Patent Publication Number: US-2023161017-A1

Title: Optoelectronic sensor

Description:
The invention relates to an optoelectronic sensor for detecting an object in a monitored zone. 
     Many optoelectronic sensors work in accordance with the sensing principle in which a light beam is transmitted into the monitored zone and the light beam reflected by an object is received again in order then to electronically evaluate the received signal, for example for a distance measurement. The time of flight is here often measured using a known phase method or pulse method to determine the distance of a sensed object. Optoelectronic sensors that use this method are frequently called light sensors, TOF (time of flight) sensors, or LIDAR (light detection and ranging) sensors. To expand the measured zone, the light beam can be moved, as is the case in a laser scanner. A light beam generated by a laser there periodically sweeps over the monitored zone with the help of a deflection unit. In addition to the measured distance information, a conclusion is drawn on the angular location of the object from the angular position of the deflection unit and the site of an object in the monitored zone is thus detected in two-dimensional polar coordinates. The scanning movement is achieved by a rotating mirror in most laser scanners. It is, however, also known to instead have the total measurement head with light transmitters and light receivers rotate. 
     Optoelectronic sensor of the initially named kind can work with visible light, but also with light in the infrared wavelength range. The use of infrared light has the advantage that the measurement is not perceived as irritating. In addition, light sources that emit infrared light, for example vertical cavity surface emitting lasers (VCSELs), are available in smaller sizes than laser diodes that emit visible light. A sensor can therefore be built as correspondingly smaller. 
     An infrared signal, however, has the disadvantage that the detection zone, in particular the specific location of the distance measurement on the respective surface, cannot be recognized by the human eye without additional aids, which in particular makes the assembly and the adjustment of the sensor more difficult. 
     A visible so-called pilot or target beam is therefore superposed on the infrared measurement beam, typically using a dichroitic mirror such as described in US 2004/0070745 A1. Such systems are, however, comparatively large and the adjustment of the measurement beam and the pilot mean is moreover complex and prone to losses of adjustment. 
     Starting from this prior art, it is the object of the invention to provide an improved optoelectronic sensor which has a compact and robust construction. 
     This object is satisfied by an optoelectronic senor having the features of claim 1. 
     The optoelectronic sensor in accordance with the invention has at least one measurement light source for transmitting at least one measurement light beam in the infrared wavelength range. Light beams are here not to be understood as beams in the sense of geometrical optics within a larger bundle of rays, but rather as light beams that generate corresponding light spots in the monitored zone on incidence on an object. An associated light receiver is able to generate received signals from reflected or remitted measurement light beams. 
     A control and evaluation unit controls the light sources and the light receiver and can evaluate the received signals of the light receiver to acquire information on the object such as a distance from the sensor. 
     To align the infrared measurement light beam that is invisible to the human eye, at least one pilot light beam is provided that is transmitted from a pilot light source in the visible wavelength range and at least one optical element for a coaxial overlapping of the measurement light beam and the pilot light beam so that they are projected into the monitored zone coaxially along an optical axis of the optoelectronic sensor and there generate substantially overlapping light spots. 
     The invention starts from the basic idea that the optical element for the coaxial superposition of the measurement light beam and of the pilot light beam has at least one first optical metasurface. This has the advantage that an improved sensor design with smaller space requirements and a more robust optics can be implemented. 
     Metasurfaces in the sense of this application are to be understood as optically effective layers that have structures having structure sizes smaller than the wavelengths of the measurement light beam and that typically have a binary height profile. Such surfaces are known from the technical literature (e.g.: Flat optics with designer metasurfaces, Nature Mater 13, 139 -150 (2014). DOI: 10.1038/NMAT3839) and, by a suitable choice of the structures, can vary properties of the measured light beams or pilot light beams such as the phase, amplitude, and thus the propagation direction and the polarization by imparting a phase profile, in particular in dependence on the wavelength of the light beams. Metasurfaces are also called “flat optics” since their space requirements in the beam direction are considerably smaller than that of conventional refractive optics. 
     In an embodiment of the invention, the first optical metasurface can be configured to set an aperture angle of the measurement light beam by imparting a substantially parabolic phase profile. A refractive lens typically used for this purpose and arranged downstream of the measurement light source can thereby be dispensed with and thus construction space saved. The measurement light beam can in particular be collimated by the first optical metasurface so that a parallel measurement light beam bundle is generated. 
     In a further embodiment of the invention, the first optical metasurface can be configured to set an aperture angle of the pilot light beam by imparting a substantially parabolic phase profile. A refractive lens typically used for this purpose and arranged downstream of the pilot light source can thereby be dispensed with and thus construction space saved. The measurement light pilot light beam can in particular be collimated by the first optical metasurface so that a parallel measurement light beam bundle is generated. 
     In a preferred embodiment of the invention, the optical element for the superposition of the measurement light beam and of the pilot light beam has at least one first and one second metasurface, with the first optical metasurface being able to be configured to set an aperture angle of the measurement light beam by imparting a first substantially parabolic phase profile and the second optical metasurface being able to be configured to set an aperture angle of the pilot light beam by imparting a second substantially parabolic phase profile. The optical element can particularly preferably have a dichroitic beam combiner to coaxially superpose the measurement light beam and the pilot light beam. A particularly effective beam combination by the dichroitic beam combiner, for example a Bragg mirror, is possible by the collimation of the measurement light beam and the pilot light beam since the transmission conditions or the reflection conditions (design of the layer structure of the Bragg mirror) only have to be optimized for an angle of incidence of the measurement light beam or pilot light beam on the dichroitic beam combiner. 
     The dichroitic beam combiner can be configured as a beam combiner cube, with the optical metasurfaces preferably being able to be applied directly to the light entry surfaces of the beam combiner cube. 
     In an alternative embodiment of the invention, the first optical metasurface can be configured to coaxially superpose the measurement light beam and the pilot light beam. A dichroitic mirror can thus be dispensed with. The measurement light beam can here be incident on the optical metasurface at a first angle and the pilot light beam can be incident thereon at a second angle with respect to the optical axis of the sensor along which the measurement light beam and the pilot light beam are projected into the monitored zone, with amounts and/or directions of the first and second angles being able to differ. The optical axes along which the measurement light beam and the pilot light beam propagate to the optical metasurface can preferably intersect at the optical metasurface. The optical metasurface is configured such that the measurement light beam or pilot light beam are deflected by different amounts, are coaxially superposed, and projected into the monitored zone along the optical axis of the sensor by imparting wavelength dependent, substantially wedge shaped phase profiles from the optical metasurface. 
     In a further embodiment of the invention, the first optical metasurface can additionally be configured to set, preferably to collimate, an aperture angle of the measurement light beam or of the pilot light beam by imparting a wavelength dependent, substantially parabolic phase profile. Additional optical elements such as refractive lenses or further optical metasurfaces for setting an aperture angle of the measured light beam and/or pilot light beam can thus be dispensed with. 
     In the different embodiments, an optical beam shaping element for varying a shape of the beam cross-section of the pilot light beam can be arranged downstream of the pilot light beam in the beam direction. The optical beam shaping element can be configured, for example, such that pilot light beams generate cruciform light spots on an incidence on an object in the monitored zone. The alignment of the sensor can be further simplified by a corresponding light spot shape. The optical beam shaping element can be configured as a diffractive optical element or as a further metasurface. The beam shaping can, however, also take place by a corresponding configuration of the first metasurface for combining the measurement light beam and pilot light beam. 
     The measurement light source and the pilot light source can be arranged on a common circuit board. They can be aligned such that the optical axes along which the measurement light beam and pilot light beam are emitted by the light sources have an angle from one another and intersect at the optical element for the superposition of the measurement light beam and of the pilot light beam. The light sources can, however, also be aligned such that the optical axes along which the measurement light beam or the pilot light beam is emitted extend in parallel, with an optical element for the deflection of the measurement light beam and/or the pilot light beam then being arranged downstream of the measurement light source and/or the pilot light source and being configured such that the optical axes along which the measurement light beam and the pilot light beam propagate to the optical element for the superposition of the measurement light beam and the pilot light beam intersect thereat. 
     The pilot light source can emit at least one pilot light beam having a wavelength adapted to the light sensitivity of the human eye. Green light at a wavelength of 550 nm, for example, is perceived as 31 times brighter for the human eye than red light at a wavelength of 670 nm. When green light is used, a pilot light source having a smaller power can therefore be used. 
     A reception optics for focusing reflected or remitted measurement light beams onto the light receiver can be arranged upstream of the light receiver. The reception optics can be designed as a metasurface and can be configured such that only reflected or remitted measurement light beams are imaged on the sensor and light beams from different wavelength ranges are deflected away from the light receiver, for example a beam trap. The reception optics then acts in a similar manner to an optical filter. 
     Alternatively or additionally, an optical filter for the suppression of interference light, in particular of reflected or remitted pilot light beams can be arranged upstream of the light receiver. The optical filter can likewise be configured as a metasurface. 
     The control and evaluation unit of the sensor can be adapted to provide different operating modes of the sensor. One operating mode can, for example, be an adjustment mode in which the measurement light beam and the pilot light beam are activated simultaneously. A further operating mode can be a measurement mode in which the pilot light beam has been deactivated so that interference light generated by the pilot light beam is reduced for the light receiver. 
     In accordance with a further development, the optoelectronic sensor is configured to determine the distance of the respective surface from the sensor from a time of flight of a pulsed light signal to the respective surface and back or from the phase shift of a modulated light signal transmitted by the sensor with respect to the light signal reflected at the respective surface. The optoelectronic sensor can therefore in particular work in accordance with the time of flight (TOF) principle. 
    
    
     
       The invention will be explained in detail in the following with reference to an embodiment and to the drawing. There are shown in the drawing: 
         FIG.  1    a schematic representation of an optoelectronic sensor in accordance with the invention; 
         FIG.  2    a schematic representation of an alternative embodiment of an optoelectronic sensor in accordance with the invention; 
         FIG.  3    a schematic representation of a further embodiment of an optoelectronic sensor in accordance with the invention; 
         FIG.  4    a schematic representation of the function of an optical metasurface for the coaxial superposition of the measurement light beam and of the pilot light beam; 
         FIG.  5    a schematic representation of a further embodiment of an optoelectronic sensor in accordance with the invention; and 
         FIG.  6    exemplary light spots of measurement light beams and pilot light beams on an incidence on an object in the monitored zone. 
     
    
    
       FIG.  1    shows a schematic representation of an optoelectronic sensor  10  in accordance with the invention in an embodiment as a light sensor. The sensor  10  has a measurement light source  12 , for example a laser diode or a vertical cavity surface emitting laser (VCSEL) that emits at least one measurement light beam  14  in the infrared wavelength range. The sensor  10  furthermore has a pilot light source  20 , for example a light emitting diode (LED) that emits at least one pilot light beam  22  in the visible wavelength range. 
     An optical element  18  is configured as a beam combiner cube  26  having a dichroitic beam combiner  28  that transmits the measurement light beam  14  and reflects the pilot light beam  22 . The measurement light beam  14  and the pilot light beam  22  are thus coaxially superposed by the optical element  18  and are projected through a window  30  in the housing  32  of the sensor  10  into a monitored zone  34  along an optical axis  24  of the sensor  10 . 
     The optical element  13  moreover has a first optical metasurface  62  and a second optical metasurface  64 , with the first optical metasurface  62  being configured to collimate the measurement light beam  14  and the second optical metasurface  64  being configured to collimate the pilot light beam  22  so that the measurement light beam  14  and the pilot light beam  22  are incident on the dichroitic beam combiner  28  as collimated beams. 
     The measurement light beams remitted or remitted at an object  36  in the monitored zone  34  are conducted as received light  38  through a reception optics  42  onto a light receiver  44  via an optical filter  40  for the suppression of interference light. 
     The light receiver  44  is preferably formed as a photodiode, APD (avalanche photodiode), or SPAD (single photon avalanche diode), or SPAD matrix (SPAD array). 
     A control and evaluation unit  46  that is connected to the measurement light source  12 , to the pilot light source  20 , and to the light receiver  44  is furthermore provided in the sensor  10 . The control and evaluation unit  46  comprises a measurement light source control  48 , a pilot light source control  50 , a time of flight measuring unit  52 , and an object distance estimation unit  54 , with them initially only being functional blocks that can also be implemented on the same hardware or in other functional units such as in the light sources  12 ,,  20  or in the light receiver  44 . The evaluation unit  46  can output measured data via an interface  56  or can conversely accept control and parameterization instructions. The control and evaluation unit  46  can also be arranged in the form of local evaluation structures on a chip of the light receiver  12  or can interact as a partial implementation with the functions of a central evaluation unit (not shown). 
     The control and evaluation unit  46  can be designed to activate and deactivate the pilot light source  20  and the measurement light source  12  independently of one another. 
       FIG.  2    shows a schematic representation of an alternative embodiment of an optoelectronic sensor  60  in accordance with the invention. Unlike the sensor  10  shown in  FIG.  1   , the measurement light source  12  and the pilot light source  20  are arranged next to one another such that the measurement light beam  14  and the pilot light beam  22  are first irradiated in parallel with one another. The optical element  18  for the coaxial superposition of the measurement light beam  14  and of the pilot light beam  22  therefore has an additional surface  66 , for example a mirror, for the deflection of the pilot light beam  22  onto the dichroitic beam combiner  28 . The arrangement of the measurement light source  12  and the pilot light source  20  next to one another has the advantage, for example, that the light sources can be mounted on a common circuit board. The detection of the measurement light beams remitted or reflected at the object  36  in the monitored zone  34  takes place as in the embodiment described in  FIG.  1   . 
       FIG.  3    shows a further embodiment of an optoelectronic sensor  70  in accordance with the invention in which a first optical metasurface  72  of the optical element  18  for the coaxial superposition of the measurement light beam  14  and of the pilot light beam  22  is configured as a beam combiner of the measurement light beam  14  and the pilot light beam  22 . The function of the optical metasurface  72  will be explained further in the following  FIG.  4   . A respective second optical metasurface  74  or a third optical metasurface  76  for the collimation of the measurement light beam  14  or of the pilot light beam  22  is arranged downstream of the measurement light source  14  or of the pilot light source  20  in the light beam direction. The collimated measurement light beam  14  and pilot light beam  22  are combined by the metasurface  72 , are coaxially superposed, and are projected through the window  30  of the sensor  70  into the monitored zone  34  along of the optical axis  24  of the sensor  70 . The detection of the measurement light beams remitted or reflected at the object  36  in the monitored zone  34  takes place as in the optoelectronic sensor  10  described in  FIG.  1   . 
       FIG.  4    shows by way of example the function of the first optical metasurface  72  of the optical element  18  for the coaxial superposition of the measurement light beam  14  and of the pilot light beam  22  from  FIG.  3   . The measurement light source  12  emits a measurement light beam  14  that is collimated by a second optical metasurface  74  by imparting a substantially parabolic first phase profile and is incident on the first optical metasurface  72  along a measurement light beam axis  78  at a first angle α with respect to the optical axis  24  of the sensor  70 . The pilot light source  20  emits a pilot light beam  22  that is collimated by a third optical metasurface  76  by imparting a substantially parabolic second phase profile and is incident on the first optical metasurface  72  along a pilot light beam axis  80  at a second angle β with respect to the optical axis  24  of the sensor  70 . The measurement light beam axis  78 , the pilot light beam axis  80 , and the optical axis  24  of the sensor  70  intersect at the first optical metasurface  72 . The first optical metasurface  72  is configured such that it imparts a substantially wedge-shaped phase profile on the measurement light beam  14  and on the pilot light beam  20 , independently of the wavelength in each case, said wedge-shaped phase profile resulting in a deflection of the measurement light beam  14  and the pilot light beam  20  so that they are projected into the monitored zone  34  coaxially along the optical axis  24  of the sensor  70  after the first optical metasurface  72  and there generate a measurement light beam spot  82  with a pilot light beam spot  84  arranged concentrically thereto on the incidence on an object. 
     The collimation function of the second and/or third optical metasurface or metasurfaces  74 ,  76  can also be integrated in the first optical metasurface  72 . In this embodiment, the first optical metasurface  72  is configured such that they can respectively impart a wavelength dependent parabolic and/or wedge-shaped phase profile on the measurement light beam  14  and the pilot light beam  20 . 
     In the exemplary sensor  90  shown in  FIG.  5   , a substantially parabolic phase profile is imparted on the measurement light beam  14  by the first optical metasurface  72  and the measurement light beam  14  is collimated. Since the measurement light beam  14  has already been emitted along the optical axis  24  of the sensor  90  by the measurement light source  12 , no deflection of the measurement light beam  14  is necessary so that no wedge-shaped phase profile has to be imparted on the measurement light beam  14 . The pilot light beam  22  that is initially emitted by the pilot light source  20  in parallel with the optical axis  24  of the sensor  90  is deflected by an optical deflection element  92 , for example a mirror, a prism, or a further optical metasurface, in the direction of the first optical metasurface  72  so that it is incident on the first optical metasurface  72  along a pilot light beam axis  80  at a second angle β with respect to the optical axis  24  of the sensor  90 . The pilot light beam axis  80 , and the optical axis  24  of the sensor  90  intersect at the first optical metasurface  72 . The first optical metasurface  72  is configured such that it imparts a substantially wedge-shaped and parabolic phase profile on the pilot light beam  20  so that the pilot light beam  20  is collimated and deflected. The measurement light beam  14  and the pilot light beam  20  are thereby projected into the monitored zone  34  coaxially along the optical axis  24  of the sensor  90  after the first optical metasurface  72 . The detection of the measurement light beams remitted or reflected at the object  36  in the monitored zone  34  again takes place as in the embodiment described in  FIG.  1   . 
       FIG.  6    shows exemplary light spots  100 ,  102 ,  104  of measurement light beams and pilot light beams on an incidence on an object in the monitored zone. In the first exemplary light spot  100 , the pilot light beam spot  100   b  is arranged concentrically in the measurement light beam spot  100   a ; in the second exemplary light spot  102 , the pilot light beam spot  92   b  is configured and is arranged concentrically about the measurement light beam spot  92   a . The third exemplary light spot  104  shows a cruciform pilot light beam spot  104   b  that is arranged centrally above a circular measurement light beam spot  104   a . It is understood that the representations are purely exemplary; the light beam spots generated in the monitored zone are in particular not sharply defined as a rule. 
     The shape of the light spots  100 ,  102  can take place, for example, by a corresponding setting of the beam diameter of the measurement light beams and of the pilot light beams. A cruciform pilot light beam spot  104   b  can be generated by diffractive elements for beam shaping known from the prior art or by a further optical metasurface formed for a corresponding beam shaping.