Apparatus For Depth-Selective Raman Spectroscopy

• US 20080129992A1
• Filed: 11/25/2005
• Published: 06/05/2008
• Est. Priority Date: 12/09/2004
• Status: Active Grant

• Current Assignee: Agilent Technologies Lda Uk Limited (Agilent Technologies, Inc.)
• Original Assignee: Science and Technology Facilities Council
• Inventors: Matousek, Pavel, Parker, Anthony William
• Granted Patent: US 7,652,763 B2
• Time in Patent Office: Days
• US Class Current: 356/301
• CPC Class Codes: A61B 5/0059 using light, e.g. diagnosis by transillumination, diascopy, fluorescence A61B5/0093, A61B5/1455, A61B5/02416 and A61B5/0261 take precedence
• CPC Class Codes: A61B 5/0071 by measuring fluorescence emission
• CPC Class Codes: A61B 5/0075 by spectroscopy, i.e. measuring spectra, e.g. Raman spectroscopy, infrared absorption spectroscopy A61B5/0071 takes precedence
• CPC Class Codes: A61B 5/441 Skin evaluation, e.g. for skin disorder diagnosis
• CPC Class Codes: A61B 5/4504 Bones A61B5/4547 takes precedence
• CPC Class Codes: G01N 2021/656 Raman microprobe
• CPC Class Codes: G01N 21/65 Raman scattering

Abstract

Apparatus and methods for detecting Raman spectral features non destructively from sub-surface regions of a diffusely scattering sample are disclosed. Incident radiation is supplied at one or more sample surface entry regions, and light is collected from one or more collection regions spaced from the entry regions. Raman features are detected in the collected light, and depth information is derived according to the entry-collection spacings.

Claims

1. A method of measuring sub-surface Raman scattering through a diffusely scattering sample, the method comprising steps of:

(a) supplying incident radiation at an entry region on a surface of the sample;

(b) collecting light scattered within the sample, from a collection region on the surface, the collection region being spaced from the entry region; and

(c) detecting, in the collected light, one or more Raman features spectrally related to the incident radiation.

2. The method of claim 1 further comprising deriving, from the one or more Raman features, one or more characteristics of a sub-surface region of the sample.

3. The method of claim 1 wherein detecting comprises spectrally dispersing the collected light to form a Raman spectrum.

4. The method of claim 2 wherein the step of collecting comprises collecting light at a plurality of spacings between an entry region and a respective collection region.

5. The method of claim 4 wherein the step of deriving includes associating the Raman features from different spacings with different depths or distributions of depth within the sample.

6. The method of claim 5 further comprising combining the Raman features from different spacings to preferentially select for a particular depth or range of depths.

7. The method of claim 1 wherein the step of collection comprises collecting light from a plurality of collection regions spaced by different distances from an entry region.

8. The method of claim 1 wherein one or more collection regions surround an entry region.

9. The method of claim 8 wherein each collection region is an annulus, and for each annulus, light is collected from at least 10%, and more preferably at least 25% of the surface area of the annulus.

10. The method of claim 8 wherein the scattered light is collected by a plurality of collection optical fibres distributed around one or more illumination optical fibres used to supply the incident radiation to the entry region.

11. The method of claim 1 wherein one or more entry regions surround a collection region.

12. The method of claim 1 further comprising adjusting collection optics disposed in the path of the collected light to adjust the distance between a collection region and an entry region.

13. The method of claim 1 further comprising disposing one or more mirror elements adjacent to the sample surface outside the collection regions to reflect light back into the sample.

14. A method of measuring a sub-surface Raman spectrum of a diffusely-scattering sample, the method comprising the steps of:

a) irradiating the sample with a light probe;

b) collecting light scattered by the sample; and

c) spectrally separating at least a portion of the collected light to detect one or more Raman spectral features,

wherein light scattered by the sample is collected from a plurality of spatial locations on the surface of the sample, each spatial location being at a different distance from the point of irradiation, at least a portion of the light collected at each spatial location being separately spectrally dispersed to form a plurality of Raman spectra and wherein the method further includes the step of;

d) analysing the plurality of Raman spectra to extract information on the Raman spectrum of a sub-surface region of the sample.

15. A method as claimed in claim 14, wherein at least two Raman spectra are collected and are analysed using a scaled subtraction, the Raman spectrum collected at a distance closest to the point of irradiation being subtracted from the Raman spectrum collected further from the point of irradiation, whereby features of the Raman spectrum for a sub-layer of the sample are identified.

16. A method as claimed in claim 14, wherein the Raman spectrum for the chemical composition of the surface of the sample is known and the Raman spectra are analysed by scaled subtraction of the known Raman spectrum from the Raman spectra of the collected light.

17. A method as claimed in claim 14, wherein the plurality of Raman spectra are analysed using multivariate data analysis.

18. A method as claimed in claim 17, wherein the plurality of Raman spectra are analysed using principal component analysis.

19. A method as claimed in claim 17, wherein at least twenty Raman spectra are collected at different distances from the point of irradiation.

20. A method as claimed in claim 14, wherein the sample is irradiated at two or more different wavelengths and the collected light is a combination of a Raman spectrum and fluorescence and wherein the method comprises the further step of extracting the Raman spectrum from the collected light.

21. A method as claimed in claim 14, wherein at least one of the sample, the collection optics, and the point or region of irradiation is moved relative to the others to enable the collection of Raman spectra at different distances from the point of irradiation.

22. A method as claimed in claim 21, wherein a movable stage is provided on which the sample is mounted and the probe beam is arranged to track the movement of the sample whereby the sample is moved relative to fixed collection optics for the collection of scattered light at different distances from the point of irradiation.

23. A method as claimed in claim 22, wherein the scattered light is collected from point regions at different distances from the point of irradiation.

24. A method as claimed in claim 22, wherein the scattered light is collected from a plurality of substantially parallel lines substantially transverse to the distance as measured from the point of irradiation.

25. A method as claimed in claim 14, wherein the probe beam is supplied using optical fibres and the scattered light is collected using optical fibres arranged in a plurality of concentric circles around the probe beam optical fibres whereby the scattered light is collected in concentric rings at differing radii from the point of irradiation.

26. A method as claimed in claim 14, wherein the collected light is spectrally dispersed using a spectrometer in combination with a CCD camera.

27. A method as claimed in claim 14, wherein the light probe or incident radiation is generated using a cw laser.

28. A method as claimed in claim 21, wherein the light probe comprises two or more separate wavelengths and is generated by one or more lasers.

29. A method as claimed in claim 28, wherein the light probe is generated by a single tunable laser.

30. A method as claimed in claim 28, wherein the two or more separate wavelengths of the light probe are generated by two or more respective lasers.

31. A method as claimed in claim 27, wherein the light probe provides radiation at a wavelength of >

200 nm and <

2000 nm.

32. Apparatus for measuring sub-surface Raman scattering through a diffusively scattering sample, comprising:

a light source arranged to supply incident radiation at an entry region on a surface of the sample;

a collector arranged to collect light scattered within the sample, from a collection region on the surface, the collection region being spaced from the entry region; and

a detector arranged to detect one or more Raman features spectrally related to the incident radiation.

33. The apparatus of claim 32 further comprising an analyzer adapted to derive, from the Raman features, one or more characteristics of a sub-surface region of the sample.

34. The apparatus of claim 33 arranged to provide multiple spacings between the incident and collected light, the analyzer being adapted to combine the Raman features from said multiple spacings to preferentially select the features for a particular depth or range of depths.

35. The apparatus of claim 32 wherein the detector comprises a spectrometer arranged to spectrally disperse the collected light to separate out said Raman features.

36. The apparatus of claim 32 wherein the detector comprises one or more filters arranged to selectively transmit said Raman features.

37. The apparatus of claim 32 wherein either the entry region or the collection region is an annular region.

38. The apparatus of claim 36 further comprising an optics arrangement adapted to controllably adjust the diameter of the annular region.

39. The apparatus of claim 32 further comprising a masking device arranged to controllably adjust the spacing between the entry region and the collection region.

40. The apparatus of claim 32 further comprising one or more mirror elements disposed adjacent to the sample surface outside the one or more collection and entry regions so as to reflect light back into the sample.

41. Apparatus for selectively measuring Raman spectra generated at different depths within a diffusely-scattering medium, the apparatus comprising:

a light source for irradiating a sample with a probe beam;

collection optics for collecting light scattered by the sample and passing it to a spectrometer;

detection means for detecting light dispersed by the spectrometer,

wherein the apparatus is adapted for scattered light to be collected at a plurality of spatial locations on the surface of the sample, each spatial location being at a different distance from the point of irradiation and at least a portion of the light collected at each spatial location being separately spectrally dispersed by the spectrometer to form a plurality of Raman spectra and wherein the apparatus further includes an analyser for identifying features specific to the Raman spectrum of a sub-layer of the sample from the plurality of Raman spectra.

42. Apparatus as claimed in claim 41, wherein the analyser is adapted to perform a scaled subtraction between Raman spectra.

43. Apparatus as claimed in claim 41, wherein the analyser is adapted to perform multivariate data analysis on the Raman spectra.

44. Apparatus as claimed in claim 43, wherein the analyser is adapted to perform principal component analysis on the Raman spectra.

45. Apparatus as claimed in claim 41, further comprising a movable stage for relative movement of at least one of the sample, the collection optics and the point of irradiation to enable the collection of Raman spectra at different distances from the point of irradiation.

46. Apparatus as claimed in claim 45, wherein the movable stage is a movable sample stage and wherein means are provided for tracking the probe beam with respect to movement of the sample whereby the sample may be moved relative to the fixed collection optics to enable scattered light to be collected at a plurality of distances from the point of irradiation.

47. Apparatus as claimed in claim 41, wherein the collection optics comprises optical fibres arranged in a plurality of concentric circles around the probe beam.

48. Apparatus as claimed in claim 41, wherein the light source is a cw laser.

49. Apparatus as claimed in claim 41, wherein the detection means comprises a CCD camera.