DE102022124375B3 - Device and method for Raman spectroscopy - Google Patents

Abstract

The present invention relates to a device and a method for Raman spectroscopy, in particular a device and a method for high-resolution Raman spectroscopy with a tunable monochromatic laser source that is 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 and modulated with the modulation signal in an associated measurement signal. A device according to the invention for Raman spectroscopy comprises a spectrally tunable, with a modulation signal (Smod1) modulable laser source (10); a spectral filter element (20); a detector (30) for measuring the intensity of an excitation radiation (L1) of the laser source (10) scattered by a sample (P) and filtered by the spectral filter element (20); a means for tuning the laser source (50); and a lock-in device (40), designed to use the modulation signal (Smod1) as a reference signal to generate a Raman signal (R) of the sample that occurs within the spectral filter width ΔvFilter of the spectral filter element (20) and is modulated with the modulation signal (Smod1). (P) from the measurement signal (I).

Description

The present invention relates to a device and a method for Raman spectroscopy, in particular a device and a method for high-resolution Raman spectroscopy with a spectrally modulated, spectrally tunable monochromatic laser source for exciting a sample and a lock-in device for improvement 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.

State of the art

Raman spectroscopy is increasingly being used in industrial process and environmental measurement technology due to its suitability for in-situ and online analysis. A disadvantage here, however, is that in practice some samples, due to fluorescence and/or due to their scattering properties, deliver spectra whose quality is impaired by a high background, especially when with measurement times of less than 1 s, without complex sample preparation and/or with small-sized apparatus is being worked on.

Typically, for Raman spectroscopy, excitation radiation of a fixed, but spectrally largely freely selectable excitation wavelength is irradiated onto a sample to be examined. The excitation radiation scattered inelastically by the sample is then collected from the sample location using a suitable spectroscopy arrangement and examined spectrally. Depending on the specific material properties of the sample, individual Raman lines appear in the recorded spectrum, which have a precisely determined spectral distance that is characteristic of the sample being examined (this is usually referred to as the wavenumber distance) from the excitation wavelength of the excitation radiation.

The spectrometers used to record a Raman spectrum must not only have a high spectral resolution but also have a sufficiently high light sensitivity to record scattering spectra. Typically, such a spectrometer has a high-resolution grating as a wavelength-selective element and a correspondingly low-noise multi-channel detector, for example a CCD camera cooled electronically or using liquid nitrogen, for detection. Corresponding spectrometers are often expensive and are not suitable for setting up particularly small, compact and robust spectroscopy arrangements.

Another disadvantage of conventional spectroscopy arrangements is their relatively low sensitivity, since due to the narrow-band filtering by the wavelength-selective element only a low intensity can be recorded in each wavelength range recorded with it and thus the measured Raman signals have a low signal-to-noise ratio ( English “signal-to-noise ratio”, SNR). Although the SNR can be increased by increasing the intensity of the excitation radiation or extending the respective integration time, such measures have strict limits depending on the application. While excessive excitation power can influence or even destroy the sample, particularly short measurement times are required, especially for monitoring chemical processes or detecting hazardous substances.

Still are out EP 3 309 538 A1 and EP 3 660 474 A1 Spectroscopy arrangements with a tunable monochromatic excitation source are known, in which a specific Raman signal of a sample is selectively detected via a spectrally narrow-band filter element. In order to improve the SNR, a specific excitation of the sample can be carried out with excitation radiation from a narrow-band wavelength range that is particularly suitable for the sample with high intensity, without the spectroscopy arrangement having to be particularly adapted. The single-channel detectors used for signal detection enable the signal to be detected quickly, whereby the detection sensitivity can be significantly increased compared to conventional setups. However, for more reliable, accurate and faster Raman spectroscopy of samples even under non-optimal excitation conditions, the SNR of the acquired Raman signals needs to be further increased for a wider range of applications.

Out of DE 10 2016 111 747 A1 are a method and a device for Raman spectroscopy, in which a directly frequency-modulated and narrow-band tunable diode laser is tuned to excite a sample over a wide spectral range of excitation wavelengths and the Raman signal of the sample is tuned via at least one narrow-band passive filter element, which is for at least one excitation wavelength that is Raman-shifted by the sample is transparent, is filtered so that a Raman signal can be recorded for at least one excitation wavelength via a single-channel detector.

US 2016 / 0 123 806 A1 relates to a technique for distinguishing parasitic interference in Raman spectroscopy, the technique described there enabling a simultaneous differentiation of ambient light and scattered laser light using components already used in typical Raman detection systems and which does not require additional optical intensity modulators.

In WO 99/63 328 A1 discloses a Raman spectroscopy apparatus comprising: (a) at least one light source capable of irradiating a sample with substantially monochromatic light; (b) a filter module for filtering light scattered by the sample, the filter module comprising at least one bandpass filter, the at least one bandpass filter capable of passing two different spectral windows; and (c) at least one detector.

Disclosure of the invention

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.

These tasks are achieved according to the invention by the features of patent claims 1, 8 and 9. Appropriate 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 sensible manner and can be supplemented by explanatory facts from the description and/or details from the figures, with further embodiment variants of the invention being shown.

A first aspect of the invention relates to a device for Raman spectroscopy, comprising a spectrally tunable laser source, set up to emit a monochromatic first excitation radiation of a first wavelength λ ₁ from a first wavelength range Δλ ₁ for exciting a sample; a spectral filter element at a filter wavelength v _filter and with a spectral filter width Δv _filter , the first wavelength range Δλ ₁ and the spectral filter width Δv _filter relating 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 λ ₁ of the laser source is modulated with a first modulation signal of frequency f _mod1 (within the associated first wavelength range Δλ ₁ ); and a lock-in device, designed to use the first 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 Δ _vFilter of the spectral filter element and modulated with the first modulation signal. The lock-in device can thus use the first modulation signal as a reference signal to improve a signal-to-noise ratio of a Raman signal of the sample occurring within the spectral filter width Δv _filter of the spectral filter element and modulated with the first modulation signal in the measurement signal.

In Raman spectroscopy, the appearance of at least one specific Raman line when a sample is excited with an excitation radiation is examined spectroscopically. In particular, in the context of the present invention, this is understood to mean a spectroscopic examination of a sample with at least one preselected Raman line of the sample. However, it is also possible to simply examine the presence of the corresponding Raman line in an unknown sample.

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

The laser source is spectrally tunable over a first wavelength range Δλ ₁ , with the respective emission wavelength (ie the first wavelength λ ₁ ) of the laser source being modulated with a first modulation signal of frequency f _mod1 within the first wavelength range Δλ ₁ . Tuning is understood to mean detuning a specific emission wavelength λ within an associated wavelength range Δλ. 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 λ within an associated wavelength range Δλ. A binary tuning between a wavelength λ and a detuned wavelength λ + δλ within a wavelength range Δλ associated with the wavelength λ is also referred to as switching (“digital” modulation). A typical modulation width is about 5 nm to 10 nm. The accessible wavelength ranges Δλ can be appropriately broad. The maximum frequency f _mod1 with which the respective emission wavelength of the excitation radiation can be modulated essentially depends on the modulation properties of the modulator, the laser source and the possible readout speed of an associated detector. A preferred modulation frequency f _mod is about 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 Δλ available for modulation of 10 nm (modulation width) corresponds to a wave number range of approximately 160 cm ^-1 .

The excitation radiation scattered by the sample is spectrally filtered using a spectral filter element at a specific filter wavelength v _filter (the symbol v refers to the wave number commonly used in the field of 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 there is no active change in the filter properties of the filter element in order to record a Raman spectrum, in particular that the filter wavelength v _filter of the spectral filter element is time-invariant.

Spectral filtering is here understood to mean, in particular, filtering in which monochromatic radiation with the filter wavelength v _filter is transmitted by the filter element with a maximum intensity and the spectral ranges adjacent to the filter wavelength v _filter are suppressed or blocked. A spectral filter element can also be a correspondingly designed reflective filter element. The filter wavelength v _filter is the central wavelength of the pass band (also referred to as the transmission band or pass band) of the filter element. For symmetrical bandpasses, 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 carried out via the transmission behavior in the pass band.

An appropriate determination of a central wavelength can be made over the middle of the spectral range by the filter element having a relative transmission of at least 0.9 compared to its maximum transmission in the pass band. This definition is particularly suitable for determining the central wavelength of spectral filter elements with non-symmetrical filter area edges. The definition of the width of the passband of the spectral filter element (ie its spectral filter width Δv _filter ) can also be done via the transmission behavior. A passband of the spectral filter element can be defined as the coherent spectral range within which the relative transmission is preferably at least 0.95 compared to a maximum transmission in the passband. 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 area of a filter element can also be done via the transmission properties of the spectral filter element within this blocking area. A spectral filter element can be viewed as blocking a wavelength if it has a relative transmission of less than 0.3 for this wavelength; 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 v _filter .

The spectral filter element is preferably designed to have a spectrally narrow band. Spectrally narrowband means that the spectral filter width Δv _filter refers to only a limited spectral range in the wavelength or wavenumber range relevant to the device. Particularly preferred passband widths (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.

The first wavelength range Δλ ₁ and the spectral filter width Δv _filter relate to disjoint spectral ranges. This means that excitation light with the first wavelength λ ₁ scattered directly by the sample is filtered from the first wavelength range Δλ ₁ by the spectral filter element. The excitation light can therefore only be transmitted or reflected through the spectral filter element if a corresponding frequency shift in the excitation light has previously taken place due to inelastic scattering (e.g. by means of a 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, which means that no resulting reduction in the SNR can occur.

The detector is a single-channel detector. This can be either a single single-channel detector or a multi-channel detector that can be read individually as a corresponding single-channel detector. This has the advantage that particularly inexpensive, compact and robust detector devices can be used. The use of a high-resolution, low-noise and sensitive CCD camera is not necessary. 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.

The device according to the invention comprises a lock-in device via the Raman signal of the sample detected, which occurs within the spectral filter width Δv _filter of the spectral filter element and is modulated with the first modulation signal as a reference signal Measurement signal can be improved by appropriate frequency-selective electronic filtering out of the Raman signal. The SNR is thus improved in terms of measurement technology in that the Raman signal to be examined is also modulated with the first modulation signal of frequency f _mod1 by an associated means for tuning the laser source, so that the lock-in device can be set to the corresponding reference signal . As a result, additional noise components occurring in the measurement signal due to ambient and residual scattered light as well as other factors that disrupt spectroscopy can be effectively suppressed, so that despite detection via a single-channel detector, the Raman signal can still be selectively recorded as a corresponding evaluation signal with low noise.

Lock-in techniques and lock-in devices are well known in the prior art and are generally used to filter out a specific frequency-modulated signal from a measurement signal based on a corresponding reference signal.

A lock-in device can in particular be a lock-in amplifier in which the filtered out signal is additionally amplified. However, the gain factor can assume any real values; in particular, the gain factor of a lock-in amplifier can also be 0, 1 or -1. A gain of 0 means that the signal filtered out by the lock-in amplifier is completely attenuated.

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 recorded and evaluated largely noise-free using a single single-channel detector. The device according to the invention can therefore be implemented in a very compact, simple and cost-effective manner without any mechanically movable components. Due to the simple beam path, the excitation power can be varied without additional measures, depending on the samples to be examined.

The spectrally tunable laser source is set up to emit a monochromatic second excitation radiation of a second wavelength λ ₂ from a second wavelength range Δλ ₂ for exciting the sample, the second wavelength λ ₂ of the laser source being controlled by the means for tuning the laser source for tuning with a second modulation signal of the frequency f _mod2 (within the associated second wavelength range Δλ ₂ ) is modulated, and wherein the lock-in device is set up to, with the second modulation signal as a reference signal, a Raman occurring within the spectral filter width Δ _vFilter of the spectral filter element and modulated with the second modulation signal -Filter out the signal of the sample from the measurement signal. The lock-in device can thus use the second modulation signal to improve a signal-to-noise ratio of a Raman signal of the sample occurring within the spectral filter width Δv _filter of the spectral filter element and modulated with the second modulation signal in the measurement signal.

Preferably, the spectrally tunable laser source is set up to emit at least one monochromatic further excitation radiation of a further wavelength λ ₃ from a further wavelength range Δλ ₃ for exciting the sample, the at least one further wavelength λ ₃ of the laser source being controlled by the means for tuning the laser source for tuning with at least a further modulation signal of the frequency f _mod3 (within the at least one associated further wavelength range Δλ ₃ ), and wherein the lock-in device is set up to use the at least one further modulation signal as a reference signal within the spectral filter width Δ _vFilter of the spectral Filter element occurring, modulated with the at least one further modulation signal Raman signal of the sample from the measurement signal. The lock-in device can thus use the at least one further modulation signal to achieve a signal-to-noise ratio of a _filter of the spectral filter within the spectral filter width Δv terelements occurring, with the at least one further modulation signal modulated Raman signal of the sample in the measurement signal.

According to the invention, the different excitation radiations are emitted at the same time, but the individual excitation radiations can also be emitted with a time delay. The excitation radiation can be provided, for example, by using multiple laser diodes within the laser source that emit at different emission wavelengths. Furthermore, different excitation radiations at different excitation wavelengths can also be generated by a single laser diode whose emission wavelength can be changed 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 broad available spectral range of the laser source can also take place.

The use of laser diodes in which different modes can be excited is preferred, so that, for example, larger wavelength ranges that can be achieved at least discretely can be achieved at a distance of several 10 nm. Directly frequency-modulated laser diodes are also preferred, in which a wavelength change can be adjusted using intrinsic diode parameters, for example via a temperature or a current.

Preferably, so-called two- or multi-wavelength lasers, for example a Y-branched two-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 is a correspondingly narrow-band diode laser that can be tuned spectrally 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.

Preferably, the wavelength ranges assigned to the at least two excitation radiations, ie all wavelength ranges assigned to the respective excitation radiations, are mutually disjoint. This means that the respective excitation radiations do not have any mutual spectral overlaps even with the modulation and the adjustable excitation radiations can therefore be clearly assigned to individual wavelength ranges. In conjunction with a spectral filter element with a predetermined filter wavelength v _Fiiter , this means that the wave number ranges resolved spectroscopically by the modulation are also disjoint with one another.

Preferably, for a predetermined sample to be spectroscopiced, at least two of the excitation radiations are selected such that different Raman signals occur within the spectral filter width Δv _filter 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. For example, a mixing ratio of the composition of the sample can then be determined from the ratio of the intensity of the measured Raman signals. However, the different Raman signals that occur within the spectral filter width Δv _filter of the spectral filter element can also be formed by different Raman lines of an individual sample component. By evaluating several Raman lines of a single sample component, the reliability of a sample determination can be increased.

Preferably, at least two of the excitation radiations are modulated via the respective modulation signals with a uniform frequency and phase position. Such an embodiment has the advantage that, for example, with two Raman signals filtered by the two excitation radiations via the spectral filter element, two different Raman lines (i.e. belonging to different wave number spacings) are detected by the lock-in device at the common modulation frequency as a sum signal become. The signal level measured via 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 compared to the detection of only one specific Raman signal by measuring several Raman signals in parallel .

In particular, the simultaneous occurrence of different Raman lines of a specific sample component (or a specific sample material) can be concluded based on a correspondingly high maximum signal level (e.g. above a specific threshold value). Through such a sum Raman spectroscopy With the shortest integration times, a high sensitivity and reliability of the spectroscopy arrangement according to the invention can be achieved, in particular for specific individual sample determination.

Preferably, the Raman signals that occur are based on both Stokes shift and anti-Stokes shift. The expression “occurring Raman signal” refers to the actual presence in the measurement signal of a Raman signal of the sample occurring within the spectral filter width Δv _filter of the spectral filter element and modulated with the first modulation signal. The expression therefore presupposes a signed spectral distance (wavenumber distance) between the wavelength of the excitation radiation modulated in the associated wavelength range and the filter wavelength v _filter of the spectral filter element used.

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 in the sample are at a higher energy level than before; 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 in the sample are at a lower energy level than before; 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 detected 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 carry out a temperature measurement on the sample. In the case of a spatially extensive sample that includes different components, a highly location- and material-specific temperature measurement of an individual component of the sample's composition can also be carried out. A state-specific temperature measurement is also possible, for example of an individual rotation and vibration state of a molecule in thermal non-equilibrium.

A device according to the invention preferably comprises at least one display device for displaying the measurement signal filtered with the lock-in device. A display device can preferably be a simple analog or digital pointer element or a binary display element (e.g. light signal when a certain threshold value is exceeded). Furthermore, a display device can preferably be an analog or digital y-t recorder for displaying the Raman signal as a time course in a corresponding diagram. Different Raman signals are preferably displayed on an assigned display device.

The embodiments mentioned can advantageously be combined with one another in whole or in part.

A further aspect of the invention relates to a method for Raman spectroscopy using a device according to the invention, wherein the spectral distance between the wavelength (λ ₁ , λ ₂ , λ ₃ ) of at least one excitation radiation (L ₁ , L ₂ , L ₃ ) and one Filter wavelength v _filter of the spectral filter element corresponds to the wavenumber spacing Δṽ of a Raman signal of a predetermined sample. In Raman spectroscopy, the appearance of at least one specific Raman line when a sample is excited with an excitation radiation is examined spectroscopically. In order to be able to carry out Raman spectroscopy of a predetermined sample with a device according to the invention, at least a spectral distance between the wavelength λ of the monochromatic excitation radiation modulated in the associated wavelength range Δλ and a filter wavelength v _filter of the spectral filter element must be equal to the wavenumber distance Δṽ of a Raman signal a predetermined sample. Therefore, if a corresponding Raman signal does not appear during the spectroscopy of a specific sample, it can at least be concluded that the sample predetermined for a corresponding test is absent.

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 that has a different composition (e.g. different molecules, chemical groups, etc.) can be measured by the ratio of the intensities of individual Raman lines of the different components. The method according to the invention can also be used to record the time course of a mixing ratio (e.g. for in-situ monitoring of chemical reactions within reactors). The mixing ratio of a sample can, for example, be used to determine measurement of the degree of saturation of a solution or to determine a degree of polymerization in a polymerizable monomer solution.

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.

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

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

Brief description of the drawings

• 1 eine schematische Darstellung einer ersten Ausführungsform einer erfindungsgemäßen Vorrichtung zur Raman-Spektroskopie mit zugehörigen Spektren;
• 2 eine schematische Darstellung einer zweiten Ausführungsform einer erfindungsgemäßen Vorrichtung zur Raman-Spektroskopie mit zugehörigen Spektren;
• 3 eine schematische Darstellung einer dritten Ausführungsform einer erfindungsgemäßen Vorrichtung zur Raman-Spektroskopie mit zugehörigen Spektren; und
• 4 eine exemplarische Darstellung der Spektren der dritten Ausführungsform einer erfindungsgemäßen Vorrichtung nach 3.

The invention and the technical environment are explained in more detail below using the accompanying figures. It should be noted that the invention is not intended to be limited by the exemplary embodiments given. 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. Show it:

• 1 a schematic representation of a first embodiment of a device according to the invention for Raman spectroscopy with associated spectra;
• 2 a schematic representation of a second embodiment of a device according to the invention for Raman spectroscopy with associated spectra;
• 3 a schematic representation of a third embodiment of a device according to the invention for Raman spectroscopy with associated spectra; and
• 4 an exemplary representation of the spectra of the third embodiment of a device according to the invention 3 .

Detailed description of the drawings

1 shows a schematic representation of a first embodiment of a device according to the invention for Raman spectroscopy with associated spectra. The device shown under a) comprises a spectrally tunable laser source 10, set up to emit a first monochromatic excitation radiation L ₁ of a first wavelength λ ₁ from a first wavelength range Δλ ₁ for exciting a sample P; a spectral filter element 20 at a filter wavelength _vFilter and with a spectral filter width _ΔvFilter , the first wavelength range Δλ ₁ and the spectral filter width _ΔvFilter relating to disjoint spectral ranges; a detector 30 for measuring the intensity of the excitation radiation L 1 'scattered by the sample P (ie the first excitation radiation L ₁ 'scattered by the sample) and filtered by the spectral filter element ₂₀ (ie the excitation radiation L scattered by the sample and filtered by the spectral filter element ₁ '') as measurement signal I; a means for tuning the laser source 50, wherein for tuning the first wavelength λ ₁ of the laser source 10 is modulated with a first modulation signal S _mod1 of the frequency f _mod1 (within the associated first wavelength range Δλ ₁ ); and a lock-in device 40, designed to use the first modulation signal S _mod1 as a reference signal to contain a Raman signal R of the sample P occurring within the spectral filter width Δ _vFilter of the spectral filter element 20 and modulated with the first modulation signal S _mod1 in the measurement signal I to filter out. The device shown further comprises a display device 60 for displaying the measurement signal I filtered with the lock-in device 40 at a modulation frequency (f _mod1 ). By filtering out, the signal-to-noise ratio of the modulated with the first modulation signal S _mod1 can be determined Raman signal R of sample P in measurement signal I can be improved.

The spectra shown under b) and c) show the spectral relationships during modulation. The spectra show the intensities of both the excitation radiation (v _L1 ) and the individual Raman lines v1 _L1 - v4 _L1 (the symbol v refers to the wave number commonly used in the field of Raman spectroscopy, the specification of which corresponds to the specification of the wavelength and was chosen for better representation) of a specific sample plotted against the respective wave number distance Δṽ compared to the excitation radiation (v _L1 ). Furthermore, the filter area of a spectral filter element 20 is shown at a filter wavelength v _filter and with a spectral filter width Δ _vfilter . The wave number distance between an output wavelength λ of the excitation radiation (v _L1 ) and the filter wavelength v _filter of the spectral filter element 20 is 1500 cm ^-1 in this example. Figure b) shows the conditions for an excitation radiation (v _L1 ) of a specific output wavelength λ without a modulation shift. The ) shows the changed conditions for an excitation radiation (v _L1 +δv ₁ ) that is shifted by a corresponding modulation of the excitation radiation v _L1 at the output wavelength λ. The entire Raman spectrum is also spectrally shifted by the same amount δv ₁ , so that shown in the th example, the fourth Raman line v4 _L1 +δv ₁ is shifted outside the spectral filter width Δ _vFilter of the spectral filter element 20. The Raman line v4 _L1 can thus be detected by simply modulating the excitation radiation between v _L1 and v4 _L1 +δv ₁ using the spectral filter element 20.

2 shows a schematic representation of a second embodiment of a device according to the invention for Raman spectroscopy with associated spectra. The device shown under a) essentially corresponds to that in 1a) embodiment shown. The reference numbers and their respective assignment to individual features of the device therefore apply accordingly. In contrast to that in 1a) However, in the embodiment shown, the spectrally tunable laser source 10 is set up to emit a second monochromatic excitation radiation L ₂ of a second wavelength λ ₂ from a second wavelength range Δλ ₂ for exciting the sample P, the second wavelength λ ₂ of the laser source 10 coming from the means for tuning the laser source 50 is modulated for tuning with a second modulation signal S _mod2 of the frequency f _mod2 (within the associated second wavelength range Δλ ₂ ), and the lock-in device 40 is set up to use the second modulation signal S _mod2 as a reference signal within the spectral filter width Δ _vFilter of the spectral filter element 20 to filter out the Raman signal R of the sample P that occurs and is modulated with the second modulation signal S _mod2 from the measurement signal I. The device shown further comprises two display devices 60, 60 'for displaying the measurement signal I filtered with the lock-in device 40 at two modulation frequencies (f _mod1 , f _mod2 ).

The spectra shown under b) and c) show the spectral relationships during the modulation of the individual excitation radiations according to those under 1b) and c ) shown conditions. The wavenumber distance between the first excitation radiation v _L1 and the second excitation radiation v _L2 is 500 cm ^-1 in this example. Thus, as shown under b), the two Raman lines v4 _L1 and v3 _L2 can be adjusted by corresponding modulation of the respective excitation radiations (v _L1 , v _L2 ) at different modulation frequencies by means of spectral tuning (e.g. by detuning, tuning or switching). Can be detected with the help of the spectral filter element 20.

3 a schematic representation of a third embodiment of a device according to the invention for Raman spectroscopy with associated spectra. The device shown under a) essentially corresponds to that in 2a) embodiment shown. The reference numbers and their respective assignment to individual features of the device therefore apply accordingly. In contrast to that in 2a) However, in the embodiment shown, the spectrally tunable laser source 10 is set up to emit at least one further monochromatic excitation radiation L ₃ of a further wavelength λ ₃ from a further wavelength range Δλ ₃ for exciting the sample P, the at least one further wavelength λ ₃ of the laser source 10 being transmitted from the means to Tuning the laser source 50 for tuning with at least one further modulation signal S _mod3 of the frequency f _mod3 (within the at least one associated further wavelength range Δλ ₃ ), and the lock-in device 40 is set up to do so with the at least one further modulation signal S _med3 as a reference signal to filter out a Raman signal R of the sample P occurring within the spectral filter width Δ _vFilter of the spectral filter element 20 and modulated with the at least one further modulation signal S _mod3 from the measurement signal I. The device shown further comprises at least two display devices 60, 60 'for displaying the measurement signal I filtered with the lock-in device 40 at three modulation frequencies (f _mod1 , f _mod2 , f _mod3 ).

4 shows an exemplary representation of the spectra of the third embodiment of a device according to the invention 3 . In this embodiment, four monochromatic excitation radiations (v _L1 , v _L2 , v _L3 , v _L4 ) are selected for a predetermined sample P to be spectroscopiced in such a way that different Raman lines (v1 _L4 , v2 _L3 , v3 _L2 , v4 _L1 ) within the spectral filter width Δ _vFilter of the spectral filter element 20 occur. According to the representations in 1b) and c ), are in 4 under a) the spectrally superimposed Raman lines (v1 _L4 , v2 _L3 , v3 _L2 , v4 _L1 ) for the unmodulated excitation radiations at certain output wavelengths λ are shown lying within the spectral filter width Δ _vFilter of the spectral filter element 20, while under b) the changed conditions with a Raman line v1 _L4 +δv 4 , v2 _L3 +δv ₃ , v3 _L2 + _{δv 2} , v4 _L1 + that is shifted by a corresponding modulation of the excitation radiation (v _L1 , v _L2 , v _L3 , v _L4 ) at the output wavelength λ δv ₁ shifted excitation radiations (v _L1 +δv ₁ , v _L2 +δv ₂ , v _L3 +δv ₃ , v _L4 +δv ₃ ) are shown. The monochromatic excitation radiations (v _L1 , v _L2 , v _L3 , v _L4 ) can be modulated independently of one another via corresponding modulation signals S _mod1 , S _mod2 , S _mod3 , S _mod4 and the associated Raman signals can be displayed on a display device 60, 60 ' become.

Reference symbol list

10

spectrally tunable laser source

20

spectral filter element

30

detector

40

Lock-in device

50

Means for tuning the laser source

60, 60'

Display devices

L1

first excitation radiation

L1'

Excitation radiation scattered by the sample

L1''

Excitation radiation scattered by the sample and filtered by the spectral filter element

L2

second excitation radiation

L2'

second excitation radiation scattered by the sample

L2''

second excitation radiation scattered by the sample and filtered by the spectral filter element

L3

further excitation radiation

L3'

further excitation radiation scattered by the sample

L3''

further excitation radiation scattered by the sample and filtered by the spectral filter element

P

sample

I

measurement signal

R

Raman signal

Smod1

first modulation signal

Smod2

second modulation signal

Smod3

third modulation signal

Claims (9)

Device for Raman spectroscopy, comprising: a spectrally tunable laser source (10), set up to emit a monochromatic first excitation radiation (L ₁ ) of a first wavelength λ ₁ from a first wavelength range Δλ ₁ for exciting a sample (P); a spectral filter element (20) at a filter wavelength v _filter and with a spectral filter width Δv _filter , the first wavelength range Δλ ₁ and the spectral filter width Δv _filter relating to disjoint spectral ranges; a detector (30) for measuring the intensity of the excitation radiation (L 1 ) 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 λ ₁ of the laser source (10) is modulated with a first modulation signal (S _mod1 ) of frequency f _mod1 ; and a lock-in device (40), designed to use the first modulation signal (S _mod1 ) as a reference signal to generate a Raman modulated with the first modulation signal (S _mod1 ) occurring within the spectral filter width Δ _vFiiter of the spectral filter element (20). to filter out the signal (R) of the sample (P) from the measurement signal (I); wherein the spectrally tunable laser source (10) is set up to emit a monochromatic second excitation radiation (L ₂ ) of a second wavelength λ ₂ from a second wavelength range Δλ ₂ for exciting the sample (P), the second wavelength λ ₂ of the laser source (10) from Means for tuning the laser source (50) is modulated with a second modulation signal (S _mod2 ) of the frequency f _mod2 , and the lock-in device (40) is set up to use the second modulation signal (S _mod2 ) as a reference signal within the spectral filter width Δ _vfilter of the spectral filter element (20) to filter out the Raman signal (R) of the sample (P) modulated with the second modulation signal (S _mod2 ) from the measurement signal (I), the different excitation radiations (L ₁ , L ₂ , L ₃ ) are emitted at the same time and the detector (30) is a single-channel detector. Device according to Claim 1 , wherein the spectrally tunable laser source (10) is set up to emit at least one further monochromatic excitation radiation (L ₃ ) of a further wavelength λ ₃ from a further wavelength range Δλ ₃ for exciting the sample (P), which has at least one further wavelength λ ₃ of the laser source (10) is modulated by the means for tuning the laser source (50) with at least one further modulation signal (S _mod3 ) of the frequency f _mod3 , and the lock-in device (40) is set up to operate with the at least one further modulation signal (S _mod3 ) as a reference signal to filter out a Raman signal (R) of the sample (P) occurring within the spectral filter width Δ _vFilter of the spectral filter element (20) and modulated with the at least one further modulation signal (S _mod3 ) from the measurement signal (I). Device according to Claim 1 or 2 , wherein the wavelength ranges (Δλ ₁ , Δλ ₂ , Δλ ₃ ) assigned to the at least two excitation radiations (L ₁ , L ₂ , L ₃ ) are disjoint with one another. Device according to one of the preceding claims, wherein for a predetermined sample (P) to be spectroscopiced, at least two of the excitation radiations (L ₁ , L ₂ , L ₃ ) are selected such that different Raman signals (R) within half of the spectral filter width Δv _filter of the spectral filter element (20) occurs. Device according to one of the preceding claims, wherein at least two of the excitation radiations (L ₁ , L ₂ , L ₃ ) are modulated with a uniform frequency and phase position via the respective modulation signals (S _mod1 , S _mod2 , S _mod3 ). Device according to one of the preceding claims, 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 distance between the wavelength (A ₁ , A ₂ , A ₃ ) of at least one excitation radiation (L ₁ , L ₂ , L ₃ ) and a filter wavelength v _filter of the spectral filter element (20) corresponds to the wavenumber spacing Δṽ of a Raman signal (R) of a predetermined sample (P). Using the procedure after Claim 8 for measuring a temperature of a sample (P) or a mixing ratio of a sample (P).