Optoelectronic sensor and method of detecting objects in a monitored zone

Abstract

An optoelectronic sensor for detecting an object in a monitored zone is provided that has a light transmitter and a transmission optics associated with the light transmitter in a transmission path for transmitting a light beam and a light receiver and a reception optics associated with the light receiver and offset from the transmission optics by a spacing in a reception path for receiving a light beam remitted by the object and for generating a received light spot on the light receiver, as well a control and evaluation unit that is configured to evaluate a received signal of the light receiver. The reception optics has at least one optical metaelement having a metasurface and/or a metamaterial and is configured such that a displacement of the received light spot on the light receiver in a near zone of the sensor dependent on a distance of the object from the sensor is no larger than a full width at half maximum of the received light spot.

Claims

1. An optoelectronic sensor for detecting an object in a monitored zone that has a light transmitter and a transmission optics associated with the light transmitter in a transmission path for transmitting a light beam and a light receiver and a reception optics associated with the light receiver and offset from the transmission optics by a spacing in a reception path for receiving a light beam remitted by the object and for generating a received light spot on the light receiver, as well as a control and evaluation unit that is configured to evaluate a received signal of the light receiver, wherein the reception optics has at least one optical metaelement having a metasurface and/or a metamaterial and the reception optics is configured such that a displacement of the received light spot on the light receiver in a near zone of the sensor dependent on a distance of the object from the sensor is no larger than a full width at half maximum of the received light spot.

2. The sensor in accordance with claim 1, wherein the reception optics is configured such that the displacement of the received light spot on the light receiver in the near zone of the sensor dependent on the distance of the object from the sensor is no larger than a half full width at half maximum of the received light spot.

3. The sensor in accordance with claim 1, wherein the reception optics is configured such that the received light spot does not have any displacement dependent on the distance of the object from the sensor.

4. The sensor in accordance with claim 1, wherein the optical metaelement has at least one metalens.

5. The sensor in accordance with claim 1, wherein the optical metaelement has at least one spaceplate.

6. The sensor in accordance with claim 1, wherein the reception optics has at least one refractive and/or diffractive optics.

7. The sensor in accordance with claim 1, wherein the optical metaelement at least partly has the function of the reception optics.

8. The sensor in accordance with claim 1, wherein the sensor has an optical corrective element in the reception path that reduces a dependence of a reception level of the light receiver on the distance of the object from the sensor.

9. The sensor in accordance with claim 8, wherein the optical metaelement at least partly has the function of both the reception optics and the optical corrective element.

10. The sensor in accordance with claim 1, wherein the reception optics images the remitted light beam for all the angles of incidence occurring over a range of the sensor on the light receiver such that an overmodulation of the light receiver is avoided.

11. The sensor in accordance with claim 1, wherein the reception optics images the remitted light beam for all the angles of incidence occurring over a range of the sensor on the light receiver such that a reception level of the light receiver remains constant.

12. The sensor in accordance with claim 1, wherein the metaelement has a first part element for a reduction of a distance dependence of the received light spot position on a distance of the object and a second part element for an at least partial homogenization of a reception level at different distances of the object.

13. The sensor in accordance with claim 1, wherein the transmission optics has a second optical metaelement.

14. The sensor in accordance with claim 13, wherein the optical metaelement and the second optical metaelement are configured as a common metaelement.

15. The sensor in accordance with claim 1, wherein the sensor is configured as a light barrier or as a distance measurement sensor in accordance with the time of light principle.

16. A method of detecting an object in a monitored zone, wherein a light beam is transmitted by a light transmitter, a light beam remitted by the object is imaged on a light receiver by a reception optics arranged offset by a distance from the light transmitter, and a received signal is generated by the light receiver, wherein the reception optics has at least one optical metaelement having a metasurface and/or a metamaterial and the reception optics is configured such that a displacement of the received light spot on the light receiver dependent on a distance of the object from the sensor is reduced such that the displacement in a near zone of the sensor is no larger than a full width at half maximum of the received light spot.

Description

[0034] The invention will be explained in more detail in the following also with respect to further features and advantages by way of example with reference to embodiments and to the enclosed drawing. The Figures of the drawing show in:

[0035] FIG. 1 a block diagram of an optoelectronic sensor;

[0036] FIG. 2 a schematic diagram to explain the distance dependence on the received light spot position and size in an optoelectronic sensor having pupil division;

[0037] FIG. 3 a schematic representation of the transmission and reception paths of an optical sensor in accordance with the invention with an optical metaelement;

[0038] FIG. 4 a schematic representation of the transmission and reception paths of an optical sensor in accordance with the invention with two part elements an optical metaelement arranged behind one another in the light beam direction; and

[0039] FIG. 5 a schematic representation of the transmission and reception paths of an optical sensor in accordance with the invention with two part elements of an optical metaelement arranged next to one another in the light beam direction.

[0040] FIG. 1 shows a schematic sectional representation of an optoelectronic sensor 10 having pupil division. A light transmitter 12 transmits a light beam 16 into a monitored zone 18 via a beam-shaping transmission optics 14. If the light beam 16 is incident onto an object in the monitored zone 18, a portion thereof moves back to the sensor 10 as a remitted light beam 20. The remitted light beam 20 is guided by a reception optics 22 onto a light receiver 24 that generates an electrical received signal therefrom. The transmission optics 14 and reception optics 22 are shown purely schematically as boxes to initially leave their specific structure open. In accordance with the invention, they are at least partly implemented by an optical metaelement, as will be explained later.

[0041] The position of incidence of the remitted light beam 20 or of the received light spot generated thereby on the light receiver 24 depends, due to the spacing d between the transmission optics 14 and the reception optics 22, on the distance of the scanned object at which the remitted light beam 20 is reflected back. The offset of the transmission optics 14 and the reception optics 22 can in particular have the result that the received light spot from a far object is registered on the light receiver 24 and that from a near object is not incident on the light receiver 24, as will be explained more exhaustively below. A control and evaluation unit 28 is connected to the light transmitter 12 and to the light receiver 24 to determine a presence and/or a distance of an object in the monitored zone 18, for example, from the electrical received signals.

[0042] The light receiver 24 in FIG. 1 can be a one-dimensional light receiver having a single light reception element, a receiver array having light reception elements or pixels arranged in a row or a matrix of light reception elements or pixels.

[0043] FIG. 2 shows a schematic diagram of the distance dependence on the received light spot position and size in an optoelectronic sensor having pupil division. For this purpose, the optical axis 32 at the transmission side and the optical axis 34 at the reception side are additionally drawn. The transmission optics 14 and the reception optics 22 are here initially still shown as refractive lenses to illustrate different distance-dependent interference effects that are then improved in accordance with the invention by the already addressed optical metaelement still to be explained in more detail.

[0044] The system sensitivity of an optoelectronic sensor 10 having pupil division initially depends on various properties of the sensor 10 itself such as the light transmitter 14, focal lengths of the transmission optics 14 and reception optics 22 or on the distance d between the light transmitter 14 and the light receiver 24. It furthermore varies with the distance of the respective object. An object is shown at three different distances by way of example in FIG. 2, an object 36a in a far zone, an object 36b in an intermediate zone, and an object 36c in a near zone. What this means in absolute distances can differ greatly and is determined by the design of the sensor 10 and the distance measurement zone to be covered or the range. The remitted light beam 20a of the object 36a in the far zone, the remitted light beam 20b of the object 36b in the intermediate zone, and the remitted light beam 20c of the object 36c in the near zone are incident on the light receiver 24 at respectively different positions or even miss it altogether as in the case of the light beam 20c that was remitted by the object 36c in the near zone. The position dependence or displacement results from the spacing d between the light transmitter 14 and the light receiver 24 and can be used for the distance determination in triangulation sensors. In the present case, however, this displacement has the result that the object 36c can no longer be detected by the light receiver 24 in the near zone since the received light spot is not incident on the light receiver 24 or its active surface. Restricted sensitivity can also occur in the intermediate zone if the received light spot is not incident on the light receiver 24 or its active surface.

[0045] Intensity profiles 38a, 38b, 38c of the received light spot are additionally shown along the displacement direction in a plane 40 defined by the light receiver 24.

[0046] The reception optics 22 is configured here such that with a far object 36a or with a remitted light beam 20a from infinity, a substantially sharp image is achieved on the light receiver 24. The received light spot for the object 36a in the far zone is accordingly diffraction limited and in particular almost punctiform. In accordance with the imaging equation 1/f=1/g+1/b, where f is the focal length, g the object width, and b the image distance, the images become blurred with a decreasing distance and the intensity profiles 38b, 38c of the received light spots become very large for the remitted light beam 20b from an object 36b from the intermediate zone or even the remitted light beam 20c from an object 36c from the near zone. In the intermediate and near zones, the intensity of the received light additionally increases, indicated schematically here by the larger surface area of the intensity profiles 38a, 38c in comparison with the intensity profile 38a, of the received light spot of the object 38a from the far zone.

[0047] FIG. 3 shows a schematic representation of the transmission and reception paths of an optical sensor in accordance with the invention having an optical metaelement 22 in the reception path that corrects the position of the received light spot. The optical metaelement 22 has the reference numeral of the reception optics of FIGS. 1 to 2 that it forms, replaces, or supplements in dependence on the embodiment. It preferably simultaneously satisfies said corrective functions. The optical metaelement or a further optical metaelement can equally form, replace or supplement the transmission optics 14 and is therefore provided with its reference numeral in its lower part. The design of the optical metaelement can be selected such that the light transmitter 12 and the light receiver 24 are arranged on a common expansion card.

[0048] The optical metaelement 22 has a metasurface 22a and is in particular structured as a metalens (flat optics). It is alternatively or additionally conceivable that the body or carrier of the optical metaelement 22 has a metamaterial, in particular that the optical metaelement 22 is a spaceplate.

[0049] Conventional optical components such as lenses, waveplates, or holograms are based on light propagation over distances that are much larger than the wavelength of the light beam 16 22 to shape wavefronts. In this way, substantial changes of the amplitude, phase, or polarization of light waves are gradually accumulated along the optical path. A metasurface 22a in contrast has structures that can be understood as miniature anisotropic light scatterers, waveguides or resonators, or optical antennas (arranged perpendicular). These structures have dimensions and distances in the nanometer range, much smaller than the wavelength of the light beam 16, 22. The metasurface 22a thereby shapes optical wavefronts in accordance with the Huygens principle into any desired shapes having sub-wavelength resolution in that the nanostructures introduce spatial variations in the optical response of the light scatterers. Effects of a conventional lens can thus be modeled, but also functionalities of other optical components such as beam splitters, polarizers, or diffraction grids. The special feature is the high flexibility of reaching a desired starting wavefront and thus the most varied optical effects through adapted nanostructures. Depending on the wavelength range, materials having a suitable transmission behavior are used, for example titanium dioxide, silicon nitride or gallium phosphide in the visible spectral range and aluminum nitride in the ultraviolet spectral range, and chalcogenide alloys in the medium and silicon in the longwave infrared range.

[0050] These considerations on a metasurface can be transferred to a metamaterial in which the interior or the carrier has corresponding nanostructures, with it being able to be combined with a metasurface. Spaceplates can thus in particular be implemented that effectively compress a light path on a smaller space for which purpose reference is again additionally made to the paper of Reshef et al. cited in the introduction. The optical metaelement 22 can consequently have a metastructure or nanostructure in the interior and/or on the front side and/or rear side.

[0051] The properties of the optical metaelement 22 are now preferably selected such that the focal length becomes a function of the angle of incidence. For this purpose, the metasurface 22a is correspondingly structured and/or a metamaterial is selected for the optical metaelement, is in particular configured as a spaceplate. The desired focal length dependence on the angle of incidence can be easily implemented with a spaceplate, for example by silicon layers or silicon oxide layers as an anisotropic element for focal length variation dependent on the angle of incidence.

[0052] The angle of incidence corresponds to the object distance, as previously shown. It may be a further design demand that an imaging in the receiver plane of the light receiver 24 takes place for all the object distances such that the light receiver is not overmodulated or the reception level is as constant as possible. The result in the embodiment shown is a varying received light spot size on the light receiver 24 in dependence on the object distance. The intensity of the light beam remitted by the object on the light receiver can thereby be set such that an overmodulation of the light receiver is avoided. In the embodiment, this means that the diameter of the received light spot is larger for light beams remitted from the near zone than for light beams remitted from the far zone.

[0053] FIG. 4 shows a schematic representation similar to FIG. 3 in which the optical metaelement 22 is divided into two part elements 22.sub.1 and 22.sub.2 arranged behind one another in the light beam direction. Both part elements can comprise metamaterial and/or a metasurface at the front side and/or rear side. The imaging and corrective functions can thus be distributed. In this example, the diameter of the received light spot is created by the first part element 22.sub.1 and the position of the received light spot is created by the second part element 22.sub.2 with a spatially resolved transmission function. The different functions of the optical metaelement 22, whether as a metamaterial and/or metasurface, in particular a focal length adjustment dependent on the angle of incidence, can be split over two or more part elements in this way or a comparable way. In this respect, functions can be separated and respectively assigned to a part element and/or the part elements can complement one another in satisfying the same function. The first part element 22.sub.1 and the second part element 22.sub.2 can, for example, be arranged on oppositely disposed sides of a common substrate 42. To simplify production processes, the substrate 42 can be built up with a typical thickness of 2 mm to 10 mm by adhering two or more layers onto one another. Alternatively, the metaelement 22 can also be formed as a surface structured volume body or as a surface structured plate.

[0054] FIG. 5 shows a schematic representation of the transmission and reception paths of a further embodiment of an optical sensor in accordance with the invention in which the optical metaelement 22 is divided into two part elements 22.sub.3 and 22.sub.4 arranged next to one another in the light beam direction. Both part elements can, as in the embodiments already shown, comprise metamaterial and/or a metasurface at the front side and/or rear side. In this example, the first part element 22.sub.3 acts on light beams that were remitted by objects in the near zone; the second part element 22.sub.4 acts on light beams that were remitted by objects in the far zone. The first part element 22.sub.3 is advantageously arranged closer to the optical axis 32 at the transmission side than the second part element 22.sub.4. This in particular avoids an external migration of the received light spot in the near zone and very steep angles of incidence of the light beam 20c on the light receiver 24. The intensity of the light remitted by the objects 36a, 36c arriving on the light receiver 24 can be influenced via the apertures of the part elements 22.sub.3, 22.sub.4. Since quadratically more light power is incident on the aperture the near zone corresponding to the smaller distance, said aperture can be selected as correspondingly smaller in the near zone than in the far zone so that the intensity is homogenized on the light receiver and thus the reception level.