Method and apparatus for compensating fiber optic lead and connector losses in a fiber optic sensor by using a dual wavelength optical source and matched polarizer

• US 4,709,145 A
• Filed: 08/20/1986
• Issued: 11/24/1987
• Est. Priority Date: 08/20/1986
• 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): Westin, Edward P.
• Assistant Examiner(s): Rooff, Jessica L.
• Application Number:
  US06/898,317
• Time in Patent Office: 461 Days
• Field of Search: 250/225, 250/227, 250/231 R, 250/231 P, 324/96, 73/705, 73/800, 350/96.29
• US Class Current: 250/208.5
• CPC Class Codes: G01D 5/268 using optical fibres G01D5/28 - G01D5/38 take precedence
• CPC Class Codes: G01L 1/241 by photoelastic stress analysis
• CPC Class Codes: G01M 11/335 using two or more input wavelengths
• CPC Class Codes: G01M 11/337 by measuring polarization dependent loss [PDL]

Abstract

Apparatus and method for compensating a photo sensor for fiber optic cable lead/connector losses and optical source power fluctuations includes a specially selected broadband optical source used in conjunction with a photoelastic fiber optic transducer that uses a polarizer and an analyzer that only polarize a portion of the broadband signal. The portion of the signal that is not polarized is simply transmitted through the photoelastic element and emerges with no modulation. The portion of the signal that is polarized is modulated according to the stress on the photoelastic element. After transmitting the output light beam through a single fiber optic cable, the beam is separated into two parts. The first part is filtered so as to transmit only that portion of the broadband signal that is not affected by the polarizer and analyzer and hence is not modulated. The filtered first part is converted to an electrical signal by a photo detector. The second part of the split beam is filtered so as to transmit only that portion of the broadband signal that is affected by the polarizer and analyzer, and hence is modulated. The filtered second part is converted to an electrical signal by a second photo detector. The second signal varies according to lead and connector losses and the stress modulation on the photoelastic element. The first signal varies according to lead and connector losses only. The ratio of the second signal to the first signal then provides a measure of the changes in the transmission due to the modulation of the transducer element, without lead and connector losses, or variations due to power source fluctuations.

Claims

1. Optical sensor apparatus, comprising:

means for providing a source light beam including a first component having a first wavelength and a second component having a second wavelength;

polarizer means for receiving said source light beam, and for polarizing said first component while leaving said second component substantially unpolarized, and for providing a light beam having polarized and unpolarized components;

optical transducer means for receiving said light beam from said polarizer means, and for modulating said polarized component in response to an externally applied condition while leaving said unpolarized component substantially unmodulated, and for providing an output light beam having modulated and unmodulated components;

analyzer means for receiving said output light beam from said optical transducer means, and for providing an analyzed light beam having a plurality of modulated and unmodulated intensities corresponding to said modulated and unmodulated components, respectively;

a single first light guiding means for transmitting said analyzed light beam;

beam splitter means for receiving said analyzed light beam from said single first light guiding means, and for splitting said analyzed light beam into first and second portions, each portion including said modulated and said unmodulated light intensities;

first filter means for receiving said first portion, and for transmitting said modulated light intensity while substantially blocking said unmodulated light intensity, and for providing a first filtered light beam having said modulated light intensity;

second filter means for receiving said second portion, and for transmitting said unmodulated light intensity while substantially blocking said modulated light intensity, and for providing a second filtered light beam having said unmodulated light intensity;

first photo detector means for receiving said first filtered light beam, and for providing a first signal corresponding to an intensity of said first filtered light beam;

second photo detector means for receiving said second filtered light beam, and for providing a second signal corresponding to an intensity of said second filtered light beam; and

ratio means for receiving said first and second signals, and for providing an output signal corresponding to a ratio of said first signal to said second signal.

2. Apparatus according to claim 1 wherein said means for providing a source light beam includes a single light source providing a broadband beam of light encompassing said first and second wavelengths.

3. Apparatus according to claim 1 wherein said means for providing a source light beam includes a first light source providing a first light beam having said first wavelength, and a second light source providing a second light beam having said second wavelength, and further including an optical coupler for coupling said first and second light beams to provide said source light beam.

4. Apparatus according to claim 1 further including a single second light guiding means for transmitting said source light beam from said means for providing a source light beam to said polarizer means.

5. Apparatus according to claim 4 wherein said first and second light guiding means comprise fiber optic cables.

6. Apparatus according to claim 1 wherein said polarizer means and said analyzer means polarize only in a predetermined wavelength region, and wherein said means for providing a source light beam includes a broadband optical source having a spectral output which lies inside and outside of said predetermined wavelength region.

7. Optical sensor apparatus, comprising:

optical source means for providing a source light beam having first and second components of first and second wavelengths, respectively;

polarization optical transfer means for (a) receiving said source light beam from said optical source means, (b) polarizing said source light beam to cause said first component to be polarized while leaving said second component effectively unpolarized, (c) modulating said first component in accordance with an external condition applied to said transducer means while leaving said second component effectively unmodulated, and (d) providing an output light beam having modulated and unmodulated components;

a single first light guiding means for transmitting said output light beam from said polarization optical transducer means;

filter means for (a) receiving said output light beam from said single first light guiding means, (b) providing a first filtered optical signal having said modulated components and being effectively free of said unmodulated components, and (c) providing a second filtered optical signal having said unmodulated components and being effectively free of said modulated components;

photo detector means for (a) receiving said first and second filtered optical signals, (b) converting said optical signals into electrical signals, (c) providing a first electrical signal corresponding to an intensity of said first filtered optical signal, and (d) providing a second electrical signal corresponding to an intensity of said second filtered optical signal; and

processing means for receiving said first and second electrical signals and providing an output signal corresponding to a ratio of said first and second electrical signals.

8. Apparatus according to claim 7 wherein said optical source means includes:

first optical source means for providing a first source light beam having said first wavelength;

second optical source means for providing a second source light beam having said second wavelength; and

coupling means for coupling said first and second source light beams to form said source light beam.

9. Apparatus according to claim 7 wherein said optical source means includes a single broadband optical source which provides said source light beam having a broadband of wavelengths including said first and second wavelengths.

10. Apparatus according to claim 7 wherein said polarization optical transducer means includes:

a first polarizer receiving said source light beam;

a photoelastic transducer optically coupled to said first polarizer; and

a second polarizer optically coupled between said photoelastic transducer and said single first light guiding means.

11. Apparatus according to claim 10 further including a quarter wave plate optically coupled between said first polarizer and said photoelastic transducer.

12. Apparatus according to claim 7 wherein said filter means includes:

beam splitter means for receiving said output light beam from said single first light guiding means, and for splitting said output light beam into first and second portions, each portion including said modulated and said unmodulated components;

first filter means for (a) receiving said first portion from said beam splitter means, (b) passing said first portion modulated components, (c) substantially blocking said first portion unmodulated components, and (d) providing said first filtered optical signal; and

second filter means for (a) receiving said second portion from said beam splitter means, (b) passing said second portion unmodulated components, (c) substantially blocking said second portion modulated components, and (d) providing said second filtered optical signal.

13. Apparatus according to claim 12 wherein said photo detector means includes:

first photo detector means for receiving said first filtered optical signal from said first filter means, and for providing said first electrical signal; and

second photo detector means for receiving said second filtered optical signal from said second filter means, and for providing said second electrical signal.

14. Apparatus according to claim 7 wherein said optical source means provides said source light beam wherein said first wavelength is between approximately 550 nanometers and approximately 700 nanometers, and wherein said second wavelength is greater than approximately 800 nanometers.

15. Apparatus according to claim 7 further including a single second light guiding means for carrying said source light beam from said optical source means to said polarization optical transducer means.

16. Apparatus according to claim 7 wherein said filter means includes first and second dichroic filter means for providing said first and second filtered optical signals, respectively.

17. Apparatus according to claim 7 wherein said polarization optical transducer means includes a photoelastic fiber optic transducer which modulates said first component in accordance with externally applied pressure.

18. Optical sensor apparatus, comprising:

means for providing a source light beam having first and second wavelengths;

first light channel means for directing said source light beam;

polarizer means for receiving said source light beam from said first light channel means, and for polarizing said source light beam at said first wavelength but leaving said source light beam at said second wavelength substantially unpolarized;

optical transducer means for receiving said polarized light and said unpolarized light, and for modulating said polarized light in response to an externally applied condition while leaving said unpolarized light substantially unmodulated;

a single second light channel means for directing said modulated light and said unmodulated light;

beam splitter means for splitting the directed modulated light and unmodulated light into first and second light beams, each including said modulated and said unmodulated light;

first filter means for receiving said first light beam, and for transmitting said modulated light while substantially blocking said unmodulated light;

second filter means for receiving said second light beam, and for transmitting said unmodulated light while substantially blocking said modulated light;

first photo detector means for receiving said modulated light from said first filter means, and for providing a first output signal corresponding to an intensity of said modulated light;

second photo detector means for receiving said unmodulated light from said second filter means, and for providing a second output signal corresponding to an intensity of said unmodulated light; and

ratio means for receiving said first and second output signals from said first and second photo detector means, respectively, and for providing an electric signal corresponding to a ratio of said first output signal and said second output signal.

19. A method of sensing, comprising the steps of:

providing a source light beam having first and second components, said first component having a first wavelength and said second component having a second wavelength;

polarizing said source light beam to cause said first component to be polarized while leaving said second component substantially unpolarized;

modulating, with an optical transducer, said first component in accordance with an external condition applied to said optical transducer while leaving said second component substantially unmodulated to provide an output light beam having modulated and unmodulated components;

transmitting, with a single light guiding channel, said output light beam from said optical transducer;

filtering said output light beam from said single light guiding channel to provide (a) a first filtered optical signal which has said modulated components but is substantially free of said unmodulated components, and (b) a second filtered optical signal which has said unmodulated components but is substantially free of said modulated components;

detecting said first and second filtered optical signals and providing (a) a first electrical signal corresponding to an intensity of said first filtered optical signal, and (b) a second electrical signal corresponding to an intensity of said second filtered optical signal; and

processing said first and second electrical signals to provide an output signal corresponding to a ratio of said first and second electrical signals.

20. A method according to claim 19 wherein said step of providing a source light beam includes the step of providing said source light beam with a broadband of wavelengths including said first and second wavelengths.

21. A method according to claim 19 wherein said step of providing a source light beam includes the steps of:

providing a first source light beam having said first wavelength;

providing a second source light beam having said second wavelength; and

coupling said first and second source light beams to provide said source light beam.

22. A method according to claim 19 wherein said filtering step includes the steps of:

splitting said output light beam to provide first and second beam portions, each portion having said modulated and said unmodulated components;

filtering said first portion to provide said first filtered optical signal; and

filtering said second portion to provide said second filtered optical signal.

23. A method according to claim 22 wherein said detecting step includes the steps of:

detecting, with a first photo detector, said first filtered optical signal to provide said first electrical signal; and

detecting, with a second photo detector, said second filtered optical signal to provide said second electrical signal.

24. A method according to claim 19 wherein said step of providing a source light beam includes the step of providing said source light beam wherein said first wavelength is between approximately 550 nanometers and approximately 700 nanometers, and wherein said second wavelength is greater than approximately 800 nanometers.

25. A method according to claim 19 further including the step of transmitting, with at least a fiber optical cable, said source light beam.

26. A method of optically sensing pressure, comprising the steps of:

generating a source light beam having a first component with a first wavelength and a second component with a second wavelength;

transmitting, with at least a first fiber optic cable, said source light beam;

polarizing the transmitted source light beam to cause said first component to be polarized while leaving said second component effectively unpolarized;

detecting, with a photoelectric transducer, said pressure;

modulating, with said transducer, the polarized first component while leaving the unpolarized second component effectively unmodulated to provide an output light beam having modulated and unmodulated components;

transmitting, with a single second fiber optic cable, said output light beam;

splitting the transmitted output light beam into first and second portions, each portion having said modulated and said unmodulated components;

filtering said first portion to pass said first portion modulated components while substantially blocking said first portion unmodulated components;

filtering said second portion to pass said second portion unmodulated components while substantially blocking said second portion modulated component;

detecting said first portion modulated components and providing a first electrical signal corresponding thereto;

detecting said second portion unmodulated components and providing a second electrical signal corresponding thereto; and

receiving said first and second electrical signals and providing an output signal corresponding to a ratio of said first and second electrical signals.