Fiber optic sensor having a multicore optical fiber and an associated sensing method

• US 5,563,967 A
• Filed: 06/07/1995
• Issued: 10/08/1996
• Est. Priority Date: 06/07/1995
• Status: Expired due to Fees

• Current Assignee: Mcdonnell Douglas Corporation (The Boeing Co.)
• Original Assignee: Mcdonnell Douglas Corporation (The Boeing Co.)
• Inventors: Haake, John M.
• Primary Examiner(s): Bovernick, Rodney B.
• Assistant Examiner(s): Sanghavi, Hemang
• Application Number:
  US08/472,740
• Time in Patent Office: 489 Days
• Field of Search: 385/12, 385/37, 250/227.14, 250/227.17, 250/227.18, 250/227.23, 250/227.24, 250/227.21, 356/345
• US Class Current: 385/12
• CPC Class Codes: G01B 11/18 using photoelastic elements
• CPC Class Codes: G01D 5/268 using optical fibres G01D5/28 - G01D5/38 take precedence
• CPC Class Codes: G01D 5/35383 using multiple sensor devices using multiplexing techniques
• CPC Class Codes: G01K 11/3206 at discrete locations in the fibre, e.g. using Bragg scattering
• CPC Class Codes: G01K 5/52 with electrical conversion means for final indication
• CPC Class Codes: G01L 1/246 using integrated gratings, e.g. Bragg gratings
• CPC Class Codes: G01M 11/083 by using an optical fiber in contact with the device under test [DUT]
• CPC Class Codes: G02B 6/02042 Multicore optical fibres
• CPC Class Codes: G02B 6/021 characterised by the core or cladding or coating, e.g. materials, radial refractive index profiles, cladding shape
• CPC Class Codes: G02B 6/02204 using thermal effects, e.g. heating or cooling of a temperature sensitive mounting body optical modulation using thermo-optic effect G02F1/0147

Abstract

A fiber optic sensor and associated sensing method includes a multicore optical fiber having first and second optical cores adapted to transmit optical signals having first and second predetermined wavelengths, respectively, in a single spatial mode. The first and second optical cores each include a respective Bragg grating adapted to reflect optical signals having the first and second predetermined wavelengths, respectively. Based upon the differences between the respective wavelengths of the optical signals reflected by the respective Bragg gratings and the first and second predetermined wavelengths, a predetermined physical phenomena to which the workpiece is subjected can be determined. For example, the fiber optic sensor can include strain determining means for determining the strain imparted to the workpiece. Alternatively, the fiber optic sensor can include temperature determining means for determining the temperature to which the workpiece is subjected. Further, the fiber optic sensor can include both strain determining means and temperature determining means for simultaneously determining the strain and temperature to which a workpiece is subjected. The fiber optic sensor also includes calibration means for measuring the predetermined physical phenomena, independent of variations in other physical phenomena.

Claims

1. A fiber optic sensor for measuring strain in a workpiece independent of variations in temperature to which the workpiece is subjected, the fiber optic sensor comprising:

a multicore optical fiber comprising;

first and second optical cores adapted to transmit optical signals having first and second predetermined wavelengths, respectively, in a single spatial mode, wherein the first and second predetermined wavelengths are different, and wherein said first and second optical cores include a respective Bragg grating at the same relative spatial location which is adapted to reflect optical signals having the first and second predetermined wavelengths, respectively; and

a cladding layer surrounding each of said first and second optical cores;

first and second optical sources for generating optical signals having first and second predetermined wavelengths, respectively, said first and second optical sources being coupled to said first and second optical cores of said multicore optical fiber, respectively, for transmitting the optical signals therethrough;

first and second detectors, coupled to said first and second optical cores of said multicore optical fiber, respectively, for detecting optical signals reflected by the respective Bragg grating of said first and second optical cores; and

strain determining means, operably connected to said first and second detectors, for determining the strain imparted to the workpiece, independent of variations in the temperature to which the workpiece is subjected, based upon the difference between the respective wavelengths of the optical signals detected by said first and second detectors and the first and second predetermined wavelengths, respectively.

2. A fiber optic sensor according to claim 1 wherein said strain determining means comprises calibration means comprising:

means for determining the respective wavelength shifts of optical signals originally having first and second predetermined wavelengths due to the application of predetermined amounts of strain to the workpiece which is maintained at a constant temperature; and

means for determining the respective wavelength shifts of optical signals originally having first and second predetermined wavelengths as a result of subjecting the workpiece to predetermined temperature variations while maintaining the strain to which the workpiece is subjected constant, such that said strain determining means determines the strain imparted to the workpiece independent of any differences in the respective wavelengths of the optical signals due to variations in the temperature to which the workpiece is subjected.

3. A fiber optic sensor according to claim 1 further comprising temperature determining means, operably connected to said first and second detectors, for determining the temperature to which the workpiece is subjected based upon the difference between the respective wavelengths of the optical signals detected by said first and second detectors and the first and second predetermined wavelengths, respectively.

4. A fiber optic sensor according to claim 1 wherein said first and second optical sources are coupled to first ends of said first and second optical cores, respectively, and wherein the respective Bragg gratings of said first and second optical cores are disposed at first and second predetermined spatial locations, respectively, relative to the respective first ends of said first and second optical cores, and wherein the first and second predetermined spatial locations are identical.

5. A fiber optic sensor according to claim 1 wherein said first optical core is adapted to transmit optical signals having a first predetermined wavelength of 670 nanometers, and wherein said second optical core is adapted to transmit optical signals having a second predetermined wavelength of 1550 nanometers.

6. A fiber optic sensor according to claim 1 further comprising:

a first optical coupler for coupling said first optical source and said first detector with said first optical core; and

a second optical coupler for coupling said second optical source and said second detector with said second optical core.

7. A fiber optic sensor according to claim 1 wherein said first and second optical cores of said multicore optical fiber are doped in different manners such that said first and second optical cores are adapted to preferentially transmit optical signals having different wavelengths.

8. A fiber optic sensor for measuring a temperature to which a workpiece is subjected independent of variations in strain imparted to the workpiece, the fiber optic sensor comprising:

a multicore optical fiber comprising;

first and second optical cores adapted to transmit optical signals having first and second predetermined wavelengths, respectively, in a single spatial mode, wherein the first and second predetermined wavelengths are different, and wherein said first and second optical cores include a respective Bragg grating at the same relative spatial location which is adapted to reflect optical signals having the first and second predetermined wavelengths, respectively; and

a cladding layer surrounding each of said first and second optical cores;

first and second optical sources for generating optical signals having first and second predetermined wavelengths, respectively, said first and second optical sources being coupled to said first and second optical cores of said multicore optical fiber, respectively, for transmitting the optical signals therethrough;

first and second detectors, coupled to said first and second optical cores of said multicore optical fiber, respectively, for detecting optical signals reflected by the respective Bragg grating of said first and second optical cores; and

temperature determining means, operably connected to said first and second detectors, for determining the temperature to which the workpiece is subjected, independent of variations in the strain imparted to the workpiece, based upon the difference between the respective wavelengths of the optical signals detected by said first and second detectors and the first and second predetermined wavelengths, respectively.

9. A fiber optic sensor according to claim 8 wherein said temperature determining means comprises calibration means comprising:

means for determining the respective wavelength shifts of optical signals originally having first and second predetermined wavelengths due to the application of predetermined amounts of strain to the workpiece which is maintained at a constant temperature; and

means for determining the respective wavelength shifts of optical signals originally having first and second predetermined wavelengths as a result of subjecting the workpiece to predetermined temperature variations while maintaining the strain to which the workpiece is subjected constant, such that said temperature determining means determines the temperature to which the workpiece is subjected independent of any differences in the respective wavelengths of the optical signals due to variations in the strain imparted to the workpiece.

10. A fiber optic sensor according to claim 8 further comprising strain determining means, operably connected to said first and second detectors, for determining the strain imparted to the workpiece based upon the difference between the respective wavelengths of the optical signals detected by said first and second detectors and the first and second predetermined wavelengths, respectively.

11. A fiber optic sensor according to claim 8 wherein said first and second optical sources are coupled to first ends of said first and second optical cores, respectively, and wherein the respective Bragg gratings of said first and second optical cores are disposed at first and second predetermined spatial locations, respectively, relative to the respective first ends of said first and second optical cores, and wherein the first and second predetermined spatial locations are identical.

12. A fiber optic sensor according to claim 8 wherein said first optical core is adapted to transmit optical signals having a first predetermined wavelength of 670 nanometers, and wherein said second optical core is adapted to transmit optical signals having a second predetermined wavelength of 1550 nanometers.

13. A fiber optic sensor according to claim 8 further comprising:

a first optical coupler for coupling said first optical source and said first detector with said first optical core; and

a second optical coupler for coupling said second optical source and said second detector with said second optical core.

14. A fiber optic sensor according to claim 8 wherein said first and second optical cores of said multicore optical fiber are doped in different manners such that said first and second optical cores are adapted to preferentially transmit optical signals having different wavelengths.

15. A method of sensing a predetermined phenomena to which a workpiece is subjected, independent of other phenomena to which the workpiece is subjected, the method comprising the steps of:

generating first and second optical signals having first and second predetermined wavelengths, respectively, wherein the first and second predetermined wavelengths are different;

coupling the first and second optical signals to first and second optical cores, respectively, of a multicore optical fiber;

transmitting the first and second optical signals in a single spatial mode through the first and second optical cores, respectively, of the multicore optical fiber;

reflecting the optical signals transmitted by the first and second optical cores by first and second Bragg gratings defined by the first and second optical cores, respectively, at the same relative spatial location to thereby create first and second resonance signals, respectively;

detecting the first and second resonance signals reflected by the respective Bragg gratings of the first and second optical cores, respectively; and

determining the magnitude of the predetermined phenomena to which the workpiece is subjected, independent of other phenomena to which the workpiece is subjected, based upon the difference between the respective wavelengths of the first and second resonance signals and the first and second predetermined wavelengths, respectively.

16. A method according to claim 15 wherein said determining step comprises the step of determining the strain imparted to the workpiece, independent of variations in the temperature to which the workpiece is subjected, based upon the difference between the respective wavelengths of the first and second resonance signals and the first and second predetermined wavelengths, respectively.

17. A method according to claim 15 wherein said determining step comprises the step of determining the temperature to which the workpiece is subjected, independent of variations in the strain imparted to the workpiece, based upon the difference between the respective wavelengths of the first and second resonance signals and the first and second predetermined wavelengths, respectively.

18. A method according to claim 15 further comprising the step of calibrating the multicore optical fiber prior to said generating step such that the predetermined phenomena can be sensed independent of any differences in the respective wavelengths of the first and second optical signals due to other phenomena, wherein said calibrating step comprises the steps of:

determining the respective wavelength shifts of optical signals originally having first and second predetermined wavelengths due to the application of predetermined amounts of strain to the workpiece while maintaining the workpiece at a constant temperature; and

determining the respective wavelength shifts of optical signals originally having first and second predetermined wavelengths as a result of subjecting the workpiece to predetermined temperature variations while maintaining the strain to which the workpiece is subjected constant.

19. A method according to claim 15 further comprising the step of writing a respective Bragg grating on each of the first and second optical cores to reflect optical signals having the first and second predetermined wavelengths, respectively, prior to said generating step.

20. A method according to claim 19 wherein said coupling step comprises the step of coupling the first and second optical signals to first ends of said first and second optical cores, respectively, wherein said writing step comprises the step of writing respective Bragg gratings on the first and second optical cores at first and second predetermined spatial locations, respectively, relative to the respective first ends of the first and second optical cores, and wherein the first and second predetermined spatial locations are identical.