Optical spectrometer with diffractive element and spatially resolving light detector

Abstract

A compact optical spectrometer comprises an input port (2), a dif- fractive element (48), and a spatially resolving light detector (58). The input port (2) and the diffractive element (48) are centered on a device axis (10) of the spectrometer. The diffractive element (48) extends around the device axis (10). The device further comprises a first and a second transparent plate (18, 20). The light path from the input port (2) to the diffractive element (48) extends through the first transparent plate (18), and the light path from the diffractive element (48) to the light detector (58) extends through the second transparent plate (20) with at least one reflection on a surface of the second transparent plate (20).

Description

[0001] Optical spectrometer with diffractive element and spatially resolving light detector

[0002] Technical Field

[0003] The invention relates to an optical spectrometer having an input port, a diffractive element, and a spatially resolving light detector.

[0004] Background Art

[0005] US20170176251A1 describes a spectrometer having a slit-shaped input port, a diffractive element, and a spatially resolving light detector. The light from the input port is diffracted at the diffractive element, from where the first-order diffracted light arrives at the light detector.

[0006] Disclosure of the Invention

[0007] In a first aspect, the problem to be solved by the present invention is to provide an optical spectrometer of this type that has a compact design yet a good sensitivity.

[0008] This problem is solved by a spectrometer comprising at least the following elements:

[0009] - An input port: This is the part of the spectrometer that receives the light to be analyzed. In particular, it may be adapted to receive light traveling along a device axis.

[0010] - A diffractive element: The diffractive element may be used to split the light from the input port into its spectral components, with the various components traveling along different directions. The diffractive element may, e.g., be a simple diffractive grating, or it may be a more complex wavelength-scale metastructure or diffractive lens.

[0011] - A spatially resolving light detector: The light detector is adapted to measure light at a plurality of different locations, and it can be used to receive the spectral light components from the diffractive element to detect their intensity as a function of the wavelength.

[0012] At least one light-guiding, transparent plate extending along the light path between the input port and the diffractive element and / or between the diffractive element and the light detector. It guides the light between the respective components.

[0013] The transparent plate comprises at least one curved surface section intersecting the light path. In this context, a "curved surface section" is a section of the curve with an extension and a curvature much larger than the wavelength, e.g., larger than 0.1mm, i.e., the curved surface section is able to change the conver- gence / divergence of the light in non-diffractive manner.

[0014] The transparent plate with the curved surface section increases the optical path length as compared to a design where the light travels through gas or vacuum because any transparent plate has a refractive index much larger than 1. In addition, the curved surface section arranged in the path of the light allows to control, e.g., the convergence / divergence of the light beam, for example for focusing and / or collimation. This provides a compact, low-dispersive device design.

[0015] The spectrometer may, e.g., comprise such a curved surface section adapted to collimate light before the diffractive element.

[0016] The spectrometer may, e.g., comprise such a curved surface section adapted to focus light from the diffractive element towards the light detector.

[0017] As it travels along the light guide, the light may be reflected several times at opposite surfaces of the transparent plate for an even more compact design. In particular, said opposite surfaces may be surfaces extending transversally, in particular perpendicularly, to the device axis.

[0018] For a simple, concentric design of the various optical components, such as any deflectors, reflectors, and / or the diffractive element, the propagation direction of the light in the at least one transparent plate may extend radially in respect to a device axis.

[0019] The "at least one transparent plate" may include a first transparent plate, wherein the light path from the input port to the diffractive element extends through the first transparent plate.

[0020] Alternatively or in addition, the "at least one transparent plate" may include a second transparent plate, wherein the light path from the diffractive element to the light detector extends through the second transparent plate.

[0021] In a second aspect, which may optionally be combined with the first aspect, the spectrometer comprises at least the following aspect, - An input port: This is the part of the spectrometer that receives the light to be analyzed. In particular, it may be adapted to receive light traveling along the device axis.

[0022] - A diffractive element: The diffractive element may be used to split the light from the input port into its spectral components, with the various components traveling along different directions. The diffractive element may, e.g., be a simple diffractive grating, or it may be a more complex wavelength-scale metastructure or diffractive lens.

[0023] - A spatially resolving light detector: The light detector is adapted to measure light at a plurality of different locations, and it can be used to receive the spectral light components from the diffractive element to detect their intensity as a function of the wavelength.

[0024] The input port and the diffractive element are centered on a device axis of the spectrometer. The diffractive element extends over at least 270° around the device axis.

[0025] This concentric design around a device axis allows to process light along all, or at least almost all, angular directions, which provides improved efficiency, thereby resulting in a device of good sensitivity.

[0026] In one embodiment, the spectrometer comprises at least one lightguiding transparent plate extending perpendicularly to the device axis. In this case, the light path of at least one of

[0027] - from the input port to the diffractive element and

[0028] - from the diffractive element to the light detector extends through the transparent plate with at least one reflection on a surface of the transparent plate.

[0029] In other words, the light radially traveling between the input port and the diffractive element and / or the light radially traveling between the diffractive element and the light detector runs through this transparent plate. This increases the optical path length as compared to a design where the light travels through gas or vacuum because any transparent plate has a refractive index much later than 1. In other words, using such a transparent plate and arranging it perpendicularly to the device axis results in a more compact design.

[0030] The diffractive element may be arranged in a first annular region concentric to the device axis. In this case, other components of the spectrometer, such as a light source or the input port, may be located within the radially inner diameter of the first annular region. The input port may be arranged in a second annular region concentric with the device axis, and the input port may extend over at least 270° around the device axis. Such an annularly shaped input port may, e.g., receive other components within its inner diameter, such as a light source, a deflector, the diffractive element, and / or the light detector.

[0031] The input port may be in direct contact with the sample, e.g., with a skin section of a human.

[0032] If the device is to be equipped with a light source, this light source may be arranged to emit light along the device axis in an emission region that fits into the second annular region. In this context, an emission region that fits into the second annular region is an emission region whose diameter is not larger than the inner diameter of the second annular region. This provides a compact and efficient design to emit probe light into a sample and to receive and spectrally analyze the returning light.

[0033] If the inner diameter of the second annular region (i.e., of the region of the input port) is larger than the radially outer diameter of the of the diffractive element, the diffractive element can be located closer to the device axis than the input port.

[0034] On the other hand, the inner diameter of the first annular region (i.e. the region of the diffractive element) may be larger than the outer diameter of the input port, in which case the input port can be located closer to the device axis than the diffractive element.

[0035] The light detector may also be arranged coaxially with the device axis. In this case, it is well suited to receive the spectrally separated light from the diffractive element.

[0036] The spectrometer may comprise first and second transparent plates.

[0037] The light path from the input port to the diffractive element extends through the first transparent plate with at least one reflection on a surface of the first transparent plate, and the light path from the diffractive element to the light detector extends through the second transparent plate with at least one reflection on a surface of the second transparent plate.

[0038] This increases the optical path length as compared to a design where the light travels through gas or vacuum because any transparent solid body has a refractive index much later than 1. In addition, the change of light direction in the diffractive element may be used to transfer the light between the first and the second transparent plate. The light may again be reflected several times at each of the opposite surfaces of the first transparent plate as it travels along the first transparent plate and also of the second transparent plate as it travels along the second transparent plate for an even more compact design.

[0039] In the first as well as in the second aspect, the spectrometer may comprise a deflector adapted to deflect light traveling along a first propagation direction in the first transparent plate into light traveling along a second propagation direction in the second transparent plate, with the first and second propagation directions being different. This again provides for a more compact design.

[0040] The deflector may, e.g., be formed by a mirror and / or a prism. In a particularly simple design, though, it may be formed by the diffractive element if the diffractive element is adapted to diffract light traveling along the first propagation direction in the first transparent plate into light traveling along a second propagation direction in the second transparent plate.

[0041] When used with a spectrometer having a device axis as in the first aspect, the first and the second propagation directions may have, in respect to the device axis (i.e., in a cylindrical coordinate system where the device axis is the cylinder axis), radial components of opposite sign.

[0042] In both aspects, the spectrometer may further comprise an input deflector arranged on the light path between the input port and the diffractive element and adapted to deflect light axially arriving from the input port into a radial direction. In other words, the input deflector deflects the axially incoming light into a propagation direction that has a component extending radially in respect to the (primary) direction of the incoming light.

[0043] The input deflector may, e.g., be formed by a conical, reflecting surface of the first transparent plate, wherein the conical surface is centered with respect to the device axis.

[0044] In all aspects, for better spectral resolution and a compact design, the spectrometer may comprise a focal element arranged on the light path between the diffractive element and the light detector or forming part of the diffractive element. This focal element is adapted to focus light of different wavelengths towards different areas of the light detector. In this manner, each spectral component of the incoming light is focused on a particular region of the light detector.

[0045] For processing light in all or most angular directions, the focal element may be arranged in a third annular region concentric to the device axis, and it may extend over at least 270° around the device axis. The spectrometer may further comprise at least one reflective aperture on a surface of the at least one transparent plate, in particular of the first transparent plate. This aperture comprises a reflector that is arranged, as seen along the propagation direction of the light in the plate, between two absorbers. Such a reflective aperture allows to spatially filter light and may, e.g., be used to improve the collimation of light arriving at the diffractive element or to prevent undesired diffraction orders or specularly reflected light from arriving at the light detector.

[0046] The reflective aperture may be arranged in a fourth annular region concentric to the device axis and extend over at least 270° around the device axis.

[0047] Brief Description of the Drawings

[0048] The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein:

[0049] Fig. 1 shows a view of a first embodiment of a spectrometer,

[0050] Fig. 2 shows a sectional view of the spectrometer of Fig. 1 along its device axis,

[0051] Fig. 3 shows a view of the bottom side of the first transparent plate of the spectrometer of Fig. 2,

[0052] Fig. 4 shows a sectional view of a second embodiment,

[0053] Fig. 5 shows a sectional view of a third embodiment,

[0054] Fig. 6 shows only part of a spectrometer and illustrates a first embodiment of the input port,

[0055] Fig. 7 shows only part of a spectrometer and illustrates a second embodiment of the input port,

[0056] Fig. 8 illustrates another embodiment of the spectrometer with a non-continuous diffractive element,

[0057] Fig. 9 illustrates yet another embodiment with non-centric design,

[0058] Fig. 10 illustrates an embodiment having rotationally symmetric design with a single transparent plate, and

[0059] Fig. 11 illustrates an example with specific device dimensions.

[0060] Modes for Carrying Out the Invention Definitions

[0061] The term "axial" is understood to designate a direction parallel to the device axis. The term "radial" is understood to designate a direction perpendicular to the device axis or to designate a plane the device axis lies in.

[0062] If an element "extends over at least 270°" around the device axis, this is to be understood such that the element extends (continuously or in several separate sections, cf. Fig. 8) along at least an angular length of 270° around the device axis.

[0063] The "vertical" direction is defined such that the input port is located at the top of the device. Hence, for example, in the embodiment of Figs. 1, and 2, the first transparent plate is located, by definition, "above" the second transparent plate.

[0064] The terms "inner diameter" and "outer diameter" designate diameters perpendicular to the device axis, with the inner diameter designating the distance between opposite delimiters facing towards the device axis (such as inner delimiters of an annular region) and the outer diameter designating the distance between opposite delimiters facing away from the device axis (such as outer delimiters of an annular or circular region).

[0065] A diffractive element is considered to be centered on a device axis if its diffractive structures, such as its grating lines, are arranged along circles centered on the device axis, in particular with an accuracy of 10% of the outer diameter of the diffractive element.

[0066] The input port is considered to be centered on the device axis if its geometric center is at the device axis, in particular within an accuracy of 10% of the outer diameter of the input port.

[0067] The term "transparent" is to be understood as transparent for all light within the spectral measurement range of the spectrometer.

[0068] The term "transversal" is understood as "non-parallel".

[0069] A "plate" is a body having two parallel surfaces and having an extension larger along the directions parallel to the surfaces than perpendicularly thereto. The surfaces may have local regions that deviate from a parallel orientation, such as inclined reflectors or curved lenses. Advantageously, the area of these local regions is less than 25% of the area of the parallel surfaces.

[0070] The term "propagation direction" of light in a transparent plate designates the average direction the light propagates along the parallel surfaces of the plate.

[0071] The term "light" is to be understood to encompass not only visible light but also light in the infrared and ultraviolet spectral range. General concepts and first embodiment

[0072] A first embodiment of a spectrometer 1 is sown in Figs. 1 - 3. It comprises an input port 2 for receiving light from a sample region 4. Sample region 4 may, e.g., include living tissue, a liquid, a gas, or some other material or object. Spectrometer 1 is designed to spectrally analyze the light received at input port 2.

[0073] In one embodiment, the light may, e.g., include stimulated radiation generated by inelastic scattering in a sample, in particular Raman radiation.

[0074] For measuring stimulated radiation, spectrometer 1 may include a light source 6 adapted to emit stimulating light into sample region 4. Advantageously, light source 6 is a narrow-band light source, in particular a laser, e.g., a VCSEL-type laser.

[0075] Light source 6 may be adapted to emit its light into an emission region 8 extending along a device axis 10 of spectrometer 1.

[0076] In the shown embodiment, input port 2 is arranged in an annular region 14, which is called the "second" annular region 14 herein. Second annular region 14 is concentric to device axis 10.

[0077] As can be seen, in the embodiment of Figs. 1 - 3, input port 2 is annular and fully extends over 360° around device axis 10.

[0078] In the shown embodiment, spectrometer 1 comprises a non-transparent cover layer 16, with input port 2 extending through cover layer 16 in order to guide the light into the interior of spectrometer 1.

[0079] Input port 2 may be of a transparent material in order to mechanically close spectrometer from above while allowing the transmission of the light to be measured.

[0080] In particular, input port 2 may be a ring of a transparent material concentric to device axis 10.

[0081] Input port 2 may extend, as shown, over a top surface 16a of cover layer 16 for better contact with the sample.

[0082] In the shown embodiment, cover layer 16 is located above a first transparent plate 18, and first transparent plate 18 is located above a second transparent plate 20.

[0083] The light arriving through input port 2 enters first transparent plate 18 through a top side 22 of first transparent plate 18 and arrives at an input deflector 24 arranged on a bottom side 26 of first transparent plate 18.

[0084] Input deflector 24 is arranged in an annular region 28, which is called the "fifth" annular region 28 herein. Fifth annular region 28 is concentric to device axis 10. In the shown embodiment, input deflector 24 is annular and fully extends over 360° around device axis 10.

[0085] Input deflector 24 is adapted to deflect the light from input port 2 into a first radial propagation direction 25, i.e., to increase the angle between device axis 10 and the light.

[0086] As shown, input deflector 24 may deflect the light from input port 2 away from device axis 10, i.e., first propagation direction 25 points away from device axis 10.

[0087] As shown, input deflector 24 may be formed by a curved surface section 24a of bottom side 26. For good reflection efficiency, it may be provided with a reflective coating 30.

[0088] Input deflector 24 has, in the present embodiment, also the function of a reflector reflecting the light back into first transparent plate 18, namely towards a first reflector 32 on top side 22 of first transparent plate 18. From there, the light is reflected back into first transparent plate 18 and to a second reflector 34 at bottom side 26 and then back again to a third reflector 36 at top side 22.

[0089] In the present embodiments, each of the reflectors 32, 34, 36 is annular and concentric to device axis 10.

[0090] As shown, each reflector 32, 34, 36 may be radially (i.e. along the propagation direction of the light) arranged between two optical absorbers 38, which a reflectance, for the light within the spectrometer range, being at least a factor 10, in particular by a factor 100, smaller than the reflectance of the reflectors 32, 34, 36. In this way, reflective "apertures" are formed in annular regions. These annular regions are called "fourth" annular regions.

[0091] The series of apertures formed in this way helps to collimate the light as it is reflected back and forth within first transparent plate 18 and to absorb light components that are not well collimated. This is illustrated in Fig. 2 with the light field 42 emerging from input port 2 being not fully collimated but becoming more collimated along its path. The collimation may, in addition or alternatively, also be supported or provided by the curved surface section 24a.

[0092] Finally, the light arrives at an exit window 44 of first transparent plate 18, from where it enters an entry window 46 of second transparent plate 20. In the shown embodiment, exit window 44 as well as entry window 46 are annular and concentric to device axis 10, and they are located facing each other to let light pass between them.

[0093] A diffractive element 48 is arranged at exit window 44, entry window 46 or between them. It acts as a dispersive element as well as a radial deflector. Advantageously, diffractive element 48 is a surface structure of first transparent plate 18 and / or second transparent plate 20.

[0094] Diffractive element 48 may comprise grating structures, such as grooves and ridges, extending coaxially around device axis 10. Thus, its local grating vector extends radially, such that it changes the radial component of the propagation vector of the light arriving at diffractive element 48, with the change being a function of the wavelength.

[0095] For good efficiency, diffractive element 48 may be optimized to generate light of only one diffraction order, e.g., the order 1 or -1, e.g., by being a properly dimensioned blaze grating. Residual light of other orders may, if necessary, be absorbed by absorbers 38 arranged on second transparent plate 20.

[0096] Diffractive element 48 is arranged in an annular region 49, which is called the "first" annular region 49 herein. First annular region 49 is concentric to device axis 10.

[0097] As can be seen, in the embodiment of Figs. 2 and 3, diffractive element 48 is annular and fully extends over 360° around device axis 10.

[0098] Since the light arriving at diffractive element 48 is well collimated, the diffracted light is collimated as well, but the direction of the diffracted light depends on the wavelength. Fig. 2 illustrates two diffracted light fields 50a, 50b, with light field 50a corresponding to a longer wavelength than light field 50b.

[0099] In second transparent plate 20, the light travels along a second propagation direction 51, which, in the shown embodiment, extends radially in respect to and towards device axis 10.

[0100] The light coming from diffractive element 48 next falls onto a focal element 52 located at a bottom side 54 of second transparent plate 20. Focal element 52 is adapted to focus the light from diffractive element 48 onto a spatially resolving light detector 58. Hence, advantageously, if each of the light fields 50a, 50b from diffractive element 48 is collimated, focal element 52 has a focal length approximately, in particular within an accuracy of 33%, in particular with an accuracy within 10%, equal to the optical path length between focal element 52 and light detector 58.

[0101] Note that the focal point can also be selected to be somewhat before or after light detector 58 is slightly larger spot sizes are desired at the pixels of the light detector.

[0102] In the shown embodiment, focal element 52 comprises a curved surface section 52a of second transparent plate 20 covered by a reflector 53. However, it may, e.g., also be a metasurface lens or diffractive lens operated in reflection or transmission. For more examples, see the section "Combined diffractive element and focal element" below.

[0103] The light from focal element 52 is reflected by a reflector 60 arranged at top side 62 of second transparent plate 20. Reflector 60 is again annular and coaxial to device axis 10.

[0104] Finally, the light arrives at light detector 58, which is a spatially resolving light detector having an array of pixels 64 extending perpendicularly to device axis 10. For more details, see the section "Light detector" below.

[0105] Second, embodiment

[0106] Fig. 4 shows a sectional, schematic view of a second embodiment of the spectrometer. It has substantially the same design as the first embodiment and is symmetric about device axis 10 with an input port 2, a first transparent plate 18, and a second transparent plate 20 arranged along device axis 10.

[0107] In the second embodiment, though, the are only two reflectors 24, 32 at first transparent plate 18, and they are slightly concave (in radial direction) for a better collimation of the light field from input port 2. There may again be absorbers (not shown) for absorbing non-collimated components or other undesired parts of the light field.

[0108] The second embodiment further differs from the first one in that diffractive element 48 is arranged on second transparent plate 20, namely at its top surface 62. Further, the light from diffractive element 46 first impinges on a flat reflector 60 at bottom surface 54 of second transparent plate 20, from where it is reflected to focal element 52, which is arranged at top surface 62 of second transparent plate 20.

[0109] Radial light propagation

[0110] In the embodiments shown so far, the light travels radially outwards, along first propagation direction 25, in first transparent plate 18 and radially inwards, along second propagation direction 51, in second transparent plate 20, with, diffractive element 48 acting as a radial deflector adapted to deflect light traveling outwards in first transparent plate 18 into light traveling inwards in second transparent plate 20.

[0111] Hence, in these embodiments, diffractive element 48 lies radially outwards from input port 2. In other words, the inner diameter Dli of first annular region 49 (the region of diffractive element 46) is larger than the outer diameter D2o of input port 2 (see Fig. 4). This design has the advantage that input port 2 can be designed to receive light from a comparatively local region of the sample.

[0112] This design may, however, also be reversed in that, in first transparent plate 18, the light travels radially inwards and, in second transparent plate 20, the light travels radially outwards. In this case, diffractive element 48 may again act as a radial deflector adapted to deflect light traveling inwards in first transparent plate 18 into light traveling outwards in second transparent plate 20.

[0113] Such an embodiment is shown in Fig. 5. Here, diffractive element 48 lies radially inward from input port 2. In other words, the inner diameter D2i of second annular region 14 (the region of input port 2) is larger than the outer diameter Dio of diffractive element 48. This design has the advantage that input port 2 can, for a given radial aperture R, have a larger input area.

[0114] In the embodiment of Fig. 5, first propagation direction 25 is directed towards device axis 10 and second propagation direction 51 is directed away from device axis 10.

[0115] In both cases, the spectrometer comprises a radial deflector (which is the diffractive element 48 in the shown embodiments) adapted to deflect light traveling along a first radial direction in first transparent plate 18 into light traveling along a second radial direction in second transparent plate 20, with the first and second radial directions extending opposite to each other.

[0116] Instead of using the diffractive element 48 as radial deflector, a mirror may, e.g., be used for the purpose.

[0117] The embodiment of Fig. 5 illustrates another element that may be used in connection with any embodiment of the spectrometer: It comprises a linear variable optical filter 66 arranged at the input side of light detector 58. This element is described in more detail in the section "Light detector" below.

[0118] Light collimation

[0119] As mentioned, the incoming light may be collimated before it arrives at diffractive element 48 in order to make it easier to spatially separate the diffracted spectral components thereof and to focus them at different locations on light detector 58.

[0120] One means to do so are the reflective apertures formed by reflectors 24, 32, 34, and / or 36 arranged radially between absorbers 38 as described above.

[0121] Another way to improve light collimation, which can be used alone or in combination with the reflective apertures, is the provision of focusing optics (i.e., optics having a positive focal length, at least in a sectional view within a radial plane), at input port 2.

[0122] A first embodiment of such light collimation at input port 2 is shown in Fig. 6 (which only shows input port 2, part of cover layer 16, and part of first transparent plate 18). Here, input port 2 faces, at its output side, a convex lens 68 (convex in a sectional view of a radial plane as shown in Fig. 6), with convex lens 68 having a higher refractive index than input port 2

[0123] A second embodiment of such light collimation is shown in Fig. 7. Here, entry surface 70 of input port 2 is convex for forming a convex lens 68.

[0124] Hence, in more general terms, the spectrometer may comprise focusing optics 68, 70 at input port 2 adapted to collimate incoming light at least partially (i.e., adapted to increase the collimation of the incoming light).

[0125] Device symmetry

[0126] The embodiments described so far, had circular symmetry in respect to device axis 10, at least for at least one of input port 2, diffractive element 48, focal element 52, and the reflectors 24, 32, 34, 36, 60. If a standard camera device is used for light detector 58, it will typically not have rotational symmetry, but it may at least intersect with device axis 10.

[0127] Instead of circular symmetry, these components may also, e.g., have discrete rotational symmetry. An example of four-fold rotational symmetry for the bottom side of first transparent plate 18 of the embodiment of Fig. 2 is illustrated in Fig. 8. As can be seen when comparing to Fig. 3, in Fig. 8 input deflector 24, reflector 34, and diffractive element 48 do not extend over an angular length of 360° within their respective annular regions, but, e.g., only over a total angular length of 300°. For example, diffractive element 48 may comprise several sections 48a, 48b, 48c, 48d, each of which extends only over part of the angular length of the device.

[0128] Advantageously, though, input port 2, diffractive element 48, focal element 52, and the reflectors 24, 32, 34, 36, 60 each extend over at least 270° around device axis 10 for processing a large amount of the available light.

[0129] In some embodiments, though, other geometries may be used, such as the one of Fig. 9. There, input port 2, diffractive element 48, focal element 52, and the reflectors 24, 32, 34, 36, 60 each extend over less than 270°, such a over 180° or even less.

[0130] In the embodiment of Fig. 9, though, the elements are still arcuate, extending along arcs that are concentric to device axis 10. In other embodiments, not shown, linear components may, e.g., be used. Diffractive element 48 may comprise subsections having different grating parameters, in particular having different grating spacings.

[0131] For example, in the embodiment of Fig. 8, at least some of the sections 48a, 48b, 48c, 48d may have grating spacings different from each other in order to cast different spectral ranges onto light detector 58. The respective sections may also have different grating shapes and / or amplitudes.

[0132] Hence, in all embodiments shown herein, a diffractive element 48 having several sections with different grating spacings allows to analyze the light over a wider spectral range or several distinct spectral ranges. For example, a first section may be used for analyzing a first spectral range of interest while a second section may be used for analyzing a second, different spectral range of interest.

[0133] The sections 48a, 48b... may be spatially separated as shown in Fig. 8, adjacent to each other, or even overlapping each other.

[0134] Transparent plates

[0135] The embodiments described herein comprise at least one transparent plate 18, 20. If the spectrometer has a symmetric arrangement about device axis 10, the transparent plates 18, 20 can be plate-shaped (i.e., they are formed by plates having substantially uniform thickness except at locations where there are surface elements, such as recesses or protrusions, e.g., for optical or mechanical purposes), and they extend perpendicularly to device axis 10.

[0136] The thickness T (see Fig. 2), which is the extension along device axis 10 for a symmetric spectrometer, of each transparent plate 18, 20 is advantageously chosen for a good spatial separation of the individual sections of the light field as it is reflected back and forth through the body.

[0137] Since the width of the light field within the transparent plates 18, 20 is a function of the aperture R (see Fig. 2) of input port 2, a larger thickness T should be used for a large radial aperture R. In particular, the thickness T is at least 1 mm, in particular at least 2 mm.

[0138] In this context, the aperture R for a device with a device axis 10 is chosen to be the extension of input port 2 in radial direction as illustrated in Fig. 2. The thickness T is given by the average extension of the transparent plates 18, 20 in axial direction.

[0139] Not that the thickness T of the two transparent plates 18, 20 may be different.

[0140] The spectrometers described so far had two plate-shaped transparent plates 18, 20 extending parallel to each other. The spectrometer may, however, also comprise more than two such transparent plates, with suitable deflector elements between them.

[0141] In another embodiment, as shown in Fig. 10, only a single transparent plate 18 may be used. In this spectrometer, input port 2, diffractive element 48, focal element 52, and the reflectors 24, 32, 34, 36 each extend over at least 270° around device axis 10 for processing a large amount of the available light.

[0142] In this embodiment, diffractive element 48 operates as a reflective grating.

[0143] This embodiment allows for a more compact design of the spectrometer along device axis 10.

[0144] The transparent plate 18 or bodies 18, 20 is / are transparent over the spectral measurement range of the spectrometer, which may, e.g., be in the infrared, visible, and / or UV spectral range.

[0145] The transparent plate / bodies may, e.g., be of glass, such as flint glass, crown glass, soda lime, quartz, BK7, BK6, sapphire, fused silica or others, if the measurement range is in the VIS / NIR / UV, or of silicon for infrared spectroscopy.

[0146] Light detector

[0147] Light detector 58 is, as mentioned, a spatially resolving light detector in order to distinguish between the different spectral components of the light diffracted from diffractive element 38. To do so, it may be provided with a two-dimensional array of pixels 64.

[0148] If the spectrometer is arranged around a device axis 10, the array of pixels 64 is best arranged perpendicularly to device axis 10 and intersects device axis 10. It may be centered on device axis 10.

[0149] Light detector 58 may be a CCD camera.

[0150] As mentioned, a linear variable optical filter 66 (see, e.g., Fig. 5) may be arranged at the input side of light detector 58, in particular a linear variable optical bandpass filter. Such filters, typically formed by wedged Fabry -Perot filters, are known to the skilled person. This type of filter can be used to improve the spectral selectivity of light detector 58 and to prevent stray light not caught by the absorbers 38 from generating false signals.

[0151] It must be noted that, due to the substantially rotational symmetric configuration, variable optical filter is rotationally symmetric, too, i.e., its transmission wavelength varies with the distance from device axis 10. In addition, it must be dimensioned to take into account that the light traverses it at a non-perpendicular angle, and this angle depends on the distance from device axis 10 as can, e.g., be seen in Fig. 2.

[0152] Curved surface section

[0153] As mentioned, in some embodiment the one or more transparent plates 18, 20 have a curved surface section 24a, 52a. Such a curved surface section, which has macroscopic, i.e., non- wavelength- scale extension and curvature of at least 0.1 mm, can be used to shape the light field by means of specular reflection or refraction.

[0154] If used in specular reflection, and as shown, the curved surface section 24a, 52a is advantageously provided with a specular reflector, such as reflector 30, 53.

[0155] The radial extension X (see, e.g., Fig. 11 for curved surface section 52a) is, e.g., at least 0.1 mm, in particular at least 0.3 mm for shaping macroscopic beams.

[0156] If scattering and / or diffraction are to be avoided, the curvature of the curved surface section 24a, 52a is at least 0.1 mm, in particular at least 0.3 mm.

[0157] In spectrometers having a device axis 10, for efficiently processing a large amount of light, the curved surface section may extend over at least 270° around device axis 10, in particular around 360° of device axis 10.

[0158] Again in spectrometers having a device axis 10, for processing the light uniformly, the curved surface section extends along a curved surface that is rotationally symmetric in respect to device axis 10. If the curved surface section extends around 360° of device axis 10, it is, in this case, per se rotationally symmetric in respect to device axis 10.

[0159] The curved surface section 24a, 52a may, e.g., be manufactured by hot-stamping or by a casting process using a suitably machined stamp or cast.

[0160] In important applications, as mentioned, the curved surface section is used for focusing or collimation, i.e., it should provide an optical element with a positive focal length. Hence, if used in reflection, the curved surface section may form a convex surface section of the respective transparent plate 18, 20.

[0161] Combined diffractive element and focal element

[0162] In the above embodiments, diffractive element 48 and focal element 52 are separate structures. Alternatively, though, they may also be formed by common structures, such as a focusing diffractive element (see, e.g., Fan et al., DOL10.1364 / OE.23.016281) or by a series of metasurface structures (see, e.g., Fa- raji-Dana et al., DOI: 10.1038 / s41467-018-06495-5). These structures again comprise a diffractive element but include further wavelength-scale structuring to implement focusing.

[0163] Spectral range and resolution

[0164] The spectral range of the spectrometer depends on its intended use. For example, it may cover a range of, e.g., at least 100 nm, which may, e.g., be centered around 550, 850, or 1550 nm.

[0165] The spectral resolution of the device may, thanks to the long light paths within the transparent plates 18, 20 be high, in particular if the light path is reflected several times at each of the opposite surfaces of the transparent plate. Multiple reflections before diffractive element 48 allow to improve beam collimation and, therefore, better angular separation of the diffracted components after diffractive element 48. Multiple reflections after focal element 52 allows a better spatial separation of the spectral regions focused on light detector 58.

[0166] This is illustrated in Fig. 11, which shows an example with six reflections on the surfaces of first transparent plate 18 and four reflections on the surfaces of second transparent plate.

[0167] Example parameters in Fig. 11 are: deflection angle a = 30°, transparent plate thickness T = 3 mm, focal length f of focusing element 52 equal 16.4 mm. Diffractive element 48 is designed to have its first-order diffraction angle at 0 = 60°, as shown, for 857.5 nm. The transparent plates 18, 20 are fused silica. With sufficient spatial resolution of light detector 58, this type of device may have a spectral resolution of around 1 nm or better over a range of 800 - 915 nm.

[0168] The wavelength of light source 6 depends on the use of the spectrometer. For biological measurements, for example, light source 6 may, e.g., emit light with a wavelength between 800 and 850 nm.

[0169] Notes

[0170] Fig. 2 shows that light source 6 is arranged in cover layer 16, but it may also be arranged in first transparent plate 18 (position 6' in Fig. 2), second transparent plate 20 (position 6" in Fig. 2), or below second transparent plate 20. In these cases case, the parts of the spectrometer located between light source 6 and sample region 4 are transparent for the light from light source 6. In the embodiments described above, reflectors 24, 32, 34... have been used. They may be formed by reflective coatings, such as metallic coatings or interferometric dielectric coatings, or, if the refractive index of the transparent plate(s) and the angle of incidence of the light allow, they may be based on total internal reflection.

[0171] In the shown embodiments, diffractive element 48 is arranged in a (flat) plane extending perpendicularly to device axis 10. Alternatively, though, it may be arranged on a curved surface section, as illustrated by dotted lines 48a in Fig. 11 , which is advantageously rotationally symmetric to device axis 10. In particular, a Fastie-Ebert configuration may be used, see, e.g., https: / / www.horiba.com / int / scien- tific / technologies / spectrometers-and-monochromators / spectrometers-monochroma- tors-and- spectrographs / .

[0172] The optical absorbers 38 may, e.g., be of black chromium. Alternatives are, e.g., black nickel, black zinc, black phosphate (6), or black molybdenum. These compounds are known to the skilled person, see, e.g. J. Takadoum in "Black coatings: a review", European Physical Journal: Applied Physics, 2010, 52 (3), 10.105 l / epjap / 2010155, hal-00646463 (https: / / hal.science / hal-00646463 / document).

[0173] The arrangement of the diffractive element, mirroring surface, and spatially resolving light detector on, e.g., one or two transparent plates provides not only for a compact design, but it also ensures a well-defined and robust relative arrangement of these components.

[0174] While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

Claims1. An optical spectrometer comprising an input port (2), a diffractive element (48), a spatially resolving light detector (58), and at least one light-guiding, transparent plate (18, 20) extending along a light path between at least one of- the input port (2) and the diffractive element (48) and- the diffractive element (48) and the light detector (58), wherein the transparent plate (18, 20) comprises at least one curved surface section (24a, 48a, 52a) intersecting said light path.

2. The spectrometer of claim 1 wherein the light path is reflected several times at each of opposite surfaces of the transparent plate (18, 20).

3. The spectrometer of any of the preceding claims wherein the curved surface section (24a, 48a, 52a) is adapted to collimate light before the diffractive element (48).

4. The spectrometer of any of the preceding claims wherein the curved surface section (24a, 48a, 52a) is adapted to focus light from the diffractive element (48) towards the light detector (58).

5. The spectrometer of any of the preceding claims comprising at least a first transparent plate (18) wherein the light path from the input port (2) to the diffractive element (48) extends through the first transparent plate (18, 20).

6. The spectrometer of any of the preceding claims comprising at least a second transparent plate (20) wherein the light path from the diffractive element (48) to the light detector (58) extends through the second transparent plate (20).

7. The spectrometer of the claims 5 and 6 wherein the first and second transparent plates (18, 20) are plates extending perpendicularly to a device axis (10) and arranged at different axial positions along the device axis (10).

8. The spectrometer of the claims 5 and 6 or of claim 7 comprising a radial deflector (48) adapted to deflect light traveling along a first radial direction in the first transparent plate (18) into light traveling along a second radial direction in the second transparent plate (20), with the first and second radial directions extending, radially, opposite to each other.

9. The spectrometer of claim 8 wherein the radial deflector (48) is formed by the diffractive element (48), with the diffractive element (48) adapted to diffract light traveling along the first radial direction in the first transparent plate (18, 20) into light traveling along a second radial direction in the second transparent plate (20).

10. The spectrometer of any of the preceding claims wherein the diffractive element (48) is a surface structure on the transparent plate (18, 20).

11. The spectrometer of claim 10 wherein the diffractive element (48) is located on the curved surface section (48a).

12. An optical spectrometer, in particular of any of the preceding claims, comprising an input port (2), a diffractive element (48), and a spatially resolving light detector (58), wherein the input port (2) and the diffractive element (48) are centered on a device axis (10) of said spectrometer and wherein the diffractive element (48) extends over at least 270° around the device axis (10).

13. The spectrometer of claim 12 further comprising at least one light-guiding transparent plate (18, 20) extending perpendicularly to the device axis (10), wherein a light path of at least one of- from the input port (2) to the diffractive element (48) and- from the diffractive element (48) to the light detector (58) extends through the transparent plate (18, 20) with at least one reflection on a surface of the transparent plate (18, 20).

14. The spectrometer of claim 13 wherein a propagation direction of the light in the at least one transparent plate extends radially in respect to the device axis (10).

15. The spectrometer of any of the claims 12 to 14 wherein the light detector (58) intersects the device axis (10), and in particular is centered on the device axis (10).

16. The spectrometer of any of the claims 12 to 15 wherein the light detector (58) comprises a two-dimensional array of pixels (64) extending perpendicularly to the device axis (10).

17. The spectrometer of any of the claims 12 to 16 wherein the diffractive element (48) is arranged in a first annular region (49) concentric to the device axis (10).

18. The spectrometer of claim 17 wherein an inner diameter (Dli) of the first annular region (49) is larger than an outer diameter (D2o) of the input port (2).

19. The spectrometer of any of the claims 12 to 18 wherein the input port (2) is arranged in a second annular region (14) concentric to the device axis (10) and extends over at least 270° around the device axis (10).

20. The spectrometer of claim 19 wherein the input port (2) is a ring of a transparent material concentric to the device axis (10).

21. The spectrometer of any of the claims 19 or 20 comprising a light source (6) adapted to emit light along the device axis (10) in an emission region (8) fitting into the second annular region (14).

22. The spectrometer of any of the claims 19 to 21 wherein an inner diameter (D2i) of the second annular region (14) is larger than an outer diameter (Dio) of the diffractive element (48).

23. The spectrometer of any of the claims 12 to 22 wherein the input port (2) is adapted to receive light traveling along the device axis (10).

24. The spectrometer of any of the claims 1 to 11 and of any of the claims 12 to 23 wherein the curved surface section (24a, 48a, 52a) has a radial extension (X) of at least 0.1 mm, in particular of at least 0.3 mm.

25. The spectrometer of any of the claims 1 to 11 and of any of the claims 12 to 24 wherein the curved section (24a, 48a, 52a) extends over at least 270° around the device axis (10), in particular around 360°.

26. The spectrometer of any of the claims 1 to 11 and of any of the claims 12 to 24 wherein the curved section (24a, 48 a, 52a) extends along a curved surface that is rotationally symmetric in respect to the device axis (10).

27. The optical spectrometer of any of the preceding claims comprising adjacent first and second transparent plates (18, 20), wherein a light path from the input port (2) to the diffractive element (48) extends through the first transparent plate (18) with at least one reflection on a surface of the first transparent plate (18) and wherein a light path from the diffractive element (48) to the light detector (58) extends through the second transparent plate (20) with at least one reflection on a surface of the second transparent plate.

28. The spectrometer of claim 27 wherein each light path is reflected several times at opposite surfaces of the first transparent plate (18) and of the second transparent plate (20).

29. The spectrometer of the claims 5 and 6 or of claim 7 or of any of the claims 27 or 28 comprising a deflector (48) adapted to deflect light traveling along a first propagation direction (25) in the first transparent plate (18) into light traveling along a second propagation direction (51) in the second transparent plate (20), with the first and second propagation directions (25, 51) being different.

30. The spectrometer of claim 29 wherein the deflector (48) is formed by the diffractive element (48) with the diffractive element (48) adapted to diffract light traveling along the first propagation direction (25) in the first transparent plate (18) into light traveling along the second propagation direction (51) in the second propagation transparent plate (20).

31. The spectrometer of any of the claims 12 to 26 and of any of the claims 29 or 30 wherein the first and the second propagation directions (25, 51) have, in respect to the device axis (10), radial components of opposite sign.

32. The spectrometer of any of the claims 27 to 31 wherein the diffractive element (48) is a surface structure on the first or the second transparent plate (20).

33. The spectrometer of any of the preceding claims further comprising an input deflector (24) arranged on a light path between the input port (2) and the diffractive element (48) and adapted to deflect light arriving axially from the input port (2) into a radial direction, and in particular to deflect the light from the input port (2) away from a device axis (10).

34. The spectrometer of any of the claims 27 to 32 and of claim 33 wherein the input deflector (24) is arranged in a fifth annular region (28) concentric to the device axis (10) and extends over at least 270° around the device axis (10).

35. The spectrometer of any of the preceding claims further comprising a focal element (52) arranged on a light path between the diffractive element (48) and the light detector (58) or being part of the diffractive element (48), wherein the focal element (52) adapted to focus light of different wavelengths towards different areas on the light detector (58).

36. The spectrometer of any of the claims 12 to 26 and of claim 35 wherein the focal element (52) is arranged in a third annular region concentric to the device axis (10) and extends over at least 270° around the device axis (10).

37. The spectrometer of any of the claims 35 or 36 wherein the focal element (52) comprises a curved surface section (52a) of the transparent plate (18, 20).

38. The spectrometer of any of the preceding claims further comprising, on a surface of at least one of the one or more transparent plates (18, 20), at least one reflective aperture comprising a reflector (24, 32, 34, 36, 53, 60) arranged, along a propagation direction of the light, between two absorbers (38)39. The spectrometer of any of the claims 12 to 26 and of claim 34 wherein the reflective aperture is arranged in a fourth annular region (40) concentric to the device axis (10) and extends over at least 270° around the device axis (10).

40. The spectrometer of any of the preceding claims further comprising a non-transparent cover layer (16) covering a transparent plate (18), wherein the input port (2) extends through the cover layer (16) to the transparent plate (18).

41. The spectrometer of any of the preceding claims wherein a thickness of the at least one transparent plate (18, 20) is at least 1 mm, in particular at least 2 mm.

42. The spectrometer of any of the preceding claims further comprising focusing optics (68, 70) at the input port (2).

43. The spectrometer of any of the preceding claims wherein the diffractive element (48) comprises at least two sections (48a, 48b, 48c, 48d) having different grating spacings.

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