US-12624999-B2 - Flowcell and system with improved collection efficiency for Raman spectroscopy

Abstract

Flowcells and Raman analysis systems provide improved signal collection dynamics through increased solid-angle geometries and improved numerical aperture for near-diffraction-limited performance. A combined excitation/collection beam passes through a first optical material, a sample conduit and a second optical material. A concave reflective aspheric surface focuses and re-collimates the combined beam to and from a region of the sample within the conduit. The optical materials may comprise separate windows or may integrally form sidewalls the conduit. The reflective surface may be spaced apart from the second window or may be integrally formed with the second optical material. The focused region in the sample may approximate a point or a line, and at least a portion of the interior wall of the conduit may be reflective, causing the combined beam to pass through the sample region more than once to enhance collection efficiency.

Inventors

• Nicholas Skriba
• James Tedesco
• JOSEPH SLATER

Assignees

• ENDRESS+HAUSER OPTICAL ANALYSIS, INC.

Dates

Publication Date

20260512

Application Date

20221107

Claims (20)

1. 1 . A flowcell for spectroscopy for use with a collimated optical beam combining a laser excitation beam and a signal collection beam into a combined beam, the flowcell comprising: a flow channel configured to convey a sample, wherein the flow channel has opposing first and second sides, and a longitudinal axis defining a flow direction; first and second optical materials disposed on the first and second sides of the flow channel, respectively; and a single concave aspheric reflective surface disposed adjacent the second side, wherein the concave aspheric reflective surface is a parabolic, biconic, or freeform optical surface, wherein the collimated beam is configured relative to the flowcell to pass through the first optical material, intersect the flow channel at a substantially transverse angle, pass through the second optical material, and then impinge upon the concave aspheric reflective surface, wherein the concave aspheric reflective surface is configured to focus the excitation beam of the combined beam to a region within the sample within the flow channel and to re-collimate the signal collection beam into the combined beam from the region within the sample, and wherein the signal collection beam includes Raman scattered radiation.
2. 2 . The flowcell of claim 1 , wherein the concave aspheric reflective surface is a biconic optical surface.
3. 3 . The flowcell of claim 1 , wherein the first and second optical materials each comprise separate windows disposed on the opposing first and second sides of the flow channel.
4. 4 . The flowcell of claim 1 , wherein the first and second optical materials form the opposing first and second sides of the flow channel, respectively.
5. 5 . The flowcell of claim 1 , wherein: the first and second optical materials comprise separate first and second windows disposed on the opposing first and second sides, respectively, of the flow channel; and the concave aspheric reflective surface is spaced apart from the second window.
6. 6 . The flowcell of claim 1 , wherein: the first and second optical materials comprise separate first and second windows disposed on the opposing first and second sides, respectively, of the flow channel; and the concave aspheric reflective surface is integrally formed with the second window.
7. 7 . The flowcell of claim 1 , wherein: the first and second optical materials at least partially define an integral block of material that surrounds the flow channel; and the concave aspheric reflective surface defines a portion of the integral block of material.
8. 8 . The flowcell of claim 7 , wherein the first and second optical materials are the same material such that the integral block is a monolithic block of material.
9. 9 . The flowcell of claim 1 , wherein: the flow channel is an elongated conduit defining a central axis; and the concave aspheric reflective surface defines an elongated reflector configured to focus and re-collimate the combined beam to and from a region around the central axis of the conduit.
10. 10 . The flowcell of claim 1 , wherein: the flow channel has an interior wall; and at least a portion of the interior wall is reflective and configured to cause the combined beam to pass through the region within the flow channel more than once as to enhance a collection efficiency of the signal collection beam.
11. 11 . The flowcell of claim 1 , wherein: the flow channel includes a spherical chamber defined at least partially by an interior wall; and at least a portion of the interior wall is reflective, causing the combined beam to pass through the region more than once as to enhance a collection efficiency of the signal collection beam.
12. 12 . The flowcell of claim 1 , further comprising: a laser operative to generate the laser excitation beam; a spectrograph operative to receive and operate on the signal collection beam; and optical components configured and arranged to combine the laser excitation beam and the signal collection beam into the combined beam.
13. 13 . The flowcell of claim 1 , wherein: the first optical material comprises a window disposed on the first side of the flow channel; the concave aspheric reflective surface is a first-surface mirror comprising the second optical material; and the concave aspheric reflective surface defines the second side of the flow channel opposite the first side.
14. 14 . The flowcell of claim 1 , wherein the concave aspheric reflective surface includes a reflective multilayer dielectric coating.
15. 15 . A Raman analysis system, comprising: a laser source operative to generate a laser excitation beam; a spectrograph operative to receive and operate on a signal collection beam, which includes Raman scattered radiation; a computer configured to receive signals from the spectrograph to analyze Raman signatures present in the signal collection beam; optical components operative and arranged to combine the laser excitation beam and the signal collection beam into a combined counter-propagating, collimated, excitation/collection beam; a flowcell configured to convey a sample, the flowcell including a flow channel therethrough, the flow channel having opposing first and second sides, and a longitudinal axis defining a flow direction; first and second optical materials disposed on the first and second sides of the flow channel, respectively; and a single concave aspheric reflective surface, wherein the concave aspheric reflective surface is a parabolic, biconic, or freeform optical surface; wherein the collimated excitation/collection combined beam is configured to pass through the first optical material, intersect the flow channel at a substantially transverse angle, pass through the second optical material, and then impinge upon the concave aspheric reflective surface; and wherein the concave aspheric reflective surface is configured to focus the laser excitation beam of the combined beam to a region within the sample within the flow channel and to re-collimate the signal collection beam into the combined beam from the region within the flow channel.
16. 16 . The system of claim 15 , wherein the concave aspheric reflective surface is a biconic optical surface.
17. 17 . The system of claim 15 , wherein: the first and second optical materials comprise separate first and second windows disposed on opposing first and second sides of the flow channel, respectively; and the concave aspheric reflective surface is integral with the second window.
18. 18 . The system of claim 15 , wherein: the first and second optical materials form a monolithic block of material that surrounds the flow channel; and the concave aspheric reflective surface defines a portion of the monolithic block.
19. 19 . The system of claim 15 , wherein: the flow channel is an elongated conduit defining a central axis; and the concave aspheric reflective surface defines an elongated reflector configured to focus and re-collimate the combined beam to and from a region around the central axis of the conduit.
20. 20 . The system of claim 15 , wherein: the flow channel includes an interior wall; and at least a portion of the interior wall is reflective, causing the combined beam to pass through the region within the sample more than once as to enhance a collection efficiency of the signal collection beam.

Description

TECHNICAL FIELD The present disclosure relates generally to spectroscopy and, in particular, to Raman spectroscopy and, more particularly, to flowcells and Raman spectroscopic systems with improved collection efficiency. BACKGROUND Induced radiation effects such as Raman scattering and fluorescence have become extremely valuable tools associated with the non-destructive determination of molecular composition of materials and media. However, Raman scattering is a notoriously weak effect, requiring advanced techniques to ensure adequate signal for quantitative analysis of the molecular compositions. A conventional Raman system includes three main components: a laser excitation source, sampling optics and a spectrometer. Because Raman instruments use lasers in the visible and near-infrared regions, optical fibers can be used to transmit radiation from the laser excitation source to a sample and to collect resulting scattered radiation from the sample. In process control and other applications, a Raman probe can be inserted into a chemical reaction process to collect the scattered radiation directly, or the Raman probe can collect the scattered radiation, including Raman spectra within the scattered radiation, though a window, for example, via an external reaction sample loop or flowcell, thereby avoiding potential sample contamination. FIG. 1 shows a schematic diagram of a conventional, fiber-based Raman probe 100. Excitation radiation (e.g., light) from a laser is transmitted into the probe over an optical fiber 102, which radiation is then collimated by a lens 104. The collimated light may then pass through a bandpass filter 108 to remove wavelengths within the collimated light that are not at the laser's wavelength, generally noise resulting from the fiber 102 and optics, such as the lens 104. The filtered light may be reflected by a mirror 106 onto a beam combiner 120, which may then be directed to a sample along a counter-propagating collimated path 122. Light scattered by the sample under investigation returns along path 122 (e.g., a collection signal), passes through beam combiner 120 and may be filtered by an optional notch filter 116 to remove Rayleigh-scattered light at the laser frequency and under interference before being focused by a lens 114 onto the end of a collection fiber 112, which may be connected to a spectrometer of the Raman system. FIG. 2 shows a simplified block diagram of a conventional flowcell arrangement 200 for Raman spectroscopy. The laser excitation source is represented by block 202, and the spectrometer with block 204. A processor 208 (e.g., computer) may be included to control system operation, to enable a user interface and to receive and analyze Raman signals (e.g., Raman spectra) separated by the spectrometer of the Raman system. Block 206 represents beam-combining optics to generate a collimated, counter-propagating, combined excitation/collection beam 210. For example, block 206 may include a fiber-coupled probe like the probe 100 of FIG. 1. In FIG. 2, the combined beam 210 is focused by an objective lens 212 to a point 214 within a conduit 216 (e.g., a flow tube), which may contain a medium 222 under investigation (e.g., a liquid). The conduit 216 may be a primary flow tube, process vessel or a capillary branch from a primary tube or vessel. Notably, the lens 212 is air-spaced from the conduit 216 such that the combined beam 210 travels from the lens 212, through free space (not necessarily a vacuum) and into the conduit 216. Conventional objective lenses for flowcells, such as the lens 212, have a numerical aperture (NA) of about 0.3, yielding a solid angle of cone 218 is approximately 0.29 Sr. Additional losses of aperture occur due to an index mismatch from lens-air-medium interfaces, resulting in a solid angle within the sample of approximately 0.16 Sr, for example (n=1.333 for pure water). A reflector 220 may be provided to achieve a ‘multi-pass’ configuration in which the excitation radiation of the beam 210 passes back through the conduit 216 toward the optics 206. While this arrangement generates additional signal through relayed imaging, this arrangement still does not increase the solid angle. Thus, 0.3 NA objective lenses can only ‘see’ 4.6% of the total hemisphere in air, 2.5% in water. As such, a need remains for improved optical geometries to maximize signal-generation capabilities in flow-cell configurations for Raman spectroscopy. SUMMARY In one aspect of the present disclosure, a flowcell for spectroscopy for use with a collimated optical beam combining a laser excitation beam and a signal collection beam into a combined beam comprises: a flow channel configured to convey a sample, wherein the flow channel has opposing first and second sides; first and second optical materials disposed on the first and second sides of the flow channel, respectively; and a concave aspheric reflective surface disposed adjacent the second side, wherein the collimated beam