LIGHT TRANSMISSION SYSTEM FOR DELIVERING LIGHT TO A RAMAN SPECTROMETER

Abstract

According to an aspect of the present inventive concept there is provided a light transmission system (100, 200) for delivering light to a Raman spectrometer (20), the system comprising: a homogenizer (110, 510) with an entrance surface (112, 512) having an entrance height and an entrance width, an exit surface (114, 514) having an exit height and an exit width, and wherein the exit height is of a larger size than the entrance height; and wherein the homogenizer (110, 510) is configured to receive, at the entrance surface (112, 512), light from a bundle (210) of optical fibers (220) and wherein each fiber (220) in the bundle (210) defines an entrance divergence angle of light; and wherein the homogenizer (110, 510) is configured to transmit light such that an exit divergence angle of light, in a plane parallel with a direction of the exit height, is smaller than the entrance divergence angle of light, in a plane parallel with a direction of the entrance height.

Claims

1. A light transmission system for delivering light to a Raman spectrometer, said light transmission system comprising: a homogenizer comprising an entrance surface and an exit surface, the entrance surface having an entrance height and an entrance width, wherein the entrance height and the entrance width are of different sizes, the exit surface having an exit height and an exit width, wherein the exit height and the exit width are of different sizes, and wherein the exit height is of a larger size than the entrance height; and wherein the homogenizer defines an optical path for propagation of light from the entrance surface to the exit surface; wherein the homogenizer is configured to receive, at the entrance surface, light from a bundle of optical fibers and wherein each fiber in the bundle of fibers defines an entrance divergence angle of light received at the entrance surface; and wherein the homogenizer is configured to transmit light such that an exit divergence angle of light exiting at the exit surface, in a plane perpendicular to the exit surface and parallel with a direction of the exit height, is smaller than the entrance divergence angle of light, in a plane perpendicular to the entrance surface and parallel with a direction of the entrance height, such that the exit divergence angle is adapted to an acceptance angle of a detector of the Raman spectrometer.

2. The light transmission system according to claim 1, further comprising a bundle of optical fibers, each optical fiber of the bundle of optical fibers comprising a transmitting end, wherein the transmitting ends of the optical fibers are arranged in a configuration adapted to a shape of the entrance surface such that light from each optical fiber is received by the homogenizer at the entrance surface.

3. The light transmission system according to claim 2, wherein each optical fiber of the bundle of optical fibers further comprises a receiving end configured to receive light, and wherein the receiving ends of the optical fibers are arranged in a circular configuration.

4. The light transmission system according to claim 3, wherein the transmitting ends of the optical fibers and the receiving ends of the optical fibers are arranged in different configurations.

5. The light transmission system according to claim 1, wherein the homogenizer is configured to provide an exit divergence angle which is less than or equal to 50% of the entrance divergence angle, and preferably less than or equal to 35% of the entrance divergence angle.

6. The light transmission system according to claim 1, wherein the homogenizer further comprises one or more side surfaces, and wherein the one or more side surfaces are configured so as to allow propagation of light from the entrance surface to the exit surface by total internal reflection of light at the one or more side surfaces.

7. The light transmission system according to claim 1, wherein at least one of the entrance surface and the exit surface of the homogenizer is provided with a glue, said glue having a refractive index which is equal to a refractive index of the homogenizer, such that refraction of light is minimized when light passes between the homogenizer and the glue.

8. The light transmission system according to claim 1, wherein the homogenizer has a shape of a trapezoidal prism.

9. The light transmission system according to claim 1, wherein the homogenizer is tapered in a direction of the width towards the exit surface, such that the entrance width is of a larger size than the exit width, the homogenizer thereby being configured to transmit light such that an exit divergence angle of light exiting at the exit surface, in a plane perpendicular to the exit surface and parallel with a direction of the exit width, is larger than the entrance divergence angle of light, in a plane perpendicular to the entrance surface and parallel with a direction of the entrance width, such that the exit divergence angle is further adapted to the acceptance angle of the detector of the Raman spectrometer.

10. An illumination and light collection system for a Raman spectrometer, the illumination and light collection system comprising: a light transmission system according to claim 1; an optical head, optically coupled to the light transmission system, and configured for guiding excitation light to a sample from which Raman light is consequently emitted, for collecting the Raman light and directing the collected Raman light along an optical collection path, for filtering out the excitation light from the optical collection path, and for coupling the collected Raman light in the optical collection path into the light transmission system.

11. The illumination and light collection system according to claim 10, wherein the optical head further comprises a collimating reflector arranged to collect the Raman light emitted from a sample into a hemisphere with a solid angle of 2π srad and directing the collected Raman light along the optical collection path.

12. The illumination and light collection system according to claim 11, wherein the collimating reflector is a compound parabolic concentrator, said compound parabolic concentrator being hollow and coated with a material of high reflectivity for at least one wavelength range.

13. The illumination and light collection system according to claim 11, wherein the optical head further comprises projection optics arranged for projecting the Raman light, collected by the collimating reflector, along the optical collection path and for coupling the collected Raman light into the light transmission system, such that a divergence angle of the collected Raman light, when reaching the light transmission system, is smaller than or equal to an acceptance angle of the light transmission system.

14. The illumination and light collection system according to claim 10, wherein the light transmission system further comprises a bundle of optical fibers, each optical fiber of the bundle of optical fibers comprising a transmitting end, wherein the transmitting ends of the optical fibers are arranged in a configuration adapted to a shape of the entrance surface such that light from each optical fiber is received by the homogenizer at the entrance surface, wherein the optical head further comprises at least one filter set for filtering out excitation light from the optical collection path, the at least one filter set comprising an interference filter configured to block excitation light having an angle of incidence, with respect to a normal of the interference filter, smaller than or equal to at least an acceptance angle of the optical fibers.

15. The illumination and light collection system according to claim 10, wherein the optical head is configured for guiding excitation light such that excitation light reaches the sample from a side being the same as a side from which Raman light is collected.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0075] The above, as well as additional objects, features and advantages of the present inventive concept, will be better understood through the following illustrative and non-limiting detailed description, with reference to the appended drawings. In the drawings like reference numerals will be used for like elements unless stated otherwise.

[0076] FIGS. 1a-1b illustrate a light transmission system for delivering light to a Raman spectrometer, the light transmission system comprising a homogenizer.

[0077] FIG. 2 illustrates a light transmission system for delivering light to a Raman spectrometer, the light transmission system comprising a homogenizer and a bundle of optical fibers.

[0078] FIG. 3 illustrates an illumination and light collection system for a Raman spectrometer, the system comprising an optical head and a light transmission system.

[0079] FIG. 4 illustrates the details of the optical head for exciting a sample, the sample consequently emitting Raman light, and for collecting and coupling the Raman light into the light transmission system delivering light to the Raman spectrometer.

[0080] FIGS. 5a-5b illustrate an alternative homogenizer.

DETAILED DESCRIPTION

[0081] In cooperation with attached drawings, the technical contents and detailed description of the present inventive concept are described thereinafter according to a preferable embodiment, being not used to limit the claimed scope. This inventive concept may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the inventive concept to the skilled person.

[0082] FIG. 1a illustrates a light transmission system 100 for delivering light to a Raman spectrometer (not illustrated in the present figure), according to an embodiment of the inventive concept. The light transmission system 100 comprises a homogenizer 110 of a solid, transparent material. The homogenizer 110 comprises an entrance surface 112 and an exit surface 114. The homogenizer defines an optical path in a direction of a central axis A of the homogenizer, from the entrance surface 112 to the exit surface 114. Further, the homogenizer 110 comprises four side surfaces 116.

[0083] Light entering the homogenizer 110 through the entrance surface 112 may propagate to the exit surface 114 of the homogenizer 110, either passing directly from the entrance surface 112 to the exit surface 114, or via one or more reflections in the side surfaces 116. In the present embodiment reflections in the side surfaces 116 rely on total internal reflection of light. However, to a person skilled in the art it is equally conceivable that reflections in the side surfaces 116 may be a result of a reflective coating provided on the side surfaces 116 or a combination of the above.

[0084] In the present embodiment, the width of the entrance surface 112 is equal to the width of the exit surface 114. However, the height of the entrance surface 112 is smaller than the height of the exit surface 114. In other words, the height of the homogenizer increases when following the propagation of light along the central axis A.

[0085] FIG. 1b illustrates a side-view of the homogenizer 110 of the light transmission system 100. Light entering the homogenizer 110 at the entrance surface 112 has an entrance divergence angle θ.sub.1 in the height direction, and light exiting the homogenizer 110 at the exit surface 114 has an exit divergence angle θ.sub.2 in the height direction. As a consequence of the increasing height of the homogenizer 110 along the direction of light propagation through the homogenizer 110, the divergence is reduced such that the exit divergence angle θ.sub.2 in the height direction is smaller than the entrance divergence angle θ.sub.1 in the height direction. The light exiting the homogenizer 110 is to be delivered to a Raman spectrometer. By the reduction of divergence described herein, it may be ensured that the divergence angle of light delivered to the Raman spectrometer falls within an acceptance angle of a detector in the Raman spectrometer such that most, and possibly all, Raman light is coupled to the detector in the Raman spectrometer.

[0086] FIG. 2 illustrates a light transmission system 200 for delivering light to a Raman spectrometer (not illustrated in the present figure), according to an embodiment of the inventive concept. The light transmission system 200 comprises the homogenizer 110 and a bundle 210 of optical fibers 220. Each of the optical fibers 220 in the bundle 210 comprise a receiving end 222 and a transmitting end 224.

[0087] In the present embodiment, the receiving ends 222 of the optical fibers 220 are arranged in a circular configuration forming a circular receiving end 212 of the bundle 210. The receiving end 212 of the bundle 210 is configured to match a cross-sectional shape of Raman light from an optical head in which Raman light is collected and projected towards the receiving end 212 of the bundle 210, such that all Raman light projected towards the receiving end 212 falls within the circular configuration of the ends 222 of the optical fibers 220, whereby all Raman light may potentially be coupled into the bundle 210 of optical fibers 220.

[0088] In the present embodiment, the transmitting ends 224 of the optical fibers 220 are arranged in a rectangular configuration forming a rectangular transmitting end 214 of the bundle 210. The transmitting end 214 of the bundle 210 is configured to match the shape of the entrance surface 112 of the homogenizer 110, such that all the transmitting ends 224 of the optical fibers 220 are arranged within the shape of the entrance surface 112. In the present manner all Raman light may potentially be coupled from the bundle 210 of optical fibers 220 into the homogenizer 110.

[0089] In the present arrangement, the ends 224 of the optical fibers 220 are attached to the entrance surface 112 of the homogenizer 110 by means of a glue with a refractive index matching that of the materials of the optical fibers 220 and the homogenizer 110. However, also other means for arranging the transmitting ends 224 of the optical fibers 220 in alignment with the entrance surface 112 of the homogenizer 110 are conceivable, such as mechanically clamping the parts together.

[0090] By the present arrangement the distribution of Raman light may be reshaped between the optical head and the Raman spectrometer. In this manner, a more flexible light transmission system 200 may be provided, since it allows for combinations of optical heads and Raman spectrometers to be made, that would otherwise not be possible due to mismatch of optical interfaces intended for coupling light between the two units.

[0091] FIG. 3 illustrates an illumination and light collection system 300 for a Raman spectrometer 20, according to an embodiment of the inventive concept. The illumination and light collection system 300 comprises an optical head 400 and a light transmission system 200. The light transmission system 200 comprises a bundle 210 of optical fibers 220 and a homogenizer 110. In the present example, the exit surface 114 of the homogenizer is optically coupled to a Raman spectrometer 20, the Raman spectrometer 20 comprising a beam shaper 21 and a spectrometer chip 22.

[0092] The optical head 400 is configured for illuminating a sample with excitation light and the sample will consequently em it Raman light. The details of the optical head 400 will be described in relation to FIG. 4.

[0093] The Raman light is coupled from the optical head 400 into circular receiving end 212 of the bundle 210 of optical fibers 220, in which the Raman light propagates through the optical fibers 220 to the rectangular transmitting end 214 of the bundle 210 of optical fibers 220. At the transmitting end 224 the Raman light is coupled to the entrance surface 112 of the homogenizer 110 and continues propagation through the homogenizer 110 to the exit surface 114.

[0094] In the present arrangement, the Raman spectrometer 20 is of the type described in EP2913663, comprising a beam shaper 21 for distributing collimated light over a discrete number of line shaped fields, and a spectrometer chip 22 wherein the spectrometer chip 22 is adapted for processing the light in a discrete number of line shaped fields coming from the beam shaper 21. The homogenizer 110 is made of a material with an index of refraction matching that of the beam shaper 21, in order to minimize reflection and refraction in the interface between the components. Further, the exit surface 114 is attached to the first surface 21a of the beam shaper by means of a glue with a refractive index matching that of the materials of the homogenizer 110 and the beam shaper 21. However, also other means for arranging the entrance surface 112 of the homogenizer 110 in alignment and optically coupled to the first surface 21a of the beam shaper 21 are conceivable, such as mechanically clamping the parts together.

[0095] The Raman light from the homogenizer 110 reaches the Raman spectrometer 20 with a high angle of incidence with respect to a normal of the surface of the spectrometer chip 22. In the present arrangement, the angle of incidence is typically in the range of 80-85%, however, another angle of incidence within another range is also conceivable. From the exit surface 114 of the homogenizer 110, the Raman light is transmitted to the Raman spectrometer 20. The exit surface 114 of the homogenizer 110 is optically coupled to a first surface 21a of the beam shaper 21 such that the Raman light in the homogenizer 110 propagates though the first surface 21a and into the beam shaper 21.

[0096] Depending on the direction of individual light rays, Raman light may reach the third surface 21c of the beam shaper 21 either by propagating directly from the first surface 21a to the third surface 21c, or by propagating from the first surface 21a to a second surface 21b in which light is reflected to propagate to the third surface 21c. The third surface 21c is facing the spectrometer chip 22, and it is through the third surface 21c that the Raman light reaches the spectrometer chip 22. The third surface 21c is not a planar surface, but presents a saw-tooth-like structure configured for distributing Raman light over a discrete number of line shaped fields as the Raman light leaves the beam shaper 21. The spectrometer chip 22 is adapted for processing the Raman light in a discrete number of line shaped fields coming from the beam shaper 21, in that the spectrometer chip comprises a plurality of detectors arranged along a number of lines on the spectrometer chip 22. The Raman spectrometer is described in further detail in EP2913663, which is incorporated herein by reference.

[0097] The detectors on the spectrometer chip 22 present a limited acceptance angle within which the detectors may receive, and thus detect, light. Further, the acceptance angle in the direction along a line of detectors may be different from the acceptance angle in the direction across a line of detectors. In the present example, the acceptance angle across a line of detectors is +/−7.5 degrees (in air), and the acceptance angle along a line of detectors is +/−30 degrees. However, due to the symmetry of the optical fibers 220, the Raman light transmitted out of the transmitting end 214 of the bundle 210 of optical fibers 220 is 24 degrees in all directions, in the present example. Hence, it is of importance that the homogenizer 110 efficiently reduces the divergence angle of the Raman light from the bundle 210 of fibers 220 in at least one direction. The direction across a line of detectors correspond to the height direction of the homogenizer 110 in the present example, and the homogenizer is thus configured to reduce the divergence angle in the height direction from 24 degrees to at least 7.5 degrees, in order for the detectors to be able to receive and detect all Raman light. By way of example, in order for the homogenizer to reduce the divergence by a factor of 4, the exit width of the exit surface 114 should be 4 times larger than the entrance height of the entrance surface 112.

[0098] In case where the cross-section of the output of the optical head 400 and the input of the Raman spectrometer 20 have different geometrical shapes, it is also important that the Raman light field is reshaped, to avoid losses. In the present example, the optical head 400 has a circular output, whereas the Raman spectrometer 20 has a rectangular input. By means of the light transmission system 200 comprising a bundle 210 of optical fibers 220 with a circular receiving end 212 and a rectangular transmitting end 214, the difference is bridged such that the Raman light field is reshaped to fit the Raman spectrometer 20 without significant losses of Raman light.

[0099] FIG. 4 illustrates the details of the optical head 400 for exciting a sample 10, the sample consequently emitting Raman light, and for collecting and coupling the Raman light into the light transmission system 200 delivering light to the Raman spectrometer 20, according to an embodiment of the inventive concept.

[0100] In the present embodiment, the source of excitation light is a laser and positioned external to the optical head 400. Excitation light from the laser is coupled into the optical head 400 via an optical fiber optically coupled to an excitation fiber input port 410. Excitation light from the input port is directed to a dichroic mirror 412. Excitation light is represented in FIG. 4 by dashed-line-arrows. The dichroic mirror is provided with a reflective coating configured to reflect light at the excitation wavelength and to transmit light at least at wavelengths longer than the excitation wavelength, such that Stokes shifted Raman light may efficiently be transmitted through the dichroic mirror.

[0101] In the present example, the excitation light has a wavelength of 785 nm. With an excitation wavelength of 785 nm, the wavelength range for detection of Raman light is approximately 830 nm to 920 nm. The reflective coating on the dichroic mirror 412 is thus configured to efficiently transmit light in at least this wavelength range. The wavelengths mentioned here are examples of a specific embodiment, however it is realized by a person skilled in the art that also other excitation wavelengths may be used, and therefore also other ranges of detection may be possible.

[0102] The excitation light reflected by the dichroic mirror 412 is directed into a collimating reflector 420 comprising a small entrance 422 and a large exit 424. The excitation light is directed through the large exit 424 and the small entrance 422 and through a transparent front glass 430 of the optical head 400. A sample 10 is positioned on the other side of the front glass 430, such that the sample is illuminated by the excitation light. Subsequently to being illuminated by excitation light, the illuminated part of the sample 10 emits Raman light in substantially all directions. Raman light is here represented by solid-line-arrows.

[0103] In the present example, the collimating reflector 420 is a compound parabolic concentrator. The collimating reflector 420 is a hollow optical component provided with a reflective coating with high reflectivity within at least the wavelength range to be detected. By way of example, a reflective coating of Gold may be provided on the collimating reflector 420, however, also other materials are conceivable for providing a high reflective coating. The collimating reflector 420 has a large acceptance angle at the small entrance 422 and a smaller angular divergence at the large exit 424, such that the Raman light emitted from the sample 10 into a hemisphere with a solid angle of 2π srad may be collected by the collimating reflector 420 through the small entrance 422. The collected Raman light is subsequently collimated to a smaller angular divergence and directed out through the large exit 424 and through the dichroic mirror 412 along an optical collection path. The collimating reflector is configured such that the Raman light exiting the large exit 424 has a divergence angle which is not larger than the acceptance angle of receiving ends 222 of the optical fibers 220. The acceptance angle of the receiving ends 222 of the fibers 220 is typically not larger than 30 degrees, and in the present example the acceptance angle is 24 degrees. The present embodiment provides an effective manner of collecting Raman light from a large solid angle, thereby ensuring a good signal-to-noise ratio.

[0104] The optical head further comprises projection optics 440, in the present example being a first lens 442 and a second lens 444. The projection optics image the large exit 424 of the collimating reflector 420 onto the receiving end 212 of the bundle 210 of optical fibers 220. The combination of the collimating reflector 420 and the projection optics 440 ensures that Raman light is efficiently collected and coupled into the bundle 210 of optical fibers 220.

[0105] In the optical collection path from the collimating reflector 420 to the receiving end 212 of the bundle 210 of optical fibers 220 is also arranged a first filter set 452 and a second filter set 454. Each of the filter sets 452, 454 comprise an interference filter configured to block light at the excitation wavelength reaching the filter with an angle of incidence, with respect to a normal of the interference filter, smaller than or equal to at least the acceptance angle of the optical fibers 220. Excitation light in the optical collection path may originate from reflections of excitation light in optical components, in inner walls of the optical head 400 or from light scattering off of the sample 10. By the combination of optical fibers 220 and interference filters configured to block excitation light of at least all angles up to the acceptance angle of the optical fibers 220, an effective manner for filtering out excitation light from the optical path of Raman light may be provided.

[0106] FIG. 5a illustrates a homogenizer 510, according to an embodiment of the inventive concept. The homogenizer 510 is of a solid, transparent material. The homogenizer 510 comprises an entrance surface 512 and an exit surface 514. The homogenizer defines an optical path in a direction of a central axis A of the homogenizer, from the entrance surface 512 to the exit surface 514. Further, the homogenizer 510 comprises four side surfaces 516.

[0107] Light entering the homogenizer 510 through the entrance surface 512 may propagate to the exit surface 514 of the homogenizer 510, either passing directly from the entrance surface 512 to the exit surface 514, or via one or more reflections in the side surfaces 516. In the present embodiment reflections in the side surfaces 516 rely on total internal reflection of light. However, it is realized by a person skilled in the art that reflections in the side surfaces 516 may be a result of a reflective coating provided on the side surfaces 516.

[0108] Similarly to what has been previously described for homogenizer 110, the height of the homogenizer 510 increases when following the propagation of light along the central axis A, resulting in a smaller divergence angle in the height direction for light at the exit surface 514 compared to at the entrance surface 512. Additionally, the width of the entrance surface 512 is larger than the width of the exit surface 514. In other words, the width of the homogenizer decreases when following the propagation of light along the central axis A.

[0109] FIG. 5b illustrates a top-view of the homogenizer 510. Light entering the homogenizer 510 at the entrance surface 512 has an entrance divergence angle θ.sub.3 in the width direction, and light exiting the homogenizer 510 at the exit surface 514 has an exit divergence angle θ.sub.4 in the width direction. As a consequence of the decreasing width of the homogenizer 510 along the direction of light propagation through the homogenizer 510, the divergence is increased such that the exit divergence angle θ.sub.4 in the width direction is larger than the entrance divergence angle θ.sub.3 in the width direction. By replacing the homogenizer 110 by the homogenizer 510 in any of the previously described light transmission systems 100, 200, a light transmission system that utilizes a larger portion of the acceptance angle in the width direction of the detectors on the spectrometer chip 22 may be provided.

[0110] In the above the inventive concept has mainly been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.