Spectroscopic apparatus using spectrum narrowed and stabilized laser with Bragg grating

• US 20050105084A1
• Filed: 11/12/2004
• Published: 05/19/2005
• Est. Priority Date: 11/13/2003
• Status: Active Grant

• Current Assignee: Metrohm Spectro Incorporated
• Original Assignee: BWT Property, Inc.
• Inventors: Wang, Sean Xiaolu, Zhou, Xin Jack
• Granted Patent: US 7,245,369 B2
• Time in Patent Office: Days
• US Class Current: 356/301
• CPC Class Codes: G01J 3/02 Details
• CPC Class Codes: G01J 3/0205 Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
• CPC Class Codes: G01J 3/0208 using focussing or collimating elements, e.g. lenses or mirrors; performing aberration correction
• CPC Class Codes: G01J 3/0218 using optical fibers
• CPC Class Codes: G01J 3/0227 using notch filters
• CPC Class Codes: G01J 3/10 Arrangements of light sources specially adapted for spectrometry or colorimetry
• CPC Class Codes: G01J 3/1895 using fiber Bragg gratings or gratings integrated in a waveguide
• CPC Class Codes: G01J 3/433 Modulation spectrometry; Derivative spectrometry
• CPC Class Codes: G01J 3/44 Raman spectrometry; Scattering spectrometry ; Fluorescence spectrometry
• CPC Class Codes: G01N 21/65 Raman scattering
• CPC Class Codes: G02F 1/3542 Multipass arrangements, i.e. arrangements to make light pass multiple times through the same element, e.g. using an enhancement cavity
• CPC Class Codes: H01S 3/109 Frequency multiplication, e.g. harmonic generation
• CPC Class Codes: H01S 5/0092 for nonlinear frequency conversion, e.g. second harmonic generation [SHG] or sum- or difference-frequency generation outside the laser cavity
• CPC Class Codes: H01S 5/141 using a wavelength selective device, e.g. a grating or etalon H01S5/146 takes precedence
• CPC Class Codes: H01S 5/146 using a fiber as external cavity
• CPC Class Codes: H01S 5/147 having specially shaped fibre, e.g. lensed or tapered end portion
• CPC Class Codes: H01S 5/2036 Broad area lasers
• CPC Class Codes: H01S 5/4012 Beam combining, e.g. by the use of fibres, gratings, polarisers, prisms

Abstract

An apparatus for measuring properties of physical matters by means of Raman spectroscopy including a laser element, a wavelength dispersion element, an array or single element detector, and a control and data processing unit. The laser element, which is used to excite Raman scattering, is spectrum narrowed and stabilized by attachment of a Bragg grating device. The grating can be either a volume Bragg grating (VBG) written inside a glass substrate or a fiber Bragg grating (FBG) written inside an optical fiber. A laser element can be provided with a wavelength modulation capability for fluorescence background suppression.

Claims

1. An apparatus for Raman spectroscopy of physical matter, the apparatus comprising:

(a) a spectrum narrowed and stabilized laser element for generating a laser beam and for causing the laser beam to enter the physical matter to stimulate Raman scattering in the physical matter and thereby to produce a Raman signal, the spectrum narrowed and stabilized laser element comprising a Bragg grating;

(b) an optical filter, disposed in a path of the Raman signal, for removing Rayleigh scattering from the Raman signal to produce a filtered Raman signal;

(c) an optical spectrometer, disposed in a path of the filtered Raman signal, for measuring the spectrum of the Raman signal and for generating a detection signal; and

(d) a microprocessor, connected to receive the detection signal, for determining properties of the physical matter from the detection signal.

2. The apparatus of claim 1, wherein the laser element comprises a diode laser.

3. The apparatus of claim 1, wherein the laser element comprises a solid-state laser.

4. The apparatus of claim 1, wherein the laser element comprises a gas laser.

5. The apparatus of claim 1, wherein the laser element comprises a fiber laser.

6. The apparatus of claim 1, wherein the Bragg grating is a volume Bragg grating.

7. The apparatus of claim 1, wherein the Bragg grating is a fiber Bragg grating.

8. The apparatus of claim 1, wherein the Bragg grating functions as a band-stop filter.

9. The apparatus of claim 1, wherein the Bragg grating functions as a band-pass filter.

10. The apparatus of claim 2, wherein the diode laser is spectrum narrowed and stabilized using said Bragg grating in external cavity configuration.

11. The apparatus of claim 10, wherein the Bragg grating is a volume Bragg grating.

12. The apparatus of claim 10, wherein the Bragg grating is a fiber Bragg grating.

13. The apparatus of claim 12, wherein the fiber Bragg grating exists in one arm of an optical fiber coupler, and wherein the other arm of said fiber coupler is used for laser output.

14. The apparatus of claim 2, wherein the diode laser is spectrum narrowed and stabilized using said Bragg grating in self-seeded injection locking configuration.

15. The apparatus of claim 3, wherein the solid-state laser is formed by disposing a nonlinear optical medium after a Bragg grating enhanced diode laser for frequency conversion.

16. The apparatus of claim 15, wherein the nonlinear optical medium is a uniform optical crystal.

17. The apparatus of claim 15, wherein the nonlinear optical medium has a periodic structure for quasi-phase matching.

18. The apparatus of claim 3, wherein the solid-state laser is formed by inserting a nonlinear optical medium between the diode laser and the Bragg grating for frequency conversion.

19. The apparatus of claim 1, wherein the Bragg grating possesses a single resonant wavelength.

20. The apparatus of claim 1, wherein the Bragg grating possesses a plurality of resonant wavelengths.

21. The apparatus of claim 1, wherein the optical spectrometer comprising:

(a) a wavelength dispersion element, disposed in a path of the filtered Raman signal, for breaking down the filtered spectrum into wavelength or wave number components containing a Raman shifted signal with a dispersed Raman shift spectrum; and

(b) a photo detector, disposed in a path of the Raman shifted signal, for detecting the dispersed Raman shift spectrum and for generating a detection signal.

22. The optical spectrometer of claim 21, wherein the dispersion element comprises a grating.

23. The optical spectrometer of claim 21, wherein the dispersion element comprises a prism.

24. The optical spectrometer of claim 21, wherein the detector comprises an array photo detector.

25. The optical spectrometer of claim 21, wherein the photo detector comprises a single element photo detector.

26. The apparatus of claim 1, wherein the optical spectrometer is a Fourier transform spectrometer.

27. The apparatus of claim 1, wherein the optical spectrometer is a fixed filter based spectrometer.

28. The apparatus of claim 1, wherein the optical spectrometer is a discrete filter set based spectrometer.

29. The apparatus of claim 1, wherein the optical spectrometer is a tunable filter based spectrometer.

30. The apparatus of claim 1, wherein the optical spectrometer is an optical transform based spectrometer.

31. The apparatus of claim 1, comprising a plurality of said laser elements.

32. The apparatus of claim 31, wherein the multiple laser elements have the same wavelength to achieve higher output power.

33. The apparatus of claim 31, wherein the multiple laser elements have different wavelengths.

34. The apparatus of claim 33, being used to achieve better signal to noise ratio.

35. The apparatus of claim 33, being used to eliminate background noise.

36. The apparatus of claim 33, being used for multi-dimensional spectroscopic analysis.

37. The apparatus of claim 31, further comprising a multiplexing element for multiplexing outputs of the lasers to form the laser beam.

38. The apparatus of claim 37, wherein the multiplexing element comprises a lens.

39. The apparatus of claim 37, wherein the multiplexing element comprises a polarizing combination.

40. The apparatus of claim 37, wherein the multiplexing element comprises a wavelength division multiplexer.

41. The apparatus of claim 37, wherein the multiplexing element comprises a fiber bundle.

42. An apparatus for Raman spectroscopy of physical matter, the apparatus comprising:

(a) a Bragg grating enhanced super luminescence diode element for emitting light to be incident on the physical matter to stimulate Raman scattering in the physical matter and thereby to produce a Raman signal;

(b) an optical filter, disposed in a path of the Raman signal, for removing Rayleigh scattering from the Raman signal to produce a filtered Raman signal;

(c) an optical spectrometer, disposed in a path of the filtered Raman signal, for measuring the spectrum of the Raman signal and for generating a detection signal; and

(d) a microprocessor, connected to receive the detection signal, for determining properties of the physical matter from the detection signal.

43. The apparatus of claim 42, wherein the Bragg gratings functions as a spectrum slicing filter.

44. A method for fluorescence suppression in Raman spectroscopy of a physical matter, comprising the steps of:

(a) providing a multi-wavelength laser array by combining multiple Bragg grating enhanced lasers with different wavelengths to produce outputs;

(b) shining the outputs of the lasers sequentially at certain frequency onto the physical matter to excite Raman scattering;

(c) providing a Raman spectrometer to detect the excited Raman scattering and to generate a detection signal; and

(d) amplifying/filtering the detection signal using a frequency selective amplification/filtering method tuned to said excitation frequency.

45. The method of claim 44, wherein the lasers are diode lasers.

46. The method of claim 44, wherein the lasers are solid-state lasers.

47. The method of claim 44, wherein the lasers are fiber lasers.

48. The method of claim 44, wherein step (d) is performed using a lock-in amplifier.

49. The method of claim 44, wherein step (d) is performed using a digital filtering technique.