Patent Publication Number: US-2023147321-A1

Title: System and method for coupling an optical beam into a light receiver

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/EP2021/058186, filed on Mar. 29, 2021, and claims benefit to European Patent Application No. EP 20169840.4, filed on Apr. 16, 2020. The International Application was published in English on Oct. 21, 2021 as WO 2021/209252 A1 under PCT Article 21(2). 
    
    
     FIELD 
     The present invention is directed to a system and a method for coupling an optical beam into a light receiver, particularly for coupling at least one optical beam, each having an essentially or exactly elliptical beam cross section into a light receiver having an essentially or exactly circular light receiving cross section. 
     BACKGROUND 
     In many applications, an optical beam generated by a laser, such as a semiconductor laser or a laser diode, needs to be coupled into a light receiver, such as an optical fibre. It is often necessary to couple more than only one optical beam into such a light receiver. For example, for many microscopy applications, such as laser scanning techniques, it is advantageous to deliver light including multiple laser lines to the microscope itself via a single-mode optical fibre. In such cases, the stability of the laser light power at the end of the optical fibre is very important. Due to the small fibre core diameters of single-mode fibres of only several microns, fibre coupling needs an accurate alignment of the corresponding optical elements directing and deflecting the optical beam(s) to the optical fibre. If several laser lines are involved, the beams are typically merged onto the same beam path using dichroic mirrors. Such a dichroic mirror allows a first beam to pass through the dichroic mirror while deflecting a second beam such that both beams will then propagate in the same direction and preferably co-axially. 
     For such applications, typically a system is used comprising mirrors/dichroic mirrors (also herein referred to as “optical beam deflectors”) for deflecting the one or more optical beams onto the same path of propagation leading to the light receiver/optical fibre. Usually, the necessary accurate alignment of the optical beams is done using precise opto-mechanics having a mirror fixed thereon by an adhesive or by a locking screw. Such mirror holders comprise a three-point bearing where at least two supporting points can be moved back and forth, e.g. by screws with a fine thread. Thus, the vertical and horizontal tilt can be set independently. State-of-the-art mirror holders can be moved very precisely and are very suitable for fibre coupling. Additionally, the mechanics can often be locked, leading to a quite good long-term-stability. There are, however, some drawbacks involved using such mirror holders. First, they are quite expensive, second, the achieved alignment of beams has to be stable in the long term in order to ensure laser power stability at the fibre end, increasing the demands on the opto-mechanics even more. Finally, typically a user has to align the mirrors for the desired coupling purposes, which is a time consuming procedure. Mirror holders or opto-mechanics of that kind are referred to in the following as “adjustment devices”. 
     Nowadays, laser diodes are typically used as compact and mostly cost-efficient sources of laser light. They have, however, a significant disadvantage in combination with fibre-coupling. After collimation, the laser beam has an elliptical shape/beam cross section, while single-mode fibres show a circular symmetric core, which means that the TEM00-mode coupling into the optical fibre is also circular. The result is that the coupling efficiencies of laser diodes into fibres are not that good due to the fact that a significant part of the elliptical laser beam focus can not couple into the circular fibre core. 
     The standard solution to solve this problem is beam-shaping, i.e. transforming the elliptical laser beam into a circular beam, by using e.g. an anamorphotic prism pair or cylindrical lenses. This increases the complexity and the price of the optical system, which is again disadvantageous. 
     SUMMARY 
     In an embodiment, the present disclosure provides a system for coupling at least one optical beam, each having an essentially or exactly elliptical beam cross section into a light receiver having an essentially or exactly circular light receiving cross section, the system comprising at least one optical beam entry port, each configured to provide an entry for each of the at least one optical beams into the system, an optical beam exit port configured to provide an exit for the at least one optical beam out of the system for coupling the at least one optical beam into the light receiver, the optical beam exit port comprising a lens configure to perform a Fourier Transform, at least one optical beam deflector for deflection of an optical beam of the at least one optical beam, and an optical base element, at least one of the at least one beam deflectors being directly fixed to the base element, wherein due to being fixed, the at least one directly fixed optical beam deflector is allowed to rotate around a rotation axis which rotation results in an additional deflection of the corresponding optical beam, wherein the corresponding at least one optical beam and/or its assigned optical beam entry port is further configured such that the semi-major axis of the elliptical cross section of the optical beam on a deflection surface of a respective optical beam deflector is oriented essentially or exactly parallel to the rotation axis, and wherein the at least one optical beam deflector is oriented such that, after having passed the optical beam exit port, the elliptical cross section of the at least one optical beam overlaps the circular cross section of the light receiver. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Subject matter of the present disclosure will be described in even greater detail below based on the exemplary figures. All features described and/or illustrated herein can be used alone or combined in different combinations. The features and advantages of various embodiments will become apparent by reading the following detailed description with reference to the attached drawings, which illustrate the following: 
         FIG.  1    schematically shows a top view of a system according to an embodiment of the present invention; 
         FIG.  2    shows an example of a state-of-the-art opto-mechanical mirror holder; 
         FIGS.  3   a - 3   d    schematically show an optical beam deflector directly fixed to a base element as used in a system according to an embodiment of the present invention; and 
         FIG.  4    schematically shows a perspective view of a part of a system shown in  FIG.  1    together with a light receiver according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In view of the problems described above, there is a need for an improved beam coupling system and a corresponding method, particularly reducing the necessity for ultra-stable opto-mechanics. 
     Embodiments of the present invention provide a system for coupling at least one optical beam, each having an essentially or exactly elliptical beam cross section, such as one or more optical beams generated by respective semiconductor lasers or laser diodes, into a light receiver, such as an optical fibre, having an essentially or exactly circular light receiving cross section, wherein the system comprises at least one optical beam entry port, each port being configured to provide an entry for each of the at least one optical beam into the system, an optical beam exit port configured to provide an exit for the at least one optical beam out of the system for coupling the at least one optical beam into the light receiver, at least one optical beam deflector, such as a mirror and/or dichroic mirror, for deflection of an optical beam of the at least one optical beam, and an optical base element, such as an optical base plate extending in an (x-y-) plane of the system. In this system, at least one beam deflector of said at least one beam deflector is directly, in other words unintermediately, particularly without using any adjustment device such as the above opto-mechanics or holder, fixed to the base element. Particularly, no adjustment of the thus fixed beam deflector after its fixation is possible. Due to the fixation, especially during or even after the fixation, as will be explained in more detail below, a directly fixed beam deflector typically might rotate—within a small rotation angle range—around a rotation axis which rotation results in an additional deflection of the corresponding optical beam. This “additional” deflection occurs in addition to the normal desired deflection. In this system, the at least one optical beam entry port and/or the at least one optical beam is further configured such that the semi-major axis of the elliptical cross section of the respective optical beam on a deflection surface of the beam deflector is oriented essentially or exactly parallel to the rotation axis. Finally, in this system, the at least one optical beam deflector is oriented such that after having passed the optical beam exit port or at the vicinity of the optical beam exit port, the elliptical cross section of the at least one optical beam overlaps the circular cross section of the light receiver. 
     Hitherto, in commercial coupling systems, each deflection element/optical beam deflector or at least the one located immediately before the optical beam exit port is delivered with its own opto-mechanics/adjustment device, increasing the costs of the system. It would be advantageous and more user friendly to align the mirrors in a factory, e.g. using special opto-mechanics, and then after having fixed the mirrors to the system release them from the opto-mechanics, which latter can then be used for the production of another system. Embodiments of the present invention achieve this very advantage. A single, some or even all beam deflectors used in the system can be directly fixed onto the base element of the system without delivering corresponding opto-mechanics to the user of the system. 
     Further, embodiments of the present invention overcome the problems related to the fixing of a mirror to the base element/plate of the system. One could think, for example, of fixing the mirror with some kind of screw or by simply using an adhesive. Each of these processes will unfortunately move the mirror a little bit while fixing it. A screw typically induces a little movement on the mirror while fastening it. On the other hand, an adhesive tends to shrink while hardening and will thus move the mirror. Further, with thermal changes of the environment, adhesives tend to shrink or expand, thus resulting in thermally induced movements of the mirror after its fixation, which can influence the coupling efficiency. Further, there are different ways of fixing a mirror to the base element/plate. The backside of the mirror can be fixed to a holder connected to the base element. Another way would be fixing the mirror with a lateral surface at one of its sides directly to the base element. The fixing could be done by an adhesive. During hardening of the adhesive and later on due to thermal fluctuations, the adhesive moves, moving the mirror and thus leading to different angular orientations. This would typically render the time consuming and cumbersome alignment process void. 
     In an embodiment, the present invention solves this problem as described in the following disclosure. 
     The idea of orienting the elliptical cross section of an optical beam in the way described above makes use of the supposed disadvantage of an elliptical beam cross section in order to compensate mirror movements related to a direct fixing of a mirror to the base element. This is described in the following with reference to  FIG.  3   . 
       FIG.  3   a    schematically shows a beam deflector/mirror  310  directly fixed via an adhesive  314  onto a base element  300 , here a base plate extending in the x-y-plane of the system. The mirror or beam deflector  310  has a deflection surface  311  and a backside  313  opposed to the deflection surface  311 , and further four lateral surfaces at the four sides of the mirror  310 , the mirror being directly fixed in this embodiment with one of its lateral surfaces  312  via the adhesive  314  to the base element  300 . It is noted that “directly” or “unintermediately” in the context of fixing a beam deflector to the base element is especially to be understood in the meaning of fixing the beam deflector without an adjustment device allowing free adjustment of the beam deflector after its fixation. Such a “direct” fixing, however, does not exclude that the beam deflector can be fitted onto a holder or into a frame, e.g. irreversibly, or that the beam deflector and the holder/frame are a compound unit acting as a unibody component, and the holder/frame can be fixed to the base element. The fixing itself can be done via an adhesive and/or with the help of at least one screw and/or welding or other fixing means. Again, it is noted that the fixing is particularly done without using any kind of adjustable holders or opto-mechanics as shown in  FIG.  2    by means of which the beam deflector would be indirectly fixed to the base element. 
       FIG.  3   a    further shows an optical beam  320  such as a laser beam travelling in an x-y-plane and being reflected by a deflection surface  311  of the mirror  310 . The illuminated area on the deflecting surface  311  is also shown. The arrows  320  denote the propagation direction of the laser beam, not the beam shape. 
     Fixing the mirror  310  by a layer of adhesive  314  to the base element  300 , the adhesive will typically create a layer between the lateral surface  312  and the base plate  300 , which layer is never really perfectly uniform, but could, for example, be in the form of a slight wedge. This wedge of adhesive will lead to a rotation of the mirror  310  around an axis parallel to the x-axis upon shrinkage or expansion of the wedge due to hardening or thermal changes. This situation is illustrated in  FIG.  3   b   . A rotation of the mirror  310  around an axis parallel to the x-axis, however, does not influence the direction of the reflected beam  320 , in other words, does not lead to an “additional” deflection as defined above. 
     Additionally or alternatively, the wedge of adhesive can be formed as shown in  FIG.  3   c   . Again, during shrinkage or expansion of the adhesive due to hardening of the adhesive or due to thermal changes, the thicker side of the wedge will expand further than the thinner side of the wedge, leading to a tilt or a rotation of the mirror  310  around a rotation axis y′ parallel to the y-axis. A movement of the mirror  310  around the y′-axis, however, leads to a deflection of the beam upon reflection out of the x-y-plane as illustrated in  FIG.  3   c    and thus to an additional undesired deflection. As explained below, embodiments of the present invention, however, are to a large extent insensitive to such a deflection of the beam out of the x-y-plane as long as the elliptical cross section overlaps the circular cross section of the light receiver. 
     As mentioned above, another possibility of fixing the mirror  310  to the base element  300  would be to use a holder which is fixed to the base element, the mirror  310  being fixed with its backside  313  at least partly to such a holder. As can be seen from  FIG.  3   , such a fixing might lead to a slight rotation of the mirror  310  around an axis parallel to the z-axis upon shrinkage or expansion of the adhesive. In this case, it would have to be made sure that the semi-major axis of the elliptical cross section of the optical beam on the deflection surface  311  is oriented (essentially) parallel to the rotation axis/z-axis. 
     As explained further below in connection with embodiments of the present invention, the situation of  FIG.  3   c   , i.e. a rotation of the mirror  310  around the y′-axis leads to a shift of the elliptical cross section with its semi-major axis along the z-direction as illustrated in  FIG.  3   d    by the arrow next to the elliptical beam cross section  330 . Such a shift hardly influences the coupling efficiency as long as the elliptical cross section  330  overlaps the circular cross section  340  of the light receiver. 
     It is noted that each focussing lens performs a Fourier transformation of the beam cross section of the beam passing the lens. In case of an elliptical shape such a Fourier transformation essentially results in a rotation of the elliptical shape of 90°. Thus, for example, the lens  170  as shown in  FIG.  1    rotates the elliptical shape of the optical beam  120 ,  121  comprising an elliptical cross section being oriented essentially parallel to the x-y-plane between the dichroic beam deflector  130  and the lens  170  such that the semi-major axis of the optical beam at the vicinity of the optical beam exit port  190 /at the light receiver  160  is oriented along the z-direction as illustrated in  FIG.  3   d    by the arrow next to the elliptical beam cross section  330 . 
     It should be noted that “essentially or exactly elliptical” or “elliptical or at least essentially elliptical” is meant to describe an elliptical form which may vary as a result of interaction with optical elements, such as (dichroic) mirrors, optical lenses etc. The same applies to the expression “essentially or exactly circular” and to the definitions of “essentially or exactly parallel” and “essentially or exactly perpendicular”. Further, deviations are allowed within typical tolerances. 
     Referring again to  FIG.  3   , an adhesive wedge between a lateral surface  312  of the beam deflector  310  and the base element  300  will only have a relevant influence if movement of the wedge results in a rotation of the beam deflector around the y′-axis (see  FIG.  3   c   ). As already discussed above, this will lead to a deflection of the beam in z-direction and thus to movement of the focal spot in z-direction on the light receiving element (see  FIG.  3   d   ). The beam deflector will almost never significantly rotate around the z-axis if the adhesive shrinks or expands. Bearing this understanding in mind, the elliptical shape of the laser beam cross section can even help to stabilise the laser coupling into the light receiver/fibre, if the beam is correctly oriented. By orienting the beam such that the semi-major axis of the elliptical cross section lies in the x-y-plane after having entered the optical beam entry port of the system, the focal spot of the optical beam will show its semi-major axis oriented parallel to the z-axis after having passed the exit port including a lens as shown in  FIG.  3   d   . Thus, if the beam moves in z-direction, the circular light receiving cross section  340  will still be significantly illuminated due to the elongation of the elliptical focal spot  330  in this direction. 
     In an advantageous embodiment, the at least one optical beam entry port or every optical beam entry port comprises a collimator lens. According to an embodiment, the optical beam exit port comprises a lens performing a Fourier Transform, in particular a condenser lens. While the collimator lens generates a beam of parallel light bundles, the condenser lens focuses a beam of parallel light bundles into a focal spot. A coupling end of the light receiver/optical fibre should be located at this focal spot. 
     In another advantageous embodiment, the optical beam exit port is further configured to receive an optical fibre as the light receiver, such that an optical fibre can be mounted to the optical beam exit port. Similarly, it is advantageous if a laser, particularly a laser diode, can be mounted to an optical beam entry port. 
     Typically, the lateral surface of a beam deflector is essentially or exactly perpendicular to the deflection surface of the optical beam deflector for deflecting the optical beam. 
     Embodiments of the present invention are particularly advantageous for coupling two or more optical beams into a light receiver such as an optical fibre. In this case, at least one of the optical beam deflectors is a dichroic beam deflector which allows a first optical beam to pass through the dichroic beam deflector and deflects a second optical beam, e.g. due to the wavelength characteristic of the dichroic beam deflector. Such a dichroic beam deflector is particularly suited for superimposing two optical beams. In case of more than two optical beams, more than one dichroic beam deflectors may be used. Such a dichroic beam deflector is advantageously arranged for directing the first and the second optical beams to the optical beam exit port. 
     In an advantageous embodiment, at least the optical beam deflector which is located immediately before the optical beam exit port is directly fixed to the base element. Typically, the system is set up starting to adjust the beam deflectors in the direction from the entry port to the exit port. Any deviations of the preceding mirrors/beam deflectors can, to a certain degree, be balanced out by adjusting the beam deflector/dichroic beam deflector located immediately before the optical beam exit port correspondingly. To this end, hitherto the last beam deflector/dichroic beam deflector is usually provided with opto-mechanics, i.e. an adjustment device. With embodiments of the present invention, however, even these opto-mechanics can be rendered superfluous. In this context, it is particularly advantageous if each of the at least one optical beam deflector is directly fixed to the base element without using any opto-mechanics. 
     In a second aspect of the present invention, a method for coupling at least one optical beam into a light receiver is provided. The method for coupling at least one optical beam, each having an elliptical or at least essentially elliptical beam cross section into a light receiver having a circular or at least essentially circular light receiving cross section, comprises the steps of: providing at least one optical beam deflector for deflection of an optical beam of the at least one optical beam, and providing an optical base element, particularly extending in a plane, wherein at least one beam deflector of said at least one beam deflector is, after an orientation step for orienting the at least one beam deflector, directly fixed to the base element, particularly without an adjustment device allowing adjustment of the directly fixed deflection element after its fixation, wherein due to the fixation the respective directly fixed optical beam deflector is allowed—within a small range—to rotate around a rotation axis which rotation results in an additional (undesired) deflection of the corresponding optical beam, and orienting the semi-major axis of the elliptical cross section of the respective optical beam on a deflection surface of the respective optical beam deflector parallel or at least essentially parallel to the rotation axis, wherein the orientation step for orienting the at least one beam deflector comprises orienting the at least one optical beam deflector such that the elliptical cross section of the at least one optical beam overlaps the circular cross section of the light receiver. 
     In a preferred embodiment, the method comprises the steps of: providing at least one optical beam deflector for deflection of an optical beam of the at least one optical beam, and providing an optical base element extending in an x-y-plane, wherein at least one beam deflector of said at least one beam deflector is, after an orientation step for orienting the at least one beam deflector, directly fixed with a lateral surface of the beam deflector to the base element, such that a deflection surface of the beam deflector extends at least essentially perpendicular to the surface of the base element/x-y-plane, and orienting the semi-major axis of the elliptical cross section of the respective optical beam parallel or at least essentially parallel to the x-y-plane when striking/hitting the deflection surface of the respective optical beam deflector. 
     It is pointed out that the features described above in relation to the system according to the first aspect represent an analogous description of the corresponding features of the method according to the second aspect. Some or all of the method steps may be executed by (or using) a hardware apparatus, like for example, a processor, a microprocessor, a programmable computer or an electronic circuit. In some embodiments, one or more of the most important method steps may be executed by such an apparatus. 
     As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items and may be abbreviated as “/”. 
     It should be noted that features of the above examples as well as of the examples explained below can—wholly or in part—be combined to other examples not explicitly mentioned herein, nevertheless being part of the present disclosure. 
     In  FIG.  1   , two optical beams  120 ,  121 , each having an elliptical beam cross section are coupled into a light receiver  160  having a circular light receiving cross section. The system is designated  100 . System  100  comprises two optical beam entry ports  180 ,  181 , each port being configured to provide an entry for each of the optical beams  120  and  121  into the system  100 . In the embodiment shown, optical beam entry port  180  comprises a collimator lens  150  and is configured for mounting a laser diode  140  for generating the optical beam  120 . Same applies to optical beam entry port  181  which comprises a collimator lens  151  and is configured to receive a laser diode  141  for generating the optical beam  121 . 
     The system  100  further comprises an optical beam exit port  190  configured to provide an exit for the merged two optical beams  120  and  121  out of the system  100  for coupling the beams into the light receiver  160  which is an optical fibre in the embodiment shown. The optical beam exit port  190  further comprises a condenser lens  170  and is further configured to receive the optical fibre  160 . Usually, the system  100  comprises not only the elements shown e.g. in  FIG.  1    as well as an optical base element preferably in the form of a base plate  300  (see e.g.  FIG.  3  or  4   ), but also at least one side wall and in particular with a top cover element, such that the system may have a box-shape. The at least one optical beam entry port  180 ,  181  and the optical beam exit port  190  are in such a case formed in a side wall. 
     The system  100  further comprises a number of beam deflectors  110 ,  111 ,  112 , and  130 , e.g. fitted into corresponding frames or onto corresponding holders, for deflection of the optical beams  120 ,  121  in order to direct them to the optical beam exit port  190 . Optical beam deflectors  110 ,  111 ,  112  may be in the form of simple mirrors, while beam deflector  130  may be a dichroic beam deflector which allows optical beam  121  to pass through it and which deflects optical beam  120  in order to merge the two optical beams  120 ,  121  onto the same optical path and preferably essentially propagating in a co-axially manner in the direction of the optical beam exit port  190 . 
     System  100  may comprise further optical elements and may comprise more than two optical beam entry ports. A person skilled in the art may easily adapt system  100  to a system for coupling more than two optical beams into a light receiving element  160 . 
     The system  100  further comprises an optical base element extending in an x-y-plane of the system, which plane corresponds to the drawing plane of  FIG.  1   . At least one beam deflector, especially the dichroic beam deflector  130  and advantageously also the mirrors  110 ,  111 ,  112 , are directly fixed with a lateral surface to the base element. This is further described with regard to  FIG.  3   . The deflection surfaces of the fixed beam deflectors extend essentially or exactly perpendicular to the base element/drawing plane of  FIG.  1   . The such fixed beam deflectors are advantageously fixed via an adhesive. Alternatively or additionally, the fixing can be achieved by at least one screw and/or by welding. 
     System  100  may advantageously be set up in a factory, first aligning mirrors  110  and  111 , second, aligning mirror  112  and finally aligning dichroic mirror  130  and fixing each of the aligned mirrors with an adhesive layer to the base element. The pre-built system  100  can then be delivered to a user without the need of a user alignment by means of opto-mechanics. 
       FIG.  2    shows a photography of an opto-mechanical holder or opto-mechanics  200  for a beam deflector typically used hitherto as an adjustment device for aligning and fixing a beam deflector. The mirror-mounting surface of the opto-mechanics  200  is designated  210 . The opto-mechanics  200  comprises a ball bearing  220 , a spring  240  and an adjustment screw  230 . At least two supporting points of the mirror-mounting surface  210  can be moved back and forth by corresponding screws  230  with a fine thread. Thus, the vertical and horizontal tilt can be set independently. This allows a highly precise movement of the beam deflector. Such opto-mechanics  200  are high in costs and the alignment process is time consuming. 
       FIG.  3    schematically shows a beam deflector  310  directly fixed to an optical base element  300  as part of an embodiment of the system  100  shown in  FIG.  1   . The beam deflector  310  can be any one of the beam deflectors  110 ,  111 ,  112 ,  130  of  FIG.  1   . Beam deflector  310  is directly fixed with its lateral surface  312  to the base element  300  via an adhesive layer  314 . As shown in  FIG.  3   a   , the semi-major axis of the elliptical cross section  330  of the optical beam  320  is oriented parallel to the x-y-plane in which the optical base element  300  extends and in which the beam  320  travels in direction of the deflecting surface  311 . Further details of  FIGS.  3   a  to  3   d    have already been discussed above. As already explained above, the fixing process and any possible thermal changes afterwards may lead to mirror movements. While movements around a z-axis are very unlikely, movements around an x-axis are not relevant for the coupling efficiency. The only relevant movement is the movement around the y′-axis as shown in  FIG.  3   c   . Such a movement leads to a deflection of the reflected beam  320  out of the x-y-plane, which results in a movement of the focal spot  330  in the z-direction as shown in  FIG.  3   d   . As long as the elliptical cross section  330  of the optical beam  320  overlaps the circular cross section  340  of the optical fibre  160 , a movement of the semi-major axis along the z-direction hardly influences the coupling efficiency. 
       FIG.  4    schematically shows in a perspective view e.g. a part of a system shown in  FIG.  1    together with a light receiver  160  according to an embodiment of the present invention. The optical beam deflector  310  can be a mirror especially in case of only one beam being coupled into a light receiver, or can be a dichroic beam deflector in case of coupling two or more optical beams into a light receiver (as shown in  FIG.  1    for two optical beams). In the latter case, optical beam  320  in  FIG.  4    corresponds to the optical beam  120  in  FIG.  1    and the beam deflector  310  corresponds to the dichroic beam deflector  130  of  FIG.  1   . Again, the arrows  320  illustrate the laser beam propagation directions and the elliptical shapes illustrate the corresponding beam cross sections. The optical beam  320 , after being reflected by the beam deflector  310 , is directed to the condenser lens  170 , the elliptical illuminated area being shown in  FIG.  4    (on the surface of the beam deflector  310  and at a side of the condenser lens  170  facing the beam deflector  310 ). The condenser lens  170  focuses the beam onto the front side of the light receiver, here an optical fibre  160  which is arranged with its front side in the focal plane of lens  170 . This arrangement results in coupling of the optical beam having an elliptical beam shape  330  into the optical fibre  160  having a circular light receiving section  340  as shown in  FIG.  3   d   . The elliptical shape of the optical beam  320  between the beam deflector  310  and the condenser lens  170  is oriented with its semi-major axis essentially parallel to the x-y-plane. Lens  170  turns the elliptical shape of the optical beam  320  about 90 degrees by the Fourier Transform function of the lens  170 , thus resulting in a (focused) elliptical shape of the optical beam  320  whose semi-major axis is oriented essentially parallel to the z-axis (as shown in  FIG.  3   d   ). 
     As can be seen from, for example,  FIGS.  3   a    and  4 , it should be made sure that the mirror dimension in the y-direction or more generally the mirror breadth is sufficient to cover the elliptical beam shape to its full extent. As the semi-major axis of the elliptical beam shape is oriented parallel to the x-y-plane, the semi-major axis is enlarged on the mirror surface, if the beam hits the mirror non-perpendicularly. (For illustration purposes, e.g., a circular beam profile would generate an elliptical illuminated area on a mirror surface, if the beam hits the mirror non-perpendicularly.) Thus, according to an embodiment of the present invention, an elliptical beam profile with its semi-major axis parallel to the x-y-plane may need a mirror with a larger dimension in the y-direction (as shown in  FIGS.  3   a    and  4 ), a fact which would normally prevent a person skilled in the art from choosing such a beam profile orientation and would rather lead to choosing a preferred orientation of the semi-major axis parallel to the z-direction. As explained above, however, the present embodiment offers significant advantages such as a better light coupling stability. 
     While subject matter of the present disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. Any statement made herein characterizing the invention is also to be considered illustrative or exemplary and not restrictive as the invention is defined by the claims. It will be understood that changes and modifications may be made, by those of ordinary skill in the art, within the scope of the following claims, which may include any combination of features from different embodiments described above. 
     The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B and C” should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise. Moreover, the recitation of “A, B and/or C” or “at least one of A, B or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C. 
     LIST OF REFERENCE SIGNS 
     
         
         
           
               100  system 
               110  beam deflector, mirror 
               111  beam deflector, mirror 
               112  beam deflector, mirror 
               130  beam deflector, dichroic beam deflector 
               120  optical beam 
               121  optical beam 
               140  laser diode 
               141  laser diode 
               150  collimator lens 
               151  collimator lens 
               160  light receiver, optical fibre 
               170  condenser lens 
               180  optical beam entry port 
               181  optical beam entry port 
               190  optical beam exit port 
               200  opto-mechanical holder, opto-mechanics 
               210  mirror-mounting surface 
               220  ball bearing 
               230  adjustment screw 
               240  spring 
               300  optical base element, base plate 
               310  optical beam deflector, mirror, dichroic beam deflector 
               311  deflecting surface 
               312  lateral surface 
               313  backside 
               314  adhesive layer, adhesive 
               320  optical beam 
               330  elliptical cross section 
               340  circular cross system