Abstract:
A device for generating multiple collimated light beams includes a carrier and at least two light generation units mounted on the carrier. The light generation units are configured to emit, each from an aperture, light beams at respectively different wavelengths and being arranged, with respect to axes of the light beams, substantially parallel and substantially in a common plane. A first lens is mounted on the carrier intersectingly to the axes of the light beams and has a principal plane. The light generation units have their apertures aligned along an alignment line substantially perpendicular to the axes of the light beams and the principal plane of the first lens is non-perpendicular to the axes of the light beams and is non-parallel to the alignment line.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to European Patent Application No. 14 178 525.3, filed on Jul. 25, 2014, the entirety of which is incorporated by reference herein. 
     BACKGROUND 
     The present invention relates to a device for generating multiple collimated light beams. 
       FIGS. 1 and 2  show such a device  1  according to the state of the art as described in e.g. WO 2012/113856 A1 and WO 2012/113883 A2. The device  1  can be used as a light source for a pixel of a display or projector and is, for example, capable of emitting selectively controlled red, green and blue light beams which superpose in the viewer&#39;s eye to yield any desired colour. To this end, three light generation units  2   a ,  2   b ,  2   c  are mounted —via a submount  3 —on a carrier  4  and emit light beams  5   a ,  5   b ,  5   c  at respectively different wavelengths λ a , λ b , λ c  (here: red, green, blue). 
     Due to physical and manufacturing constraints, the light beams  5   a ,  5   b ,  5   c  emitted from the light generation units  2   a ,  2   b ,  2   c  each diverge around an axis (“mean” or “primary” axis)  6   a ,  6   b ,  6   c . If light generation units based on conventional edge emitting laser diodes are used, the emitted light beams  5   a ,  5   b ,  5   c  will diverge “fast” (broadly) along a direction d f  corresponding to a p-n transition (“edge”)  7  in the laser diodes, and will diverge “slowly” (narrowly) in a direction d s  perpendicular thereto. To collimate the light beams  5   a ,  5   b ,  5   c  in the direction d f , a so-called fast axis collimation (FAC) lens  8  is mounted in front of the light generation units  2   a ,  2   b ,  2   c . Alternatively or additionally, there can also be arranged a slow axis collimation (SAC) lens to collimate the light beams  5   a ,  5   b ,  5   c  in the slowly diverging direction d f  (not shown). 
     Lenses, such as the FAC (and SAC) lens  8  mentioned, have wavelength dependent focal points (focal lengths) due to the wavelength dependency of their refraction index, known as chromatic aberration. This is depicted in  FIG. 3 , where the non-linear wavelength dependency of the back focal length is shown for a specific lens. On the horizontal axis, the wavelength λ of the light traversing the lens is depicted, while the vertical axis depicts the focal length f. As can readily be seen at curve  9 , in the visible light range (depicted by the vertical, dashed lines) the wavelength dependency is non-linear, leading to complicated chromatic aberrations. 
     According to  FIG. 2 , the state of the art overcomes this problem by mounting each of the light generation units  2   a ,  2   b ,  2   c  further away from the lens  8  to move their apertures  10   a ,  10   b ,  10   c  into a distance  11   a ,  11   b ,  11   c  from the lens  8  corresponding to the back focal length of the lens  8  for the respective wavelength λ a , λ b , λ c  used. However, this solution brings several problems. Firstly, manufacturing a submount  3  with individually backwards shifted light generation units  2   a ,  2   b ,  2   c  turns out to be a complex and minute task, since the light generation units  2   a ,  2   b ,  2   c  need to be applied with very small tolerances away from the lens  8  somewhere in the middle of the submount  3 . Once the light generation units  2   a ,  2   b ,  2   c  are bonded onto the submount  3 , there is no way of calibrating the device  1 , while there is also no way of readjusting them to compensate e.g. aging effects. 
     Secondly, because of the fast divergence of the light beams  5   a ,  5   b ,  5   c  in the vertical direction d f , a lower part of the light beams  5   a ,  5   b ,  5   c  gets cut off when the light generation units  2   a ,  2   b ,  2   c  are mounted away from an edge of the submount  3 , whereupon only a fraction of the light beams  5   a ,  5   b ,  5   c  can be used for collimation. This problem is aggravated by the fact that conventional edge-emitting laser diodes used for the light generation units  2   a ,  2   b ,  2   c  are mounted with their p-side facing downwards for thermal reasons and thus have their edges  7  and consequently their apertures  10   a ,  10   b ,  10   c  near the bonding submount  3 , so that an even larger percentage of the light beams  5   a ,  5   b ,  5   c  gets cut off upon moving backwards on the submount  3 . 
     Additionally, because the individual light generation units  2   a ,  2   b ,  2   c  have to be moved back by different lengths  11   a ,  11   b ,  11   c , the intensity of the collimated light beams  5   a ,  5   b ,  5   c  will differ since a larger percentage of a light beam is cut off the further back the light generation unit is shifted. Thus, the light generation units  2   a ,  2   b ,  2   c  have to be corrected in output intensity, too. 
     SUMMARY 
     It is an object of the invention to provide an improved device for generating multiple collimated light beams which overcomes the above-mentioned drawbacks of the state of the art. 
     To this end, the invention provides for a device for generating multiple collimated light beams, comprising: a carrier; at least two light generation units mounted on the carrier, the light generation units being configured to emit, each from an aperture, light beams at respectively different wavelengths and being arranged, with respect to axes of the light beams, substantially parallel and substantially in a common plane; and a first lens mounted on the carrier intersectingly to the axes of the light beams and having a principal plane; wherein the light generation units have their apertures aligned along an alignment line substantially perpendicular to the axes of the light beams; wherein said principal plane of the first lens is non-perpendicular to the axes of the light beams and is non-parallel to the alignment line. 
     The invention thus provides for a light beam generation device which is easy to calibrate, since the light generation units can be aligned along a straight line, thus simplifying the manufacturing process, while compensating for the chromatic aberration by tilting a common (FAC or SAC) lens. The alignment can be done with a larger tolerance than in the state of the art, since misalignments or aging effects can be corrected by tilting the lens around a correction angle. Furthermore, a subsequent calibration after the bonding can be achieved by newly adjusting the angle of the lens, if the latter is mounted releasably. 
     Because the light generation units can be arranged along a common alignment line such as e.g. an edge of the submount or carrier, the diverging light beams do not get cut off. Thus, the entire cross section of the light beams can be used for collimation, yielding a highest possible, uniform intensity of the collimated beams without the further need of readjusting individual light generation units in intensity. 
     Another advantage of the device according to the invention is that the lens is at a slant with respect to the propagation direction of the light beams. The amount of light reflected from the lens into the light generation unit is thereby reduced and unwanted effects such as hysteresis phenomena or a prolongation of the relaxation oscillation otherwise encountered in laser diodes are thereby avoided. In view of this, there is no need to apply a high-quality anti-reflection coating with a large number of dielectric layers onto the lens, thereby reducing costs of the device. Higher intensities of light beams emerging from the lens can also be achieved. 
     In a first-order approximation, the light generation units can be spaced at equal distances from each other on the alignment line, which yields a simple manufacturing processes. However, in another embodiment the apertures may be spaced at unequal distances from each other along the alignment line. Thereby, the light generation units can be adjusted to the non-linear wavelength-dependent focal length of the lens as precisely as desired, while still being aligned along a straight line. Since the uneven spacing gives a further degree of freedom additional to the tilting of the lens and the distance of the lens to the light generation units, the tolerance limits are effectively split up between said degrees of freedom. 
     In a further embodiment of the invention, there are three light generation units mounted on the carrier, configured to emit a green, a red and a blue light beam. This setup enables the device to be used as a basis for pixels of a full colour display or projector. Alternatively, it is also possible to use two, four, or even more light generation units, which may also emit light beams at different wavelengths than the ones mentioned, e.g. which do not lie in the visible light spectrum. 
     With the device according to the invention, it is particularly favourable when the light generation units are mounted on the carrier via a submount. Thereby, ready-made submounts with bonded light generation units can be manufactured, which can in turn be mounted onto the carrier of the device. The device can then be built using ready-made sets of individual lenses and submounts, which can be sold separately and combined individually. 
     In a further embodiment, the alignment line lies parallel to an edge of the submount, e.g. on the edge. The light generation units can thus be mounted as easily as possible, while gaining an extra clearance from the carrier, making it possible to avoid a “cut-off” of the light beam, even though the light generation units are not arranged on the edge of the carrier. 
     In another embodiment, the light generation units each comprise a laser diode. Such laser diodes can be bonded onto or even integrated into the submount to further reduce the build height. 
     Several types of lenses and lens combinations can be used for the device according to the invention to collimate the light beams emitted by the light generation units. The lenses could even be made out of a material having a graded refraction index (GRIN), however, for ease of manufacture materials with a homogeneous refraction index can be used. 
     In one embodiment, a single FAC lens can be used on the carrier. Such a lens can be a rod lens having a constant profile along a longitudinal direction of the lens. These sort of lenses are especially easy to produce. The lens may be an acylindrical rod lens to reduce the spherical aberration. 
     In an alternative embodiment, a single SAC lens can be mounted on the carrier in the inventive way, i.e. such a lens can have a planar side and at least two, in particular three, parallel cylindrical bulges on an opposite side thereof. 
     It is also possible to combine the above-mentioned FAC and SAC lenses, namely a first (FAC) lens collimates the diverging light beam into a light fan, whereupon a second (SAC) lens collimates the light fan into a collimated light beam, or the other way around, respectively. The FAC lens can, for example, be at a non-zero angle and the SAC lens at an arbitrary angle towards the alignment line, i.e. the first lens is an acylindrical rod lens and a second lens, having a planar side and at least two, in particular three, cylindrical bulges on an opposite side thereof, is mounted before or after the first lens on the carrier when seen in the propagation direction of the light beams. 
     Alternatively the SAC lens can be at a non-zero angle and the FAC lens at an arbitrary angle towards the alignment line, i.e. the first lens has a planar side and two or more parallel cylindrical bulges on an opposite side thereof and the second lens, being an acylindrical rod lens, is mounted before or after the first lens on the carrier when seen in the propagation direction of the light beams. 
     As just mentioned, said second lens could be parallel to the alignment line, but in several embodiments it is not, i.e. also the second lens has a principal plane which is non-perpendicular to the axes of the light beams and is non-parallel to the alignment line. If the first and the second lens are made out of a different material with a different refraction index and/or the focal lengths are different due to different radii of curvature, the principal plane of the first (FAC or SAC) lens could lie at a different angle to the alignment line than the principal plane of the second (SAC or FAC) lens. 
     In all those embodiments, a planar side of the first lens can optionally be joined—e.g. monolithically—to a planar side of the second lens, to form an especially compact component. 
     In some cases, e.g. when the FAC lens and the SAC lens are at a different angle to each other and are to be joined together, a transparent wedge can be used to join a planar side of the first lens and the planar side of the second lens. In particular, the wedge can be joined with the lenses monolithically, i.e. forming a single piece. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention shall now be explained in more detail on the basis of exemplary embodiments thereof with reference to the accompanying drawings, in which: 
         FIG. 1  shows a device according to the state of the art in a perspective view; 
         FIG. 2  shows the device of  FIG. 1  according to the state of the art in a schematic plan view; 
         FIG. 3  shows the wavelength dependency of the back focal length of a lens by means of a diagram; 
         FIG. 4  shows a first embodiment of the device according to the invention in a schematic plan view; 
         FIG. 5  shows a second embodiment of the device according to the invention in a schematic plan view; 
         FIGS. 6 a  and 6 b    show two different embodiments of a lens of the devices of  FIGS. 4 and 5  in perspective views; and 
         FIGS. 7 a , 7 b , 8 a , 8 b , 9 a , 9 b , 10 a  and 10 b    show different combinations of the lenses of  FIGS. 6 a  and 6 b    as can be used in the devices of  FIGS. 4 and 5  in schematic plan views. 
     
    
    
     DETAILED DESCRIPTION 
     Regarding  FIGS. 1 to 3  about the state of the art it is referred to the introduction. 
       FIG. 4  shows a first embodiment of an inventive device  12 , where the same reference numerals denote the same components as in the device  1  of  FIGS. 1 and 2 . The device  12  can be used as a light source for a pixel of a display or projector or, e.g., for lighting, medical and optical communication applications. 
     In the embodiment of  FIG. 4 , the apertures  10   a ,  10   b ,  10   c  of the light generation units  2   a ,  2   b ,  2   c  of the device  12  are aligned along an alignment line  13  which in this case also corresponds to an edge  14  of the submount  3 . In other embodiments, the alignment line  13  could also be parallel at a distance to the edge  14 . The apertures  10   a ,  10   b ,  10   c  are spaced at equal distances  15 ,  15 ′ from each other, while the axes  6   a ,  6   b ,  6   c  of the light beams  5   a ,  5   b ,  5   c  lie substantially parallel and substantially in a common plane. 
     The light generation units  2   a ,  2   b ,  2   c  can be of any kind, e.g., light emitting diodes (LEDs), laser diodes, in particular edge-emitting laser diodes emitting parallel to a p-n transition or vertical-cavity surface-emitting laser diodes (VCSEL diodes) emitting perpendicular to a p-n transition, etc. 
     To cope with the chromatic aberration, the lens  8 , or more specifically its principal plane  16 , is substantially mounted at an angle α to the alignment line  13 . The principal plane of a rod lens has the property that a ray emerging from a corresponding focal point on one side of the lens seems to bend at the principal plane only (instead of the more complicated optical path within the lens) and then travels perpendicular with respect to the collimation direction (e.g. perpendicular to the direction d f  in case of a FAC lens) after emerging from the principal plane. 
     In a first embodiment, the lens  8  is a fast axis collimation (FAC) lens, e.g. embodied as a collective rod lens as shown in  FIG. 6 a   , which is used to collimate an incoming beam  5   a ,  5   b ,  5   c  with respect to the direction d f . The lens  8  has a planar side  17  facing the light generation units  2   a ,  2   b ,  2   c  and a convex side  18  opposite thereto. To reduce the spherical aberration, the lens  8  is embodied as an acylindrical rod lens having a constant profile (cross-section) along its longitudinal direction  22 . The lens  8  could optionally be made out of a material having a graded refraction index (GRIN) in order to assist and/or eliminate the convexity of the side  18 . 
     As the principal plane  16  is aligned at an angle α to the alignment line  13 , the three focal points—each one corresponding to the wavelength used—of the lens  8  substantially lie at each one of the apertures  10   a ,  10   b ,  10   c  of the light generation units  2   a ,  2   b ,  2   c , i.e. the red focal point substantially lies at the aperture  10   a  of the light generation unit  2   a  emitting a red light beam  5   a  and so forth. As known to the person skilled in the art, the actual “source points” of the light beams  5   a ,  5   b ,  5   c  can in reality lie a bit further behind the apertures  10   a ,  10   b ,  10   c  within the light generation units  2   a ,  2   b ,  2   c . 
     As was already discussed in the introduction, the chromatic aberration is non-linear. In a further embodiment depicted in  FIG. 5 , this can be overcome by introducing unequal intervals  15 ,  15 ′ between the apertures  10   a ,  10   b ,  10   c . Together with the angle α of the principal plane  16  towards the alignment line  13  and the distance of the lens  8  from the apertures  10   a ,  10   b ,  10   c , manufacturing tolerances can be divided between these degrees of freedom. 
       FIG. 6 b    shows a slow axis collimation (SAC) lens  8 , having a planar side  19  and three parallel cylindrical bulges  20   a ,  20   b ,  20   c  on its opposite side. This lens  8  serves as to collimate the light beams  5   a ,  5   b ,  5   c  with respect to the direction d s . Since the divergence of the light beams  5   a ,  5   b ,  5   c  is not as large in this direction d s , there is no need to balance the spherical aberration by forming the bulges acylindrical, however, it could still be done to improve the quality of the optical system. In this embodiment, the lens  8  can thus be seen as a combination of three cylindrical lenses, the focal lengths of which are each wavelength-dependent. The principal planes of the individual bulges coincide spatially for a specific wavelength considered, which is why they are all assigned here to the lens  8  as a (common) principal plane  16 . 
     Again, the principal plane  16 , and thereby the lens  8 , is aligned at an angle α to the alignment line  13 , whereupon a focal point of each bulge  20   a ,  20   b ,  20   c  coincides with one aperture  10   a ,  10   b ,  10   c  of the light generation units  2   a ,  2   b ,  2   c . 
       FIGS. 7 a -10 b    show further embodiments of combinations of lenses, where a first lens  8 —in these cases a FAC lens  8  according to  FIG. 6 a   -is used to collimate the light beams  5   a ,  5   b ,  5   c  in the first (rapidly diverging) direction d f  and a second lens  21 —in the form of a SAC lens  8  as discussed in  FIG. 6 b   —is used to collimate the light beam  5   a ,  5   b ,  5   c  in the second (slowly diverging) direction d s . 
     It is understood, however, that the first (front) lens  8  could be a SAC lens as shown in  FIG. 6 b   , collimating the light beam  5   a ,  5   b ,  5   c  in the direction d s , and that the second lens  21  could be a FAC lens as shown in  FIG. 6 a    to collimate the light beam  5   a ,  5   b ,  5   c  in the direction d f  (embodiments not shown). 
     In  FIGS. 7 a  and 7 b   , the first and second lenses  8 ,  21  are mounted in succession having their principal planes  16  aligned at the same angle α towards the alignment line  13 . 
     In the embodiment of  FIG. 7 a   , the two lenses  8 ,  21  are mounted with a spatial displacement so as not to be in contact with each other. Thereby, the lenses  8 ,  21  can be mounted removably on the carrier  4 , making an individual replacement of each one of the lenses  8 ,  21  possible. The planar sides  17 ,  19  of the FAC and SAC lenses  8 ,  21  are facing the light generation units  2   a ,  2   b ,  2   c . 
     In the alternative embodiment of  FIG. 7 b   , the two lenses  8 ,  21  can be combined to a single component  23  by joining, e.g. gluing, their planar sides  17 ,  19  together, so that the acylindrical side of the FAC lens  8  faces the light generation units  2   a ,  2   b ,  2   c . Alternatively, the component  23  can be readily formed as a monolithic or one-piece lens, i.e. there is no need for gluing two lenses  8 ,  21  together. 
     In  FIGS. 8 a  and 8 b   , the lenses  8 ,  21  are mounted in succession having their principal planes  16  aligned at different angles α, β towards the alignment line  13 . This has a wide series of applications, since the angles α, β are, as known to the person skilled in the art, dependent on several factors, e.g. the choice of material of the lens, the radius of curvature of each segment or bulge, the distance of respective the principal plane from the apertures  10   a ,  10   b ,  10   c , the respective spacings  15 ,  15 ′ between the apertures  10   a ,  10   b ,  10   c , and so forth. 
     As above, in the embodiment of  FIG. 8 a   , the two lenses  8 ,  21  are mounted with a spatial displacement to each other so as not to be in contact with each other. Upon joining the lenses  8 ,  21  together, as shown in  FIG. 8 b   , a transparent wedge  24  is inserted between the lenses, joining the planar side  17  of the first lens  8  to the planar side  19  of the second lens  21 , e.g. by gluing the wedge  24  onto the planar sides  14 ,  18 . Thereby it is ensured that the principal planes  16  of the lenses  8 ,  21  keep different angles α, β with respect to the alignment line  13 . 
       FIGS. 9 a  and 9 b    show a combination of a lens  8  tilted at an angle α towards the alignment line  13 , after which there is mounted a second lens  21  whose principal plane  16  is parallel to the alignment line  13 . To this end, the bulges  20   a ,  20   b ,  20   c  can optionally be differently shaped, e.g. have different radii of curvature, or can each be made out of different materials to compensate for the chromatic aberration with regard to the lens  21 . Again,  FIG. 9 a    shows the lenses  8 ,  21  having a spatial displacement, while they are joined together (or embodied as a single component) by a wedge  24  in  FIG. 9   b.    
     Similarly,  FIGS. 10 a  and 10 b    show the second SAC lens  21  tilted at an angle α towards the alignment line  13 , while the principal plane  16  of the first FAC lens  8  is parallel to the alignment line  13 . To compensate for the chromatic aberration with regard to the first lens  8 , the first lens  8  can optionally be manufactured in a special way, e.g. with a skewed acylindrical curvature on its side  18 . Again,  FIG. 10 b    shows the lenses  8 ,  21  joined together by wedge  24 . 
     The invention is not restricted to the specific embodiments described in detail herein, but encompasses all variants, combinations and modifications thereof that fall within the framework of the appended claims.