Compact spatial heterodyne spectrometer

Abstract

A spectrometer comprises a spectrometer beam splitter (38), a first diffractive grating (40a), with a first arm (42a) of the spectrometer extending between the spectrometer beam splitter (38) and the first diffractive grating (40a), a second diffractive grating (40b), with a second arm (42b) of the spectrometer extending between the spectrometer beam splitter (38) and the second diffractive grating (40b), and a camera (48) at an exit (38b) of the spectrometer beam splitter (38). The camera (48) is directly mounted to the beam splitter (38), and its distance from the center of the beam splitter (38) is smaller than the beam diameter in the spectrometer. The camera image is subjected to a spatial frequency analysis that accounts for wave-length-specific overlaps of the pairs of diffracted components at the camera (48) by using wavelength-depending overlap parameters depending on these overlaps.

Description

[0001]P193358PC00 DRAFT 2024-06-28 REV.DOCX 1 Compact Spatial Heterodyne Spectrometer Technical Field The invention relates to a Raman spectrometer with a light source, illumination optics, collimation optics, and a spatial heterodyne spectrometer. Background Art US10908023B2 and US11719626B2 describe devices for perform- ing spatial heterodyne spectrometry (SHS). SHS spectrometers comprise a beam splitter dividing the incoming light along two arms of an interferometer. At the end of each arm, the light is re- flected by means of a tilted diffractive grating. Each spectral component of the light returns to the beam splitter where it is overlapped with its counterpart from the other arm. The pairs of diffracted spectral components generate interference patterns having different spatial frequencies. These interference patterns can be de- tected by a camera, and the one- or two-dimensional camera image can be subject to spectral analysis in order to detect the amplitudes of the different spatial frequencies and, thereby, the amplitudes of the diffracted spectral components. Disclosure of the Invention The problem to be solved by the present invention is to provide a compact spatial heterodyne spectrometer as well as a method for operating a compact spatial heterodyne spectrometer. This problem is solved by the independent claims. Accordingly, in a first aspect, a spatial heterodyne spectrometer is provided that comprises at least the following elements: - A beam splitter: The beam splitter may be used to split an incom- ing light beam to be analyzed into a first and a second part. - A collimator section arranged at an input side of the spectrometer beam splitter: It is adapted to generate the light for the spectrometer. - A first diffractive grating, with a first arm of the spectrometer ex- tending between the beam splitter and the first diffractive grating. P193358PC00 DRAFT 2024-06-28 REV.DOCX 2 - A second diffractive grating, with a second arm of the spectrome- ter extending between the beam splitter and the second diffractive grating. - A camera: The camera is arranged at the exit of the beam splitter and may be used to measure an interference pattern generated by the overlapping pairs of diffracted spectral components returning from the two arms and overlapping, at least partially, at the location of the camera. In a plane of the first arm and the second arm, the collimator section is adapted to generate light having, at the beam splitter, a diameter (i.e., a width) D. Further, the distance between the center of the spectrometer beam splitter and the camera is smaller than D. This aspect is based on the understanding that placing the camera so close to the spectrometer beam splitter, in units of the diameter of the light field from the collimator section, increases the overlap of the pairs of diffracted spectral compo- nents of the light that interfere at the camera. In a second aspect, a method is provided for operating a spatial het- erodyne spectrometer, such as a spatial heterodyne spectrometer of the first aspect. The spatial heterodyne spectrometer comprises at least the following elements: - A spectrometer beam splitter. - A first diffractive grating, with a first arm of the spectrometer ex- tending between the spectrometer beam splitter and the first diffractive grating. - A second diffractive grating, with a second arm of the spectrome- ter extending between the spectrometer beam splitter and the second diffractive grat- ing. - A camera at an exit of the spectrometer beam splitter. The method comprises at least the following: - Sending light to be analyzed into the spectrometer beam splitter. The light to be analyzed is split into the two arms and diffracted into diffracted spec- tral components at the gratings. The returning diffracted components of the two arms are recombined by the spectrometer beam splitter and interfering at the camera. - Performing spatial frequency analysis on an image recorded by the camera, i.e., performing an analysis that generates parameters indicative of the spec- trum of spatial frequencies that are apparent on the recorded image. The spatial frequency analysis accounts for wavelength-specific overlaps of the pairs of diffracted components at the camera by using a plurality of wavelength-depending overlap parameters depending on said overlaps. This method is based on the understanding it is possible to compen- sate for the fact that the overlap of each pair of diffracted spectral components from P193358PC00 DRAFT 2024-06-28 REV.DOCX 3 the two arms at the location of the camera varies with the wavelength. Hence, the method allows to obtain more accurate results and / or to reduce the expensive and comparatively large components conventionally used for physically compensating the wavelength-dependence of the overlap. Brief Description of the Drawings The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following de- tailed description thereof. Such description makes reference to the annexed drawings, wherein: Fig.1 shows a sectional view of an embodiment of a spectrometer, Fig.2 illustrates light components propagating through a spectrom- eter, Fig.3 illustrates the image recorded by the camera for monochro- matic light at three different wavelengths, Fig.4 illustrates some overlap parameters. Modes for Carrying Out the Invention Definitions The term "transversal" is to be understood as "non-parallel". The term "prism beam splitter" designates a beam splitter formed at the interface of two prism-shaped elements, such as a cube beam splitter. Spectrometer Fig.1 illustrates the design of some embodiments of a spectrometer, such as it can, e.g., be used for Raman spectroscopy. The shown spectrometer may comprise functional sections includ- ing an illumination section 2, a collimator section 4, and a spectrometer section 6. Illumination section 2 is adapted to send light at an excitation wave- length range onto a target 8 and to receive returning light scattered from target 8. The scattered light may include Raman-scattered light. Collimation section 4 is adapted to collimate the returning light, preparing it for entry into spectrometer section 6. P193358PC00 DRAFT 2024-06-28 REV.DOCX 4 Spectrometer section 6 is adapted to spectrally analyze the light from collimator section 4. The spectrometer further comprises a support structure 10 including several molded, interconnected support elements 12 – 24. In the following sections, the functional sections 2 – 6 as well as the support structure 10 are described in more detail. Illumination Section The shown illumination section 2 comprises a light source 26 emit- ting light at an excitation wavelength range. The excitation wavelength range is centered around a center excita- tion wavelength^0and has a full-width-half-maximum range ^^. For Raman spectroscopy, ^^ may be small for good resolution, such as smaller than 5 nm or even smaller than 1 nm. ^0depends on the application. For glucose detection, for example,^0may be between 780 and 790 nm, such as 785 nm, with a corresponding spectral analyzation range between 800 nm and 950 nm (corresponding to a Stokes shift of about 239 to 2213 cm–1). In another example,^0may be between 825 and 835 nm, such as 830 nm, where cost-effective lasers are available and lay parasitic fluorescence may be generated), and the spectral analyzation range may be between 845 nm and 960 nm (corresponding to a Stokes shift of about 214 to 1415 cm–1). Other values of^0may, e.g., be 795, 808, 830, or 850 nm because lasers are, e.g., available for these wavelengths. A typical range of^0may be between 750 and 880 nm. It may also be useful to extend the spectral analyzation range, e.g., up to 3000 cm–1in order to use further parts of the Raman spectrum to improve the quality of the sensor signal (e.g., of a biomarker, such as using the water peak to de- termine the excited tissue volume). In other examples,^0may be in the UV for detecting resonance ef- fects of proteins, or^0may be around 1065 nm, again for reducing fluorescence, or it may be around 720 nm for resonance with hemoglobin. Light source 26 may, e.g., be a vertical-cavity surface-emitting laser (VCSEL) for its compact size, narrow bandwidth, mass-production capabilities, and large light power. P193358PC00 DRAFT 2024-06-28 REV.DOCX 5 Illumination section 2 further comprises illumination optics 28a – 28d, which is adapted to project light from light source 26 through a sensing port 30 towards target 8 in order to generate Raman scattered light in the target. Note that the number of lenses in the illumination optics 28a – 28d depends on implementation and may also differ from four. For example, only a single lens may be used before and / or after the beam splitter. Sensing port 30 may be formed by an opening in support structure 10. Sensing port 30 may be closed by a window 34 that is transparent for the light from light source 26 as well as for the Raman-scattered light from target 8. Window 34 may be held by support structure 10, in particular by the second illumi- nation support element 14 as described in more detail below. To reduce undesired Raman scattering, window 34 may be of glass or of another material with low Raman scattering. In the shown type of embodiments, part of illumination optics 28a – 28d (namely the lenses 28c, 28d in the shown embodiment) process both the light from light source 26 on its way to target 8 as well as the scattered light returning from target 8. Therefore, illumination section 2 further comprises a dichroic beam splitter 32 for separating light in the excitation wavelength range from the returning Raman- scattered light. In the shown type of embodiments, dichroic beam splitter 32 passes light at the excitation wavelength range along the axis Ai of illumination section 2 while it reflects Raman-scattered light towards the collimation optics in collimator section 4. In alternative embodiments, though, dichroic beam splitter 32 may be adapted to reflect light at the excitation wavelength range while passing Raman- scattered light, in which case light source 26 and collimator section 4 may be swapped in their positions. In yet other embodiments, target 8 may, e.g., be illuminated from one direction while the Raman-scattered light is collected along another direction, in which case separate optics are used to project the excitation light towards the target and to receive the scattered returning light. In that case, dichroic beam splitter 32 may be dispensed with. In yet other embodiments, the spectrometer may not comprise an il- lumination section at all, and the light to be spectrally analyzed may be generated by other means. P193358PC00 DRAFT 2024-06-28 REV.DOCX 6 If a dichroic beam splitter 32 is used, as shown, a first part 28a, 28b of the illumination optics may be arranged on the path of the light between light source 26 and dichroic beam splitter 32, and a second part 28c, 28d of the illumina- tion optics may be arranged on the path of the light between dichroic beam splitter 32 and sensing port 30. The first part 28a, 28b is adapted to collimate the light from light source 26 at dichroic beam splitter 32 while the second part 28c, 28d is adapted to fo- cus the light from dichroic beam splitter 32 to a focal point 33 at sensing port 30. It must be noted that this collimation and focusing, as it is imple- mented by illumination optics 28a – 28d, will not be perfect because light source 26 is an extended light source, e.g., with an area in a range between 0.1 ^ 0.1 mm2and 2 ^ 2 mm2(as compared to an aperture diameter Da of the illumination optics of, e.g., between 3 and 6 mm). As mentioned, the illumination optics 28a – 28d may be adapted to focus the light from the light source 28 at a focal point 33. For safety reasons, and as shown in Fig.6, focal point 33 may be located within an exterior side 30a of the sens- ing port, i.e., within the convex hull of the section of the support structure 10 that forms the sensing port 30 (i.e., within the convex hull of second end 14b of second il- lumination support element). This reduces the risk of excessively high radiation on target 8 because the light leaving the spectrometer is divergent. Collimation Section Collimation section 4 receives scattered light from illumination sec- tion 2 or from another source of radiation. In the shown embodiment, it comprises collimation optics 34a – 34c, with 34a, 34b being convex lenses and 34c being a pin- hole. The distances between pinhole 34c and each lens 34a and 34b are equal to the focal lengths of the lenses 34a, 34b, respectively. Collimation optics 34a – 34c is adapted to improve the collimation of the light scattered back from target 8 in order to send light of better collimation, along a collimation axis Ac, into spectrometer section 6. To do so, the already partially collimated light entering collimator section 4 is focused at pinhole 34c, which spatially filters out the poorly collimated light components, whereupon it is again collimated by lens 34b. The aperture diameter of pinhole 34c may, e.g., be less than 3 mm or even less than 1 mm. In other embodiments, pinhole 34c may be annular to collect light from a ring area at the skin and not necessarily from the focal spot. In a compact design, collimation axis Ac may extend transversally, in particular perpendicularly, to the illumination axis Ai of illumination section 2, P193358PC00 DRAFT 2024-06-28 REV.DOCX 7 with the illumination axis Ai being defined as extending from light source 26 to sens- ing port 30. Spectrometer Section Spectrometer section 6 is adapted to analyze the spectrum of the light from collimator section 4 within a spectral analyzation range. This range may, e.g., lie somewhere within ^0and ^0+ 200 nm (with ^0being the center excitation wavelength as mentioned above) and have a spectral width of 50 and 150 nm. For ex- ample, if ^0= 785 nm, the spectral analyzation range may be from 800 nm to 950 nm. In a typical example, the spectral analyzation range may be between 239 cm–1and 2213 cm–1. Spectrometer section 6 comprises a spatial heterodyne spectrometer 36. Such interferometers are, e.g., described by C.-A. Stöckling et al., " Optical Simu- lation and Design of Spatial Heterodyne Spectrometers for Remote Sensing Applica- tions", EPJ Web of Conferences 238, 12018 (2020), For further references, see US5059027A or US10908023B2. As described here, spatial heterodyne spectrometers, if dimensioned properly, may be well suited for compact, wearable devices. The spatial heterodyne spectrometer comprises a spectrometer beam splitter 38 and two gratings 40a, 40b. The spectrometer beam splitter 38 and the grat- ings 40a, 40b form a Michelson interferometer with two arms 42a, 42b. The spectrometer may further comprise two field-widening prisms 44a, 44b, with one prism arranged in each arm 42a, 42b between spectrometer beam splitter 38 and the grating 40a and 40b, respectively. The light from collimator section 4 enters through an entry side sur- face 38a of spectrometer beam splitter 38. In the beam splitter, the light is split equally between the two arms 42a, 42b. Beam splitter 38 may be achromatic over the spectral analyzation range. At the end of each arm 42a, 42b, the light is diffracted at the grating 40a and 40b respectively. Each grating 40a, 40b may be arranged in first-order Littrow-con- figuration for a wavelength ^Llying in the spectral analyzation range ^min... ^maxof the spectrometer, i.e., light at said wavelength is first-order diffracted back parallel to the direction of the incoming light. This configuration reduces the angular spread of the diffracted components in respect to the axis of the arms. P193358PC00 DRAFT 2024-06-28 REV.DOCX 8 The prisms 44a, 44b improve, as known, the quality of the recon- structed spectrum and allow more throughput (etendue, especially angle of inci- dence). The diffracted components from the gratings 40a, 40b travel back along the arms 42a, 42b. In beam splitter 38, half of their intensity is directed towards an exit 38b of beam splitter 38. The diffracted components may pass optional camera optics (not shown) and arrive at a camera 48, where they generate interference patterns. The interference patterns of the different pairs of diffracted compo- nents of equal wavelength have different spatial frequencies along camera 48. Spectral analysis of the spatial spectral components in the image recorded by camera 46 allows to determine the spectral components of the light arriv- ing in spatial heterodyne spectrometer 36. More details on techniques for such a spec- tral analysis are provided in the section "Camera Image Processing" below. The images recorded by camera 48 are processed by a computing device (not shown), which determines the spectrum of the light arriving at spatial het- erodyne spectrometer 36. Support Structure As mentioned, the spectrometer may comprise a support structure 10 including several molded, interconnected support elements 12 – 24. Support structure 10 may include several features that, alone and / or in combination, simplify the manufacturing process and allow to build a highly com- pact spectrometer. For example, building a support structure of several molded support elements and, optionally, other parts, streamlines the manufacturing process because molded support elements are easy to manufacture in large numbers even if they have complex shape. In the following, the support elements of the various functional sec- tions 2, 4, and 6 are described in more detailed. In the shown embodiment, illumination section 2 has three illumina- tion support elements 12, 14, and 16. For example, there may be a first illumination support element 12 holding a first part 28a, 28b of the illumination optics. This first part may, as shown, be the lens or lenses 28a, 28b of the illumination optics that is / are arranged along the light path between light source 26 and dichroic beam splitter 32. At a first end 12a, first illumination support element 12 may be con- nected to light source 26. In the shown example, the first end 12a of first illumination P193358PC00 DRAFT 2024-06-28 REV.DOCX 9 support element 12 is connected to a carrier member 50, such as a plate, that holds light source 26. Further, there may be a second illumination support element 14 holding a second part 28c, 28d of the illumination optics. This second part may, as shown, be the lens or lenses 28a, 28b of the illumination optics that is / are arranged along the light path between dichroic beam splitter 32 and sensing port 30. Further, there may be a third illumination support element 16 hold- ing dichroic beam splitter 32. The third illumination support element 16 may be ar- ranged (along the light path from light source 26) between the first and second illumi- nation support elements 12, 14. If dichroic beam splitter 32 is used in transmission for the light from light source 26, the first and the second illumination support elements, 12, 14 are mounted to opposite sides of the third illumination support element 16. The first and second illumination support elements 12, 14 may have cylindrical or at least rotationally symmetric outer surfaces and / or be provided with screw connectors for connecting them to third illumination support element 16 and / or carrier member 50. On the other hand, third illumination support element 16 may have one or more substantially planar outer surfaces, e.g., on its side facing collimator sec- tion 4. The wall 16a on that side may have an opening 16b for the passage of the scat- tered light from target 8. This simplifies connecting third illumination support ele- ment 16 to collimator section 4. In particular if dichroic beam splitter 32 is a prism beam splitter, third illumination support element 16 may have four substantially flat walls extend- ing, e.g., parallel to illumination axis Ai, between the first and second illumination support elements 12, 14. Collimation section 4 may have a single collimation support ele- ment 18 holding the collimation optics 34a, 34b, 34c. Collimation support element 18 may be directly mechanically mounted to third illumination support element 16, which simplifies mutual alignment. Collimation support element 18 may have a cylindrical or at least rotationally symmetric outer surface and / or be provided with a screw connector at a first end 18a for connecting it the illumination section 2, e.g., to the third illumination support element 16 as shown in Fig.1. In addition or alternatively thereto, collima- tion support element 18 may be provided with a screw connector at a second end 18b for connecting it to spectrometer section 6. P193358PC00 DRAFT 2024-06-28 REV.DOCX 10 Collimation support element 18 may hold the two collimation lenses 34a, 34b as well as pinhole 34c between them for ease of assembly and accu- rate mutual alignment. Spectrometer section 6 may have an analyzer support element 20 that holds the spatial heterodyne spectrometer 10. Collimation support element 18 and analyzer support element 20 may be directly mechanically mounted to each other, which simplifies mutual align- ment. Analyzer support element 20 may hold spectrometer beam splitter 38. The support structure of spectrometer section 6 may further com- prise two arm support elements 22a, 22b, each of which holds one grating 40a, 40b and one prism 44a, 44b, thereby allowing to separately assembly, in accurate mutual alignment, each grating 40a, 40b and its respective prism 44a, 44b. More details on the arm support elements 22a, 22b are provided below. Alternatively, the prisms and / or gratings may be directly mounted to analyzer support element 20 and / or to spectrometer beam splitter 38. The support structure of spectrometer section 6 may further com- prise a camera support element, which is mounted to analyzer support element 20, e.g., screwed thereto. Camera support element 24 may hold camera 48 as well as (if pre- sent) camera optics. Filter Assembly The spectrometer may comprise a filter assembly, e.g., arranged in the support structure 10, which is positioned to receive light returning from target 8 and adapted to block light in the excitation wavelength range, i.e., to block light from light source 26. There are various sections in the spectrometer where elements of the filter assembly may be located. For a good suppression of the light in the excita- tion wavelength, several such elements may be arranged in series. Some such ele- ments are described in the following – they may be used individually or in combina- tion. These elements may not only block light in the excitation wave- length range. They may also block light at other wavelengths if these wavelengths fall outside the spectral analyzation range. For example, they may block visible light or far-infrared light if the spectral analyzation range is in the near infrared. P193358PC00 DRAFT 2024-06-28 REV.DOCX 11 However, the filter assembly should be adapted to pass light in the spectral analyzation range of the spectrometer. One element of the filter assembly may be the dichroic beam split- ter 32. As mentioned, it is designed to prevent light in the excitation wavelength range from entering collimator section 4. Since dichroic beam splitter 32 may not be able to prevent all light in the excitation wavelength and / or in other undesired wavelength ranges from enter- ing collimator section 4, further elements may be added to the filter assembly. If dichroic beam splitter 32 is a prism beam splitter, one such ele- ment may, e.g., be a filter coating 58 (see Fig.1) arranged on the exit surface 32a of the dichroic beam splitter 32, with the exit surface 32a being the surface facing colli- mation optics 34a, 34b, 34c. Such a filter coating obviates the need to separately in- stall a filter component. In addition, or alternatively, such a filter coating 60 (Fig.1) may be arranged on any of the surfaces of spectrometer beam splitter 38, such as on entry side surface 38a. In addition, or alternatively, such a filter coating may also be ar- ranged on any of the surfaces of the spectrometer beam splitter 38. In addition or alternatively thereto, the filter assembly may com- prise separately mounted filter elements 62a, 62b, e.g., arranged in collimator section 4, as shown in dashed lines in Fig.1. Spectrometer Geometry Fig.2 illustrates some further aspects that may be used in the design of the spectrometer 36. The figure shows beam splitter 38, the two arms 42a, 42b of the spectrometer, and camera 48. Note: In the shown embodiments, the planes of the gratings 40a, 40b are perpendicular to the plane of the arms 42a, 42b and the grating vectors lie substantially within the plane of the arms 42a, 42b. The multispectral light 70 coming from collimator section 4 is shown in continuous lines. At spectrometer beam splitter 38, it is split into two beam parts 72a, 72b of substantially equal intensity that propagate through the prisms 44a, 44b (if present) to the gratings 40a, 40b. At the gratings 40a, 40b, the beam parts 72a, 72b are diffracted, whereby they are split into diffracted spectral components. As mentioned, the gratings 40a, 40b may be arranged under Littrow-configuration for a wavelength^Llying in the spectral analyzation range of P193358PC00 DRAFT 2024-06-28 REV.DOCX 12 the spectrometer. Fig.4 shows the paths of two diffracted spectral components. The first spectral component is denoted by 74a in first arm 42a and 74b in second arm 42b and has the wavelength ^1= ^L. The second spectral component is denoted by 76a in first arm 42a and 76b in second arm 42b and has a wavelength ^2> ^L. As can be seen, the first spectral component 74a, 74b propagates along the same path as, but in opposite direction to, the beam parts 72a, 72b, respectively. As they reach spectral beam splitter 38, part of these diffracted spectral components 74a, 74b, 76a, 76b propagate towards camera 48. As can be seen, the first diffracted spectral components 74a, 74b co- incide and overlap fully on camera 48 while the second diffracted spectral compo- nents 76a, 76b do not fully coincide and overlap only partially at camera 48. For all pairs of diffracted spectral components of equal wavelength, the wavefronts from the two arms are non-parallel because the gratings 40a, 40b are arranged at a non-zero tilt angle ^ in respect to the incident direction of the beam parts 72a, 72b. Fig.2 illustrates the wavefronts 78a, 78b of the first diffracted spectral components 74a, 74b. Hence, each pair of diffracted spectral components generates interference fringes in the image recorded by camera 48, but only where the pair of diffracted spectral components overlaps. This is illustrated in Fig.3, which shows the image recorded by camera 48 for monochromatic incoming light 70 at three different wavelengths ^1= ^L, ^2> ^L, and ^3> ^2. The regions of overlap are shown in white-dotted lines and denoted by O(^1), O(^2), and O(^3). As can be seen, the region of overlap becomes smaller for wavelengths farther away from the Littrow-angle wavelength. Most of the conventional SHS designs combine two measures to keep the regions of overlap as large as possible over the spectral analyzation range of the spectrometer: - The prisms 44a, 44b reduce the angle between diffracted spectral components of different wavelengths without substantially reducing the angles be- tween the wavefronts. - Camera imaging optics arranged between beam splitter 38 and camera 48 are used to project the regions of the beam splitters 40a, 40b into the plane of the camera, thereby increasing the overlap even for diffracted spectral components farther away from the Littrow wavelength. However, as described here, there are other measures to deal with the issue of partial overlap that may obviate the need for camera optics and / or prisms, and / or at least allow to use simpler versions of these components, thereby providing a P193358PC00 DRAFT 2024-06-28 REV.DOCX 13 simpler and / or more compact spectrometer design. Such measures, which may be used in combination or alternatively, are described in the following sections. Distance Between Camera and Beam Splitter One of the mentioned possible measures includes placing camera 48 close to spectrometer beam splitter 48. As illustrated in Fig.2, collimator section 4 is adapted to generate, at least in the plane of the two arms 42a, 42b, a light field with a diameter (i.e., a width) D. This light field is substantially collimated, even though it might still have a slight divergence of, e.g., 5°. Camera 48 is, when compared D, close to the center 80 of the beam splitter 38, with the center 80 being defined as the location where the center axis 82 of the collimated light 70 from collimator section 4 intersects beam splitter 38. For a good overlap of all diffracted spectral components, the dis- tance L between center 80 of spectrometer beam splitter 38 and camera 48 should be smaller than D. In a particularly robust and compact design, and as shown in Fig.1, if spectrometer beam splitter 38 is a prism beam splitter, camera 48 may be mounted directly at the surface at the exit 38b of the prism beam splitter. The diameter D of the collimated light field should be sufficiently large for the desired spectral resolution (see below). Even though camera optics may be used between beam splitter 38 and camera 48 for projecting the regions of the gratings 40a, 40b onto camera 48 and thereby increasing the overlap of pairs of diffracted spectral components at the cam- era, the measures described herein make using such optics optional. Hence, in some embodiments, the spectrometer may not have any imaging optics arranged between the spectrometer beam splitter 38 and the camera 48, which allows for a more com- pact spectrometer. Similarly, to increase the overlap of the pairs of diffracted spectral components at camera 48, the first and second arms 42a, 42b may be short. Hence, the optical lengths La, Lb of each arm 42a, 42b (defined by the integral of the product of distance and refractive index and as measured for axial light traveling from center 80 of the beam splitter 38 to the gratings 40a, 40b) may be smaller than 2.D. (Note: Fig.2 is not to scale and the arms 42a, 42b may be shown to be longer than they actually are.) A somewhat higher limit for the arm lengths La, Lb than for the distance L of the camera is provided for accommodating the tilt of the gratings as well as for the presence of the prisms 44a, 44b. P193358PC00 DRAFT 2024-06-28 REV.DOCX 14 Grating Spacing and Tilt Angle Another possible measure to increase the overlap for each pair of diffracted spectral components while keeping the device compact is to adjust the grat- ing spacing S of the gratings and use a large grating spacing and a small tilt angle^. The wavelength^Lfor first-order Littrow-configuration should still lie in the spectral analyzation range of the spectrometer. Note: in the present text, the tilt angle ^ is defined as the angle be- tween the normal vector of the grating plane and the direction of the light 72a, 72b as it arrives at the grating 40a, 40b. In some embodiments, a low tilt angle ^ as achieved if the grating spacing S of the gratings 40a, 40b fulfills the following condition S = k.^L, with k between 5 and 10. (1) Such grating spacings S are much larger than those that are typically used in SHS spectrometers. For too small values of k, the diffraction angles at wavelengths away from the wavelength^Lbecome large, which reduces the overlap at the camera. For too large values of k, the resolution of the spectrometer is affected unless very large gratings are used. k in the above range between 5 and 10 provides a sufficient spectral analyzation range of approximately 150 nm in the near infrared (e.g., at around 800 nm), which is a typical range for a Raman spectrometer, while it keeps the size of the spectrometer small. The relation between grating spacing S, tilt angle ^, and the first- order Littrow wavelength^Lis sin(^) =^L / (2.S). (2) Hence, a value of k within 5 and 10 corresponds to a tilt angle ^ be- tween 2.9° and 5.8°. Since the overlap of the spectral component pairs on camera 48 is largest at the (first-order) Littrow wavelength^L, the accuracy of the spectrometer is best at the Littrow wavelength^Lbecause a strong signal can be derived from the camera image (see below). The size of the gratings (along the grating vector) and the width D of the beam (along the grating vector) should be large enough to provide sufficient P193358PC00 DRAFT 2024-06-28 REV.DOCX 15 spectral resolution. Assuming that ^0= (^min+ ^max) / 2 is the center wavelength of the spectral analyzation range and ^^ is the desired resolution, the number G of grating periods within the width D should fulfill m is the diffraction order to be used. Usually, m = 1. For typical Raman measurements within a range of ^min = 800 nm and ^max = 915 nm and with a desired resolution ^^ of 0.8 nm, we have N = 1071, i.e., the number G of grating periods should be at least somewhat more than 1000 within the width D. When using ^L = ^min = 800 nm, the grating spacing S given by Eq. (1) is, for k = 5...10, between 3.2 µm and 6.4 µm and, therefore, D should be at least 3.2 mm ...6.4 mm. This illustrates that a Raman spectrometer can be implemented us- ing beam diameters of less than 10 mm, e.g., between 3.2 mm and 6.4 mm. Therefore, the spectrometer is readily suitable for integration into handheld or wearable devices. Camera Image Processing Another possible measure to deal with the issue of partial overlap lies in the spatial frequency analysis of the images recorded by camera 48. Digital Fourier Transform: In a simple approach, spatial frequency analysis may be based on calculating the regular digital Fourier transform of the brightness values ^^,^of the pixels m, n of the camera image, i.e., where m, n are the pixel indices, assuming that m extends perpendicularly to the inter- ference fringes (cf. Fig.3(A)), M is the number of pixels along index m and N is the number of pixels along index n. The sum extends over all pixels, and u,v are the spa- tial frequencies of the fringes in the Fourier transform. If we assume the fringes have a grating vector parallel to index m, a 1-dimensional analysis may be used, i.e. P193358PC00 DRAFT 2024-06-28 REV.DOCX 16 where m, n are the pixel indices, assuming that m extends perpendicularly to the inter- ference fringes (cf. Fig.3(A)), M is the number of pixels along index m and N is the number of pixels along index n. The sum extends over all pixels, and u is the spatial frequencies of the fringes in the Fourier transform. The following examples assume that a 1D Fourier transform of the type of Eq. (4b) is used, but they can be readily expanded to 2D Fourier transforms of the type of Eq. (4a). The signal strength I at a given spatial frequency u, can, e.g., be as- sessed from the amplitude of the complex value X(u), i.e., Further, the known relation between spatial frequency u and wave- length ^(u) (or ^(u,v) if a 2D Fourier transform is used) can be used to convert the values from Eq. (5) to the wavelengths ^ of the light to be analyzed. However, when the overlap O(^) varies strongly over the wave- lengths ^ in the spectral analyzation range, the method of Eqs. (4) will yield too small values for the wavelengths ^ with small overlap O(^). To overcome the issue of varying overlap, the spatial frequency analysis may account for wavelength-specific overlaps O(^) of the pairs of diffracted components from the two arms 42a, 42b as they interfere at the camera 48 by using wavelength-depending "overlap parameters" depending on said overlaps O(^). In the following, some examples of such overlap parameters on how they are used in the spatial frequency analysis are described. Scaled, Non-Weighted Digital Fourier Transforms: In a first, simple approach, the "overlap parameters" may be the ar- eas A(^) covered by the overlaps O(^) as shown with crosshatch in Fig.4. In that case, for example, Eq. (4) may be re-formulated as P193358PC00 DRAFT 2024-06-28 REV.DOCX 17 with Aubeing the area A(^(u)) and A0being an optional scaling factor, such as the area at full overlap or the area of the image. In another approach, the overlap parameters may be the linear ex- tensions L(^), e.g., along index m of the overlaps O(^) as shown in Fig.4. Eq. (4) may in that case be re-formulated as with Lu being the extension L(^(u)) and L being an optional scaling factor, such as the extension at full overlap or the extension of the image along index m. Note that Eq. (7) is but an approximation. For large overlaps, the extension Lu varies substantially linearly with the overlap area Au, but for small over- laps, it deviates from the linearity and Eq. (7) loses accuracy. Alternatively, for better linearity in Lu, the sum of Eq. (7) may ex- tend only over an elongate subset Ps of the pixels centered on the center of the over- laps and extending along index m as illustrated in Fig.4. Within this subset Ps, the area of the overlap is substantially proportional to Lu also for small overlaps. Hence, in more general terms, the overlap parameters may be de- scriptive of at least one of the following values: - the area (Au) of overlap of the pairs of diffracted components at the camera 48 and - the (linear) extension (Lu) of overlap (e.g., perpendicular to the in- terference fringes in the image) of the pairs of diffracted components at the camera 48. Using non-weighted, regular digital Fourier transforms as in Eqs. (6) and (7) and subsequently scaling the spectral components has the advantage that it is possible to use efficient FFT algorithms, which makes processing fast. Hence, in some embodiments, the spatial frequency analysis may comprise calculating a non-weighted digital Fourier transform of at least part of the image. The Fourier components calculated in this way may then be scaled with the overlap parameters. Weighted Digital Fourier Transform: In yet another approach, the simple non-weighted digital Fourier transform of Eqs. (4), (6), or (7) may be replaced by a weighted digital Fourier trans- form as follows: P193358PC00 DRAFT 2024-06-28 REV.DOCX 18 with wm,n(u) (or wm,n(u,v)) being a weight function specifying the weight of each pixel for the spatial frequency u. ^^is a scaling factor depending on the spatial frequency u. In a simple embodiment, wm,n(u) may, e.g., be zero outside the over- lap O(^(u)) and 1 within the overlap. In a more advanced embodiment, wm,n(u) may be proportional to the interference contrast of the interference pattern on camera 48 at the region around pixel m, n of the pair of diffracted components at wavelength ^(u). The scaling factor Sf may be proportional to 1 / Pu, with Pu being the sum of wm,n(u) over all pixels m, n, i.e. Using a weighted digital Fourier transform as in Eq. (8) allows to computationally suppress any noise signals received by pixels outside the overlap re- gion O(^(u)), thereby providing better signal quality. Methods using regular Fourier transforms and FFT (as with Eqs. (6) and (7)) may be combined with methods using weighted digital Fourier transforms (as in Eq. (8)). The former may, e.g., be used for fast, real-time processing while the latter may be used when more accurate results are required. Hence, in some embodiments, the "overlap parameters" may com- prise weight functions wm,n(u) descriptive of a strength of interference of the pairs of diffracted components at the camera at the image locations m, n. Spectral Modelling: In yet other embodiments, techniques other than Fourier transforms may be used. For example, the pixel intensities xm,n of the image can be modeled as a sum of Raman spectra, e.g., of different expected compounds the concentration of which is to be measured, as follows: ^^,^ = ∑^ ^^ ∙ F^(^, ^) , (10)where functions Fr(m,n) describe the interference patterns generated by one Raman spectrum r with r = 1... R and R being the number of modeled Raman spectra. ar are unknown parameters describing the strengths of the spectra. P193358PC00 DRAFT 2024-06-28 REV.DOCX 19 The Raman spectra r have different wavelength distributions and / or spectral widths, and the functions Fr(m,n) may be determined experimentally (e.g., by feeding the respective spectrum to the spectrometer and recording the image by means of camera 48) or numerically (e.g., by using ray tracing to calculate the camera image using the known spectrum of the spectrum and the imaging properties of the spectrometer. The Raman spectra r may correspond to the Raman spectra of one or more expected compounds in the target to be measured, or they may, e.g., be equally spaced over the spectral analyzation range. The parameters arare indicative of the amplitudes of the spectra. Once camera 48 has recorded an image and, therefore, the values of xm,nare known, the parameters arcan be determined using linear fitting techniques. In this variant, the "overlap parameters" are represented by the func- tions Fr(m,n) because each of them depends on the overlap of the respective pairs of diffracted components within the Raman spectrum r. Hence, in these embodiments, the spatial frequency analysis may comprise fitting several parameters arwith r = 1... R and R > 1 to the image recorded by the camera, wherein with xm,n being pixel brightness values at coordinates m, n in the image and with Fr(m,n) being functions giving the brightness of the pixel values for R predefined spectra r. As mentioned, the functions Fr(m,n) may correspond, in their center wavelength and spectral width, to a number R of expected Raman spectra in the spec- trum of the light to be analyzed. Note that functions Fr(m,n) may comprise one or more further pa- rameters that may also be fitted. Such parameters may include, e.g., device parame- ters of the spectrometer that account for manufacturing tolerances, temperature pa- rameters if temperature affects the optical properties of the spectrometer and / or the target, a pressure parameter if pressure affects the properties of the target, etc. Determining the Overlap Parameters: The various "overlap parameters" as described above may be deter- mined, as already mentioned, experimentally or by means of modelling the spectrom- eter. This is explained in the following. P193358PC00 DRAFT 2024-06-28 REV.DOCX 20 In a first class of embodiments, the method may comprise the step of running calibration measurements with light of known spectral composition for de- termining the overlap parameters. In this first class of embodiments, for example, monochromatic light at different wavelengths may be subsequently fed to the analyzer, and the im- ages recorded by camera 48 may be analyzed in order to determine the overlap pa- rameters. Alternatively, in some embodiments, white light (i.e., a known mix of a large number of wavelengths over the spectral analyzation range) may be fed to the spectrometer and, again, the image recorded by camera 48 may be analyzed, e.g., using local Fourier analysis or fitting techniques, in order to determine the overlap pa- rameters. In the second class of embodiments, the spectrometer may, e.g., be modeled by means of known ray tracing techniques for a plurality of wavelengths in the spectral analyzation range, calculating the interference pattern at the location of camera 48 for each one of them. In this case, the overlap parameters can be obtained from the results of the modeled interference patterns. The second class may also easily incorporate imperfections from manufacturing or changes in the spectrum. Further notes: Some important embodiments of image processing include the fol- lowing: - As mentioned, it is possible to calibrate and automatically correct for misalignment or grating imperfections, e.g., by using the techniques mentioned in the section "determining the Overlap Parameters". - Grating misalignments may be corrected for by rotating the image. - Bandpass filters may be used because the range of the fringe fre- quencies in the image is known. By running bandpass filters over the image data, all irrelevant frequencies can be suppressed. - Image processing can be applied to the image for removing arti- facts (defects such as dots, lines, scratches,...). - Automatic optimization of the region of interest to which the FT may be applied; as the spectral range to be analyzed is often known, it is possible to optimize the overlap (e.g., for max. resolution (small analysis window or more signal strength), but less resolution (larger analysis window)). P193358PC00 DRAFT 2024-06-28 REV.DOCX 21 - Any curvature of fringes may be detected, e.g., by image pro- cessing, and be corrected for (since it is known that the fringes are ideally straight), or image regions with strongly curved fringes may be ignored. - Pixel interpolation may be used to convert the discrete values of the pixels into a continuous distribution. Notes The above device and method are, e.g., suited for Raman spectrum analysis because they can be well adapted to the wavelength range of typical Raman spectra. Hence, the method may further comprise the following steps: - Illuminating the target 8 at an excitation wavelength range. - Feeding Raman-scattered light from the target 8 to the spectrome- ter. - Using a result of the spatial frequency analysis for determining the spectrum of the Raman-scattered light. Similarly, the spectrometer may be used for Raman spectroscopy. The spectrometer and method are, as mentioned, particularly suited for wearable applications. Since they don't need imaging optics at the camera and / or can cope with strongly varying overlaps, they have the potential to be built at small size, low weight, and low cost. While there are shown and described presently preferred embodi- ments of the invention, it is to be distinctly understood that the invention is not lim- ited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

P193358PC00 DRAFT 2024-06-28 REV.DOCX 22 Claims 1. A spatial heterodyne spectrometer comprising a spectrometer beam splitter (38), a collimator section (4) arranged at an input side of the spectrometer beam splitter (38), a first diffractive grating (40a), with a first arm (42a) of the spec- trometer extending between the spectrometer beam splitter (38) and the first diffrac- tive grating (40a), a second diffractive grating (40b), with a second arm (42b) of the spectrometer extending between the spectrometer beam splitter (38) and the second diffractive grating (40b), and a camera (48) at an exit (38b) of the spectrometer beam splitter (38), wherein, in a plane of the first arm (42a) and the second arm (42b), the collimator section (4) is adapted to generate, at the beam splitter (38), light having a diameter D, and wherein a distance L between a center (80) of the spectrometer beam splitter (38) and the camera (48) is smaller than D.

2. The spectrometer of claim 1 wherein the spectrometer beam split- ter (38) is a prism beam splitter and wherein the camera (48) is mounted to a surface of the exit (38b) of the prism beam splitter.

3. The spectrometer of any of the preceding claims wherein the gratings (40a, 40b) are arranged at a tilt angle α in respect to the light arriving at the diffractive gratings (40a, 40b), wherein the tilt angle α is between 2.9° and 5.8°.

4. The spectrometer of any of the preceding claims wherein the spectrometer has a spectral analyzation range extending from a minimum wavelength ^minto a maximum wavelength ^maxand wherein the gratings (40a, 40b) are arranged in first-order Littrow-configuration for a wavelength ^Llying between ^minand ^max.

5. The spectrometer of any of the preceding claims further compris- ing, in each arm (42a, 42b), a prism (44a, 44b) arranged between the spectrometer beam splitter (38) and the grating (40a, 40b).P193358PC00 DRAFT 2024-06-28 REV.DOCX 23 6. The spectrometer of any of the preceding claims without imaging optics arranged between the spectrometer beam splitter (38) and the camera (48).

7. The spectrometer of any of the preceding claims wherein an opti- cal length La, Lb of each arm (42a, 42b) is smaller than 2.D.

8. Use of the spectrometer of any of the preceding claims for Ra- man spectroscopy.

9. A method for operating a spatial heterodyne spectrometer, in par- ticular a spatial heterodyne spectrometer of any of the preceding claims, wherein the spectrometer comprises a spectrometer beam splitter (38), a first diffractive grating (40a), with a first arm (42a) of the spec- trometer extending between the spectrometer beam splitter (38) and the first diffrac- tive grating (40a), a second diffractive grating (40b), with a second arm (42b) of the spectrometer extending between the spectrometer beam splitter (38) and the second diffractive grating (40b), and a camera (48) at an exit (38b) of the spectrometer beam splitter (38), the method comprising sending light to be analyzed into the spectrometer beam splitter (38), wherein the light is split into the two arms (42a, 42b), diffracted into diffracted components at the gratings (40a, 40b), and pairs of diffracted components of equal wavelength from the two arms (42a, 42b) are recombined by the spectrometer beam splitter (38) and interfering at the camera (48), performing spatial frequency analysis on an image recorded by the camera (48), wherein the spatial frequency analysis accounts for wavelength-spe- cific overlaps of the pairs of diffracted components at the camera (48) by using a plu- rality of wavelength-depending overlap parameters depending on said overlaps.

10. The method of claim 9 comprising running calibration measurements with light of known spectral composition for determining the overlap parameters.P193358PC00 DRAFT 2024-06-28 REV.DOCX 24 11. The method of any of the claims 9 or 10 comprising performing model calculations on the spectrometer for determining the overlap parameters.

12. The method of any of the claims 9 to 11 wherein the overlap pa- rameters are descriptive of at least one of - an area (Af) of overlap of the pairs of diffracted components at the camera (48) and - a extension (Lf) of overlap of the pairs of diffracted components at the camera (48).

13. The method of any of the claims 9 to 12 wherein the overlap pa- rameters comprise weight functions wm,n(f) descriptive of a strength of interference of the pairs of diffracted components at the camera (48) at image locations m, n.

14. The method of any of the claims 9 to 13 wherein the spatial fre- quency analysis comprises calculating a non-weighted digital Fourier transform of at least part of the image.

15. The method of any of the claims 9 to 14 wherein the spatial fre- quency analysis comprises calculating a weighted digital Fourier transform of at least part of the image.

16. The method of any of the claims 9 to 15 wherein the spatial fre- quency analysis comprises fitting several parameters arwith r = 1... R and R > 1 to the image recorded by the camera, whereinwith xm,n being pixel brightness values at coordinates m, n in the image and with Fr(m,n) being functions descriptive of the brightness of the pixel values for R prede- fined spectra r.

17. The method of any of the claims 9 to 16 further comprising illuminating a target (8) at an excitation wavelength range, feeding Raman-scattered light from the target (8) to the spectrome- ter, andP193358PC00 DRAFT 2024-06-28 REV.DOCX 25 using a result of the spatial frequency analysis for determining a spectrum of the Raman-scattered light.

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