Raman Spectrometer

Abstract

Raman spectrometer with a high-power laser diode (16) for emission of laser light (31) which has a central wavelength (λ) La ) in the blue spectral range, with a measuring chamber (20) through which a gas (22) flows, with first focusing means (18) for focusing the laser light (31) into the measuring chamber (20) and with a spectrally resolving detector (70) and with second focusing means (21) for imaging Raman scattered light (34) generated in the measuring chamber (20) into the spectrally resolving detector (70), characterized in that an optical short-pass filter (19) is arranged in the beam path of the laser light (31) between the high-power laser diode (16) and the measuring chamber (20), the cutoff wavelength (λ) of which KP ) is longer than the central wavelength (λ La ) of the laser light (31).

Description

State of the art

[0001] Raman spectrometers are already known from the prior art, for example from DE 10 2021 107 229 A1 or EP 3 748 339 B1. They are based on the principle that focused laser light is inelastically scattered by the molecules of a gas in a measuring cell, producing so-called Raman scattered light. They further rely on the spectrally resolved detection of this Raman scattered light. In this way, properties of the gas in the measuring cell can be determined.

[0002] From DE 10 2009 026 744 A1 it is known to provide a bandpass filter and / or a longpass filter in a diode laser of a Raman spectrometer in order to greatly minimize the emission of the diode laser outside its laser wavelength and to achieve a stronger and more stable measurement signal. Disclosure of the invention

[0003] The inventors recognized that the emission from high-power laser diodes with a central wavelength in the blue spectral range inevitably includes, in addition to the actual laser radiation (whose wavelength is very close to the central wavelength), autofluorescence components. These components exhibit spectral energy densities over a relatively large interval around the central wavelength. While these spectral energy densities are small compared to the spectral energy density of the actual laser radiation at the central wavelength, the energy density of the autofluorescence components is not small compared to the expected spectral energy density of the Raman scattered light at low partial pressures of the gas in the measuring chamber. This limits the measurement accuracy and resolution of the Raman spectrometer.

[0004] The solution known from the above-mentioned prior art of using a bandpass filter is not usable for blue high-power diode lasers, since optical bandpass filters for the relevant powers and wavelengths are not available with the bandwidth required in view of batch and operating parameter-related variations in the central wavelength of such blue high-power diode lasers.

[0005] To improve the measurement accuracy and resolution of the Raman spectrometer, the invention provides for an optical shortpass filter in the laser light path between the high-power laser diode and the measuring chamber. The cutoff wavelength of this shortpass filter is longer than the central wavelength of the laser light. In this way, the actual laser light passes through the shortpass filter unimpeded, while the autofluorescence components of the high-power laser diode's emission in the long-wavelength range are significantly suppressed. This creates a wavelength interval in which the Raman scattered light can be sensitively detected by the detector even with low background radiation.

[0006] The advantageous effects of the invention become even more effective when, instead of a single optical short-pass filter, two or more optical short-pass filters are arranged in series.

[0007] In a further development, it is proposed that an optical long-pass filter be placed in the beam path of the Raman scattered light between the measuring chamber and the spectrally resolving detector. The cutoff wavelength of this filter is longer than the central wavelength of the laser light. This further improves the measurement accuracy and resolving power of the Raman spectrometer. In particular, light with the wavelength of the actual laser light (central wavelength; e.g., Rayleigh scattering) is prevented from reaching the spectrally resolving detector and interfering with the detection of the Raman scattered light due to its comparatively high spectral energy density and / or total energy.

[0008] It can be advantageous to design the long-pass filter with a shorter cutoff wavelength than the short-pass filter. This ensures that spectral components attributable to Raman scattering are not excessively blocked by the long-pass filter.

[0009] Alternatively, the cutoff wavelength of the long-pass filter can be longer than or equal to the cutoff wavelength of the short-pass filter. In this case, it is ensured that spectral components attributable to the autofluorescence of the high-power diode laser are almost completely prevented from reaching the detector. The measurement accuracy and resolution of the Raman spectrometer are thus further improved.

[0010] It can be advantageously designed so that the cutoff wavelength of the long-pass filter, the cutoff wavelength of the short-pass filter, and the central wavelength of the laser light lie within a wavelength interval extending over less than 10 nm. In this case, it is simultaneously ensured that, within a certain wavelength interval, the autofluorescence components of the high-power laser diode's emission are effectively blocked, and spectral components attributable to Raman scattering are not excessively blocked.

[0011] Further developments of the invention are based on the finding that the highest efficiency or the most effective optical density of optical filters (short-pass filters and long-pass filters) are achieved when the filters are illuminated by a collimated (also: parallelized) beam.

[0012] For this reason, it is advantageous if the high-power laser diode emits laser light in the form of a divergent beam, and if the first focusing means have a first optic that transforms the divergent beam into a collimated beam, and a second optic that transforms the collimated beam into a convergent beam, and if the optical shortpass filter(s) is / are arranged in the collimated beam.

[0013] For the same reason, it is advantageous if the second focusing means are provided to have a first optic and if the Raman scattered light generated in the measuring chamber is at least partially transformed by the first optic of the second focusing means into a collimated beam, and if the second focusing means have a second optic which transforms the collimated beam into a convergent beam, and if the long-pass filter is arranged in the collimated beam between the first optic and the second optic of the second focusing means.

[0014] A high-power laser diode can be a laser diode capable of emitting light with an optical power of at least one watt. Wavelengths in the blue spectral range can be understood as wavelengths in the interval from 380 nm to 500 nm, particularly in the interval from 440 nm to 460 nm.

[0015] The high-power laser diode can in particular be a "free-running" laser diode, meaning that measures to stabilize the wavelength of its emission, such as frequency-selective feedback to external components, visual Bragg grating, or similar techniques, can be omitted.

[0016] The single figure in the application shows an exemplary embodiment of the present invention.

[0017] A Raman spectrometer 30 has a high-power laser diode 16 for emitting laser light 31, which has a central wavelength λ. LaThe example shows a wavelength of 447 nm, see diagram (a). In addition to the spectral component at this wavelength, which is also referred to here as the actual laser radiation 31L, the emission also exhibits autofluorescence components 31E, which extend on the long-wavelength side from the central wavelength, for example up to the wavelength 490 nm and exhibit a measurable spectral power density there.

[0018] The high-power laser diode 16 emits spatially divergent laser light 31. The spatial properties divergence and beam diameter of the laser light 31 typically differ in two beam profile directions perpendicular to the propagation direction of the laser light 31, which can be compensated for by suitable cylindrical optics, which will not be discussed further here.

[0019] The Raman spectrometer 30 has a measuring chamber 20 through which a gas 22 flows - perpendicular to the plane of the drawing in the figure.

[0020] The Raman spectrometer 30 has first focusing means 18 for focusing the laser light 31 into the measuring chamber 20. The first focusing means 18 include, for example, a first optic 18.1 that transforms the divergent beam into a collimated beam, which has, for example, an approximately round or approximately square beam profile. The first optic 18.1 of the first focusing means 18 can, for example, include the cylindrical optic mentioned above.

[0021] The first focusing means 18 include, for example, a second optical element 18.2 that transforms the collimated beam into a convergent beam which is focused in the measuring chamber 20. The second optical element 18.2 can, for example, be a spherical plano- or biconvex lens.

[0022] In the collimated beam, i.e., between the first optic 18.1 and the second optic 18.2 of the first focusing device 18, a short-pass filter 19 is arranged in the beam path in this example. The short-pass filter 19 has a cutoff wavelength λ in this example. KP from 454 nm. As can be seen in diagram (b) and by comparison with diagram (a), the effect of the short-pass filter 19 is that it halves the spectral power density of the emission from the high-power laser diode 16 at its cutoff frequency of 454 nm and attenuates it even more at longer wavelengths. The actual laser radiation 31L, which has a wavelength of 447 nm, is transmitted unimpeded by the short-pass filter 19.

[0023] As an alternative to the single short-pass filter 19 shown in the figure and explained above as an example, it would also be possible to arrange two such short-pass filters 19 in series at the same location. The transmission of the longer-wavelength components of the emission from the high-power laser diode 16 through the short-pass filters 19 is then reduced even further compared to a single short-pass filter 19.

[0024] In the measuring chamber 20, Raman scattering 34, which has characteristic wavelengths, is generated by the inelastic interaction of the molecules of gas 22 with the focused laser light 31. Diagram (c) shows, for example, two Raman lines 34.1, 34.2, whose wavelengths are longer than those of the actual laser light 31L. Furthermore, the laser light 31 is also elastically scattered by the molecules of gas 22. The laser radiation 31 deflected in this way has the same wavelength as the incident laser light 31 and is called Rayleigh scattering 37, see also diagram (c). The Rayleigh scattering 37 is spatially superimposed on the Raman scattering 34 in the measuring chamber 20 and at the exit from the measuring chamber 20.

[0025] The part of the laser light 31 that is not deflected by interaction with the molecules of the gas 22 is absorbed in the example in a beam absorber 32, which is arranged behind the measuring chamber 20 in the direction of propagation of the laser light 31.

[0026] The Raman spectrometer 30 comprises a detector 70 that is capable of quantitatively, spectrally resolved, and dynamically analyzing incident light. For example, it includes a grating spectrometer and a CCD detector.

[0027] The Raman spectrometer 30 further comprises second focusing means 21 for imaging Raman scattered light 34 generated in the measuring chamber 20 into the spectrally resolving detector 70. For example, the second focusing means 21 comprise a first optic 21.1, which collimates the Raman scattered light 34 generated in the measuring chamber 20, and a second optic 21.2, which transforms the collimated light into a convergent beam and images it into the detector 70. The first and second optics 21.1, 21.2 of the second focusing means 21 can each be, for example, a spherical plano- or biconvex lens.

[0028] In the collimated beam, i.e., between the first optic 21.1 and the second optic 21.2 of the second focusing device 21, a long-pass filter 45 is arranged in the beam path in this example. The long-pass filter 45 has a cutoff wavelength λ in this example. LPfrom 451nm. As can be seen in diagram (d) and by comparison with diagram (c), the effect of the longpass filter 45 is that it significantly suppresses the Rayleigh scattering (@447nm), while the Raman scattering lines (> 451nm) are transmitted unhindered. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2021 107 229 A1

[0001] EP 3 748 339 B1

[0001] DE 10 2009 026 744 A1

[0002]

Claims

[1] Raman spectrometer with a high-power laser diode (16) for emission of laser light (31) which has a central wavelength (λ La ) in the blue spectral range, with a measuring chamber (20) through which a gas (22) flows, with first focusing means (18) for focusing the laser light (31) into the measuring chamber (20) and with a spectrally resolving detector (70) and with second focusing means (21) for imaging Raman scattered light (34) generated in the measuring chamber (20) into the spectrally resolving detector (70), characterized by , that an optical short-pass filter (19) is arranged in the beam path of the laser light (31) between the high-power laser diode (16) and the measuring chamber (20), the cutoff wavelength (λ) of which KP ) is longer than the central wavelength (λ La ) of the laser light (31). [2] Raman spectrometer according to claim 1, characterized by, that in the beam path of the Raman spectrometer (30) between the measuring chamber (20) and the spectrally resolving detector (70) an optical long-pass filter (45) is arranged, the cutoff wavelength (λ) of which LP ) is longer than the central wavelength (λ La ) of the laser light (31). [3] Raman spectrometer according to claim 2, characterized by , that the cutoff wavelength (λ LP ) of the longpass filter (45) is shorter than the cutoff wavelength (λ KP ) of the shortpass filter (19). [4] Raman spectrometer according to claim 2 or 3, characterized by , that the cutoff wavelength (λ LP ) of the longpass filter (45), the cutoff wavelength (λ KP ) of the shortpass filter (19) and the central wavelength (λ La ) of the laser light (31) lie in a wavelength interval extending over less than 10nm. [5] Raman spectrometer according to any one of the preceding claims, characterized by, that in the beam path of the laser light (31) between the high-power laser diode (16) and the measuring chamber (20) there are two optical short-pass filters (19) whose cutoff wavelengths (λ KP ) are longer than the central wavelength (λ La ) of the laser light (31), are arranged in a row. [6] Raman spectrometer according to any one of the preceding claims, characterized by , that the high-power laser diode (16) emits laser light (31) in the form of a divergent beam, that the first focusing means (18) comprise a first optics (18.1) that transforms the divergent beam into a collimated beam, and a second optics (18.2) that transforms the collimated beam into a convergent beam. [7] Raman spectrometer according to claim 6, characterized by that the collimated beam has an approximately round or square beam profile, or an elongated elliptical or rectangular beam profile. [8] Raman spectrometer according to claim 6 or 7, characterized by , that the optical shortpass filter (19) is arranged in the collimated beam, in particular the two optical shortpass filters (19) are arranged in the collimated beam. [9] Raman spectrometer according to any one of the preceding claims, characterized by , that the second focusing means (21) have a first optic (21.1) and that the Raman scattered light (34) generated in the measuring chamber (20) is at least partially transformed by the first optic (21.1) of the second focusing means (21) into a collimated beam, and that the second focusing means (21) have a second optic (21.2) which transforms the collimated beam into a convergent beam. [10] Raman spectrometer according to one of claims 2 to 4 and also according to claim 9, characterized by, that the longpass filter (45) is arranged in the collimated beam between the first optics (21.1) and the second optics (21.2) of the second focusing means (21).

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