METHOD, DEVICE AND SYSTEM FOR DETECTING RAMAN STRAIGHT LIGHT

Description

[0001] The invention relates to a method, a device and a system for detecting Raman scattered light.

[0002] Raman spectroscopy is one of the most important non-invasive and rapid methods for material identification today. A Raman spectrometer, which has very high spectral resolution, is typically used, allowing the material to be identified based on its characteristic Raman shifts.

[0003] Raman spectroscopy is based on the Raman effect – a type of inelastic light scattering. In inelastic light scattering, a molecule is polarized by excitation light. This gives the molecule additional energy, which is significantly smaller than the energy of the excitation of higher electronic states and is very short-lived, and which is composed of a quantum mechanical superposition of all the states involved.

[0004] For Raman spectroscopy, the wavelength of the excitation light does not necessarily have to correspond to the material-specific absorption bands of the object under investigation. That is, the wavelength can be selected from any spectral range. Typically, light sources with wavelengths of 365 nm, 405 nm, 546 nm, 670 nm, 783 nm, or 1064 nm are used as excitation light.

[0005] The Raman-scattered light exhibits a material-specific spectral shift relative to the wavelength of the excitation light, corresponding to the characteristic energies of rotational, vibrational, phonon, or spin-flip processes. This results in very small wavelength shifts and therefore requires high spectral resolution in the detection instruments.

[0006] These instruments are commercially available in the price range of €60,000 to €500,000, contain high-quality and expensive optical components, and, apart from less expensive handheld or portable devices, require a laboratory environment with highly skilled operators. With state-of-the-art equipment and a 785 nm excitation beam, a spectral resolution of approximately 0.1 nm can be achieved. Typical values ​​for the spectral resolution of laboratory instruments range from approximately 0.3 nm at an excitation wavelength λexc = 546 nm to 1.1 nm at λexc = 1064 nm.

[0007] In contrast, more affordable handheld devices are disadvantageous because they achieve their smaller size and cheaper components at the expense of spectral resolution and sensitivity.

[0008] The spectrally resolved measurement signal is usually detected using a CCD sensor. The optical sensitivity of the CCD sensor is the limiting factor for the sensitivity of a Raman spectrometer. The technical complexity of spectrometers using CCD sensors increases with the number of individual detectors, i.e., the number of individual pixels of the sensor, since each individual detector is electronically amplified. Consequently, not only do the requirements for trained personnel and the environmental demands for operating these spectrometers increase, but so do the costs of the corresponding instruments. This effect is particularly limiting when it comes to relatively fast dynamic measurements of colloidal or highly dilute liquid samples, especially in flow applications.In this case, elastic scattering at the colloidal particles and additional refractive effects at the interfaces between media with different refractive indices impair the signal yield from the samples or the sample container. For such samples, exceptionally high sensitivity is required without the possibility of a longer integration time.

[0009] Ivleva et al. (NP Ivleva, AC Wiesheu, & R. Niessner (2017), Microplastic in aquatic ecosystems, Angewandte Chemie International Edition, 56(7), 1720-1739) describe an attempt to extend the application of conventional measurement methods to the environmental problem of microplastics using a µ-Raman spectrometer, i.e., image analysis and Raman measurement of individual colloidal particles. However, the physical limitations of imaging, such as the diffraction limit or the limited mechanical resolution of microscopic stages, restrict this method. Due to their complexity, µ-Raman spectrometers require a laboratory environment and corresponding experience in their operation. Furthermore, such instruments are associated with high costs. Further prior art is disclosed in US4648714 A, US2018275064 A, 1, US4784486 A and US4176916 A.

[0010] An additional problem with increasing sensitivity is that the possibilities for further signal amplification and increasing the signal-to-noise ratio are already largely exhausted in current high-quality cooled CCD sensors. This is partly due to the number of individual pixels and the physical limitations of the materials used.

[0011] The only type of non-dispersive Raman detector realized to date is the FT-Raman spectrometer. Instead of a dispersive element, such as a grating, this device uses a spectrally tunable filter, e.g., a Fabry-Perot filter, to split the Raman signal spectrally (see Nondispersive Raman Spectrometers, in "Raman Spectroscopy for Chemical Analysis" 2005). The Raman signal is then superimposed with the excitation light, and the resulting interferogram is recorded by a single detector or an array of detectors. However, such a device suffers from the disadvantages mentioned above, namely the expensive optical components and the need for a laboratory environment.The idea of ​​developing a non-dispersive Raman detector based on conventional interference filters is already known, but has not yet gained widespread acceptance due to a particularly stringent requirement for spectral resolution (width of the transmission function). When using an interference filter with a transmittance range wider than the line to be detected, the intensity to be detected is averaged together with the background within the filter's transmittance range by a detector, resulting in an overall reduced detection signal.

[0012] The object of the invention is to propose an improved non-dispersive method for the detection of Raman scattered light. Compared to the prior art, the improvement includes the feasibility of a handy and portable device without compromising resolution or sensitivity, and which can also be used independently of an expensive and complex laboratory environment.

[0013] The problem is solved by the method according to claim 1 and a device according to claim 8.

[0014] The terms "scattered light" and "scattered light signal" are used below. Both scattered light and scattered light signal describe the same thing, namely photons emitted by the sample under investigation. "Scattered light" refers to its physical wave or beam nature—namely, its ability to be diffracted, refracted, or interfere. "Scattered light signal," on the other hand, describes its mathematical nature—namely, its ability to be processed using signal processing methods, and in particular, its ability to be transformed, integrated, convolutional, or correlated. The terms "light" and "signal" are used synonymously within the scope of this invention.

[0015] In a first aspect, the invention relates to a method for detecting Raman scattered light using at least one interference filter and a detection unit, wherein the Raman scattered light to be detected comprises an incoming scattered light signal. The method comprises the steps defined in claim 1.

[0016] For this method, a Raman device can be used which, by means of a light source, in particular a laser, excites particles in a sample to emit Raman scattered light. Both the emitted Raman scattered light and the excitation light can be deflected and / or guided by optical components. For carrying out the method according to the invention, only the Raman scattered light is required, which, unlike elastic Rayleigh scattering, is angle-independent and exhibits a homogeneous scattering intensity over the entire solid angle.

[0017] The at least one interference filter is preferably a narrowband interference filter, wherein the at least one interference filter is a bandpass filter with a spectral width between 0.5 nm and 4 nm, in particular between 1.5 nm and 2 nm, and preferably 2 nm, in the visible and / or near-infrared spectrum. The spectral width of an interference filter is determined by the spectral band δλ, which is defined by a transmission function assigned to an optical path length L through the interference filter in question. The physical and mathematical characteristics of both light and the at least one interference filter are therefore closely linked.

[0018] The term "at least one interference filter" can refer to a single first interference filter or to multiple interference filters. If multiple interference filters are used in the process, individual filters are designated as "first interference filter," "second interference filter," etc.

[0019] An interference filter can, for example, consist of a substrate layer, preferably made of glass or quartz glass, which is plane-parallel and coated on one or both sides. The coating(s) can, for example, comprise several layers of different materials applied alternately and are transparent only to the spectral band δλ. All coatings together with the substrate form a Fabry-Perot filter with a preferably quasi-rectangular transmission function.

[0020] An interference filter therefore filters light with specific wavelengths through constructive and destructive interference. Which wavelengths are filtered by the interference filter depends on the optical path length L of the light through the at least one interference filter. The optical path length L is the sum of the optical path lengths Lj of the individual layers j, where each optical path length Lj is the product of the refractive index nj of the material of the respective layers j and the actual length Ij of the path of the light through the layer j. The optical path length L is thus defined as: L = ∑ j n j ∗ l j .

[0021] The lengths Ij depend on the geometric configuration of the at least one interference filter or its layers. Preferably, each layer j has a thickness dj, whereby the layers j can have different thicknesses, or the thicknesses dj can differ. If the layers of the at least one interference filter are planar, the lengths Ij are determined based on the thicknesses dj, the refractive indices nj, nj-1, and the angle of incidence αj of the Raman scattered light onto the corresponding layer j, where the refractive index nj-1 is the refractive index of the medium upstream of the respective layer j. In the case of the first layer, the medium upstream of the respective layer j is usually air with a refractive index of approximately 1. In the case of all other layers, the refractive index nj-1 is the refractive index of the respective upstream layer jj-1.The angles of incidence αj are always determined relative to the normal of the entrance surface of the Raman scattered light into the corresponding layer j. The following applies to the length Ij: . l j = d j cos sin − 1 n j − 1 n j sin α j , where sin − 1 n j − 1 n j sin α j The angle of the ray path to the normal through the respective layer j is described according to Snell's law of refraction.

[0022] For the sake of simplicity, the at least one interference filter is described below as an optical component with a (total) length I and a (total) thickness d, onto which the Raman scattered light strikes at the angle of incidence α. The optical path length L thus depends on the thickness d of the at least one interference filter, the angle of incidence α of the Raman scattered light on the at least one interference filter, and the refractive indices nj of the layer materials.

[0023] In signal processing terms, each optical path length L is assigned a transmission function. For example, a first optical path length L1 is assigned a first transmission function, a second optical path length L2 is assigned a second transmission function, and so on. The assignments of transmission functions and optical path lengths L1 are bijective, meaning they are unique and invertible.

[0024] A transmission function defines a spectral band δλ for light with specific wavelengths that transmits along the optical path length L through one of at least one interference filter. The spectral band δλ defined by a transmission function is arranged around a mean wavelength λ₀ and is bounded above by an upper limit δλ⁺ and below by a lower limit δλ⁻. The following holds true: δλ = δλ + − δλ − .

[0025] The upper limit δλ+< and the lower limit δλ-< can be arranged symmetrically or asymmetrically around the mean wavelength λ₀. The mean wavelength λ₀ can be, for example, the arithmetic mean, the mode, the median, or a weighted average of the transmitted wave spectrum. Crucially, the mean value must be determined in the same way for all transmission functions. In the context of rectangular transmission functions, the mean wavelength λ₀ is also commonly referred to as the "central wavelength" of the transmission function.

[0026] A transmission function can be approximately described by a rectangular function, where the transmission function f(λ) is then defined as: f λ = 0 , wenn λ > λ 0 + δλ + 0 , wenn λ < λ 0 − δλ − 1 , wenn λ 0 − δλ − ≤ λ ≤ λ 0 + δλ + .

[0027] In particular, |λ 0 + δλ +< | = |λ 0 - δλ -< | holds if the upper limit and the lower limit are arranged symmetrically around the mean wavelength λ 0, i.e., if it is indeed the central wavelength.

[0028] It should be noted here that the term "transmission function," as used in this document, is not synonymous with the technical specification for an interference filter, which is also frequently referred to as the "transmission function." While the technical specification always refers to the transmission property of an interference filter under perpendicular incidence and thus a constant material property, the term used here refers to a transmission function that depends on a variable optical path length. This function also ultimately describes a quasi-rectangular function, but it can be shifted to other frequencies, for example, due to different angles of incidence through the same interference filter or due to different layer thicknesses of the interference filter. And it is precisely such a shift that the invention utilizes.

[0029] The increased spectral resolution is achieved by using a series of transmission functions that are minimally spectrally shifted from each other to map the incoming scattered light signal, in other words the incoming Raman scattered light, onto the detection unit.

[0030] A "spectrally shifted transmission function" is understood to be a transmission function that is assigned to a second optical path length L₂ ≠ L₁, which differs from a first optical path length L₁, and defines a second spectral band δλ₂ whose mean wavelength λ₂₀ is shifted relative to the mean wavelength λ₁₀ of the first spectral band with spectral width δλ₁₀. The spectral shift by wavelength Δλ₂ is always specified relative to the first mean wavelength λ₁₀, with the first mean wavelength λ₁₀ serving as the reference wavelength. The following applies: Δλ 2 = λ 2 , 0 − λ 1 , 0 .

[0031] The displacement itself is also specified using the unit "wavelength".

[0032] A "minimal" spectral shift of a transmission function by the wavelength Δλ₂ means that the spectral shift of the transmission function by the wavelength Δλ₂ is smaller, and preferably much smaller, than the spectral width δλ₂ of the transmission function itself. The shift is preferably at least 0.1 nm. Furthermore, the shift is preferably at most 2 nm, and particularly preferably less than 1 nm. For reasons of resolution, the ratio of the spectral width δλ₁ and δλ₂ of each of the transmission functions to the spectral shift Δλ₂ is preferably at least 2 and particularly preferably at least 3. Furthermore, for practical reasons, the ratio of the spectral width δλ₁ and δλ₂ of each of the transmission functions to the spectral shift Δλ₂ is preferably at most 10, particularly preferably at most 6, and most particularly preferably not more than 4.

[0033] While a first interference filter with a first thickness d1 and / or consisting of layers with first refractive indices n1,j is used to generate the first filtered scattered light signal, a second interference filter, distinct from the first, with a second thickness d2 and / or consisting of layers with second refractive indices n2,j can be used to generate the second filtered scattered light signal. Alternatively, a path for the Raman scattered light through the first interference filter with a second optical path length L2 ≠ L1 can be chosen for generating the second filtered scattered light signal. For example, the angle of incidence α2 of the Raman scattered light on the first interference filter can be changed when generating the second filtered scattered light signal compared to the angle of incidence α1 when generating the first filtered scattered light signal.In other words, the Raman scattered light strikes the surface of the first interference filter at a different angle of incidence α 2 ≠ α 1 to generate the second filtered scattered light signal than when generating the first filtered scattered light signal.

[0034] The two spectral bands δλ₁ and δλ₂, defined by the transmission functions and used to generate the scattered light signals, partially overlap. Analysis of all filtered scattered signals then provides information about the Raman lines in the sample, information that can normally only be obtained using high-resolution Raman instruments. By combining the detected scattered light signals, this information can be extracted from the detected scattered light and assembled into a Raman spectrum. While a single filtered scattered light signal integrates the measured spectrum over the corresponding band δλ₁, two or more signals provide support points spaced less than the bandwidth δλ₁ of the transmission functions, from which the spectrum can be reconstructed with higher resolution.In particular, this allows the positions of Raman peaks to be determined precisely, even when they are narrower than the spectral bands of the transmission functions.

[0035] Thus, it is possible to detect a high-resolution Raman spectrum using components that are relatively inexpensive compared to costly laboratory equipment, namely at least one interference filter and the detection unit. Since the method requires only one or a few interference filters and a detection unit with one or a few detector cells, a corresponding detection device can be built in a compact, and especially portable, size.

[0036] The method is not limited to generating a first and a second filtered scattered light signal. It can also include generating any number of additional filtered scattered light signals, generated analogously to the second filtered scattered light signal. The method then further comprises the following step: Generating further filtered scattered light signals by applying further transmission functions to the incoming scattered light signal, wherein each of the further transmission functions is assigned to a further optical path length L i ≠ L 1 through the first interference filter or through a further interference filter, wherein the further transmission functions each define a further spectral band δλ i, wherein the further spectral bands δλ i comprise light with further wavelengths by further mean wavelengths λ i, wherein the further mean wavelengths λ i are each shifted by one wavelength Δλ i relative to the first mean wavelength λ 1, wherein the light of the further wavelengths is transmitted through the first interference filter or one of the further interference filters.

[0037] When manufacturing interference filters on wafers with a diameter of ≥ 4", the thickness of the individual coating layers varies from the center to the edge of the wafer due to production limitations. This can result in individual layers being thinner at the edge than in the center of the wafer, leading to a shift in the mean wavelength λ₀. This effect, which is generally considered a production defect, can be utilized according to the invention by dividing the wafer into sections, each section having its own mean wavelength λ₀. Interference filters formed from such sections, when positioned in the same location, transmit slightly shifted spectral ranges of Raman scattered light due to the different mean wavelengths λ₀ and the same spectral width. This effect can lead to a shift in the transmission function of an interference filter by up to 10 nm.

[0038] A large-area wafer can be mapped with relatively little effort with respect to the spectral shift and separated to the required size. In this way, a filter kit for an approximately 12 nm wide spectral range (2 nm spectral width + 10 nm shift) can be created. Such a filter kit comprises the first, the second, and subsequent interference filters, where the transmission functions of the second and subsequent interference filters are shifted relative to the first mean wavelength λ₁ by wavelengths Δλ₂ and Δλ₭, respectively.

[0039] Advantageously, a production error can be used for the claimed method by manufacturing interference filters with different optical path lengths Li in a single production process. This makes the components required for the method less expensive.

[0040] Furthermore, the use of additional interference filters can advantageously simplify the design of a device for carrying out the method. No moving parts are required, for example, for a swiveling motion to change the angle of incidence α. Moreover, the detection unit can be designed as a single detector cell, which further reduces costs.

[0041] In one embodiment, the method further comprises the following steps: Generating a reference light signal, generating a first filtered reference light signal by applying the first transmission function to the reference light signal, wherein the filtered reference light signal is associated with the first filtered scattered light signal, generating a second filtered reference light signal by applying the second transmission function to the reference light signal, wherein the second filtered reference light signal is associated with the second filtered scattered light signal, detecting the first and second filtered reference light signals by the detection unit, generating corrected filtered scattered light signals by subtracting the filtered reference light signals from the respective associated filtered scattered light signals.

[0042] Subtracting the filtered reference light signals from the respective filtered scattered light signals advantageously reduces errors in the signals detected by the detection unit. In particular, this correction can reduce inherent system errors, especially errors caused by optical impurities in the components used or alignment errors. The reference light signal is generated by the same excitation light as the measurement signal, which passes through the device along the same path and, where applicable, with the same medium (air, water, microscope slide, container, etc.), but without the substance being sought.

[0043] In another embodiment, a Raman spectrum is generated from the detected scattered light signals or from the corrected scattered light signals.

[0044] The Raman spectrum can contain one or more peaks, each associated with a peak height and a peak position within the spectrum. At least one material is associated with each peak position. The peak height provides information about the amount of material in the sample associated with that peak position. If the Raman spectrum contains no peaks, then no materials whose Raman lines lie within the analyzed spectrum can be detected in the sample.

[0045] The intensity of Raman-scattered light is proportional to the number of scattering molecules in the sample. The following applies: I ∼ ν 4 I 0 N ∂ a ∂ q 2 , where I is the intensity of the Raman scattering light, v is the frequency of the exciting laser, I0 is the intensity of the exciting laser, N is the number of scattering molecules and ∂ a ∂ q The polarizability change is...

[0046] In another embodiment, a multivariate data analysis, in particular chemometrics, is used to generate the Raman spectrum.

[0047] Multivariate data analyses are methods in which several variables are examined simultaneously. These variables can be weighted differently, and the weighting of each variable is not necessarily known. Multivariate data analyses include methods such as structure-discovering techniques, particularly factor analysis, cluster analysis, or multidimensional scaling, and structure-testing techniques, particularly analysis of variance, artificial neural networks, discriminant analysis, and conjoint analysis.

[0048] Chemometrics, or chemometric methods, refers to methods based on multivariate data analysis that extract a maximum amount of chemical information from experimental measurement data. Spectra of material mixtures in near-infrared, visual, or UV spectroscopy can generally only be evaluated using chemometric methods. Examples of such methods include principal component analysis, cluster analysis, and multiple linear regression.

[0049] The spectral resolution can be advantageously increased by using chemometric methods. The spectral width of the peaks in the Raman spectrum can thus be significantly smaller than the width of the spectral bands δλ of the applied transmission functions.

[0050] Higher-resolution peaks increase the accuracy of determining materials or material mixtures and their quantities in a sample, enabling such determinations to be performed even on samples in flow. Similarly, chemometric methods can contribute to resolving a total spectrum formed from minimally shifted and superimposed (partial) spectra. In this case, the shift between the spectra leads to a higher resolution of the overall spectrum to be determined. Particularly in environmental engineering, the results can be applied to environmentally relevant material mixtures, allowing, for example, the determination of the pollution of a body of water with microparticles, especially microplastics, and the differentiation between the contributions of organic and inorganic particles.

[0051] In one embodiment, the incoming scattered light signal has a propagation direction, wherein the method between generating the first filtered scattered light signal and generating the second filtered scattered light signal comprises pivoting the first interference filter relative to the propagation direction of the incoming scattered light signal, wherein the pivoting of the first interference filter causes a change in the first optical path length L 1 to the second optical path length L 2.

[0052] As explained above, the shift of the second transmission function relative to the first transmission function by the wavelength Δλ₂ depends on the difference between the second optical path length L₂ and the first optical path length L₁. If the first interference filter is tilted relative to the propagation direction of the incoming scattered light signal, the angle of incidence α of the Raman scattered light changes from a first angle of incidence α₁ to a second angle of incidence α₂. This also changes the optical path length L₂ from the first optical path length L₁ to the second optical path length L₂. The angle of incidence α can therefore be changed by tilting the first interference filter relative to the propagation direction of the incoming scattered light signal by an angle ε. The wavelength Δλ₂ of the spectral shift can be estimated as follows: Δ λ 2 = λ 2 − λ 1 mit λ 2 ≈ λ 1 1 − sin 2 ε n 1 2 ,

[0053] The angle ε for generating the second or subsequent filtered scattered light signals can be continuously adjusted or set in interval steps. The interval steps can be, but are not limited to, 0.1°, 0.5°, 1°, or >1°.

[0054] The angle ε for generating the second filtered scattered light signal, or further filtered scattered light signals, can be set, for example, in a range of 0.1° to 20°, and preferably from 0.1° to 15°. It should be noted that the angle ε describes a change in the position of the first interference filter from a first position to a second position. The angles ε for generating the second filtered scattered light signal, and optionally the further filtered scattered light signals, always refer to a first position of the first interference filter, which can also be called the reference position.

[0055] Advantageously, this embodiment requires only the first interference filter, which is used to generate the second filtered scattered light signal and, if necessary, further filtered scattered light signals. The filtered scattered light signals are generated sequentially at different angles ε, with the first interference filter being pivoted to a corresponding position for each filtered scattered light signal. Furthermore, this embodiment requires only a single detector cell as the detection unit, onto which all the filtered Raman scattered light is imaged, typically focused. This reduces the number of components required for the method and consequently the costs.

[0056] An alternative embodiment of the method provides that the first interference filter is replaced by the second interference filter between the generation of the first filtered scattered light signal and the generation of the second filtered scattered light signal.

[0057] The second optical path length L₂ ≠ L₁ is achieved by the second interference filter through a thickness d₂ ≠ d₁ that differs from that of the first interference filter, or through refractive indices n₂,j ≠ n₁,j that differ from those of the first interference filter, or through a combination of different thicknesses and different refractive indices. In this case as well, the device requires only a single detector cell as the detection unit, onto which the entire filtered scattered light signal or the Raman scattered light to be detected is imaged / focused, thus requiring a small number of components.

[0058] Instead of exchanging them, two or more interference filters can alternatively be positioned at different locations to generate the first, second, and, if necessary, further filtered scattered light signals, so that the differently filtered scattered light signals can be detected simultaneously. This requires several detection units, each assigned to one of the individual interference filters.

[0059] Another embodiment combines the features of both preceding embodiments. Here, several pivotable interference filters with different thicknesses d and / or coatings with different refractive indices nj are used. This advantageously increases the potential range of the spectral shifts. By using several pivotable interference filters, the number of possibilities for applying differently spectrally shifted transmission functions, and thus the scope of the detectable Raman spectrum, can be increased.

[0060] The following example illustrates this: A first interference filter with thickness d1 and refractive indices n1,j can be positioned by pivoting so that transmission functions with spectral bands δλAB, which together extend from wavelength λA to wavelength λB, can be used. A second interference filter, for example with a different thickness d2 ≠ d1 and / or different refractive indices n2,j ≠ n1,j, can be positioned by pivoting so that transmission functions with spectral bands δλCD, which together extend from wavelength λC to wavelength λD, can be used. The entire spectral range that can be captured with these two interference filters thus encompasses the wavelengths between λA and λB as well as between λC and λD.

[0061] Another alternative embodiment of the method provides that the incoming scattered light signal propagates divergently or convergently, and that the first filtered scattered light signal is generated at a first angle α1 relative to a reference beam, and the second filtered scattered light signal is generated at a second angle α2 ≠ α1 relative to the reference beam. The light of the first wavelengths and the light of the second wavelengths are transmitted through the first interference filter.

[0062] The normal to the entrance surface of the first interference filter is preferably used as the reference beam when generating the first filtered scattered light signal.

[0063] For example, the angle of incidence α at which the Raman scattered light strikes the surface of the first interference filter can be changed by a first converging lens positioned in front of the first interference filter. From an approximately parallel, for example collimated, Raman scattered light, the first converging lens creates a beam focus in front of, within, or behind the interference filter, but not on the detector. The Raman scattered light can then be considered as a sum of infinitesimally wide partial beams. Each of the partial beams strikes the first interference filter at an individual angle of incidence αi. Due to the different angles of incidence αi, different optical path lengths Li through the first interference filter result for each partial beam. Accordingly, several filtered scattered light signals are generated from the incoming scattered light signal using the first converging lens.The procedure then further includes the following step before generating the first filtered scattered light signal: . Focusing the incoming scattered light signal using a first converging lens.

[0064] The angular range that can be covered with this embodiment depends on the optical properties, in particular the refractive index, of the first converging lens.

[0065] For this method, the detection unit comprises an array of several detector cells, whereby the filtered scattered light signals are detected by the detector cells.

[0066] As an alternative to a converging lens, a diverging lens can also be used, which spreads out the incoming scattered light beam.

[0067] In another aspect, the invention relates to a device for detecting Raman scattered light according to claim 8, wherein the device is configured to perform the method described above, the device comprising: a first interference filter, a first lens and a detection unit.

[0068] For the claimed device, interference filters are suitable, the substrate of which is preferably made of glass, in particular quartz glass, for wavelengths in the ultraviolet, visible, or infrared spectrum, or specifically for the infrared spectrum, of sapphire glass or germanium. The interference filters are preferably between 0.5 and 2 mm thick. The first and optionally the second interference filter have a spectral width of a few nanometers, in particular between 0.5 nm and 4 nm, and preferably between 1.5 nm and 2 nm, and most preferably a spectral width of 2 nm.

[0069] The detection unit can, for example, but is not limited to, be designed as a photomultiplier tube (PMT), in particular silicon photomultiplier, charge coupled device (CCD), avalanche photodiode (APD), in particular silicon APD or single photon avalanche diode (SPAD) or near-infrared diode, in particular made of indium gallium arsenide.

[0070] According to the invention, the device comprises a first lens, wherein the detection unit comprises an array of detector cells, also called pixels, wherein the array is aligned relative to the first lens such that light is imaged convergently or divergently onto the array through the first lens, wherein the first interference filter is positioned between the first lens and the detection unit.

[0071] The first lens can be configured as either a converging lens or a diverging lens.

[0072] The array can be configured, in particular, as a one-dimensional array of detector cells. Furthermore, the array is preferably positioned such that the Raman scattered light or the partial beams illuminate the array as completely as possible.

[0073] The first lens focuses or defocuses the Raman scattered light such that, as described above, the Raman scattered light strikes the first interference filter in partial beams, each beam striking the surface of the first interference filter at a different angle of incidence αi. This results in a different transmission function being applied to each partial beam. The transmission functions are each shifted by the wavelength Δλi, corresponding to the angle αi, relative to the first transmission function.

[0074] In a further embodiment, the first lens is designed as a cylindrical lens with a first cylindrical axis, wherein the device further comprises a second lens, wherein the second lens is designed as a cylindrical converging lens with a second cylindrical axis, wherein the first and the second cylindrical axes are oriented perpendicular to each other and wherein the array is positioned along the focal line of the second lens.

[0075] Advantageously, the second cylindrical converging lens focuses the filtered scattered light signal onto the detection unit, thereby increasing the signal strength.

[0076] The first lens can preferably also be designed as a cylindrical converging lens and be positioned such that the focal line of the first lens lies in front of, on, in or behind the surface of the first interference filter.

[0077] In an advantageous way, this configuration of the first and second cylindrical converging lenses allows the light yield of the scattered light through the detection unit to be increased.

[0078] The second lens is preferably positioned between the first interference filter and the array of detector cells.

[0079] In one embodiment, the first interference filter is pivotably mounted.

[0080] The pivoting of the first interference filter can be fixed either by means of a mounting frame (especially a miniaturized one) or another suitable device, or by periodic rotation about the pivoting axis. Periodic rotation creates a "tunable" interference filter.

[0081] This embodiment implements a variant of the method described above, in which the optical path length L is achieved by changing the angle of incidence α of the Raman scattered light. Advantageously, this embodiment requires only one detector cell as the detection unit, further reducing the cost of the device. Furthermore, the individual detector cell can be provided with a larger detection area, thereby increasing the signal strength.

[0082] In a further embodiment, the device can have a first converging lens, a second converging lens and a pivotable first interference filter in combination according to the embodiments mentioned above.

[0083] In one embodiment, the first interference filter is designed to be interchangeable with the second interference filter and / or further interference filters.

[0084] In this embodiment, the spectral shift by the wavelength Δλ 2 or Δλ i is achieved by the second or further interference filters.

[0085] The device can, for example, include a holder for the interference filters, with which the interference filters are sequentially positioned to generate the filtered scattered light signals. The holder can, for example, be designed as a magazine in which the interference filters are positioned one behind the other.

[0086] In an alternative embodiment, several interference filters are arranged, each with a detector cell, so that the generation of the first and second scattered light signals, as well as optionally the further filtered scattered light signals, can take place simultaneously.

[0087] Furthermore, the device can include a closed housing with an inlet transparent to Raman scattered light in order to protect the optical components of the device from dust or damage.

[0088] In another aspect, the invention relates to a system for detecting Raman scattered light, comprising a light source, in particular a laser source, a device for positioning a sample and an optical device, wherein the optical device is configured to direct the light of the light source onto the sample, and wherein the system further comprises one of the devices described above.

[0089] The following describes embodiments of the invention with reference to figures. The figures show: Figure 1 is a schematic representation of an embodiment with a first converging lens; Figure 2 is a schematic representation of the embodiment made of Figure 1from another perspective; Figure 3 a schematic representation of a further embodiment with a swiveling first interference filter; Figure 4 a schematic representation of an embodiment with multiple detectors.

[0090] Figure 1 Figure 1 shows an embodiment of the device 01. The device 01 comprises a first interference filter 20, a detection unit 30, and a first converging lens 22. The detection unit 30 comprises several detector cells 32 arranged side by side in a row. Raman scattered light, comprising an incoming scattered light signal 10, strikes the first converging lens 22. The incoming scattered light signal 10 is composed of the incoming partial beams 11 to 15. The converging lens 22 refracts and focuses the incoming scattered light signal 10. The focus of the first converging lens 22 lies within the first interference filter 20.

[0091] The incoming partial beams 11 to 15 strike the surface of the first interference filter 20 at different angles of incidence α. The central incoming partial beam 13 strikes the first interference filter 20 at an angle of incidence α 13 = 0° (not shown here) and is referred to here as the reference beam. The incoming partial beam 11 strikes the first interference filter 20 at an angle of incidence α11. The incoming partial beam 12 strikes the first interference filter 20 at an angle of incidence α12. The incoming partial beam 14 strikes the first interference filter 20 at an angle of incidence α14. The incoming partial beam 15 strikes the first interference filter 20 at an angle of incidence α15. The angles of incidence α11, α12, α14, and α15 are not equal to the angle of incidence α13 and therefore not equal to 0°. In the following, the angles of incidence α11 to α15 differ from each other.In other words, the incoming scattered light signal propagates convergently to the converging lens 22.

[0092] The incoming partial beams 11, 12, 13, 14, and 15 pass through the first interference filter 20. Since they strike the first interference filter 20 at different angles of incidence α11 to α15, the incoming partial beams 11 to 15 pass through the first interference filter 20 along different paths with optical path lengths L11, L12, L13, L14, and L15 (not shown here). The different optical path lengths L11 to L15 result in a different transmission function being applied to each of the incoming partial beams 11 to 15. If, for example, the first transmission function is applied as a reference to the incoming partial beam 13, the transmission functions applied to the incoming partial beams 11, 12, 14 and 15 are spectrally shifted by the wavelengths Δλ 11 , Δλ 12 , Δλ 14 and Δλ 15 relative to the first transmission function.By applying the transmission functions to the incoming partial beams 11 to 15, the filtered partial beams 41 to 45 are generated.

[0093] The filtered partial beam 41 is generated from partial beam 11 at an angle α11 ≠ α13 relative to the reference beam. The filtered partial beam 42 is generated from partial beam 12 at an angle α12 ≠ α13 relative to the reference beam. Partial beam 43 is generated from partial beam 13 as the reference beam. The filtered partial beam 44 is generated from partial beam 14 at an angle α14 ≠ α13 relative to the reference beam. The filtered partial beam 45 is generated from partial beam 15 at an angle α15 ≠ α13 relative to the reference beam. The filtered partial beams together form the filtered scattered light signal 40, whereby the filtered partial beam 43 is detected as the first scattered light signal and the remaining, partially superimposed, filtered partial beams 41, 42, 44 and 45 are detected as further scattered light signals by the detection unit.

[0094] The filtered partial beams 41 to 45 strike the detection unit 30, which in this embodiment comprises several detector cells 32 arranged side by side. The filtered partial beams 41 to 45 are detected by the detector cells 32. Using chemometric methods, a Raman spectrum of the measured sample can be generated from the filtered scattered light signal 40, in particular from the filtered partial beams 41 to 45, after detection.

[0095] In the embodiment shown here, an optional second converging lens is positioned between the first interference filter 20 and the detection unit 30. The converging lens is a cylindrical converging lens whose focal line lies on the detection unit 30 and which focuses the filtered partial beams 41–45 in a plane perpendicular to the image plane onto the detector cells 32 of the detection unit 30.

[0096] A view of the embodiment rotated by 90° Figure 1 is in Figure 2 The partial beams 11-15 pass through the first converging lens 22 and the first interference filter 20 without being refracted in the image plane shown. The second converging lens 24 focuses the filtered partial beams 41-45 onto the detector cells 32 of the detector unit 30. The individual detector cells 32 are arranged one behind the other in this view.

[0097] Figure 3Figure 1 shows a further schematic representation of an embodiment of the device 01. The device 01 comprises a detection unit 30, which is formed from a single detector cell 32. Furthermore, the device 01 comprises a first interference filter 20. The first interference filter 20 is pivotably mounted, being pivotable in the pivoting direction R. The position of the pivot axis, and thus the specific design of the filter holder and the pivoting mechanism, is not relevant here. A first interference filter 20 pivoted by the angle α is shown with a dashed line.

[0098] When the incoming scattered light signal 10 strikes the surface of the first interference filter 20, the wavelength Δλ of a spectral shift in the transmission function of the first interference filter 20 depends on the angle of incidence α between the propagation direction of the incoming scattered light signal 10 and the normal to the surface of the first interference filter 20. If the first interference filter 20 is pivoted about the pivot axis (here located within the first interference filter 20) in the pivot direction R, the angle of incidence α of the incoming scattered light signal on the first interference filter 20 changes. If the first interference filter is pivoted within the depicted image plane, as indicated here by the dashed line, the angle ε by which the first interference filter is pivoted is equal to the change in the angle of incidence α.This setup allows the application of different transmission functions that are spectrally shifted relative to each other by the wavelength Δλ, depending on the angle ε.

[0099] The first interference filter 20 generates a filtered scattered light signal 40 from the incoming scattered light signal 10 by applying the transmission function, which depends on the angle ε. The filtered scattered light signal 40 is detected by the detector cell 32 of the detection unit 30. In order to apply different transmission functions to the incoming scattered light signal 10, several measurements are carried out in this embodiment with different positions of the first interference filter 20, so that in each measurement the incoming scattered light signal 10 strikes the first interference filter 20 at a different angle of incidence α.

[0100] Figure 4Figure 1 shows a schematic representation of another embodiment of the device 01 and how it is used for the detection of Raman scattered light. The device 01 is placed in the beam path of a laser beam 54, the laser beam 54 being generated by a laser source 52.

[0101] A capillary 56 runs perpendicular to the image plane in the center of the device 01. A mixture containing a sample to be examined flows through the capillary 56, either into or out of the image plane. The wall of the capillary 56, which is transparent to laser light, is in Figure 4 Shown hatched.

[0102] The laser beam 54 passes through the device 01, penetrating the capillary 56. The laser light of the laser beam 54 is scattered, at least partially, in all directions by the sample within the capillary 56, whereby the as-yet-undetected scattered light is propagated in all directions as a (detectable and incoming) scattered light signal 10. A transmission detector 58 is positioned behind the device 01, which detects and / or absorbs the unstratified laser light. The propagation directions of the incoming scattered light signals 10 and the laser beam 54 are indicated here by arrows.

[0103] The device further comprises several detectors 60 arranged in a circle around the capillary 56. The arrangement of the detectors 60 is interrupted in the regions of the laser beam path 54, so that the laser beam 54 is not blocked by the detectors 60 or the detectors 60 detect the unscratched laser light.

[0104] The detectors 60 each comprise at least one detection unit and at least one interference filter. Optionally, the detectors 60 may comprise one or more converging lenses. The detection units may further comprise one or more detection cells.

[0105] The detectors 60 differ from one another in that their interference filters filter different spectral ranges. For example, the interference filters in the detectors 60 can be oriented differently relative to the propagation direction of the incoming scattering signals 10, so that the incoming scattered light signals 10 strike the interference filters at different angles α. Alternatively or additionally, the interference filters can have coatings of different thicknesses. In other words, the detectors 60 differ from one another by different optical path lengths L due to their interference filters.

[0106] The incoming scattered light signals 10 are detected as filtered scattered light signals by the detection units of the detectors 60 after transmission through the interference filters. A Raman spectrum can then be generated from the detected scattered light signals.

[0107] Advantageously, the embodiment of the device according to Figure 4 Several measurements can be carried out in parallel. Reference symbol list

[0108] 01 Device 10 Incoming scattered light signal 11 Incoming partial beam 12 Incoming partial beam 13 Incoming partial beam 14 Incoming partial beam 15 Incoming partial beam 20 First interference filter 22 First converging lens 24 Second converging lens 30 Detection unit 32 Detector cell 40 Filtered scattered light signal 41 Filtered partial beam 42 Filtered partial beam 43 Filtered partial beam 44 Filtered partial beam 45 Filtered partial beam 52 Laser source 54 Laser beam 56 Capillary 58 Transmission detector 60 Detector R = direction of rotation Δλ 1,2,i = wavelength of the spectral shift λ 1,2,i = mean wavelength L 1,2,i = optical path length δλ 1,2,i = spectral band α = angle of incidence ε = angle

Claims

1. Method for detecting Raman scattered light using a Raman device having a light source for generating excitation light by means of which particles in a sample are excited to emit the Raman scattered light, having at least one interference filter and a detection unit (30), wherein the Raman scattered light to be detected comprises an incoming scattered light signal (10), wherein the method comprises the following steps: • generating a first filtered scattered light signal (40) by means of a first interference filter as a function of a first optical path length L1 of the light through the first interference filter, wherein a first transmission function is applied to the incoming scattered light signal (10), wherein the first transmission function is associated with the first optical path length L1 through the first interference filter (20), wherein the first transmission function defines a first spectral band δλ1, wherein the first spectral band δλ1 comprises light with first wavelengths around a first average wavelength λ1, wherein the light of the first wavelengths transmits through the first interference filter (20), • generating a second filtered scattered light signal (40) by means of the first interference filter or by means of a second interference filter as a function of a second optical path length L2 ≠ L1, wherein a second transmission function is applied to the incoming scattered light signal (10), wherein the second transmission function is assigned to the second optical path length L2 ≠ L1 through the first interference filter (20) or through a second interference filter, wherein the second transmission function defines a second spectral band δλ2, wherein the second spectral band δλ2 comprises light with second wavelengths around a second average wavelength λ2, wherein the second average wavelength λ2 for the second transmission function is determined in the same way as the first average wavelength λ1 for the first transmission function, wherein the second average wavelength λ2 is offset relative to the first average wavelength λ1 around a wavelength Δλ2 that is smaller than the width of the second spectral band δλ2 of the second transmission function, and wherein the light of the second wavelengths transmits through the first interference filter (20) or through the second interference filter, • detecting the first and second filtered scattered light signal (40) through the detection unit (30).

2. Method according to claim 1, characterised in that the first average wavelength λ1 is optionally the arithmetic mean, the mode, the median or a weighted average of the first wavelengths of the transmitted light, and the second average wavelength λ1 is accordingly the arithmetic mean, the mode, the median or a weighted average of the second wavelengths of the transmitted light.

3. Method according to one of the preceding claims, characterised in that the method further comprises the following steps: • generating a reference light signal, • generating a first filtered reference light signal by means of the first interference filter as a function of the first optical path length L1 of the light through the first interference filter, wherein the first transmission function is applied to the reference light signal, wherein the first filtered reference light signal is associated with the first filtered scattered light signal (40), • generating a second filtered reference light signal by means of the first interference filter or by means of the second interference filter as a function of the second optical path length L2 ≠ L1, wherein the second transmission function is applied to the reference light signal, wherein the second filtered reference light signal is associated with the second filtered scattered light signal (40), • detecting the first and second filtered reference light signal through the detection unit (30), • generating corrected filtered scattered light signals by subtracting the filtered reference light signals from the respectively associated filtered scattered light signals (40).

4. Method according to one of the preceding claims, characterised in that a Raman spectrum is generated from the detected scattered light signals or from the corrected scattered light signals.

5. Method according to claim 4, characterised in that a multivariate data analysis, in particular chemometrics, is used for generating the Raman spectrum.

6. Method according to one of claims 1 to 5, characterised in that between the generation of the first filtered scattered light signal (40) and the generation of the second filtered scattered light signal (40), the first interference filter (20) is replaced by the second interference filter.

7. Method according to one of claims 1 to 6, characterised in that the incoming scattered light signal (10) is divergently or convergently propagated and the generation of the first filtered scattered light signal (40) takes place at a first angle α1 relative to a reference beam and the generation of the second filtered scattered light signal (40) takes place at a second angle α2 ≠ α1 relative to the reference beam and transmits the light of the first wavelengths and the light of the second wavelengths through the first interference filter (20).

8. Device (01) for detecting Raman scattered light, characterised in that the device is designed to carry out the method according to claim 1, wherein the device comprises: - a light source for generating excitation light • a first interference filter (20), wherein the first interference filter is configured to generate a first filtered scattered light signal (40) as a function of a first optical path length L1 of the light through the first interference filter, wherein a first transmission function is applied to the incoming scattered light signal (10), wherein the first transmission function is a function of the first optical path length L1 through the first interference filter (20), wherein the first transmission function defines a first spectral band δλ1, wherein the first spectral band δλ1 comprises light with first wavelengths around a first average wavelength λ1, wherein the light of the first wavelengths transmits through the first interference filter (20), • a first lens (22) and • a detection unit (30), wherein the detection unit (30) comprises an array of detector cells (32), wherein the array is oriented relative to the first lens (22) such that light through the first lens (22) is imaged onto the array in a convergent or divergent manner, wherein the first interference filter (20) is positioned between the first lens (22) and the detection unit (30), whereby the first interference filter (20) is configured to generate a second filtered scattered light signal (40) as a function of a second optical path length L2 ≠ L1 of the light through the first interference filter, wherein a second transmission function is applied to the incoming scattered light signal (10), wherein the second transmission function is a function of the second optical path length L2 ≠ L1 through the first interference filter (20), wherein the second transmission function defines a second spectral band δλ2, wherein the second spectral band δλ2 comprises light with second wavelengths around a second average wavelength λ2, wherein the second average wavelength λ2 for the second transmission function is determined in the same way as the first average wavelength λ1 for the first transmission function, wherein the second average wavelength λ2 is offset relative to the first average wavelength λ1 around a wavelength Δλ2 which is smaller than the width of the second spectral band δλ2 of the second transmission function, and wherein the light of the second wavelengths transmits through the first interference filter (20).

9. Device (01) according to claim 8, characterised in that the first lens (22) is designed as a cylindrical lens with a first cylinder axis, wherein the device further comprises a second lens, wherein the second lens is designed as a cylindrical convergent lens with a second cylinder axis, wherein the first and second cylinder axes are oriented perpendicular to one another, and wherein the array (32) is positioned along the focal line of the second lens.

10. Device (01) according to claim 9, characterised in that the second lens is positioned between the first interference filter (20) and the array of detector cells (32).

11. System for detecting Raman scattered light, comprising a light source, in particular a laser source, a device for positioning a sample and an optical device, wherein the optical device is designed to direct the light of the light source onto the sample, characterised in that the system further comprises a device according to one of claims 8 to 10.