Abstract:
The invention relates to a device for adjusting a beam profile of a laser beam, which is guided in a light guide of a fibre optic cable provided with a protective sleeve, wherein the device comprises a deformation device for deforming the light guide in a section of the fibre optic cable not enclosed by the protective sleeve, a housing, which is designed to surround the light guide at least in the section not enclosed by the protective sleeve, and also two holders provided at opposite ends of the housing for fixing in each case one end of a section of the protective sleeve, wherein the holders each comprise an opening for feeding the light guide to the deformation device. The invention also relates to a laser processing machine having such a device and to an associated production method.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a device for adjusting a beam profile, a laser processing machine having such a device, and to a method for producing such a device. 
       BACKGROUND OF THE INVENTION 
       [0002]    If light from a beam source is coupled into one end of a light guide, the beam profile at the other end of the light guide is determined not only by the beam source but also by the homogenisation properties of the light guide. The near field power distribution desired for a laser processing machine at the output end of the light guide can take the shape (in a radial direction) of, for example, a mountain peak (conical tip with marked intensity maximum at the beam centre) or a flat-top mountain peak, (cylindrical shape, i.e. plateau of substantially constant intensity, “top-hat”). By setting a beam profile with a marked intensity maximum at the beam centre, the welding penetration depth in a laser welding process can be increased. By setting a top-hat profile on the other hand, a wide seam and hence a larger joint width between the two workpieces to be welded can be achieved. 
         [0003]    US 2010/0150201 A1 describes the coupling of laser radiation of a fibre bundle into a single fibre of a fibre optic cable. For better alignment of the fibres relative to the radiation entering therein, a mode mixer is used, which comprises a means for altering the curvature of the fibre optic cable. It is intended to achieve a homogenisation of the energy density of the laser beam by winding the fibre optic cable in loops. 
         [0004]    KR 10-2010-0068694 (Kim Jom Sool) discloses a device in which a section of a fibre optic cable is bent in order to achieve a uniform brightness distribution of a laser beam guided in the fibre optic cable. For that purpose the fibre optic cable is fixed at two points to a fixing plate and a movement device arranged therebetween displaces the section of the fibre optic cable running between said two points in order to adjust the curvature of the fibre optic cable. 
         [0005]    In order to protect light guides against damage from external influences, they are surrounded inter alia by a protective tube and form a fibre optic cable. The protective tube serves to withstand tensile, bending and transverse compressive stresses of the fibre optic cable. In particular when the light guide is used for transporting a high power laser beam, the protective tube should also ensure that no forces leading to a distortion or deformation of the light guide are able to act on the light guide. Such a protective tube is known, for example, from DE 20 2004 004 817 U1. The protective tube described therein has a high stiffness so that it can be bent only as far as a minimal bend radius of 150 mm, at which no damaging forces act on the light guide with the light guide diameters used there. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention addresses the problem of specifying a device for adjusting a beam profile, which on the one hand permits a flexible beam forming and on the other hand protects the light guide satisfactorily against damage due to external influences. 
         [0007]    This problem is solved according to the invention by a device for adjusting a beam profile of a laser beam, which is guided in a light guide of a fibre optic cable (also called a laser light cable, LLK) having a protective sleeve, wherein the device comprises: a deformation device for deforming the light guide in a section of the fibre optic cable not enclosed by the protective sleeve, a housing, which is designed to surround the light guide at least in the section not enclosed by the protective sleeve, and two holders provided at opposite ends of the housing for fixing in each case one end of a section of the protective sleeve, wherein the holders each comprise an opening for feeding the light guide to the deformation device. 
         [0008]    The adjustment of the beam profile is effected in the case of the device according to the invention by the application of a force, e.g. by bending, a section of the light guide not enclosed by the protective sleeve, whereby bend radii that are smaller than in the sections in which the light guide is enclosed by a protective sleeve can be achieved. At the same time, the arrangement inside the housing of the section that is not enclosed by the protective sleeve ensures that the light guide is protected also in said region against external influences. The protective sleeve provided outside the housing prevents forces that interfere with the transport of the high power laser radiation from being able to act on the light guide outside the housing. By means of the device according to the invention the protective sleeve of the fibre optic cable can be fixed in the holders, whilst the section of the light guide not enclosed by the protective sleeve is fed to the deformation device and deformed there. 
         [0009]    Preferably, the deformation device has a passage for passage of the light guide, which passage is movable transversely to the holders. The holders for fixing the protective sleeve are spaced apart from each other in the longitudinal direction of the fed-in light guide and the deformation device is arranged between the two holders, which may consist of a plurality of components which are fixed in their position relative to one another. The holders are preferably aligned along a common axis. The movement of the passage is effected in this case typically in a plane perpendicular to this common axis. By moving the passage relative to the holders, the light guide fed through the passage of the deformation device can be bent, since the holders are rigidly connected to one another via the housing. The deformation device can be designed to effect a linear displacement of the passage relative to the holders and in this manner bend the light guide. As an alternative to bending the light guide, the deformation device can also be designed to alter the beam profile by torsion of the light guide or by exerting pressure on the outer surface of the light guide. 
         [0010]    An especially preferred embodiment provides for the deformation device to be rotatable relative to the holders about an axis of rotation that is offset with respect to a feed axis formed by the openings in the holders, i.e. is spaced from the feed axis. The openings, which in particular can be in the form of feed channels, define the feed axis. The axis of rotation and the feed axis typically run parallel to one another. The degree of deformation or bending of the light guide can in this case be adjusted via the angle of rotation of the deformation device, wherein the adjustment is typically effected continuously and at exactly one rotated angle (starting position) the light guide is fed without any bending through the deformation device. A good homogenisation of the output (top-hat profile) can be achieved, for example, by a U-shaped or S-shaped bending of the light guide. A U-shaped bending is understood to be a bending in which the light guide comes, for example, from the left, is bent downwards, runs back again in a U-shape and then continues to the right. 
         [0011]    In a development of the invention, the deformation device is in the form of a hollow cylinder, which has an eccentric bore as the passage. The passage, or rather the bore, runs eccentrically with respect to the centre line of the hollow cylinder that forms the axis of rotation, so that the hollow cylinder embodies a simple form of a cam. The hollow cylinder can in particular comprise two bore portions tapering towards the passage in order to facilitate feeding of the light guide through the passage. 
         [0012]    In order to permit an easy and convenient operation, it is advantageous if the deformation device comprises a control element that extends through an opening in the housing and can be operated from outside the housing. The control element is preferably rigidly connected to the deformation device, so that a movement of the control element is converted directly into a movement of the deformation device. The control element can be, for example, a pin, which is guided through an opening in the housing, for example, in the form of a slot extending over a specific angular range, and can be turned along this opening relative to the housing, wherein the rotational movement can be effected, for example, over an angular range of at most 180°. In order to seal the opening or the slot with respect to the surroundings, and to permit an additional guidance of the control element, a covering, for example, a collar-like covering, can be provided on the exterior of the housing and can be rigidly connected to the control element. It goes without saying that a scale, e.g. an angle scale, can be provided on the exterior of the housing and/or on the covering in order to indicate the degree of deformation or bending of the light guide. 
         [0013]    In a preferred embodiment, the deformation device is designed to bend the light guide in the section of the fibre optic cable with no sleeve up to a minimum bend radius of 40 mm, preferably 25 mm, especially preferably 10 mm, in order in this manner to permit a significant change of the beam profile. The radii of curvature specified here can be effected typically not by bending a fibre optic cable, in which the light guide is enclosed by the protective sleeve, since the covering is intended to prevent too great a bending of the light guide. The (minimum) bend radius required for an ideal beam profile adjustment also depends on the diameter of the light guide (e.g. quartz fibre). The values specified here are typical for a diameter of more than about 600 μm. In the case of smaller diameters (e.g. of more than about 100 μm), correspondingly smaller (minimal) bend radii are normally used. 
         [0014]    A preferred embodiment of the device comprises a fibre optic cable, the light guide of which is guided through the holders and the deformation device arranged therebetween, wherein the protective sleeve comprises two sections, which enclose respective sections of the light guide outside the deformation device, wherein the end of each section facing the deformation device is fixed in a respective one of the holders. The sections of the protective sleeve (part-sleeves) protect the light guide against damage in the region outside the deformation device and are fixed in the holders, e.g. are pushed therein, so that the protective sleeve is prevented from slipping off the light guide. Inside the deformation device meanwhile, the desired beam profile adjustment can be made, irrespective of the properties of the protective sleeve. A seal, e.g. in the form of an O-ring, which surrounds the outer surface of the protective sleeve fixed in the holders, can optionally be mounted in the region of the holders. It goes without saying that each section of the protective sleeve can also comprise a plurality of part-sleeves nested one inside the other, e.g. a radially inner part-sleeve which is surrounded by a radially outer part-sleeve. 
         [0015]    Fibre optic cables that are used for transporting laser radiation from a laser source to a laser processing head in a laser processing machine are generally provided with a protective sleeve that prevents a bending of the light guide with a bend radius of less than 200 mm or less than 150 mm. A beam profile adjustment of the desired quality is not generally possible with such bend radii, so that a bending of the light guide relative to the protective sleeve in the above-described manner is required. 
         [0016]    The invention also relates to a laser processing machine having a device as described above, in which the fibre optic cable is typically used for transport of laser radiation from a laser source to a laser processing head. 
         [0017]    A further aspect of the invention relates to a method for producing the above-described device, comprising the following steps: feeding the light guide of the fibre optic cable through the passage of the deformation device, providing the light guide with a section of the protective sleeve at each of the opposite ends of the housing and fixing one end of each section in a respective one of the holders. In this manner a device can be provided in which the light guide, at least in the region of the deformation device, is not enclosed by a protective sleeve. 
         [0018]    Further advantages of the invention are apparent from the description and the drawings. Features mentioned above and hereafter can likewise be used alone or in any combination. The exemplary embodiments illustrated and described are not to be understood as an exhaustive list but are merely of an exemplary nature for explanation of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  shows a sectional view of a device for beam profile adjustment comprising a deformation device in a first angular position, in which a light guide is fed, without being bent, through a passage in the deformation device; 
           [0020]      FIG. 2  shows a sectional view of the device for beam profile adjustment comprising a deformation device in a second angular position, in which the light guide has a curvature in the region of the passage; 
           [0021]      FIG. 3  shows an exterior view of the device of  FIG. 1  and  FIG. 2 ; and 
           [0022]      FIG. 4  shows a laser processing machine with a device for beam profile adjustment according to  FIG. 1  to  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIGS. 1 and 2  show a device  20  for beam profile adjustment of a laser beam (not shown), which is guided in a light guide  1  of a fibre optic cable  2 , in two different angular positions of a deformation device  6 , which in the present example is designed to bend the light guide  1  and which has a passage  10  for the light guide  1 . 
         [0024]    A housing  14  surrounds the region of the deformation device  6  and also two holders  4   a,    4   b,  through which the light guide  1  of the deformation device  6  can be fed, by introducing the light guide  1  into the first holder  4   a  at the angular position of the deformation device shown in  FIG. 1 , The light guide  1  is here fed via a first opening  5   a  of the first holder  4   a  to the passage  10  of the deformation device  6 . The light guide  1  passes via a second opening  5   b  in the second holder  4   b  and is led out of the housing  14  at the second holder  4   b.  In the angular position of the deformation device  6  shown in  FIG. 1 , the openings  5   a,    5   b  of the holders  4   a,    4   b  and the passage  10  are arranged along a common feed axis  11 , so that the light guide  1  can be passed without problems through the openings  4   a,    4   b  and the passage  10  of the deformation device  6 . The deformation device  6  in the present example is in the form of a circular cylinder with an eccentric bore in the form of a double funnel, the minimum diameter of which forms the passage  10 . 
         [0025]    The fibre optic cable  2  comprises in the present example a protective sleeve  7   a,    7   b,  which annularly surrounds the light guide  1  and protects it against external influences, in particular damage and undue bending. The protective sleeve comprises two sections  7   a,    7   b  which, after the light guide  1  has been introduced into the deformation device  6 , are pushed over the ends of the light guide  1 . The ends of the sections  7   a,    7   b  facing the housing  14  are secured in the holders  4   a,    4   b,  in the present example, are firmly clamped therein. In order to provide a seal against the surroundings, seals  3   a  and  3   b  in the form of O-rings are provided on the holders  4   a  and  4   b  respectively. Since the sections or part-sleeves  7   a,    7   b  terminate at or at least in portions in the holders  4   a,    4   b,  the fibre optic cable  2  has a section  9  between the holders  4   a,    4   b  with no sleeve, in which the deformation device  6  is provided. Both the section  9  of the light guide  1  with no sleeve and also the transitions from the section  9  with no sleeve to the sections  7   a,    7   b  of the protective sleeve is protected against external influences by the housing  14 . 
         [0026]    By means of a control element  12 , which in the example shown is rigidly connected to the deformation device  6 , the deformation device  6  can be rotated about an axis of rotation  13  that forms the axis of symmetry of the cylindrical deformation device  6  and of the housing  14 . The rotation of the deformation device  6  can also be effected in a different manner e.g. by means of a screw or a motor. The deformation device  6  can be rotated by means of the control element  12  into a second angular position, which is shown in  FIG. 2  and in which the deformation device  6  has been rotated into an angular position rotated through 180° with respect to the angular position shown in  FIG. 1 . Since the passage  10  is arranged eccentrically with respect to the axis of rotation  13 , when the deformation device  6  is rotated the passage  10  moves along a circular path perpendicular to the axis of rotation  13 . Although the passage  10  still runs parallel to the feed axis  11  here, it is spaced apart therefrom. As a result, the section  9  of the light guide  1  that is located inside the deformation device  6  is bent, as shown in  FIG. 2 . By actuating the control element  12 , the light guide  1  can therefore be brought out of the extended position into a bent or curved position, the curvature of the light guide  1  increasing as the distance between the feed axis  11  and the passage  10  increases. In the angular position shown in  FIG. 2 , this distance is the maximum distance. The dimensions of the housing  14  and in particular the distance between the feed axis  11  and the axis of rotation  13  and the length of the passage  10  are here selected so that in this angular position a (minimum) bend radius of 40 mm, preferably of 25 mm, in particular of 10 mm, can be achieved. 
         [0027]    Due to the bending of the light guide  1  a compressive stress is generated on the part of the light guide  1  disposed inwardly with respect to the bend, i.e. on the part of the U-shaped section  9  of the light guide  1  facing upwards in  FIG. 2 , whereas a tensile stress is generated on the opposite part of the U-shaped section on the outside with respect to the bend. These differences in stress can cause e.g. a change in the refractive index within the light guide  1 , so that the change in the degree of bending (change in the radius of curvature) causes a change of the beam profile. In this manner the beam profile of the laser beam guided in the light guide  1  can be matched to a desired application. The more marked the curvature (that is, the smaller the bend radius), the more closely the beam profile can be approximated to a top-hat profile. 
         [0028]    Using the deformation device  6  shown in  FIG. 1  and  FIG. 2  the light guide  1  can be brought into a shape of which the curvature undergoes a sign change twice. It is also possible, however, using a correspondingly designed deformation device  1 , to effect more than two curvature changes, in order to optimise the beam profile. Alternatively or additionally to bending the light guide  1 , the deformation device  6  can also be designed to twist the light guide  1  or to exert pressure on the (unbent) light guide  1 ). This measure also alters the refractive index of the light guide  1  in the region of the deformation, which can be used for an adjustment of the beam profile. 
         [0029]    It goes without saying that the device  20  described here can also be used with fibre optic cables that have merely a single protective sleeve or more than two protective sleeves nested one inside another. The holders then have to have a corresponding number of fixing devices for the particular protective sleeve. The protective sleeve can be constructed as described, for example, in DE 20 2004 004 817 U1. It serves only to provide mechanical stability, and has no beam-guiding action. The minimum bend radius of the light guide  1  that can be achieved with the protective sleeve  7   a,    7   b  is approximately 150 mm, and is therefore considerably larger than the bend radii that can be produced at the section  9  of the light guide  1  not enclosed by the protective sleeve  7   a,    7   b.    
         [0030]      FIG. 3  shows an exterior view of the beam-forming device  20  of  FIG. 1  and  FIG. 2 . The control element  12  is in the form of a pin, which is connected to a collar  15  mounted on the exterior of the housing  14 , so that a rotation of the deformation device  6  can be achieved by applying force to the pin  12  and/or to the collar  15 . The collar  15  also serves as a covering for a slot-shaped opening in the housing  14  (see  FIG. 1  and  FIG. 2 ) through which the pin  12  is guided and which in the present example extends over an angular range of 180° For a reproducible beam profile adjustment, the collar  15  or the housing  14  can be provided with an (angle) scale and can function as a setting wheel. 
         [0031]      FIG. 4  shows part of a laser processing machine  17  with a device  20  according to  FIGS. 1-3  for adjusting the beam profile, in which the laser radiation is fed from a laser beam source (not shown) by means of the fibre optic cable  2  to a beam guide  19  for free propagation of the laser beam  18 . The beam guide feeds the laser beam  18 , which has been imparted the desired beam profile in the device  20 , to a laser processing head  21 . By means of the device  20  the shape of the intensity distribution of the laser beam  18  can be adjusted depending on the type of laser processing to be carried out. 
         [0032]    All in all, the above-described device  20  permits a reliable transport of laser radiation by means of a fibre optic cable  2 , the laser radiation having a beam profile that is adjustable substantially independently of the beam source.