Ultracompact optical system for 3-d imaging

Abstract

The invention relates to an optical system (1) for 3-D 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), and - a detector (5), wherein the input aperture (2) is configured to collimate object light (10) coming from an examination object (S) in a first spectral range and set said light into a predefined polarization state, wherein the object light (10) propagates at a first inclination angle (β) in relation to the optical axis (OA) of the input aperture (2) to the first metalens array (3), wherein the first metalens array (3) is configured and arranged to focus a first component (101) of the object light (10) and leave a second component (102) of the object light (10) unchanged, wherein the second metalens array (4) is configured and arranged to collimate the focused first component (101) and transmit the second component (102) unchanged such that the first component (101) and the second component (102) enclose a second inclination angle (β') with one another in relation to their respective propagation direction and are incident on the detector (5), forming an interference pattern in the process, wherein the second inclination angle (β') corresponds to twice the first inclination angle (β) in terms of magnitude.

Description

[0001] Ultra-compact optical system for 3D imaging

[0002] Description

[0003] The invention relates to an optical system for 3D imaging according to claim 1.

[0004] Optical 3D imaging systems are known from the state of the art that obtain information about the three-dimensional position, surface structure, or composition of an object by evaluating a holographic interference pattern. These holographic systems often operate with a scanning light beam, the so-called object light, and a reference light beam, the so-called reference light, which are combined in a holographic unit. Due to the temporal and spatial coherence of the light, they result in an interference pattern on a detector. Through various evaluation measures, conclusions can be drawn about the wavefronts and thus about 3D information about the object being examined scanned with the object light.

[0005] However, these holographic systems have several disadvantages. They often require moving components and a relatively large installation space, have comparatively poor spatial resolution, or are complex and costly to manufacture.

[0006] Nevertheless, these systems are of great importance, especially in minimally invasive medicine. 3D imaging applications have also recently arrived in the mobile phone sector. However, completely different systems are used there, ones that are not based on a holographic principle.

[0007] The object of the invention is therefore to provide a 3D imaging system which overcomes the aforementioned disadvantages.

[0008] The problem according to the invention is solved by a system according to claim 1.

[0009] Advantageous embodiments of the invention are specified in the subclaims and described below. According to this, an optical system for 3D imaging comprising at least the following components is provided:

[0010] An entrance aperture having an optical axis, a first metal lens array and a second metal lens array, a detector, wherein the entrance aperture is configured to collimate object light coming from an examination object in a first spectral range, in particular when the entrance aperture has a lens and the object light originates from a focal plane of the entrance aperture, and to fix it in a predefined polarization state composed of two mutually conjugate polarization states, wherein the object light propagates to the first metal lens array at a first inclination angle, β, with respect to the optical axis of the entrance aperture, wherein the first metal lens array is configured and arranged to focus a first portion of the object light, which comprises a first polarization state of the two mutually conjugate polarization states, and a second portion of the object light,which comprises a second polarization state of the two mutually conjugated polarization states, to leave unchanged, in particular to leave it collimated unchanged, wherein the second metal lens array is configured and arranged behind the first metal lens array in the propagation direction of the object light, to collimate the focused first portion and to transmit the second portion unchanged, in particular to transmit it collimated unchanged, so that the first portion and the second portion, after being propagated through the first and second metal lens arrays, each have the same wavefront curvature and are in particular collimated, and enclose a second inclination angle, ß - 2*ß, with respect to their respective propagation direction and strike the detector to form an interference pattern,wherein the second angle of inclination corresponds in magnitude to twice the first angle of inclination and based on the interference pattern, three-dimensional position information of an object area of ​​the object under investigation can be created.

[0011] The system according to the invention, through the use of metal lenses, provides a holographic imaging system that solves the aforementioned problems. Metal lenses or metal lens arrays can be manufactured on a planar structure without the need for complex grinding processes for the production of curvature radii of conventional lenses. Even more important, however, is the possibility of manufacturing metal lenses in such a way that they can perform different optical operations or exhibit different properties depending on polarization, wavelength, or angle [1].

[0012] In this context, polarization is of particular importance because it can be controlled across wavelengths. Ling Li et al. [2] describe how individual metal lenses change their focal length depending on the polarization. Depending on the design of the metal lens, it can also operate in a broadband spectral range without changing its focusing properties. This is a fundamental difference to diffraction structures such as optical gratings or holograms, which inherently diffract light in a wavelength-dependent manner. The metal lens properties are possible, among other things, due to the structures, which are smaller than the optical wavelength and significantly smaller than the typical camera pixel dimension of 2 - 5 pm. In summary, the state of the art describes that metal lenses can be designed up to a numerical aperture of NA > 0.5 and optical bandwidths > 100 nm for various color bands in the red, green, and blue (RGB) color range.

[0013] The entrance aperture can, in particular, contain optical components such as a lens or a plurality of lenses arranged in a lens arrangement, for example, an objective. Alternatively, the entrance aperture can also have no lens, but merely comprise a pinhole, which—provided the object under examination is at a sufficiently large distance (e.g., in the range of approximately 100 mm to 500 mm) from the pinhole—images the incident object light from the object under examination into the system in a sufficiently collimated manner.

[0014] Furthermore, the entrance aperture has an optical element configured to impart a predefined polarization state to the object light arriving from the object under examination. Such an optical element can be, for example, a polarizer. In the context of the present specification, the term "entrance aperture" refers in particular to an area in front of the first metal lens array. This means that the entrance aperture does not necessarily refer only to one opening of the system, but rather that the term "entrance aperture" can extend to all optical components and elements of the system that are arranged in front of the first metal lens array in the propagation direction of the object light.

[0015] The term "metal lens array" refers specifically to the arrangement of a plurality of metal lenses whose respective optical axes are aligned essentially parallel to each other. The optical axes of the individual metal lenses are aligned, in particular, parallel to the optical axis of the entrance aperture.

[0016] The dimensions, for example the diameter, of a single metal lens are known in the state of the art and can vary in particular in the range of a few millimeters, for example 0.2 mm to 10 mm.

[0017] In particular, each metal lens in a metal lens array can be assigned at least one focal length. This focal length depends, for example, on the polarization state of the incident object light.

[0018] Furthermore, the focal length of the metal lenses and thus also the focal length assigned to the first and / or second metal lens array can be wavelength-dependent.

[0019] According to one embodiment of the invention, the focal lengths of all metal lenses of the first metal lens array are the same.

[0020] According to a further embodiment of the invention, the focal lengths of all metal lenses of the second metal lens array are the same.

[0021] According to a further embodiment of the invention, the mean focal lengths of the first and second metal lens arrays are equal.

[0022] The term “average focal length” refers in particular to a focal length which is assigned to the respective metal lens array and which results in particular from an average of all focal lengths of the metal lenses arranged in the array, in particular wherein the average focal length corresponds to this average.

[0023] According to one embodiment of the invention, the detector comprises a camera having a plurality of photosensitive pixels, wherein these photosensitive pixels are configured to register at least object light from the first spectral range. In this way, an interference pattern can be recorded by means of the detector.

[0024] The recorded interference pattern can then be transmitted in the form of data to an evaluation unit assigned to the system or included in the system, which generates three-dimensional information or a three-dimensional representation of at least the object under investigation or a region thereof from the transmitted data.

[0025] The term "collimated" and related terms, in the context of this specification, are to be interpreted in particular to mean that the light exhibits only minimal wavefront curvature, at least for one wavelength, in particular for a wavelength or spectral range. At different wavelengths, the convergence or divergence of the light beam or wave field typically increases depending on the wavelength. These chromatically induced deviations from ideal collimation are also encompassed by the term "collimated" in the context of this invention. Furthermore, deviations due to alignment and system tolerances are also to be understood under the term "collimated."

[0026] In particular, if the system does not have a lens in the entrance aperture, the statements in the previous paragraph apply in the understanding assumed by the person skilled in the art, namely that the collimation does not have to be perfect, i.e. the light is imaged into the system in a slightly divergent or convergent manner.

[0027] In particular, it should be noted here that the system is designed to image and / or process non-collimated object light onto the detector in an analogous manner, in particular, wherein the system utilizes non-collimated light beams within the meaning of the specification to generate depth information, i.e., 3D information about the object under investigation. The description of the invention based on collimated light beams serves, in particular, merely to clearly disclose the relative position and function of the system components, but in particular does not preclude the recording and / or processing of light beams not collimated by the input aperture.

[0028] In particular, the term “collimated” and related terms also include a convergence and / or divergence of the light beam up to a divergence or convergence angle in the range 0°< < 2*ß max is to be understood within the scope of the invention, where ß ma x is the maximum first inclination angle that can be imaged or recorded by the system.

[0029] Alternatively or additionally, the term "collimated" and related terms within the scope of the specification can also be understood to mean that a light beam diameter (in this context, a light beam bundle) has a minimum divergence angle at its narrowest point. This image for defining a collimated light beam is used in the wave-optical description of a laser beam and can be applied analogously to the system. This definition is particularly applicable to input apertures that have at least one lens.

[0030] According to the invention, the first inclination angle can be measured with respect to a propagation direction and the optical axis of the input aperture.

[0031] In terms of a wave representation of the object light, this means that the plane wavefronts of the collimated object light after the entrance aperture have an angle of 90°+ ß with respect to the optical axis of the entrance aperture.

[0032] The predefined polarization state of the object light after the entrance aperture comprises two mutually conjugated polarization states. These two polarization states can, in particular, be two linearly polarized polarization states, in particular a p-polarized and an s-polarized state.

[0033] Alternatively, the two polarization states can also represent a right-handed and a left-handed circularly polarized state. It should be noted that the predefined polarization state is, in particular, a superposition of these two conjugated polarization states.

[0034] If the object light with the predefined polarization state now strikes the first metal lens array, this first metal lens array will, due to its optical properties, focus the object light, which has the first polarization state of the two conjugate polarization states, in particular onto the focal plane assigned to the first metal lens array. In contrast, the first metal lens array, also due to its optical properties, allows the object light, which has the second polarization state of the two conjugate polarization states, to be transmitted essentially unchanged. This means that the first metal lens array does not lead to any increased convergence or divergence for object light that has the second polarization state, but essentially behaves like an optically transparent medium without diffraction properties, i.e. like a neutral optical medium, for example like a homogeneous pane of glass.

[0035] As a result, the object light incident on the first metal lens array is split into a first portion consisting of object light of the first polarization state and a second portion consisting of object light of the second polarization state.

[0036] Ideally, the ratio of this splitting is approximately 1:1, i.e. the object light is split into two equally intense parts.

[0037] The two components of the object light then hit the second metal lens array, which is equipped with identical, or at least analogous, optical properties to the first metal lens array.

[0038] The second metal lens array is arranged in such a way that it recollimates the first portion and allows the already collimated second portion to transmit essentially unchanged. Here, too, the second metal lens array essentially behaves as a transparent, neutral optical medium with respect to the second portion, as already explained for the first metal lens array.

[0039] In particular, the system is designed, e.g., by means of appropriate optical components, such that the angle at which the first component impinges on the first metal lens array is inverted compared to the first inclination angle when it propagates through the second metal lens array. In this way, the first component and the second component enclose the second inclination angle. The second inclination angle can be measured either with respect to the propagation direction of the first and second components, or alternatively, equivalently, as the angle between the wavefronts of the first and second components.

[0040] Due to the inventive design, the second angle of inclination is twice as large as the first angle of inclination. This is particularly the case when the focal lengths assigned to the first and second metal lens arrays are equal.

[0041] The first and second components are superimposed behind the second metal lens array, forming an interference pattern on the detector. Based on the interference pattern and its analysis, three-dimensional information about an area of ​​the object under investigation can be created.

[0042] The object area comprises one or more illumination areas with object light, wherein the illumination area on the object under examination is in particular essentially circular with a diameter in the range of 1 mm to 50 mm.

[0043] The term “focusing” or related terms refer in particular to adjusting the wavefront curvature, which causes the associated light beam or light wave bundle to be convergent, i.e. to be imaged onto a smallest beam diameter (focal point) at a position in space.

[0044] Complete three-dimensional information about the object under examination can be generated, for example, by optical scanning, particularly by moving the system relative to the object under examination. Additionally or alternatively, the system can simultaneously image, record, and evaluate multiple areas of the object under examination.

[0045] It is noted that object light that is reflected from the object under investigation at a distance from the entrance aperture that causes the object light to not be collimated by the entrance aperture, but has a different wavefront curvature, so that diverging or converging object light is produced.

[0046] This situation is often the case with an entrance aperture that comprises only a pinhole instead of a lens, since a pinhole does not have an associated focal length. However, if the object light falls onto the pinhole from a sufficiently large distance, the pinhole causes a sufficiently high degree of collimation or a sufficiently low degree of divergence of the object light, so that it is considered collimated within the meaning of the invention.

[0047] But even in the case that the object light is not collimated within the scope of the definition of this specification, this light is imaged and recorded by the system according to the physical principles and can be included in a corresponding evaluation in order to obtain 3D information about an object area (e.g. a surface of the object under investigation).

[0048] The second component of the object light will propagate unchanged through the first and second metal lens arrays in any case (collimated or uncollimated) according to the principles outlined. The first component focused by the first metal lens array behaves analogously, whereby the first component therefore does not fall on a focal plane associated with the first metal lens array, but rather in front of or behind it, and the second metal lens array generates a first component with a correspondingly modified wavefront curvature according to the wavefront curvature.

[0049] These originally non-collimated first and second components also lead to an interference pattern on the detector and can be evaluated accordingly to obtain three-dimensional information about the object area.

[0050] According to a further embodiment of the invention, it is provided that the first and the second portion of the object light are each linearly polarized perpendicular to one another, in particular s- and p-polarized, in particular wherein the predefined polarization state is a linearly polarized polarization state which consists of a superposition of the first and the second portion. Linearly polarized light can be generated comparatively easily. Furthermore, a polarization state can be determined comparatively easily with regard to its polarization if the polarization is linear along one direction. In contrast, it can be more difficult to distinguish a circularly polarized state from an elliptically polarized state. A plurality of optical elements is configured to split or combine linearly polarized light which is conjugated to one another.to be manipulated differently, so that a linear polarization of the first and second parts can be advantageous.

[0051] According to a further embodiment of the invention, the system comprises a polarization-dependent beam splitter, in particular a polarization-dependent beam splitter cube, between the first and the second metal lens array, the system further comprising the following components:

[0052] A first mirror, in particular wherein the first mirror is planar, A reflector array comprising a plurality of reflective retroreflectors,

[0053] A first λ / 4 element arranged between the polarization-dependent beam splitter and the first mirror,

[0054] A second λ / 4 element arranged between the polarization-dependent beam splitter and the reflector array, wherein the polarization-dependent beam splitter is arranged with respect to object light incident from the first metal lens array such that the first portion is reflected by the beam splitter and the second portion is transmitted through the beam splitter, wherein the reflector array is arranged such that it reflects the first portion back towards the beam splitter and to the second metal lens array, wherein the first mirror is arranged such that it reflects the second portion back towards the beam splitter and to the second metal lens array, in particular wherein the reflected first and reflected second portions propagate through the polarization-dependent beam splitter in the direction of the second metal lens array due to the polarization states exchanged by the respective λ / 4 elements,in particular, wherein the first and the second portion each propagate twice through either the first or the second λ / 4 element, so that the first and the second portion have exchanged polarization states after the second passage through the respective λ / 4 element.

[0055] In other words, the polarization-dependent beam splitter is arranged with respect to the object light incident from the first metal lens array such that the first portion is reflected by the beam splitter and the second portion is transmitted through the beam splitter, wherein the reflector array is arranged on a side of the beam splitter to which the first portion coming from the first metal lens array and reflected by the beam splitter propagates, wherein the first portion striking the reflector array is reflected back towards the beam splitter and to the second metal lens array, wherein the first mirror is arranged on a side of the beam splitter opposite the first metal lens array, i.e. on the side to which the second portion coming from the first metal lens array and transmitted by the beam splitter propagates,wherein the first mirror reflects the second portion incident on the first mirror back toward the beam splitter and to the second metal lens array, in particular wherein the back-reflected first and back-reflected second portions propagate toward the second metal lens array due to the exchanged polarization states through the polarization-dependent beam splitter.

[0056] The polarization-dependent beam splitter is designed in particular to reflect one of the two components of the object light and to transmit the other.

[0057] For example, an A / 4 plate can be used as an A / 4 element. This is an optical delay element that has different refractive indices for different polarization directions. This allows the polarization state of the object light to be changed. According to the invention, when the first and / or second component passes through twice, the respective component assumes its conjugate polarization state. For example, when passing through the A / 4 element twice, an s-polarized state would be converted into a p-polarized state and vice versa.

[0058] As a result, the back-reflected components at the polarization-dependent beam splitter both propagate toward the second metal lens array. With this configuration, it is important to note that the second metal lens array must be designed with its focusing properties, depending on the polarization of the object light, in such a way that it collimates the focused first component. This means that, unless another optical element is arranged in front of the second metal lens array that converts the polarization states of the first and second components back to their original polarization states imprinted after the entrance aperture, the focusing property of the second metal lens array should be directed toward the respective conjugate polarization state compared to the first metal lens array.

[0059] According to a further embodiment, the number of retroreflectors included in the reflector array is identical to the number of metal lenses of the first and second metal lens arrays.

[0060] According to a further embodiment of the invention, it is provided that a first λ / 2 element is arranged between the polarization-dependent beam splitter and the second metal lens array, which element is designed to exchange the polarization states of the first and second components, in particular so that the polarization states of the first and second components again correspond to the polarization states of the first and second components after the input aperture.

[0061] The first A / 2 element is in particular arranged such that it is only passed through in the propagation direction of the first and second components after the first and second components have propagated twice through the beam splitter or a beam splitter surface of the beam splitter.

[0062] This embodiment allows the use of a second metal lens array that is identical to the first metal lens array and does not have to have inverse properties with respect to the polarization states as described in the previous paragraph, but rather has identical properties with respect to the polarization states as the first metal lens array.

[0063] In particular, the first A / 2 element is an A / 2 plate.

[0064] This allows for cost-effective and simplified production of the system. According to a further embodiment of the invention, the system comprises an actuator arrangement configured to adjust a position of the first mirror and / or the reflector array, so that a phase between the wavefronts associated with the first and second portions can be adjusted.

[0065] This design allows for the adjustment of a relative phase position between the first and second components, so that, in particular, a constant light component on the detector can be avoided. In particular, the actuator arrangement is configured to adjust the position of the first mirror and / or the reflector array such that phase shifts of more than 2TT are possible. This has particular advantages for the color resolution of the system.

[0066] The actuator arrangement should be configured to shift the position of the first mirror and / or the reflector array in fractions of wavelengths of the first spectral range, particularly along the optical axis. It may be advantageous, particularly for adjustment purposes, if the actuator arrangement is configured to tilt the first mirror and / or the reflector array relative to the optical axis of the input aperture.

[0067] Furthermore, it may be advantageous that the actuator arrangement is configured to adjust a position of the reflector array perpendicular to the optical axis of the input aperture.

[0068] The actuator arrangement serves in particular to lengthen or shorten an optical path length of the first and / or the second portion of the object light.

[0069] The actuator arrangement can be controlled via an external control unit assigned to the system.

[0070] According to one embodiment of the invention, the actuator arrangement comprises at least one piezo element. In particular, the actuator arrangement comprises at least one ring piezo arrangement.

[0071] Although a piezo element is strictly speaking a moving part, due to the comparatively monolithic design of a piezo element, there is no particular risk of wear due to exposed precision mechanics, so that the system can be considered to be largely free of moving components despite the piezo element.

[0072] The use of piezo elements contributes particularly to the increased robustness of the system. Furthermore, piezo actuators can be controlled and adjusted with particular accuracy and precision.

[0073] According to an alternative embodiment in which no beam splitter is required, it is provided that the system comprises a transparent solid element which points with a first surface in the direction of the first metal lens array and with a second surface, opposite the first surface, in the direction of the second metal lens array, in particular wherein a volume encompassed by the transparent solid element is free of selectively reflecting and selectively diffracting surfaces, in particular wherein the transparent element is cuboid-shaped or plate-shaped.

[0074] This embodiment can be advantageous if a particularly compact design is desired along a construction direction, for example along the optical axis of the entrance aperture.

[0075] The solid transparent element can be a simple glass plate or a simple polymer plate that is transparent in the first spectral range.

[0076] According to one embodiment of the invention, the solid transparent element is made of a material selected from the group consisting of glass, polymer or crystal.

[0077] While in an embodiment with a beam splitter the metal lens arrays form an angle of 90° to each other, in this embodiment the metal lens arrays are located exactly opposite each other and enclose the solid transparency between them.

[0078] According to a further embodiment of the invention, the system comprises at least one liquid crystal configured to adapt a phase between the wavefronts of the object light assigned to the first and second components, in particular wherein the at least one liquid crystal is configured to change the phase between the wavefronts of the first and second components via a control module. The liquid crystal can, for example, be arranged in addition to a solid transparent element along the optical axis of the entrance aperture between the first and second metal lens arrays. Alternatively, the solid transparent element can comprise the liquid crystal or consist of the liquid crystal. In the latter embodiment, the liquid crystal should have birefringent properties.

[0079] If it is an embodiment with a beam splitter, the liquid crystal can be arranged on one of the sides that have the first mirror or the reflector array.

[0080] In a further embodiment, if the beam splitter is a beam splitter cube, one of the triangular prisms constituting the beam splitter can comprise the liquid crystal or consist of the liquid crystal.

[0081] Alternatively, both triangular prisms constituting the beam splitter can contain a liquid crystal. This allows for individual phase adjustments for both the first and second portions of the object light relative to the other portion. Furthermore, the use of two liquid crystals enables a longer overall optical path, allowing for larger phase shifts between the first and second portions.

[0082] The use of a liquid crystal to adjust the relative phases of the first and second components to each other enables the creation of a system that does not require any moving components and therefore has an extremely high degree of robustness.

[0083] According to an embodiment of the invention, which has already been described in a previous paragraph, it is provided that the transparent solid element comprises the at least one liquid crystal or consists of the at least one liquid crystal.

[0084] According to an embodiment of the invention, which has already been described in a previous paragraph and which comprises a polarization-dependent beam splitter cube according to at least one of the previous embodiments, it is provided that the polarization-dependent beam splitter comprises a first and a second prism which form a beam splitter cube of the beam splitter, wherein the first and / or the second prism comprises the at least one liquid crystal, in particular wherein both the first and the second prism have a liquid crystal.

[0085] According to one embodiment of the invention, a focal plane of the first metal lens array and a focal plane of the second metal lens array lie on top of one another.

[0086] According to one embodiment of the invention, it is provided that an analyzer is arranged behind the second metal lens array and in front of the detector in the propagation direction, which analyzer is designed to change the polarization states of the first and second components, so that an interference of the first component with the second component on the detector is achieved.

[0087] This embodiment enables a higher interference contrast on the detector.

[0088] According to one embodiment of the invention, it is provided that the entrance aperture comprises a polarizer which is configured to bring the object light from the first spectral range into the predefined polarization state.

[0089] According to one embodiment of the invention, the input aperture comprises at least one lens which is configured to collimate the object light.

[0090] According to an alternative embodiment of the invention, it is provided that the entrance aperture comprises a pinhole as an imaging element, and in particular wherein the entrance aperture is free of lenses or refractive optical elements that collimate the object light originating from the object under examination.

[0091] According to one embodiment of the invention, it is provided that the system is configured to deflect the propagation direction of the object light from the first spectral range incident on the system in a wavelength-dependent manner, so that the object light, as well as the first and second components, enclose a wavelength-dependent angle with the optical axis in addition to the first angle of inclination.

[0092] This can be achieved, for example, by a corresponding metal lens design of the first metal lens array.

[0093] This embodiment enables an improved color resolution of the system, since object light of the first spectral range is imaged onto different areas of the detector depending on the wavelength.

[0094] According to one embodiment of the invention, it is provided that the object light of the first spectral range consists of two or more disjoint wavelength ranges and / or wherein the system is configured to filter the object light into two or more disjoint wavelength ranges that form the first spectral range, wherein there are gaps between the wavelength ranges, in particular wherein these gaps are each at least 50 nm wide, so that for each wavelength range an interference pattern is generated on the detector, from which three-dimensional position information and a color composition with respect to the wavelength ranges of an object region of the object under examination can be created.

[0095] In particular, the first spectral range is split into the three primary colors red, green and blue, which can be translated into the following wavelength ranges, for example:

[0096] The wavelength range of the first spectral range for the blue color channel extends in particular from 420 nm to 480 nm, the wavelength range of the first spectral range for the green color channel extends in particular from 520 nm to 565 nm, and the wavelength range of the first spectral range for the red color channel extends in particular from 630 nm to 680 nm.

[0097] According to one embodiment of the invention, it is provided that the object light comprises at least one further spectral range which is different and disjoint from the first spectral range, wherein the first and the second metal lens array as well as the polarization-dependent beam splitter are transparent and optically inactive, i.e. neutral, for light from the at least one further spectral range, wherein the polarization-dependent beam splitter further comprises a VPH (volume phase hologram) which is designed to diffract the light from the at least one further spectral range in a polarization- and angle-dependent manner and to be transparent and optically inactive, i.e. neutral, for the light from the first spectral range.

[0098] The first and the at least one further spectral range can occupy wavelength ranges that vary along the spectrum. Particularly with regard to the previous embodiment, the further spectral range can be arranged, for example, between the green and red color channels, and in particular be limited to a spectral range between 570 nm and 620 nm. Alternatively and / or additionally, the at least one further spectral range can extend from the near infrared range, in particular from 700 nm or 800 nm upwards, into the infrared range of more than 1300 nm.Because the VPH is specifically arranged in a Littrow configuration and designed for wavelengths from the wider spectral range, incident object light from the wider spectral range (also referred to as the second spectral range in the context of the specification) propagating along the optical axis of the entrance aperture and impinging on the VPH is diffracted at an angle of 90° toward the detector. The incident object light from the second spectral range is specifically s-polarized when it impinges on the VPH.

[0099] In this embodiment, in particular reference light, which is coupled into the beam splitter via a reference arm, is caused to interfere with the object light, which is coupled via the so-called object arm, on the detector.

[0100] According to one embodiment of the invention, it is therefore provided that the system is configured to guide reference light from the second spectral range, in particular via a reference arm, via a side of the beam splitter opposite the entrance aperture to the VPH, wherein the first mirror is transparent in particular for reference light, i.e. in particular for light from the second spectral range.

[0101] According to this embodiment, on the side of the beam splitter opposite the entrance aperture, reference light, which is also s-polarized when it strikes the VPH, can be collimated from the second spectral range and also 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 at 90° in the opposite direction from the detector. There, it is reflected by a mirror or a prism arrangement, wherein the mirror and / or the prism arrangement enclose an angle of more or less than 45°, for example 45° ± 0.2°, with the VPH. As a result, the reflected reference light is at least partially transmitted at the VPH and interferes with the object light of the second spectral range.This interference pattern can be advantageously used to generate increased spatial resolution along the optical axis of the entrance aperture. In particular, the extended spectral range consists of a multitude of disjoint spectral lines with very narrow linewidths, for example, on the order of sub-nanometers, and extending over a range of, for example, 10 nm to 50 nm. Since the VPH diffracts each of these spectral lines slightly differently, this type of "optical comb" can be used to generate increased spatial resolution along the optical axis from the resulting interference patterns on the detector.

[0102] Similar holographic systems are known, but instead of a VPH they have, for example, conventional transmission diffraction gratings, which, however, have disadvantageous properties with regard to the desired properties.

[0103] It is noted that for the functionality of this embodiment, with respect to the light of the second spectral range, the first mirror is advantageously at least partially transparent.

[0104] Furthermore, it can be advantageous if the second A / 4 element is optically neutral, i.e. has no significant effect on the light.

[0105] Alternatively, a third λ / 4 element can be provided, which is arranged in front of the first mirror, for example as seen from the incident reference light, and is therefore in particular not accessible to object light of the first spectral range, wherein the third λ / 4 element is designed, together with the second λ / 4 element, to rotate the polarization of the reference light by 90°, in particular such that the reference light advantageously strikes the VPH in an s-polarized state.

[0106] This is described in the following embodiment of the invention, according to which the system comprises a third λ / 4 element which is arranged on a side of the first mirror facing away from the beam splitter and which is designed, in cooperation with the second λ / 4 element, to bring the reference light into a predefined polarization state so that the reference light is linearly s-polarized when it strikes the VPH coming from the first mirror.

[0107] Furthermore, it is advantageous if the polarization-dependent beam splitter also has no optical effect on light from the second spectral range, i.e., is also optically neutral. Furthermore, the first and second metal lens arrays are advantageously optically neutral for light from the second spectral range.

[0108] The reference light of the second spectral range can be provided by a reference light source, such as a laser, which can also serve as an object light source. Furthermore, the system may include a collimating lens for the reference light, allowing the collimated reference light to propagate toward the VPH. A polarizer may also be provided to ensure that the reference light hits the VPH in an s-polarized state.

[0109] 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, which prism arrangement is designed to reflect light, in particular the reference light of the at least one further spectral range diffracted by the VPH in the direction of the reflector array, at a prism angle in the direction of the detector, and wherein the prism arrangement is transparent and optically inactive for light from the first spectral range.

[0110] As already mentioned, the prism angle is set so that a reflection surface of the prism arrangement forms an angle of not 45° with the VPH, whereby in particular the angle is included with an amount greater than 45.2°.

[0111] This "tilt" ensures that the reference light beam reflected back by the prism array is transmitted through the VPH to a sufficiently high degree, providing sufficient reference light for interference on the detector with the object light of the second spectral range. This extension of the system can improve the resolution along the optical axis to the sub-millimeter or even sub-micrometer range.

[0112] Further features and advantages of the invention are explained below with reference to the figures describing exemplary embodiments. They show:

[0113] Fig. 1 is a schematic view of a first embodiment of the invention;

[0114] Fig. 2 is a schematic view of a second embodiment of the invention with color separation;

[0115] Fig. 3 is a schematic view of a third embodiment of the invention VPH;

[0116] Fig. 4 is a schematic view of a fourth embodiment of the invention without a beam splitter;

[0117] Fig. 1 schematically illustrates a system 1 according to an exemplary embodiment of the invention. The system 1 is configured for use in holographic imaging applications. In particular, the system 1 is suitable for three-dimensional color imaging.

[0118] A particular advantage of the system shown in Fig. 1 is its extremely high structural compactness, which is achieved in particular by the fact that no moving components or fault-prone precision mechanical components are required.

[0119] The system 1 shown in Fig. 1 is also referred to as a compact 3D color module in the context of the invention. The system 1 is configured to three-dimensionally capture and, if necessary, also display a surface or an area below the surface of an object under examination S—alternatively referred to as an object in this specification.

[0120] For this purpose, the object S is illuminated, for example, in a point-like area with an external light source 18, which can be included in the system 1 or arranged separately. The light 10 with which the object S is illuminated is also referred to as object light 10 in the context of holography. Natural ambient light can also serve as the light source 18—although this does not necessarily illuminate the object S in a point-like area.

[0121] In response to the illumination, the object S reflects the object light 10 through various processes such as scattering, reflection, or luminescence. The object light 10 originating from the object S is collected via the input aperture 2 of the system 1.

[0122] In the following and also in parts of the description, the case is considered in which the object light S originating from the object under investigation S originates from or near a plane E of the object under investigation S, which lies in a focal plane E of the entrance aperture 2. In cases in which the object light 10 originates from a plane close to the focal plane E of the entrance aperture 2, the system 1 behaves according to the differently curved wavefronts of the object light 10 at the entrance aperture 2, which is known to the person skilled in the art.

[0123] The entrance aperture 2 is configured to collimate the object light 10 originating from the object under examination S, in particular the object light 10 originating from the focal plane E of the entrance aperture 2. It should be noted that the entrance aperture 2 can have a collimating optic 2a, e.g., in the form of one or more lenses 2a. In the case illustrated in Fig. 1, the entrance aperture 2 comprises a collimating lens 2a. However, it is equally possible for the collimating optic 2a to comprise only a pinhole (not illustrated). With a correspondingly small pinhole and / or a sufficiently large object distance, the incident object light 10 is also collimated or at least exhibits a sufficiently high degree of collimation.

[0124] Regardless of the design of the entrance aperture 2, collimated light is defined as light with essentially flat wavefronts.

[0125] In addition, the entrance aperture 2 has an optical axis OA. The optical axis OA in Fig. 1 extends centrally and perpendicularly to the entrance aperture 2 or to the collimating lens 2a. The collimating lens 2a is configured at least to collimate object light 10 from a first spectral range. Furthermore, the collimating lens 2a can also be configured to collimate object light 10 from a second spectral range. In this case, the collimating lens 2a can comprise, for example, an achromat, an apochromat, or a superapochromat.

[0126] The figures depict light in the form of ray optics. This means that the wavefronts and the curvatures of the wavefronts of the light rays depicted in the figures are generally not shown. However, those skilled in the art will know how light wavefronts are influenced by the various optical components of system 1 and how the direction of propagation and the properties of the light are thereby influenced.

[0127] Fig. 1 shows two different light beams 10a, 10b of the object light 10. The first situation concerns a first object light beam 10a, which originates from a region of the object under examination S that lies on the optical axis OA of the entrance aperture 2. The second situation concerns a second object light beam 10b, which originates from a region of the object under examination that is laterally offset from the optical axis of the entrance aperture 2. The term “laterally offset” can, for example, be described using a Cartesian coordinate system (x, y, z as indicated in Fig. 1) that is assigned to the entrance aperture 2. The z-axis extends along the optical axis OA of the entrance aperture 2, and the x- and y-axes extend perpendicular to it, i.e., lateral to the optical axis OA. The designations “first” and “second” serve merely to differentiate between the two, but not to indicate a sequence.

[0128] The first object light beam 10a is collimated by the collimating lens 2a and then propagates parallel to the optical axis OA of the collimating lens 2a, ie, it encloses an angle of 0° with the optical axis OA of the entrance aperture 2. The angle enclosed by the collimated object light 10, 10a, 10b with the optical axis OA is also referred to as the first inclination angle ß in this specification.

[0129] The first inclination angle ß is defined in particular by the angle enclosed between the optical axis OA of the entrance aperture 2 and the propagation direction of the collimated object light beam 10, 10a, 10b. The second object light beam 10b is similarly collimated by the collimating lens 2a and then propagates further, however, at a first inclination angle to the optical axis OA of the collimating lens 2a, which is not equal to 0°. According to the laws of ray optics, the first inclination angle ß (in conjunction with the entrance aperture 2) contains information about the lateral position on the object under investigation from which the respective object light beam 10b originates.

[0130] A polarizer 15 is arranged behind the collimating lens 2a, which is configured and optionally adjustable to bring the object light 10 into a predefined polarization state. The polarizer 15 is configured, in particular, to bring the object light 10 into the predefined polarization state—at least for object light 10 from the first spectral range.

[0131] It should be noted that the polarizer 15 can also be arranged in front of the collimating lens 2a. And the incident object light 10 is only collimated by the collimating lens 2a arranged downstream of the polarizer 15. It is also conceivable that the collimating lens 2a or the entrance aperture 2 itself has a corresponding polarizing property.

[0132] The terms “behind” and “in front” are to be understood in particular as referring to an arrangement with respect to the propagation direction of the object light 10 and less to its geometric sequence or arrangement, which may deviate from the “purely optical” beam path due to folding by beam splitters or mirrors.

[0133] In the present case, the polarizer 15 is arranged and adjusted (e.g., at a rotation angle of 45°) such that the object light 10, regardless of the direction of incidence or the first inclination angle β, has s- and p-polarized light in approximately equal proportions. This means that in this example, the predefined polarization state is composed of a first polarization state comprising s-polarized object light and a second polarization state comprising p-polarized object light. It is assumed that the person skilled in the art is familiar with the terms s- and p-polarized light as conjugate linearly polarized polarization directions. The assignment of the s-polarized object light to the first polarization state and the p-polarized object light to the second polarization state can also be done exactly the other way around and is for illustrative purposes only.

[0134] In general, the polarizer 15 can be configured to convert the incident object light 10 into object light 10 having two conjugate polarization states. These could, for example, also be left- and right-rotating circularly polarized object light 10.

[0135] In the following, the case of s- and p-polarized polarization states will be discussed without loss of generality. This is generally valid because the object light originating from the object under investigation can also be unpolarized, whereby this object light is always "seen" by the first metal lens array as the first and second components - even without polarizer 15 - since unpolarized light also contains s- and p-polarized components.

[0136] The object light 10 with the predefined polarization state just described now strikes a first metal lens array 3 according to the invention. The first metal lens array 3 comprises a plurality of metal lenses 30 arranged in an array. The first metal lens array 3 is now arranged and configured according to the invention such that it focuses a first portion 101 of the object light having the first polarization state, i.e. at least increases the wavefront curvature, so that the object light is convergent, while in contrast a second portion 102 of the object light having the second polarization state propagates essentially unchanged through the first metal lens array. This means that if the object light has been collimated by the entrance aperture 2, the second portion 102 remains collimated after it has propagated through the first metal lens array 3.

[0137] The term "unchanged" in the context of the metal lens array 3, 4 refers in particular to the fact that the wavefronts of the second portion 102 or the second polarization state can ideally pass through the metal lens array 3, 4 completely unchanged, so that the wavefronts of the second portion 102 or the second polarization state are unchanged in front of and behind the metal lens array. It is clear to the person skilled in the art that slight changes in the wavefronts may nevertheless occur due to imperfections in the metal lens array 3, 4. This is intended to be reflected in the term "essentially." The synonymous term used within this specification is "optically neutral."

[0138] The focused first portion 101 is in particular focused such that it is focused on a focal plane 3B assigned to the first metal lens array 3.

[0139] A polarization-dependent beam splitter 6 is arranged behind the first metal lens array 3, in particular in front of the focal plane 3B of the first metal lens array 3. In the example of Figure 1, the polarization-dependent beam splitter 6 is a polarization-dependent beam splitter cube. The beam splitter cube 6 comprises two triangular prisms, e.g., a first and a second triangular prism 6A, 6B, which are connected at their base surfaces and define a beam splitter surface 6F along the base surfaces. The beam splitter surface 6F extends at a 45° angle to the optical axis OA of the input aperture 2 or the first metal lens array 3.

[0140] The first object light beam 10a, comprising a first and second portion, and the second object light beam 10b, also comprising a first and second portion 101, 102, now strike the first triangular prism 6A of the beam splitter 6 and propagate through it to the beam splitter surface 6F.

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

[0142] In the following, the object light rays of the first portion 101 are considered first. These rays were focused by the first metal lens array 3, i.e., they have the first polarization state behind the first metal lens array 3, and are thus reflected at the beam splitter surface 6F. This light propagates further in the first triangular prism 6A and then strikes a first λ / 4 plate 9a, which is arranged, for example, on a surface of the beam splitter cube 6.

[0143] The first λ / 4 plate 9a causes a change in the polarization state of the first portion 101; in this example, the polarization state is changed from linearly polarized to circularly polarized.

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

[0145] The reflector array 8 comprises the same number of retroreflectors 80 as the first metal lens array 3 comprises metal lenses 30, and also the same number of retroreflectors 80 as the second metal lens array 4 comprises metal lenses 40,

[0146] The first portion 101 reflected back by the reflector array 8 now propagates again through the first λ / 4 plate 9a, whereby the polarization state of the first portion 101 is again changed, in such a way that the polarization state of the first portion 101 now corresponds to the second polarization state of the object light behind the entrance aperture 2 - in this example, the polarization state of the first portion changes from s-polarized light to p-polarized light after the first portion 101 has propagated a total of twice through the first λ / 4 plate 9a.

[0147] The first portion 101 then propagates through the first triangular prism 6A and impinges on the beam splitter surface 6F, where the first portion 101 is now transmitted due to the changed polarization. The first portion 101 then propagates further through the second triangular prism 6B and subsequently impinges on a second metal lens array 4, which is arranged opposite the reflector array 8 on the beam splitter 6.

[0148] Let us now turn to the second portion 102 of the object light. The second portion 102 of the object light, i.e., the portion exhibiting the second polarization state, propagates behind the beam splitter surface 6F through the second triangular prism 6B of the beam splitter 6 and then through a second λ / 4 plate 9b, which is arranged, for example, on a side of the beam splitter cube 6 opposite the first metal lens array 3.

[0149] The second λ / 4 plate 9b causes a change in the polarization state of the second component 102; in this example, the polarization state is changed from linearly polarized to circularly polarized. Behind the second λ / 4 plate 9b, a planar mirror 7 is arranged perpendicular to the optical axis OA of the entrance aperture 2, which reflects the second component 102 back. The reflected second component 102 then propagates again through the second λ / 4 plate 9b, thereby changing the polarization state of the second component 102 in such a way that the polarization state of the second component 102 now corresponds to the first polarization state of the object light behind the entrance aperture 2 – in this example, the polarization state of the second component 102 changes from p-polarized light to s-polarized light after the second component 102 has propagated twice through the second λ / 4 plate 9b.

[0150] The second portion 102 reflected back by the first mirror 7 then propagates through the second triangular prism 6B and again strikes the beam splitter surface 6F, where, due to the changed polarization of the second portion 102, the second portion 102 is reflected. The second portion 102 thus continues to propagate through the second triangular prism 6B and then, like the first portion 101, strikes the second metal lens array 4.

[0151] The second metal lens array 4 has essentially the same properties as the first metal lens array 4 and comprises a plurality of metal lenses 40 arranged in the array. In contrast to the first metal lens array 3, however, in this example the second metal lens array 4 has the property of leaving light in the polarization state of the second portion 102 unchanged, in particular collimated unchanged, and of collimating light from the first portion 101. This means that behind the second metal lens array 4, the light from both the first and second portions 101, 102 is collimated, provided that the object light was collimated by the entrance aperture 2 and directed towards the first metal lens array 3.

[0152] Behind the second metal lens array 4, an analyzer 14 is arranged, which aligns the polarization states of the first and second portions 101, 102 so that the light beams of the first and second portions 101, 102 can interfere with each other. The analyzer 14 is set, for example, at an angular position of 45° so that the aligned first and second portions 101, 102 have the same polarization directions. Behind the analyzer 14, a detector 5 is arranged, which is configured to record the interfering first and second portions 101, 102 of the object light. The detector 5 can be a camera, for example. The interfering first and second portions 101, 102 of the object light form an interference pattern on the detector 5, which can be evaluated using an evaluation unit (not shown) to generate three-dimensional image information.

[0153] With reference to object light 10, which encloses a first inclination angle unequal to 0° with the optical axis OA, the following should now be noted. Due to the special and inventive arrangement of the optical components of system 1, the object light rays of the first portion 101 and the second portion 102 of the object light, which enter system 1 at the first inclination angle β, i.e., enclose the first inclination angle β with the optical axis OA behind the entrance aperture 2, enclose a second inclination angle β' after reflection by the reflector array 8 or the planar mirror 7. This second inclination angle β' is twice as large as the first inclination angle β and is achieved by using the combination of planar mirror 7 and reflector array 8.While the first component 101 has an angle of reflection ß at an angle of incidence ß after reflection at the reflector array 8, the second component 102 has an angle of reflection of -ß after reflection at the planar mirror 7. These angles then add up to twice the first inclination angle 2ß, which corresponds to the second inclination angle ß'.

[0154] The advantage of this arrangement is the improved lateral resolution of System 1 compared to other systems. Furthermore, such a System 1 does not contain any moving precision mechanical components, making System 1 extremely robust and enabling a very compact design.

[0155] The system 1 according to the invention allows the person skilled in the art to calculate the wavefronts of the object light from the local wavefront angles per metal lens of the second metal lens array or from the interference frequencies of the interference pattern generated on the detector, which in turn allow conclusions to be drawn about a z-deviation from the focal plane of the entrance aperture; thus, in addition to the lateral spatial resolution, image information regarding a z-position of the area of ​​the object under examination can be generated. As an alternative to the optical properties of the second metal lens array 4, it can also be considered to design the second metal lens array 4 identically to the first metal lens array 3, i.e., in particular, identical also with regard to its optical properties with regard to the polarization states.

[0156] In this case, an λ / 2 plate 11 (indicated by dashed lines) would be arranged behind the beam splitter cube 6 on the side of the second metal lens array 4 and in front of the second metal lens array 4. This plate rotates the polarization states of the first and second components 101, 102 by 90° so that they return to their original, predefined polarization states. This embodiment with two completely identical metal lens arrays 3, 4 has the advantage that the system 1 and the metal lens arrays 3, 4 can be manufactured comparatively cost-effectively.

[0157] In a further advantageous embodiment, which is illustrated in Fig. 1 with at least one embodiment, the system 1 comprises a means for shifting a light wave phase for the first and / or second portion 101, 102 of the object light. This makes it possible to avoid an interfering DC or constant light signal on the detector 5. For this purpose, the phase in the first or second portion is typically achieved, for example, by a slight change in the optical path length (also referred to as the optical path length).

[0158] The change in the optical path length can be achieved by an actual geometric extension of the path length for the first and / or the second part, or by an adjustment or variation of the refractive index through which the first and / or the second part runs.

[0159] In Fig. 1, a phase adjustment is realized by an actuator 12, which can move the reflector array 8 at least along the optical axis OA of the input aperture 2 (even if this is folded by the beam splitter cube).

[0160] The actuator 12 can, for example, comprise a piezo element that can be controlled via an electrical actuator. In particular, the piezo element can be a piezo ring element.

[0161] The actuator 12 thus enables a change in the optical path length for the first portion 101 of the object light, which is reflected from the input aperture 2 via the beam splitter 6 toward the reflector array 8. By shifting the reflector array 8 along the optical axis OA, a relative phase to the second portion 102 of the object light can be adjusted.

[0162] Alternatively or additionally, a further actuator 12' can also be arranged on the side of the first mirror 7 and can be configured to move the mirror 7 at least along the optical axis OA of the entrance aperture 2.

[0163] It is also possible for both an actuator 12 to control the reflector array 8 and another actuator 12' to control the first mirror 7, so that the optical paths of both the first 101 and the second part 102 can be changed.

[0164] Alternatively or additionally, it would also be possible to create a change in the refractive index in one or both triangular prisms 6A, 6B of the beam splitter cube 6, so that the light wave phases of the first and / or second portions 101, 102 can be adjusted (not shown). The refractive index can be changed by applying an electrical voltage to the first and / or second triangular prism 6A, 6B. For this purpose, the triangular prisms 6A, 6B must be made of a material familiar to those skilled in the art.

[0165] Alternatively, the phase change in one of the two polarization states can be adjusted by the first and / or second metal lens arrays 3, 4. This can be achieved, for example, by a phase-shifting element or by new developments in the metal lens of the array itself, which allow the phase of a polarization direction to be varied by applying an electrical or magnetic quantity.

[0166] Regardless of how the phase shifts are generated, system 1 is particularly designed to enable relative phase shifts of more than 2μΩ, so that in addition to constant light suppression in the interferograms of individual wavelengths or wavelength ranges, it is also possible to separate different colors or more widely separated wavelength ranges via the phase position. The corresponding separation is possible, for example, using a Fourier transformation and is generally known to those skilled in the art. In Fig. 2, based on Fig. 1, an extension of the invention for all three primary colors RGB of system 1 is presented.Although the phase shift over a larger area (several 2TT) has already been described in the previous paragraphs, this can limit the dynamic range of the detector 5, since the signals of all primary colors exhibit a high degree of interference preferentially in a central area around the optical axis OA of the system (now relative to the detector) (and less at the edges), thus leading to undesirable signal enhancement. Therefore, Fig. 2 shows an embodiment that slightly deflects the three primary colors in a wavelength-selective manner from the central area around the optical axis OA in the area of ​​the first metal lens array 3. This can be achieved using a local prism, an optical grating, or a VPH (not shown), which is superimposed on the actual lens effect of the first metal lens array 3.

[0167] Alternatively or additionally, this property can also be fulfilled by the first metal lens array 3 itself. In Fig. 2, the wavelength-selective deflection via the first metal lens array 3 for three different wavelengths (also referred to as three colors RGB in the context of the specification) is shown in the form of rays (arrows 21, 22, 23).

[0168] Each of the three wavelengths / colors per object point or object area generates an interferogram or an interference pattern with a spatial frequency and a direction on the detector 5. Together with phase shifts as described for Fig. 1, the respective spectral image components can be separated by the person skilled in the art, so that by evaluating the interference pattern on the detector 5 via an evaluation unit, spatially resolved 3D color image information can be generated.

[0169] To increase the resolution of system 1, particularly along the optical axis OA, system 1 can be modified with a beam splitter 6 configured as follows. This system then allows for extremely high resolution along the z-axis, particularly in the submicrometer range.

[0170] In the embodiment illustrated in Fig. 3, the beam splitter cube 6 comprises a transmission diffraction grating arrangement in the form of at least one volume phase hologram grating (VPH) 16 along the beam splitter surface. VPHs 16 are commonly known to those skilled in the art as volume phase hologram (transmission) gratings. Furthermore, the system comprises a wavelength-selective prism arrangement 17, in particular a wavelength-selective double prism arrangement 17, on the side of the beam splitter cube 6 opposite the detector 5.

[0171] It should be noted that, according to this example, the VPH 16 and the prism arrangement 17 are configured for a second spectral range, and in particular, are transparent to the first spectral range of the object light, i.e., the wavefronts of the first spectral range pass through the VPH 16 and the prism arrangement 17 unchanged. Conversely, the prism arrangement 17 is configured such that light from the second spectral range is reflected by a prism surface 17F, while light from the first spectral range propagates through the prism surface 17F and thus passes through the prism arrangement 17 unchanged.

[0172] Likewise, the VPH 16 is designed to diffract light from the second spectral range depending on angle and wavelength, and to allow light from the first spectral range to propagate through it unchanged.

[0173] Conversely, the metal lens arrays 3, 4, the polarizer 15, the λ / 4 plates 9a, 9b, and optionally also the λ / 2 plate 11 are equally transparent (and in particular optically neutral), leaving the wavefronts and polarization states of the object light of the second spectral range unchanged. The light from the second spectral range is used by system 1 to generate a particularly high spatial resolution, particularly along the optical axis.

[0174] Instead of the beam path for the object light of the first spectral range shown in Fig. 1, Figure 3 instead shows the beam path for the object light 20-1 and reference light 20-2, which in this example also lies in the second spectral range. The reference light 20-2, which is preferably also s-polarized, at least when it strikes the VPH, is emitted by a reference light source 19, e.g. a single-mode aperture, and collimated via collimation optics 25. The optical axes of the collimation optics 25 and the entrance aperture 2 lie on top of one another. The reference light 20-2 then enters the beam splitter cube 6 in this collimated state from a side of the beam splitter cube 6 opposite the entrance aperture 2. To make this possible, the planar first mirror 7, which reflects object light 10 from the first spectral range, must be transparent to the reference light 20 from the second spectral range.The mirror 7 is at least a dichroic mirror and does not change the wavefronts of the reference light. The second λ / 4 plate 9a can also be configured not to change the polarization state of the reference light 20-2, or, in conjunction with another delay element, e.g., a third λ / 4 plate 9c, can be configured so that the reference light 20-2 is s-polarized upon impingement on the VPH 16.

[0175] The mode of operation of the VPH 16 in interaction with the prism arrangement 17 is now described below.

[0176] In the example shown, the object light 20-1 from the second spectral range, collimated by the entrance aperture 2, strikes the VPH 16 at an angle of approximately 45°. Since the VPH 16 is positioned in a so-called Littrow arrangement and optimized for the second spectral range, the incident object light 20-1 from the second spectral range is diffracted by the VPH 16 at an angle of approximately 90° (relative to the incident object light 20-1) or approximately 45° (relative to the beam splitter surface 6F) toward the detector 5. Ideally, the object light 20-1 of the second spectral range is linearly polarized when it strikes the VPH 16, specifically s-polarized, since the diffraction efficiency of VPHs is then at its highest.

[0177] On the other side, the collimated reference light 20-2 also strikes the VPH 16 and is diffracted by it toward the prism array 17, which is arranged on the side of the beam splitter 6b opposite the detector 5. Here, too, the VPH 16 is positioned at a 45° angle relative to the reference light 20-2, resulting in a Littrow configuration.

[0178] The reference light 20-2 propagates toward the prism arrangement 17 and is reflected there by a reflection surface 17F of the prism arrangement 17, which forms an angle α with the beam splitter surface and thus the VPH. The angle α (not shown, instead the differential angle (prism angle) αa = 45°-α) is not equal to 45° in this case, but is either greater or less than 45°, in particular more than 0.2° greater or less than 45°. As a result, the reference light, which is diffracted at an angle of 45° by the VPH 16 toward the prism arrangement 17, is reflected by the prism arrangement 17 back to the VPH 16, where the reference light 20-2 strikes the VPH 16 at an angle different from 45°.As a result, however, at least a part of the reference light 20-2 reflected back by the VPH 16 is not diffracted back towards the reference light source 19, but propagates through the VPH 16 towards the detector 5, where it interferes at an angle corresponding to twice the angle a relative to the diffracted object light 20-1 on the detector 5.

[0179] The core idea of ​​this embodiment is that the beam splitter cube 6 has a VPH 16 along its beam splitter surface 6F, which forms an angle a of not equal to 45° with the reflection surface 17F of the prism arrangement 17. As a result, reference light 20-2 is at least partially and to a sufficiently high degree transmitted from the VPH 16 toward the detector 5 upon its second impact on the VPH 16 (after reflection by the prism arrangement 17).

[0180] In particular, it is provided that the light from the second spectral range is in the form of spectral line combs comprising a plurality of disjoint, narrow-band spectral lines - in particular with line widths in the subnanometer range.

[0181] Due to the properties of the VPH 16, the multitude of spectral lines are then diffracted slightly differently depending on the wavelength (cf. beams to 20-T, 20-2'), resulting in a dispersive splitting of the spectral lines on the detector side 5, enabling highly precise spatial resolution along the optical axis OA. The information about the z-position of the object S is contained in particular in the phase data of the individual spectral lines. Combined with the information from the interference pattern of the first spectral range, such a system 1 enables the determination of high-resolution 3D color information of an object S.

[0182] The second spectral range typically lies in the near-infrared or infrared range. For example, in the range from 700 to 900 nm, or even in a range of 1300 nm and beyond. In the latter case, measurements can even be taken beneath biological tissue surfaces. It is, of course, also possible to provide a separate arrangement for the VPH 16 described in the previous paragraphs, which only contains the optical components necessary for the second spectral range, in particular the VPH 16 and the prism arrangement, which can then also be designed as a mirror.

[0183] Additionally, system 1 can be equipped with an additional VPH (not shown), where the additional VPH is optically active in a different, third spectral range and diffracts the incident light there in a wavelength- and angle-dependent manner. In this way, even a third spectral range can be scanned with appropriate components and transmission properties of these components.

[0184] For example, it would be possible to capture a surface profile in the near infrared and simultaneously a structure below the surface in the infrared range, whereby in the visible (first spectral) range the system is operated via the described metal lens array mode of operation.

[0185] Alternatively, it is also possible for the second spectral range and the first spectral range to be nested within one another, but without overlapping, ie the first spectral range comprises, for example, three wavelength ranges which are characteristic, for example, of the color channels (or colors) red, green and blue, while the second spectral range lies in at least one wavelength range which is located between these three wavelength ranges.

[0186] The wavelength range of the first spectral range for the blue color channel extends in particular from 420 nm to 480 nm, the wavelength range of the first spectral range for the green color channel extends in particular from 520 nm to 565 nm, and the wavelength range of the first spectral range for the red color channel extends in particular from 630 nm to 680 nm. The second spectral range can therefore extend, for example, in a wavelength range from 505 nm to 515 nm, and / or from 570 nm to 625 nm, or from 690 nm upwards. A significantly less complex embodiment of the invention is shown in Figure 4. In this variant, the beam splitter cube can be omitted without having to forego the basic concept of the invention. The advantage of the embodiment shown in Figure 4 is the possibility of an ultra-compact design.

[0187] In Fig. 1, the edge length of the beam splitter cube determines the dimension in all three spatial directions. In Fig. 4, however, the spatial direction perpendicular to the optical axis can be selected to be smaller. Instead of the beam splitter cube, a transparent solid element is provided, which points with a first surface towards the first metal lens array and with a second surface opposite the first surface towards the second metal lens array, in particular wherein a volume encompassed by the transparent solid element is free of selectively reflecting and selectively diffracting surfaces, in particular wherein the transparent element is cuboid-shaped or plate-shaped.

[0188] The element can therefore be, for example, a glass plate or a polymer plate. On the transparent element, the first metal lens array is arranged on a planar surface opposite the second metal lens array, which is arranged on a planar surface on a side opposite the first surface. The entrance aperture and the polarization states generated there, which are impressed on the object light, have already been described in connection with Fig. 1.

[0189] Metal lenses can easily be manufactured with a numerical aperture NA = 0.5. For a typical metal lens diameter of 1 mm, the focal length at 1 mm to a metal lens plane, i.e. the transparent element separating the metal lens arrays would have to be about 2 mm (for the sake of simplicity, the refractive index of the transparent element was not taken into account). This would be significantly smaller than, for example, the height of one edge of a beam splitter cube (e.g. 5 mm). Furthermore, the height and lateral dimensions are decoupled in this example, which is particularly advantageous for mobile phone applications, as height is an absolute premium here, while size along the lateral direction is not a problem. This means that with a compact design along the z-axis (height), a large-area detector can still be used because it extends along the x- and y-directions.

[0190] For the beam path in detail, reference is made to Figure 1 with regard to the polarization states and their generation at the entrance aperture.

[0191] Figure 4 depicts two different light beams of the object light. The first situation concerns a first object light beam 31, which originates from a region of the object under examination that lies on the optical axis OA of the entrance aperture 2. The second situation concerns a second object light beam 32, which originates from a region of the object under examination that lies laterally offset from the optical axis OA of the entrance aperture 2.

[0192] The first object light beam 31 is collimated by the entrance aperture 2 and then propagates parallel to the optical axis OA of the entrance aperture 2, ie, it encloses an angle of 0° with the optical axis OA of the entrance aperture 2. The angle enclosed by the collimated object light with the optical axis is also referred to as the first inclination angle ß in this specification.

[0193] The first inclination angle ß is defined in particular by the angle enclosed between the optical axis OA of the entrance aperture 2 and the propagation direction of the collimated object light beam.

[0194] The second object light beam 32 is also collimated by the entrance aperture 2 and then propagates further, however, at a first inclination angle ß to the optical axis OA of the collimating lens 2a, which is not equal to 0°. According to the laws of ray optics, the first inclination angle ß (together with an associated focal length of the entrance aperture) contains information about the lateral position on the object under investigation from which the respective object light beam originates.

[0195] The entrance aperture 2 further comprises a polarizer 15 configured to bring the object light into a predefined polarization state. The polarizer 15 is configured, in particular, to bring the object light into the predefined polarization state—at least for object light from the first spectral range.

[0196] In the present case, the polarizer 15 is arranged and adjusted (e.g., at a rotation angle of 45°) such that the object light, regardless of the direction of incidence or the first inclination angle β, has approximately equal proportions of s- and p-polarized light. This means that in this example, the predefined polarization state is composed of a first polarization state comprising s-polarized object light and a second polarization state comprising p-polarized object light. The assignment of the s-polarized object light to the first polarization state and of the p-polarized object light to the second polarization state can also be done the other way around and is for illustrative purposes only.

[0197] In the following, without loss of generality, the case of s- and p-polarized polarization states is discussed.

[0198] According to the invention, the object light with the predefined polarization state just described now strikes the first metal lens array 3. According to the invention, the first metal lens array 3 is now arranged and configured such that it focuses a first portion 31-1 of the object light having the first polarization state, while a second portion 31-2 of the object light having the second polarization state propagates essentially unchanged through the first metal lens array 3.

[0199] The focused first portion 31-1 is in particular focused such that it is focused on a focal plane 3B assigned to the first metal lens array 3.

[0200] This applies to both the first and the second object light beam 31, 32.

[0201] The second metal lens array 4 is now arranged such that a focal plane 4B associated with the second metal lens array 4 lies on the focal plane 3B associated with the first metal lens array 3. Furthermore, the second metal lens array 4 is configured such that it collimates object light 31-1, which has the first polarization state and is thus focused on the focal plane 3B of the first metal lens array 3, and transmits the second portion 31-2 of the object light, which has the second polarization state, essentially unchanged—the second portion 31-2 is therefore still collimated thereafter.

[0202] For the first object light beam 31, which encloses an angle of 0° with the optical axis OA, this means that it comes out of the second metal lens array 4 at an angle of 0° and hits the detector 5 at this angle.

[0203] For the second object light beam 32, which propagates at a first inclination angle ß of not equal to 0° to the optical axis OA, it follows that the first portion 32-1 of the second object light beam 32, after appropriate focusing and recollimation of the first portion 32-1 with the first polarization state, includes a second inclination angle ß' with the second portion 32-2 of the second object light beam, which propagates essentially unchanged through the first and second metal lens arrays 3, 4, which is twice as large as the first inclination angle ß. The first and second portions 32-1, 32-2 of the second object light beam 32 therefore strike the detector, which is arranged behind the second metal lens array 4, at this second inclination angle.In particular, an analyzer 14 can also be arranged in front of the detector 5, which analyzer 14 adjusts the polarization states of the first and second components, in particular in a 45° rotation, so that an improved interference and thus an improved interference pattern on the detector 5 is created.

[0204] To achieve a relative phase adjustment of the wavefronts with respect to the first component and the second component, as described in connection with Fig. 1, the solid transparent element can comprise a liquid crystal (not shown) that has a different refractive index for object light of the first and / or second polarization state, so that a phase relationship with respect to the wavefronts can be set via the liquid crystal. A control unit can be provided in the system for this purpose.

[0205] The embodiment described in Fig. 4 is particularly advantageous for metal lens arrays 3, 4 whose metal lenses have a comparatively high numerical aperture; for example, a numerical aperture greater than 0.4.

[0206] Regardless of the specific embodiment, the system 1 can

[0207] Laser light source 18 which is designed to controllably illuminate the object under examination and in particular to illuminate the object S in regions so that, for example, a complete image of the object under examination can be generated via an optical scanning process.

[0208] For this purpose, it can be provided that the laser light source 18 emits different wavelengths in a sequence and thus sequentially illuminates the object under examination with different wavelengths, so that color information can be obtained from the sequential illumination.

[0209] Alternatively, the laser light source can be configured to emit light from the first and second spectral ranges simultaneously or at different times.

[0210] References

[0211] [1] Jangwoon Sung et al, “Progresses in the practical metasurface for holography and lens,” Nanophotonics 2019, 8(10), p. 1701-1718.

[0212] [2] Ling Li, et al., “Polarization-Switchable Multi-Focal Noninterleaved Metalenses in the visible,” Laser&Photonics reviews, 2021, 15, 2100198,

[0213] List of reference symbols

[0214] 1 system

[0215] 2 Entrance aperture

[0216] 2a Collimation optics / lens / objective

[0217] 3 first metal lens array

[0218] 30 metal lenses

[0219] 3B Focal plane of the first metal lens array

[0220] 4 second metal lens array

[0221] 40 metal lenses

[0222] 4B Focal plane of the second metal lens array

[0223] 5 Detector

[0224] 6 beam splitters

[0225] 6A first prism of the beam splitter

[0226] 6B second prism of the beam splitter

[0227] 6F reflection surface

[0228] 8 reflector array

[0229] 80 retroreflectors

[0230] 9a first A / 4 element

[0231] 9b second A / 4 element

[0232] 11 A / 2 element

[0233] 12, 12' actuator arrangement

[0234] 13 solid transparent element

[0235] 13-1 first side of the solid transparent element

[0236] 13-2 second side of the solid transparent element

[0237] 14 Analyzer

[0238] 15 Polarizer

[0239] 16 VPH

[0240] 17 prism arrangement, double prism

[0241] 17F reflection surface

[0242] 18 Object light source

[0243] Reference light source

[0244] 25 Reference light collimation optics

[0245] 21 , 22, 23 Light rays red (21), green (22), blue (23

[0246] 31 first ray of light

[0247] 31-1 first share

[0248] 31-2 second share

[0249] 32 second light beam

[0250] 32-1 first share

[0251] 32-2 second share

[0252] 10 Object light

[0253] 101 first share

[0254] 102 second share

[0255] 10a first ray of light

[0256] 10b second light beam

[0257] 20-1 , 20-1 ' Object light beam in the second spectral range 20-2, 20-2' Reference light beam in the second spectral range E Focal plane of the entrance aperture

[0258] OA optical axis

[0259] S Object of investigation x, y, z directions of a Cartesian coordinate system

[0260] Aa prism angle ß first inclination angle ß' second inclination angle

Claims

Optical system (1) for 3D imaging comprising at least the following components: An entrance aperture (2) having an optical axis (OA), a first metal lens array (3) and a second metal lens array (4), a detector (5), wherein the entrance aperture (2) is configured to collimate object light (10) coming from an examination object (S) in a first spectral range and to fix it in a predefined polarization state composed of two mutually conjugated polarization states, wherein the object light (10) propagates at a first inclination angle (ß) with respect to the optical axis (OA) of the entrance aperture (2) to the first metal lens array (3), wherein the first metal lens array (3) is configured and arranged to focus a first portion (101) of the object light (10), which comprises a first polarization state of the two mutually conjugated polarization states, and a second portion (102) of the object light (10),which comprises a second polarization state of the two mutually conjugated polarization states, wherein the second metal lens array (4) is designed and arranged to collimate the focused first portion (101) and to transmit the second portion (102) unchanged, so that the first portion (101) and the second portion (102), after being propagated through the first and second metal lens arrays (3, 4), each have the same wavefront curvature, and in particular are each collimated, and enclose a second angle of inclination (ß') with each other with respect to their respective propagation direction and strike the detector (5) to form an interference pattern,wherein the second angle of inclination (ß') corresponds in magnitude to twice the first angle of inclination (ß) and based on the interference pattern, three-dimensional position information of an object region of the object under examination (S) can be created. The system (1) according to claim 1, characterized in that the first and the second portion (101, 102) of the object light (10) are each perpendicularly linear, polarized to each other, in particular wherein the predefined polarization state is a linearly polarized polarization state consisting of a superposition of the first and the second portion (101, 102).

3. The system (1) according to one of the preceding claims, wherein the system (1) comprises a polarization-dependent beam splitter (6), in particular a polarization-dependent beam splitter cube, between the first and the second metal lens array (3), wherein 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) arranged between the polarization-dependent beam splitter (6) and the first mirror (7), A second λ / 4 element (9b) arranged between the polarization-dependent beam splitter (6) and the reflector array (8), wherein the polarization-dependent beam splitter is arranged with respect to object light (10) incident from the first metal lens array (3) such that the first portion (101) is reflected by the beam splitter (6) and the second portion (102) is transmitted through the beam splitter (6), wherein the reflector array (8) is arranged on a side of the beam splitter (6) to which the first portion (101) coming from the first metal lens array (3) and reflected by the beam splitter (6) propagates, wherein the first mirror is arranged on a side of the beam splitter (6) opposite the first metal lens array (3), wherein the first mirror (7) redirects the second portion (102) striking the first mirror (7) in the direction of the beam splitter (6) and reflected back to the second metal lens array (4),in particular, wherein the back-reflected first and the back-reflected second portion (101, 102) propagate through the polarization-dependent beam splitter (6) in the direction of the second metal lens array due to the exchanged polarization states.

4. The system (1) according to claim 3, characterized in that between the beam splitter (6) and the second metal lens array (4) an A / 2 element (11) is arranged, which is configured to exchange the polarization states of the first and second portions (101, 102). The system (1) according to one of claims 3 or 4, characterized in that the system (1) comprises an actuator arrangement (12) configured to adjust a position of the first mirror (7) and / or the reflector array (8) such that a phase between the wavefronts assigned to the first and second portions (101, 102) can be adjusted.The system (1) according to one of claims 1 or 2, characterized in that the system (1) comprises a transparent solid element (13) which points with a first surface (13-1) in the direction of the first metal lens array (3), and with a second surface (13-2) opposite the first surface (13-1) in the direction of the second metal lens array (4), in particular wherein a volume encompassed by the transparent solid element (13) is free of selectively reflecting and selectively diffracting surfaces, in particular wherein the transparent element is cuboid-shaped or plate-shaped.The system (1) according to one of the preceding claims, characterized in that the system (1) comprises at least one liquid crystal configured to adapt a phase between the wavefronts associated with the first and second portions (101, 102), in particular wherein the at least one liquid crystal is configured to change the phase between the wavefronts of the first and second portions (101, 102) via a control module. The system according to claims 6 and 7, wherein the transparent solid element (13) comprises the at least one liquid crystal or consists of the at least one liquid crystal.The system according to claim 7 and one of claims 3, 4 or 5, characterized in that the polarization-dependent beam splitter (6) comprises a first and a second prism (6A, 6B) which form a beam splitter cube of the beam splitter (6), wherein the first and / or the second prism (6A, 6B) comprises the at least one liquid crystal, in particular wherein both. the first and the second prisms (6A, 6B) each comprise a liquid crystal according to claim 7. The system (1) according to one of the preceding claims, characterized in that a focal plane (3B) of the first metal lens array (3) and a focal plane (4B) of the second metal lens array (4) lie on top of one another. The system (1) according to one of the preceding claims, characterized in that an analyzer (14) is arranged behind the second metal lens array (4) and in front of the detector (5) in the propagation direction, which analyzer is configured to match the polarization states of the first and second components (101, 102) such that interference of the first component (101) with the second component (102) on the detector (5) is achieved.The system (1) according to one of the preceding claims, characterized in that the input aperture (2) comprises a polarizer (15) configured to bring the object light (10) from the first spectral range into the predefined polarization state. The system (1) according to one of the preceding claims, characterized in that the input aperture (2) comprises at least one lens (2a) configured to collimate the object light (10) coming from the object under examination (S). The system (1) according to one of the preceding claims, wherein the system (1) is configured to deflect the propagation direction of the object light (10) from the first spectral range incident on the system (1) in a wavelength-dependent manner, such that the object light (10) and the first and second components (101, 102) enclose a wavelength-dependent angle with the optical axis (OA) in addition to the first inclination angle (β).The system (1) according to one of the preceding claims, characterized in that the object light (10) of the first spectral range consists of. two or more disjoint wavelength ranges and / or wherein the system (1) is configured to filter the object light (10) into two or more disjoint wavelength ranges which form the first spectral range, wherein there are gaps between the wavelength ranges, wherein for each wavelength range an interference pattern is generated on the detector (5), from which three-dimensional position information and a color composition with respect to the wavelength ranges of an object region of the object under examination (S) can be created.

16. The system (1) according to one of the preceding claims, if dependent on claim 3, characterized in that the object light (10) comprises at least one further spectral range which is different and disjoint from the first spectral range, wherein the first and the second metal lens array (3, 4) and the polarization-dependent beam splitter (6) are transparent and optically inactive for light from the at least one further spectral range, wherein the polarization-dependent beam splitter (6) further comprises a volume phase hologram (16) which is designed to diffract the light from the at least one further spectral range in a polarization- and angle-dependent manner and to be transparent and optically inactive for the light from the first spectral range, in particular wherein the volume phase hologram (16) extends along a reflection surface (6F) of the beam splitter (6).

17. The system according to claim 16, characterized in that the system is designed to guide reference light from the second spectral range, in particular via a reference arm, via a side of the beam splitter (6) opposite the entrance aperture to the VPH, wherein the first mirror is transparent in particular to reference light.

18. The system according to claim 17, characterized in that the system (1) comprises a third λ / 4 element (9c) which is arranged on a side of the first mirror (7) facing away from the beam splitter (6) and which is designed, in cooperation with the second λ / 4 element (9b), to convert the reference light (20-2, 20-2') into a predefined polarization state. so that the reference light (20-2, 20-2') is s-polarized when it strikes the VPH (16) coming from the first mirror. The system (1) according to one of claims 16 to 18, wherein the system (1) has a wavelength-selective prism arrangement (17) between the reflector array (8) and the beam splitter (6), which prism arrangement is configured to reflect light of the at least one further spectral range at a prism angle (Aa) in the direction of the detector (5), and wherein the prism arrangement (17) is transparent and optically inactive for light from the first spectral range.

Images