Patent Publication Number: US-2023146283-A1

Title: Modular deflection units in mirror symmetrical arrangement

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to laser processing technologies, such as additive manufacturing. In particular, the invention refers to an optical deflection module and to an optical modular deflection system with deflection units which are designed in pairs having a mirror symmetry for improved coordinated operation and enhanced compactness. 
     BACKGROUND OF THE INVENTION 
     Additive manufacturing processes, in which a material is added layer by layer and thermally processed to produce a component, are becoming more and more important in industrial production as compared to classical subtractive manufacturing processes such as milling, drilling and turning, in which a component is produced by subtracting material from an initial material bulk. The layer-by-layer production method that is characteristic of additive manufacturing processes enables the production of highly complex geometric structures with a high degree of design flexibility that subtractive processes cannot achieve. 
     The increase in the industrial importance of additive manufacturing processes is driven by the increasing efficiency of the light sources used for thermally processing the starting materials. Accordingly, the market is currently experiencing a transition from the use of additive manufacturing processes for the production of prototypes (“rapid prototyping”) to a mass industrial use of this technology for series production (“rapid manufacturing”). This development can be observed in numerous sectors of technology, such as the aerospace industry, the automotive industry, medical technology and prosthetics. 
     A special type of additive manufacturing is based on powder-bed-based processes in which a powder starting material is sequentially applied in layers to the component to be manufactured and melted and processed by a working light beam, typically a laser beam. The powder layers typically have thicknesses in the micrometre range. Scanning units are used for directing the laser light in a controlled manner for melting the powder starting material at a series of target positions according to a predefined process so as to form the desired workpiece. 
     Scanning units typically comprise galvanometers, i.e. mirrors movable around an axis, for scanning the laser light in different directions by reflecting it at corresponding reflection angles. By combining two mirrors movable around two perpendicular axes, the laser light can be scanned throughout a two-dimensional work field. The movement of the galvanometer mirror or mirrors of such scanning units about the respective axes is respectively driven by a precision galvanometer motor. The mirrors are typically attached to a permanent magnet that is configured to inductively interact with a coil wound within the corresponding galvanometer motor when an electric current flows through the coil. In many applications, the galvanometer motors are considerably larger in size than the mirrors, for which the galvanometer motors pose increased space requirements and design issues. 
     The possibility of simultaneously forming or laser-processing a component by several laser devices plays a major role for increasing the efficiency of systems for powder-bed-based additive production of components in technologies such as direct powder fusion, vat photopolymerisation or directed energy deposition. Such parallelisation allows achieving higher output rates. However, the benefits of the combined use of several laser devices for simultaneously processing a component (parallelisation) must be balanced against the aforesaid space requirements and design issues related to the use of a plurality of scanning mirrors brings about. 
     Therefore, there is a need for improvement in the field of additive production of components with regard to deflection devices for parallel processing of a component by several laser devices. 
     U.S. 2019/0283332 A1 describes an additive manufacturing apparatus comprising a plurality of optical modules configured for directing lasers generated by a plurality of laser modules for melting powder. Each of the optical modules comprises a pair of tiltable mirrors. In each optical module, one of the mirrors is tiltable to steer a laser beam in an X-direction and the other tiltable mirror is tiltable to steer the laser beam in a Y-direction perpendicular to the X-direction. 
     Two deflection units that are arranged next to each other such that their working areas are overlaid on a common working area, which can be processed jointly and simultaneously by both deflection units are known from U.S. 2019/310463 A1. 
     U.S. 2017/173883 A1 describes using lower powered laser beam to melt a powder-based build material in synchronisation with a higher powered laser beam used to pre-heat the powder-based build material. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the aforementioned technical disadvantages and provides a solution based on a deflection module according to claim 1, a modular deflection system according to claim 18 and a method of laser processing one or more workpieces according to claim 25. Preferred embodiments of the invention are defined in the appended dependent claims. 
     A deflection module according to the present invention comprises a first deflection unit and a second deflection unit. Each deflection unit comprises a scanning device configured for scanning a respective working beam over a corresponding working field: the first deflection unit comprises a first scanning device configured for scanning a first working beam over a first working field and the second deflection unit comprises a second scanning device configured for scanning a second working beam over a second working field. The working beam may be a light beam for processing one or more workpieces, in particular a laser beam for laser processing one or more workpieces, for example in an additive manufacturing process or in other laser processes such as welding or laser-marking. 
     Each of the scanning devices may be a galvanometer scanning device. In preferred embodiments, the first scanning device may comprise a first movable mirror for scanning the first working beam in a first direction by tilting around a first axis and a second movable mirror for scanning the first working beam in a second direction by tilting around a second axis. The first axis may be perpendicular to the second axis. The second scanning device may comprise a first movable mirror for scanning the second working beam in the first direction by tilting around a third axis and a second movable mirror for scanning the second working beam in the second direction by tilting around a fourth axis. The third axis may be perpendicular to the fourth axis and/or parallel to the first axis. 
     In other embodiments, one or both of the first and second scanning devices may comprise a movable mirror for scanning the corresponding working beam in the first direction and in the second direction by tilting around two different corresponding axes, preferably two mutually perpendicular axes. Embodiments in which one of the scanning devices comprises a first movable mirror and a second movable mirror, each movable around one corresponding axis as previously described, and the other one of the scanning devices comprises a single movable mirror movable around two axes are also foreseen according to the invention. Although the invention will be described here mainly with reference to embodiments in which each of the scanning devices comprises a first movable mirror and a second movable mirror, the principles of the invention equally apply to embodiments in which at least one of the first and second scanning devices comprises a movable mirror for scanning the corresponding working beams in two directions by tilting about respective axes. In particular, all properties regarding the mutual arrangement and distance of the second movable mirrors of different deflection units may equally apply to the mutual arrangement and distance of the movable mirrors (each movable about one or two axes) of different deflection units in corresponding embodiments. 
     In any case, each of the first and second deflection units is configured for scanning the respective working beam in two independent and different directions, the first direction and the second direction. This allows scanning the first and second working beams over respective two-dimensional working fields, the first working field and the second working field. The first direction may be perpendicular to the second direction. For example, the first direction may correspond to an x-direction and the second direction may correspond to a y-direction in a Cartesian coordinate system. However, the skilled person will understand that the coordinate choice is arbitrary, so that the first direction may correspond to a y-direction and the second direction may correspond to an x-direction. 
     The first deflection unit and the second deflection unit may have a similar or identical structure and may have similar or identical optical components. Each of the first and second deflection units may further comprise a light source, in particular a laser source, for generating the corresponding working beam. Thus, the first and second working beams might be independently generated. In some embodiments, however, one (the same) light source may be connected to the first deflection unit and to the second deflection unit to provide both the first working beam and the second working beam. 
     According to some embodiments, in particular if one or both of the first and second scanning devices comprises first and second movable mirrors, the second movable mirror of the first scanning device may be arranged after the first movable mirror of the first scanning device along the beam path of the first working beam towards the first working field. In other words, when propagating towards the first working field, the first working beam may first be reflected by the first movable mirror of the first scanning device and then be reflected by the second movable mirror of the first scanning device, such that the second movable mirror of the first scanning device constitutes the last point of the optical system at which the direction in which the first working beam propagates is transmitted and/or may be changed before the first working beam reaches the first working field. The second movable mirror of the first scanning device may hence form an optical output window of the first deflection unit. 
     Likewise, the second movable mirror of the second scanning device may be arranged along the beam path of the second working beam towards the second working field after the first movable mirror of the second scanning device. Thus, when propagating towards the second working field, the second working beam may first be reflected by the first movable mirror of the second scanning device and then be reflected by the second movable mirror of the second scanning device, such that the second movable mirror of the second scanning device constitutes the last point of the optical system at which the direction in which the second working beam propagates is transmitted and/or may be changed before the second working beam reaches the second working field. The second movable mirror of the second scanning device may hence form an optical output window of the second deflection unit. 
     The first working field, which can be scanned by the first working beam deflected by the first deflection unit, and the second working field, which can be scanned by the second working beam deflected by the second deflection unit, have a common overlap area, i.e. a region that is comprised both in the first working field and in the second working field. The common overlap area is an area reachable by the first working beam and by the second working beam. Any point in space within the common overlap area may hence be laser-processed by the first working beam and by the second working beam. The first deflection unit and the second deflection unit may operate in cooperation with each other to form and laser process one or more workpieces in the common overlap area with high efficiency. The first working field and the second working field may lie on the same plane, at a given distance from the last movable mirror along the path of the corresponding working beam, e.g. from the second movable mirror of the first scanning device and/ or from the second movable mirror of the second scanning device. This distance is referred to as “scan radius”. The scan radius may correspond to a distance between the second movable mirror of the first scanning device and/ or of the second scanning device and the corresponding working field along the vertical z-direction, i.e. a distance between the second and/or fourth axis and the respective working field in the vertical direction (z-direction). 
     In the deflection module according to the invention, the second movable mirror of the first scanning device and the second movable mirror of the second scanning device (or the movable mirror of the first scanning device — movable around two axes — and the movable mirror of the first scanning device — movable around two axes) may be arranged mirror—symmetrically with respect to each other and with respect to a common plane of mirror symmetry. Further, the second axis, around which the second movable mirror of the first scanning device is rotatable for scanning the first working beam in the second direction, and the fourth axis, around which the second movable mirror of the second scanning device is rotatable for scanning the second working beam in the second direction (or the respective axes of movable mirrors movable around two axes), may be aligned with each other, i.e. may lie on the same line. As a consequence, the first working field and the second working field may be aligned with each other, in particular in the second direction and/or in a direction perpendicular to the common plane of mirror symmetry. The second and fourth axes (or the respective axes of movable mirrors movable around two axes) may in particular be perpendicular to the common plane of mirror symmetry and mutually aligned, meaning that the second movable mirror of the first scanning device and the second movable mirror of the second scanning device may be movable around the same rotation axis arranged perpendicular to the common plane of mirror symmetry. 
     The symmetric and aligned arrangement of at least the second movable mirrors of the first and second scanning devices as well as of the second and fourth axes according to the invention allows for a very compact arrangement of the deflection module, in particular with respect to the integration of the first deflection unit and the second deflection unit within a single deflection module for a cooperative use of the first and second working beams for simultaneous laser-processing. Thanks to the mirror-symmetric arrangement of the invention, the distance separating the second movable mirror of the first scanning device from the second movable mirror of the second scanning device, in particular the distance separating an optical centre of the second movable mirror of the first scanning device from an optical centre of the second movable mirror of the second scanning device, can be reduced to a minimum while avoiding any overlapping or obstruction of one of the optical components by other optical components, for example avoiding the possibility of collisions between the second movable mirror of the first scanning device and the second movable mirror of the second scanning device during the operation thereof. 
     “Optical centre” refers herein to the centre of the beam path defined by the respective deflection unit for the corresponding working beam when the working beam is directed down vertically in the z-direction and/or to the geometrical centre of the corresponding working field. The movable mirrors are arranged such that, when a working beam is reflected at a corresponding movable mirror and deflected towards the centre of the corresponding working field, said working beam is reflected at the optical centre thereof and around said optical centre, depending on a cross-sectional intensity distribution and/or on the spot size of the working beam. For example, when a working beam having a Gaussian intensity distribution in its cross-section is reflected at a movable mirror and deflected towards the centre of the corresponding working field, the centre (the maximum) of said intensity distribution is reflected precisely at the “optical centre” of said movable mirror. The distance between the second movable mirror of the first scanning device, in particular the optical centre thereof, and the second movable mirror of the second scanning device, in particular the optical centre thereof, may thereby be as small as mounting and/or manipulation requirements allow. The same may apply to the physical separation between the edges of the second movable mirror of the first scanning device and the second movable mirror of the second scanning device. 
     The reduced distance between the second movable mirrors of the first and second scanning devices further allows maximising the size of the common overlap area in which the first deflection unit and the second deflection unit can operate cooperatively at the same time without having to increase the scan radius, which would otherwise lead to increased inaccuracy, in particular at the edges of the workings fields, where the movable mirrors work with largest tilt angles, The reduced scan radius may allow integrating the deflection module of the invention in a laser processing system with a reduced overall vertical dimension. The deflection module of the invention hence provides a solution for parallelised laser-processing which is compact and easy to transport and ship, which is advantageous in terms of providing a rapid and satisfactory customer service. The deflection module according to the invention thus provides an improved balance between compactness and high yield/productivity achieved through the simultaneous action of a plurality of working beams. 
     The first and second scanning devices may be rigidly fixed with respect to each other, such that a relative spatial position of the first scanning device with respect to the second scanning device, in particular of the respective movable mirrors thereof with respect to each other, may be fixed. 
     According to some embodiments of the invention, each of the first and second scanning devices may further comprise a respective galvanometer motor for tilting the respective second movable mirror. The first scanning device may comprise a first galvanometer motor for tilting the second movable mirror of the first scanning device and the second scanning device may comprise a second galvanometer motor for tilting the second movable mirror of the second scanning device. The first and second scanning devices may further comprise or be connected to corresponding control units controlling an operation of the galvanometer motors and hence a corresponding tilting movement of the respective second movable mirror. The first movable mirrors of the first and second scanning devices may also be connected to respective galvanometer motors driving their tilting movement and, optionally, control units controlling their operation. 
     The mirror-symmetric and aligned arrangement of at least the second movable mirrors according to the invention allows arranging the first galvanometer motor and the galvanometer stepper motor on opposite sides of the respective second movable mirror with respect to the common plane of mirror symmetry, such that the first galvanometer motor and the second galvanometer motor are arranged mirror-symmetrically with respect to each other and to the common plane of mirror symmetry, while keeping a minimal distance between the second movable mirrors. Although the galvanometer motors are relatively voluminous devices, the arrangement according to the invention allows for their spatial distribution in a space-efficient manner such that they do not come in contact with each other, obstruct the mobility of other optical components, in particular, the movable mirrors or block any beam path inside the deflection module. 
     The first and second galvanometer motors may be arranged extending substantially perpendicular to the common plane of mirror symmetry, in particular respectively extending from a first end proximal to the corresponding second movable mirror to a second end distal from said corresponding second movable mirror (and from the common plane of mirror symmetry). The second movable mirror of the first scanning device may hence be arranged, in a direction substantially perpendicular to the common plane of mirror symmetry, between the first galvanometer motor and the common plane of mirror symmetry, while the second movable mirror of the second scanning device may be arranged, in said direction substantially perpendicular to the common plane of mirror symmetry, between the second galvanometer motor and the common plane of mirror symmetry. The first galvanometer motor may further be aligned with the second galvanometer motor in said direction substantially perpendicular to the common plane of mirror symmetry. 
     Notably, the rotation axis of the first and second galvanometer motors needs not coincide with the respective rotation axis of the corresponding second movable mirror, although this can be the case in some embodiments. The axis of rotation of the first and/or second galvanometer motors may be arranged parallel — with an offset — to the respective one of the second and fourth axes or with an inclination with respect thereto of up to 15°. 
     Although the mirror-symmetry and the alignment of the deflection module of the invention is at least formed by the second movable mirrors of the first and second scanning devices, further components of the first and second deflection units may display the same mirror-symmetry — with respect to the same common plane of mirror symmetry — and/or may be aligned or parallel with respect to each other. For example, the first movable mirror of the first scanning device and the first movable mirror of the second scanning device may be arranged mirror-symmetrically with respect to each other and to the common plane of mirror symmetry. The first axis may however be arranged essentially in parallel to the third axis, wherein the first and third axes may be arranged essentially parallel to the common plane of mirror symmetry, for example aligned with the vertical z-direction or arranged with respect to the vertical z-direction with an inclination from about 0° to about 15°. 
     In some embodiments, the first and third axes may be arranged parallel to each other and to the common plane of mirror symmetry and respectively perpendicular to the second and fourth axes, and the second and fourth axes may be arranged aligned with each other and perpendicular to the common plane of mirror symmetry and to the first and third axes. 
     According to some embodiments, the first working beam may be incident on the first scanning device, in particular on the first movable mirror thereof, propagating in a first incidence direction perpendicular to the common plane of mirror symmetry, and the beam path of the second working beam may be incident on the second scanning device, in particular on the first movable mirror thereof, propagating in a second incidence direction perpendicular to the common plane of mirror symmetry, wherein the first incidence direction may be aligned with and opposed to the second incidence direction. Thus, the first working beam and the second working beam may be in line with each other, at least until they reach the respective scanning device. 
     According to some embodiments, the first deflection unit and the second deflection unit may be arranged mirror-symmetrically with respect to the common plane of mirror symmetry and/or with respect to each other, such that a beam path of the first working beam, at least before being scanned by the first scanning device, and a beam path of the second working beam, at least before being scanned by the second scanning device are mirror symmetric with respect to each other and to the common plane of mirror symmetry. Thus, the beam path followed by the first working beam within the first deflection unit, at least until the first working beam is deflected by the first scanning unit, may be mirror-symmetric to the beam path followed by the second working beam within the second deflection unit, at least until the second working beam is deflected by the second scanning unit. Thus, the beam path followed by the first working beam within the first deflection unit and the beam path followed by the second working beam within the second deflection unit may be specular images of each other with respect to the common plane of mirror symmetry. 
     The “mirror symmetry” of the first and second deflection units with respect to each other may refer to the position and/or settings of each of the optical components thereof, such as mirrors and lenses, and to the corresponding light beam paths they define for the respective working beam, in particular when the orientation and/or settings of each of the optical components of the first deflection unit — with which the first working beam interacts on its way to the first working beam — corresponds to the corresponding orientation and/or settings of the respective optical components of the second deflection unit — which the second working beam interacts on its way to the second working field, wherein the latter optical components may be a specular image of the former components with respect to the common plane of mirror symmetry. “Settings” may refer to the optical properties of an optical component, such as focal length, diameter or size, shape, aperture, etc. For example, a first optical lens of the first deflection unit arranged such as to be a specular image of a second optical lens of the second flection unit with respect of the common plane of mirror symmetry may have the same focal length, size and shape as said second optical lens. 
     This notwithstanding, the mirror symmetry does not necessarily imply that the first and second deflection units must always be configured such as to maintain this symmetry with respect to each and every one of the optical components and settings at any time, in particular with respect to the first scanning device and the second scanning device, which may operate independently form each other. For example, it needs not be the case that whenever the first deflection unit, by means of the first scanning device, directs the first working beam to a point on the first working field, the second deflection unit correspondingly directs, by means of the second scanning device, the second working beam to a point on the second working field corresponding to a specular image of the first working beam with respect to the axis of mirror symmetry. Instead, each of the first and second deflection units may be configured to operate independently, such that the optical components thereof, in particular the lenses and/or mirrors of the first and second scanning devices, may take during operation different positions and orientations that may break the general mirror symmetry of the deflection module. 
     For example, a tilt of the first movable mirror around the first axis may be different, at any given time, from a tilt of the first movable mirror around the third axis (i.e. not correspond to a specular image thereof), and a tilt of the second movable mirror around the second axis may be different from a tilt of the second movable mirror around the fourth axis are arranged mirror-symmetrically with respect to each other and to a common plane of mirror symmetry. 
     In some embodiments, the beam path of the first working beam before being scanned by the first scanning device may be aligned with the beam path of the second working beam before being scanned by the second scanning device in a direction perpendicular to the common plane of mirror symmetry. In other words, before being deflected by the respective scanning devices, the first working beam and the second working beam may be coplanar, i.e. lie on a common plane, wherein the common plane may in particular be perpendicular to the common plane of mirror symmetry. This provides a particularly compact configuration by having both the first working beam — and the corresponding optical elements defining a beam path of the first working beam at least up to the first scanning device — and the second working beam — and the corresponding optical elements defining a beam path of the second working beam at least up to the second scanning device — arranged on a plane, which may allow for a reduction of the width of the deflection module and hence of the volume thereof. 
     According to some embodiments of the invention, a separation between the second movable mirror of the first scanning device and the second movable mirror of the second scanning device may correspond to not more than ⅓ of a diameter of the second movable mirror of the first scanning device and/or of the second movable mirror of the second scanning device, preferably not more than ¼ thereof, more preferably not more than ⅕ or ⅙ thereof. “Separation” may refer herein to a shortest definable Euclidian distance. The separation between the second movable mirror of the first deflection unit and the second movable mirror of the second deflection unit may be a separation distance between an edge of the second movable mirror of the first deflection unit and an edge of the second movable mirror of the second deflection unit in a direction perpendicular to the common plane of mirror symmetry, in particular in the direction in which the second and fourth axes extend. In some embodiments, the diameter of the second movable mirror of the first scanning device may be equal to the diameter of the second movable mirror of the second scanning device. “Diameter”, as used herein for mirrors, may refer not only to the size of movable mirrors having a circular shape, but any geometrical quantity defining the length of a main axis of the respective movable mirror. For example, if a movable mirror has an oval or elliptical shape, “diameter” may refer to the minor or major axis thereof. If a movable mirror has a square or rectangular form, “diameter”, as used herein, may refer to the length or width thereof. 
     According to some embodiments, a shape and size of the second movable mirror of the first scanning device and of the second movable mirror of the second scanning device may be equal. Irrespectively of whether the second movable mirrors of the first and second scanning devices are equal or not in shape and size, each of them may have a circular, elliptical, square, rectangular, rhombic or polygonal shape, in particular the reflection surface thereof. The second movable mirrors may be arranged such that the corresponding axis around which the movable mirror is tiltable, for example when correspondingly driven by a respective galvanometer motor, coincides with a main axis of the movable mirror. For example, if the second movable mirrors have an elliptic shape, the second and fourth axes may be aligned with the major axes of the ellipses defined by the second movable mirrors (and aligned with each other). If the second movable mirrors have a rectangular shape, the second and fourth axes may be aligned with the longitudinal axis of symmetry of the rectangle defined by the second movable mirrors (and aligned with each other). If the second movable mirrors have circular shape, the second and fourth axes may be aligned with the diameter of the circle defined by the second movable mirrors (and aligned with each other). 
     Similar considerations may apply to the first movable mirrors of the first and second scanning devices, wherein the first movable mirrors may be arranged such that the corresponding axis around which the movable mirror is tiltable, for example when correspondingly driven by a respective galvanometer motor, coincides with a minor axis of the movable mirror. For example, if the first movable mirrors have an elliptic shape, the first and third axes may be aligned with the minor axes of the ellipses defined by the first movable mirrors (and aligned with each other). If the first movable mirrors have a rectangular shape, the first and third axes may be aligned with the shorter axis of symmetry of the rectangle defined by the first movable mirrors. 
     According to some embodiments, a diameter of the second movable mirror of the first scanning device and/or of the second movable mirror of the second scanning device may be from 5 mm to 50 mm, preferably from 10 mm to 40 mm, more preferably from 20 mm to 30 mm. The same considerations may apply to the first movable mirrors of the first and second scanning devices. 
     According to some embodiments of the invention, a distance between a an optical centre of the second movable mirror of the first scanning device and an optical centre of the second movable mirror of the second scanning device may correspond to not more than 4 times an aperture of the first movable mirror of the first scanning device and/or of the first movable mirror of the second scanning device, preferably not more than 3 times thereof, more preferably not more than 2.5 or 2 times thereof. 
     In embodiments in which the second movable mirror of each of the first and second scanning devices is arranged after the corresponding first movable mirror, the “aperture” of the respective first movable mirror may refer to the extension (diameter) of the respective first movable mirror in a direction parallel to corresponding axis of rotation, i.e. to the first or third axis, respectively. For example, if a first movable mirror has an oval or elliptical shape, the aperture may correspond to the minor axis of the oval or ellipse. If the first movable mirror has a square or rectangular shape, the aperture may correspond to the shorter side or “width” thereof. The aperture of the first movable mirrors may be adapted to working beams of a given beam diameter, for example of a given 1/e 2  diameter. 
     The dimensions of the corresponding second movable mirrors may be configured for reflecting working beams of a given diameter, for example of a given 1/e 2  diameter, after being reflected by the corresponding first movable mirror at a given angle of incidence. For example, if a second movable mirror has an oval or elliptical shape, the major axis thereof, which may be aligned with the corresponding axis of rotation, i.e. with the second or fourth axis, may be dimensioned such as to be able to reflect the working beam coming from the respective first movable mirror taking into account the separation distance between the first and second movable mirrors and the range of possible angles of incidence of the working beam on the second movable mirror depending on a tilt angle of the respective first movable mirror. The dimensions of the second movable mirror may further be adapted to working beams of a given beam diameter, for example of a given 1/e 2  diameter. 
     The dimensions of the second movable mirrors, in particular the diameter thereof, may be greater than the dimensions of the corresponding first movable mirror. For example, if the first movable mirror and the second movable mirror both have an oval shape, the minor axis of the second movable mirror may be greater than the minor axis of the first movable mirror. 
     For instance, assuming a Gaussian distribution of the intensity profile of the working beams in the cross-sections thereof, the first working beam may be incident on the first scanning device, in particular on the first movable mirror of the first scanning device, having a first 1/e 2  beam diameter, and the second working beam may be incident on the second scanning device, in particular on the first movable mirror of the second scanning device, having a second 1/e 2  beam diameter. The second beam diameter may preferably be equal to the first beam diameter. The aperture of the first movable mirror of the first scanning device and/or the aperture of the first movable mirror of the second scanning device may be defined herein to correspond to at least 1.1 times, preferably at least 1.3 times, more preferably at least 1.5 times the respective 1/e 2  beam diameter. For example, for first and second working beams having each a 1/e 2  beam diameter of 20 mm, the corresponding apertures of the respective first movable mirrors may be 30 mm (1.5 times the 1/e 2  beam diameter) and the corresponding distance between the optical centres of the respective second movable mirrors may be 75 mm. 
     According to some embodiments, a distance between the optical centre of the second movable mirror of the first scanning device and the optical centre of the second movable mirror of the second scanning device may be not more than 120 mm, preferably not more than 80 mm, more preferably not more than 60 mm. 
     According to some embodiments, a distance between the second movable mirror of the first scanning device and the second movable mirror of the second scanning device may correspond to not more than ⅓ of the aperture of the first movable mirror of the first scanning device and/or of the second scanning device, preferably not more than ¼ thereof, more preferably not more than ⅕ or ⅙ thereof. 
     According to some embodiments, a separation between the second movable mirror of the first scanning device and the second movable mirror of the second scanning device may be not more than 50 mm, preferably not more than 30 mm, more preferably not more than 10 mm. 
     Each of the first and second working fields may cover an area of 100 mm x 100 mm to 1000 mm x 1000 mm, preferably from 300 mm x 300 mm to 700 mm x 700 mm, more preferably from 400 mm x 400 mm to 600 mm x 600 mm. The first and second working fields may have the same shape and size. The first working field and the second working field may be aligned with each other in a direction parallel to the common plane of mirror symmetry, such that they cover the same distance and extension on said direction parallel to the common plane of mirror symmetry, i.e. extend in said direction from a first common corner point to a second common corner point. In said direction parallel to the common plane of mirror symmetry, the common overlap area may hence have an extension corresponding to 100% of the extension covered by the first and/or second working field in said direction parallel to the common plane of mirror symmetry. 
     In some embodiments of the invention, the first and second working fields may partly overlap in an overlap direction perpendicular to the common plane of mirror symmetry, wherein the common overlap area may have an extension in the overlap direction corresponding to at least 75%, preferably at least 80%, more preferably at least 90% of the extension covered by the first and/or second working field in the overlap direction. Thus, the common overlap area may have an extension in the overlap direction from 75 mm to 900 mm and an extension in a direction parallel to the common plane of mirror symmetry of 100 mm to 1000 mm, preferably an extension in the overlap direction from 225 mm to 630 mm and an extension in a direction parallel to the common plane of mirror symmetry of 300 mm to 700 mm, more preferably an extension in the overlap direction from 300 mm to 540 mm, and an extension in a direction parallel to the common plane of mirror symmetry of 400 mm to 600 mm. The common overlap area may for example be 400 mm x 500 mm. 
     While other solutions known from the prior art for having more than one deflection unit operating on a common working area of overlapping working fields are based on increasing the scan radius, the inventors of the present invention have realised that small separations between the second movable mirrors of the first and second scanning devices and/or of the optical centres thereof — in particular separations within the aforementioned ranges — can be achieved in combination with the aforementioned areas of the common overlapping area while keeping a compact size of the deflection module, and notably, without having to increase the scan radius, thanks to the symmetrical arrangement and configuration of the first and second deflection units according to the invention. By avoiding an increase in the scan radius, a large common overlap area of the working fields of different deflection units can be achieved without increasing the scan radius. 
     According to some embodiments of the invention, a height of the second movable mirror of the first scanning device over the first working field and/or a height of the second movable mirror of the second scanning device over the second working field, i.e. the respective scan radii, may be not more than 800 mm, preferably not more than 600 mm, more preferably not more than 400 mm. The height of the second movable mirror of the first scanning device over the first working field and the height of the second movable mirror of the second scanning device over the second working field may be equal to each other. Scan radii within the aforementioned ranges can be achieved in combination with small separations between the second movable mirrors of the first and second scanning devices, in particular separation within the aforementioned ranges, by means of the symmetric arrangement of the components of the first and second scanning units according to the invention. The aforesaid scan radii may be implemented by the first and second deflection units by correspondingly setting the focal length of their respective optical systems, such that the first and second working beams be focused on the first and second working fields, respectively, i.e. at distances from the corresponding scanning device corresponding to the aforesaid scan radii. 
     Thus, the invention allows combining the technical advantages of a large common overlap area in which the first deflection unit and the second deflection unit may cooperatively operate to laser-process one or more workpieces simultaneously and of a reduced scan radius, which results in a more compact design and higher optical accuracy, due for a example to a reduced inclination of the working beams when laser-processing a workpiece in the peripheral portions of the respective working fields. 
     Further, by allowing a reduced scan radius, the deflection module of the invention allows having a reduced working volume (i.e. the three-dimensional vertical projection of the working fields, in particular of the common overlap area). When a fluid flow through the working volume is used to carry away gaseous residues resulting from the laser processing of the workpiece within the working volume, which could otherwise negatively affect the laser work by absorbing part of the light of the working beams, the required amount of fluid in the working volume can be reduced due to the decreased working volume. The fluid flow may for example be a flow of an inert gas, such as argon, which allows suppressing an oxidation of the material used for forming the workpiece, for example metallic powder, during the laser processing. Reducing the amount of inert gas required contributes to a considerable cost reduction of the entire laser-processing process in view of the high costs of inert gases. In addition, thanks to the reduced working volume, the flowing behaviour of the fluid flow within the working volume, for example the formation of turbulent phenomena, may be easier to control. 
     In some embodiments, the deflection module may further comprise a housing, wherein the first deflection unit and the second deflection unit may be enclosed within the housing. The housing, which may for example be of a material comprising aluminium and/or stainless steel, may preferably be waterproof and/or dustproof. This way, a thermal shift of the optical components of the deflection module, which may result from the accumulation of dust, dirt, humidity and/or water, can be reduced, whereby the working precision and definition of the deflection module can be preserved due to the sealing effect of the housing. Further, the housing may protect the deflection module during transportation or maintenance, thereby enhancing the advantageous modular design and easy replaceability of the deflection module according to the invention. 
     According to some embodiments, the housing may comprise a first transparent window configured for letting though the first working beam propagating from the first scanning device to the first working field and a second transparent window configured for letting through the second working beam propagating from the second scanning device to the second working field. The first transparent window may be arranged below the second movable mirror of the first scanning device and aligned therewith in a vertical direction (i.e. in a z-direction perpendicular to the first working field), such that the first transparent window may hence form an optical output window of the first deflection unit, through which the first working beam is last transmitted before reaching the first working field. The second transparent window may be arranged below the second movable mirror of the second scanning device and aligned therewith in a vertical direction (i.e. in a z-direction perpendicular to the second working field), such that the second transparent window may hence form an optical output window of the second deflection unit, through which the second working beam is last transmitted before reaching the second working field. The first transparent window and/or the second transparent window may comprise a glass plate. In some embodiments the first transparent window and the second transparent window may be integral with each other. For example, the first transparent window and the second transparent window may be formed by one and the same glass plate. 
     The first transparent window and the second transparent window may be adjacent to each other. Additionally or alternatively, the first transparent window and the second transparent window may be adjacent to the same lateral wall of the housing, such that the first and second transparent windows and said lateral wall of the housing, which may be arranged perpendicular to the first and second transparent windows, may share a common edge. As will be explained below, this configuration has the advantage of providing a configuration in which the first and second transparent windows of a deflection module are arranged adjacent to the first and second transparent windows of another deflection module, when both deflection modules abut each other by the corresponding lateral wall (the lateral wall that is adjacent to the respective first and second transparent windows). 
     According to some embodiments, the first deflection unit and/or the second deflection unit may further comprise an optical element, preferably a dichroic and/or reflective mirror, for reflecting light in a first wavelength range of the first and/or second working beam, respectively, at least partially, wherein the respective scanning device is arranged in the beam path of the corresponding working beam between the corresponding working field and the corresponding optical element, such that the corresponding working beam propagates to the corresponding scanning device being reflected at the corresponding optical element. The optical element may hence operate as a deviator/reflector for the corresponding working beam for deflecting the working beam coming from a first direction, for example from an input window and/or from a laser light source, to a second direction, in particular towards the corresponding scanning device. 
     For example, if a working beam (the first and/or second working beam) is generated by a light source arranged with respect to the respective working field such that the working beam comes out from the light source propagating in the vertical direction (z-direction), i.e. perpendicular to the respective working field, the optical element may be arranged at a 45° angle with respect to said vertical direction, such as to deviate the working beam from the vertical direction to a horizontal direction (e.g. x- and/or y- direction), such that it reaches the scanning device in said horizontal direction. However, other configurations are possible. In particular, the working beam (the first and/or second working beam) can also be generated by a light source arranged with respect to the respective working field such that the working beam comes out from the light source propagating in the horizontal direction (e.g. x-and/or y-direction) or propagating in a diagonal direction having a vertical component and a horizontal component. 
     The optical element may further be configured for transmitting light in a second wavelength range of the first and/or second working beam at least partially. This allows the respective deflection unit to define a detection beam path followed by a detection beam in the second wavelength range, for example from the corresponding working field to a corresponding detection device, such that, when said detection beam propagates back from the working field, it is directed (transmitted) to the detection device instead of being reflected back towards the light source. The detection beam can then propagate from said corresponding working field to said corresponding detection device being reflected by the corresponding scanning device and transmitted by the corresponding optical element. 
     Thus, the optical element may be used as a reflection element for redirecting the corresponding working beam, formed by light in the first wavelength range, for example between 1000 nm and 1100 nm, towards the corresponding scanning device on its way towards the corresponding working field. At the same time, the optical element may further be used as a transmission element letting through a detection beam coming from the respective working field, formed by light in the second wavelength range, for example below 1000 nm or over 1100 nm, towards a detection unit configured for detecting said detection beam. The detection beam may be used to obtain information about the workpiece being formed and hence to monitor the laser-processing and/or about the conditions of the laser-processing by the working beams. For example, the detection beam may be used to calibrate and/or synchronise the working beams. 
     The optical element may hence decouple a working beam from the corresponding detection beam, thereby allowing to control the optical settings thereof independently as explained in European patent application EP 3 532 238 A1 (e.g. paragraphs [0018] and [0019] thereof). Thereby, aberrations of the detection beam can be avoided and the monitoring functionalities can remain focused. The detection unit may comprise an optical sensor, an optical camera, a diode, a pyrometric device, an optical coherent tomography detector and the like. The optical element and the detection unit may respectively correspond to an optical element (“optisches Element”) and a detection device (“Detektionseinrichtung”) as described in European patent application EP 3 532 238 A1. 
     According to some embodiments, the first deflection unit and/or the second deflection unit may further comprise a focusing device for focusing, zooming and/or collimating the respective working beam. The focusing device may be arranged along the beam path followed by the respective working beam before the respective scanning device and before the respective optical element. The focusing unit may have a variable focal length. The focusing unit may comprise a first fixed lens, a first movable lens and a further fixed or movable lens. The focusing device may in particular correspond to a focussing device (“Fokussiervorrichtung”) as described in EP 3 532 238 A1. The skilled person shall understand that any of the aforementioned “lenses” may be formed by a corresponding group of lenses and needs not be formed by a single lens. The focusing device may be configured for setting the focal length of the respective optical systems (i.e. the first or second deflection unit, respectively), such that the first and second working beams be focused on the first and second working fields, respectively, i.e. at distances from the corresponding scanning device corresponding to the corresponding scan radius, in particular to a scan radius in the previously described ranges. 
     Another aspect of the present invention refers to a modular deflection system comprising a first deflection module and a second deflection module, wherein the first deflection module and the second deflection module may correspond to any of the embodiments of a deflection module according to the invention described above. In some embodiments, the first deflection module and the second deflection module may have an identical or at least similar configuration, i.e. may have identical or at least similar or equivalent optical components and settings. 
     The first deflection module and the second deflection module of the modular deflection system of the invention may be mutually attached or attachable. In some embodiments, the first deflection module and the second deflection module may be configured to be removably attached to each other. When the first deflection module and the second deflection module are attached to each other, the common overlapping area of the first deflection module and the common overlapping area of the second deflection module overlap, thereby forming a common overlap field. The common overlap field hence constitutes an overlap area of the first and second working fields of the first deflection module and of the first and second working fields of the second deflection module. 
     Thus, up to four deflection units, the first and second deflection units of the first deflection module and the first and second deflection units of the second deflection module can cooperate to laser-process one or more workpiece simultaneously in the common overlap field, thereby achieving a high degree of parallelisation. At the same time, due to the compact configuration of the deflection modules according to the invention, an overall size of the modular deflection system can remain rather moderate, in particular a vertical dimension thereof, in view of the small ratio of the respective scan radii to the respective common overlap areas provided by the configuration of each of the first and second deflection modules. The modular deflection system of the invention hence provides a highly compact arrangement of four independent deflection units that may operate simultaneously on the common overlap field to laser-process the same workpiece or workpieces. The modular deflection system may profit from all technical advantages of the deflection module according to the invention referred to above. 
     The compact configuration of each of the deflection modules according to the invention allows mutually attaching the first deflection module and the second deflection module such that the scanning devices thereof, and in particular the second movable mirrors thereof, can be arranged very close to each other. 
     Further, the modular structure of the modular deflection system of the invention allows for an improved design flexibility and for reduced maintenance complexity. For example, if one of the deflection modules of the modular deflection system must be inspected or repaired by the manufacturer, it can be easily detached from the modular deflection system, replaced by a corresponding replacement deflection module, and transported to a manufacturer site, such that the modular deflection system may continue to be employed at a customer end and production needs not be interrupted. 
     According to some embodiments, the first deflection module and the second deflection module may be mirror-symmetrical with respect to each other, when the first deflection module and the second deflection module are mutually attached. When the first and second deflection modules are mutually attached, the common plane of mirror symmetry of the first deflection module may be aligned with the common plane of mirror symmetry of the second deflection module. 
     The modular and symmetric design of the modular deflection system of the invention allows having a reduced separation, not only between the movable mirrors of each of the first and second deflection modules, for example between the second movable mirror of the first scanning device of the first deflection module and the second movable mirror of the second scanning device of the first deflection module, but further between a movable mirror of the first deflection module and a movable mirror of the second deflection module, in particular between the second movable mirror of the first deflection module and the second movable mirror of the second deflection module. This is particularly the case when the second movable mirrors of each of the first and second deflection modules are arranged offset from a longitudinal axis of the respective deflection module, for example when the second movable mirror and a corresponding transparent window located below said second movable mirror are arranged adjacent to a lateral wall of a respective housing of the deflection module. Having the same situation, actually a specular image thereof, in the other deflection module of the modular deflection system, allows for a small separation distance between the second movable mirrors of different deflection modules and hence contributes to the compact design of the modular system and to an increased ratio of the size of the common overlap field to the scan radii of the modular system. 
     According to some embodiments of the invention, a distance between an optical centre of the second movable mirror of the first scanning device of the first deflection module and an optical centre of the second movable mirror of the first or second scanning device of the second deflection module may correspond to not more than 4 times an aperture of the first movable mirror of the first scanning device and/or of the first movable mirror of the second scanning device of the first or second deflection module, preferably not more than 3 times thereof, more preferably not more than 2.5 or 2 times thereof. 
     According to some embodiments, a separation between the second movable mirror of the first scanning device of the first deflection module and the second movable mirror of the first or second scanning device of the second deflection module may correspond to not more than ⅓ of a diameter of the second movable mirror of the first scanning device of the first deflection module and/or of the second movable mirror of the first or second scanning device of the second deflection module, preferably not more than ¼ thereof, more preferably not more than ⅕ or ⅙ thereof, wherein the aperture may be defined as explained above. 
     A diameter or aperture of the first or second movable mirror of the first and/or second scanning device of the first deflection module may be equal to a diameter or aperture of the first or second movable mirror of the first and/or second scanning device of second deflection module. In particular, all four first or second movable mirrors of the modular deflection system may have the same diameter, aperture, size and/or shape. 
     In some embodiments, a distance between the optical centre of the second movable mirror of the first scanning device of the first deflection module and the optical centre of the second movable mirror of the first or second scanning device of the second deflection module may be greater than a distance between the optical centres of the second movable mirrors of the first or second deflection module, preferably up to 20% greater, more preferably up to 10% greater, most preferably up to 5% greater. However, the distance between the optical centre of the second movable mirror of the first scanning device of the first deflection module and the centre of the second movable mirror of the first or second scanning device of the second deflection module may be essentially equal to the distance between the optical centres of the second movable mirrors of the first or second deflection module. The same applies to a distance between the optical centre of the second movable mirror of the second scanning device of the first deflection module and the centre of the second movable mirror of the first or second scanning device of the second deflection module. 
     According to some embodiments, a distance between the optical centre of the second movable mirror of the first or second scanning device of the first deflection module and the optical centre of the second movable mirror of the first or second scanning device of the second deflection module may be not more than 120 mm, preferably not more than 80 mm, more preferably not more than 60 mm. 
     According to some embodiments, a separation between the second movable mirror of the first scanning device of the first deflection module and the second movable mirror of the first or second scanning device of the second deflection module may be not more than 50 mm, preferably not more than 30 mm, more preferably not more than 10 mm. The separation between the second movable mirror of the first scanning device of the first deflection module and the second movable mirror of the first or second scanning device of the second deflection module may be greater than a distance between the second movable mirrors of the first or second deflection module, preferably up to 20% greater, more preferably up to 10% greater, most preferably up to 5% greater. This may allow avoiding a risk of collision or interference between second movable mirrors of different deflection modules. 
     The first deflection module may comprise a first housing, wherein the first deflection unit and the second deflection unit of the first deflection module are enclosed within the first housing. The first housing may be dustproof and/or waterproof. The second deflection module may comprise a second housing, wherein the first deflection unit and the second deflection unit of the second deflection module may be enclosed within the second housing. The second housing may be dustproof and/or waterproof. The first housing and the second housing may be mutually attachable in such a manner that the first housing and the second housing are arranged adjacent to each other, when the first deflection module and the second deflection module are attached to each other. The second housing may be a specular image of the first housing, when the first and second housings are attached together. 
     According to some embodiments, the first housing may comprise a first transparent window through which the respective first working beam propagates from the first scanning device of the first deflection module to the respective first working field and a second transparent window through which the respective second working beam propagates from the second scanning device of the first deflection module to the respective second working field. Further, the second housing may comprise a third transparent window through which the respective first working beam propagates from the first scanning device of the second deflection module to the respective first working field and a fourth transparent window through which the respective second working beam propagates from the second scanning device of the second deflection module to the respective second working field. The first transparent window, the second transparent window, the third transparent window and/or the fourth transparent window may be arranged adjacent to each other when the first deflection module and the second deflection module are attached to each other. 
     In other embodiments, the modular deflection system may comprise a common housing, wherein the first and second deflection units of the first deflection module and the first and second deflection units of the second deflection module may be enclosed within the common housing. The common housing may be dustproof and/or waterproof. 
     Each of the first and second working fields of the first deflection module and each of the first and second working fields of the second deflection module may cover an area of 100 mm x 100 mm to 1000 mm x 1000 mm, preferably from 300 mm x 300 mm to 700 mm x 700 mm, more preferably from 400 mm x 400 mm to 600 mm x 600 mm. The first and second working fields of the first deflection module may be aligned with each other in a first overlap direction, and the first and second working fields of the second deflection module may be aligned with each other in said first overlap direction as well. Said first overlap direction may be parallel to the common plane of mirror symmetry of the first and second deflection modules. Further, the first working field of the first deflection module may be aligned with one of the first and second working fields of the second deflection module in a second overlap direction perpendicular to the first overlap direction and the second working field of the first deflection module may be aligned with the other one of the first and second working fields of the second deflection module in the second overlap direction. 
     In each of the first and second overlap directions, the common overlap field may have an extension corresponding to at least 75%, preferably at least 80%, more preferably at least 90% the extension covered by the first and/or second working fields of the first and/or second deflection modules in the corresponding overlap direction. Thus, in some embodiments of the invention, the common overlap field may have an extension in each of the first and second overlap directions from 70 mm to 800 mm (covering an area from 70 mm x 70 mm to 800 mm x 800 mm), preferably from 220 mm to 600 mm (covering an area from 220 mm x 220 mm to 600 mm x 600 mm), more preferably from 300 mm to 540 mm (covering an area from 300 mm x 300 mm to 540 mm x 540 mm). The common overlap field may for example be 330 mm x 330 mm. 
     A further aspect of the invention refers to a method of laser processing one or more workpieces using a deflection module or a modular deflection system according to any of the embodiments of the invention previously described. The method comprises laser processing the work piece by a first working beam deflected by a first deflection unit of the deflection module or the modular deflection system, wherein the first working beam has a first power density. The method further comprises laser processing the work piece by the second working beam deflected by a second deflection unit of the deflection module or the modular deflection system, wherein the second working beam has a second power density, wherein the second power density is higher than the first power density. A “power density” refers herein to surface power density, i.e. to beam power divided by unit area of the corresponding working field. The second power density may be 1.5 times, 3 times, 5 times or 10 times higher than the first power density. 
     According to the method of the invention, at least in a subregion of the common overlapping area (or of the common overlap field if the used deflection module is integrated in a modular deflection system according to any of the previously described embodiments of the invention), the workpiece is laser processed by the first working beam, at a lower power density, before or after being processed by the second working beam, at a higher power density. 
     Thus, the method according to the invention allows using the first working beam deflected by the first deflection unit for warming up the material used for forming a given one of the one or more workpieces in a warm-up phase by using a lower beam power before using the second working beam deflected by the second deflection unit for laser processing the work piece at regions of the workpiece that have been previously warmed at by the first working beam. Additionally or alternatively, the first working beam deflected by the first deflection unit may be used for progressively cooling down the material used for forming the one or more workpieces in a cool-down phase by using a lower beam power density after using the second working beam deflected by the second deflection unit. Thereby, the thermal variations of the material used for forming the one or more workpieces can be smoothed or flattened in time by being divided into more than one progressive stages: e.g. warming up to a lower temperature by the first working beam, then melting at a higher temperature by the second working beam, and then cooling down to a lower temperature by the first working beam. Partial melting may also occur during the warm-up and/or during the cool-down phase. This allows reducing the temperature gradients undergone by the material used for forming the workpiece and hence prevents the formation of irregularities due to strong thermal gradients. 
     According to some embodiments, the first working beam and the second working beam may have the same beam power, wherein the first working beam may have a greater spot size than the second working beam. Thus, the different beam power densities can be implemented using working beams having the same beam power, for example by using identical laser sources for generating both the first working beam and a second working beam. Additionally or alternatively, the first working beam and the second working beam may have the same spot size, wherein the first working beam may have a smaller beam power than the second working beam. Thus, it is also possible to implement the different beam power densities by using identical spot sizes but different beam powers. 
     The aforesaid method may further be implemented with a modular deflection system according to any of the previously described embodiments, i.e. a modular deflection system comprising a first and a second deflection module, with each deflection module comprising two deflection units. The method may comprise laser processing the work piece by at least one (of the four available) working beams and laser processing the work piece by the remaining working beams. For example, the first working beam deflected by the first deflection unit of the first deflection module and the first working beam deflected by the first deflection unit of the second deflection module may be operated with the first beam power and used for warming up the material used for forming a given one of the one or more workpieces and the second working beam deflected by the second deflection unit of the first deflection module and the second working beam deflected by the second deflection unit of the second deflection module may subsequently be operated with the second beam power for laser-processing said given one of the one or more workpieces. All working beams, i.e. all deflection units, may in any case operate simultaneously. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows perspective view of the interior of a deflection module according to embodiments of the invention. 
         FIG.  2    shows a schematic top view of a deflection module like the deflection module of  FIG.  1    according to some embodiments of the invention. 
         FIG.  3    shows a schematic side view of a deflection module like the deflection module of  FIG.  1    according to some embodiments of the invention. 
         FIG.  4    shows a schematic illustration of the working fields and the common overlapping area of a deflection module according to some embodiments of the invention. 
         FIG.  5    is a schematic flow diagram of a method of laser processing a work piece according to some embodiments of the invention. 
         FIG.  6    shows schematic perspective views of the exterior of a deflection module according to some embodiments of the invention.  FIG.  6   a    shows a superior perspective view and  FIG.  6   b    shows an inferior perspective view. 
         FIG.  7    shows a schematic perspective view of the exterior of a modular deflection system according to some embodiments of the invention.  FIG.  7   a    shows a superior perspective view and  FIG.  7   b    shows an inferior perspective view. 
         FIG.  8    shows a schematic top view of the interior of a modular deflection system like the modular deflection system of  FIG.  7    according to some embodiments of the invention. 
         FIG.  9    shows a schematic illustration of the working fields and the common overlap field of a modular deflection system like the modular deflection system of  FIG.  7    according to some embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to a preferred embodiment illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated apparatus and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur now or in the future to one skilled in the art to which the invention relates. 
       FIG.  1    shows a schematic perspective view of the interior of a deflection module according to some embodiments of the invention, in particular of the optical components included therein. The first deflection module comprises a first deflection unit  10   a  and a second deflection unit  10   b . The first deflection unit  10   a  comprises a first scanning device  12   a , which comprises a first movable mirror  12   a - 1  and a second movable mirror  12   a - 2 .  FIG.  2    and  FIG.  3    respectively show a top view and a side view of a deflection module according to embodiments of the invention like the deflection module shown in  FIG.  1   , wherein the same reference numerals are used for the same components. For the following description,  FIGS.  1  to  3    may be considered in combination to the extent that they show the same components. 
     The first movable mirror  12   a - 1  is configured for scanning a first working beam  50   a  in a first direction, which in the embodiment shown in  FIG.  1    corresponds to the x-direction, by tilting around a first axis A 1 , which in the embodiment shown in  FIG.  1    is arranged with an inclination relative to the vertical z-direction of about 15°. The movement or tilting of the first movable mirror  12   a - 1  of the first deflection unit  10   a  is driven by a galvanometer motor  14   a - 1  that is arranged extending along the first axis A 1 , i.e. having a longitudinal axis corresponding to the largest dimension of the stepper motor  14   a - 1  extending along the first axis A 1 . 
     The second movable mirror  12   a - 2  is configured for scanning the first working beam  50   a , after the first working beam  50   a  is reflected by the first movable mirror  12   a - 1 , in a second direction, which in the embodiment shown in  FIG.  1    corresponds to an y-direction perpendicular to the x- and z-directions, by tilting around a second axis A 2 , which in the embodiment shown in  FIG.  1    is aligned with the x-direction. The movement or tilting of the second movable mirror  12   a - 2  of the first deflection unit  10   a  is driven by a galvanometer motor  14   b - 2  that is arranged extending substantially parallel to the second axis A 2 , i.e. substantially perpendicular to the common plane of mirror symmetry M. 
     The first movable mirror  12   a - 1  and the second movable mirror  12   a - 2  thus form an XY-scanning device configured for scanning the first working beam  50   a  in the x- and y-directions over a two-dimensional working field  40   a . One or more workpieces or start materials located within the working field  40   a  can hence be laser-processed by the first working beam  50   a  deflected by the first deflection unit  10   a . 
     The first working beam  50   a  is generated by a first laser source  28   a  that is optically connected to the first deflection unit  10   a  and/or, in some embodiments, integrated in the first deflection unit  10   a . In the embodiment under consideration, the first laser source  28   a  is configured for generating laser light with a wavelength of 1070 nm forming the first working beam  50   a . 
     After being generated by the first laser source  28   a , the working beam  50   a  propagates through a first focusing device  20   a  that is configured for focusing, zooming and collimating the working beam  50   a . The focusing device  20   a  comprises a first movable lens  22   a , a second movable lens  24   a  and a fixed lens  26   a , wherein the movable lenses  22   a  and  24   a  can be shifted in the z-direction for adjusting a variable focal length of the first focusing device  20   a  and for zooming and collimating the first working beam  50   a , thereby adjusting, for example, a beam diameter of the first working beam  50   a . The first lens  22   a  may be a fixed lens in other embodiments. The first focusing device  20   a  operates as a focusing and zooming unit setting the focal length of the entire optical system of the first deflection unit  10   a  such that the first working beam  50   a  is focused on the first working field  40   a , at a distance SR from the second movable mirror  12   a - 2  (cf.  FIG.  3   ). 
     After propagating through the first focusing device  20   a , the first working beam  50   a  is reflected by a first optical element  16   a , which in the embodiment shown in  FIG.  1    is a dichroic mirror configured for reflecting light in a first wavelength range from 1020 nm to 1080 nm, such that the first working beam  50   a  is deflected from the z-direction from which it arrives from the first laser source  28   a  to the x-direction, towards the first scanning device  12   a  (cf.  FIG.  3   , showing a side view in the zx-plane corresponding to the perspective view of  FIG.  1   ). 
     In the embodiments shown in  FIGS.  1  to  3   , the working beam  50   a  is generated by the first laser source  28   a  and fed into the first deflection unit in the vertical direction (in the z-direction). Therefore, the first optical element  16   a  is arranged at a 45° angle in the xz-plane with respect to each of the z- and x-directions (cf.  FIG.  3   ). However, other configurations and corresponding arrangements of the first optical element  16   a  are possible. In other embodiments, the first working beam  50   a  may enter the first deflection units  10   a  in the horizontal x-direction or in a diagonal direction, for example a diagonal direction in the xz-plane, i.e. a direction having an x-component and a z-component, for instance at a 45° angle, although other angles are possible. The first optical element  16   a  may then be arranged at a corresponding angle for directing the first working beam  50   a  towards the first scanning device  12   a , in particular towards the first movable mirror  12   a - 1 . The same applies to the second deflection unit  10   b  (to be described in the following) with respect to the arrangement of the second laser source  28   b  and the second optical element  16   b . 
     The deflection module further comprises a second deflection unit  10   b  having a structure, arrangement and optical components corresponding, possibly identical, to the components of the first deflection unit  10   a . For example, the lenses  22   b ,  24   b  and  26   b  of the second focusing device  20   b  can be identical to the corresponding lenses  22   a ,  24   a  and  26   a , respectively, of the first focusing device  20   a . Likewise, the second optical element  16   b  of the second deflection unit  10   b  can be identical to the corresponding first optical element  16   a  of the first deflection unit  10   a  and be arranged accordingly to as to fulfil the same function. The second focusing device  20   b  operates as a focusing and zooming unit setting the focal length of the entire optical system of the second deflection unit  10   b  such that the second working beam  50   b  is focused on the second working field  40   b , at a distance SR from the second movable mirror  12   b - 2  (cf.  FIG.  3   ). 
     The second deflection unit  10   a  comprises a second scanning device  12   b , which comprises a first movable mirror  12   b - 1  and a second movable mirror  12   b - 2 , which respectively correspond in terms of function and structure to the first movable mirror  12   a - 1  and the second movable mirror  12   a - 2  of the first deflection unit  10   a . The first movable mirror  12   b - 1  is configured for scanning a second working beam  50   b , which is generated by a second laser source  28   b  that is functionally identical to the first laser source  28   a , in the first direction (x-direction), by tilting around a third axis A 3 , which in the embodiment shown in  FIG.  1    is parallel to the first axis A 1 , i.e. also arranged with respect to the vertical z-direction with an inclination of about 15°. The movement or tilting of the first movable mirror  12   b - 1  of the second deflection unit  10   b  is driven by a galvanometer stepper motor  14   b - 1  that is arranged extending along the first axis A 3 , corresponding to the galvanometer motor  14   a - 1 . 
     The second movable mirror  12   b - 2  is configured for scanning the second working beam  50   b , after the second working beam  50   b  is reflected by the second optical element  16   b  and the first movable mirror  12   b - 1 , for scanning the second working beam  50   b  in the second direction (y-direction), by tilting around a fourth axis A 4 , which is aligned with the second axis A 2  in the x-direction (cf.  FIG.  2   , showing a top view in the xy-plane corresponding to the perspective view of  FIG.  1   ). The movement or tilting of the second movable mirror  12   a - 2  of the first deflection unit  10   a  is driven by a galvanometer motor  14   b - 2  that is arranged extending substantially parallel to the fourth axis A 4  (and the second axis A 2 ) and hence substantially perpendicular to the common plane of mirror symmetry M, corresponding to the galvanometer stepper motor  14   a - 2 . 
     The first movable mirror  12   b - 1  and the second movable mirror  12   b - 2  form an XY-scanning device configured for scanning the second working beam  50   b  in the x- and y-directions over a two-dimensional working field  40   b . One or more workpieces or start materials located within the working field  40   b  can hence be laser-processed by the second working beam  50   b  deflected by the second deflection unit  10   b . 
     The first deflection unit  10   a  and the second deflection unit  10   b  are arranged mirror-symmetrically with respect to each other and with respect to a common plane of mirror symmetry M, which in  FIG.  1    extends in the yz-plane, i.e perpendicular to the x-direction. As schematically seen in the top view of  FIG.  2    and in the frontal view of  FIG.  3   , the beam path followed by the first working beam  50   a  before being scanned by the first scanning device  12   a , i.e. between the first laser source  28   a  and the first scanning device  12   a , is mirror symmetric, with respect to the common plane of mirror symmetry M, to the beam path followed by the second working beam  50   b  before being scanned by the second scanning device  12   b , i.e. between the second laser source  28   b  and the second scanning device  12   b . In the schematic views shown in  FIGS.  1  to  3   , the first working beam  50   a , before being reflected by the first movable mirror  12   a - 1 , is mirror symmetric to the second working beam  50   b , before it is reflected by the first movable mirror  12   b - 1 , and aligned therewith in the x-direction. 
     The portion of the first working beam  50   a  propagating in the z-direction from the first laser source  28   a  to the first optical element  16   a  propagates parallel to the portion of the second working beam  50   b  that propagates also in the z-direction from the second laser source  28   b  to the second optical element  16   b . The portion of the first working beam  50   a  that propagates from the first optical element  16   a  to the first movable mirror  12   a - 1  and the portion of the second working beam  50   b  that propagates from the second optical element  16   b  to the first movable mirror  12   b - 1  propagate aligned with each other in the x-direction and directed towards each other, i.e. towards the common plane of mirror symmetry M. 
     The mirror symmetry between the first deflection unit  10   a  and the second deflection unit  10   b  with respect to the common plane of mirror symmetry M may be broken along the beam path followed, respectively, by the first working beam  50   a  and the second working beam  50   b , from the corresponding scanning device  12   a  or  12   b  on, inasmuch as the movable mirrors  12   a - 1  and  12   a - 2  of the first scanning device  12   a  might be tilted at a given time differently than or without corresponding to a mirror-symmetric tilting state of the movable mirrors  12   b - 1  and  12   b - 2  of the second scanning device  12   b , i.e. without corresponding to a specular image thereof with respect to the common plane of mirror symmetry M. However, the first movable mirror  12   a - 1  and the second movable mirror  12   b - 1  of the first scanning device  12   a  are, in their o-tilt positions, arranged, respectively, mirror-symmetrically with respect to the first movable mirror  12   b - 1  and the second movable mirror  12   b - 2  of the second scanning device  12   b  in their o-tilt positions. 
     Such mirror-symmetric arrangement of the first and second deflection units  10   a  and  10   b  allows for an arrangement of the first scanning device  12   a  and the second scanning device  12   b , and in particular of the respective second movable mirrors  12   a - 2  and  12   b - 2 , in which a distance d oc  between the optical centre of the second movable mirror  12   a - 2  and the optical centre of the second movable mirror  12   b - 2  is reduced to a minimum. The second movable mirrors  12   a - 2  and  12   b - 2  are positioned very close to each other and are mutually separated in the x-direction by a small distance d. Consequently, the first working field  40   a  of the first deflection unit  10   a  and the second working field  40   b  of the second deflection unit  10   b  overlap with each other in at least a respective subregion thereof forming a common overlap area  42 . The common overlap area  42  belongs both to the first working field  40   a  and to the second working field  40   b . 
     In the embodiments illustrated in  FIGS.  1  to  3   , the movable mirrors  12   a - 1 ,  12   a - 2 ,  12   b - 1  and  12   b - 2  all have a polygonal shape configured to reflect a corresponding working beam having an 1/e 2  diameter of up to 30 mm. The first and second working beams  50   a  and  50   b  have a Gauss-distributed intensity profile in their cross-sections and are incident on the first mirror  12   a - 1  of the first scanning device  12   a  and on the first mirror  12   b - 1  of the second scanning device  12   b , respectively, having a first 1/e 2  beam diameter of 20 mm. The first mirrors  12   a - 1  and  12   b - 1  are designed to have an aperture corresponding to 1.5 times the aforesaid 1/e 2  beam diameter, i.e. an aperture of 30 mm, such that they can respectively reflect about 99.5% of the light of the first and second working beams  50   a  and  50   b , respectively. The optical centres of the second movable mirrors  12   a - 2  and  12   b - 2  are separated from each other in the x-direction by the distance doc = 65 mm and the edges of the second movable mirrors  12   a - 2  and  12   b - 2  are separated from each other in the x-direction by the distance d = 5 mm. 
     As seen in  FIG.  2   , each of the second movable mirrors  12   a - 2  and  12   b - 2  is separated from the corresponding first movable mirror  12   a - 1  and  12   b - 1 , respectively, in the y-direction. This separation does however not affect the separation d between the second movable mirrors  12   a - 2  and  12   b - 2 . 
     The mirror-symmetric and aligned arrangement of the second movable mirrors  12   a - 2  and  12   b - 2  allows minimising the distance doz between the optical centres of the second movable mirrors  12   a - 2  and  12   b - b , thereby increasing the size of the common overlap area  42  without having to increase a distance between each of the second movable mirrors  12   a - 2  and  12   b - 2  and the plane on which the first working field  40   a  and the second working field  40   b  (and hence the common overlap area  42 ) lie, i.e. without having to increase the scan radius.. 
     As shown in  FIG.  4   , which shows a schematic view of the first working field  40   a  and the second working field  40   b  in the xy-plane, each of the first and second working fields  40   a  and  40   b  has a square shape with a side length L A  = L B  = 500 mm covering an area of 500 mm x 500 mm. The first working field  40   a  and the second working field  40   b  are aligned with each other in the y-direction: in the view illustrated in  FIG.  4   , the left edges of the first and second working fields  40   a  and  40   b  are aligned with each other in the y-direction and so are the corresponding right edges. Thus, in the y-direction the first and second working fields  40   a  and  40   b  have a 100% overlap. In the y-direction, the first working field  40   a  and the second working field  40   b  overlap over a distance of 500 mm. In the x-direction, the first and second working fields  40   a  and  40   b  partly overlap (81% overlap) over a distance L c  = 435 mm. The common overlap area  42  hence covers an area of 500 mm x 435 mm. 
     Such large overlap of the first and second working fields  40   a  and  40   b  is compatible, thanks to the mirror-symmetric and aligned arrangement of the first and second deflection units  10   a  and  10   b , and in particular of the second movable mirrors  12   a - 2  and  12   b - 2 , with a rather small scan radius SR (cf.  FIG.  3   ). In the embodiments considered in  FIGS.  1  to  3   , the scan radius is SR = 620 mm. 
     The galvanometer motors  14   a - 2  and  14   b - 2  for tilting the second movable mirrors  12   a - 2  and  12   b - 2  respectively, are arranged on opposite sides of the corresponding second movable mirror  12   a - 2 ,  12   b - 2 : as seen in  FIGS.  1  and  2   , the second movable mirror  12   a - 2  is arranged between the common plane of mirror symmetry M and the stepper motor  14   a - 2 . As seen in  FIG.  2    in the xy-plane, the galvanometer motor  14   a - 2  is arranged to the left of the second movable mirror  12   a - 2 . Likewise, the second movable mirror  12   b - 2  is arranged between the common plane of mirror symmetry M and the galvanometer motor  14   b - 2 , such that, as seen in  FIG.  2    in the xy-plane, the galvanometer motor  14   b - 2  is arranged to the right of the second movable mirror  12   b - 2 . This configuration, with the galvanometer motors longitudinally extending in the x-direction perpendicular to the common plane of mirror symmetry M is space-saving and favours a reduced distance between the second movable mirrors  12   a - 2  and  12   b - 2  while avoiding any obstruction or collision between the stepper motors  14   a - 2  and  14   b - 2  and other components of the deflection module and also between the second movable mirrors  12   a - 2 - and  12   b - 2 . 
     The schematic view of  FIG.  2    does not include the galvanometer motors associated to the first movable mirrors  12   a - 1  and  12   b - 1  for illustrative purposes. For the same reason, the schematic view of  FIG.  3    does not include any of the galvanometer motors of the deflection module and only shows the first and second scanning devices as a schematic superposition of the corresponding movable mirrors  12   a - 1 ,  12   a - 2  and  12   b - 1 ,  12   b - 2 , respectively. 
     As shown in  FIG.  3   , each of the first and second deflection units  10   a  and  10   b  further defines a corresponding detection beam path for a first detection beam  52   a  and a second detection beam  52   b , respectively. The optical elements  16   a  and  16   be , besides being reflective in the aforesaid wavelength range between 1000 nm and 1100 nm, have a high transmittance for wavelengths below 1000 nm and over 1100 nm. As a consequence, reflection light originated in the working fields, for example by a reflection of illumination light or of the light of the first and/or second working beams  50   a ,  50   b , and reflected back by the scanning devices  12   a  and  12   b  are transmitted by the respective optical element  16   a  and  16   b , such that the corresponding detection beams  52   a  and  52   b  propagate from the respective working field  40   a  and  40   b  to a respective detection device  70   a ,  70   b  that is configured for receiving and detecting the detection beams  52   a  and  52   b  for monitoring the laser processing by the corresponding deflection unit  10   a  or  10   b . In the embodiment under consideration, the detection devices  70   a  and  70   b  each comprise a camera. Further, the first and second deflection units  10   a  and  10   b  each comprise a set of movable lenses  72   a ,  72   b  and fixed lenses  74   a ,  74   b , respectively, for focusing the respective detection beam  52   a  and  52   b  on the corresponding detection device  70   a ,  70   b  for each position on the work fields  40   a ,  40   b , from which reflection light might reach the detection devices  70   a  and  70   b  depending on the settings of the corresponding scanning devices  12   a  and  12   b . 
       FIG.  5    is a flow diagram of a method  200  of laser processing one or more workpieces using a deflection module like the deflection module described with respect to  FIGS.  1  to  3    above. The workpiece can be formed from a basis material such as metal powder by laser-processing successive layers of the basis material within the common overlap area  42  using the first deflection unit  10   a  and the second deflection unit  10   b  of the deflection unit. 
     In the method  200 , the first deflection unit  10   a  of the deflection module is used for scanning the working beam  50   a , which is generated as a laser beam with a first power density of 4 MW/cm 2 , and the second deflection unit  10   b  of the deflection module is used for scanning the working beam  50   b , which is generated as a laser beam with a second power density of 40 MW/cm 2 . The first working beam  50   a  and the second working beam  50   b  may be generated by identical laser sources having the same beam power. The higher power density of the second working beam is implemented by configuring the second working beam  50   b  having a smaller spot size than the first working beam  50   a . 
     The first working beam  50   a  is used for warming up the basis material and the second working beam  50   b  is subsequently used for laser-processing the basis material at points at which the basis material has previously been warmed up by the first working beam  50   a . The first and second working beams  50   a  and  50   b  can operate simultaneously, such that the first working beam  50   a  goes on to warm up other points of the basis material while the second working beam  50   b  is laser-processing points of the basis material already warmed-up by the first working beam  50   a . 
     For each layer of basis material to be laser-processed, at given points of the basis material, the first working beam  50   a  is first used, at  202 , for warming up the basis material. Then, at  204 , the second working beam  50   b  is used at the same points of the basis material for laser-processing the warmed-up basis material. 
     In other embodiments (not shown), the first working beam  50   a  can further be employed for slowing down the cooling-off of points of the basis material that have previously been laser-processed by the second working beam  50   b . 
     If more than one deflection modules are combined for cooperative operation (see description of  FIG.  7    to below), there are more than two working beams available, for which more than one working beams, for example two, can be used in  202  to warm up and/or cool down the basis material and more than one working beam, for example two, can be used in  204  to laser-process points of the basis material already warmed-up or to be cooled-down by the other working beams. 
       FIG.  6    schematically illustrates two different perspective exterior views of a deflection module according to embodiments of the invention, comprising a first deflection unit  10   a  and a second deflection unit  10   b  as described for the embodiments shown in  FIGS.  1  to  3   . As seen in  FIG.  6   , the deflection module comprises a housing  60 . All optical components described with respect to  FIGS.  1  to  3   , with the exception of the laser sources  28   a  and  28   b , are housed within the housing  60 , in the arrangement illustrated in  FIGS.  1  to  3   , wherein the longitudinal direction of the housing  60 , i.e. the direction in which the housing  60  extends longest, corresponds to the x-direction. In the embodiment shown in  FIG.  6   , the housing  60  comprises an optical inlet  68   a ,  68   b  in the form of an optical connector for receiving a laser source like the laser sources  28   a  and  28   b  described for  FIGS.  1  to  3   , wherein, when the laser source is coupled to the optical inlet  68   a ,  68   b , the laser source is arranged in a diagonal position in the xz-plane, forming a 30° angle with respect to each of the z- and x-axes. Thus, in the embodiments considered in  FIG.  6   , the laser light generated by the laser sources enters the deflection module in such diagonal direction. 
     The housing  60  is waterproof and dustproof and implements IP64 sealing protection according to the International Protection Rating, such that the interior thereof is isolated from the outside environment of the housing  60  due to the sealing effect provided by the housing  60 . 
     The housing  60  comprises a first transparent window  62   a  and a second transparent window  62   b , which are respectively formed by glass plates arranged at the bottom of the housing  60 , as shown in  FIG.  6   b   . The first transparent window  62   a  is arranged below the second movable mirror  12   a - 2  of the first scanning device  12   a  of the first deflection module  10   a , aligned with the second movable mirror  12   a - 2  in the xy-plane (cf.  FIGS.  1  to  3   ), at a distance of about 55 mm from the second movable mirror  12   a - 2  in the z-direction, such that the first working beam  50   a  can be transmitted through the first transparent window  62   a  for any targeted point of the first working field  40   a , i.e. for any deflection setting of the first scanning device  12   a . Likewise, the second transparent window  62   b  is arranged below the second movable mirror  12   b - 2  of the second scanning device  12   b  of the second deflection module  10   b , aligned with the second movable mirror  12   b - 2  in the xy-plane (cf.  FIGS.  1  to  3   ), at a distance of about 55 mm from the second movable mirror  12   b - 2  in the z-direction, such that the second working beam  50   b  can be transmitted through the second transparent window  62   b  for any targeted point of the second working field  40   b , i.e. for any deflection setting of the second scanning device  12   b . 
     The first transparent window  62   a  and the second transparent window  62   b  are arranged adjacent to each other, such that they share a common edge  65 . In the embodiment shown in  FIG.  6   , the first transparent window  62   a  and the second transparent window  62   b  are formed by independent glass plates. However, in other embodiments, the first transparent window  62   a  and the second transparent window  62   b  may be integral with each other and a single glass plate may cover both the first and second transparent windows  62   a  and  62   b . 
     As seen in  FIG.  6   , the first transparent window  62   a  and the second transparent window  62   b  are arranged adjacent to a lateral wall  63  of the housing  60  instead of being arranged in the middle of the bottom part of the housing or centred in the y-direction. In other words, the first and second transparent windows are not arranged at equal distances from the lateral wall  63  of the housing and the opposite lateral wall of the housing. As shown in  FIG.  7   , this allows mutually attaching two deflection modules  102 ,  104  like the deflection modules illustrated in  FIGS.  1  to  3    (interior views) and 6 (exterior view) to form a modular deflection system having a minimal distance between the transparent windows  62   a  and  62   b  of a first housing  60   a  of a first deflection module and the corresponding transparent windows  62   c  and  62   d  of a second housing  60   b  of a second deflection module.  FIGS.  7   a  and  7   b    respectively show perspective views from different angles of a first deflection module  102  and a second deflection module  104 , which are removably attached to each other forming a modular deflection system. 
     As seen in  FIG.  7   b   , since the transparent windows  62   a  and  62   b  of the first deflection module  102  and the transparent windows  62   c  and  62   d  of the second deflection module  104  are arranged offset from a central position with respect to the longitudinal axis of the respective deflection module, without being equidistant with respect to opposing lateral walls of the respective housings  60   a  and  60   b , when the first and second deflection modules  102 ,  104  are attached together, the transparent windows  62   a  and  62   b  of the first housing  60   a  are adjacent, respectively, to the transparent windows  62   c  and  62   d  of the second housing  60   b . The housings  60   a  and  60   b  comprise an attachment mechanism (not shown) for detachably or removably attaching the first and second deflection modules  102 ,  104  to each other. 
       FIG.  8    shows a schematic front view of the interior of the modular deflection system shown in  FIG.  7    when the first deflection module  102  and the second deflection module  104  are mutually attached. Each of the first and second deflection modules  102  and  104  corresponds to a deflection module like reflection module described with respect to  FIGS.  1  to  3   , comprising the same components in a corresponding arrangement. Thus,  FIG.  8    corresponds to a doubling of the schematic top view of  FIG.  2   . The first deflection module  102  and the second deflection module  104  are arranged mirror symmetrical with respect to each other and with respect to a further plane of mirror symmetry O that is indicated in  FIG.  8   . 
     Due to the symmetric arrangement of each of the first and second deflection modules  102  and  104 , wherein the first deflection module  102  defines a first common plane of mirror symmetry M1 corresponding to the plane M in  FIGS.  1  to  3    and the second deflection module  104  defines a second common plane of mirror symmetry M2 corresponding to the plane M in  FIGS.  1  to  3   , and due to the arrangement of the respective second movable mirrors  12   a - 2 ,  12   b - 2 ,  12 C- 2  and  12   d - 2 , which are arranged adjacent to one of the lateral edges of the respective deflection module (corresponding to the arrangement of the transparent windows  62   a - 62   d  described with respect to  FIG.  7   ), the separation between each two of the second movable mirrors  12   a - 2 ,  12   b - 2 ,  12 C- 2  and  12   d - 2  is reduced to a minimum. The separation between the second movable mirrors  12   a - 2  and  12   b - 2  of the first deflection module  102  and between the second movable mirrors  12 C- 2  and  12   d - 2  of the second deflection module  104  as well as the separation distances between the respective optical centres thereof correspond to the separations distances d, doc that have been described for  FIGS.  1  to  3   . 
     Further, the separation d′ between the second movable mirror  12   a - 2  of the first scanning device  12   a  of the first deflection module  102  and the second movable mirror  12 C- 2  of the first scanning device  12 C of the second deflection module  104  and between the second movable mirror  12   b - 2  of the second scanning device  12   b  of the first deflection module  102  and the second movable mirror  12   d - 2  of the second scanning device  12   d  of the second deflection module  104  is of about 10 mm. The distance d′ oc  between the optical centre of the second movable mirror  12   a - 2  of the first scanning device  12   a  of the first deflection module  102  and the optical centre of the second movable mirror  12 C- 2  of the first scanning device  12 C of the second deflection module  104  and between the optical centre of the second movable mirror  12   b - 2  of the second scanning device  12   b  of the first deflection module  102  and the optical centre of the second movable mirror  12   d - 2  of the second scanning device  12   d  of the second deflection module  104  is of about 65 mm. 
     As a consequence, the size of a common overlap field  44 , in which the working field  40   a  of the first deflection unit  10   a  of the first deflection module  102 , the working field  40   b  of the second deflection unit  10   b  of the first deflection module  102 , the working field  40   c  of the first deflection unit  10   c  of the second deflection module  104 , and the working field  40   d  of the second deflection unit  10   d  of the second deflection module  104  overlap as shown in  FIG.  9    can be increased for a given scan radius. 
     In the embodiment illustrated in  FIG.  9   , each of the working fields  40   a ,  40   b ,  40   c  and  40   d  is a square field covering an area of 500 mm x 500 mm. The first and second working fields  40   a  and  40   b  of the first deflection module  102  are aligned with each other in a first overlap direction (the x-direction). The first and second working fields  40   c  and  40   d  of the second deflection module  104  are likewise aligned with each other in the first overlap direction (the x-direction). The first and second working fields  40   a  and  40   b  of the first deflection module  102  and the first and second working fields  40   c  and  40   d  of the second deflection module  104  overlap with each other in the first overlap direction to 87%, i.e. for a length of 435 mm. Further, the first working fields  40   a  and  40   c  and the second working fields  40   b  and  40   d  overlap with each other, respectively, in a second overlap direction (the y-direction) to 87%, i.e. for a length of 435 mm. Thus the common overlap field  44  covers an area of 435 mm x 435 mm, while the scan radius between each of the second movable mirrors  12   a - 2 ,  12   b - 2 ,  12 C- 2  and  12   d - 2  (in their respective o-tilt positions) and the plane of the working fields  40   a ,  40   b ,  40   c  and  40   d  is of 620 mm. 
     Although preferred exemplary embodiments are shown and specified in detail in the drawings and the preceding specification, these should be viewed as purely exemplary and not as limiting the invention. It is noted in this regard that only the preferred exemplary embodiments are shown and specified, and all variations and modifications should be protected that presently or in the future lie within the scope of protection of the invention as defined in the claims.