VCSEL device for an SMI sensor for recording three-dimensional pictures

A Vertical Cavity Surface Emitting Laser (VCSEL) includes a VCSEL array, a multitude of detectors, a first electrical laser contact, and at least one second electrical laser contact. The VCSEL array comprises a multitude of laser diodes, each laser diode including an optical resonator having a first distributed Bragg reflector, a second distributed Bragg reflector and an active layer for light emission, the active layer being arranged between the first distributed Bragg reflector and the second distributed Bragg reflector. The first electrical laser contact and the at least one second electrical laser contact are arranged to provide an electrical drive current to electrically pump the optical resonators of the laser diodes. Each detector is arranged to generate an electrical self-mixing interference measurement signal associated to at least one laser diode upon reception of the laser light.

FIELD

The invention relates to a Vertical Cavity Surface Emitting Laser (VCSEL) device for a self-mixing interference (SMI) sensor for recording three-dimensional (3D) pictures, the SMI sensor and a mobile communication device comprising the VCSEL device or the SMI sensor. The invention further relates to a corresponding method of fabricating such a VCSEL device.

BACKGROUND

State of the art optical 3D sensors need high power light sources and corresponding detectors. Optical alignment complexity, power consumption and eye safety are critical points.

WO 2017/016888 A1 discloses a laser sensor module for particle density detection. The laser sensor module may comprise an array of a multitude of laser diodes.

US 2011/0064110 A1 discloses a vertical cavity surface emitting laser device with a monolithically integrated photodiode.

US 2003/0021327 A1 discloses a vertical cavity surface emitting laser device comprising a photodetector deposited directly onto a top surface of the laser structure.

SUMMARY

The present disclosure provides an improved VCSEL device for an SMI sensor for recording three-dimensional pictures.

In an embodiment, the present invention provides a Vertical Cavity Surface Emitting Laser (VCSEL) device for a self-mixing interference sensor for recording three-dimensional pictures. The VCSEL device includes a VCSEL array, a multitude of detectors, a first electrical laser contact, and at least one second electrical laser contact. The VCSEL array comprises a multitude of laser diodes, each laser diode including an optical resonator having a first distributed Bragg reflector, a second distributed Bragg reflector and an active layer for light emission, the active layer being arranged between the first distributed Bragg reflector and the second distributed Bragg reflector. The first electrical laser contact and the at least one second electrical laser contact are arranged to provide an electrical drive current to electrically pump the optical resonators of the laser diodes. The first electrical laser contact is a common contact for all laser diodes of the VCSEL array. The at least one second electrical laser contact is arranged to electrically contact at least a subgroup of the multitude of laser diodes of the VCSEL array. Each detector is arranged to generate an electrical self-mixing interference measurement signal associated to at least one laser diode upon reception of the laser light.

According to a first aspect a Vertical Cavity Surface Emitting Laser (VCSEL) device for a self-mixing interference sensor for recording three-dimensional pictures is provided. The VCSEL device comprises a VCSEL array, a multitude of detectors (e.g. optical detector like photodiodes or photo transistors), a first electrical laser contact and at least one second electrical laser contact. The VCSEL array comprises a multitude of laser diodes (VCSEL). The multitude of laser diodes may be arranged on a common substrate. The common substrate may, for example, be a growth substrate for growing semiconductor layers the VCSEL array consists of or a substrate which is bonded to the VCSEL array in a subsequent processing step. Each laser diode comprises an optical resonator. The optical resonator comprises a first distributed Bragg reflector, a second distributed Bragg reflector and an active layer for light emission. The active layer is arranged between the first distributed Bragg reflector and the second distributed Bragg reflector. The first electrical laser contact and the at least one second electrical laser contact are arranged to provide an electrical drive current to electrically pump the optical resonators of the laser diodes. The first electrical laser contact is a common contact for all laser diodes of the VCSEL array. The at least one second electrical laser contact is arranged to electrically contact at least a subgroup of the multitude of laser diodes of the VCSEL array. Each detector may be arranged to receive laser light from at least one laser diode of the VCSEL array (optical detector). Each detector is arranged to generate a self-mixing interference measurement signal associated to the at least one laser diode upon reception of the laser light (optical detector, electrical detector etc.). Each detector may be associated to one, two, three, four or more laser diodes. A subgroup of the multitude of laser diodes of the VCSEL array may comprise, for example, a column or a row of the VCSEL array. A self-mixing interference signal is generated if a laser diode (VCSEL) emits laser light and a part of the emitted laser light is reflected back to the optical resonator of the respective laser diode. The reflected laser light interferes with the standing wave pattern within the optical resonator or laser cavity resulting in the self-mixing interference signal. The self-mixing interference signal can be detected by means of the detector (e.g. variations of the laser intensity within the optical resonator which can, for example, be detected by means of a photodiode).

The multitude of laser diodes may comprise more than 100 laser diodes. The multitude of laser diodes may especially comprise at least 1000, preferably at least 5000 and most preferably at least 10000 laser diodes. The VCSEL array may even comprise more than 100000 laser diodes (e.g. VGA resolution of 640×480 pixel and the like).

The VCSEL device may comprise two, three, four or more second electrical laser contacts. Each second electrical laser contact is in this embodiment arranged to electrically drive a corresponding subgroup of laser diodes of the VCSEL array. The second electrical laser contacts may, for example, be an anode laser contacts. A first anode contact may in combination with the first electrical laser contact (common cathode contact) provide an electrical drive current to a first column or row of the VCSEL array. The anode contacts may therefore enable switching on or off the respective subgroup independent from the other subgroups. The detectors may, for example, be arranged in a complementary arrangement with respect to the arrangement of the subgroup of the VCSEL array. The detectors may, for example, be arranged in rows if the laser diodes of the VCSEL array are arranged in columns. This matrix arrangement enables read out of a single self-mixing interference measurement corresponding to one laser diode signal by means of one detector even if one complete column of laser diodes emits laser light and generates a corresponding self-mixing interference signal within each optical resonator. The matrix arrangement simplifies contacting of the detectors but requires a corresponding switching scheme. An alternative approach is to provide one dedicated detector for each laser diode such that it is clear which laser diode generated the respective self-mixing interference measurement signal.

The VCSEL device may according to an alternative embodiment comprise one second electrical laser contact, wherein the first electrical laser contact and the second electrical laser contact are arranged to provide a common electrical drive current to all laser diodes of the VCSEL array. Each detector is in this embodiment associated to one dedicated laser diode to enable identification of the respective self-mixing interference measurement signal. This embodiment may enable a very fast measurement of a 3-D setting or scene.

The detector may be integrated in the optical resonator. The detectors may, for example, be integrated in the first distributed Bragg reflector or the second distributed Bragg reflector. The detectors may, for example, be integrated in an unstructured DBR such that one common detector may be arranged to receive self-mixing interference signals from two, three, four or more VCSELs as described above. The detector may, for example, be integrated in the first (lower) DBR to provide a common detector for neighboring VCSELs.

Each optical resonator may comprise one dedicated detector. Assignment between laser diode and corresponding dedicated detector is in this case unambiguous such that identification of the self-mixing interference signal is simplified.

Each detector may comprise a first detector electrode. The first detector electrode and at least one additional electrode are arranged to read out the electrical self-mixing interference measurement signal. The additional electrode may be comprised by the first electrical laser contact or the second electrical laser contact. The respective laser diode and detector share in this case one electrode or contact. The additional electrode may alternatively be a separate second detector electrode independent from the first electrical laser contact and the second electrical laser contact.

The at least one additional electrode may be the at least one second electrical laser contact(s). The detector is integrated in the second distributed Bragg reflector. The VCSEL array is arranged such that the laser light is emitted via the first distributed Bragg reflector during operation of the VCSEL device. The laser diodes (VCSELs) are in this case so-called bottom emitters which emit the laser light through the substrate if the substrate is not removed in a direction opposite to the side of the substrate next to the optical resonators. The emission wavelength is in this case such that the (GaAs) substrate is transparent (emission wavelength greater than 900 nm in case of GaAs) for example 940 nm. The substrate may alternatively be at least locally removed. The first detector electrodes of the detectors may be contacted by wire bonds. The self-mixing interference measurement signal can be read from each individual detector (e.g. photodiode or photo transistor).

The VCSEL device may alternatively be arranged as a flip chip device. The topmost first detector electrodes may (e.g. completely) cover the mesa of the respective laser diode (VCSEL). The first detector electrodes are (e.g. full) metal contacts that can be flip chipped. The second electrical laser contact(s) of the subgroup(s) (e.g. whole laser array, columns or rows) may, for example, be electrically contacted by means of wire bonds so that each detector (Pixel) can be read by a simple electrical connection. All of the laser diodes (VCSELs) may be operated simultaneously or subgroup by subgroup.

The VCSEL array may according to an alternative embodiment be mounted on a detector chip. The detector chip comprises the detectors. The detector chip may, for example, be a silicon-based detector chip comprising an array of photodetectors, wherein each photodetector is aligned with a corresponding laser diode (VCSEL) comprised by the VCSEL array. The detector chip may alternatively comprise, for example, rows or columns of photodetectors which may be arranged in a matrix arrangement with respect to the subgroups of laser diodes as discussed above.

The VCSEL device comprises at least one optical device arranged to redirect the laser light. The optical device may, for example, comprise a lens, an array of lenses, a diffusor or the like to spread the laser light emitted by the VCSEL array in a defined field-of-view. The at least one optical device may be an optical structure etched in a substrate (e.g. gallium arsenide growth substrate) comprised by the VCSEL array.

According to a further aspect a self-mixing interference sensor for recording three-dimensional pictures comprising a VCSEL device according to any embodiment discussed above is provided. The three-dimensional self-mixing interference sensor further comprises a driving circuit and an evaluator. The driving circuit is arranged to electrically drive the laser diodes by means of the first electrical laser contact and the at least one second electrical laser contact. The evaluator is arranged to evaluate the electrical self-mixing interference measurement signals. The evaluator may, for example, be arranged to determine distances, velocities and/or acceleration of objects based on self-mixing interference signals in each optical resonator or laser cavity of the laser devices comprised by the VCSEL array. The evaluator may further be arranged to reconstruct a three-dimensional picture of the field of view and objects in the field of view based on the multitude of distances, velocities and/or accelerations.

According to a further aspect a mobile communication device may comprise a VCSEL device according to any embodiment described above or a three-dimensional self-mixing interference sensor as described above. The mobile communication device is arranged to present a three-dimensional picture of a scene to a user of the mobile communication device based on self-mixing interference measurement signals of the scene as described above. The evaluation of the self-mixing interference measurement signals may be performed by the laser sensor and/or the mobile communication device. The VCSEL device or the three-dimensional self-mixing interference sensor may further be comprised by 3-D cameras.

According to a further aspect a method of fabricating a Vertical Cavity Surface Emitting Laser (VCSEL) device for a self-mixing interference sensor for recording three-dimensional pictures is provided. The method comprising the steps of: providing a substrate, providing a VCSEL array comprising a multitude of laser diodes on the substrate, wherein each laser diode comprises an optical resonator, wherein the optical resonator comprises a first distributed Bragg reflector, a second distributed Bragg reflector and an active layer for light emission, wherein the active layer is arranged between the first distributed Bragg reflector and the second distributed Bragg reflector, providing a first electrical laser contact, wherein the first electrical laser contact is a common contact for all laser diodes of the VCSEL array, providing at least one second electrical laser contact, wherein the first electrical laser contact and the at least one second electrical laser contact are arranged to provide an electrical drive current to electrically pump the optical resonators of the laser diodes, and wherein the at least one second electrical laser contact is arranged to electrically contact at least a subgroup of the multitude of laser diodes of the VCSEL array, and providing detectors, wherein each detector is arranged to generate an electrical self-mixing interference measurement signal associated to the at least one laser diode upon reception of laser light.

The steps need not be performed in the order given above. The different layers may be deposited by epitaxial methods like MOCVD, MBE and the like. The substrate may be removed in a subsequent processing step.

It shall be understood that the VCSEL device according to any embodiment described above and the method of fabricating the VCSEL device have similar and/or identical embodiments.

Further advantageous embodiments are defined below.

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

DETAILED DESCRIPTION

Various embodiments will now be described by way of the Figures.

Self-mixing interference is used for detecting movement of and distance to an object. Background information about self-mixing interference is described in “Laser diode self-mixing technique for sensing applications”, Giuliani, G.; Norgia, M.; Donati, S. & Bosch, T., Laser diode self-mixing technique for sensing applications, Journal of Optics A: Pure and Applied Optics, 2002, 4, S. 283-S. 294 which is incorporated by reference. Detection of movement of a fingertip relative to a sensor in an optical input device is described in detail in International Patent Application No. WO 02/37410 which is incorporated by reference. The principle of self-mixing interference is discussed based on the examples presented in International Patent Application No. WO 02/37410. A diode laser having a laser cavity or optical resonator is provided for emitting a laser or measuring beam. At its upper side, the device is provided with a transparent window across which an object, for example a human finger, is moved. A lens is arranged between the diode laser and the window. This lens focuses the laser beam at or near the upper side of the transparent window. If an object is present at this position, it scatters the measuring beam. A part of the radiation of the measuring beam is scattered in the direction of the illumination beam and this part is converged by the lens on the emitting surface of the laser diode and re-enters the cavity of this laser. The radiation re-entering the cavity of the diode laser induces a variation in the gain of the laser and thus in the intensity of radiation emitted by the laser, and it is this phenomenon which is termed the self-mixing effect in a diode laser.

The change in intensity of the radiation emitted by the laser or of the optical wave in the laser cavity can be detected by a photodiode or a detector arranged to determine an impedance variation across the laser cavity. The diode or impedance detector converts the radiation variation into an electric signal, and electronic circuitry is provided for processing this electric signal.

FIG.1shows a principal sketch of a first VCSEL device100with integrated photodiode. The first VCSEL device100comprises a multitude of VCSELs with integrated photodiode which are arranged on a common substrate110. The first VCSEL device100is a bottom emitter emitting laser light (indicated by the arrow) through the substrate110. Each of the VCSELs with integrated photodiode comprises a first DBR115, an active layer120, a first part of a second DBR135-1, a second part of the second DBR135-2and a detector140. The detector140comprises at least one layer which is arranged to detect a variation of the optical wave within the optical resonator or laser cavity built by the first DBR115and the second DBR135-1,135-2and the semiconductor layers sandwiched between the first DBR115and the second DBR135-1,135-2. The single VCSELs are in this embodiment separated by means of an oxidation opening130or trench which is subsequently filled with an electrically isolating material. The oxidation opening130may, for example, be used to provide a current aperture (not shown) in the first DBR115or the second DBR135-1,135-2. The VCSELs are electrically contacted by means of a first electrical laser contact105and at least one second electrical laser contact127. The first electrical laser contact105electrically contacts in this embodiment a common current distribution layer (not shown) which is integrated in the non-etched part of the first DBR115to provide the current to all VCSELs comprised by the VCSEL array. The first VCSEL device100comprises a multitude of second electrical laser contacts127, wherein each second electrical laser contact127contacts one row of VCSELs comprised by the VCSEL array. The rows are arranged perpendicular with respect to the plane ofFIG.1. The first DBR115comprises 30 pairs of layers with alternating refractive index to provide a reflectivity of 98.5% such that the emission of laser light through the substrate110is enabled. The first DBR115may, for example, comprise AlGaAs (AlxGa(1-x)As) layers with different Al concentration (e.g. variation between 15% and 90%) to provide different refractive indices. The first part of the second DBR135-1may comprise 30 pairs of layers with alternating refractive index and the second part of the second DBR135-2may comprise another 20 pairs of layers with alternating refractive index to provide sufficient reflectivity. The second part of the second DBR135-2of each mesa of the VCSELs is covered by a first detector electrode150such that each detector140can be read out by means of the corresponding second electrical laser contact127and the corresponding first detector electrode150. A solder bump160is provided on top of each first detector electrode150such that the first VCSEL device100can be mounted in a flip chip arrangement on top of, for example, a submount (not shown) which may comprise evaluations circuitry to evaluate measurement signals generated by means of the first VCSEL device100.

FIG.2shows a principal sketch of the electrical contacting scheme of the first VCSEL device100shown inFIG.1. The VCSELs are electrically pumped by means of a current source190which is contacted to the common first electrical laser contact105and to the second electrical laser contacts127-1,127-2such that each row comprising a multitude of laser diodes122can be operated independent from the other rows. The second electrical laser contacts127and the first detector electrodes150are electrically connected with an evaluator (not shown) such that each detector140can be read out separately to determine the self-mixing measurement signals.

FIG.3shows a principal sketch of a top view of the first VCSEL device100. The first VCSEL device100is electrically contacted by means of the common first electrical laser contact105, the second electrical laser contacts127-1,127-2. . . and the solder bumps160.

FIG.4shows a principal sketch of a second VCSEL device100comprising a detector chip144. The second VCSEL device100comprises in this embodiment a bottom emitting VCSEL array comprising an optical resonator with a first DBR, a second DBR135and an active layer120sandwiched between the first DBR115and the second DBR135. The active layer120typically comprises one or several quantum well layers. A first electrical laser contact105(usually n-contact) is provided on a backside of the substrate110opposing the side of the substrate110on which the optical resonators are provided. An at least one second electrical laser contact127(usually p-contact) is provided on top of the second DBR135. The first and the second electrical laser contacts105,127are arranged to provide an electrical drive current (electrically pump) the optical resonator. The VCSEL device100may comprise further layers as, for example, current distribution layers, current confinement layers and the like which are not explicitly shown inFIG.4but well known to those skilled in the art. The first electrical laser contact105as well as the second electrical laser contact(s)127(both may comprise metal layers) may surround a hole through which laser light10can be emitted if a drive current above a laser threshold current of the VCSEL device100is supplied. Emission wavelength of the laser light10is above 900 nm, preferably above 930 nm such that the substrate110(GaAs) is essentially transparent for laser light10. The VCSEL array of the VCSEL device100is mounted with the second electrode(s)127on a detector chip144. The detector chip144is according to this embodiment arranged to provide electrical connection of the first electrical laser contact105and the second electrical laser contact (not shown). The detector chip144further comprises a multitude of detectors140(e.g. photodiodes). The detectors140are aligned with the openings of the first electrical laser contact105and the second electrical contact(s)127. The reflectivity of the first DBR115and the second DBR135is arranged such that the laser light10is emitted through the substrate110and an optical device170(lens) which is etched in the substrate110. The lenses are arranged to focus the laser light10to a field-of-view. The laser beams are spread with respect to each other by shifting the lenses with respect to a center of the corresponding VCSEL. The reflectivity of the second DBR135is such that a defined amount of laser light is received by the respective detector140to determine the self-mixing interference measurement signal. Each detector is electrically contacted to two contact pads148which are arranged on the opposite side of the detector chip144. The contact pads148may enable mounting of the VCSEL device on, for example, a PCB.

FIG.5shows a principal sketch of a third VCSEL device100with detector chip144. The third VCSEL device100comprises an array of top emitting VCSELs each comprising a first DBR115, a second DBR135and an active layer120arranged between the first DBR115and the second DBR135. Reflectivity of the second DBR135is in this case somewhat lower than the reflectivity of the first DBR115. Majority of the laser light10is therefore emitted through e.g. ring shaped second electrical laser contacts127which are arranged on top of the second DBR135to electrically contact the rows or columns of VCSELs comprised by the VCSEL array similar as discussed with respect toFIG.2or3. The second electrical laser contacts127are electrically isolated with respect to the first DBR115and the active layer120by means of an isolating structure108(e.g. oxide layer). A first electrical laser contact105is deposited on a backside of substrate110opposing the side of the substrate110on which the layer stacks of the first DBR115and subsequently the active layer120, second DBR135and second electrical laser contact127are processed. The first electrical laser contact105comprises openings which are aligned with the VCSELs such that laser light10can be emitted via the substrate110through the corresponding opening. The substrate110may alternatively be locally removed to avoid absorption of laser light10by the substrate110. The third VCSEL device100is similarly as discussed with respect toFIG.4mounted on the detector chip144which comprises an electrical driver (not shown) which is arranged to provide an electrical drive current to the VCSELs by means of the first electrical laser contact105and the second electrical laser contact127. The detector chip144further comprises detectors140, wherein the detectors140are aligned with the openings of the first electrical contact105. The detector chip144further comprises an evaluator323which is arranged to receive self-mixing interference measurement signals from the detectors140to determine distances to, for example, an object to determine a three-dimensional picture of the object. The third VCSEL device100further comprises one common optical device170. The common optical device120is integrated on wafer level by means of a transparent material. The transparent material is deposited on top of the VCSELs. The transparent material is further shaped such that laser light10emitted by different VCSELs is directed to different directions. The common optical device120may further comprise integrated micro lenses (e.g. structured layers of different transparent materials) which are arranged to focus the laser light10. The optical devices as discussed with respect toFIG.4andFIG.5may also be combined with the embodiment discussed with respect toFIG.1.

FIG.6shows a cross-section of self-mixing interference sensor300according to a first embodiment. The self-mixing interference sensor300is arranged to determine presence, distances and movements of objects by means of self-mixing interference measurements. The optical sensor300comprises a VCSEL device100as discussed above, a transmission window310and a driving circuit320for electrically driving the VCSEL device100. The driving circuit320is electrically connected to the VCSEL device100to supply electrical power to the VCSEL device100in a defined way. The driving circuit320comprises a memory device for storing data and instructions to operate the driving circuit320and a processing unit for executing data and instructions to operate the driving circuit320. The self-mixing interference sensor300further comprises an evaluator323. The detectors140(e.g. photodiodes) comprised by the VCSEL device100are arranged to determine variations in the standing wave pattern within the laser cavity coupled to the respective photodiode. The evaluator323comprises at least one memory device like a memory chip and at least one processing device like a micro-processor. The evaluator323is adapted to receive electrical signals from the VCSEL device100and optionally from the driving circuit320to determine distances or movements of one or more objects based on the interference of laser light10which is reflected by the respective object and the optical standing wave within the respective laser cavity. The evaluator may optionally be arranged to reconstruct a 3-D picture of a scene which is illuminated by means of the self-mixing interference sensor300.

FIG.7shows a principal sketch of a mobile communication device380comprising a self-mixing interference sensor300. The self-mixing interference sensor300can, for example, be used in combination with a software application running on the mobile communication device380. The software application may use the self-mixing interference sensor300for providing a 3-D picture or movie of a scene illuminated by means of the self-mixing interference sensor300.

FIG.8shows a principal sketch of a process flow of a method of fabricating a VCSEL device100according to the present invention. A substrate110is provided in step410. A VCSEL array comprising a multitude of laser diodes is provided in step415on the substrate. Each laser diode comprises an optical resonator. The optical resonator comprises a first distributed Bragg reflector, a second distributed Bragg reflector and an active layer for light emission. The active layer is arranged between the first distributed Bragg reflector115and the second distributed Bragg reflector. A first electrical laser contact is provided in step420. The first electrical laser contact105is a common contact for all laser diodes of the VCSEL. An at least one second electrical laser contact is provided in step425. The first electrical laser contact and the at least one second electrical laser contact are arranged to provide an electrical drive current to electrically pump the optical resonators of the laser diodes. The at least one second electrical laser contact is further arranged to electrically contact at least a subgroup of the multitude of laser diodes122of the VCSEL array. In step430detectors are provided. Each detector is arranged to generate an electrical self-mixing interference measurement signal associated to the at least one laser diode upon reception of laser light.

The layers of the first DBR115, the active layer120, the second DBR135and the electrical contacts and any other layer as current injection layers and the like may be deposited by epitaxial methods like MOCVD or MBE.

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.

LIST OF REFERENCE NUMERALS

10laser light100VCSEL device105first electrical laser contact108isolating structure110substrate115first DBR120active layer122laser diode127,127-1,127-2second electrical contact130oxidation opening135second DBR135-1first part of second DBR135-2second part of second DBR140detector144detector chip148contact pad150first detector electrode160solder bump170optical device300self-mixing interference sensor310transmission window320driving circuit323evaluator380mobile communication device410step of providing a substrate415step of providing a VCSEL array420step of providing a first electrical laser contact425step of providing a second electrical laser contact430step of providing a detector