Optical grating sensor and method of monitoring with a multi-period grating

• US 4,985,624 A
• Filed: 02/09/1989
• Issued: 01/15/1991
• Est. Priority Date: 05/11/1988
• Status: Expired due to Term

• Current Assignee: Simmonds Precision Products, Inc. (Raytheon Technologies Corporation d/b/a RTX)
• Original Assignee: Simmonds Precision Products, Inc. (Raytheon Technologies Corporation d/b/a RTX)
• Inventors: Spillman, William B. Jr.
• Primary Examiner(s): Nelms, David C.
• Application Number:
  US07/308,239
• Time in Patent Office: 705 Days
• Field of Search: 250/226, 250/221, 250/231 SE, 250/237 G, 250/231.14, 250/227.23, 356/374, 356/395, 356/354, 356/355, 356/356, 350/162.17, 350/162.24
• US Class Current: 250/237.00G
• CPC Class Codes: G01D 5/38 by diffraction gratings

Abstract

The sensor and method of monitoring of the invention uses light diffracted from diffracting elements having at least one region of nonuniformity. The sensor is useful for monitoring displacement, stress, temperature and pressure. In one form of the invention, a diffraction grating is fabricated from first and second gratings and has a step-wise change in the grating period at an inter-grating boundary. The grating is illuminated with broadband light that is diffracted as a function of the irradiated position on the grating with the wavelength of the diffracted light and the change in wavelength being a function of the diffraction grating position. In another form of the present invention, a diffraction grating is provided with a variable grating period that can very linearly, exponentially, or in accordance with some other function with the wavelength of the diffracted light being a function of the relative displacement of the grating. Additionally, gratings can be combined to provide a transducer that provides an output that can be compensated for temperature variations.

Claims

1. A method of optical displacement detection comprising:

providing a diffraction grating having a first grating period and a different second grating period, a first optical fiber and a detecting means having a second optical fiber having input and output ends, means for focusing diffracted light into said input end of said second optical fiber and means coupled to said output end of said second optical fiber for determining at least the wavelength of the diffracted light;

irradiating said diffraction grating at a predetermined angle with broadband optical radiation passing through said first optical fiber; and

detecting a change in a characteristic of the light diffracted from said diffraction grating as a consequence of irradiation of the boundary between the first and second gratings at the boundary is displaced relative to the irradiating light.

2. The method of optical displacement detection according to claim 1, wherein said diffraction grating is a reflective diffraction grating.

3. The method of optical displacement detection according to claim 1, wherein the means for irradiating comprises:

an optical source for producing broadband light;

an optical fiber having an input coupled to said optical source and having an output; and

means for directing the broadband light from said output of said optical fiber onto said diffraction grating at said predetermined angle.

4. The method of optical displacement detection according to claim 1, wherein the wavelength determining means comprises:

a transmission diffraction grating;

means for focussing the light emitted from said output end of said second optical fiber onto said transmission diffraction grating; and

an array of light-responsive devices for detecting the diffracted light, the wavelength of the diffracted wave sensed as a function of selected ones of the light responsive devices irradiated by the diffracted light.

5. A method of optical displacement detection comprising:

providing a diffraction grating having a linearly variable grating period;

irradiating said diffraction grating at a predetermined angle with broadband optical radiation; and

detecting a change in a characteristic of the light diffracted from said diffraction grating as a consequence of the displacement of the diffraction grating relative to the irradiating light.

6. The method of optical displacement detection according to claim 5 wherein said diffraction grating is a reflective diffraction grating.

7. The method of optical displacement detection according to claim 5, wherein the means for irradiating comprises:

an optical source for producing broadband light;

an optical fiber having an input coupled to said optical source and having an output; and

means for directing the broadband light from said output of said optical fiber onto said diffraction grating at said predetermined angle.

8. The method of optical displacement detection according to claim 5, wherein said detecting means comprises:

a second optical fiber having an input end and an output end;

means for focussing the diffracted light into said input end of said second optical fiber; and

means coupled to said output end of said second optical fiber for determining at least the wavelength of the diffracted light.

9. The method of optical displacement detection according to claim 8, wherein the wavelength determining means comprises:

a transmission diffraction grating;

means for focussing the light emitted from said output end of said second optical fiber onto said transmission diffraction grating; and

an array of light responsive devices for detecting the diffracted light, the wavelength of the diffracted wave sensed as a function of selected ones of the light responsive devices irradiated by the diffracted light.

10. A method of detection using an optical transducer comprising:

providing an optical fiber, a diffraction grating having a linearly variable grating period, and means responsive to a physical parameter coupled to said diffraction grating to apply a force thereto to strain said diffraction grating in response to the physical parameter;

irradiating said diffraction grating at a first predetermined angle with broadband optical radiation passing through said optical fiber; and

detecting a change in a characteristic of the light diffracted from said diffraction grating as a consequence of the strain experienced by said diffraction grating.

11. The method of detection according to claim 10, wherein said means responsive to a physical parameter is responsive to pressure.

12. The method of detection of claim 10 wherein said means responsive to a physical parameter is responsive to stress.

13. The method of detection of claim 10 wherein said means responsive to a physical parameter is responsive to strain.

14. The method of detection according to claim 10, wherein said means responsive to a physical parameter is responsive to temperature.

15. A method of detection comprising:

providing a diffraction grating having a linearly variable first grating period and a nonlinearly variable second grating period, and

means responsive to a physical parameter coupled to said diffraction gratings to apply a force thereto to strain said diffraction gratings in response to the physical parameter;

irradiating said diffraction gratings at a predetermined angle with broadband optical radiation; and

detecting a change in a characteristic of the light diffracted from said diffraction gratings as a consequence of irradiation of the boundary between the first and second gratings as a consequence of the strain experienced by said diffraction gratings.

16. The method according to claim 15, wherein said means responsive to a physical parameter is responsive to pressure.

17. The method according to claim 15, wherein said means responsive to a physical parameter is responsive to temperature.

18. A method of optical displacement detection comprising:

providing a first diffraction grating having a variable grating period; and

a second diffraction grating having a fixed grating period;

irradiating said first and second diffraction gratings with broadband optical radiation; and

detecting light refracted from said first diffraction grating and said second diffraction grating, said means detecting a change in a characteristic of the light diffracted from the first diffraction grating as a consequence of displacement of the first diffraction grating relative the irradiating light and compensating for any error in the detected characteristic common to both said first and second diffraction gratings using the detected light from said second diffraction grating.

19. A method of optical displacement sensing comprising:

providing a first diffraction grating having a first linearly variable grating period and a second nonlinearly variable grating period separated by a boundary;

a second diffraction grating having a fixed grating period;

irradiating said first and second diffraction gratings with broadband optical radiation; and

detecting light refracted from said first diffraction grating and second diffraction grating, said means detecting a change in a characteristic of the light diffracted from the first diffraction grating as a consequence of displacement of the boundary relative to the irradiating light and compensating for any error in the detected characteristic common to both said first and second diffraction gratings using the detected light from said second diffraction grating.

20. A method of optical displacement sensing comprising:

providing a diffraction grating having a linearly variable first grating period and a nonlinearly variable second grating period;

irradiating said diffraction grating at a predetermined angle with broadband optical radiation;

detecting a change in a characteristic of the light diffracted from said diffraction grating as a consequence of irradiation of the boundary between the first and second gratings as the boundary is displaced relative the irradiating light; and

sensing a portion of the broadband light and normalizing the detected characteristic of the diffracted light in response thereto.

21. A method of optical displacement sensing comprising:

providing an optical fiber a diffraction grating having a linearly variable grating period;

irradiating said diffraction grating at a predetermined angle with broadband optical radiation passing through said optical fiber;

detecting a change in a characteristic of the light diffracted from said diffraction grating as a consequence displacement of the diffraction grating relative the irradiating light; and

sensing s portion of the broadband light and normalizing the detected characteristic of the diffracted light in response thereto.

22. An electromagnetic radiation sensor comprising a source of electromagnetic radiation, an optical fiber, diffracting means for diffracting electromagnetic radiation and an electromagnetic radiation detector,

said source being positioned to propagate electromagnetic radiation through said optical fiber to said diffracting means,

said detector being positioned to detect at least a portion of said electromagnetic radiation from said diffracting means,

said diffracting means comprising diffracting elements said elements having at least one region of nonuniformity

said region being effective to change at least one detectable characteristic of the radiation detected by said detector.

23. The sensor of claim 22 wherein said diffracting comprises at least one diffraction grating, said grating comprising diffracting elements having a variable period.

24. The sensor of claim 23 wherein said diffracting means comprises at least two diffraction gratings, each said grating comprising a plurality of diffracting elements.

25. The sensor of claim 24 wherein the spacing of said elements is detectably different in each said grating.

26. The sensor of claim 22 wherein said diffracting means reflects said detected radiation.

27. The sensor of claim 22 wherein said diffracting means transmitted said detected radiation.

28. The sensor of claim 22 wherein said characteristic is wavelength.

29. The sensor of claim 22 wherein said characteristic is intensity.

30. The sensor of claim 22 wherein said electromagnetic radiation comprises infrared radiation, visible light or untraviolet radiation.

31. The sensor of claim 22 wherein said diffracting means comprises a planar upper surface, and said source is positioned to propagate electromagnetic radiation to intersect said planar upper surface of said diffracting means at an acute angle.

32. The sensor of claim 22 in combination with a switch.

33. The sensor of claim 22 in combination with a display.

34. The sensor of claim 22 wherein said detector is connected to a microprocessor.

35. A method of monitoring, comprising;

providing an optical displacement sensor comprising;

a diffraction grating having a linearly variable first grating period and a nonlinearly variable second grating period;

means for irradiating said diffraction grating at a predetermined angle with broadband optical radiation; and

means for detecting a change in a characteristic of the light diffracted from said diffraction grating as a consequence of irradiation of the boundary between the first and second gratings as the boundary is displaced relative the irradiating light;

said detectable characteristic of said detected radiation changing consistently with changes in a monitored condition, whereby said condition is determined from said characteristic of said radiation.

36. The method of claim 35 wherein said monitored condition is temperature, pressure, stress or displacement.

37. The method of claim 35 wherein said diffracting means is a planar diffraction grating having nonuniformly positioned diffracting elements, and said radiation is propagated to said grating at an acute angle to the plane of said grating.

38. The method of claim 35 wherein said detected radiation is diffracted by a portion of said diffrating means which is exposed to radiation from said source and said portion of said diffracting means in said exposed changes with changes in said monitored condition.

39. The method of claim 38 wherein said radiation from said comprises a substantially broader range of wavelengths than the range of wavelengths in said detected radiation.