Device and method for raman spectroscopy

Abstract

The invention relates to a device for Raman spectroscopy, comprising a spectrally tunable laser source (10) which can be modulated using a modulation signal (Smod1); a spectral filter element (20); a detector (30) for measuring the intensity of an excitation radiation (L1) of the laser source (10), said excitation radiation being scattered by a sample (P) and filtered by the spectral filter element (20); means for tuning the laser source (50); and a lock-in device (40) which is designed to filter a Raman signal (R) of the sample (P) out of the measurement signal (I) when using the modulation signal (Smod1) as a reference signal, said Raman signal occurring within the spectral filter width AvFhter of the spectral filter (20) and being modulated by means of the modulation signal (Smod1).

Description

[0001] title

[0002] Apparatus and method for Raman spectroscopy

[0003] Description

[0004] The present invention relates to a device and a method for Raman spectroscopy, in particular to a device and a method for high-resolution Raman spectroscopy with a spectrally tunable monochromatic laser source spectrally modulated by a modulation signal for exciting a sample and a lock-in device for improving a signal-to-noise ratio of a Raman signal of the sample occurring within the spectral filter width of a narrow-band spectral filter element in an associated measurement signal.

[0005] State of the art

[0006] Raman spectroscopy is increasingly being used in industrial process and environmental measurement technology due to its suitability for in-situ and online analysis. However, one disadvantage is that some samples, due to fluorescence and / or their scattering properties, produce spectra whose quality is compromised by a high background, especially when measuring with measurement times of less than 1 s, without complex sample preparation, and / or with small-scale equipment.

[0007] Typically, Raman spectroscopy involves irradiating a sample under investigation with excitation radiation of a fixed, yet largely spectrally selectable, wavelength. The excitation radiation, inelastically scattered by the sample, is then collected from the sample location using a suitable spectroscopy setup and spectrally analyzed. Depending on the specific material properties of the sample, individual Raman lines appear in the recorded spectrum. These lines exhibit a precisely defined spectral separation (usually referred to as the wavenumber separation) from the excitation wavelength of the excitation radiation, which is characteristic of the particular sample under investigation.

[0008] The spectrometers used to record a Raman spectrum must not only have a high spectral resolution but also a sufficiently high light sensitivity to record scattering spectra. Typically, such a spectrometer has a high-resolution grating as the wavelength-selective element and a correspondingly low-noise multi-channel detector, for example a CCD camera cooled electronically or by liquid nitrogen, for detection. Such spectrometers are often expensive and are not suitable for the construction of particularly small, compact and robust spectroscopy arrangements. A further disadvantage of conventional spectroscopy arrangements is their relatively low sensitivity. Due to the narrowband filtering by the wavelength-selective element, only a low intensity can be detected in each wavelength range. Thus, the measured Raman signals have a low signal-to-noise ratio.The SNR can be increased by increasing the intensity of the excitation radiation or extending the respective integration time, but such measures have strict limitations depending on the application. While excessive excitation power can affect or even destroy the sample, particularly short measurement times are required for monitoring chemical processes or detecting hazardous substances.

[0009] Furthermore, EP 3309538 A1 and EP 3660474 A1 disclose spectroscopy arrangements with a tunable monochromatic excitation source, in which a specific Raman signal from a sample is selectively detected via a spectrally narrowband filter element. To improve the SNR, the sample can be specifically excited with excitation radiation from a narrowband wavelength range particularly suitable for the sample at high intensity, without requiring any special adaptation of the spectroscopy arrangement. The single-channel detectors used for signal acquisition enable rapid signal acquisition, significantly increasing detection sensitivity compared to conventional setups.However, for more reliable, accurate and faster Raman spectroscopy of samples even under suboptimal excitation conditions, the SNR of the acquired Raman signals must be further increased for a wider range of applications.

[0010] Disclosure of the invention

[0011] It is therefore an object of the present invention to provide a device and a method for high-resolution Raman spectroscopy, which enables an increase in the SNR compared to the prior art and thus leads to a shortening of the required integration times while simultaneously increasing the sensitivity of the spectroscopy arrangement.

[0012] These objects are achieved according to the invention by the features of patent claims 1, 9, and 10. Expedient embodiments of the invention are contained in the dependent claims. The features listed individually in the patent claims can be combined with one another in a technologically expedient manner and can be supplemented by explanatory facts from the description and / or details from the figures, whereby further embodiments of the invention are shown.A first aspect of the invention relates to a device for Raman spectroscopy, comprising a spectrally tunable laser source, configured to emit a monochromatic first excitation radiation of a first wavelength Ai from a first wavelength range AAi for exciting a sample; a spectral filter element at a filter wavelength vpiiter and with a spectral filter width Avpiiter, wherein the first wavelength range AAi and the spectral filter width Avpiiter relate to disjoint spectral ranges; a detector for measuring the intensity of the excitation radiation scattered by the sample and filtered by the spectral filter element as a measurement signal; a means for tuning the laser source, wherein for tuning the first wavelength Ai of the laser source with a first modulation signal of the frequency f. mOdi (within the associated first wavelength range AAi); and a lock-in device configured to filter out, using the first modulation signal as a reference signal, a Raman signal of the sample occurring within the spectral filter width AvFiiter of the spectral filter element and modulated with the first modulation signal from the measurement signal. Thus, using the first modulation signal as a reference signal, the lock-in device can improve the signal-to-noise ratio of a Raman signal of the sample occurring within the spectral filter width Avpiiter of the spectral filter element and modulated with the first modulation signal in the measurement signal.

[0013] In Raman spectroscopy, the occurrence of at least one specific Raman line upon excitation of a sample with excitation radiation is spectroscopically investigated. In particular, within the scope of the present invention, this refers to a spectroscopic examination of a sample using at least one preselected Raman line. However, it can also be used to examine only for the presence of the corresponding Raman line in an unknown sample.

[0014] Monochromatic excitation radiation is defined as radiation with a narrow spectral width at a specific central wavelength. Such excitation radiation can typically be emitted by diode lasers or laser diodes in single-mode operation. The spectral width should be selected to match the spectral resolution of the measurement system and the spectral width of the Raman line under investigation. Central wavelengths around 785 nm with a full width at half maximum (FWHM) of less than 1 nm are particularly preferred.

[0015] The laser source is spectrally tunable over a first wavelength range AAi, wherein the respective emission wavelength (ie the first wavelength Ai) of the laser source is modulated with a first modulation signal of frequency f mOdi is modulated within the first wavelength range AAi. Tuning is understood as detuning a specific emission wavelength A within an associated wavelength range AA. This can, for example, be a continuous or discontinuous tuning (e.g., a quasi-continuous tuning with at least one spectral jump range) of a specific emission wavelength A within an associated wavelength range AA. Binary tuning between a wavelength A and a detuned wavelength A + ÖA within a wavelength range AA associated with wavelength A is also referred to as switching (“digital” modulation). A typical modulation width is approximately 5 nm to 10 nm. The accessible wavelength range AA can be correspondingly wide. The maximum frequency f mOd, with which the respective emission wavelength of the excitation radiation can be modulated, depends essentially on the modulation properties of the modulator, the laser source, and the possible readout speed of an associated detector. A preferred modulation frequency f mO d is approximately 0.01 Hz, more preferably 0.1 Hz, more preferably 1 Hz, more preferably 10 Hz, more preferably 100 Hz, and even more preferably 1 kHz. Assuming an emission wavelength of 785 nm, a wavelength range AA of 10 nm (modulation width) available for modulation corresponds to a wavenumber range of approximately 160 cm -1 .

[0016] The excitation radiation scattered by the sample is spectrally filtered by a spectral filter element at a specific filter wavelength vpiiter (the symbol v refers to the wavenumber commonly used in Raman spectroscopy, the specification of which is equivalent to the specification of the wavelength). The spectral filter element can preferably be a passive filter element, in particular a dichroic filter, a Bragg filter (e.g., Bragg grating, VBG, FBG), or a Fabry-Perot filter. The use of a diffractive grating, an etalon, or a Mach-Zehnder interferometer is also preferred. Passive means that no active change in the filter properties of the filter element occurs to record a Raman spectrum, in particular that the filter wavelength vpiiter of the spectral filter element is time-invariant.

[0017] Spectral filtering is understood here in particular to mean filtering in which monochromatic radiation with the filter wavelength vpiiter is transmitted by the filter element with maximum intensity, and the spectral ranges adjacent to the filter wavelength vpiiter are suppressed or blocked. A spectral filter element can also be a suitably designed reflective filter element. The filter wavelength vpiiter is the central wavelength of the passband (also referred to as the transmission range or passband) of the filter element. For symmetrical bandpass filters, this results from the spectral position of the center of the bandpass. An alternative determination of the central wavelength of a filter element can also be made via the transmission behavior in the passband.A convenient determination of a central wavelength can be made via the middle of the spectral range if the filter element has a relative transmission of at least 0.9 compared to its maximum transmission in the passband. This definition is particularly suitable for determining the central wavelength of spectral filter elements with non-symmetrical filter range edges. The definition of the width of the passband of the spectral filter element (i.e., its spectral filter width Avpiiter) can also be made via the transmission behavior. In this case, a passband of the spectral filter element can be defined as the contiguous spectral range within which the relative transmission is preferably at least 0.95 compared to a maximum transmission in the passband.Also preferred are contiguous spectral ranges in which the relative transmission is at least 0.7; at least 0.8; at least 0.9; or at least 0.99. A corresponding definition of the blocking range of a filter element can also be made via the transmission properties of the spectral filter element within this blocking range. A spectral filter element can be considered to block a wavelength if, for this wavelength, it has a relative transmission of less than 0.3; less than 0.2; less than 0.1; less than 0.5; or less than 0.01 in relation to the maximum transmission of the spectral filter element in the passband of its filter wavelength vpiiter.

[0018] Preferably, the spectral filter element is designed to be spectrally narrowband. Spectrally narrowband means that the spectral filter width Avpiiter refers to only a limited spectral range in the wavelength or wavenumber range relevant for the device. Particularly preferred passbandwidths (FWHM) are below 10 nm, below 5 nm, below 1 nm, and below 0.1 nm. Depending on the wavelength range, this corresponds to frequency widths in the lower THz range down to the MHz range.

[0019] The first wavelength range AAi and the spectral filter width Avpiiter refer to disjoint spectral ranges. This means that excitation light scattered directly by the sample with the first wavelength Ai from the first wavelength range AAi is filtered by the spectral filter element. The excitation light can thus only be transmitted or reflected by the spectral filter element if a corresponding frequency shift in the excitation light has previously occurred due to inelastic scattering (e.g., due to Stokes or anti-Stokes shift occurring in the sample). Therefore, direct or elastically scattered excitation light (e.g., due to Rayleigh scattering or Mie scattering on the sample) is filtered out by the spectral filter element, preventing a resulting reduction in the SNR.

[0020] The detector is preferably a single-channel detector. This can be either a single single-channel detector or a multi-channel detector that can be individually read as a single-channel detector. This has the advantage of allowing the use of particularly cost-effective, compact, and robust detector devices. The use of a high-resolution, low-noise, and sensitive CCD camera is not required. If a multi-channel detector with individual channels is operated as a single-channel detector, several channels of the multi-channel detector can also be combined into a single single-channel detector.

[0021] The device according to the invention comprises a lock-in device via which, with the first modulation signal as a reference signal, a signal-to-noise ratio of a Raman signal of the sample occurring within the spectral filter width Avpiiter of the spectral filter element and modulated with the first modulation signal in the detected measurement signal can be improved by a corresponding frequency-selective electronic filtering of the Raman signal. The SNR is thus improved metrologically by the Raman signal to be examined also being matched with the first modulation signal of frequency f by an associated means for tuning the laser source. mOdi is modulated so that the lock-in device can be adjusted to the corresponding reference signal. This effectively suppresses additional noise components in the measurement signal caused by ambient and residual scattered light, as well as other factors that interfere with spectroscopy, so that, despite detection via a single-channel detector, the Raman signal can still be selectively captured as the corresponding evaluation signal with low noise. Lock-in techniques and lock-in devices are well known in the art and are generally used to filter out a specific frequency-modulated signal from a measurement signal based on a corresponding reference signal.

[0022] A lock-in device can, in particular, be a lock-in amplifier, in which the filtered signal is further amplified. However, the gain factor can take on any real value; in particular, the gain factor of a lock-in amplifier can also be 0, 1, or -1. A gain factor of 0 means that the signal filtered out by the lock-in amplifier is completely attenuated.

[0023] The main idea of ​​the present invention is therefore to achieve a significant improvement in SNR compared to the prior art through the use of lock-in techniques, thereby enabling shorter integration times and increased sensitivity compared to comparable spectroscopy arrangements. The Raman signal can be detected and evaluated largely noise-free via a single single-channel detector. The device according to the invention can thus be implemented very compactly, simply, and cost-effectively without any mechanically moving components. Due to the simple beam path, the excitation power can be varied without additional measures, depending on the samples to be examined.Preferably, the spectrally tunable laser source is configured to emit a monochromatic second excitation radiation of a second wavelength A2 from a second wavelength range AA2 for exciting the sample, wherein the second wavelength A2 of the laser source is selected by the means for tuning the laser source for tuning with a second modulation signal of frequency f. mOd2 (within the associated second wavelength range AA2), and wherein the lock-in device is configured to use the second modulation signal as a reference signal to filter out a Raman signal of the sample occurring within the spectral filter width AvFiiter of the spectral filter element and modulated with the second modulation signal from the measurement signal. The lock-in device can thus use the second modulation signal to improve the signal-to-noise ratio of a Raman signal of the sample occurring within the spectral filter width Avpiiter of the spectral filter element and modulated with the second modulation signal in the measurement signal.

[0024] Preferably, the spectrally tunable laser source is configured to emit at least one monochromatic further excitation radiation of a further wavelength A3 from a further wavelength range AA3 for exciting the sample, wherein the at least one further wavelength A3 of the laser source is modulated by the means for tuning the laser source for tuning with at least one further modulation signal of the frequency fmod3 (within the at least one associated further wavelength range AA3), and wherein the lock-in device is configured to use the at least one further modulation signal as a reference signal to filter out from the measurement signal a Raman signal of the sample occurring within the spectral filter width AvFiiter of the spectral filter element and modulated with the at least one further modulation signal.The lock-in device can thus use the at least one further modulation signal to improve a signal-to-noise ratio of a Raman signal of the sample in the measurement signal that occurs within the spectral filter width Avpiiter of the spectral filter element and is modulated with the at least one further modulation signal.

[0025] The different excitation radiations are preferably emitted simultaneously, but a time-delayed emission of the individual excitation radiations can also occur. The excitation radiations can be provided, for example, by using multiple laser diodes emitting at different emission wavelengths within the laser source. Furthermore, different excitation radiations at different excitation wavelengths can also be generated by a single laser diode with a variable emission wavelength or by a correspondingly spectrally tunable diode laser. In addition to the discrete provision of individual emission wavelengths spaced apart from one another (for example, at a distance of 10 nm or 20 nm), a corresponding selection of individual emission wavelengths from a wide available spectral range of the laser source can also be made.

[0026] Preferred is the use of laser diodes in which different modes can be excited, so that, for example, larger, at least discrete, wavelength ranges can be achieved at intervals of several tens of nm. Also preferred are directly frequency-modulated laser diodes, in which a wavelength change can be adjusted using intrinsic diode parameters, for example, via temperature or current.

[0027] Preferably, so-called dual- or multi-wavelength lasers can be used, for example a Y-branched dual-wavelength DBR diode laser (e.g., Maiwald et al., "Dual-Wavelength Y-Branch Distributed Bragg Reflector Diode Laser at 785 Nanometers for Shifted Excitation Raman Difference Spectroscopy," Appl. Spectrosc. 69, 1144-1151 (2015)). Furthermore, it is preferred that the laser source be a correspondingly narrowband diode laser that is spectrally tunable over a wide range. This can be, for example, an ECDL system or a spectrally tunable diode-pumped solid-state laser. Furthermore, appropriately tunable dye or fiber lasers are particularly suitable.

[0028] Preferably, the wavelength ranges assigned to the at least two excitation radiations, i.e., all wavelength ranges assigned to the respective excitation radiations, are disjoint from one another. This means that the respective excitation radiations exhibit no mutual spectral overlap, even with the modulation, and the adjustable excitation radiations can thus be clearly assigned to individual wavelength ranges. In conjunction with a spectral filter element with a predetermined filter wavelength vpiiter, this means that the wavenumber ranges spectroscopically resolved by the modulation are also disjoint from one another.

[0029] Preferably, for a predetermined sample to be spectroscoped, at least two of the excitation radiations are selected such that different Raman signals occur within the spectral filter width Avpiiter of the spectral filter element. This embodiment can be applied in particular to samples that have a different composition (e.g., different molecules, chemical groups, etc.). In these cases, the different excitation radiations can be set up to measure different Raman lines of the individual components of the composition. From the ratio of the intensity of the measured Raman signals, for example, a mixing ratio of the sample composition can then be determined. However, the different Raman signals occurring within the spectral filter width Avpiiter of the spectral filter element can also be formed by different Raman lines of a single sample component.By evaluating several Raman lines of a single sample component, the reliability of a sample determination can be increased.

[0030] Preferably, at least two of the excitation beams are modulated by the respective modulation signals with a uniform frequency and phase position. Such an embodiment has the advantage that, for example, if two of the two excitation beams are filtered by the spectral filter element, Raman signals from two different Raman lines (i.e., belonging to different wavenumber spacings) are detected by the lock-in device at the common modulation frequency as a sum signal. The signal level measured by the lock-in device then represents a superposition of two different Raman signals from a single sample component. The SNR can thus be significantly increased by the parallel measurement of multiple Raman signals compared to the detection of only one specific Raman signal.

[0031] In particular, a correspondingly high maximum signal level (e.g., above a certain threshold) can be used to infer the simultaneous occurrence of different Raman lines of a specific sample component (or a specific sample material). Such sum Raman spectroscopy allows for high sensitivity and reliability of the spectroscopy arrangement according to the invention, particularly for specific individual sample determination, with minimal integration times.

[0032] Preferably, the Raman signals that occur are based on both Stokes shift and anti-Stokes shift. The term "occurring Raman signal" refers to the actual presence of a Raman signal from the sample in the measurement signal that occurs within the spectral filter width Avpiiter of the spectral filter element and is modulated with the first modulation signal. The term thus assumes a signed spectral spacing (wavenumber spacing) between the wavelength of the excitation radiation modulated in the corresponding wavelength range and the filter wavelength vpiiter of the spectral filter element used.

[0033] During the Stokes shift, energy is transferred from photons of the excitation radiation to the scattering sample. After the scattering process, the molecules or similar particles in the sample are at a higher energy level than before, and the energy and frequency of the scattered photons are lower than those of the exciting photons. During the anti-Stokes shift, energy is transferred from the scattering sample to the photons of the excitation radiation. After the excitation process, the molecules or similar particles in the sample are at a lower energy level than before, and the scattered photons have a higher energy and a higher frequency than the exciting photons. In the preferred embodiment, both Stokes and anti-Stokes shifts are thus recorded as Raman signals.The ratio of the intensities of the respective Stokes and anti-Stokes shifts of the Raman signals can be used, in particular, to perform a temperature measurement on the sample. For a spatially extended sample comprising different components, a highly location- and material-specific temperature measurement of a single component of the sample's composition can thus be performed. A state-specific temperature measurement is also possible, for example, of a single rotational and vibrational state of a molecule in thermal nonequilibrium.

[0034] A device according to the invention preferably comprises at least one display device for displaying the measurement signal filtered by the lock-in device. A display device can preferably be a simple analog or digital pointer element or a binary display element (e.g., a light signal when a specific threshold value is exceeded). Furthermore, a display device can preferably be an analog or digital yt recorder for displaying the Raman signal as a time profile in a corresponding diagram. Different Raman signals are preferably displayed on a respective display device.

[0035] The above-mentioned embodiments can advantageously be combined in whole or in part.

[0036] A further aspect of the invention relates to a method for Raman spectroscopy using a device according to the invention, wherein the spectral spacing between the wavelength (Ai, A2, A3) of at least one excitation radiation (Li, L2, L3) and a filter wavelength vpiiter of the spectral filter element corresponds to the wavenumber spacing Av of a Raman signal of a predetermined sample. In Raman spectroscopy, the occurrence of at least one specific Raman line upon excitation of a sample with excitation radiation is spectroscopically investigated.Therefore, in order to perform Raman spectroscopy of a predetermined sample using a device according to the invention, at least one spectral separation between the wavelength A of the monochromatic excitation radiation modulated in the corresponding wavelength range AA and a filter wavelength vpiiter of the spectral filter element must be specified to the wavenumber separation Av of a Raman signal of a predetermined sample. Therefore, the absence of a corresponding Raman signal during spectroscopy of a specific sample can at least be used to conclude that the sample specified for a corresponding test is absent.

[0037] A further aspect of the invention relates to the use of a method according to the invention for measuring a temperature of a sample or a mixing ratio of a sample. The temperature of a sample can be measured, in particular, via the ratio of the intensities of the respective Stokes and anti-Stokes shifts of the Raman signals. The mixing ratio of a sample having a different composition (e.g., different molecules, chemical groups, etc.) can be measured via the ratio of the intensities of individual Raman lines of the different components. The method according to the invention can accordingly also be used to record the temporal progression of a mixing ratio (e.g., for in-situ monitoring of chemical reactions within reactors).The mixing ratio of a sample can be used, for example, to determine the degree of saturation of a solution or to determine the degree of polymerization in a polymerizable monomer solution.

[0038] Furthermore, further preferred embodiments of the method according to the invention result directly from the features mentioned in the description of the device according to the invention.

[0039] Further preferred embodiments of the invention result from the features mentioned in the respective subclaims.

[0040] The various embodiments of the invention mentioned in this application can be advantageously combined with one another, unless otherwise stated in the individual case.

[0041] Brief description of the drawings

[0042] The invention and the technical environment are explained in more detail below with reference to the accompanying figures. It should be noted that the invention is not intended to be limited by the exemplary embodiments cited. In particular, unless explicitly stated otherwise, it is also possible to extract partial aspects of the facts explained in the figures and combine them with other components and findings from the present description. They show:

[0043] Fig. 1 is a schematic representation of a first embodiment of an apparatus for Raman spectroscopy according to the invention with associated spectra;

[0044] Fig. 2 is a schematic representation of a second embodiment of an apparatus for Raman spectroscopy according to the invention with associated spectra;

[0045] Fig. 3 is a schematic representation of a third embodiment of a device according to the invention for Raman spectroscopy with associated spectra; and

[0046] Fig. 4 is an exemplary representation of the spectra of the third embodiment of a device according to the invention according to Fig. 3. Detailed description of the drawings

[0047] Figure 1 shows a schematic representation of a first embodiment of an apparatus according to the invention for Raman spectroscopy with associated spectra. The apparatus shown under a) comprises a spectrally tunable laser source 10, configured to emit a first monochromatic excitation radiation Li of a first wavelength Ai from a first wavelength range AAi for exciting a sample P; a spectral filter element 20 at a filter wavelength vpiiter and with a spectral filter width AvFiiter, wherein the first wavelength range AAi and the spectral filter width AvFiiter relate to disjoint spectral ranges; a detector 30 for measuring the intensity of the excitation radiation Li' scattered by the sample P (i.e., the first excitation radiation L scattered by the sample) and filtered by the spectral filter element 20 (i.e.,the excitation radiation Li) scattered by the sample and filtered by the spectral filter element as the measurement signal I; a means for tuning the laser source 50, wherein for tuning the first wavelength Ai of the laser source 10 is modulated with a first modulation signal Smodi of the frequency fmodi (within the associated first wavelength range AAi); and a lock-in device 40, configured to use the first modulation signal Smodi as a reference signal to filter out a Raman signal R of the sample P, which occurs within the spectral filter width AvFiiter of the spectral filter element 20 and is modulated with the first modulation signal Smodi, in the measurement signal I. The device shown further comprises a display device 60 for displaying the Raman signal R generated by the lock-in device 40 at a modulation frequency (f. mOdi) filtered measurement signal I. By filtering out, the signal-to-noise ratio of the Raman signal R of the sample P modulated with the first modulation signal Smodi in the measurement signal I can be improved.

[0048] The spectra shown under b) and c) illustrate the spectral relationships during modulation. In the spectra, the intensities of both the excitation radiation (vi_i) and the individual Raman lines v1 i_i - V4LI (the symbol v refers to the wavenumber commonly used in Raman spectroscopy, which corresponds to the wavelength and was chosen for clarity) of a specific sample are plotted against the respective wavenumber spacing Av relative to the excitation radiation (vi_i). Furthermore, the filter range of a spectral filter element 20 is shown at a filter wavelength vpiiter and with a spectral filter width AvFiiter. The wavenumber spacing between an output wavelength A of the excitation radiation (vi_i) and the filter wavelength vpiiter of the spectral filter element 20 is 1500 cm in this example. -1. Figure b) shows the conditions for excitation radiation (vi_i) of a specific output wavelength A without a modulation shift. Figure c) shows the changed conditions for excitation radiation (VLI+ÖVI) shifted by a corresponding modulation of the excitation radiation VLI at the output wavelength A. The entire Raman spectrum is also spectrally shifted by the same amount övi, so that in the example shown, the fourth Raman line V4LI+ÖVI is shifted outside the spectral filter width ÄvFiiter of the spectral filter element 20. The Raman line V4LI can thus be detected by simply modulating the excitation radiation between VLI and V4LI+ÖVI using the spectral filter element 20.

[0049] Figure 2 shows a schematic representation of a second embodiment of an apparatus according to the invention for Raman spectroscopy with associated spectra. The apparatus shown under a) corresponds essentially to the embodiment shown in Fig. 1 a). The reference numerals and their respective assignment to individual features of the apparatus therefore apply accordingly. In contrast to the embodiment shown in Fig. 1 a), however, the spectrally tunable laser source 10 is configured to emit a second monochromatic excitation radiation L2 of a second wavelength A2 from a second wavelength range AA2 for exciting the sample P, wherein the second wavelength A2 of the laser source 10 is selected by the means for tuning the laser source 50 for tuning with a second modulation signal S mO d2 of the frequency f mOd2 (within the associated second wavelength range AA2), and the lock-in device 40 is configured to receive the second modulation signal S mO d2 as a reference signal, a signal occurring within the spectral filter width ÄvFiiter of the spectral filter element 20, with the second modulation signal S mO d2 modulated Raman signal R of the sample P from the measurement signal I. The device shown further comprises two display devices 60, 60' for displaying the Raman signal R of the sample P obtained with the lock-in device 40 at two modulation frequencies (f mO di , fmod2) filtered measurement signal I.

[0050] The spectra shown under b) and c) show the spectral relationships during the modulation of the individual excitation radiations according to the conditions shown in Fig. 1 b) and c). The wavenumber separation between the first excitation radiation VLI and the second excitation radiation VL2 is 500 cm in this example.-1 . Thus, as shown under b), the two Raman lines V4LI and V3L2 can be detected by appropriate modulation of the respective excitation radiations (VLI, vi_2) at different modulation frequencies by means of spectral tuning (e.g., by detuning, tuning through, or switching) with the aid of the spectral filter element 20.

[0051] Figure 3 shows a schematic representation of a third embodiment of an apparatus according to the invention for Raman spectroscopy with associated spectra. The apparatus shown under a) essentially corresponds to the embodiment shown in Fig. 2 a). The reference numerals and their respective assignment to individual features of the apparatus therefore apply accordingly. In contrast to the embodiment shown in Fig. 2 a), however, the spectrally tunable laser source 10 is configured to emit at least one further monochromatic excitation radiation L3 of a further wavelength A3 from a further wavelength range AA3 for exciting the sample P, wherein the at least one further wavelength A3 of the laser source 10 is selected by the means for tuning the laser source 50 for tuning with at least one further modulation signal S mO d3 of the frequency f mOd3 (within the at least one associated further wavelength range AA3), and the lock-in device 40 is configured to work with the at least one further modulation signal S mO d3 as a reference signal, a signal occurring within the spectral filter width AvFiiter of the spectral filter element 20, with which at least one further modulation signal S mO d3 modulated Raman signal R of the sample P from the measurement signal I. The device shown further comprises at least two display devices 60, 60' for displaying the Raman signal R of the sample P modulated by the lock-in device 40 at three modulation frequencies (f mO di, fmod2, fmods) filtered measurement signal I.

[0052] Figure 4 shows an exemplary representation of the spectra of the third embodiment of a device according to the invention according to Fig. 3. In this embodiment, for a predetermined sample P to be spectroscoped, four monochromatic excitation radiations (VLI, VL2, VL3, VL4) are selected such that different Raman lines (v11_4, V2L3, V3L2, V4LI) occur within the spectral filter width AvFiiter of the spectral filter element 20. According to the representations in Fig. 1 b) and c), in Fig.4, under a), the spectrally superimposed Raman lines (v11_4, V2L3, V3L2, V4LI) for the unmodulated excitation radiation at specific output wavelengths A lying within the spectral filter width AvFiiter of the spectral filter element 20 are shown, while under b), the changed conditions are shown for a Raman line v1i_4+öv4, V2L3+ÖV3, V3L2+ÖV2, V4LI+ÖVI shifted by a corresponding modulation of the excitation radiation (VLI+ÖVI, vi_2+öv2, VLS+ÖVS, VL4+ÖV3) at the output wavelength A. The monochromatic excitation radiation (VLI, VL2, VL3, VL4) can be modulated using corresponding modulation signals Smodi, S. mO d2, S mO d3, S mO d4 are modulated independently of each other and the corresponding Raman signals are displayed on a respective display device 60, 60'. List of reference symbols

[0053] 10 spectrally tunable laser source

[0054] 20 spectral filter element

[0055] 30 detector

[0056] 40 Lock-in device

[0057] 50 means for tuning the laser source

[0058] 60, 60' display devices

[0059] Li first excitation radiation

[0060] Li' excitation radiation scattered by the sample

[0061] Li" excitation radiation scattered by the sample and filtered by the spectral filter element

[0062] L2 second excitation radiation

[0063] L2' second excitation radiation scattered by the sample

[0064] L2" second excitation radiation scattered by the sample and filtered by the spectral filter element

[0065] L3 further excitation radiation

[0066] L3' further excitation radiation scattered by the sample

[0067] L3" further excitation radiation scattered by the sample and filtered by the spectral filter element

[0068] P Sample

[0069] I Measurement signal

[0070] R Raman signal

[0071] Smodi first modulation signal

[0072] Smod2 second modulation signal

[0073] Smod3 third modulation signal

Claims

Patent claims 1 . Apparatus for Raman spectroscopy, comprising: a spectrally tunable laser source (10) configured to emit a monochromatic first excitation radiation (Li) of a first wavelength Ai from a first wavelength range AAi for exciting a sample (P); a spectral filter element (20) at a filter wavelength vpiiter and with a spectral filter width Avpiiter, wherein the first wavelength range AAi and the spectral filter width Avpiiter relate to disjoint spectral ranges; a detector (30) for measuring the intensity of the excitation radiation (Li) scattered by the sample (P) and filtered by the spectral filter element (20) as a measurement signal (I); a means for tuning the laser source (50), wherein the first wavelength Ai of the laser source (10) is modulated with a first modulation signal (Smodi) of frequency f mOdi; and a lock-in device (40) configured to use the first modulation signal (Smodi) as a reference signal to generate a filter width A V Fit er of the spectral filter element (20) and modulated with the first modulation signal (Smodi) to filter out from the measurement signal (I).

2. Device according to claim 1, wherein the spectrally tunable laser source (10) is configured to emit a monochromatic second excitation radiation (L2) of a second wavelength A2 from a second wavelength range AA2 for exciting the sample (P), the second wavelength A2 of the laser source (10) is tuned by the means for tuning the laser source (50) with a second modulation signal (S mO d2) the frequency f mO d2, and the lock-in device (40) is arranged to be connected to the second modulation signal (S mOd2) as a reference signal within the spectral filter width A V Fit er of the spectral filter element (20) occurring with the second modulation signal (S mO d2) filtering out the modulated Raman signal (R) of the sample (P) from the measurement signal (I).

3. Device according to claim 2, wherein the spectrally tunable laser source (10) is configured to emit at least one further monochromatic excitation radiation (L3) of a further wavelength A3 from a further wavelength range AA3 for exciting the sample (P), which at least one further wavelength A3 of the laser source (10) is tuned by the means for tuning the laser source (50) with at least one further modulation signal (S mO d3) the frequency f mO d3 is modulated, and the Lock-in device (40) is designed to be connected to the at least one further modulation signal (S mOd3) as a reference signal, a signal occurring within the spectral filter width ÄvFiiter of the spectral filter element (20) with which at least one further modulation signal (S mO d3) filtering out the modulated Raman signal (R) of the sample (P) from the measurement signal (I). Device according to claim 2 or 3, wherein the wavelength ranges (AA1, AA2, AA3) assigned to the at least two excitation radiations (Li, L2, L3) are disjoint from one another. Device according to one of claims 2 to 4, wherein for a predetermined sample (P) to be spectroscoped, at least two of the excitation radiations (Li, L2, L3) are selected such that different Raman signals (R) occur within the spectral filter width Avpiiter of the spectral filter element (20). Device according to one of claims 2 to 5, wherein at least two of the excitation radiations (Li, L2, L3) are modulated via the respective modulation signals (Smodi, S mOd2, Smods) are modulated with a uniform frequency and phase position. Device according to one of claims 2 to 6, wherein the occurring Raman signals (R) are based on both Stokes shift and anti-Stokes shift. Device according to one of the preceding claims, further comprising at least one display device (60, 60') for displaying the measurement signal (I) filtered with the lock-in device (40). Method for Raman spectroscopy using a device according to one of the preceding claims, wherein the spectral spacing between the wavelength (Ai, A2, A3) of at least one excitation radiation (Li, L2, L3) and a filter wavelength vpiiter of the spectral filter element (20) corresponds to the wavenumber spacing Av of a Raman signal (R) of a predetermined sample (P). Use of the method according to claim 9 for measuring a temperature of a sample (P) or a mixing ratio of a sample (P).

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