Laser-based light source

Abstract

A laser-based light source comprises a laser, an optical fiber, a conversion device, and a detector, the laser for emitting laser light with a laser peak emission wavelength, the optical fiber for guiding the laser light for being received at a first surface of the conversion device, the conversion device for converting at least a part of the laser light to converted light with a peak emission wavelength being longer than the laser peak emission wavelength, the optical fiber further for guiding a part of the converted light back in the direction of the laser and the detector for detecting at least a part of the back guided converted light. A vehicle headlight comprises such a laser-based light source, and a lighting system comprises such a vehicle headlight.

Claims

1. A laser-based light source comprising at least one laser, at least one optical fiber, at least one conversion device and at least one detector, wherein the at least one laser is adapted to emit laser light with a laser peak emission wavelength, wherein the at least one optical fiber is adapted to guide the laser light such that the laser light is received at a first surface of the at least one conversion device, wherein the conversion device is adapted to convert at least a part of the laser light to converted light, wherein a peak emission wavelength of the converted light is in a longer wavelength range than the laser peak emission wavelength, wherein the at least one optical fiber is further adapted to receive from the conversion device and to guide back in the direction of the at least one laser a part of the converted light, and wherein the detector is arranged to detect at least a part of the back guided converted light.

2. The laser-based light source according to claim 1, wherein the at least one optical fiber comprises at least a first light guiding structure with a first numerical aperture and at least a second light guiding structure with a second numerical aperture with respect to an optical axis of the at least one optical fiber, and wherein the detector is arranged to receive the part of the back guided converted light which is received by the at least one optical fiber from the conversion device at an angle with respect to the optical axis which is bigger than a first boundary angle with respect to the optical axis, wherein the first boundary angle is defined by the smaller of the first numerical aperture and the second numerical aperture.

3. The laser-based light source according to claim 2, wherein the detector is arranged to receive the part of the back guided converted light which is received by the at least one optical fiber from the conversion device at an angle with respect to the optical axis which is bigger than a second boundary angle with respect to the optical axis, wherein the second boundary angle is defined by the bigger of the first numerical aperture and the second numerical aperture.

4. The laser-based light source according to claim 2, wherein the at least one optical fiber is a double cladded optical fiber, wherein the first light guiding structure is a core of the double cladded optical fiber, wherein the second light guiding structure is an inner cladding of the double cladded optical fiber and wherein the double cladded optical fiber further comprises an outer cladding.

5. The laser-based light source according to claim 4, wherein a first refractive index of the core is bigger than a second refractive index of the inner cladding, and wherein the second refractive index of the inner cladding is bigger than a third refractive index of the outer cladding.

6. The laser-based light source according to claim 5, wherein the first numerical aperture of the core is smaller than the second numerical aperture of the inner cladding.

7. The laser-based light source according to claim 6, wherein the laser-based light source is arranged such that the laser light is received by the core within a first range of angles smaller than the first boundary angle defined by the first numerical aperture such that the laser light is guided to the conversion device within the core, and wherein the laser-based light source is arranged such that a part of the converted light is received by the core or the inner cladding within a second range of angles bigger than the first boundary angle such that this part of the converted light is back guided to the detector within the core and the inner cladding, and wherein the detector is arranged to receive this part of the back guided converted light at an angle with respect to the optical axis which is bigger than the first boundary angle.

8. The laser-based light source according to claim 7, wherein the laser-based light source is arranged such that a part of the converted light is received by the core or the inner cladding within a third range of angles such that this part of the converted light is back guided to the detector within the core and the inner cladding, wherein the third range of angles is bigger than a second boundary angle defined by the second numerical aperture and wherein the detector is arranged to receive this part of the back guided converted light at an angle with respect to the optical axis which is bigger than the second boundary angle and smaller than a maximum angle defined by the first refractive index of the core and the third refractive index of the outer cladding.

9. The laser-based light source according to claim 7, wherein the laser-based light source comprises a first optical device and a second optical device, wherein the first optical device is arranged to focus the laser light to the core such that the laser light is received by the core within the first range of angles, and wherein the second optical device is arranged such that at least a part of the converted light is focused to the core or the inner cladding such that this part of the converted light is received by the core or the inner cladding at an angle which is bigger than a second boundary angle defined by the second numerical aperture.

10. The laser-based light source according to claim 5, wherein the first numerical aperture of the core is bigger than the second numerical aperture of the inner cladding.

11. The laser-based light source according to claim 1, wherein the laser-based light source comprises at least one controller arranged to receive detector signals from the detector, and wherein the at least one controller is arranged to provide control signals to control the at least one laser based on the received detector signals.

12. The laser-based light source according to claim 11, wherein the at least one controller being arranged to switch off the at least one laser if an intensity of the back guided converted light measured by the detector is below a predetermined threshold value.

13. The laser-based light source according to claim 5, wherein the inner cladding is further arranged such that laser light reflected at the first surface of the conversion device, but not the converted light, is absorbed in order to reduce the intensity of back guided reflected laser light received by the core or the inner cladding at bigger angles than the first boundary angle.

14. A vehicle headlight comprising at least one laser-based light source according to claim 1.

15. A lighting system comprising at least one vehicle headlight according to claim 14 and at least one light emission control device, wherein the light emission control device is adapted to submit control signals to the at least one vehicle headlight.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

(2) The invention will now be described, by way of example, based on embodiments with reference to the accompanying drawings.

(3) In the drawings:

(4) FIG. 1 shows a principal sketch of a first embodiment of a laser-based light source;

(5) FIG. 2 shows a principal sketch of a second embodiment of a laser-based light source;

(6) FIG. 3 shows a principal sketch of a cross-section parallel to an optical axis of a first optical fiber;

(7) FIG. 4 shows a principal sketch of a cross-section perpendicular to the optical axis of the first optical fiber;

(8) FIG. 5 shows a principal sketch of a cross-section parallel to an optical axis of a second optical fiber;

(9) FIG. 6 shows a principal sketch of a cross-section perpendicular to the optical axis of the second optical fiber.

(10) In the Figures, like numbers refer to like objects throughout. Objects in the Figures are not necessarily drawn to scale.

DETAILED DESCRIPTION OF EMBODIMENTS

(11) Various embodiments of the invention will now be described by means of the Figures.

(12) FIG. 1 shows a principal sketch of a first embodiment of a laser-based light source 100. The laser-based light source 100 comprises a laser 110 which is arranged to emit laser light 10. The laser 110 is controlled by means of a controller 150 which comprises an electrical driver to drive the laser 110. The controller 150 receives detector signals from detector 140 which is arranged to detect light which is back guided by means of optical fiber 120. Laser light 10 is focused by means of a first optical device 151 which is in this case a lens to an entrance facet of the optical fiber 120. The entrance facet of the optical fiber 120 may be arranged to avoid reflection of the laser light 10. The laser 110, the first optical device 151, the controller 150 and the detector 140 are arranged on the same side of the optical fiber 120 and may be regarded as a laser module. The laser module may comprise an interface (not shown) to permanently or detachably couple the optical fiber 120 to the laser module. The optical fiber 120 is a single cladded optical fiber which is arranged to guide the laser light 10 to a conversion device 130. The optical fiber 120 is further arranged to back guide light received from the side of the conversion device 130 to the side of the laser 110 or more general of the laser module such that a feedback signal can be generated by means of back guided light received by detector 140.

(13) Laser light 10 guided by optical fiber 120 is focused by means of a second optical device 152, e.g. a lens, to a first surface of the conversion device 130. The conversion device 130 comprises light converting material like a phosphor which is arranged to convert laser light 10 to converted light 20 which has a different wavelength or has a wavelength within a different, especially a longer, wavelength range than the wavelength of laser light 10. The peak wavelength of the laser light 10 is preferably within the blue wavelength range and a peak wavelength of the converted light 20 is preferably within the yellow wavelength range of the visible spectrum. The conversion device 130 is further arranged such that essentially all of the laser light 10 received by the conversion device 130 is converted by means of the light converting material to converted light 20. The angular distribution of the converted light 20 may be influenced by means of a surface structure of a light emission surface of the conversion device 130. The light emission surface may be identical to the first surface to which the laser light 10 is focused. The conversion device 130 and the second optical device 152 may be regarded as a conversion module which can be permanently or detachably coupled to optical fiber 120 by means of an accordingly adapted interface (not shown). A part of the converted light 20 is focused by means of the second optical device 152 to an exit facet (with respect to laser light 10) of the optical fiber 120. A part of the converted light 20 is back guided by means of optical fiber 120. The detector 140 in the laser module is arranged such that a part of the back guided converted light 20 is detected.

(14) FIG. 2 shows a principal sketch of a second embodiment of a laser-based light source 100. The configuration is very similar to the first embodiment discussed with respect to FIG. 1. The single cladded optical fiber 120 is replaced by a double cladded optical fiber which comprises a core 121 with a first refractive index n1, an inner cladding 122 with a second reflective index n2<n1, and an outer cladding 123 with a third reflective index n3<n2. A main difference is that a part of the laser light 10 is reflected at the first surface of the conversion device 130. This reflected laser light 11 is also received by the exit facet of the optical fiber 120 such that a part of the reflected laser light 11 is back guided by the optical fiber 120, too. A ratio between the back guided converted light 20 and back guided reflected light 11 may be influenced by means of the angular distribution of the converted light 20 and the reflected light 11 or by, for example, separating the first surface which reflects a part of the laser light 10 and the light emission surface by, for example, providing a reflective coating at this first surface which is arranged to reflect converted light 20 and extract the converted light 20 at a different surface of the phosphor. Converted light 20 and reflected laser light 11 may in this case be mixed by an accordingly adapted optical mixer which may be comprised by the laser-based light source 100 or which may be arranged externally. The double cladded optical fiber enables an at least partial decoupling between converted light 20 and reflected laser light 11.

(15) FIG. 3 shows a principal sketch of a cross-section parallel to an optical axis 125 of a first optical fiber 120 which is configured as a double cladded optical fiber as shown in FIG. 2. The double cladded optical fiber is arranged such that the numerical aperture NAc of the core which is defined as NAc={square root over (n1.sup.2 n2.sup.2)} is smaller than the numerical aperture NAic of the inner cladding which is defined NAic={square root over (n2.sup.2 n3.sup.2)}. Light received at the light entrance surface of the core 121 with an angle of incidence between the optical axis 125 and the first boundary angle 161 which is defined by NAc is guided within the core 121. Light received at the light entrance surface of the core 121a (with respect to light coming from the conversion device 130) with an angle of incidence between the optical axis 125 and the maximum angle 163 which is determined by the maximum numerical aperture NAmax of the double cladded optical fiber which is defined as NAmax={square root over (n1.sup.2 n3.sup.2)} is guided within the inner cladding 122. Light received by a light entrance surface of the inner cladding 122a (with respect to light coming from the conversion device 130) at an angle of incidence smaller than the second boundary angle 162 which is defined by NAic is guided within the inner cladding 122. Light received at the light entrance surface of the core 121a at an angle bigger than the maximum angle 163 or received at the light entrance surface of the inner cladding 122a at an angle bigger than the second boundary angle 162 with respect to an inner cladding's axis 126 parallel to optical axis 125 will be absorbed after entering the outer cladding 123.

(16) The double cladded optical fiber can be used to at least partially decouple the back guiding of the converted light 20 and the reflected laser light 11 for bigger angles than the first boundary angle 161. The inner cladding 122 may be further arranged such that reflected laser light 11, as e.g. blue laser light, (but not the converted light 20) is absorbed in order to reduce the intensity of back guided reflected laser light 11 received at bigger angles than the first boundary angle 161. Alternatively or in addition, filters or absorbers for the blue laser light may be arranged on a part of the facets of the double cladded optical fiber on the side of the conversion device and especially on the side of the laser in order to avoid or reduce back guiding of reflected laser light 11 at bigger angles than the first boundary angle 161.

(17) FIG. 4 shows a principal sketch of a cross-section perpendicular to the optical axis 125 of the first optical fiber 120 as discussed with respect to FIG. 3. The light entrance surface of the core 121a with the smaller numerical aperture NAc has in this case a rectangular shape wherein the light entrance surface of the inner cladding 122a surrounding the light entrance surface of the core 121a has a circular outer boundary to the outer cladding 123. The rectangular shape of the core 121 is used to define in combination with the second optical device a predefined illumination pattern on the conversion device 130 and especially the conversion material (phosphor) comprised by the conversion device 130. The core 121 (low NA region) which is used for transporting the laser light 10 (blue laser light) emitted by the laser 110 is placed inside the high NA region of the inner cladding 122. The region of the conversion device 130, or more precisely the conversion material comprised by the conversion device 130, that is detected by detector 140 is larger than the area excited by means of laser light 10 guided in the core 121. If the detector 140 is detecting under an angle larger than the second boundary angle 162 but smaller than the maximum angle 163 an image of the light entrance surface of the core 121a is detected.

(18) FIG. 5 shows a principal sketch of a cross-section parallel to an optical axis 125 of a second optical fiber 120 which is configured as a double cladded optical fiber as shown in FIG. 2. The general configuration is quite similar as discussed with respect to FIG. 3, but the numerical aperture NAc of the core 121 is bigger than the numerical aperture NAic of the inner cladding 122. The double cladded optical fiber is arranged such that the numerical aperture NAc of the core 121 is bigger than the numerical aperture NAic of the inner cladding 122. Light received at the light entrance surface of the core 121a with an angle of incidence between the optical axis 125 and the second boundary angle 162 is guided within the core 121. Light received at the light entrance surface of the core 121a with an angle of incidence between the optical axis 125 and the maximum angle 163 is guided within the inner cladding 122. Light received by a light entrance surface of the inner cladding 122a at an angle of incidence smaller than the first boundary angle 161 is guided within the inner cladding 122. Light received at the light entrance surface of the core 121a at an angle bigger than the maximum angle 163 or received at the light entrance surface of the inner cladding 122a at an angle bigger than the first boundary angle 161, which is in this case smaller than the second boundary angle 162, will be absorbed after entering the outer cladding 123.

(19) FIG. 6 shows a principal sketch of a cross-section perpendicular to the optical axis 125 of the second optical fiber 120 as discussed with respect to FIG. 5. The light entrance surface of the core 121a with the bigger numerical aperture NAc has in this case a circular shape wherein the light entrance surface of the inner cladding 122a surrounding the light entrance surface of the core 121a has a rectangular outer boundary to the outer cladding 123. The core 121 (high NA region) which is used for transporting the laser light 10 (blue laser light) is placed inside the low NA region of the inner cladding 122. The region of the conversion device 130, or more precisely the conversion material comprised by the conversion device 130, that is detected by detector 140 is in this case a small part of the area of the conversion material excited by means of laser light 10. If the detector 140 is detecting under an angle larger than the second boundary angle 162 but smaller than the maximum angle 163 an image of the light entrance surface of the core 121a is detected which receives only converted light 20 from the excited area of the conversion material. The configuration of the optical fiber 120 may therefore be used in order to define the detection area on the conversion material of the conversion device 130.

(20) It is a basic idea of the present invention to use only one optical fiber 120 to guide laser light 10 emitted by a laser 110 to a conversion device 130 and converted light 20 which is converted by means of the conversion device 130 back in the direction of the laser 110. Extensive wiring is thus avoided by using only one optical fiber 120 instead of two optical fibers or one optical fiber in combination with an electrical signal line which may be used to transfer detector signals back to a controller 150 in order to control the laser 110. Furthermore, a malfunction like a break of the optical fiber 120 emitting the laser light 10 or a displacement of the conversion device 130 can be detected by means of a detector 140, e.g. a photo diode or the like, arranged at the side of the laser 110 if the back guided converted light 20 falls below a predefined threshold value. Detection of back guided converted light 20 may be improved by using a double cladded or even multi cladded optical fiber 120 (three, four or more claddings). Different numerical apertures of the core and of at least one cladding, e.g. of the inner cladding, may be used in order to decouple the angular distribution of the laser light from the one of the back guided converted light 20, and, optionally, from the one of the back guided laser light 10, i.e., e.g. of the one of the reflected laser light 11.

(21) While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive.

(22) From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the art and which may be used instead of or in addition to features already described herein.

(23) Variations to the disclosed embodiments can be understood and effected by those skilled in the art, from a study of the drawings, the disclosure and the appended claims. In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality of elements or steps. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

(24) Any reference signs in the claims should not be construed as limiting the scope thereof.

LIST OF REFERENCE NUMERALS

(25) 10 laser light 11 reflected laser light 20 converted light 100 laser-based light source 110 (first) laser 120 optical fiber 121 core 121a light entrance surface of core 122 inner cladding 122a light entrance surface of inner cladding 123 outer cladding 125 optical axis of optical fiber 126 inner cladding's axis 130 conversion device 140 detector 150 controller 151 first optical device 152 second optical device 161 first boundary angle 162 second boundary angle 163 maximum angle