Patent Publication Number: US-9845930-B2

Title: Fiber-optic conversion module

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 14/670,012 filed on Mar. 26, 2015, which is a continuation of International Application No. PCT/EP2013/070074 filed on 26 Sep. 2013, which claims the benefit of U.S. Provisional Application Ser. No. 61/706,131 filed 27 Sep. 2012 and claims benefit under 35 U.S.C. §119(a) of German Patent Application No. 10 2012 109 088.2 filed 26 Sep. 2012. The entire contents of all of which are hereby incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The invention relates to a fiber-optic conversion module as part of a lighting device on a vehicle, in particular as part of a headlight. 
     2. Description of Related Art 
     A light emitting device as part of a headlight is known from WO2008000208A1 (DE 10 2006 029 203 A1) and comprises an excitation light source and coupled thereto an optical fiber and downstream thereof a converter material for converting the excitation light of shorter wavelengths into useful light of an on average longer wavelength having an appearance of white light. The useful light, in this case, is composed of a longer wavelength fraction of converted light and an unconverted fraction of the excitation light. In detail, a plug-in connection to a transparent body, a converter in transmission mode, and an optical component for emitting the converted useful light in a preferred direction are provided. A cooling element for the converter is not provided. When in such a light emitting device an optical fiber becomes damaged, this may lead to dangerous situations in case lasers are used as the excitation light sources. Therefore, the light emitting device is provided with additional detection means for detecting damage to the one or more optical fibers and, if necessary, to shut off the power supply to the excitation light source. The detection means may be provided on the light input end for excitation light to receive light fed back from the converter, which indicates that the optical fiber is operational and intact. Another method of failure detection is to check for current flow in a network of wires including one or more wires running through the cladding regions of the one or more optical fiber(s). When an optical fiber breaks, the co-extending wire should also break, which would cause an interruption of current flow and lead to failure detection. In the event of a failure, the radiation source is shut off. The light emitting device has a configuration such that the conversion material only works in transmission. Cooling of the converter material is very difficult to accomplish in this case. However, effective cooling is indispensable because of the heat produced during conversion. 
     From DE 10 2010 034 054 A1 a laser-based white light source is known, in which a laser produces a light spot in a converter by means of a converging lens, for delivering white useful light in a transmission mode. In order to avoid high temperatures at the light spot, the converter is produced using thermally conductive material and is additionally cooled by air and/or is rotated to cause the light spot to move around in the converter material. 
     The use of a plurality of optical fibers adjacent to each other in a light emitting device that includes a converter in transmission mode is known from US 2010/0254153 A1. 
     US 2008/0075406 A1 discloses an optical component comprising a converter, an optical fiber leading thereto, a holding member for the optical fiber, and a cap for securing the converter to the holding member. The cap and the holding member are made of a material of high thermal conductivity for preferably removing heat from the converter which is operated in transmission. 
     Light emitting devices for headlights, in part also in remission mode, are known from US 2011/0148280 A1, US 2011/0279007 A1, US 2011/0280033 A1, US 2012/0106178 A1, US 2012/0106183 A1, and US 2012/0069593 A1. Here, the converter material is arranged in form of a block in the focus of the parabolic headlight mirror, with a transparent plate extending transversely through the parabolic mirror for fixing the converter (US 2011/0148280 A1, US 2011/0279007 A1, US 2011/0280033 A1, US 2012/0069593 A1), or a support wall is formed along the axis of the parabolic mirror, to which the converter is mounted (US 2012/0106178 A1, US 2012/0106183 A1). A fiber-optic conversion module provided with connectors for injecting light at one end and with a light exit window for emitting useful light at the other end and mountable as a whole in a working unit, for example a parabolic mirror, is not specified in the cited documents. 
     SUMMARY 
     An object of the invention is to provide a fiber-optic conversion module which is suitable as a subassembly in a lighting device between an excitation light source and a headlight for delivering useful light and which provides for good heat dissipation. 
     Another object is to improve beam safety when using laser light in lighting devices. 
     In this context, a fiber-optic conversion module is to be created that prevents dangerous excitation light from exiting when lasers or laser diodes are used as light sources, even if the car headlight is destroyed, as happens in car accidents, and optical fibers that lead into the headlight are broken. 
     Another object of the inventors is to provide a fiber-optic conversion module which forms part of a failure detection means which is able to detect fractures in optical fibers and to cause a shut-off of the excitation light source. 
     The fiber-optic conversion module according to the invention comprises at least one optical fiber, a light exit head, a converter, a cooling element, and a light exit window. The optical fiber has a light input end provided with a connector for being connected to an associated excitation light source. The light exit head ensures that the light exit end of the optical fiber directs the excitation light onto a light spot that is located on the surface of the converter. The shorter wavelength excitation light mostly enters into the converter where it is converted into longer wavelength converted light which is emitted together with scattered excitation light into a useful radiation angle, as useful light, while disturbing reflected excitation light (the so-called Fresnel reflection) is trapped by a light stop and is made harmless. In the case of a car headlight, the useful light is delivered to a parabolic mirror. The fiber-optic conversion module may as well comprise a plurality of optical fibers. 
     Furthermore, a supporting case may be formed by the light exit head and a manifold housing and may serve to support the at least one optical fiber. In case a plurality of optical fibers are provided, the light exit head ensures that the respective light exit ends of the optical fibers direct the excitation light onto a light spot. The supporting case is configured so that heat produced in the converter can be discharged and dissipated through the supporting case. 
     Optical fiber connectors have a spatial radial dimension that is greater than that of the optical fibers, and the same applies to excitation light sources, e.g. laser diodes. Accordingly, at the light input end the supporting case of the fiber-optic conversion module will have a minimum width which is considerably greater than the width of the light spot to be produced on the converter. Therefore, if a plurality of optical fibers are provided, the optical fibers fed with excitation light must be brought together, and for this purpose the manifold housing is used to bundle the optical fibers that extend from the connectors to the respective light exit ends. 
     For optically monitoring the converter, a further optical fiber may be provided in addition to the optical fibers carrying the excitation light, which further optical fiber extends substantially in parallel to the excitation light optical fibers and feeds back converted light that is relayed via a connector to a photodetector, for monitoring purposes. The photodetector is part of monitoring means for monitoring the operability of the lighting device. If during operation of the excitation light sources, one or more optical fibers break, the brightness of the light spot on the converter will decrease or the light spot will disappear, which will be detected by the photodetector, whereupon the excitation light sources are switched off. 
     A breakage of optical fibers may as well be detected electrically. For this purpose, the optical fibers are provided with an electrically conductive coating, and in case a plurality of optical fibers are provided they are insulated from each other but electrically connected together at a node close to the light exit ends. So a breakage in any optical fiber will cause an interruption of the electrical circuit path which can be detected by a lack of current or a decreasing current in detection means and is used for switching off the excitation light sources. 
     The means for detecting electrical failure may as well comprise a frangible conductive path on an exit window for useful light of the light exit head. When the exit window for useful light undergoes damage, the conductive path thereon will be interrupted which leads to error detection as described. 
     In order to prevent the excitation light (laser light) from escaping even if optical fibers should break, metallic protective tubes are provided in one embodiment of the invention. Such metallic protective tubes extend from the respective connector to the manifold housing, each one accommodating an individual optical fiber, while a shared metallic protective tube extends from the manifold housing to the light exit head to enclose the combined optical fibers. In another embodiment of the invention, a crescent-shaped manifold housing is provided in which the optical fibers extent in a curve within tubes in order to provide for a compensation in length. A compensation in length may be necessary to compensate for effects of the different thermal expansion of the optical fibers and the metallic protective tubes. A compensation in length may moreover be necessary due to a bend, expansion, or compression of the protective tubes. Such bends may occur when installing the conversion module, or even in operation if the headlight is configured so that it can move relative to the excitation light source. If only a single optical fiber is provided, the metallic protective tube may as well extend between the connector and the light exit head. Such metallic tubes are capable to resist intense laser light without being damaged, at least for a short time, and thus to prevent laser light from escaping if optical fibers in the conversion module should break due to an accident. 
     When the fiber-optic conversion module is employed in a car headlight, the optical fibers will experience heavy vibration stress. For this reason, an insulating plastic tube is provided as an inner protective tube in the metallic protective tube, which plastic tube surrounds the optical fibers and thereby prevents them from striking against the inner wall of the outer protective tube. In addition, the optical fiber is better protected from environmental factors such as humidity. 
     For guiding and supporting the optical fibers within the light exit head, different measures may be taken. The optical fibers may be supported in V-shaped grooves between a lower part and an upper part of the light exit head. It is also possible to support a plurality of the optical fibers in a larger V-shaped groove. Instead of grooves, bores in the light exit head may be used for guiding and holding the optical fibers. Appropriately, the light exit head includes an insert part, in which the optical fibers are supported and guided to the optical fiber concentration point of the light exit head. 
     Feeding of the excitation light to the converter may as well be accomplished via a focusing optical element. This may be a rod lens, a spherical lens, or an aspherical lens. A rod lens may have a flat ground light input end for excitation light and a lens-shaped light exit end, also for excitation light. The focusing optical element enables to focus divergent light from the optical fibers onto the converter. This allows to increase the spacing between the light exit end of the optical fiber and the converter. 
     In a preferred embodiment of the invention, the light exit head has a housing step at the light exit end of the optical fibers, which step faces an inclined surface of the converter. 
     The converter may be backed up with a mirror and may be provided with a light stop to trap a margin of converted yellow light that is excessive for the generation of white light. 
     The excitation light that is incident onto the converter as a beam of rays is partially reflected at the surface thereof as a main reflection, called Fresnel reflection, and this largely monochromatic reflected excitation light is a safety hazard when it penetrates to the outside. Therefore, it is intercepted by a beam trap comprising a light stop attached to the light exit head at a distance from the converter. 
     If polarized light is used, such as laser light, the unwanted Fresnel reflection can be reduced by orienting the polarization in parallel to the reflecting surface. Light polarized in parallel is reflected less, over the entire angular range, than light polarized vertically. If the angle of incidence is close to the polarization angle, the so-called Brewster&#39;s angle, reflectivity becomes almost zero. Since with short lengths of the fiber, such as in a range from 30 to 50 cm, the polarization of the laser light is largely maintained within the fiber, this property of the light may be used in a fiber-coupled device as presently provided in order to reduce Fresnel reflection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described with reference to the drawings. In the drawings: 
         FIG. 1  is a partially sectional view of a fiber-optic conversion module; 
         FIG. 2  is a top plan view of the conversion module; 
         FIG. 3  is a longitudinal sectional view through a light exit head; 
         FIG. 4  is a perspective view of a light exit head; 
         FIG. 5  shows a first exemplary embodiment of an insert in the light exit head; 
         FIG. 6  shows a second exemplary embodiment of an insert in the light exit head; 
         FIG. 7  shows a third exemplary embodiment of an insert in the light exit head; 
         FIG. 8  shows a fourth exemplary embodiment of an insert in the light exit head; 
         FIG. 9  illustrates the configuration of an optical fiber; 
         FIG. 10  is a sectional view through a cable including an optical fiber; 
         FIG. 11  shows a first embodiment of a ferrule; 
         FIG. 12  shows a second embodiment of a ferrule; 
         FIG. 13  shows a third embodiment of a ferrule; 
         FIG. 14  shows a fourth embodiment of a ferrule; 
         FIG. 15  shows a manifold housing; 
         FIG. 16  is a schematic sectional perspective view of details of the light exit head; 
         FIG. 17  is a sectional perspective view of another light exit head; 
         FIG. 18  is a longitudinal sectional view through the light exit head of  FIG. 17 ; 
         FIG. 19  shows a cover sheet on the light exit head; 
         FIG. 20  illustrates a fiber breakage monitoring circuit; and 
         FIG. 21  illustrates a fiber breakage and cover sheet breakage monitoring circuit. 
     
    
    
     DETAILED DESCRIPTION 
     The fiber-optic conversion module as a whole is shown in  FIGS. 1 and 2  for an embodiment with three optical fibers  6 , comprising a light exit head  1 , a manifold housing  2 , a connecting cable  3  between the manifold housing and the light exit head, and three feed cables  4  each one provided with an SMA connector  5 .  FIG. 1  illustrates the light exit head  1  in a partially magnified and sectional view. Light exit head  1  and manifold housing  2  define a supporting case for the optical fibers  6  in their converged state. The three optical fibers  6  which have a light input end at the SMA connectors  5  and a light exit end in light exit head  1  extend within the conversion module. SMA connectors  5  are adapted to be coupled to individual laser diodes  60  ( FIGS. 20, 21 ) which inject their excitation light into the individual optical fibers  6 . Both the laser diodes  60  and the connectors  5  require a radial spacing relative to the optical fibers  6 , i.e. they cannot be packed as close to one another as the optical fibers. Therefore, at the light input end the conversion module has larger space requirements than in the region of manifold housing  2  or light exit head  1 . Therefore, for converging the optical fibers  6  without producing sharp bends, some curvature in the optical fibers is necessary between the light input end at connectors  5  and the light exit end within light exit head  1 , if two or more optical fibers  6  fed by laser diodes are used next to each other. 
     The excitation light exiting from optical fibers  6  is incident in an angle on a converter  8  which converts the shorter wavelength excitation light into longer wavelength converted light and additionally scatters excitation light, so that the remitted light, on average, is useful light of longer wavelengths. As to the useful light, usually a white light character is desired. The useful light passes through a cover sheet  9  to the working unit part which in the case of headlights is a parabolic mirror, a segment of a parabolic mirror, or a modified parabolic mirror surface. If the headlight is intended for generating substantially parallel useful light, a small light spot  80  ( FIGS. 16, 18 ) is produced on converter  8 . 
       FIG. 3  is a perspective longitudinal sectional view through a light exit head  1  which comprises a lower part  10  with good heat conduction, an upper part  11 , and an insert  12 . The connecting cable  3  to manifold housing  2  has a metallic protective tube  30  which is glued into light exit head  1  in a manner so as to be at least dustproof, preferably resistant to pressurized water. The optical fibers  6  extending inside protective tube  30  are additionally protected by plastic tubes  31 . After leaving plastic tubes  31 , optical fibers  6  converge towards insert  12 , with certain curvatures, and towards an optical fiber concentration point  120  on insert  12 . 
       FIG. 4  is a perspective view of light exit head  1 , which includes a mounting flange  10   a  at the lower part  10 . 
       FIG. 5  shows a first embodiment of an insert  12 . For supporting optical fibers  6 , three V-shaped grooves  121 ,  122 , and  123  are provided, which lead to the optical fiber concentration point  120 , with outer grooves  121  and  123  extending in a slight curve. A further groove  124  which can be used for supporting a further, feed-back optical fiber  7  extends in parallel to groove  123 . 
       FIG. 6  shows the guidance and support of the excitation light carrying optical fibers  6  and of feed-back optical fiber  7  in an insert  12  which is slightly modified as compared to  FIG. 5 . 
     The fibers are polished to be flush with the light exit face. Therefore, the optical fiber end face of the inclined optical fibers extends in an angle to the axis of the optical fibers, resulting in refraction of the exiting light. Thus, the angles of the grooves can be reduced as compared to a design with straightly severed ends of the optical fibers, which has a beneficial effect on the width of the component. 
       FIG. 7  shows an embodiment of an insert  12  having a deep V-groove  125  for accommodating the excitation light carrying optical fibers  6 . The further, feed-back optical fiber  7  extends in groove  124  in parallel to the bundle of optical fibers  6 . 
       FIG. 8  shows a further embodiment of insert  12  having a bore passage  126  for accommodating optical fibers  6  and a bore passage  127  for accommodating optical fiber  7 . Optical fibers  6  extend in parallel to each other within insert  12  and produce a composed light spot  80  consisting of individual partially overlapping light spots of the individual optical fibers. 
     In all embodiments of  FIGS. 5 to 8 , insert  12  has a front face  128  which when the insert is installed in light exit head  1  forms a step, from where a free space extends to converter  8 . The excitation light emerging from optical fibers  6  is superimposed on light spot  80 . 
     As an alternative to the configuration of insert  12  shown in  FIGS. 5 to 8 , a rod lens (not shown) may be used as well, which may consist of a cylindrical rod having a flat polished light input face and a lens-shaped curved exit face for focusing the excitation light onto light spot  80  at converter  8 . In this case, the optical fibers  6  carrying the excitation light are held together by a ferrule at the input end of the rod lens. 
       FIG. 9  is a perspective sectional view of one of optical fibers  6  comprising a fiber core  61  made of quartz glass of a higher refractive index, a fiber cladding  62  also made of quartz glass but of a lower refractive index, and a conductive coating  63 . The conductive coating  63  may comprise a brittle material which in case of breakage of the glass fiber  61 ,  62  also breaks, thus allowing to signal a fiber breakage by interrupting a current path. Preferably, however, the conductive layer  63  is made of aluminum, which allows to signal fiber breakage and moreover protects the surface of the optical fiber particularly well. In this way, the risk of breakage can be further reduced. Other measures for this purpose include to coat the glass fiber with Tefzel® on polyimide, Tefzel® on aluminum, or Tefzel® on acrylate. 
       FIG. 10  is a cross-sectional view through one of feed cables  4  which extend between connectors  5  and manifold housing  2 . Each of the optical fibers  6  is surrounded by an insulating plastic tube  41  which in turn is surrounded by a metallic protective tube  40 . Thus, optical fibers  6  are fairly well protected against breakage. If nevertheless breakage should occur so that short-wave excitation light exits from the break point, then it can be assumed that the high-energy excitation light will not be capable to destroy the metallic protective tube to escape to the outside which would be dangerous. 
       FIGS. 11 to 14  show variations of a ferrule  50  for optical fiber  6  at the light input end at connector  5 . Ferrule  50  is stepped comprising a connecting end portion  51  and a stop portion  52 . The ferrule  50  is to be designed in a manner so that in case of misalignment no damage occurs to connector  5  between the input face of the optical fiber  6  and the exit face of the excitation light source. This issue can be addressed with a crimp sleeve in combination with an Al-coated optical fiber, or by a larger cladding diameter, or by multi-step gluing of the optical fiber.  FIGS. 11 to 14  illustrate exemplary embodiments in which the optical fiber  6  is anchored in ferrule  50  by means of an adhesive layer  55 . 
     In the exemplary embodiment of  FIG. 11 , the connecting end portion  51  of ferrule  50  is crimped at front end  51   a , thereby displacing the adhesive layer  55  so that glue gap  55   a  disappears. In the exemplary embodiment of  FIG. 12 , an aperture stop  53  is secured to the end face of ferrule  50  in a manner so that only the fiber  61  remains uncovered. Aperture stop  53  tolerates the very intense excitation radiation. In the exemplary embodiment of  FIG. 13 , a ceramic socket  54  is used instead of the aperture stop, which closely surrounds the fiber cladding  62  of the optical fiber. Finally, the embodiment of  FIG. 14  uses a thickened fiber cladding (as compared to the embodiments of  FIGS. 11, 12, 13 ). 
       FIG. 15  shows a manifold housing  2  having a crescent-shaped inner space  20 . Manifold housing  2  is arranged adjacent to a housing  59  that accommodates connectors  5  and light sources  60  as well as a light sensor  70 . Feed cables  4  extend from of housing  59 , each one with an insulating protective tube  41  ( FIG. 10 ) inside, which in turn protects the corresponding optical fiber  6 . The light input end  21  of crescent-shaped manifold housing  2  is considerably wider than the light exit end  22 . At the light exit end  22 , optical fibers  6  together with their protective tubes  41  are fed into connecting cable  3  which leads into light exit head  1 . The amount of thermal expansion of the optical fiber  6  on the one hand and of the protective tube  41  on the other may differ considerably, so that a compensation in length is needed to avoid tensile stresses in the fiber. For this purpose, protective tube  41  may comprise two portions  41   a ,  41   b  having different diameters, which telescopically engage each other thus defining a displacement range of the protective tube  41 , which provides for a thermal compensation in length between the optical fiber and the protective tube. 
     A compensation in length should also be allowed between the individual optical fibers in order to enable to compensate for movements of the fibers relative to each other which may occur when the conversion module moves. In case of the crescent-shaped interior  20  this means that the tubes  41  must be able to extend along different curves. The minimum and maximum curvatures are indicated at  41   c  and  41   d.    
     Instead of extending in a curve as shown in  FIG. 15 , the protective tubes  41  may as well take a helical path in regions between the excitation light source and the light exit head. In this helical region, a possibility of yielding is provided by an increase or decrease of the pitch of the helix. This may be combined with an integrated displacement range for helical sections. 
     Another way of compensation in length is to have the protective tube and the optical fiber extend in a loop, wherein the length of the loop may change in case of a temperature change and the differential expansion between the optical fiber and protective tube may be compensated for by a displacement range of the protective tube. 
       FIG. 16  is a perspective sectional view of the front portion of the light exit head  1 . Converter  8  is mounted on the cooled surface of a cooling element  14  which is arranged obliquely to the longitudinal extent of the light exit head  1  and is joined and in good thermal communication with the lower part  10  of light exit head  1 . The optical fibers  6  extend along the longitudinal extension of light exit head  1  and define the predetermined beam direction of the excitation light in an angle to light spot  80 . Because of heavy vibrations in vehicles, permanent mechanical stability is ensured by an undercut press connection. In addition, a heat conductive adhesive may be employed. The converter  8  is backed by a mirror  141  which is fixed to the main body  140  of cooling element  14  by means of a heat conductive adhesive (e.g. thermally conductive silver adhesive). For the purposes of heat dissipation, metal is preferred as the material of the light exit head  1  including the cooling element  14 . 
     The excitation light of shorter wavelength emerging from optical fibers  6  skips step  128  and produces a round oval light spot  80  on converter  8 , from which useful light of longer wavelengths, on average, is emanating in form of a light lobe  81 , in an emission mode. If parallel headlight or spotlight is desired, the light spot  80  is restricted by an annular aperture stop  82  to avoid a yellow margin. The interior of light exit head  1  in which the converter  8  is placed, is covered by light exit sheet  9  ( FIG. 3 ) which forms a window for the exiting useful light. Major fractions of the excitation light beam will penetrate into the converter material, besides, however, a reflection is formed on the surface of the converter  8 , which is referred to as a Fresnel reflection, and which may be shielded by a light stop  93  on light exit sheet  9 . Between the surface of cooling element  14  and the lower surface of light stop  93  a radiation trap is defined, optionally involving portions of the converter, which causes the reflected short-wave excitation light to fizzle out between the lower surface of light stop  93  and the upper surface of converter  8 . 
       FIGS. 17, 18  are longitudinal sectional views through a further embodiment of light exit head  1 . Parts similar to those of the embodiment of  FIG. 16  are denoted by the same reference numerals. In this embodiment of the converter head, heat dissipation from converter  8  is improved as compared to the embodiment of  FIG. 16 . In fact, the triangularly-shaped lateral surface of cooling element  14  is again in surface contact with the highly conductive lower part  10  of light exit head  1 , but in addition a heat-conducting body  15  is provided extending from the bottom surface of cooling element  14  to a heat sink, not shown, so that heat removal is not only accomplished via the housing of the light exit head but another heat conduction path is established. Thus, there are no longer major requirements imposed on the thermal conductivity of housing part  10 , so that it may be made of stainless steel, for example. To provide for a particularly good thermal contact between cooling element  14  and the heat sink, not illustrated, the cooling element  14  may protrude slightly beyond the plane of the lower surface of the housing part, but only to an extent so that the heat conduction path  15  is able to compensate for the height difference. Cooling element  14  may be made of copper. In order to increase corrosion resistance, it may be provided with a Ni/Ag coating or another anti-corrosion layer. 
     In all embodiments of light exit head  1 , the insert  12  has a front face  128  which forms a step across which the exiting beam of excitation light passes through the free space between front face  128  and the inclined upper surface of converter  8 . In order to shift the light spot  80  closer to the center of converter  8  as compared to the embodiment of  FIG. 16 , insert  12  has an inclined surface below step  128 , which matches the inclination of the converter  8 . 
     To protect against escaping laser light in the event the light exit head should be damaged, laser protection plates  16  are additionally provided in the region of insert  12 . 
       FIG. 18  shows the light exit head  1  in relation to a reflector  17  of an automotive headlight. Light spot  80  is located in the focal point of reflector  17 . For adjusting this, a reflector reference surface  18  is defined on the light exit head. Another reference surface  19  serves to correctly install the light exit head. Furthermore, the radiation angle α of useful light between the boundary rays of light lobe  81  is indicated. A fraction of the useful light is trapped by blue-light stop  93 , because in front thereof the useful light mixes with the Fresnel reflection resulting from reflection of the excitation light on the surface of the converter. This Fresnel reflection is in part absorbed by blue-light stop  93  and is in part reflected back to impinge on the converter surface and to be absorbed. 
       FIG. 19  shows the light exit sheet  9  in a plan view. In embodiments of the fiber-optic conversion module with integrated monitoring of the integrity of this light exit sheet  9 , tempered glass is the preferred material for the light exit sheet. An anti-reflection layer is applied onto both surfaces of the glass, and additionally a conductive trace  91 , e.g. in form of a sputtered ITO layer. Conductive trace  91  extends in parallel to the edges of light exit sheet  9  and terminates in two opposing solder pads  92  which serve to connect test wires. 
     If in the event of an accident the headlights breaks, the tempered light exit sheet  9  will entirely decay into small crumbs, some of which near the edge will leave the composite of the sheet, so that conductive trace  91  will be reliably interrupted. This can be detected by a monitoring circuit which is described below. 
       FIG. 20  shows a fiber breakage monitoring circuit. Shown are SMA connectors  5  and  57  of optical fibers  6  and  7 , respectively, furthermore the conductive coatings  63  of these optical fibers and the light exit head  1  with cover sheet  9 . The connectors  5  of optical fibers  6  each have associated therewith a respective laser diode  60  which injects the shorter wavelength excitation light into the respective optical fiber  6 . Optical fiber  7  for optical monitoring has associated therewith a photodiode  70  which receives useful light as emitted by the converter via optical fiber  7 . Electrically conductive coatings  63  are electrically connected together at a node  67 . There is a power feed  65  to each of connectors  5  via series resistors  64 , and a return path  66  via the conductive coating of optical fiber  7  and the associated connector. As long as current flows, there will be a voltage drop at each of resistors  64 . If an optical fiber  6  breaks, the voltage at the associated resistor  64  will break down which can be detected for detecting a breakage of the corresponding optical fiber  6 . The detection means are well known to skilled persons and therefore are not shown. If one of the optical fibers  6  should break, the light spot generated on the converter will become darker, which is detectable by photodiode  70 . If such a break of the respective optical fiber  6  is found, at least the corresponding laser diode  60  is switched off. In this manner, damaging effects of the high-energy laser light are avoided. 
       FIG. 21  shows the circuit of  FIG. 20  with the enhancement of monitoring the cover sheet  9 . The electrical return path from node  67  runs via conductive trace  91 , so that a breaking of cover sheet  9  ultimately also leads to a shut-off of laser diodes  60 . 
     In summary it can be stated that in the invention shorter wavelength excitation light is directed onto a converter  8  which is arranged inclined or in an angle with respect to the beam direction, with the angle of inclination relative to the beam direction being such that, on average, longer wavelength useful light is emitted in a radiation angle which is substantially outside the reflection angle of the Fresnel reflex, so that this Fresnel reflection can be trapped and neutralized by a light stop  93 .