WO2021037577A1

WO2021037577A1 - Optical system for light steering

Info

Publication number: WO2021037577A1
Application number: PCT/EP2020/072763
Authority: WO
Inventors: Stefan Bernet; Monika Ritsch-Marte; Martin BAWART
Original assignee: Medizinische Universität Innsbruck
Priority date: 2019-08-23
Filing date: 2020-08-13
Publication date: 2021-03-04

Abstract

The present invention relates to an optical system for light steering comprising a first decentered optical element having a negative optical power, an aperture diameter, a center of curvature and a focal point, and defining a first optical axis passing through the center of curvature and the focal point; and a first holder, the first optical element being mounted on the first holder such that the first optical element is rotatable about a first axis of rotation. The optical system further comprises a second decentered optical element having a positive optical power, an aperture diameter, a center of curvature and a focal point, and defining a second optical axis passing through the center of curvature and the focal point; and a second holder, the second optical element being mounted on the second holder such that the second optical element is rotatable about a second axis of rotation. The first and second optical elements can be rotated independently from each other. The first axis of rotation is parallel to the first optical axis with a distance between the first axis of rotation and the first optical axis being at least 5% of the aperture diameter of the first decentered optical element and the second axis of rotation is parallel to the second optical axis with a distance between the second axis of rotation and the second optical axis being at least 5% of the aperture diameter of the second decentered optical element.

Description

Optical System for Light Steering

Devices which deflect light beams are implemented in a wide variety of technical systems such as laser scanners, laser printers, scan heads for laser writing, welding and marking, laser show systems, and in other fields. Generally, systems for deflecting beams employ galvo mirror systems in which a set of rotating galvo mirrors are used to deflect light beams along a two- dimensional plane. However, galvo systems present numerous drawbacks, for instance, certain minimum space requirements. Galvo system mirrors must be placed a certain distance away from each other resulting in an undesired beam displacement as a function of the distance between the two mirrors. Other beam deflection systems include rotating elements that use a pair of prisms which are both rotated. However, rotation of such bulky parts leads to limited rotation speeds.

Furthermore, in any optical system there is a certain amount of misdirected light that is reflected or diffracted from optical elements. In typical beam steering systems this misdirected light is capable of being diffracted or reflecting off of one or more optical elements and then being emitted from the system, such as a laser cutting system, in the direction of a workpiece. In other words, the misdirected light emitted from typical beam steering systems is still collimated. This light would then pass through a successive focusing lens to be focused on the work piece in an undesired location. Thus, the misdirected light arrives in focus and at the wrong location on the workpiece, resulting in potentially marking the workpiece in the wrong positions

It is an object of the present invention to provide an optical system which is particularly suited for light steering and provides an improvement over known light steering systems.

This object is achieved with the features of the independent claims. Dependent claims refer to preferred embodiments.

The present invention, inter alia, relates to an optical system for light steering comprising a first decentered optical element having a negative optical power, an aperture diameter, a center of curvature, and a focal point, and defining a first optical axis passing through the center of curvature and the focal point. The optical system further comprises a first holder, wherein the first holder optical element is mounted on the first holder such that the first optical element is rotatable about a first axis of rotation. The optical system comprises a second decentered optical element having a positive optical power, an aperture diameter, a center of curvature and a focal point, and defining a second optical axis passing through the center of curvature and the focal point, and a second holder, the second optical element being mounted on the second holder such that the second optical element is rotatable about a second axis of rotation. The first and second optical elements can be rotated independently from each other. The center of curvature represents a point on a surface of the optical element which, when viewed in profile, is the apex of the curvature of the optical element. In other words, the first derivative has a value of zero at the center of curvature. The center of curvature can also he at a virtual point outside of the dimensions of the physical optical element for certain types of optical elements.

The optical system of the present invention allows for precise light steering by simply rotating the first and second optical elements which can be done manually or automatically, e.g., by means of actuators or motors. Moreover, the system is very compact while at the same time achieving a large span of steering angles. Thus, the system is more versatile and can be more easily implemented than, e.g., the galvo mirror systems known in the art.

The aperture of a decentered optical element is the largest circle which can be inscribed within the optical element perpendicular to the optical axis of the optical element. The aperture diameter is defined as the diameter of the aperture circle. The geometric center of an optical element is the center point of the aperture circle. Thus, in the case of a circular optical element, the aperture diameter is simply the diameter of the optical element and the geometric center is simply the center point of the circular optical element.

The system utilizes two decentered optical elements. The actual amount of decentering depends generally on the optical requirements of the system, its geometry and the like. It is, however, preferred that the first axis of rotation is parallel to the first optical axis with a distance between the first axis of rotation and the first optical axis being at least 5% of the aperture diameter of the first decentered optical element. It is further preferred that the second axis of rotation is parallel to the second optical axis with a distance between the second axis of rotation and the second optical axis being at least 5% of the aperture diameter of the second decentered optical element.

The aperture diameter of the first and/or second decentered optical element may assume any size and may be determined by the specific use of the optical system. The aperture diameter of the first and/or second decentered optical element may typically be within a range from 2 mm to 10 cm.

One advantage of the present invention over previous systems is that any misdirected light passing through the optical system for light steering will not be collimated when it is emitted from the optical system and thus, after passing through a focal lens will not have a focal point in the same plane as the intended workpiece. In other words, misdirected light beams will not be in focus in the plane of the work piece and will thus have reduced potential for marking the workpiece in an undesired location. This argument also applies to the case of decentered optical elements realized as diffractive optical elements. Such diffractive optical elements typically have a limited diffraction efficiency, and therefore they diffract some light into undesired diffraction orders. However, this misdirected light will have a different divergence as compared to the correctly diffracted light, and thus will not be in focus in the plane of the work piece.

Preferably, the distance between the first axis of rotation and the first optical axis is at least 10% of the first aperture diameter, more preferably at least 20% of the first aperture diameter, even more preferably at least 30% of the first aperture diameter. Preferably, the distance between the second axis of rotation and the second optical axis is at least 10% of the second aperture diameter, more preferably at least 20% of the second aperture diameter, even more preferably at least 30% of the second aperture diameter.

Preferably the first and second holders are configured to releasably interlock, such that the first and second holders are enabled to rotate together as a unit. Having optical elements enabled to rotate independently controls the degree of deflection of the light. Optical elements that rotate together allow for control of the polar direction of the light. Thus, holders that releasably interlock allow for simultaneous and precise control of both the degree of deflection and the direction of light. Preferably, the first axis of rotation and the second axis of rotation are parallel. Preferably, the first axis of rotation and the second axis of rotation are aligned. For most general applications the first and second optical elements would normally be provided parallel to one another. In order to minimize space considerations the first and second optical elements would preferably be aligned.

Preferably, the absolute value of the optical power of the first optical element is equal to the absolute value of the optical power of the second optical element.

Preferably, the absolute value of the optical power of the first decentered optical element satisfies the equation B > 0.01

where Bi is the optical power and Di is the aperture diameter of the first decentered optical element.

Preferably, the absolute value of the optical power of the second decentered optical element satisfies the equation B₂ > 0.01 D₂ ^X where B2 is the optical power and D2 is the aperture diameter of the second decentered optical element.

Preferably, the smallest distance between the first optical element and second optical element is no greater than 1/5 of the aperture diameter of the larger of the first and second decentered optical elements (or preferably both decentered optical elements), preferably no greater than 1/10 of the aperture diameter of the larger of the first and second decentered optical elements (or preferably both decentered optical elements), more preferably no greater than 1/20 of the aperture diameter of the larger of the first and second decentered optical elements (or preferably both decentered optical elements). A small distance between the two optical elements allows for a more compact optical system.

Preferably, the optical system is adapted and configured for controlling the angle of deflection and the polar direction of a light beam passing through the optical system by changing the rotational orientation of the first optical element and/or the rotational orientation of the second optical element. Preferably, the polar direction of light emerging from the optical system can be varied between 0° and 360°.

Preferably, the angle of deflection of light emerging from the optical system can be varied between 0° and 10°, preferably between 0° and 30°, more preferably between 0° and 60°.

Preferably, the first optical element and/or the second optical element are circular lenses. Preferably, the first lens and/or the second lens has a parabolic lens profile and/or a spherical lens profile. Circular lenses present a more easily produced configuration.

Preferably the first lens is a plano-convex lens and the second lens is a plano-concave lens.

Preferably the first lens and second lens are mounted such that a convex side of the first lens faces toward a concave side of the second lens. Such an arrangement provides the smallest optical aberrations. However, in order to achieve a particularly compact system it is also preferred to have the plane sides facing each other.

Preferably, the first optical element and/or the second optical elements comprise diffractive structures. Preferably, the first and/or second optical elements comprise blazed diffractive structures. A diffractive configuration of the optical system may reduce the amount of space required for the optical system, and the weight of its optical components. Additionally, diffractive optical elements can reduce optical aberrations. Preferably, the first and/or second diffractive structures are first order diffractive structures or higher order diffractive structures. Said diffractive structures can provide for a more coordinated transmission of specific light wavelengths.

Preferably, each optical element comprises a structured surface and a planar surface.

Preferably, the optical system further comprises a drive system, wherein the drive system comprises at least one motor configured to rotate the first and/or second optical elements and a controller for controlling the motor. A drive system provides for precise adjustment of the light angle and direction and allows for automation of the light steering. The present invention is also directed to a laser scanning device comprising the optical system as described herein.

The present invention is also directed to an imaging system comprising the optical system as described herein.

The present invention is also directed to a laser machining apparatus comprising the optical system as described herein.

The present invention is also directed to a decentered diffractive lens comprising a center of curvature; a focal point; an optical axis passing through the center of curvature and the focal point; an aperture comprising an aperture diameter and a geometric center, the geometric center being the center of the aperture; and a geometric axis passing through the geometric center and being parallel to the optical axis; wherein a distance between the geometric axis and the first optical axis is at least 5% of the aperture diameter. In case the decentered diffractive lens comprises one planar and one curved surface, the center of curvature is to be defined with regard to the curved surface. In case the decentered diffractive lens comprises two curved surfaces, the above condition shall hold true for the center of curvature of at least one of the curved surfaces.

The present invention is also directed to use of an optical system for light steering as described herein which includes the steps of providing the optical system as described herein, directing light through the optical system such that the light passes through both the first optical element and the second optical element. Further steps include adjusting the angle of deflection of the light by rotating either the first optical element or the second optical element. Further steps include adjusting the polar direction of the light by rotating the first optical element and second optical element together. It will be evident to the skilled person that the first and second optical elements need not necessarily be rotated simultaneously, even though this is preferred.

The subject-matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments which are illustrated in the attached drawings, in which: Fig. 1 illustratively shows an embodiment of the optical system for light steering;

Fig. 2a illustratively shows a circular lens version of the first decentered optical element;

Fig. 2b illustratively shows a circular lens version of the second decentered optical element;

Fig. 3 depicts an embodiment of the optical system including two decentered circular lenses;

Fig. 4a schematically illustrates a decentered diffractive lens with a positive optical power and a decentered diffractive lens with a negative optical power;

Fig. 4b schematically illustrates a cross-section of a decentered diffractive lens with a positive optical power and a cross-section of a decentered diffractive lens with a negative optical power; and

Fig. 5 schematically illustrates the diffractive patterns of light passing through a diffractive lens version of the optical system dependent on the relative angle between the first and second decentered optical lenses.

In principle, identical parts are provided with the same reference signs in the figures.

The terms “beam deflection” and “light steering” may be interchangeably used within the description. Both terms refer to using a system to redirect incident light such that it leaves the system at an angle relative to the angle of incidence. Light, in this context may refer to light of any wavelength within the electromagnetic spectrum. Specifically, light having a visible wavelength may be steered with the present system, as well as infrared light, ultraviolet light, and other wavelengths may also be used together with the present system.

Fig. 1 provides an illustrative embodiment of the optical system for light steering 100. The system 100 comprises a first decentered optical element 110 and a second decentered optical element 150. Both decentered optical elements are described in more detail with respect to Fig. 2a and Fig. 2b. The optical system 100 further comprises a first holder 120, which is adapted to securely hold the first decentered optical element 110. The first holder 120 is configured such that the first optical element 110 is rotatable about a first axis of rotation Ri. The optical system 100 also comprises a second holder 160, which is adapted to securely hold the second decentered optical element 150. The second holder 160 is configured such that the second optical element 150 is rotatable about a second axis of rotation R2. In the embodiment shown the first and second axes of rotation are identical. Yet, the skilled person will understand that the first and second axes of rotation may also be parallel to each other with the two axes being displaced relative to each other or that the first and second axes of rotation may even be arranged not to be parallel to each other.

Light incident upon the optical system 100 is shown as two parallel arrows moving from left to right through the system 100. As is schematically shown, the light emerging from the optical system 100 is at an angle with respect to the direction of light incident upon the system, thus the optical system 100 can be used for light steering.

Use of the optical system 100 for light steering involves providing the optical system as described herein then directing incident light through the optical system 100 such that the light passes through both the first decentered optical element 110 and the second decentered optical element 150. The angle of deflection of the light, i.e. the angle between light emerging from the optical system 100 with respect to the direction of light incident upon the system, is then adjusted by rotating either the first decentered optical element 110 around its first axis of rotation Ri or the second decentered optical element 150 around its second axis of rotation R2. The polar direction of the light emitted from the optical system 100 is controlled by rotating the first decentered optical element 110 and the second decentered optical element 150 together as a single unit. Thus, the two optical elements being independently rotatable allows for independently adjusting both the angle of deflection and the polar direction of the emerging light.

In the illustration of Fig. 1 the optical elements 110 and 150 are shown to both be circular lenses and of the same diameter, however such a configuration is not necessary to realize the benefits of the present invention. The first decentered optical element 110 and the second decentered optical element 150 may also be formed as cylindrical lenses, or other non-circularly shaped lenses.

Generally, the axis of rotation Ri of the first decentered optical element 110 can be roughly parallel to the direction of light incident upon the optical system 100, however, this does not need to be the case. Other orientations of the axis of rotation Ri are envisioned as long as the axis of rotation Ri is not perpendicular to the direction of light incident upon the optical system 100. In Fig. 1 the axis of rotation Ri is shown as passing through the geometric center of the optical element, which may be the most straightforward arrangement, however, this also need not be the case.

Light which emerges from the optical system 100 will in general provide a cone of accessibility, within which each position can be accessed. Thus in a polar direction, the light emerging from the optical system can be varied between 0° and 360°. The angle of deflection of light emerging from the optical system is dependent on the precise configuration of the optical system 100, but may be varied between 0° and 10°, preferably between 0° and 30°, more preferably between 0° and 60°.

A basic requirement of the optical system 100 is that the first and second optical elements are decentered, i.e. that the first optical axis Oi and the first axis of rotation Ri are not aligned or positioned within a negligible distance of one another, as may be the case, for instance, in an unintentionally decentered or aberrant lens. Preferably, the axis of rotation Ri is displaced from the first optical axis Oi by a distance of at least 5% of the aperture diameter of the first decentered optical element 110. Preferably, the distance between the axis of rotation Ri and the first optical axis Oi may be even larger, such as at least 10% of the first aperture diameter, at least 20% of the first aperture diameter, or at least 30% of the first aperture diameter. The optical axis can even be positioned outside of the physical dimensions of the optical element in configurations wherein the center of curvature lies outside of the physical optical element.

The aperture of a decentered optical element is the largest circle which can be inscribed within the optical element perpendicular to the optical axis of the optical element. The aperture diameter is the diameter of the aperture circle. The geometric center of an optical element is the center point of the aperture circle. Thus, in the case of a circular optical element, the aperture diameter is simply the diameter of the optical element and the geometric center is simply the center of the circular optical element.

The aperture diameter of the first and/or second decenter ed optical element may assume any size and may be determined by the specific use of the optical system. The aperture diameter of the first and/or second decenter ed optical element may typically be within a range from 2 mm to 10 cm.

Similarly, the second optical axis O2 and the second axis of rotation R2 may not align or be positioned within a negligible distance of one another. Preferably, the axis of rotation R2 must be displaced from the second optical axis O2 by a distance of at least 5% of the aperture diameter of the second decentered optical element 110. Preferably, the distance between the axis of rotation R2 and the second optical axis O2 may be even larger, such as at least 10% of the second aperture diameter, at least 20% of the second aperture diameter, or at least 30% of the second aperture diameter. The optical axis can be even outside of the physical dimensions of the optical element.

In the illustration of Fig. 1 the first axis of rotation Ri and the second axis of rotation R2 are aligned, in that they form one unitary axis of rotation R. However, such a configuration is only one possible configuration of the system, other configurations in which the first axis of rotation Ri and the second axis of rotation R2 are separated by a distance are also within the scope of the invention. An aligned configuration may be beneficial in certain circumstances, for instance, when the space occupied by the optical system 100 should be minimized and/or when the first and second decentered optical elements 110, 150 are of the same size and shape.

While the first and second holders 120, 160 are configured to rotate independently of one another, it is preferred that the first and second holders 120, 160 are also configured to releasably interlock and rotate together as a unit. Interlocking between the first and second holders 120, 160 may be performed by a latch, a hook, a spring and pin, abutment, or any other type of reversible engagement. The optical system 100 may also comprise a drive system with at least one motor configured to rotate the first and/or second optical elements, and a controller for controlling the motor. For many implementations of the optical system 100, having precise and automated control of light deflection and direction can be very beneficial. For instance, a laser beam deflection system will often need quick and accurate deflection of the laser beam. The controller in this instance could be a computer or processor.

As shown in Fig. 1, the first decentered optical element 110 can be separated a distance from the second decentered optical element 150. While the space between the two optical elements can be very small, wherein in some configurations the first decentered optical element 110 and the second decentered optical element 150 may even abut, or be directly adjacent to one another, the exact distance between the first decentered optical element 110 and the second decentered optical element 150 can be determined by the requirements of the optical setup. In some configurations the smallest distance between optical elements measured from any point on the first decentered optical element 110 to any point on the second decentered optical element 150 is no greater than 1/5 of the aperture diameter of the larger of the first and second decentered optical elements, preferably no greater than 1/10 of the aperture diameter of the larger of the first and second decentered optical elements, more preferably no greater than 1/20 of the aperture diameter of the larger of the first and second decentered optical elements. Optionally the smallest distance between the first optical element and second optical element is no greater than 10 mm, preferably no greater than 8 mm, preferably no greater than 5 mm.

One example of the first decentered optical element 110 is depicted in more detail in Fig. 2a. The first decentered optical element 110 has a negative optical power. A negative optical power is defined in that incoming parallel light rays, as shown in the three arrows on the left of Fig. 2a, are divergent after passing through the first decentered optical element 110. A negative optical power can also be described by the location of the focal point 114 of the first optical element 110, which is positioned in the space on the light- incident side of the first optical element 110.

The optical power of the first decentered optical element 110 may be any negative number. Naturally, the optical power of the first decentered optical element 110 is dependent on the particular use of the optical system 100, more particularly, dependent on the wavelengths of light used with the optical system 100. Thus, the optical power of the first decentered optical element 110 may satisfy the equation satisfies the equation

B₁ < -0.01

where Bi is the optical power and Di is the aperture diameter of the first optical element. In specific implementations of the optical system 100 for light steering, the optical power of the first optical element 110 may have an optical power within the range of -0.5 m ¹ to -0.01 m ¹, preferably within the range of -5 m ¹ to -0.5 m ¹, and more preferably within the range of -50 m ¹ to -5 m ¹.

An example of the second decentered optical element 150 is depicted in Fig. 2b. The second decentered optical element 150 has a positive optical power. A positive optical power is defined in that incoming parallel light rays, as shown in the three arrows on the left of Fig. 2b, are convergent after passing through the second decentered optical element 150. A positive optical power can also be described by the location of the focal point 154 of the second optical element 150, which is positioned in the space opposite of the light-incident side of the second optical element 150.

The optical power of the second decentered optical element 150 may be any positive number. Similar to the first optical element 110, the optical power of the second decentered optical element 150 is dependent on the particular use of the optical system 100, more particularly dependent on the aperture and the wavelengths of light used with the optical system 100. Thus, the optical power of the second decentered optical element 150 may satisfy the equation

B₂ > 0.01 Z⁾ ₂ ¹ where B2 is the optical power and D2 is the aperture diameter of the second decentered optical element. In specific implementations of the optical system 100, the optical power of the second optical element 150 may have an optical power within the range of 0.5 m ¹ to 0.01 m ¹, preferably within the range of 5 m ¹ to 0.5 m ¹, and more preferably within the range of 50 m ¹ to 5 m ¹. In some configurations of the optical system 100, it is advantageous to provide a second decentered optical element 150 with an optical power that is equal to the absolute value of the optical power of the first decentered optical element 110.

As shown in Fig. 2a the first decentered optical element 110 comprises a center of curvature 112. The center of curvature 112 is a point on a surface of the optical element when viewed in profile which is the apex of the curvature of the optical element. In certain configurations, the center of curvature can also he at a virtual point outside of the dimensions of the physical optical element. Mathematically, the center of curvature of the optical element is a point on the surface of the optical element when viewed in profile from any polar angle that would have a first derivative value of zero. Generally, the center of curvature 112 is a point of the surface of the optical element, or on a virtual plane continuing the surface of the optical element and extending beyond the physical optical element, around which there is some degree of rotational symmetry of the surface of the optical element. For example, if the optical element consists of a simple lens, then the center of curvature corresponds to the center of the lens. In the case of a diffractive lens, the center of curvature may be the center of concentric rings of diffractive structures. Together the focal point 114 and the center of curvature 112 of the first optical element 110 define the first optical axis Oi.

A corresponding center of curvature 152 is depicted for the second decentered optical element 150 in Fig. 2b. The second center of curvature 152 has all of the features of the first center of curvature 112.

Fig. 3 depicts one possible configuration of the optical system for light steering 100. In this configuration the first decentered optical element 110 and the second decentered optical element 150 are both circular lenses wherein the flat surfaces of the lenses are facing toward one another. Such a configuration may be beneficial for saving space in an optical configuration. Alternatively, the two circular lenses may be arranged with flat surfaces facing away from one other. Additionally, the order of the first decentered optical element 110 and the second decentered optical element 150 in the direction of the incident light may be interchanged. In this illustration the first and second holders 120, 160 are not shown, but would attach to the first and second optical elements 110, 150, respectively, at the periphery. Incoming light is depicted in Fig. 3 as two parallel rays entering from the left and exiting from the right. Due to the rotational displacement of the second optical element 150 relative to the first optical element, the light is emitted at an angle compared to the angle of the incident light. Rotation of the first and second optical elements 110, 150 allows for the optical system 100 to direct light to be emitted at any angular position within a cone with a maximal opening angle. The maximum opening angle of the cone depends on the focal length (1 /optical power) of both the first and second decentered optical elements 110, 150 as well as the displacement of the optical axis Oi from the axis of rotation Ri, and the displacement of the optical axis O2 from the axis of rotation R2.

In the exemplary configuration shown in Fig. 3 the axis of rotation Ri of the first decentered optical element 110 is aligned with the axis of rotation R2 of the second decentered optical element 150 although this is not a requirement of the optical system 100.

Fig. 4a schematically illustrates a diffractive version of the first and second decentered optical elements 210, 250. Therein, the black point density corresponds to the phase shifts in an interval between 0 and 2p for single-order diffractive elements. The same can be observed between 0 and multiples of 2p for multi-order diffractive elements. As shown in Fig. 4a, one optical element has a positive optical power and the other optical element has a negative optical power. Consequently, in the assembled optical system the two elements are arranged adjacent to one another.As describe above, the aperture diameter Di, D2 of a decentered optical element is the diameter of the largest circle which can be inscribed within the optical element perpendicular to the optical axis of the optical element. Thus, in the case of a circular optical element, for example, as depicted in Fig. 4a, the aperture diameter is simply the diameter of the optical element. Although the first optical element 210 and the second optical element 250 are depicted as circular optical elements, they may take on other geometric shapes as described above. The first optical element 210 further comprises a first geometric center 216, which is the geometric center of the largest circle which can be inscribed within the optical element perpendicular to the optical axis of the optical element. Thus, in the case of a circular optical element, the geometric center 216 is simply the center of the optical element. Analogously, the second optical element 250 also comprises a second geometric center 256 which is defined in the same manner. The first rotational axis Ri passes perpendicularly through the first optical element 210 and the second rotational axis R2 passes perpendicularly through the second optical element 250. Preferably the first rotational axis Ri passes perpendicularly through the first geometric center 216. Preferably the second rotational axis R2 passes perpendicularly through the second geometric center 256.

The center of curvature 212 of the first optical element 210 is depicted as the center of the dark circle. Similarly, the center of curvature 252 of the second optical element 250 is the center of the white circle.

The first optical element 210 is decentered, i.e. that the first optical axis Oi and the geometric center 216 are not aligned or positioned within a negligible distance of one another, as may be the case, for instance, in an unintentionally decentered or aberrant lens. Preferably, the geometric center 216 (or equivalently, the axis of rotation Ri) is displaced from the first optical axis Oi by a distance of at least 5% of the aperture diameter Di of the first decentered optical element 210. Preferably, the distance between the geometric center 216 (or axis of rotation Ri) and the first optical axis Oi may be even larger, such as at least 10% of the first aperture diameter, at least 20% of the first aperture diameter, or at least 30% of the first aperture diameter. The optical axis can even be positioned outside of the physical dimensions of the optical element in configurations wherein the center of curvature lies outside of the physical optical element.

The second optical element 250 is decentered, i.e. that the second optical axis O2 and the geometric center 256 are not aligned or positioned within a negligible distance of one another, as may be the case, for instance, in an unintentionally decentered or aberrant lens. Preferably, the geometric center 256 (or equivalently, the axis of rotation R2) is displaced from the second optical axis O2 by a distance of at least 5% of the aperture diameter D2 of the second decentered optical element 250. Preferably, the distance between the geometric center 256 (or axis of rotation R2) and the second optical axis O2 may be even larger, such as at least 10% of the second aperture diameter, at least 20% of the second aperture diameter, or at least 30% of the second aperture diameter. The optical axis can even be positioned outside of the physical dimensions of the optical element in configurations wherein the center of curvature lies outside of the physical optical element.

In the diffractive configuration of the optical system for beam steering, the optical elements may be very thin, for example, maximally 0.5 mm thick each, and placed very close to one another such that the combined optical system may be no thicker than 1 mm combined. In some configurations the smallest distance between optical elements measured from any point on the first decentered optical element 210 to any point on the second decentered optical element 250 is no greater than 1/5 of the aperture diameter of the larger of the first and second decentered optical elements, preferably no greater than 1/10 of the aperture diameter of the larger of the first and second decentered optical elements, more preferably no greater than 1/20 of the aperture diameter of the larger of the first and second decentered optical elements. Optionally the smallest distance between the first optical element and second optical element is no greater than 10 mm, preferably no greater than 8 mm, preferably no greater than 5 mm. The first and second optical elements 210, 250 may have optical powers as described in connection with Figs. 1-3, above.

It should be understood that the diffractive first optical element 210 and the diffractive second optical element 250 may be provided in place of the first and second optical elements as described in the context of the optical system 100 of Figs. 1-3 as described above and therefore be used in combination with the first and second holders 120, 160 and/or drive system as previously described. In other words, it is envisioned that the diffractive first and second optical elements 210, 250 are optical elements that fulfill all the requirements of the inventive optical system.

Fig. 4b schematically illustrates a representative cross-section through the diffractive version of the first and second decentered optical elements 210, 250 shown in Fig. 4a. In the cross-section the height of the diffractive blazing correspond to the phase shifts in an interval between 0 and 2p for single-order diffractive elements. The same can be observed between 0 and multiples of 2p for multi-order diffractive elements. Fig. 4b also illustrates the center of curvature 212 of the first optical element 210 and the center of curvature 252 of the second optical element 250. Furthermore, it the geometric center 216 of the first optical element 210 is shown as positioned a distance away from the center of curvature 212 as has previously been described. The geometric center 256 of the second optical element 250 is positioned a distance away from the center of curvature 252 as has previously been described. Although one surface of each optical element is depicted as structured, while the other is depicted as flat, this is not a necessary requirement of the optical elements.

An analytical description of the configuration depicted in Figs. 4a and 4b, wherein the diffractive optical elements are placed on top of one another, is presented hereafter to aid in understanding the functionality of the optical system 100. However, the analysis presented may differ depending on the configuration of the optical system 100.

The first decentered optical element of Figs. 4a and 4b has a phase function Pi and the second decentered optical element has a phase function P2. For the case of diffractive optical elements, these phase function are wrapped to a selected phase interval, which is typically done by applying a Modulo-Operation to the phase functions, as for example a Modulo^ operation to obtain a first order diffractive optical element, or a Modulo-(N 2p) operation to obtain an N¹¹¹ order diffractive optical element. For the sake of convenience, these Modulo-operations are omitted in the following discussion, but it is understood that the discussion also applies to such wrapped phase functions which characterize diffractive optical elements.

In this example the first optical element is a positive lens with a focal length / and with a displacement of the optical axis Oi from the rotational axis Ri of a to the right side (along the x- axis). The diffractive elements are designed for transmitting light having a wavelength _d. The Cartesian coordinates are given by X and Y. The phase function Pi is written as:

The second decentered optical element 150/250 with negative focal length f and a displacement of the optical axis O2 from the rotational axis R2 by a distance a to the left (negative x-direction) has the phase function:

P₂ = - -⁽{X + a)² + Y²). (2) If the first element is rotated by an angle of Q around a shared rotational axis Ri, R2, the phase function then becomes:

The phase Ptot of the total transmission function is given by adding the phase of the rotated first decentered optical element 110/210 to the stationary second decenter ed optical element 150/250, resulting in:

The phase of the corresponding transmission function corresponds to that of a blazed grating with grating vector G of:

which has a magnitude of

The grating vector points in a direction <Pdeflect measured from the x-axis (horizontal) in polar coordinates of:

which is also the direction into which the beam is deflected.

The total beam deflection angle F of a perpendicularly incident collimated beam is then given by the standard formula for diffraction at a plane grating, where l denotes the wavelength of the incoming light:

Thus, the maximal deflection angle is obtained for 0 = 0, and corresponds to

If such a deflected beam is focused by an additional standard lens with a focal length f_s, the ( x,y ) coordinates of the focused spot in the focal plane are given by:

Since refractive elements show a much lower wavelength dependency than diffractive elements, one can set l = _d for the case of refractive elements.

It should be noted that in order to address certain (x,y) coordinates the above formulas have to be inverted. The above mathematical inversion does have an analytical solution, however the procedure is quite lengthy, and thus only the basic procedure is provided. First a person must calculate the desired magnitude of the deflection angle from the desired focal spot positioned in the plane of the object of focus. Then the correct relative rotation angle Q between the two elements can be obtained by inverting equation 10 above. This angle is then adjusted by rotating the first decentered optical element 110/210 correspondingly. According to equation 7, adjusting Q goes along with adjusting a certain deflection direction F _def_lect = —Q/2. In order to address the correct deflection direction, which has again been calculated from the desired spot position, one has to rotate the set of the two optical elements 110/210, 150/250 as a whole by a correction angle, which is the difference of the desired angle to F deflect. Of course, this rotation as a whole unit can also be performed if the rotation angles of the first and second decentered optical elements 110/210, 150/250 are controlled individually. In this case, the two elements are just rotated by the same correction angle in the same direction. Of note is that in the calculation of the rotation angles for the two optical elements by inverting equation 10, two possible solutions for the relative rotation angle Q arise, namely, a positive and a negative solution, both with the same absolute value. Either of the solutions can be used, however, they result in different absolute rotation angles of the two elements. A natural option would be to choose the one of the two solutions which is closer to the actual angular positions of the two elements, and which would thus minimize the additional rotation angles needed to achieve the new position.

Fig. 5 depicts an example of the mutual phase function of two diffractive elements as shown in Fig. 4a and Fig. 4b and rotating one of them by an angle indicated in Fig. 5 and according to the equations as described above. For a mutual rotation angle of 0° one obtains a blazed grating with minimal grating constant and maximal magnitude of the deflection angle. For rotation angles of 45°, 90° and 135° the resulting blazed grating successively increases its grating constant, rotates, and decreases the angle of deflection. For the opposite angles (-45°, -90°, and -135°) the same grating constants are achieved, however the directions of the grating vector and thus, deflection direction is different. For a mutual rotation angle of 180° the phase function is constant, producing no beam deflection.

The angle of deflection Q of a light beam relative to the optical axis Oi, Ch in the diffractive version of the optical system 100 is given by sin Q = l/d, where l is the wavelength of the incoming light and d is the grating constant of the diffractive decentered optical element.

For higher rotation angles, the grating constant increases as an inverse cosine- function of the half rotation angle until the grating completely disappears at an angle of 180°, resulting in no beam deflection. The direction of the grating vector with respect to the horizontal direction as shown in Fig. 5, corresponds to one half of the relative rotation angle between the first decentered optical element 110/210 and the second decentered optical element 150/250.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and non-restrictive; the invention is thus not limited to the disclosed embodiments. Variations to the disclosed embodiments can be understood and effected by those skilled in the art and practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality and may mean “at least one”.

The following are preferred aspects of the invention:

1. An optical system for light steering comprising: a first decentered optical element having a negative optical power, an aperture diameter, a center of curvature and a focal point, and defining a first optical axis passing through the center of curvature and the focal point; a first holder, the first optical element being mounted on the first holder such that the first optical element is rotatable about a first axis of rotation; a second decentered optical element having a positive optical power, an aperture diameter, a center of curvature and a focal point, and defining a second optical axis passing through the center of curvature and the focal point; and a second holder, the second optical element being mounted on the second holder such that the second optical element is rotatable about a second axis of rotation; wherein the first and second optical elements can be rotated independently from each other.

2. The optical system of aspect 1, wherein the first axis of rotation is parallel to the first optical axis with a distance between the first axis of rotation and the first optical axis being at least 5% of the aperture diameter of the first decentered optical element.

3. The optical system of any one of the previous aspects, wherein the second axis of rotation is parallel to the second optical axis with a distance between the second axis of rotation and the second optical axis being at least 5% of the aperture diameter of the second decentered optical element. 4. The optical system of any one of the previous aspects, wherein the first optical element and second optical element comprise diffractive structures, preferably wherein the first and second optical elements comprise blazed diffractive structures.

5. The optical system of aspect 4, wherein the first and second diffractive structures are first order diffractive structures or higher order diffractive structures.

6. The optical system of aspect 4 or aspect 5, wherein each optical element comprises a structured surface and a planar surface.

7. The optical system of any one of the previous aspects, wherein the distance between the first axis of rotation and the first optical axis is at least 10% of the first aperture diameter, preferably at least 20% of the first aperture diameter, more preferably at least 30% of the first aperture diameter.

8. The optical system of any one of the previous aspects, wherein the distance between the second axis of rotation and the second optical axis is at least 10% of the second aperture diameter, preferably at least 20% of the second aperture diameter, more preferably at least 30% of the second aperture diameter.

9. The optical system of any one of the previous aspects, wherein the first and second holders are configured to releasably interlock, such that the first and second holders are enabled to rotate together as a unit.

10. The optical system of any one of the previous aspects, wherein the first axis of rotation and the second axis of rotation are parallel, preferably aligned.

11. The optical system of any one of the previous aspects, wherein the absolute value of the optical power of the first decentered optical element is equal to the absolute value of the optical power of the second decentered optical element.

12. The optical system of any one of the previous aspects, wherein the absolute value of the optical power of the first decentered optical element satisfies the equation

B₁ > 0.01 where Bi is the optical power and Di is the aperture diameter of the first decentered optical element.

13. The optical system of any one of the previous aspects, wherein the absolute value of the optical power of the second decentered optical element satisfies the equation

B₂ > 0.01 D- ¹ where B2 is the optical power and D2 is the aperture diameter of the second decentered optical element.

14. The optical system of any one of the previous aspects, wherein the smallest distance between the first optical element and second optical element is no greater than 1/5 of the aperture diameter of the larger of the first and second decentered optical elements, preferably no greater than 1/10 of the aperture diameter of the larger of the first and second decentered optical elements, more preferably no greater than 1/20 of the aperture diameter of the larger of the first and second decentered optical elements.

15. The optical system of any one of the previous aspects, wherein the system is adapted and configured for controlling the angle of deflection and the polar direction of a light beam passing through the optical system by changing the rotational orientation of the first optical element and/or the rotational orientation of the second optical element.

16. The optical system of any one of the previous aspects, wherein the polar direction of light emerging from the optical system can be varied between 0° and 360°.

17. The optical system of any one of the previous aspects, wherein the angle of deflection of light emerging from the optical system can be varied between 0° and 10°, preferably between 0° and 30°, more preferably between 0° and 60°.

18. The optical system of any one of the previous aspects, wherein the first optical element and/or the second optical element are circular lenses, preferably wherein the first lens and/or the second lens has a parabolic lens profile and/or a spherical lens profile.

19. The optical system of aspect 18, wherein the first lens is plano-convex and the second lens is plano-concave. 20. The optical system of aspect 19, wherein the first lens and the second lens are mounted such that a convex side of the first lens faces toward a concave side of the second lens.

21. The optical system of any one of the previous aspects, wherein the optical system further comprises a drive system, wherein the drive system comprises at least one motor configured to rotate the first and/or second optical elements, and a controller for controlling the motor.

22. A laser scanning device comprising the optical system of any of aspects 1-21.

23. An imaging system comprising the optical system of any of aspects 1-21.

24. A laser machining apparatus comprising the optical system of any of aspects 1-21.

25. Use of an optical system for light steering according to any one of the previous aspects, comprising the steps of: providing the optical system according to any one of aspects 1-21; directing light through the optical system such that the light passes through both the first optical element and the second optical element; adjusting the angle of deflection of the light by rotating either the first optical element or the second optical element; and adjusting the polar direction of the light by rotating the first optical element and second optical element together.

26. A decentered diffractive lens comprising: a center of curvature; a focal point; an optical axis passing through the center of curvature and the focal point; an aperture comprising an aperture diameter and a geometric center, the geometric center being the center of the aperture; and a geometric axis passing through the geometric center and being parallel to the optical axis; wherein a distance between the geometric axis and the first optical axis is at least 5% of the aperture diameter.

Claims

1. An optical system for light steering comprising: a first decentered optical element having a negative optical power, an aperture diameter, a center of curvature and a focal point, and defining a first optical axis passing through the center of curvature and the focal point; a first holder, the first optical element being mounted on the first holder such that the first optical element is rotatable about a first axis of rotation; a second decentered optical element having a positive optical power, an aperture diameter, a center of curvature and a focal point, and defining a second optical axis passing through the center of curvature and the focal point; and a second holder, the second optical element being mounted on the second holder such that the second optical element is rotatable about a second axis of rotation; wherein the first and second optical elements can be rotated independently from each other; wherein the first axis of rotation is parallel to the first optical axis with a distance between the first axis of rotation and the first optical axis being at least 5% of the aperture diameter of the first decentered optical element; and wherein the second axis of rotation is parallel to the second optical axis with a distance between the second axis of rotation and the second optical axis being at least 5% of the aperture diameter of the second decentered optical element.

2. The optical system of claim 1, wherein the first optical element and second optical element comprise diffractive structures, preferably wherein the first and second optical elements comprise blazed diffractive structures.

3. The optical system of claim 2, wherein the first and second diffractive structures are first order diffractive structures or higher order diffractive structures.

4. The optical system of any one of the previous claims, wherein the distance between the first axis of rotation and the first optical axis is at least 10% of the first aperture diameter, preferably at least 20% of the first aperture diameter, more preferably at least 30% of the first aperture diameter.

5. The optical system of any one of the previous claims , wherein the distance between the second axis of rotation and the second optical axis is at least 10% of the second aperture diameter, preferably at least 20% of the second aperture diameter, more preferably at least 30% of the second aperture diameter.

6. The optical system of any one of the previous claims, wherein the first and second holders are configured to releasably interlock, such that the first and second holders are enabled to rotate together as a unit.

7. The optical system of any one of the previous claims, wherein the absolute value of the optical power of the first decentered optical element satisfies the equation

B_± > 0.01

where Bi is the optical power and Di is the aperture diameter of the first decentered optical element; and/or wherein the absolute value of the optical power of the second decentered optical element satisfies the equation

B₂ > 0.01 1⁾ _;,^·1 where B2 is the optical power and D2 is the aperture diameter of the second decentered optical element.

8. The optical system of any one of the previous claims, wherein the smallest distance between the first optical element and second optical element is no greater than 1/5 of the aperture diameter of the larger of the first and second decentered optical elements, preferably no greater than 1/10 of the aperture diameter of the larger of the first and second decentered optical elements, more preferably no greater than 1/20 of the aperture diameter of the larger of the first and second decentered optical elements.

9. The optical system of any one of the previous claims, wherein the first optical element and/or the second optical element are circular lenses, preferably wherein the first lens and/or the second lens has a parabolic lens profile and/or a spherical lens profile.

10. The optical system of claim 9, wherein the first lens is plano-convex and the second lens is plano-concave, preferably wherein the first lens and the second lens are mounted such that a convex side of the first lens faces toward a concave side of the second lens.

11. The optical system of any one of the previous claims, wherein the optical system further comprises a drive system, wherein the drive system comprises at least one motor configured to rotate the first and/or second optical elements, and a controller for controlling the motor.

12. An imaging system comprising the optical system of any of claims 1-11.

13. A laser machining apparatus comprising the optical system of any of claims 1-11.

14. A decentered diffractive lens comprising: a center of curvature; a focal point; an optical axis passing through the center of curvature and the focal point; an aperture comprising an aperture diameter and a geometric center; and a geometric axis passing through the geometric center and being parallel to the optical axis; wherein a distance between the geometric axis and the first optical axis is at least 5% of the aperture diameter.

15. Use of the optical system for light steering according to any one of claims 1 to 11, the use preferably comprising the steps of: providing the optical system according to any one of the previous claims; directing light through the optical system such that the light passes through both the first optical element and the second optical element; adjusting the angle of deflection of the light by rotating either the first optical element or the second optical element; and adjusting the polar direction of the light by rotating the first optical element and second optical element together.