COMBINED SPECTROSCOPY SYSTEM INCLUDING RAMAN AND ATR-FTIR

Abstract

System for simultaneous measurement Raman and mid-infrared absorption signals from a sample, the system comprising an ATR crystal adapted for holding a sample thereon, at least one Raman excitation light source for Raman excitation, at least one FTIR excitation light source for FTIR excitation, at least one photodetector configured for collecting signals with a wavelength comprised at least in one of the IR spectrum or the Raman spectrum, a wavelength-dispersive device, such as a spectrometer, for collecting Raman signals, an excitation lens, and collection optics comprising a first collection lens.

Claims

1. System for simultaneous measurement of Raman and infrared absorption signals from a sample, the system comprising: an ATR crystal adapted for holding a sample thereon, at least one Raman excitation light source for Raman excitation, at least one FTIR excitation light source for FTIR excitation, at least one photodetector configured for collecting signals with a wavelength comprised at least in one of the IR spectrum or the Raman spectrum, a wavelength-dispersive device, such as a spectrometer, for the collected Raman signals, an excitation lens, and collection optics comprising a first collection lens, wherein the ATR crystal is configured to receive a light beam from the FTIR excitation light source, to redirect the light beam to the sample, to receive modified reflected radiation due to interaction of the evanescent field with the sample and to redirect it so that it can be received by at least one photodetector, wherein the excitation lens and the ATR crystal are positioned so that a first optical path is defined, such that the excitation lens is configured to focus into the sample a light beam emitted from the Raman excitation light source, wherein the ATR crystal and the collection optics are positioned so that a second optical path is defined, such that the collection optics is configured to collect Raman scattered light emitted by the sample and to redirect said Raman scattered light to at least one photodetector through the wavelength-dispersive device, wherein the Raman subsystem is an off-axis system, wherein the collection lens is configured to be movable along a direction parallel to the second optical path, and wherein the excitation lens is configured to be movable along a direction parallel to the first optical path and/or in a direction transverse to the first optical path.

2. The system according to claim 1, wherein the collection lens has a high numerical aperture value, preferably the numerical aperture value being greater than 0.4.

3. The system according to claim 1, wherein the excitation lens and/or the collection lens has a low numerical aperture value, preferably lower than 0.2.

4. The system according to claim 2 wherein the excitation lens has a high numerical aperture value, preferably greater than 0.4.

5. The system according to claim 1, further comprising: a first prism configured to receive a light beam from the FTIR excitation light source and to redirect said light beam to the ATR crystal; and/or a second prism configured to receive the modified reflected radiation from the ATR crystal, and to redirect said modified reflected radiation.

6. The system according to claim 1, further comprising: a first mirror configured to receive a light beam from the FTIR excitation light source, and to redirect said light beam; and/or a second mirror configured to redirect the modified reflected radiation to the at least one photodetector.

7. The system according to claim 6, wherein the first mirror and/or the second mirror is a paraboloidal mirror.

8. The system according to claim 1, further comprising a fixed mirror, a beam splitter and a movable mirror, wherein the beam splitter is configured to split the beam emitted by the FTIR excitation light source into two split beams, wherein the fixed mirror is arranged to receive one split beam and to reflect it back to the beam splitter, wherein the movable mirror is arranged to receive the other split beam and to reflect it back to the beam splitter and wherein the first mirror is arranged to receive the beam resulting from the interference of the two split beams and to redirect it.

9. The system according to claim 1, wherein the at least one photodetector comprises a first photodetector configured for collecting signals with a wavelength comprised in the IR spectrum and a second photodetector configured for collecting signals with a wavelength comprised in the Raman spectrum together with the wavelength-dispersive device, wherein the ATR crystal is configured to redirect the modified reflected radiation so that it can be received by the first photodetector, and wherein the collection optics is configured to redirect Raman scattered light to the second photodetector through the wavelength-dispersive device.

10. The system according to claim 1, comprising a single photodetector configured for collecting signals with a wavelength comprised both in the IR spectrum and in the Raman spectrum, wherein the ATR crystal is configured to redirect the modified reflected radiation due to interaction of the evanescent field with the sample so that it can be received by said photodetector, and wherein the collection optics is configured to redirect Raman scattered light to said photodetector through the wavelength-dispersive device.

11. The system according to claim 1, further comprising a processing unit in data communication with the at least one photodetector and the wavelength-dispersive device, the processing unit comprising a spectral data library for identifying the signals measured by the at least one photodetector.

12. The system according to claim 1, further comprising a band pass filter for filtering the light beam from the Raman excitation light source, wherein the band pass filter is located on the first optical path between the Raman excitation light source and the excitation lens.

13. The system according to claim 1, wherein the collection optics further comprises a long pass filter and a second collection lens, wherein the first collection lens, the long pass filter and the second collection lens are located on the second optical path such that: the first collection lens is configured to collect and collimate Raman scattered light from the sample, the long pass filter is configured for filtering the Raman scattered light collected by the first collection lens, and the second collection lens is configured for collecting the filtered Raman scattered light from the long pass filter and to redirect said filtered Raman scattered light to at least one photodetector through the wavelength-dispersive device.

14. The system according to claim 13, wherein the first collection lens, the long pass filter, the second collection lens comprised in the collection optics, the wavelength dispersive device and the second photodetector are assembled so as to be movable in a direction transverse to the second optical path.

15. The system according to claim 10, wherein: the collection optics further comprises a long pass filter and a second collection lens, wherein the first collection lens, the long pass filter and the second collection lens are located on the second optical path such that: the first collection lens is configured to collect and collimate Raman scattered light from the sample, the long pass filter is configured for filtering the Raman scattered light collected by the first collection lens, and the second collection lens is configured for collecting the filtered Raman scattered light from the long pass filter and to redirect said filtered Raman scattered light to at least one photodetector through the wavelength-dispersive device; and wherein the second collection lens is configured to redirect the filtered Raman scattered light to the single photodetector through the wavelength-dispersive device.

16. The system according to claim 1, further comprising a controller configured to control the position of the excitation lens and/or of the collection optics.

17. The system according to claim 1, wherein the light beams in both the Raman subsystem and the FTIR subsystem are transmitted through free space without using optical fibers.

18. A method for adding a new record to the spectral data library provided in claim 11, wherein the method comprises the steps of: i. providing a system according to claim 1; ii. placing the new sample on the ATR crystal to be hold thereon; iii. activate both the Raman excitation light source for Raman excitation and the FTIR excitation light source for FTIR excitation; iv. collecting both signals by the at least one photodetector; v. associating such measured signal to such sample thus forming the new record; and vi. adding such new record to the spectral data library stored in the processing unit of the system.

Description

DESCRIPTION OF THE DRAWINGS

[0103] These and other characteristics and advantages of the invention will become clearly understood in view of the detailed description of the invention which becomes apparent from a preferred embodiment of the invention, given just as an example and not being limited thereto, with reference to the drawings.

[0104] FIG. 1 shows a system for measuring Raman and infrared absorption signals from a sample according to an embodiment of the invention.

[0105] FIG. 2 shows a system for measuring Raman and infrared absorption signals from a sample according to an embodiment of the invention.

[0106] FIG. 3 shows an example of Raman excitation light and FTIR excitation paths within a schematic ATR crystal according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0107] FIGS. 1 and 2 depict a system (1) for simultaneous measurement of Raman and infrared absorption signals from a sample (2) according to two embodiments of the invention. The systems (1) shown in both figures are in an operative mode obtaining a Raman and an infrared absorption signal from a single sample (2) simultaneously. In these figures the light beams are represented with arrows, wherein the Raman subsystem light beams are represented with dotted lines and the FTIR subsystem light beams are represented with dashed lines.

[0108] In particular, the systems (1) shown in both FIGS. 1 and 2 comprise an ATR crystal (3), onto which the sample (2) is provided. Said ATR crystal (3) is configured to receive a light beam from the FTIR excitation light source (7), to redirect the light beam to the sample (2), and to receive modified reflected radiation due to interaction of the evanescent field with the sample (2). The high refractive index of the ATR crystal (3) helps to attain the total internal reflection at the sample (2) interface, where an evanescent wave, which extends into the sample (2), is generated due to attenuated total reflection. Then, the infrared beam is directed, so that it can be received by a first photodetector (9) configured for collecting signals with a wavelength comprised in the IR spectrum.

[0109] Additionally, for performing the excitation function of both the FTIR and the Raman subsystems, the systems (1) in both FIGS. 1 and 2 comprise one Raman excitation light source (6) for Raman excitation, and one FTIR excitation light source (7) for FTIR excitation, respectively.

[0110] For obtaining the infrared absorption signal, an infrared light source (7), which in this embodiment is a broadband source, as for example a SiC light source, operating from visible to mid infrared wavelengths (400 nm - 25000 nm), emits an infrared beam. Preferably, this broadband source, e.g. the SiC light source, operates at mid-infrared wavelengths (2.5 .Math.m - 25 .Math.m).

[0111] Further, in the excitation side of the Raman spectroscopy subsystem, the system (1) comprises an excitation lens (15). Said excitation lens (15) along with the ATR crystal (3) is positioned so that a first optical path is defined. The excitation lens (15) is configured to focus into the sample (2) a light beam emitted from the Raman excitation light source (6).

[0112] In turn, in the collection side of the Raman subsystem, the system (1) comprises collection optics comprising a first collection lens (17). The ATR crystal (3) and the collection optics are positioned so that a second optical path is defined, such that the collection optics is configured to collect Raman scattered light emitted by the sample (2) and to redirect said Raman scattered light to a wavelength-dispersive device (20), such as a Raman spectrometer. A second photodetector (8) registers the Raman signal. In the system (1) of FIG. 1 the second photodetector (8) is embodied integrated with the wavelength-dispersive device (20), namely comprised in the second photodetector (8). In the system (1) of FIG. 2 the second photodetector (8) and the wavelength-dispersive device (20) are embodied as separate devices connected one to another.

[0113] Additionally, for obtaining optically sectioned Raman signals from the sample (2), the collection optics of the systems (1) shown in both FIGS. 1 and 2 comprise a long pass filter (18) configured for filtering the Raman scattered light collected by the first collection lens (17) and a second collection lens (19) configured for collecting the filtered Raman scattered light from the long pass filter (18) and to redirect said filtered Raman scattered light to the wavelength-dispersive device (20) and the photodetector (8) configured for collecting signals with a wavelength comprised in the Raman spectrum.

[0114] Compared to the system (1) shown in FIG. 1, the system (1) shown in FIG. 2 additionally comprises a first (4) and a second (5) prism which are shown coupled at the side facets of the ATR crystal (3) for coupling the FTIR light beam. The first (4) and a second (5) prisms and the ATR crystal (3) are shaped and assembled resulting in a geometry which allows integration of the collection optics of the Raman subsystem.

[0115] Further, the system (1) shown in FIG. 2 comprises additional elements belonging to the FTIR subsystem. In particular, the system (1) comprises a beam splitter (13) configured to split the beam emitted by the FTIR excitation light source (7) into two split beams; a fixed mirror (12) arranged to receive one split beam and to reflect it back to the beam splitter (13); and a movable mirror (14) arranged to receive the other split beam and to reflect it back to the beam splitter (13). The system (1) shown in FIG. 2 further comprises a first mirror (10) arranged to receive the beam resulting from the interference of the two split beams and to redirect it to the first prism (4); and a second mirror (11) configured to redirect the modified reflected radiation to the first photodetector (9) configured for collecting signals with a wavelength comprised in the IR spectrum.

[0116] The beam splitter (13) shown in FIG. 2 splits the infrared light beam emitted by the FTIR excitation light source (7) into a first and second split beam. The first split beam is then directed towards the fixed mirror (12), whereas the second split beam is directed towards the movable mirror (14). Afterwards, reflections corresponding to both split beams interfere, and the resulting infrared beam is then directed towards the first mirror (10) which, in turn, redirects said infrared beam towards the first prism (4), which is in optical connection with the ATR crystal (3). The infrared beam goes through total internal reflection in the ATR crystal (3), which in the present embodiment can be made, for example, of diamond.

[0117] The high refractive index of the ATR crystal (3) and the first prism (4) helps to attain the total internal reflection at the sample interface, where an evanescent wave which extends into the sample is generated due to total internal reflection. Then, the infrared beam is directed, through the second prism (5) connected to the ATR crystal (3), towards a second mirror (11) which, in turn, redirects the infrared beam towards the first photodetector (9), the infrared absorption signal being collected thereby.

[0118] Compared to the system (1) shown in FIG. 1, for obtaining optically sectioned Raman signal from the sample (2), the system (1) shown in FIG. 2 comprises additional elements belonging to the Raman spectroscopy subsystem. In particular, the system (1) further comprises a band pass filter (16) configured for filtering the light beam emitted by the Raman excitation light source (6).

[0119] In particular, the Raman excitation light source (6), which in this particular embodiment is a laser, emits a light beam towards a band pass filter (16) which is used to block unwanted background or sidelobes from the laser (6). Then, after passing through the band pass filter (16), the light beam passes through the excitation lens (15) which is used to focus the light beam onto the sample (2) as well as to excite several layers in the sample (2).

[0120] The light beam is then directed to the sample (2) interface after passing through the ATR crystal (3). Then, the first collection lens (17) collects and collimates the Raman scattered light and redirects it into the second collection lens (19) after passing through the long pass filter (18), which blocks scattered laser light and leakage from the filter.

[0121] Afterwards, the second collection lens (19) redirects and focuses the resulting Raman scattered light towards the wavelength-dispersive device (20) and the second photodetector (8), registering the Raman signal thereby.

[0122] With the advent of miniaturizing technologies, the system (1) according to the invention can be easily implemented as portable Raman and FTIR spectrometers for real time in situ chemical analyses. A portable combined Raman and FTIR system (1) has the potential of providing robust information to first responders, such as law enforcement, military and healthcare workers in order to estimate, or mitigate impending chemical threats.

[0123] The unmatched advantage of such a combined system (1) is that reliable chemical identification of multiple constituents can be obtained in seconds, including complex mixtures.

[0124] FIG. 3 depicts a schematic ATR crystal (3) according to an embodiment of the invention where respective paths of Raman excitation light and FTIR excitation light within are shown.

[0125] The ATR crystal (3) has a (isosceles) trapezoid shape in cross-section and comprises two plane-parallel surfaces and two side facets, wherein one of the plane-parallel surfaces is intended for supporting the sample. As it is depicted, the light beam emitted from the Raman excitation light source enters the ATR crystal through the other plane-parallel surface and goes through plane-parallel surfaces of the ATR crystal without being guided by total internal reflection (TIR) inside the crystal (3). Therefore, Raman excitation light will not enter the detection path of the FTIR subsystem.

[0126] It is to be noted that, regardless the incident angle of the Raman excitation light, the light cannot be transported by TIR within the ATR crystal as long as the light is coupled through the long surface (i.e. lower surface in the figure), which is plane-parallel to the surface at the sample side.

[0127] On the other hand, the FTIR light beam enters the ATR crystal through one of the side facets and the high refractive index of the ATR crystal helps to attain the total internal reflection at the sample interface. Then, the modified reflected radiation is redirected without interfering with the light beam emitted from the Raman excitation light source.