Ultra-compact optical system for 3D imaging

Abstract

The present invention relates to an optical system (1) for three-dimensional imaging, the optical system (1) comprising at least the following components: - an input aperture (2) having an optical axis (OA), a first metalens array (3) and a second metalens array (4); - detector (5), Equipped with the input aperture (2) is configured to collimate an object beam (10) coming from an investigation object (S) in a first spectral range and fix it in a predetermined polarization state, wherein the object beam (10) propagates into the first metalens array (3) at a first tilt angle (β) with respect to an optical axis (OA) of the input aperture (2); the first metalens array (3) is constructed and arranged to focus a first portion (101) of the object beam (10) and leave a second portion (102) of the object beam (10) unchanged; The second metalens array (4) is constructed and arranged to collimate the focused first portion (101) and transmit the second portion (102) unchanged, such that the first portion (101) and the second portion (102) subtend a second tilt angle (β') with each other relative to their respective propagation directions and form interference fringes upon impinging on the detector (5), wherein the second tilt angle (β') corresponds to twice the magnitude of the first tilt angle (β), of the optical system (1).

Description

[Technical Field]

[0001] The invention relates to an optical system for three-dimensional imaging as claimed in claim 1. [Background technology]

[0002] Three-dimensional optical imaging systems are known from the state of the art for obtaining information about the three-dimensional position, surface structure, or composition of an object by evaluating holographic interference patterns. These holographic systems often work with a scanning beam (the so-called object beam) and a reference beam (the so-called reference beam), which are combined in a holographic unit, and the temporal and spatial coherence of the light generates an interference pattern on the detector. Through various evaluation means, conclusions can be drawn about the wavefront of the investigated object scanned by the object beam, and thus about its three-dimensional information.

[0003] However, these holographic systems suffer from various drawbacks: they often require moving parts and a relatively large installation space, have relatively poor spatial resolution, or are complex and costly to manufacture.

[0004] Nevertheless, these systems are of great importance, especially in minimally invasive medicine. Now, 3D imaging applications are also appearing in the mobile phone sector, although here completely different systems are used that are not based on holographic principles. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] [1]Jangwoon Sung et al, “Progresses in the practical metasurface for holography and lens,” Nanophotonics 2019,8(10),p.1701-1718 [Non-patent document 2] [2]Ling Li,et al.,“Polarization-Switchable Multi-Focal Noninterleaved Metalenses in the visible,”Laser&Photonics reviews,2021,15,2100198 Summary of the Invention [Problem to be solved by the invention]

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a three-dimensional imaging system that overcomes the aforementioned drawbacks. [Means for solving the problem]

[0007] The problem of the present invention is solved by a system according to claim 1.

[0008] Advantageous embodiments of the invention are provided in the dependent claims and described below.

[0009] This includes at least the following components: an input aperture having an optical axis; a first metalens array and a second metalens array; a detector; An optical system for three-dimensional imaging is provided, comprising: In particular, when the input aperture comprises a lens and the object beam emanates from a focal plane of the input aperture, the input aperture is configured to collimate object beam coming from an object under investigation in a first spectral range and fix the light to a predetermined polarization state comprised of two mutually conjugate polarization states, wherein the object beam propagates through the first metalens array at a first tilt angle β with respect to the optical axis of the input aperture, and wherein the first metalens array is configured to focus a first portion of the object beam having a first polarization state of the two mutually conjugate polarization states and fix the light to a predetermined polarization state comprised of a second polarization state of the two mutually conjugate polarization states. a second metalens array configured and arranged to collimate the focused first portion and transmit (transmit) the second portion unchanged, particularly with its collimation unchanged, and a second metalens array configured and arranged behind the first metalens array in the propagation direction of the object beam to collimate the focused first portion and transmit (transmit) the second portion unchanged, particularly with its collimation unchanged, such that the first and second portions each have the same wavefront curvature, particularly collimated, after propagating through the first and second metalens arrays, and are tilted relative to each other at a second tilt angle β'=2 * β, forming an interference pattern that strikes the detector, where the magnitude of the second tilt angle is twice the first tilt angle, and based on the interference pattern, three-dimensional position information of the object region under investigation can be generated.

[0010] By using metalenses, the system of the present invention provides a holographic imaging system in which the aforementioned problems are solved.

[0011] Metalenses or metalens arrays can be fabricated in planar structures without the complex grinding processes required to produce the radius of curvature of conventional lenses. But more importantly, metalenses can be fabricated to perform different optical operations or exhibit different properties depending on the polarization, wavelength, or angle of light [1].

[0012] In this context, polarization is particularly important due to its control over wavelength. Ling Li et al. [2] describe how individual metalenses can change their focal length depending on the polarization. Depending on the design, metalenses can even operate over a broad spectral range without changing their focusing properties. This is fundamentally different from diffractive structures such as optical gratings or holograms, which inherently diffract light in a wavelength-dependent manner. The properties of metalenses are made possible, among other things, by their structures being smaller than the optical wavelength, significantly smaller than the 2-5 μm pixel size of a typical camera. In summary, the current state of the art describes that metalenses can be designed with numerical apertures > 0.5 and optical bandwidths exceeding 100 nm for different color bands in the red, green, and blue (RGB) color range.

[0013] The input aperture may include, among other things, an optical component, such as a lens or multiple lenses arranged in a lens arrangement, e.g., an objective lens. Alternatively, the input aperture may not include a lens, but may only include a pinhole aperture that images the object light entering the system from the object under study in a sufficiently collimated manner, provided that the object under study has a sufficiently long distance from the pinhole aperture, e.g., in the range of about 100 mm to 500 mm.

[0014] Furthermore, the input aperture comprises an optical element configured to impose a predetermined polarization state on the object beam arriving from the object under investigation. Such an optical element can be, for example, a polarizer.

[0015] In the context of this specification, the term "input aperture" refers specifically to the region in front of the first metalens array, i.e., the input aperture does not necessarily refer to only one opening in the system, but the term "input aperture" can extend to all optical components and elements of the system that are positioned in front of the first metalens array in the direction of propagation of the object beam.

[0016] The term "metalens array" particularly refers to an arrangement of multiple metalenses, where the optical axes of each of the multiple metalenses are essentially parallel to one another, and the optical axes of the individual metalenses are particularly aligned parallel to the optical axis of the input aperture.

[0017] The dimensions, such as the diameter, of the individual metalenses are known in the art and can be, in particular, a few millimeters, for example, in the range of 0.2 millimeters to 10 millimeters.

[0018] In particular, each metalens in a metalens array can have at least one focal length associated with it, which focal length depends, for example, on the polarization state of the incident object light.

[0019] Furthermore, the focal length of the metalens, and therefore the focal length associated with the first and / or second metalens arrays, may also be wavelength dependent.

[0020] According to embodiments of the present invention, all of the metalenses in the first metalens array have the same focal length.

[0021] According to another embodiment of the present invention, all of the metalenses in the second metalens array have the same focal length.

[0022] According to another embodiment of the present invention, the average focal lengths of the first and second metalens arrays are the same.

[0023] The term "average focal length" particularly refers to the focal length associated with each metalens array, which in particular results from the average of all of the focal lengths of the metalens arranged in the array, and in particular the average focal length to which this average corresponds.

[0024] According to one embodiment of the present invention, the detector comprises a camera with a plurality of light-sensitive pixels configured to record object light from at least a first spectral range, in this way an interference pattern can be recorded by the detector.

[0025] The recorded interference pattern can then be transmitted in the form of data to an evaluation unit associated with or provided by the system, which generates from the transmitted data three-dimensional information or a three-dimensional representation of at least the object under investigation or a region thereof.

[0026] In the context of this specification, the term "collimated" and related terms should be interpreted to mean, in particular, that light has only minimal wavefront curvature for at least one wavelength, particularly for one wavelength or spectral range. As the wavelength shifts, the convergence or divergence of the light rays or wave field typically increases with wavelength. These chromatically induced deviations from ideal collimation are also encompassed by the term "collimated" in the context of the present invention. Furthermore, the term "collimated" also includes deviations due to adjustment and system tolerances.

[0027] In particular, if the system does not have a lens at the input aperture, the description in the previous paragraph applies as will be understood by those skilled in the art: collimation does not need to be perfect, i.e., the light may be imaged into the system slightly diverging or converging.

[0028] In particular, it should be pointed out here that the system is also designed to image and / or process non-collimated object light on the detector in a similar manner, and in particular that the system uses non-collimated light rays to generate depth information, i.e., three-dimensional information, about the object under investigation. In particular, the description of the invention based on collimated light rays serves only to clearly disclose the location and function of the system's components relative to one another, and in particular does not exclude the recording and / or processing of light rays that are not collimated by the input aperture.

[0029] In particular, the term "collimate" and related terms are used in the context of the present invention in the range 0°<ξ≦2 * β maxβ max is the maximum first tilt angle that can be imaged or recorded by the system.

[0030] Alternatively, or in addition, the term "collimated" and related terms in the context of this specification can also be understood to mean that the diameter of the light beam (understood in this context as a ray bundle) has the smallest divergence angle at its narrowest point. This image for the definition of a collimated light beam applies in the field of wave optics description of laser beams and can be applied in a similar way to systems. This definition applies particularly to input apertures with at least one lens.

[0031] According to the present invention, the first tilt angle can be measured relative to the direction of propagation and the optical axis of the input aperture.

[0032] In terms of the wave representation of the object beam, this means that the plane wavefront of the collimated object beam after the input aperture has an angle of 90°+β with respect to the optical axis of the input aperture.

[0033] The predetermined polarization state of the object beam after the input aperture consists of two mutually conjugate polarization states, which can in particular be two linearly polarized polarization states, in particular p-polarization and s-polarization states.

[0034] Alternatively, the two polarization states can refer to a right-handed circular polarization state and a left-handed circular polarization state.

[0035] Note that a given polarization state is specifically a superposition of these two conjugate polarization states.

[0036] When object light having a given polarization state strikes the first metalens array, the optical properties of the first metalens array cause the object light having a first of two mutually conjugate polarization states to be focused, specifically onto a focal plane associated with the first metalens array. In contrast, the optical properties of the first metalens array allow the object light having a second of the two mutually conjugate polarization states to be transmitted essentially unchanged. In other words, the first metalens array does not result in an increase in the convergence or divergence of the object light having the second polarization state, and behaves like an optically transparent medium with essentially no diffractive properties, i.e., like a neutral optical medium, such as a homogeneous glass plate.

[0037] As a result, the object beam impinging on the first metalens array is split into a first portion consisting of the object beam in the first polarization state and a second portion consisting of the object beam in the second polarization state.

[0038] Ideally, this splitting ratio is approximately 1:1, i.e. the object beam is split into two equal intensity parts.

[0039] The two portions of the object beam then impinge on a second metalens array, the second metalens array having the same or at least similar optical properties as the first metalens array.

[0040] In particular, the second metalens array is positioned to re-collimate the first portion and allow the already collimated second portion to be transmitted essentially unchanged, and again the second metalens array behaves as an essentially transparent neutral optical medium with respect to the second portion, as previously described for the first metalens array.

[0041] In particular, the system is designed, e.g., by corresponding optics, such that the angle at which the first portion strikes the first metalens array is reversed compared to the first tilt angle as it propagates through the second metalens array. In this manner, the first and second portions mutually surround a second tilt angle, where the second tilt angle can be measured relative to the direction of propagation of the first and second portions, or alternatively but equivalently, can be measured as the angle between the wavefronts of the first and second portions.

[0042] In the present design, the second tilt angle is twice the first tilt angle, which is particularly true when the focal lengths associated with the first and second metalens arrays are the same.

[0043] The first and second portions are superimposed behind the second metalens array such that an interference pattern is formed on the detector, and based on the interference pattern and its analysis, three-dimensional information about the object region under investigation can be generated.

[0044] The object region comprises one or more illumination regions with the object beam, the illumination region of the investigated object being essentially circular, in particular with a diameter in the range of 1 mm to 50 mm.

[0045] "Focusing" or related terms refer specifically to the setting of wavefront curvature, which causes the ray bundle of associated light rays or light waves to converge, i.e., image to a location in space with a minimum ray diameter (focus).

[0046] Complete three-dimensional information about the study object can be generated, for example, by optical scanning, in particular by relative displacement of the system with respect to the study object. Additionally or alternatively, multiple object regions of the study object can be simultaneously imaged, recorded and analyzed by the system.

[0047] It should be noted that the object light reflected from the object under investigation that is located far from the input aperture is not collimated by the input aperture but has a different wavefront curvature, resulting in diverging or converging object light.

[0048] This situation is common for input apertures that only comprise a pinhole aperture instead of a lens, since there is no focal length associated with the pinhole aperture. However, if the object beam enters the pinhole aperture from a sufficiently long distance, the pinhole aperture induces a sufficiently high degree of collimation or a sufficiently low divergence of the object beam, and is therefore considered collimated in the context of the present invention.

[0049] However, even if the object light is not collimated within the scope of the definition in this specification, this light can be imaged and recorded by the system according to physical principles and included in the corresponding evaluation in order to obtain three-dimensional information about the object region of the object under investigation (e.g., the surface of the object under investigation).

[0050] The second portion of the object beam will in any case be propagated unaltered (collimated or uncollimated) by the first and second metalens arrays, in accordance with the principles described above, with the first portion focused by the first metalens array exhibiting similar behavior in that it does not lie on a focal plane associated with the first metalens array but falls in front of or behind it, and the second metalens array responding to the wavefront curvature to produce a first portion with a corresponding change in wavefront curvature.

[0051] These originally uncollimated first and second portions also result in interference patterns at the detector, which can be evaluated accordingly to obtain three-dimensional information about the object region.

[0052] According to a further embodiment of the invention, it is envisaged that the first and second portions of the object beam are linearly polarised perpendicular to each other, in particular s-polarised and p-polarised, and in particular the predetermined polarization state is a linear polarization state consisting of a superposition of the first and second portions.

[0053] Linearly polarized light is relatively easy to generate. Furthermore, the polarization state for that polarization can be relatively easily determined if the polarization is linear along one direction. In contrast, it can be more difficult to distinguish between circular and elliptically polarized states. The optical elements are configured to split or separately manipulate the linearly polarized light that is conjugate with one another, such that the linear polarization of the first and second portions can be advantageous.

[0054] According to further embodiments of the present invention, the system comprises a polarization-dependent beamsplitter, in particular a polarization-dependent beamsplitter cube, between the first metalens array and the second metalens array, wherein the system also comprises the following components: a first mirror, in particular a planar first mirror; a reflector array including a plurality of reflective retroreflectors; a first λ / 4 element arranged between the polarization dependent beam splitter and the first mirror; a second λ / 4 element positioned between the polarization dependent beam splitter and the reflector array; Also equipped, Here, the polarization-dependent beamsplitter is positioned such that, for object light incident from the first metalens array, a first portion is reflected by the beamsplitter and a second portion is transmitted through the beamsplitter; the reflector array is positioned to back-reflect the first portion towards the beamsplitter to the second metalens array; and the first mirror is positioned to back-reflect the second portion towards the beamsplitter to the second metalens array; in particular, the back-reflected first portion and the back-reflected second portion propagate through the polarization-dependent beamsplitter towards the second metalens array with polarization states reversed by their respective λ / 4 elements; and in particular, the first portion and the second portion each propagate twice through either the first or second λ / 4 element such that the first portion and the second portion have reversed polarization states after passing through their respective λ / 4 elements twice.

[0055] In other words, for object light incident from the first metalens array, the polarization-dependent beam splitter is positioned such that a first portion is reflected by the beam splitter and a second portion is transmitted by the beam splitter, and the reflector array is positioned on a side of the beam splitter along which the first portion coming from the first metalens array and reflected by the beam splitter propagates, where the first portion striking the reflector array is reflected back towards the beam splitter to the second metalens array, where the first mirror is positioned on the side of the beamsplitter opposite the first metalens array, i.e., the side of the beamsplitter along which the second portion coming from the first metalens array and transmitted by the beamsplitter propagates, where the first mirror reflects the second portion striking the first mirror back towards the beamsplitter to the second metalens array, and in particular the reversed polarization state causes the back-reflected first and back-reflected second portions to propagate through the polarization-dependent beamsplitter in the direction of the second metalens array.

[0056] The polarization dependent beam splitter is specifically configured to reflect one of the two portions of the object beam and transmit the other.

[0057] For example, a λ / 4 plate can be used as a λ / 4 element. In other words, it is an optical retardation element with different refractive indices for different polarization directions. This can change the polarization state of the object beam. According to the present invention, it is provided that after two passes of the first and / or second portions, each portion assumes its conjugate polarization state. For example, two passes through the λ / 4 element converts the s-polarization state to the p-polarization state, and vice versa.

[0058] As a result, any portion of the light that is reflected back from the polarization-dependent beamsplitter propagates towards the second metalens array.

[0059] It should be noted that with this configuration, the second metalens array should be designed to collimate the focused first portion in terms of its focusing properties as a function of the polarization of the object beam. In other words, unless a further optical element is placed before the second metalens array to convert the polarization states of the first and second portions back to the original polarization states imposed after the input aperture, the focusing properties of the second metalens array should be towards their respective conjugate polarization states compared to the first metalens array.

[0060] According to another embodiment, the number of retroreflectors included in the reflector array is the same as the number of metalenses in each case of the first and second metalens arrays.

[0061] According to further embodiments of the present invention, a first λ / 2 element is disposed between the polarization dependent beamsplitter and the second metalens array, and is configured to invert the polarization states of the first and second portions, in particular, configured so that the polarization states of the first and second portions again correspond to the polarization states of the first and second portions after the input aperture.

[0062] In particular, the first λ / 2 element is positioned so as to only pass in the propagation direction of the first and second portions after the first and second portions have propagated twice through the beam splitter or the beam splitter surface of the beam splitter.

[0063] This embodiment may use a second metalens array that is identical to the first metalens array, with the second metalens array having the same properties with respect to polarization states as the first metalens array, although the second metalens array does not necessarily have the opposite properties with respect to polarization states as described in the previous paragraph.

[0064] In particular, the first λ / 2 element is a λ / 2 plate.

[0065] This allows for cost-effective and simplified production of the system.

[0066] According to a further embodiment of the invention, it is provided that the system comprises an actuator arrangement configured to set the position of the first mirror and / or reflector array, thereby making it possible to set the phase between the wavefronts associated with the first and second portions.

[0067] This design allows the relative phase position between the first and second portions to be set, thereby avoiding in particular uniform light areas on the detector. In particular, the actuator arrangement is configured to set the position of the first mirror and / or reflector array to allow a phase shift of more than 2π. This has particular advantages for the color resolution of the system.

[0068] The actuator arrangement should be configured to shift the position of the first mirror and / or reflector array in proportion to the wavelengths of the first spectral range, in particular along the optical axis. It can be particularly advantageous for adjustment purposes if the actuator arrangement is configured to tilt the first mirror and / or reflector array with respect to the optical axis of the input aperture.

[0069] Furthermore, it may be advantageous for the actuator arrangement to be configured to adjust the position of the reflector array perpendicular to the optical axis of the input aperture.

[0070] The actuator arrangement serves in particular to lengthen or shorten the optical path length of the first and / or second part of the object beam.

[0071] Actuator placement can be controlled via an external control unit associated with the system.

[0072] According to one embodiment, the actuator arrangement comprises at least one piezo element, in particular the actuator arrangement comprises at least one ring piezo arrangement.

[0073] Although the piezoelectric element is technically a moving part, due to its relatively monolithic design there is no particular risk of wear from exposed precision mechanisms, and the system can be considered to have very few moving parts, despite the piezoelectric element.

[0074] The use of piezo elements contributes particularly to the robustness of the system. Furthermore, piezo actuators can be controlled and set particularly accurately and precisely.

[0075] According to another embodiment in which a beam splitter is not required, the system comprises a transparent solid element having a first surface facing towards the first metalens array and a second surface opposite the first surface facing towards the second metalens array, and in particular the volume comprised by the transparent solid element is free of selectively reflecting and selectively diffracting surfaces, and in particular the transparent element is cuboid or plate shaped.

[0076] This embodiment may be advantageous when a particularly compact design is desired along the installation direction, for example along the optical axis of the input aperture.

[0077] The solid transparent element can be a simple glass plate or a simple polymer plate that is transparent in the first spectral range.

[0078] According to an embodiment of the present invention, the solid transparent element is made of a material selected from the group consisting of glass, polymer or crystal.

[0079] In one embodiment involving a beamsplitter, the metalens arrays surround a 90° angle from one another, however, in this embodiment, the metalens arrays are positioned diametrically opposite one another and surround a solid transparent portion between them.

[0080] According to a further embodiment of the invention, it is provided that the system comprises at least one liquid crystal configured to adjust the phase between the wavefronts of the object beam associated with the first and second portions, in particular the at least one liquid crystal configured to change the phase between the wavefronts of the first and second portions via the control module.

[0081] For example, the liquid crystal may be disposed along the optical axis of the input aperture between the first and second metalens arrays in addition to the solid transparent element. Alternatively, the solid transparent element may comprise or consist of the liquid crystal. In the latter embodiment, the liquid crystal should have birefringent properties.

[0082] In the embodiment with a beam splitter, the liquid crystal may be placed on one of the sides with the first mirror or reflector array.

[0083] In another embodiment, when the beam splitter is a beam splitter cube, one of the triangular prisms that make up the beam splitter may comprise or consist of a liquid crystal.

[0084] Alternatively, both triangular prisms that make up the beam splitter may also contain liquid crystals. In this way, the phase of both the first and second portions of the object beam can be set independently relative to the other portion. Using two liquid crystals also allows for a longer overall optical path, which allows for a larger phase shift between the first and second portions.

[0085] Using liquid crystals to set the relative phase of the first and second parts to each other makes it possible to produce a system that requires no moving parts and is therefore very robust.

[0086] According to an embodiment of the invention already described in the previous paragraph, it is provided that the transparent solid element comprises or consists of at least one liquid crystal.

[0087] According to an embodiment of the invention already described in the previous paragraph, there is provided a polarization-dependent beam splitter cube according to at least one of the previous embodiments, wherein the polarization-dependent beam splitter comprises a first and a second prism forming the beam splitter cube of the beam splitter, wherein the first and / or the second prism comprises at least one liquid crystal, in particular both the first prism and the second prism comprise a liquid crystal.

[0088] According to one embodiment of the invention, the focal plane of the first metalens array and the focal plane of the second metalens array are above each other.

[0089] According to one embodiment of the present invention, there is provided an analyzer positioned behind the second metalens array and in front of the detector in the direction of propagation, the analyzer being configured to change the polarization states of the first and second portions, such that interference between the first and second portions is achieved on the detector.

[0090] This embodiment allows for a higher interference contrast on the detector.

[0091] According to one embodiment of the present invention, it is provided that the input aperture comprises a polarizer configured to direct the object beam from a first spectral range into a predetermined polarization state.

[0092] According to an embodiment of the present invention, it is provided that the input aperture comprises at least one lens configured to collimate the object beam.

[0093] According to another embodiment of the invention, it is provided that the input aperture comprises a pinhole aperture as imaging element, in particular the input aperture does not include a lens or a refractive optical element for collimating the object beam originating from the object under investigation.

[0094] According to one embodiment of the present invention, it is provided that the system is configured to wavelength-dependently deflect the propagation direction of object light from a first spectral range impinging on the system, whereby the object light and the first and second portions enclose a wavelength-dependent angle with the optical axis in addition to the first tilt angle.

[0095] This can be achieved, for example, by a corresponding metalens design in the first metalens array.

[0096] This embodiment allows for improved color resolution of the system because the object light in the first spectral range is imaged onto different areas of the detector depending on wavelength.

[0097] According to one embodiment of the present invention, it is provided that the object light in the first spectral range consists of two or more separate wavelength ranges and / or the system is configured to filter the object light into two or more separate wavelength regions forming the first spectral range, with gaps between the wavelength ranges, in particular each of these gaps being at least 50 nm wide, whereby for each wavelength range an interference pattern is generated on the detector from which three-dimensional position information and color composition can be generated for the wavelength range of the object region of the investigated object.

[0098] In particular, the first spectral range is divided into three primary colors, red, green and blue, which can be translated, for example, into the following wavelength ranges: The wavelength range of the first spectral range of the blue color channel extends in particular from 420 nm to 480 nm, the wavelength range of the first spectral range of the green color channel extends in particular from 520 nm to 565 nm, and the wavelength range of the first spectral range of the red color channel extends in particular from 630 nm to 680 nm.

[0099] According to one embodiment of the present invention, it is provided that the object beam includes at least one further spectral range that is different from and separate from the first spectral range, and wherein the first and second metalens arrays and the polarization-dependent beam splitter are transparent and optically inert, i.e., neutral, to light from the at least one further spectral range, and wherein the polarization-dependent beam splitter further includes a VPH (Volume Phase Hologram), which is configured to diffract light from the at least one further spectral range as a function of polarization and angle, and to be transparent and optically inert, i.e., neutral, to light from the first spectral range.

[0100] The first spectral range and the at least one further spectral range can occupy different wavelength ranges along the spectrum. In particular, as with the previous embodiment, the further spectral range can be limited, for example, to a spectral range between 570 nm and 620 nm between the green and red color channels. Alternatively and / or additionally, the at least one further spectral range can extend into the near-infrared range, i.e., from 700 nm or 800 nm or higher, to the infrared range beyond 1300 nm. The VPH is arranged in a Littrow configuration and specifically designed for wavelengths in the further spectral range, so that incident object light from the further spectral range (also referred to as the second spectral range in the context of this specification) propagating along the optical axis of the input aperture and impinging on the VPH is diffracted at an angle of 90° toward the detector. In particular, incident object light from the second spectral range is s-polarized upon impinging on the VPH.

[0101] In this embodiment, the reference beam coupled into the beam splitter via the reference arm causes interference with the object beam coupled via the so-called object arm onto the detector.

[0102] According to one embodiment of the present invention, the system is therefore configured to direct reference light from the second spectral range, in particular via the reference arm, through the side of the beam splitter opposite the input aperture to the VPH, wherein the first mirror is particularly transparent to the reference light, i.e. particularly to light from the second spectral range.

[0103] According to this embodiment, the reference light, which is also s-polarized when it strikes the VPH, can be collimated from the second spectral range at the side of the beam splitter opposite the input aperture and can also be sent to the VPH in a Littrow configuration (for this purpose, the first mirror must be transparent to the second spectral range). There, the reference light is also diffracted 90° in the opposite direction from the detector. There, it is reflected by a mirror or prism arrangement that surrounds the VPH at an angle greater than or less than 45°, e.g., 45°±0.2°. As a result, the back-reflected reference light is at least partially transmitted by the VPH and interferes with the object light in the second spectral range. This interference pattern can be advantageously used to generate increased spatial resolution along the optical axis of the input aperture. In particular, the further spectral range consists of multiple separated spectral lines with very small linewidths, e.g., on the sub-nanometer order, spanning, e.g., a range of 10 nm to 50 nm. Because the VPH diffracts each of these spectral lines slightly differently, using this type of "optical comb" the resulting interference pattern on the detector yields improved spatial resolution along the optical axis.

[0104] Similar holographic systems are known, but for example have conventional transmission gratings instead of VPHs, but they have disadvantageous properties with respect to the desired properties.

[0105] It is noted that for the functionality of this embodiment, the first mirror is advantageously at least partially transparent with respect to light in the second spectral range.

[0106] Furthermore, it may be advantageous if the second λ / 4 element is optically neutral, ie, does not significantly affect the light.

[0107] Alternatively, a third λ / 4 element can be provided, which is, for example, arranged in front of the first mirror from the perspective of the incident reference light, i.e., inaccessible to the object light, particularly in the first spectral range, and which is configured, together with the second λ / 4 element, to rotate the polarization of the reference light by 90°, in particular so that the reference light advantageously strikes the VPH in an s-polarized state.

[0108] This is illustrated in the following embodiment of the invention, whereby the system comprises a third λ / 4 element, which is positioned on the side of the first mirror facing away from the beam splitter and is configured to operate in conjunction with the second λ / 4 element to put the reference light into a predetermined polarization state, so that the reference light is linearly s-polarized when coming from the first mirror and striking the VPH.

[0109] It would also be advantageous if the polarization-dependent beamsplitter also has no optical effect on light in the second spectral region, i.e., is optically neutral. Further, it would be advantageous if the first and second metalens arrays were optically neutral with respect to light from the second spectral range.

[0110] The reference light in the second spectral range can be provided by a reference light source, such as a laser, which can also function as an object light source. The system can also include a collimation lens for the reference light so that the reference light propagates collimated toward the VPH. If necessary, a polarizer can also be provided to ensure that the reference light impinges on the VPH in an s-polarized state.

[0111] According to a further development of the previous embodiment, it is provided that the system has a wavelength-selective prism arrangement between the reflector array and the beam splitter, the prism arrangement being configured to reflect light of at least one further spectral range, in particular the reference light, diffracted by the VPH in the direction of the reflector array at a prism angle in the direction of the detector, wherein the prism arrangement is transparent and optically inactive to light from the first spectral range.

[0112] As already mentioned, the prism angle is set so that the reflective surface of the prism arrangement forms an angle with the VPH that is not equal to 45°, and here particularly encompasses angles greater than 45.2°.

[0113] This "tilted position" allows the reference beam reflected back from the prism arrangement to be transmitted through the VPH with a sufficiently high power so that enough reference light is available to interfere on the detector with the object beam in the second spectral range. Extensions of such systems can improve the resolution along the optical axis to the submillimeter and even submicrometer range.

[0114] Further features and advantages of the invention will be explained below on the basis of the description of the figures of exemplary embodiments. [Brief explanation of the drawings]

[0115] [Figure 1] FIG. 1 shows a schematic diagram of a first embodiment of the present invention. [Figure 2] FIG. 2 shows a schematic diagram of a second embodiment of the invention with color separation. [Figure 3] FIG. 3 shows a schematic diagram of a third embodiment of the VPH of the present invention. [Figure 4] FIG. 4 shows a schematic diagram of a fourth embodiment of the invention without a beam splitter. DETAILED DESCRIPTION OF THE INVENTION

[0116] 1 illustrates a schematic diagram of a system 1 according to an exemplary embodiment of the present invention. The system 1 is configured for use in holographic imaging applications. In particular, the system 1 is suitable for three-dimensional color imaging.

[0117] A particular advantage of the system shown in Figure 1 is that it requires no moving parts or precision mechanical parts that are prone to failure, thereby allowing for a very compact design.

[0118] 1, system 1 is also referred to as a miniature 3D color module in the present invention, and is configured to capture and optionally display the surface or subsurface area of ​​an object under study S (alternatively referred to herein as an object) in three dimensions.

[0119] For this purpose, the object S is illuminated, for example, in a point-shaped area with an external light source 18, which may be provided by the system 1 or may be arranged separately. The light 10 with which the studied object S is illuminated is also called object light 10 in the context of holography. Natural ambient light may also serve as light source 18 (even though this does not necessarily have to illuminate the object S in a point-shaped area).

[0120] In response to illumination, the object S reflects object light 10 through various processes such as scattering, reflection, or emission. The object light 10 emanating from the object S is collected through an input aperture 2 of the system 1.

[0121] Below and in some parts of the description, it is considered that the object beam S emanating from the object under study S originates from or near a plane E of the object under study S, said plane being in the focal plane E of the input aperture 2. If the object beam 10 emanates from a plane close to the focal plane E of the input aperture 2, the system 1 will behave according to a different curved wavefront of the object beam 10 at the input aperture 2, which is known to those skilled in the art.

[0122] The input aperture 2 is configured to collimate the object beam 10 emanating from the object under investigation S, in particular the object beam 10 emanating from the focal plane E of the input aperture 2. It should be noted that the input aperture 2 may comprise a collimation optical unit 2a, for example in the form of one or more lenses 2a. In the case shown in Fig. 1, the input aperture 2 comprises a collimation lens 2a. However, it is also possible that the collimation optical unit 2a comprises only a pinhole aperture (not shown). Accordingly, if the pinhole aperture is small and / or the object distance is sufficiently long, the incident object beam 10 will also be collimated, or at least have a sufficiently high collimation.

[0123] Regardless of the design of the input aperture 2, collimated light specifically refers to light that has an essentially flat wavefront.

[0124] The input aperture 2 also has an optical axis OA. The optical axis OA in FIG. 1 extends centrally and perpendicularly to the input aperture 2 or to the collimation lens 2a. The collimation lens 2a is configured to collimate at least the object beam 10 from a first spectral range. The collimation lens 2a may also be configured to collimate the object beam 10 from a second spectral range. In this case, the collimation lens 2a may include, for example, an achromatic lens, an apochromatic lens, or a superapochromatic lens.

[0125] In the figures, a representation of light in the form of ray optics is used. This means that the wavefronts and wavefront curvatures in the rays depicted in the figures are generally not depicted. However, a person skilled in the art knows how the wavefront of light is affected by the various optical components of system 1 and how the direction or properties of light propagation are affected as a result.

[0126] In FIG. 1, two different rays 10a, 10b of the object beam 10 are shown. The first situation concerns the first object ray 10a, which originates from a region of the object S under study that is on the optical axis OA of the input aperture 2. The second situation concerns the second object ray 10b, which originates from a region of the object S that is laterally offset with respect to the optical axis OA of the input aperture 2. For example, the term "laterally offset" can be described using a Cartesian coordinate system (x, y, z as shown in FIG. 1) associated with the input aperture 2. The z-axis extends along the optical axis OA of the input aperture 2, while the x- and y-axes extend perpendicular thereto, i.e., transversely to the optical axis OA. The terms "first" and "second" are used only to distinguish them and not to indicate an order.

[0127] The first object beam 10a is collimated by the collimation lens 2a and then propagates parallel to the optical axis OA of the collimation lens 2a, i.e., it subtends an angle of 0° with the optical axis OA of the input aperture 2. The angle subtended by the collimated object beam 10, 10a, 10b and the optical axis OA is also referred to in the context of this specification as the first tilt angle β.

[0128] The first tilt angle β is defined in particular by the angle enclosed between the optical axis OA of the input aperture 2 and the propagation direction of the collimated object rays 10, 10a, 10b.

[0129] The second object ray 10b is also collimated by the collimation lens 2a and then propagates further at a first tilt angle β relative to the optical axis OA of the collimation lens 2a, which is not equal to 0°. According to the laws of ray optics (relative to the input aperture 2), the first tilt angle β contains information about the lateral position on the studied object from which the associated object ray 10b originates.

[0130] A polarizer 15 is arranged behind the collimation lens 2a, which can be optionally set to impart a predetermined polarization state to the object beam 10. In particular, the polarizer 15 is configured for the object beam 10 from at least a first spectral range to impart a predetermined polarization state to the object beam 10.

[0131] It should be noted that the polarizer 15 can also be placed before the collimation lens 2a. The incident object beam 10 is first collimated by the collimation lens 2a which is subordinate to the polarizer 15. It is also conceivable that the collimation lens 2a or the input aperture 2 itself has corresponding polarizing properties.

[0132] The terms "after" and "in front" should be understood to refer in particular to the arrangement relative to the propagation direction of the object beam 10, and to indicate that the geometric order or arrangement is less likely to deviate from a "purely optical" optical path due to convolution by beam splitters or mirrors.

[0133] In this case, polarizer 15 is positioned and configured (e.g., rotated 45°) so that object beam 10 has approximately equal proportions of s-polarized and p-polarized light, regardless of the direction of incidence or first tilt angle β. That is, in this example, the predetermined polarization state is composed of a first polarization state comprising s-polarized object beam and a second polarization state comprising p-polarized object beam. The terms s-polarized light and p-polarized light are considered to be known to those skilled in the art as linearly conjugated polarization directions. The assignment of s-polarized object beam to the first polarization state and p-polarized object beam to the second polarization state can also be done in an opposite manner and is for illustrative purposes only.

[0134] In general, polarizer 15 can be configured to convert the incident object beam 10 into an object beam 10 having two polarization states that are conjugate to each other, which can also be, for example, left- or right-hand circularly polarized object beam 10.

[0135] In what follows, without limiting generality, we will discuss the cases of s-polarization and p-polarization states, which is generally valid insofar as the object light emanating from the object under investigation can also be unpolarized, such that this object light will in any case be "seen" by the first metalens array as first and second portions, even in the absence of polarizer 15, since unpolarized light also contains s-polarized and p-polarized components.

[0136] As just described, object beam 10 having a predetermined polarization state impinges on first metalens array 3 in accordance with the present invention. First metalens array 3 comprises a plurality of metalenses 30 arranged in an array. In accordance with the present invention, first metalens array 3 is arranged and configured to focus a first portion 101 of the object beam having a first polarization state, i.e., to at least increase the wavefront curvature so that the object beam converges, while a second portion 102 of the object beam having a second polarization state propagates through first metalens array 3 essentially unchanged. That is, if the object beam was collimated by input aperture 2, then second portion 102 will remain collimated after propagating through first metalens array 3.

[0137] The term "unaltered" in the context of the metalens arrays 3, 4 particularly refers to the fact that the wavefront or second polarization state of the second portion 102 can ideally pass through the metalens array 3, 4 completely unchanged, such that the wavefront or second polarization state of the second portion 102 is unchanged before and after the metalens array. Nevertheless, it will be clear to those skilled in the art that slight changes in the wavefront may occur due to imperfections in the metalens arrays 3, 4. This is intended to be reflected in the expression "essentially." A synonymous term used herein is the term "optically neutral."

[0138] In particular, the focused first portion 101 is focused so as to be focused at a focal plane 3B associated with the first metalens array 3.

[0139] A polarization-dependent beamsplitter 6 is positioned behind the first metalens array 3, and in particular in front of the focal plane 3B of the first metalens array 3. In the example of FIG. 1 , the polarization-dependent beamsplitter 6 is a polarization-dependent beamsplitter cube. The beamsplitter cube 6 comprises two triangular prisms, e.g., first and second triangular prisms 6A, 6B, which are joined at their base surfaces and define a beamsplitter surface 6F along the base surfaces. The beamsplitter surface 6F extends at an angle of 45° with respect to the input aperture 2 or the optical axis OA of the first metalens array 3.

[0140] The first object ray 10a, which includes the first and second portions, and the second object ray 10b, which also includes the first and second portions 101, 102, now strike the first triangular prism 6A of the beam splitter 6 and propagate therethrough to the beam splitter surface 6F.

[0141] At the beam splitter surface 6F, the object beam 101 of the first and second object beams 10a, 10b in a first polarization state is reflected, while the object beam 102 in a second polarization state is transmitted.

[0142] In the following, a first object ray in the first portion 101 will be considered, which is focused by the first metalens array 3, i.e., has a first polarization state after the first metalens array 3, and is therefore reflected at the beamsplitter surface 6F. This light propagates further within the first triangular prism 6A and then strikes a first λ / 4 plate 9a, which is, for example, disposed on the surface of the beamsplitter cube 6.

[0143] The first λ / 4 plate 9a changes the polarization state of the first portion 101. In this example, the polarization state changes from linear to circular polarization.

[0144] Behind the first λ / 4 plate 9a, a reflector array 8 is positioned perpendicular to the (correspondingly folded) optical axis OA of the input aperture 2. The reflector array 8 comprises a plurality of retroreflectors 80 arranged in an array, which are configured to reflect incident light 101 back in the direction from which it came, largely independent of the direction of incidence of the light. Furthermore, the reflector array 8 is positioned near but offset along or parallel to the focal plane 3B of the first metalens array 3.

[0145] Reflector array 8 includes the same number of retroreflectors 80 as first metalens array 3 includes metalenses 30, and the same number of retroreflectors 80 as second metalens array 4 includes metalenses 40.

[0146] The first portion 101 that is reflected back by the reflector array 8 now propagates again through the first λ / 4 plate 9a, which then changes the polarization state of the first portion 101 so that the polarization state of the first portion 101 now corresponds to the second polarization state of the object beam behind the input aperture 2. In this example, the polarization state of the first portion 101 changes from s-polarized to p-polarized light after the first portion 101 has propagated through the first λ / 4 plate 9a a total of two times.

[0147] First portion 101 then propagates again through first triangular prism 6A and strikes beamsplitter surface 6F, where first portion 101 is now transmitted due to the changed polarization. First portion 101 then propagates further through second triangular prism 6B and then strikes second metalens array 4, which is positioned on the opposite side of beamsplitter 6 from reflector array 8.

[0148] We will now describe second portion 102 of the object beam. Second portion 102 of the object beam, i.e., the portion having the second polarization state, propagates through second triangular prism 6B of beamsplitter 6, behind beamsplitter surface 6F, and then through second λ / 4 plate 9b, which may be located, for example, on one side of beamsplitter cube 6 opposite first metalens array 3.

[0149] The second λ / 4 plate 9b causes a change in the polarization state of the second portion 102. In this example, the polarization state changes from linearly polarized light to circularly polarized light. Behind the second λ / 4 plate 9b, a plane mirror 7 is positioned perpendicular to the optical axis OA of the input aperture 2, which back-reflects the second portion 102. The back-reflected second portion 102 now propagates again through the second λ / 4 plate 9b, thereby changing the polarization state of the second portion 102 so that it now corresponds to the first polarization state of the object beam behind the input aperture 2. In this example, after the second portion 102 has propagated twice through the second λ / 4 plate 9b, the polarization state of the second portion 102 changes from p-polarized to s-polarized light.

[0150] Second portion 102 is reflected back by first mirror 7, then propagates through second triangular prism 6B and again strikes beamsplitter surface 6F, where the change in polarization of second portion 102 causes it to be reflected. Second portion 102 continues to propagate through second triangular prism 6B and then, like first portion 101, strikes second metalens array 4.

[0151] The second metalens array 4 has essentially the same properties as the first metalens array 4, comprising a plurality of metalenses 40 arranged in an array. However, in contrast to the first metalens array 3, in this example the second metalens array 4 has the property of leaving the polarization state of the second portions 102 unchanged, particularly in terms of collimation, and collimating the light of the first portions 101. That is, after the second metalens array 4, the light of both the first and second portions 101, 102 is collimated, provided that the object light from the input aperture 2 is collimated and directed towards the first metalens array 3.

[0152] An analyzer 14 is positioned behind the second metalens array 4 to align (match) the polarization states of the first and second portions 101, 102, thereby allowing the light beams of the first and second portions 101, 102 to interfere with each other. For example, the analyzer 14 is set at a 45° angle so that the aligned first and second portions 101, 102 have the same polarization direction.

[0153] A detector 5 is arranged behind the analyzer 14, which is configured to record the interfering first and second portions 101, 102 of the object beam. The detector 5 can be, for example, a camera. The interfering first and second portions 101, 102 of the object beam form an interference pattern on the detector 5, which is evaluated by an evaluation unit (not shown) to generate three-dimensional image information.

[0154] With reference to the object beam 10, which encloses a first tilt angle unequal to 0° with respect to the optical axis OA, it should be noted that due to the special arrangement of the optical components of the system 1 according to the invention, the object rays of the first portion 101 and the second portion 102 of the object beam, which enter the system 1 at the first tilt angle β, enclose the first tilt angle β with the optical axis OA behind the input aperture 2, and enclose a second tilt angle β' after reflection on the reflector array 8 or the plane mirror 7. This second tilt angle β' is twice the first tilt angle β and is achieved using a combination of the plane mirror 7 and the reflector array 8. While the first portion 101 has an incident angle β after reflection on the reflector array 8, the second portion 102 has an exit angle -β after reflection on the plane mirror 7. These angles then add up to twice the first tilt angle, 2β, which corresponds to the second tilt angle β'.

[0155] The advantage of this arrangement is that it improves the lateral resolution of the system 1 compared to other systems. Furthermore, such a system 1 does not contain any moving precision mechanical parts, making the system 1 very robust and allowing for a very compact design.

[0156] System 1 of the present invention allows one skilled in the art to calculate the wavefront of the object beam from the local wavefront angle per metalens of the second metalens array, or from the interference frequency of the interference pattern produced on the detector, thereby estimating the z-deviation from the focal plane of the input aperture, i.e., generating image information regarding the z-position of the region of the investigated object in addition to the lateral spatial resolution.

[0157] As an alternative to the optical properties of second metalens array 4, it is also conceivable to design second metalens array 4 to be identical to first metalens array 3, i.e., specifically with regard to its optical properties with respect to polarization states.

[0158] In this case, a λ / 2 plate 11 (indicated by a dash) is placed behind the beamsplitter cube 6 on the side of the second metalens array 4 and in front of the second metalens array 4, such that the λ / 2 plate rotates the polarization states of the first and second portions 101, 102 by 90° so that they once again correspond to their original predetermined polarization states. This embodiment, with two identical metalens arrays 3, 4, has the advantage that the system 1 and metalens arrays 3, 4 are relatively inexpensive to manufacture.

[0159] In a further advantageous embodiment, which for at least one embodiment is shown in Fig. 1, the system 1 comprises means for shifting the light wave phase of the first and / or second parts 101, 102 of the object beam, thereby avoiding interference with the DC or DC light signal of the detector 5. For this purpose, the phase of the first or second parts is typically achieved, for example, by a slight change in the optical path (also called optical path length).

[0160] The change in the optical path can be achieved by the actual geometric length of the optical path of the first and / or second portion, or by adjusting or changing the refractive index through which the first and / or second portion passes.

[0161] In FIG. 1, the phase adjustment is achieved by an actuator 12, which is able to shift the reflector array 8 at least along the optical axis OA of the input aperture 2 (even when folded by the beam splitter cube).

[0162] The actuator 12 may for example comprise a piezo element which can be controlled via an electrical control, in particular the piezo element may be a piezo ring element.

[0163] The actuator 12 therefore enables the optical path of the first portion 101 of the object beam to be changed, which is reflected from the input aperture 2 via the beam splitter 6 towards the reflector array 8. By shifting the reflector array 8 along the optical axis OA, the relative phase of the object beam with respect to the second portion 102 can be set.

[0164] Alternatively, or additionally, a further actuator 12 ′ may be arranged to the side of the first mirror 7 and configured to move the mirror 7 at least along the optical axis OA of the input aperture 2 .

[0165] It is also possible for an actuator 12 to control the reflector array 8 and for another actuator 12' to control the first mirror 7, thereby changing the optical path of both the first part 101 and the second part 102.

[0166] Alternatively or additionally, it is possible to induce a refractive index change in one or both triangular prisms 6A, 6B of the beam splitter cube 6, so that the light wave phase of the first and / or second portions 101, 102 can be set (not shown). The refractive index can be changed by applying a voltage to the first and / or second triangular prisms 6A, 6B. For this purpose, the triangular prisms 6A, 6B need to be made of a material familiar to those skilled in the art.

[0167] Alternatively, a phase change in one of the two polarization states can be set by the first and / or second metalens arrays 3, 4. This can be achieved, for example, by a phase shifter element or by novel developments in the metalens of the array itself, which allow the phase of the polarization direction to be changed by applying an electric or magnetic variable.

[0168] Regardless of how the phase shift is generated, system 1 is specifically designed to allow for relative phase shifts greater than 2π, so that in addition to uniform light suppression in the interferogram of individual wavelengths or wavelength ranges, it is also possible to separate different colors or wavelength ranges further apart using the phase position. Corresponding separation is possible, for example, by Fourier transform and is generally known to those skilled in the art.

[0169] In FIG. 2 , an extension of the present invention to all three primary colors RGB of system 1 is shown, building on FIG. 1 . Shifting the phase over a larger area (several 2π) has already been discussed in the previous paragraph, but this can limit the dynamic range of detector 5. This is because the signals of all primary colors preferably exhibit a high degree of interference in the central area about the optical axis OA of the system (and less at the edges), which can result in unwanted signal amplification (as related here to the detector). Thus, FIG. 2 shows an embodiment in which the three primary colors are wavelength-selectively deflected from the central area about the optical axis OA in the area of ​​first metalens array 3. This can be achieved via a local prism, optical grating, or VPH (not shown), which is superimposed on the actual lensing effect of first metalens array 3.

[0170] Alternatively, or additionally, this property may be satisfied by the first metalens array 3 itself. In Figure 2, wavelength-selective deflection of three different wavelengths (also referred to in the context of this specification as three colors, RGB) through the first metalens array 3 is shown in the form of light rays (arrows 21, 22, 23).

[0171] Each of the three wavelengths / colors per object point or object region generates an interferogram or interference pattern with spatial frequency and direction on the detector 5. In conjunction with the phase shifts described in Figure 1, one skilled in the art can separate each spectral image component, thereby generating spatially resolved three-dimensional color image information by evaluating the interference pattern on the detector 5 using an evaluation unit.

[0172] To increase the resolution of the system 1, especially along the optical axis OA, the system 1 can be modified with a beam splitter 6 designed as follows: This system allows for very high resolution along the z-axis, especially in the sub-micron range.

[0173] 3, the beamsplitter cube 6 comprises a transmission grating arrangement in the form of at least one volume phase hologram grating (VPH) 16 along the beamsplitter surface. The VPH 16 is commonly known to those skilled in the art as a volume phase holographic (transmission) grating. Additionally, the system comprises a wavelength-selective prism arrangement 17, specifically a wavelength-selective double prism arrangement 17, on the side of the beamsplitter cube 6 opposite the detector 5.

[0174] According to this example, VPH 16 and prism arrangement 17 are configured for a second spectral range, and in particular, a first spectral range of the object beam, i.e., a wavefront in the first spectral range, passes unchanged through VPH 16 and prism arrangement 17. Conversely, prism arrangement 17 is configured such that light from the second spectral range is reflected at prism surface 17F, while light from the first spectral range propagates through prism surface 17F and thus passes unchanged through prism arrangement 17.

[0175] Similarly, VPH 16 is configured to diffract light from a second spectral range according to angle and wavelength, and to allow light from the first spectral range to propagate through it unchanged.

[0176] Conversely, metalens arrays 3, 4, polarizer 15, and λ / 4 plates 9a, 9b, and, if applicable, λ / 2 plate 11, similarly behave transparently (in particular, optically neutrally), leaving the wavefront and polarization state of the object light in the second spectral range unchanged. Light from the second spectral range is used by system 1 to generate particularly high spatial resolution, particularly along the optical axis.

[0177] Instead of the optical path for the object beam 10 in the first spectral range shown in FIG. 1, FIG. 3 shows optical paths for the object beam 20-1 and the reference beam 20-2, which in this example are in the second spectral range. The reference beam 20-2 is preferably s-polarized, at least when it strikes the VPH, emitted by the reference light source 19, e.g., a single-mode aperture, and collimated via the collimation optical unit 25. The optical axes of the collimation optical unit 25 and the input aperture 2 overlap. The reference beam 20-2 then enters the beam splitter cube 6 in this collimated state from one side of the beam splitter cube 6 opposite the input aperture 2. To enable this, the planar first mirror 7, which reflects the object beam 10 from the first spectral range, must be transparent to the reference beam 20 from the second spectral range. This means that the mirror 7 is at least a dichroic mirror and does not change the wavefront of the reference beam. The second λ / 4 plate 9a may be configured to leave the polarization state of the reference beam 20-2 unchanged or to interact with other retardation elements, such as the third λ / 4 plate 9c, so that the reference beam 20-2 is configured to be s-polarized when it strikes the VPH 16.

[0178] The mechanism of action of VPH16 interacting with the prism arrangement 17 is described below.

[0179] In the example shown, object light 20-1 from a second spectral range collimated by input aperture 2 strikes VPH 16 at an angle of approximately 45°. VPH 16 is arranged in the so-called Littrow configuration and is optimized for the second spectral range, so that incident object light 20-1 from the second spectral range is diffracted by VPH 16 toward detector 5 at an angle of approximately 90° (relative to incident object light 20-1) or approximately 45° (relative to beam splitter region 6F). Ideally, object light 20-1 from the second spectral range is linearly polarized, and in particular s-polarized, when it strikes VPH 16, since the diffraction efficiency of the VPH is then maximized.

[0180] Meanwhile, collimated reference beam 20-2 also strikes VPH 16 and is diffracted towards prism arrangement 17, which is located on the side of beam splitter 6b opposite detector 5. Again, VPH 16 is at 45° to reference beam 20-2, so a Littrow configuration is also present here.

[0181] The reference beam 20-2 propagates toward the prism arrangement 17, where it is reflected by the beam splitter surface and thus by the reflecting surface 17F of the prism arrangement 17, which surrounds the VPH 16 at an angle α. The angle α in this case (not shown, but instead the difference angle (prism angle) Δα = 45° - α) is not equal to 45°, but is either greater than or less than 45°, in particular, 0.2° greater than or less than 45°. As a result, the reference beam diffracted at an angle of 45° from the VPH 16 toward the prism arrangement 17 is reflected by the prism arrangement 17 back toward the VPH 16, where the reference beam 20-2 strikes the VPH 16 at an angle different from 45°. However, as a result, at least a portion of the reference beam 20-2 reflected back from the VPH 16 is not diffracted back toward the reference source 19, but propagates through the VPH 16 toward the detector 5, where it interferes with the diffracted object beam 20-1 on the detector 5 at an angle corresponding to twice the angle α.

[0182] The core idea of ​​this embodiment is that beam splitter cube 6 has VPH 16 along its beam splitter surface 6F, which VPH subtends an angle α that is not equal to 45° with reflective surface 17F of prism arrangement 17. As a result, when reference beam 20-2 hits VPH 16 for the second time (after reflection at prism arrangement 17), it is at least partially, and to a sufficiently high extent, transmitted from VPH 16 towards detector 5.

[0183] In particular, it is intended that the light from the second spectral range is present in the form of a spectral line comb comprising a plurality of separated narrowband spectral lines, in particular with linewidths in the sub-nanometer range.

[0184] Due to the properties of the VPH 16, the spectral lines are diffracted slightly differently due to wavelength selectivity (see beams 20-1', 20-2'), resulting in a dispersive splitting of the spectral lines at the detector 5, enabling high-precision spatial resolution along the optical axis OA. Information about the z-position of the object S can be found specifically in the phase data of the individual spectral lines. In combination with information from the interference pattern in the first spectral range, such a system 1 enables the determination of high-resolution three-dimensional color information of the object S.

[0185] The second spectral range is typically the near-infrared or infrared range, for example the range of 700-900 nm, or even above 1300 nm, in which case measurements can be made even below the surface of biological tissue.

[0186] Of course, it is also possible to provide a separate arrangement for the VPH16 described in the previous paragraph, which has only the optical components required for the second spectral range, i.e., in particular the VPH16 and the prism arrangement, which can therefore also be designed as a mirror.

[0187] Additionally, system 1 can be equipped with an additional VPH (not shown) that is optically active in a different, third spectral range, where it diffracts incident light in a wavelength- and angle-dependent manner. Thus, with appropriate components and their transmission (transmission) properties, even a third spectral range can be scanned.

[0188] For example, it is possible to obtain a surface profile in the near infrared and simultaneously obtain the structure below that surface in the infrared region, where the system is operated using the metalens array mechanism described in the visible (first spectral) range.

[0189] Alternatively, the second spectral range and the first spectral range can be nested but non-overlapping, i.e. the first spectral range can include, for example, three wavelength ranges characteristic of the color channels (or colors) red, green and blue, while the second spectral range can be in at least one wavelength range located between these three wavelength ranges.

[0190] The wavelength range of the first spectral range of the blue color channel extends in particular from 420 nm to 480 nm, the wavelength range of the first spectral range of the green color channel extends in particular from 520 nm to 565 nm, the wavelength range of the first spectral range of the red color channel extends in particular from 630 nm to 680 nm, and thus the second spectral range can extend, for example, from 505 nm to 515 nm and / or from 570 nm to 625 nm, or even above 690 nm.

[0191] A very low-complexity embodiment of the invention is shown in Figure 4. In this variant, the beam splitter cube can be omitted without abandoning the basic idea of ​​the invention. The advantage of the embodiment shown in Figure 4 is the possibility of a very compact design.

[0192] In FIG. 1, the edge length of the beamsplitter cube determines the dimensions in all three spatial directions. However, in FIG. 4, the spatial direction perpendicular to the optical axis can be chosen to be smaller. Instead of the beamsplitter cube, a transparent solid element is provided, with a first surface of the element facing the first metalens array and a second surface opposite the first surface facing the second metalens array. In particular, the volume contained in the transparent solid element is free of selectively reflective and selectively diffractive surfaces, and in particular, the transparent element here is cuboid or plate-shaped.

[0193] Thus, the element can be, for example, a glass plate or a polymer plate. In a transparent element, a first metalens array is disposed on a planar surface opposite a second metalens array, which is disposed on a planar surface on the side opposite the first surface. The input aperture and the polarization state imposed on the object beam and generated therein have already been described in connection with FIG. 1.

[0194] Metalenses can be easily fabricated with a numerical aperture of NA=0.5. For a typical metalens diameter of 1 mm, the focal length would be 1 mm relative to the metalens surface, meaning that the transparent elements separating the metalens array would need to be approximately 2 mm (for simplicity, the refractive index of the transparent elements is not considered). This is again significantly smaller than, for example, the height of one edge of a beamsplitter cube (e.g., 5 mm). Furthermore, the height and lateral dimensions are decoupled in this example, which is particularly advantageous for mobile phone applications, where the structure height is absolutely critical and the design size along the lateral direction is not an issue.

[0195] This means that for a compact design along the z-axis (height), one can still use large area detectors extending along the x- and y-directions.

[0196] For details of the beam path, please refer in particular to FIG. 1, which shows the polarization state at the input aperture and its creation.

[0197] 4 shows two different rays of the object beam. The first situation concerns a first object beam 31, which originates from a region of the object under study that lies on the optical axis OA of the input aperture 2. The second situation concerns a second object beam 32, which originates from a region of the object under study that is laterally offset with respect to the optical axis OA of the input aperture 2.

[0198] The first object beam 31 is collimated by the input aperture 2 and then propagates further parallel to the optical axis OA of the input aperture 2, i.e., it subtends an angle of 0° with the optical axis OA of the input aperture 2. The angle subtended by the optical axis and the collimated object beam is also referred to herein as the first tilt angle β.

[0199] The first tilt angle β is defined in particular by the angle enclosed by the optical axis OA of the input aperture 2 and the propagation direction of the collimated object ray.

[0200] The second object ray 32 is also collimated by the input aperture 2 and then propagates further, but with a first inclination angle β not equal to 0° relative to the optical axis OA of the collimation lens 2a. According to the laws of radiation optics, the first inclination angle β contains information about the lateral position of the object under investigation (together with the associated focal length of the input aperture) from which the associated object ray originates.

[0201] The input aperture 2 also includes a polarizer 15, which is configured to put the object beam into a predetermined polarization state. In particular, the polarizer 15 is configured to put the object beam into a predetermined polarization state for at least the object beam from a first spectral range.

[0202] In this case, polarizer 15 is positioned and configured (e.g., at a rotation angle of 45°) so that the object beam has approximately equal proportions of s-polarized and p-polarized light, regardless of the direction of incidence or the first tilt angle β. That is, in this example, the predetermined polarization state is composed of a first polarization state that includes s-polarized object beam and a second polarization state that includes p-polarized object beam. The assignment of s-polarized object beam to the first polarization state and p-polarized object beam to the second polarization state can also be done in the opposite manner, and is for illustrative purposes only.

[0203] In the following, without limiting generality, the cases of s-polarization and p-polarization states will be discussed.

[0204] In accordance with the present invention, the object beam having the predetermined polarization state just described now impinges on first metalens array 3. In accordance with the present invention, first metalens array 3 is now arranged and configured to focus a first portion 31-1 of the object beam, having the first polarization state, while a second portion 31-2 of the object beam, having a second polarization state, propagates through first metalens array 3 essentially unchanged.

[0205] In particular, the focused first portion 31-1 is focused so as to be focused at a focal plane 3B associated with the first metalens array 3.

[0206] This applies to both the first and second object beams 31, 32.

[0207] The second metalens array 4 is positioned such that a focal plane 4B associated with the second metalens array 4 is above the focal plane 3B associated with the first metalens array 3. Additionally, the second metalens array 4 is configured to collimate the object beam 31-1 having a first polarization state, and thus be focused into the focal plane 3B of the first metalens array 3, and transmit essentially unchanged a second portion 31-2 of the object beam having a second polarization state, such that second portion 31-2 is thereafter still collimated.

[0208] For the first object ray 31 that brackets an angle of 0° with the optical axis OA, this means that it exits the second metalens array 4 at an angle of 0° and strikes the detector 5 at this angle.

[0209] As for the second object beam 32, it propagates at a first tilt angle β relative to the optical axis OA that is not equal to 0°, and a first portion 32-1 of the second object beam 32, after appropriate focusing and recollimation of the first portion 32-1 in the first polarization state, is enclosed by a second tilt angle β' that is twice as large as the first tilt angle β with a second portion 32-2 of the second object beam, which propagates essentially unchanged through the first and second metalens arrays 3, 4. Thus, the first and second portions 32-1, 32-2 of the second object beam 32 impinge at the second tilt angle on a detector that is positioned behind the second metalens array 4. In particular, an analyzer 14 can also be positioned before the detector 5, which matches the polarization states of the first and second portions, in particular at a 45° rotation, so that improved interference is created on the detector 5.

[0210] 1, in order to achieve relative phase adjustment of the wavefront for the first and second portions, the solid transparent element may comprise a liquid crystal (not shown) having different refractive indices for the object beam of the first and / or second polarization states, so that the phase relationship for the wavefront can be controlled and set by the liquid crystal. For this purpose, a control unit may be provided in the system.

[0211] The embodiment described in FIG. 4 is particularly advantageous for metalens arrays 3, 4 that include metalenses with relatively high numerical apertures, for example, numerical apertures greater than 0.4.

[0212] Regardless of the particular embodiment, the system 1 may have a laser light source 18 configured to illuminate the object under investigation in a controllable manner, and in particular configured to illuminate the object S in a specific area, so that a complete image of the object under investigation can be generated, for example by an optical scanning process.

[0213] For this purpose, it is provided that the laser light source 18 emits successively different wavelengths and thus successively illuminates the object under investigation with different wavelengths, so that color information can be obtained from the successive illuminations.

[0214] Alternatively, the laser source may be configured to emit light from the first and second spectral ranges simultaneously or in a time-delayed manner.

[0215] References [1] Jangwoon Sung et al, “Progresses in the practical metasurface for holography and lens,” Nanophotonics 2019, 8(10), p.1701-1718. [2] Ling Li,et al.,“Polarization-Switchable Multi-Focal Noninterleaved Metalenses in the visible,”Laser&Photonics reviews,2021,15,2100198

[0216] List of reference symbols 1 System 2 Input Aperture 2a Collimation optical unit / lens / objective lens 3 The first metalens array 30 Metalens 3B Focal plane of the first metalens array 4 Second metalens array 40 Metalens 4B Focal plane of the second metalens array 5. Detector 6 Beam Splitter 6A First Prism of Beam Splitter 6B Second prism of beam splitter 6F reflective surface 7. First Mirror 8 Reflector Array 80 Retroreflector 9a First λ / 4 element 9b Second λ / 4 element 11 λ / 2 element 12, 12' Actuator Arrangement 13 Solid transparent elements 13-1 First side of the solid transparent element 13-2 Second aspect of solid transparent elements 14 Analyzer 15 Polarizer 16 VPH 17 Prism arrangement, double prism 17F reflective surface 18 Object light source 19 Reference light source 25 Reference beam collimation optical unit 21, 22, 23 Red (21), green (22), blue (23) rays 31 First Ray 31-1 First Part 31-2 Second Part 32 Second Ray 32-1 First Part 32-2 Second Part 10 Object light 101 First Part 102 Second Part 10a First Ray 10b Second Ray 20-1, 20-1' object rays in the second spectral range 20-2, 20-2' Reference beam in the second spectral range E Focal plane of the input aperture OA optical axis S Survey object x,y,z Cartesian coordinate system directions Δα Prism angle β First tilt angle β' Second tilt angle

Claims

1. An optical system (1) for three-dimensional imaging, comprising at least the following components: an input aperture (2) with an optical axis (OA), a first metalens array (3) and a second metalens array (4); - detector (5) and Equipped with the input aperture (2) is configured to collimate an object beam (10) coming from an investigation object (S) in a first spectral range and fix it in a predetermined polarization state composed of two polarization states that are conjugate with each other, wherein the object beam (10) propagates into the first metalens array (3) at a first tilt angle (β) with respect to an optical axis (OA) of the input aperture (2); the first metalens array (3) is constructed and arranged to focus a first portion (101) of the object beam (10) that includes a first polarization state of two mutually conjugate polarization states and to leave a second portion (102) of the object beam (10) that includes a second polarization state of the two mutually conjugate polarization states unchanged; the second metalens array (4) is constructed and arranged to collimate the focused first portion (101) and transmit the second portion (102) unchanged, so that the first portion (101) and the second portion (102) each have the same wavefront curvature after propagating through the first and second metalens arrays (3, 4) and surround each other at a second tilt angle (β') with respect to their respective propagation directions, forming an interference pattern that strikes the detector (5), where the second tilt angle (β') corresponds to twice the first tilt angle (β) in terms of magnitude; and based on the interference pattern, three-dimensional position information of an object region of the investigated object (S) can be generated. The system (1).

2. The first and second portions (101, 102) of the object beam (10) are each linearly polarized perpendicularly to each other. A system (1) according to claim 1.

3. The system (1) comprises a polarization-dependent beam splitter (6) between the first metalens array and the second metalens array (3), and the system (1) further comprises the following components: a first mirror (7), a reflector array (8) comprising a plurality of reflective retroreflectors (80); a first λ / 4 element (9a) placed between the polarization dependent beam splitter (6) and the first mirror (7); a second λ / 4 element (9b) placed between the polarization dependent beam splitter (6) and the reflector array (8); Equipped with the polarization-dependent beam splitter is positioned such that, for the object beam (10) incident from the first metalens array (3), a first portion (101) is reflected by the beam splitter (6) and a second portion (102) is transmitted through the beam splitter (6); the reflector array (8) is positioned on one side of the beam splitter (6) along which the first portion (101) coming from the first metalens array (3) and reflected by the beam splitter (6) propagates; and the first mirror is positioned on one side of the beam splitter (6) opposite the first metalens array (3), and the first mirror (7) reflects the second portion (102) so that it strikes the first mirror (7) and returns toward the beam splitter (6) to the second metalens array (4); A system (1) according to claim 1 or 2.

4. 4. The system (1) of claim 3, wherein a λ / 2 element (11) is disposed between the beam splitter (6) and the second metalens array (4) and is configured to invert the polarization states of the first and second portions (101, 102).

5. 5. The system (1) according to claim 3 or 4, characterized in that the system (1) comprises an actuator arrangement (12) configured to set the position of the first mirror (7) and / or the reflector array (8) so that the phase between the wavefronts associated with the first and second parts (101, 102) can be set.

6. The system (1) according to claim 1 or 2, characterized in that the system (1) comprises a transparent solid element (13), a first surface (13-1) of the transparent solid element (13) facing towards the first metalens array (3), and a second surface (13-2) of the transparent solid element (13) opposite the first surface (13-1) facing towards the second metalens array (4).

7. The system (1) according to any one of claims 1 to 6, characterized in that the system (1) comprises at least one liquid crystal configured to adjust the phase between the wavefronts associated with the first part and the second part (101, 102).

8. 8. A system according to claim 6 or 7, wherein the transparent solid element (13) comprises or consists of at least one liquid crystal.

9. 6. The system (1) according to claim 7 and any one of claims 3, 4 or 5, characterized in that the polarization dependent beam splitter (6) comprises a first prism and a second prism (6A, 6B) forming a beam splitter cube of the beam splitter (6), the first prism and / or the second prism (6A, 6B) comprising at least one liquid crystal.

10. 10. The system (1) of any one of claims 1 to 9, wherein the focal plane (3B) of the first metalens array (3) and the focal plane (4B) of the second metalens array (4) overlap each other.

11. The system (1) according to any one of claims 1 to 10, wherein the analyzer (14) is arranged behind the second metalens array (4) and in front of the detector (5) in the propagation direction to match the polarization states of the first and second portions (101, 102), and the analyzer is configured such that interference between the first and second portions (101) and (102) is achieved on the detector (5).

12. The system (1) according to any one of claims 1 to 11, characterized in that the input aperture (2) comprises a polarizer (15) configured to direct the object beam (10) into a predetermined polarization state from a first spectral range.

13. The system (1) according to any one of claims 1 to 12, characterized in that the input aperture (2) comprises at least one lens (2a) configured to collimate the object beam (10) coming from the object (S) under study.

14. The system (1) according to any one of claims 1 to 13, wherein the system (1) is configured to wavelength-dependently deflect the propagation direction of the object beam (10) from a first spectral region impinging on the system (1), so that the object beam (10) and the first and second portions (101, 102) surround a wavelength-dependent angle with the optical axis (OA) in addition to the first tilt angle (β).

15. 15. The system (1) according to any one of claims 1 to 14, characterized in that the object light (10) in the first spectral range consists of two or more separate wavelength ranges and / or the system (1) is configured to filter the object light (10) into two or more separate wavelength regions forming the first spectral range, with gaps between these wavelength ranges, and an interference pattern is generated on the detector (5) for each wavelength range, from which three-dimensional position information and color composition can be generated for the wavelength range of the object region of the object (S) under investigation.

16. Referring back to claim 3, the system (1) of any one of claims 1 to 15 is characterized in that the object beam (10) comprises at least one further spectral range different from and separate from the first spectral range, and wherein the first and second metalens arrays (3, 4) and the polarization-dependent beam splitter (6) are transparent and optically inactive to light from the at least one further spectral range, and wherein the polarization-dependent beam splitter (6) further comprises a volume phase hologram VPH (16), which is configured to diffract light from the at least one further spectral range in a polarization- and angle-dependent manner and to be transparent and optically inactive to light from the first spectral range.

17. 17. The system (1) according to claim 16, characterized in that the system is configured to direct the reference light from the second spectral range towards the VPH via one side of the beam splitter (6) opposite the input aperture, the first mirror being particularly transparent to the reference light.

18. The system (1) according to claim 17, characterized in that it comprises a third λ / 4 element (9c) arranged on one side of the first mirror (7) facing away from the beam splitter (6) and configured to set the reference light (20-2, 20-2') to a predetermined polarization state in cooperation with the second λ / 4 element (9b) so that the reference light (20-2, 20-2') is s-polarized when coming from the first mirror and striking the VPH (16).

19. 19. The system (1) according to any one of claims 16 to 18, wherein the system (1) comprises a wavelength-selective prism arrangement (17) between the reflector array (8) and the beam splitter (6), the prism arrangement being configured to reflect light of at least one further spectral range at a prism angle (Δα) towards the detector (5), and the prism arrangement (17) being transparent and optically inactive to light from the first spectral range.

20. 20. The system (1) according to any one of claims 1 to 19, wherein the first portion (101) and the second portion (102) are collimated after being transmitted through the first and second metalens arrays (3, 4), respectively.

21. Referring back to claim 2, the system (1) according to any one of claims 2 to 20, characterized in that the predetermined polarization state is a linearly polarized polarization state consisting of a superposition of the first part and the second part (101, 102).

22. Referring back to claim 3, the system (1) of any one of claims 3 to 21, characterized in that the back-reflected first portion and the back-reflected second portion (101, 102) due to the inverted polarization states propagate towards the second metalens array via the polarization-dependent beamsplitter (6).

23. Referring back to claim 16, a system (1) according to any one of claims 16 to 22, characterized in that the volume phase hologram (16) extends along the reflective surface (6F) of the beam splitter (6).

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