Patent Application
20200028329
The present invention relates to an inherently safe laser arrangement comprising a Vertical Cavity Surface Emitting Laser (VCSEL), a lighting device comprising such a laser arrangement and a method of manufacturing the laser arrangement.
Laser arrangements including a VCSEL or a VCSEL array can be used as infrared illumination device. Using short pulses VCSEL arrays are, for example, applied in time of flight applications. Such applications include e.g. short-range gesture recognition for portable devices and 3D space recognition. VCSEL arrays of about 1 mm2 area with output power in the 1-10 W range are discussed for such applications. A specific field of illumination is defined by the application (e.g. a time of flight camera observing, e.g., 70°×50°). The typical emission pattern of a VCSEL array is rotational symmetric with a cone of e.g. 20°.
Laser safety especially in case of a laser arrangement included in a consumer product is essential in order to increase customer acceptance and to fulfill safety regulations. It is especially to be avoided that laser safety measures can be bypassed in a simple way, e.g., by manipulation of the laser arrangement.
US 2007/0103643 A1 discloses in this respect a laser light source unit in which, when a laser is taken out of the chassis or the like, a laser thereof is assuredly rendered incapable of emitting. The laser light source unit includes a laser light source portion having a light oscillation portion emitting light, a fixing member that fixes the laser light source portion, an interrupting means that, simultaneously with a movement of removing the laser light source portion off the fixing member, interrupts a current path for supplying a current to the light oscillation portion in the laser light source portion.
US 2007/0071056 A1 discloses a laser ranging and detection by sequentially emitting a plurality of beams from a vertical cavity surface-emitting laser (VCSEL) structure, re-directing the beams through optical elements such that they are fanned out over the region of view, and detecting any beams that may be reflected by objects in the region of view. The range and bearing of such objects can be determined from the beam time-of-flight and beam angle.
An embodiment of the present invention has a laser arrangement having: an array of Vertical Cavity Surface Emitting Lasers (VCSELs) and an optical structure. The VCSELs are on a semiconductor substrate and arranged to emit laser light. Each of the VCSELs comprise: a first electrode; a first distributed Bragg reflector; an active layer; a second distributed Bragg reflector; and a second electrode. The active layer is sandwiched between the first distributed Bragg reflector and the second distributed Bragg reflector. The first electrode and the second electrode are arranged to provide an electrical current across the active layer to generate the laser light. The VCSELs are each bottom emitters which are arranged to emit the laser light through the semiconductor substrate. The optical structure increases a laser emission angle of the laser light for eye safety of the laser arrangement. The optical structure is an integrated part of a semiconductor layer structure of the laser arrangement. The optical structure includes a surface structure of the semiconductor substrate. A thickness of the semiconductor substrate is arranged such that laser light emitted by neighboring ones of the VCSELs intersect with each other in a plane of the surface structure. The surface structure is arranged such that a homogeneous emission of an emission surface of the semiconductor substrate is enabled.
The present invention will be described in even greater detail below based on the exemplary figures. The invention is not limited to the exemplary embodiments. All features described and/or illustrated herein can be used alone or combined in different combinations in embodiments of the invention. The features and advantages of various embodiments of the present invention will become apparent by reading the following detailed description with reference to the attached drawings which illustrate the following:
The present invention provides an improved inherently safe laser arrangement including a VCSEL.
According to a first aspect, a laser arrangement including at least one Vertical Cavity Surface Emitting Laser being arranged to emit laser light is provided. The at least one Vertical Cavity Surface Emitting Laser includes a first electrode, a semiconductor substrate, a first distributed Bragg reflector, an active layer, a second distributed Bragg reflector and a second electrode. The active layer is sandwiched between the first distributed Bragg reflector and the second distributed Bragg reflector. The first electrode and the second electrode are arranged to provide an electrical current across the active layer. The laser arrangement includes an optical structure arranged to increase a laser emission angle of the laser light for increasing eye safety of the laser arrangement. The optical structure is an integrated part of a semiconductor layer structure of the laser arrangement. The semiconductor layer structure of the laser arrangement may optionally include further semiconductor layers supporting the functions of the functional elements like electrodes, reflectors, active layer and the like described above.
The optical structure may be arranged to reduce brightness of the laser arrangement to make the laser arrangement no longer subject to laser safety regulations. Safety regulations for Light Emitting Diodes (LED) may be applicable which are less restricted and which are well accepted in consumer products in which it may not be possible to deal with laser safety. Integration of an optical structure arranged to increase the laser emission angle of the laser light in the semiconductor layer structure provides an inherently safe device because the optical structure cannot be removed without destruction of the laser arrangement. Furthermore, the high refractive index of the semiconductor layers and especially the gallium arsenide substrate (n˜3.5) of the laser arrangement guarantees that the laser emission angle is sufficiently increased even in case of submersion with a liquid which is usually characterized by a refractive index of n<1.5. The laser emission angle is defined by the full angle enclosing an optical axis of the respective VCSEL or VCSEL array at which the intensity of the laser light in the far field is reduced to 50% in comparison to the maximum intensity of the laser light (Full Width at Half Maximum—FWHM) if the laser emission is characterized by a fundamental Gaussian beam. An optical structure may be arranged to increase the emission cone of around 20°. The laser emission angle may, for example, be increased to a cone of 40° around the optical axis of the laser arrangement.
The optical structure may include a refractive diffusor or a diffractive diffusor.
A refractive diffusor is arranged such that the laser light is refracted by means of geometric structures, which are integrated in the semiconductor layer structure. The geometric structures may include microlens arrays, especially random microlens arrays with different diameters and/or focal length and/or decenter and/or apertures, micro-prisms, micro-pyramids, micro-axicons, wavy Freeform Surface to randomly increase the emission angle, cylinder lenses and the like. The feature size of the individual structure is typically between 5 μm and 100 micrometer.
A diffractive diffusor is arranged to diffract the laser light and may include two- or multi-level gratings with a feature size in a plane of the substrate between 0.1 μm and 10 μm and a height difference perpendicular to the plane of the substrate in the order of a quarter of a emission wavelength of the laser arrangement to several wavelength.
The Vertical Cavity Surface Emitting Laser may be a bottom emitter which is arranged to emit laser light through the semiconductor substrate. The optical structure includes a surface structure of the semiconductor substrate. The surface structure may include geometric structures as discussed with respect to the refractive diffusor or gratings as discussed with respect to the diffractive diffusor. The first distributed Bragg reflector, the active layer and the second distributed Bragg reflector may be arranged on a first side of the semiconductor substrate. The surface structure may be arranged on a second side of the semiconductor substrate opposite to the first side of the semiconductor substrate.
Providing the surface structure or optical structure on the second side of the semiconductor substrate, which is not used for processing the semiconductor layers of the VCSEL or VCSELs, may enable a simple processing or manufacturing of the surface structure. Furthermore, the semiconductor substrate usually consists of gallium arsenide with a refractive index of n=3.5. The semiconductor substrate does therefore usually have the highest refractive index which further improves efficiency of the surface structure or optical structure.
The surface structure may, for example, be etched in the second side of the semiconductor substrate. The etching process may be performed prior or after processing the semiconductor layers of the VCSEL or VCSELs or more general the laser arrangement.
The surface structure may alternatively be processed by grinding, lapping, etching, soft-cushion imprint lithography followed by etching, UV-replication of a master structure on the wafer followed by an etch-transfer of the replicated structure in the substrate, electron or ion beam writing or Grey-scale lithography and etching and the like.
Use of very small feature sizes of the optical structure or surface structure may be preferred for better randomization and also to reduce direct back reflections in the laser cavity which may alter the laser emission characteristics. Optionally an AR-coating can be applied on the optical structure. Alternatively the optical structure includes at least two different geometrical features. A first feature acting as a diffusing structure (e.g. microlenses with dimensions of around 20 μm) and a second feature with a scale below the laser wavelength acting as AR-coating (reference: artificial moth-eye structures).
The laser arrangement may include an array of Vertical Cavity Surface Emitting Lasers arranged on the same semiconductor substrate. A thickness of the semiconductor substrate may be arranged such that laser light emitted by neighboring Vertical Surface Emitting Lasers intersect with each other in a plane of the optical structure. The surface structure may in this case be arranged such that a homogeneous emission of an emission surface of the semiconductor substrate is enabled.
Essentially the whole chip area or size of the semiconductor substrate of the bottom emitting VCSEL array may be used in this case to emit laser light emitted by a multitude of VCSELs. The thickness of the semiconductor substrate in combination with the optical structures is arranged in this case such that the VCSEL array appears to be one light source in which no individual VCSEL is visible.
The thickness of the semiconductor substrate may, for example, be at least the nearest distance between neighboring Vertical Surface Emitting Lasers (pitch of the VCSELs) divided by two times the tangent of halve of the laser emission angle of the laser light in the semiconductor substrate. The laser emission angle in the semiconductor substrate is transformed by means of the optical structure to the laser emission angle of the laser arrangement. This may enable using an optical structure or diffusor with a very small diffusion angle. The diffusion angle is defined as the full angle enclosing the optical axis and the angle at which the intensity is 50% of the maximum intensity behind the optical structure (with respect to the illumination source) if the optical structure is illuminated with a collimated (essentially parallel) laser beam.
The surface structure may be characterized by a diffusion angle between 2° and 20°, preferably between 2° and 8° and most preferably between 4° and 8°. The diffusion angle would in this case essentially be equal to the laser divergence angle in the semiconductor substrate. A surface structure with such a small diffusion angle in combination with a semiconductor substrate of sufficient thickness may result in a laser arrangement which can be evaluated in accordance with the lamp standard according to IEC 62471 instead of the laser standard IEC 60825. This may result in a less restrictive safety regulations and better acceptance of products including such a laser arrangement.
There may be essentially two different cases. The VCSEL array can in the first case (very small diffusion angle of around 2° or 3°) be evaluated in accordance with the lamp standard. The VCSEL array appears in this case to be a strong, potentially dangerous LED light source. A diffusion angle of more than 3° may enable a completely eye save laser (class 1 laser).
The Vertical Cavity Surface Emitting Laser or Lasers of the laser arrangement may alternatively be a top emitter which is arranged to emit laser light through a second distributed Bragg reflector in a direction pointing away from the semiconductor substrate. The second distributed Bragg reflector is arranged on a first side of the active layer. The semiconductor substrate is arranged on a second side of the active layer opposite to the first side of the active layer. The optical structure includes at least one semiconductor layer arranged on a side of the second distributed Bragg reflector opposite to the side attached to the active layer.
The optical structure is characterized by a feature size which is smaller than the individual laser beam on the light-emitting facet of the individual VCSEL (typically diameter of 10 μm) to lower brightness of each individual VCSEL. This feature size has the consequence that the optical structure cannot be part of the second distributed Bragg reflector. The reflectivity of the second distributed Bragg reflector has therefore to be adapted such that each VCSEL starts lasing without an additional contribution to the reflectivity by means of the semiconductor to air interface. Furthermore, there may be one or more intermediate layers arranged between the second distributed Bragg reflector and one, two, three or more semiconductor layers included by the optical structure.
The at least one semiconductor layer may be arranged to increase the laser emission angle to at least 40°. The laser emission angle after passing the optical structure may be between 20° and 180°.
The second distributed Bragg reflector may be arranged to provide a reflectivity of at least 95%, more preferably at least 98% and most preferably at least 99%. A reflectivity of at least 95% may be sufficient to enable lasing of each VCSEL without an additional contribution to the reflectivity by means of any optional layer between the semiconductor layer or layers of the optical structure and the second distributed Bragg reflector, the semiconductor layer or layers of the optical structure and the semiconductor air interface.
The optical structure may be a diffractive diffusor. The at least one semiconductor layer may be characterized in this case by a thickness of at least half of an emission wavelength of the at least one Vertical Cavity Surface Emitting Laser. Using a semiconductor layer with a thickness of at least half of the emission wavelength enables use of only one semiconductor layer in order to provide a diffractive diffusor to increase the laser emission angle of the laser arrangement including the VCSEL or the VCSEL array.
The thickness of the at least one semiconductor layer may be two, three, four or more times and emission wavelength of the VCSEL in order to provide the required emission angle. The optical structure shall be preferably optimized for minimized back reflection in the laser cavity e.g. by designing the diffractive optical element such that the back reflection for the laser wavelength is suppressed by deconstructive interference at the different facets. Alternatively, the diffusor is designed to have only back reflection with large angle to the optical axis, such that they do not hit the active area or are not coupled back in the laser cavity. An AR-coating can be applied on or included by the optical structure according to an alternative embodiment as described above.
According to a second aspect, a method of fabricating a laser arrangement including at least one Vertical Cavity Surface Emitting Laser is provided. The method includes the steps of:
The steps need not necessarily be performed in the order given above. Metal layers which are used to process, for example, the first and the second electrode are processed after the epitaxial semiconductor layers. Integration of the optical structure may, for example, be performed before or after processing the laser cavity. Processing step of the first electrode may depend on whether a top emitter or a bottom emitter is manufactured.
The optical structure may be integrated by several methods, among them grinding, lapping, etching, soft-cushion imprint lithography followed by etching, UV-replication of a master structure on the wafer followed by a etch-transfer of the replicated structure in the substrate and the like.
The optical structure may be covered by a planarization layer. The planarization layer is characterized by a lower refractive index than a material of the optical structure. The optical structure may be designed with respect to the refractive index of the planarization layer in order to provide the desired laser emission angle. The planarization layer may be used to smooth the surface irregularities caused by the optical structure. The planarization layer may include materials like silicone, epoxy, polyimide, SiN and the like. The difference between the refractive index of the semiconductor layer structure (substrate or one or more semiconductor layers) which is used to manufacture the optical structure and the refractive index of the planarization layer is sufficient to increase the laser emission angle to fulfill the safety requirements. The planarization layer protects the optical structure and inhibits manipulation, for example, by means of silicone oils.
According to a further aspect a light emitting device is provided. The light emitting device includes at least one laser arrangement as described above and a driver for driving the laser arrangement.
It shall be understood that the disclosed laser arrangements and the methods have similar and/or identical embodiments.
It shall be understood that a preferred embodiment of the invention can also be any combination of the disclosed features.
Further advantageous embodiments are defined below.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
The invention will now be described, by way of example, based on embodiments with reference to the accompanying drawings.
In the Figures, like numbers refer to like objects throughout. Objects in the Figures are not necessarily drawn to scale.
The laser emission angle in the semiconductor substrate 101 laser light emitted by a VCSEL is not perfectly collimated as discussed above. The profile of laser light 150 emitted through the optical structure 140 is therefore a convolution of a profile of the, for example, laser light emitted by a VCSEL without the optical structure 140 and a diffusion profile of the optical structure 140. The diffusion profile of the optical structure 140 is blurred by the profile of the laser light. An optical structure 140 which provides, for example, a top hat diffusion profile may, for example, be transformed, to a trapezoidal shape (looking at the cross section). The laser emission angle 156 may therefore be wider in comparison to the case of the collimated laser light 152. Of course the diffusion profile should be optimized such that the laser emission profile is adapted to the requirements of the application. As the laser beam of the VCSEL itself is known, this can be included in the diffusor design.
It is a basic idea of the present invention to integrate an optical structure 140 arranged to increase a laser emission angle of laser light 150 emitted by the laser arrangement in a semiconductor layer structure of the laser arrangement. The semiconductor layer structure includes a semiconductor substrate 101 on which further epitaxial semiconductor layers are processed. The laser arrangement may finally be encapsulated by means of a package. Integration of the optical diffusing optical structure 140 in the semiconductor layer structure makes it nearly impossible to get access to the optical structure 140. Furthermore, the semiconductor layer structure of the laser arrangement has to be manipulated in order to avoid increasing of the laser emission angle. Manipulation of the sensitive semiconductor layer structure does usually have the effect that the laser light 150 emitting structure of laser arrangement is destroyed. Circumventing the optical structure 140 by means of a material (e.g. liquids) which may be used to smooth the optical structure 140 is essentially ineffective because it is nearly impossible to get access to the optical structure 140 and because of high refractive index of the semiconductor layers, especially the semiconductor substrate 101.
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.
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.
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.
While the invention 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. It will be understood that changes and modifications may be made by those of ordinary skill within the scope of the following claims. In particular, the present invention covers further embodiments with any combination of features from different embodiments described above and below. Additionally, statements made herein characterizing the invention refer to an embodiment of the invention and not necessarily all embodiments.
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.
Any reference signs in the claims should not be construed as limiting the scope thereof.
The following is a list of reference numerals used herein:
| Number | Date | Country | Kind |
|---|---|---|---|
| 17164121.0 | Mar 2017 | EP | regional |
This application is a continuation of International Patent Application No. PCT/EP2018/058274, filed on Mar. 30, 2018, which claims priority to European Patent Application No. EP 17164121.0, filed on Mar. 31, 2017. The entire disclosure of both applications are hereby incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2018/058274 | Mar 2018 | US |
| Child | 16585078 | US |
