Fiber optic laser-induced breakdown spectroscopy sensor for molten material analysis

• US 20030174325A1
• Filed: 03/18/2002
• Published: 09/18/2003
• Est. Priority Date: 03/18/2002
• Status: Active Grant

• Current Assignee: SNC Technologies, Inc. (General Dynamics Corporation), Mississippi State University
• Original Assignee: Mississippi State University
• Inventors: Yueh, Fang-Yu, Singh, Jagdish P., Zhang, Hansheng, Rai, Awadesh K.
• Granted Patent: US 6,762,835 B2
• Time in Patent Office: Days
• US Class Current: 356/318
• CPC Class Codes: G01J 3/443 Emission spectrometry
• CPC Class Codes: G01N 21/718 Laser microanalysis, i.e. with formation of sample plasma

Abstract

A fiber optic laser-induced breakdown spectroscopy (LIBS) sensor, including a laser light source, a harmonic separator for directing the laser light, a dichroic mirror for reflecting the laser light, a coupling lens for coupling the laser light at an input of a multimode optical fiber, a connector for coupling the laser light from an output of the multimode optical fiber to an input of a high temperature holder, such as a holder made of stainless steel, and a detector portion for receiving emission signal and analyzing LIBS intensities. In one variation, the multimode optical fiber has silica core and silica cladding. The holder includes optical lenses for collimating and focusing the laser light in a molten alloy to produce a plasma, and for collecting and transmitting an emission signal to the multimode optical fiber.

Claims

1. A laser-induced breakdown spectroscopy probe for producing plasma in a molten alloy and collecting emission from the plasma, the probe comprising:

a collimating lens to collimate a laser beam;

a focusing lens to focus the collimated beam on the molten alloy to produce the plasma; and

a holder for containing the collimating lens and the focusing lens;

wherein the holder includes a gas valve to adjust the amount of inlet flow of a gas in the holder.

2. The probe of claim 1, wherein the holder further comprises a plurality of tubes.

3. The probe of claim 2, wherein the tubes comprise stainless steel.

4. The probe of claim 2, wherein the tubes are connected to each other by male and female threads.

5. The probe of claim 1, wherein the holder includes a bottom piece for housing the focusing lens.

6. The probe of claim 1, wherein the holder includes an internal snap ring to prevent movement of the focusing lens.

7. The probe of claim 6, wherein the holder includes an upper portion, wherein the gas enters the holder via the upper portion, and wherein the tubes include passages that provide an outlet flow of nitrogen gas.

8. The probe of claim 7, wherein the flow of the gas through the passages cools the focusing lens and the snap ring and prevents the molten alloy from reaching the focusing lens surface.

9. The probe of claim 1, further comprising a flange to support the holder.

10. The probe of claim 9, wherein the flange comprises aluminum.

11. The probe of claim 1, wherein a flowmeter is connected to the gas valve.

12. The probe of claim 1, further comprising:

a flowmeter to control an inlet flow of the gas from the holder.

13. The probe of claim 1, wherein the gas comprises nitrogen.

14. The probe of claim 1, wherein the gas comprises a nitrogen-argon helium gas.

15. The probe of claim 1, wherein the gas comprises argon.

16. The probe of claim 1, wherein the gas comprises helium.

17. The probe of claim 12, wherein the molten alloy has a surface, and

wherein the inlet and outlet flows of the gas are adjusted so as to locate the molten alloy surface at a focal plane of the focusing lens.

18. The probe of claim 1, wherein the molten alloy comprises one selected from a group consisting of glass, steel, and aluminum.

19. The probe of claim 1, wherein the holder contains the collimating lens with a spiral lock ring that provides fine adjustment of a distance between an output end of the optical fiber and the collimating lens.

20. The probe of claim 1, wherein the holder has a bottom end, the probe further comprising:

a spacer connected to the bottom end of the holder, the spacer having an internal diameter that is equal to an outer diameter of the holder and a female thread that fits into the male thread of the holder, wherein the spacer provides distance adjustment between the focusing lens and the molten alloy surface to focus the laser beam at a selected location on the molten alloy surface.

21. The probe of claim 20, wherein the spacer comprises stainless steel.

22. The probe of claim 1, wherein the holder comprises stainless steel.

23. A fiber optic laser-induced breakdown spectroscopy sensor, comprising:

a laser light source generating laser light;

a harmonic separator for directing the laser light from the laser light source;

a dichroic mirror for reflecting the laser light from the harmonic separator;

a coupling lens for coupling the laser light at an input end of a multimode optical fiber;

a connector for coupling the laser light from an output end of the multimode optical fiber to an input end of a high temperature holder, the high temperature holder comprising optical lenses for collimating and focusing the laser light in a molten alloy to produce a plasma, and for collecting and transmitting an emission signal to the multimode optical fiber; and

a detector portion for receiving the emission signal and analyzing the laserinduced breakdown spectroscopy spectra intensities of the emission signal.

24. The sensor of claim 23, wherein the multimode optical fiber has a silica core and silica cladding

25. The sensor of claim 23, wherein the high temperature holder comprises stainless steel.

26. The sensor of claim 23, wherein the connector comprises stainless steel.

27. The sensor of claim 23, wherein the detector portion includes a detector, the detector being selected from a group consisting of a charge coupled detector and an intensified charge coupled detector.

28. The sensor of claim 23, wherein the dichroic mirror transmits wavelengths in a range of about 180-1,000 nm and reflects wavelengths in a range of about 500-540 nm.

29. The sensor of claim 23, wherein the laser light source is a neodymium yttrium-aluminum garnet laser source.

30. The sensor of claim 23, wherein the dichroic mirror is positioned at about a 45°

angle relative to a line between the detector portion and a sample to be analyzed.

31. The sensor of claim 23, wherein the coupling lens has a focal length of about 10 cm.

32. The sensor of claim 23, wherein the laser light source has a maximum pulse energy of about 180 mJ.

33. The sensor of claim 23, wherein the multimode optical fiber has a numerical aperture of about 0.16.

34. The sensor of claim 23, wherein the multimode optical fiber includes a first end and a second end, the sensor further comprising:

stainless steel fiber connectors located at the first end and the second end of the multimode optical fiber.

35. The sensor of claim 23, wherein the multimode optical fiber has a core diameter of about 1.0 mm.

36. The sensor of claim 23, wherein the optical lenses include a plano convex fused silica collimating lens having a focal length of about 10 cm.

37. The sensor of claim 36, wherein the optical lenses include a plano convex fused silica focusing lens having a focal length of about 7.62 cm.

38. The sensor of claim 37, wherein the high temperature holder further comprises an internal snap ring for preventing movement of the focusing lens.

39. The sensor of claim 37, wherein the collimating lens and the focusing lens are separated by at least 2 m.

40. The sensor of claim 23, wherein the high temperature holder includes a gas valve for controlling flow of a purging gas to cool the lens holder.

41. The sensor of claim 23, wherein the multimode optical fiber is used for transmitting the laser light to the molten alloy and for transmitting the emission signal to the detector portion.

42. The sensor of claim 23, further comprising:

a spacer connected to the bottom end of the high temperature holder.

43. The sensor of claim 23, further comprising:

a beam dump for absorbing the laser light passing through the harmonic separator.

44. The sensor of claim 23, wherein the detector portion includes at least one selected from a group consisting of a spectrometer, an intensified charged couple device, a controller, a pulse generator, and a processor.

45. The sensor of claim 44, wherein the processor is selected from a group consisting of a personal computer, a minicomputer, a microcomputer, and a main frame computer.