Fiber optic refractive index monitor

• US 6,356,675 B1
• Filed: 12/01/1995
• Issued: 03/12/2002
• Est. Priority Date: 12/01/1995
• Status: Expired due to Term

• Current Assignee: National Technology & Engineering Solutions Of Sandia LLC (Honeywell International Inc.)
• Original Assignee: Sandia Corporation (Martin Marietta Corporation)
• Inventors: Weiss, Jonathan David
• Primary Examiner(s): Sanghavi, Hemang
• Application Number:
  US08/566,340
• Time in Patent Office: 2,293 Days
• Field of Search: 385/12, 385/13, 356/128-137, 356/361, 250/227.11, 250/227.13, 250/227.14, 250/227.16, 250/227.25
• US Class Current: 385/12
• CPC Class Codes: G01N 21/431 Dip refractometers, e.g. using optical fibres
• CPC Class Codes: G02B 6/2817 using reflective elements to split or combine optical signals
• CPC Class Codes: G02B 6/4298 coupling with non-coherent light sources and/or radiation detectors, e.g. lamps, incandescent bulbs, scintillation chambers

Abstract

A sensor for measuring the change in refractive index of a liquid uses the lowest critical angle of a normal fiber optic to achieve sensitivity when the index of the liquid is significantly less than the index of the fiber core. Another embodiment uses a liquid filled core to ensure that its index is approximately the same as the liquid being measured.

Claims

1. A device for detecting a change Δ

n in the index of refraction n_l of a liquid between the values n_l and n_l −

Δ

n, comprising;

a short length of optical guide having refractive index n_g, said guide having a smooth exterior surface along its length covered by the liquid;

means for inputting a light beam into said guide,

the angle of the light beam with respect to said optical guide being selected so that light impinges on the interface between said exterior surface and the liquid at the lowest critical angle φ

_c of said guide, where φ

_c =sin<sup>−

1(n_l −

Δ

n)</sup>/_ng; and

means for detecting the change in light passing through said guide.

2. The device of claim 1 wherein said guide is straight.

3. The device of claim 2 wherein the input end of said guide is beveled and light is input along the axis of said guide.

4. The device of claim 3 wherein said means for inputting a light beam comprises a first optical fiber having one end aligned with said optical guide and another end receiving light from a light source;

and collimating means between said one end and said guide for collimating light entering said guide.

5. The device of claim 4 wherein said means for detecting the change in light passing through said guide comprises a second optical fiber having one end receiving light from said guide and another end for communicating light from said guide to a detector.

6. The device of claim 5 wherein said means for detecting further comprises means for reflecting light output of said guide to said second optical fiber.

7. The device of claim 1 wherein Δ

n <

(5%)(n_l).

8. The device of claim 7 where Δ

n <

(5%)(n_l).

9. A device for detecting a change Δ

n in the index of refraction n_l of a liquid between the values n_l and n_l −

Δ

n, comprising;

a short length of optical guide having a smooth exterior surface along its length covered by the liquid, said guide consisting of a thin walled container containing a liquid having an index of refraction =n_l;

means for inputting a light beam into said guide; and

means for detecting the change in light passing through said guide.

10. The device of claim 9 wherein said means for inputting includes an optical fiber having one end aligned with an input end of said guide.

11. The device of claim 10 wherein said container is a cylinder.

12. The device of claim 11 wherein the diameter of said cylinder is approximately the same as the diameter of said fiber.