 31. In principle, this can be taken into account when designing a gradient index plate 31 with, for example, different gradients in different directions. However, if a single rotatable element that can rotate in different directions is used, preferably if the single rotatable element is positioned in the beam path directly in front of the gradient index plate 31, the design of the gradient index plate can be significantly simplified (e.g., radially symmetric).

[0016] FIG. 2A shows further details of optical system 100. In some embodiments, for example, as shown, rotation of rotatable element 22 through angle δ causes a change in beam direction of the reflected beam by angle α=2δ. In other or further embodiments, for example, as shown, the reflected beam along a first direction having a first beam angle α is received by gradient index plate 31, which can cause a slight deflection of the transmitted beam along a second beam direction B2 having a second beam angle β. As shown, gradient index plate 31 receives the beam from rotatable element 22 at a position R0 that depends on the first beam angle α. For example, the 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 small (short), for example, less than 10 centimeters, preferably less than 5 centimeters, less than 3 centimeters, or even less than 2 centimeters. This can provide a relatively compact arrangement and / or allow a relatively small plate to receive beams at various angles. In other or further embodiments, distance A1 may be greater than 5 centimeters or greater than 10 centimeters. Using a larger distance can facilitate manufacturability, e.g., requiring a less steep gradient. At each position R0, gradient index plate 31 is configured to redirect the beam received from first beam direction B1 along a respective second beam direction B2 that intersects target plane P (preferably directly; alternatively, via additional intermediate optics). More specifically, gradient index plate 31 has a gradient refractive index along its surface such that the position at which the beam intersects target plane P, e.g., radial coordinate R, depends linearly on rotation angle δ. For example, the ideal linear dependence can be expressed as R(δ) = C δ, where “C” is a constant. For example, in FIG. 3A , C ≈ 97 mm / 10°, or 9.7 millimeters per degree of rotation of rotatable element 22.

[0017] FIG. 2B shows details of a gradient index plate for linearizing beam position R and simulated light beams traversing such a plate at different angles. In some embodiments, the gradient index plate 31 comprises a gradient index n along the surface of the plate that is positioned perpendicular to the central axis A. In one embodiment, the gradient 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% (a factor of 1.01), at least 5% (a factor of 1.05), or up to 10% (a factor of 1.1) 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 illustrated simulation, the refractive index of the plate as a function of radial distance from the center (central axis A) of the plate is calculated using the quadratic equation n(r) = n0 + n1 · r + n2 · r^. 2 The constant n0 was set to 1.5, and the constants n1 and n2 were fitted to linearize the position dependence of the beam at a distance of 300 mm across the plate at a distance of 20 mm. This resulted in the constant n1 = 8.6476 x 10^ -5 and n2=-2.1919×10^ -3 is obtained. The resulting plate, with a diameter of 16 mm (radius of 8 mm), therefore has a refractive index "n" that varies between 1.5 at the center and 1.36 at the edge. Of course, other gradients can be used, for example resulting in smaller or larger proportionality constants "C" and / or adapted to any other optical setup.

[0018] FIG. 3A shows a graph comparing the linearity of beam position R for different simulated systems. The line labeled "GRIN" corresponds to the beam position R for a system with a gradient-index (GRIN) plate, as shown in FIG. 2B. The line labeled "fθ" corresponds to a simulation of an fθ lens. Similar to the GRIN plates described herein, fθ lenses can, in principle, be used to linearize the dependence of beam focus as a function of the angle of incidence onto the lens. Unlike the GRIN plates described herein, fθ lenses are typically designed to accept collimated beams and focus the beam, which is not desirable here. Also, fθ lenses are typically designed to maintain the focus at the target plane, which is not necessary for the focusing optics described herein, which have a relatively large depth of focus generated before the GRIN plate. Yet another difference is that, compared to fθ lenses, the GRIN plates described herein can reduce or completely avoid the introduction of undesirable additional aberrations into the beam that could adversely affect the depth of focus generated by the focusing optics described herein. GRIN plates may also be relatively lightweight and / or compact, e.g., capable of being placed close to a rotatable element. Finally, GRIN plates may be relatively inexpensive and / or may conserve material.

[0019] For comparison, this figure also shows what happens when no further 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 that the beam traverses along the central axis A between the rotatable element 22 and the target plane P. Although the tangent function appears to resemble a linear dependence for a small range of angles up to α=2δ=20°, in reality there is a large deviation, as shown in the graph below.

[0020] Figure 3B shows the deviation of the graphs from the ideal linear dependence. As shown, the deviation from the linear fit is relatively small for the GRIN plate and f-theta lens, but is significantly larger when no linearization element is used (denoted by "none"). The deviation, or error, ε, is defined here as the difference between the respective dependence R(δ) and the linear fit C·δ, i.e., the line through the origin with directional coefficient C that minimizes the least-squares difference with the function R(δ) over the operating range δ = [δmin, δmax], e.g., ±10 degrees. The overall 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—a significant improvement.

[0021] In general, a GRIN plate can be considered to linearize the dependence R(δ) of the radial coordinate R as a function of the rotation angle δ if the GRIN plate reduces the standard deviation of the difference ε(δ) between this dependence R(δ) and a linear fit C·δ to this dependence compared to the situation where no linearization element is used. For example, the linearization should be effective over the operating range (δmin, δmax) of the rotatable element, e.g., the maximum angle at which the 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, with the present GRIN plate, the standard deviation is 0.03 mm, which is only about 0.015% of the operating range Rp (≈2 × 94 mm).

[0022] FIG. 4 shows a focusing optical system 10 that introduces spherical aberration to produce a focused beam with an extended focal region Fr.

[0023] In some embodiments, focusing optics 10 is configured to generate focused beam B having a relatively long focal length A (the path length between the last focusing element and target plane P), e.g., a focal point at least two times greater than the working range R = |R(δmin) - R(δmax)|. The relatively long focal length A may correspond to a relatively long focal region and / or depth of focus. In other or further embodiments, focusing optics 10 includes one or more optical elements configured to introduce aberrations, e.g., spherical aberration, into the beam. The inventors have found that by adjusting these aberrations, it is possible to significantly extend the depth of focus, e.g., the length Lr of the focal region Fr.

[0024] Preferably, the focusing optical system 10 comprises at least one spherical element. For example, the focusing optical system 10 comprises at least one spherical optical surface configured to introduce spherical aberration into the generated focused beam B, the spherical aberration configured to maximize the length Lr of the focal region Fr, as defined, for example, with reference to FIGS. 5A and 5B below. More preferably, the focusing optical system 10 comprises at least two spherical optical surfaces on one or more optical elements. Most preferably, the focusing optical system 10 comprises at least a first optical element 11 having at least one spherical surface 11s that introduces a first set of spherical aberrations into the beam and a second optical element 12 having at least one spherical surface 12s that introduces a second set of spherical aberrations into the beam, the second optical element 12 being positioned at a distance from the first optical element 11, the different sets of spherical aberrations adjusted to interfere with each other to maximize the length Lr of the focal region Fr, as defined, for example, with reference to FIGS. 5A and 5B below. Instead of or in addition to lenses and / or mirrors with one or more spherical optical surfaces, other optical elements, such as metalenses, that can introduce spherical aberration are also contemplated. A focal point between the first optical element 11 and the second optical element 12 is not necessarily, but is preferred. For example, the first optical element 11 is configured to focus the beam at a position before the second optical element 12. This causes the diverging beam to strike the spherical surface of the second optical element 12, resulting in a relatively large angular spread, e.g., improved spherical aberration. The desired angular spread can also be provided by other or additional configurations, e.g., including a combination of negative and positive lenses. This may depend on the incident beam and lens strength. Alternatively or additionally, the angular spread can be variable, e.g., set by a controller to determine the spherical aberration, focal characteristics, and / or other applications.

[0025] In one embodiment, as shown, the focusing optical system 10 includes a first optical element 11 with one spherical surface 11s facing the collimated beam received from the light source 5, e.g., a laser. This spherical surface can introduce a first set of spherical aberrations into the beam. In another or further embodiment, as shown, the first optical element 11 has a flat optical surface opposite the spherical surface. Alternatively, this can be a second spherical surface. In another or further embodiment, as shown, the focusing optical system 10 includes a second optical element 12 with a spherical surface 11s facing the 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 positioned beyond the focal point of the first optical element 11. Thus, the second optical element 12 can receive a diverging beam from the first optical element 11. Alternatively, the first optical element 11 can be a defocusing element, such as a negative lens or a mirror. Preferably, at least the second optical element 12 is a focusing element, which together with the first optical element 11 results in a converging non-diffracting beam. Alternatively, or in addition to a light beam with a relatively large focal area based on spherical aberration, other non-diffracting beams, such as Bessel beams, can be used. For example, Bessel beams can be generated using axicons and / or other optical systems.

[0026] As described above and shown in the figures, GRIN plate 31 is flat. Due to its flatness, i.e., lack of curved optical surfaces, the GRIN plate does not introduce additional aberrations that might otherwise interfere with its focusing function. In particular, the flat GRIN plate does not interfere with the spherical aberration introduced into the beam by at least one spherical optical surface of focusing optics 10, e.g., the spherical surface of optical elements 11 and / or 12. In this way, the length Lr of focal region Fr can be relatively unaffected, particularly when the angle of the beam impinging on the plate's surface is changed by 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 a preceding focusing optics 10 specifically tuned to maximize the length Lr of focal region Fr. The flatness of the GRIN plate also allows it to be positioned at a relatively close distance A1 to the rotatable element 22 and accept a focused beam along direction B1 at a relatively large angle of incidence, although a curved optical surface positioned at this location would significantly affect the focusing characteristics of the beam.

[0027] Figure 5A shows the intensity profile along the axial and lateral coordinates of a focused beam with spherical aberration. Figure 5B shows the percentage of beam power Ir / Iot passing through a circular area with different radii as a function of the axial coordinate Z. Figure 6 shows the cross-sectional profiles Ia, Ib, and Ic of the focused beam at different axial positions shown in Figure 5A.

[0028] As described herein, focusing optics are configured to focus a light beam to a focal region Fr. The focal region Fr is understood to be the region where the beam width is relatively narrow, typically the region around its focal point (waist) where the beam width is narrowest. For a Gaussian beam profile, the width of the beam is typically 1 / e^ of its maximum relative intensity (irradiance). 2The width of a beam at any axial position along the beam's optical axis can be defined as the diameter of a (circular) area that contains a certain percentage of the beam's power or irradiance. For example, the D86 width is defined as the diameter of a circle centered at the center of gravity of the beam profile and containing 86% of the beam power. This percentage is 1 / e of that maximum value. ^2 corresponds to the relative amount of power contained in a circular Gaussian beam profile integrated up to 1000 . The latter definition is more generally applicable to beam profiles having multiple peaks, particularly beam profiles having a set of concentric rings with different intensity peaks, such as those shown in Figure 6. For example, such concentric rings can refer to spherical aberration and / or other aberrations in a focused beam that are intentionally introduced, for example, by focusing optics, as described herein.

[0029] In some embodiments, as shown in FIG. 5A , the length Lr (also referred to as focal depth or depth of focus) of the focal region Fr is defined as the length of a section of the focused beam along the beam's optical axis, with a D86 width (diameter) of less than 400 μm (i.e., a radius of less than 200 μm). Preferably, this length is at least 1 centimeter, more preferably at least 5 centimeters, e.g., up to 10 centimeters or more. For example, as shown for the present beam profile, this length Lr is approximately 100 mm, i.e., 10 cm. Of course, other or further definitions can also be used, such as a focal region length with a D50 width of less than 200 μm (i.e., a radius of less than 100 μm). Preferably, this length is at least 1 centimeter, more preferably at least 5 centimeters. For example, as shown for the present beam profile, this length is approximately 8 cm. As will be appreciated, having a beam with a relatively large (long) focal depth can provide a greater tolerance for overlapping the focal region Fr with the target plane P. For example, the light beams may be directed at various angles (α, β) relative to the target plane while maintaining a relatively small beam profile and overlap, and / or the position of the target plane P may be variable within the focal region Fr. For example, this can facilitate various applications such as laser marking.

[0030] 7A and 7B illustrate the control of the optical system 100 for laser marking of a product. As will be appreciated, the optical system 100 described herein may be particularly suitable for laser marking of a product. In particular, the linear dependence of the (radial) position of the beam focus on the rotation angle of the rotatable element 22, e.g., a mirror, may make the system relatively easier to control and / or improve accuracy. As will also be appreciated, a relatively large depth of focus may facilitate marking of various surfaces that may vary in height and / or shape relative to the target plane.

[0031] 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 is controllable by inputting a control signal Sc, e.g., an electrical signal. For example, this can be an analog control signal that executes a specific rotation within the actuator. Alternatively or additionally, a digital control signal can be used. For example, a motor, such as a stepper motor, that can only rotate in discrete steps with a fixed step size can be used. Preferably, by using a linearized GRIN plate, the fixed step size of the actuator rotation can be translated into a corresponding fixed step in the translation of the focal region along the target plane. For example, this can prevent deviations in the step size of the applied markers at the outer edges of their range.

[0032] In one embodiment, the optical system 100 includes a controller 50 configured to receive an image to be marked on the product. For example, the image includes a line drawing defined by a set of X, Y coordinates. In another or further embodiment, the controller 50 is configured to control the at least one rotatable element 22, for example, to set or change the rotation angle δ by sending a control signal Sc to an actuator of the rotatable element 22. In another or further embodiment, the controller 50 is configured to apply the marking M to the product by rotating the at least one rotatable element 22 through a respective rotation angle δ that is linearly dependent on the coordinates of the image, for example, the X, Y coordinates of the line drawing.

[0033] In some embodiments, as shown, the controller 50 is configured to apply the laser marking M to the product as it moves through the marking region W. For example, the marking region W is defined by the length Lr of the focal region Fr and / or the rotation range δ=[δmin, δmax] of the rotatable element 22 over which the gradient index plate 31 linearizes the beam position. In one embodiment, the laser marking M is applied to the product as it moves through the marking region W at a constant velocity V. For example, the product is moved by a conveyor belt 40 or other transport mechanism. In another or further embodiment, the controller 50 is configured to apply a constant rotational speed (e.g., applying a continuously or intermittently increasing angle at a constant rate) to the rotatable element 22 in addition to a variable rotation depending on the pattern to be applied, the constant rotational speed being set to cause the focused beam B to track the velocity V of the product as it moves through the marking region W. As will be appreciated, this allows for significantly faster and / or more efficient application of the marker compared to systems that require the product velocity to be intermittently stopped to apply the marking.

[0034] For clarity and conciseness, features may be described herein as part of the same or separate embodiments, but it will be understood that the scope of the present invention may include embodiments having all or any combination of the described features. For example, while an embodiment of a laser marking system is shown, the optical system may also be used for other or additional applications, such as laser writing or cutting. Various elements of the embodiments described above and shown provide certain advantages, such as improved control of laser beam position. Of course, it will be understood that any one of the above embodiments or processes may be combined with one or more other embodiments or processes to further refine the design and discovery and adaptation of advantages. It will be understood that the present disclosure provides particular advantages for laser marking and may be generally applicable to any application where precise control of a laser beam is important.

[0035] For clarity and conciseness, features may be described herein as part of the same or separate embodiments, but it will be understood that the scope of the present invention may include embodiments having all or some combination of the described features. For example, while the present teachings have particular advantages in optical systems that generate focused beams and control the position of focal regions, alternative systems may be envisioned by those skilled in the art with the benefit of this disclosure for achieving similar functions and results. For example, alternative optical systems may be configured to control the position of a substantially collimated beam. For example, alternative optical systems may include a light source and / or optical elements configured to generate such a collimated beam. Preferably, the collimated beam has a relatively small, constant beam width, e.g., a substantially constant D86 beam width of less than one millimeter, preferably less than half a millimeter, and more preferably less than 250 μm. Alternatively or additionally, non-diffracting beams, such as Bessel beams, may be used. Relatively small, constant beams may similarly be used in various applications, such as laser marking of objects. As will be appreciated, alternative optical systems may include the same or similar beam steering optics and gradient index plates as those described herein. For example, the beam steering optical system may include at least one rotatable element configured to receive a collimated beam at an incident angle and redirect the collimated beam along a first beam direction, the first beam direction having a first beam angle relative to a central axis, the central axis being perpendicular to a target plane, the first beam angle being controllable by setting a rotation angle of the rotatable element to change the incident angle. For example, the gradient index plate may be configured to receive the collimated beam along the first beam direction and redirect the collimated beam along a second beam direction, the second beam direction having a second beam angle relative to the central axis, the collimated beam redirected along the second beam direction having 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, and the gradient index plate may be configured to linearize the dependence of the radial coordinate as a function of the rotation angle.It will of course be understood that any one of the above embodiments can be combined with one or more other embodiments (including alternative optical systems) to further improve the design and discover and adapt advantages. It will be understood that the present disclosure provides particular advantages to laser marking, but may be applied generally to any application where improved positional control of a beam is desired.

[0036] When interpreting the appended claims, it should be understood that the word "comprising" does not exclude the presence of other elements or acts than those listed in a given claim. The words "a" or "an" preceding an element do not exclude the presence of a plurality of such elements. Any reference signs in the claims do not limit their scope. Several "means" may be represented by the same or different items or implemented structures or functions. Any of the disclosed apparatuses or parts thereof may be combined together or separated into further parts, unless otherwise specified. When a claim refers to another claim, this may indicate synergistic advantages achieved by the combination of their respective features. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be advantageously used. Thus, the present embodiments may include all actual combinations of claims, where each claim may in principle refer to any preceding claim, unless clearly excluded by the context.

Claims

1. a focusing optical system (10) comprising a set of focusing elements (11, 12) configured to generate a focused beam (B), the set of focusing elements (11, 12) comprising at least one spherical surface (11s, 12s) configured to introduce a respective set of spherical aberrations into the focused beam (B), the spherical aberrations being adjusted to maximize a length (Lr) of a focal region (Fr); a beam steering optical system (20) comprising at least one rotatable element (22) configured to receive the focused beam (B) at an angle of incidence (θ) and to redirect the focused beam (B) along a first beam direction (B1), the first beam direction (B1) having a first beam angle (α) with respect to a central axis (A), the central axis (A) being perpendicular to a target plane (P), the first beam angle (α) being controllable by setting a rotation angle (δ) of the rotatable element (22) to change the angle of incidence (θ); a flat gradient index plate (31) configured to receive the focused beam (B) along the first beam direction (B1) and to redirect the focused beam (B) along a second beam direction (B2), the second beam direction (B2) having a second beam angle (β) with respect to the central axis (A), the focused beam (B) redirected along the second beam direction (B2) having a focal region (Fr) overlapping with the target plane (P) at a radial coordinate (R) relative to an intersection of the central axis (A) and the target plane (P), the gradient index plate (31) being configured to linearize the dependence of the radial coordinate (R) as a function of the rotation angle (δ); An optical system (100) comprising:

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

3. 3. The optical system of claim 1, wherein the rotatable element (22) is positioned in the path of the focused beam (B) directly in front of the gradient index plate (31) and can rotate around at least two different rotation axes.

4. 4. The optical system of claim 1, wherein the axial distance (A1) between the center of the rotatable element (22) and the gradient index plate (31) is less than 5 centimeters.

5. 5. The optical system of claim 1, comprising at least one actuator configured to rotate the at least one rotatable element (22) in discrete steps having a step size fixed with respect to the rotation angle (δ).

6. 6. The optical system of claim 1, wherein the gradient index plate (31) comprises a gradient index (n) having a refractive index that varies along a direction perpendicular to the central axis (A) of the plate, the gradient index (n) being radially symmetric about the central axis (A).

7. 7. The optical system of claim 1, wherein the focusing optical system comprises at least a first optical element having at least one spherical surface that introduces a first set of spherical aberrations into the beam, and a second optical element having at least one spherical surface that introduces a second set of spherical aberrations into the beam, the second optical element being positioned at a distance from the first optical element, and wherein the different sets of spherical aberrations are adjusted to interfere with each other to maximize the length of the focal region.

8. 8. The optical system of claim 1, wherein a first length (Lr) of the focal region (Fr) is defined as the length of a section of the focused beam along the optical axis of the beam having a D86 width of less than 400 μm, and a second length (Lr) of the focal region (Fr) is defined as the length of a section of the focused beam along the optical axis of the beam having a D50 width of less than 200 μm, and wherein both the first length and the second length are at least 5 centimeters.

9. 9. The optical system of any one of claims 1 to 8, wherein the focusing optics (10) is configured to receive a collimated light beam from a laser (5).

10. An optical system (100) according to any one of claims 1 to 9, a controller (50) configured to receive an image of a marking to be applied to a surface and to control the at least one rotatable element (22) to apply the marking; A laser marking system comprising:

11. 11. The laser marking system of claim 10, wherein the controller (50) is configured to apply the marking by rotating the rotatable element (22) through a respective rotation angle (δ) linearly dependent on respective coordinates of features in the received image.

12. 12. The laser marking system of claim 10 or 11, wherein the controller (50) is configured to apply the marking while the product moves through a marking area (W), the marking being applied to the product while the product moves through the marking area (W) at a constant velocity (V), and the controller (50) is configured to add a constant rotational speed to the rotatable element (22) in addition to a variable rotation depending on a pattern to be applied, the constant rotational speed being configured so that the focused beam (B) tracks the product moving through the marking area (W) at the constant velocity (V).

13. 13. The laser marking system of claim 10, further comprising a laser (5) configured to direct a collimated laser beam to the focusing optics (10), and the controller (50) configured to control the light intensity of the focused beam (B) intersecting the target plane (P).

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