Optical sensor system utilizing bragg grating sensors

• US 5,680,489 A
• Filed: 06/28/1996
• Issued: 10/21/1997
• Est. Priority Date: 06/28/1996
• Status: Expired due to Term

• Current Assignee: Navy The United Stated of America As Represented By The Secretary of The
• Original Assignee: the united states of america as represented by the secretary of the navy
• Inventors: Kersey, Alan D.
• Primary Examiner(s): NGO, HUNG NHAT
• Application Number:
  US08/672,910
• Time in Patent Office: 480 Days
• Field of Search: 356/354, 356/35.5, 750/227.18, 750/227.14, 750/227.19, 750/227.23, 750/227.27, 385/12, 385/37
• US Class Current: 385/12
• CPC Class Codes: G01D 5/35383 using multiple sensor devices using multiplexing techniques
• CPC Class Codes: G01M 11/086 Details about the embedment of the optical fiber within the DUT
• CPC Class Codes: G02B 6/022 using mechanical stress, e.g. tuning by compression or elongation, special geometrical shapes such as "dog-bone" or taper G02B6/02204 takes precedence; optical modulation using photo-elastic effect G02F1/0131

Abstract

An optical system for detecting perturbations indicative of the performancef the piece of equipment being monitored is disclosed. The optical system comprises sensors each of which use Bragg gratings induced therein and wherein the Bragg gratings are arranged into a preselected distribution and each Bragg grating returns, when subjected to incident light, a narrowbeam signal identified by a predetermined wavelength. The optical system utilizes at least one interferometer whose operation is interlinked with optical multiplexing techniques, such as differentiate-cross-multiplying and time-division multiplexing.

Claims

1. A system for detecting perturbations to which a piece of equipment is being subjected thereto, said system comprising:

(a) an optical sensor placed along said piece of equipment and having a plurality of reflective Bragg gratings induced in the sensor and each grating acting as a sensor element, each grating having a unique location along said piece of equipment and a unique wavelength component for reflecting incident light, each of said unique wavelength components of said sensor elements being altered when being subjected to perturbations;

(b) a light source for providing input light to said sensor to cover all of said wavelength components so as to reflect incident light from each of said gratings;

(c) an unbalanced interferometer for receiving said reflected incident light from each of said gratings, said unbalanced interferometer having first and second arms and generating an interferometric signal for each wavelength component of said incident light reflected by each of said Bragg gratings;

(d) means for generating a modulation signal;

(e) a phase modulator for receiving said modulation signal, said phase modulator being placed in one of said arms of said unbalanced interferometer, said phase modulator modulating each of said interferometric signals; and

(f) a demodulator for receiving each of said modulated interferometric signals and producing output signals representative of the amount each of said unique wavelength components of said reflected incident light is altered when subjected to said perturbations.

2. The system for detecting perturbations according to claim 1, wherein said unique wavelength components of said plurality of Bragg gratings are separated from each other by a wavelength of about 1 nm.

3. The system for detecting perturbations according to claim 1, wherein said sensor is selected from the group consisting of a free-field sensor, a diaphragm sensor, an accelerometer type, a magnetostrictive type, an electrostrictive type, and a piezoelectric type.

4. The system for detecting perturbations according to claim 1, wherein said light source comprises a first broadband source of the type selected from the group comprising an edge-emitting LED (ELED), a superluminescent diode, a superluminescent fiber source and a fiber Erbium source.

5. The system for detecting perturbations according to claim 4, wherein said light source further comprises a second broadband source of the type selected from the group comprising an edge-emitting LED (ELED), a superluminescent diode, a superluminescent fiber source and a fiber Erbium source, and wherein said first and second broadband sources have operating wavelengths of about 1.3 μ

m and 1.55 μ

m respectively, and each of said first and second broadband sources has a bandwidth of about 50 nm.

6. The system for detecting perturbations according to claim 1, wherein said demodulator comprises:

(a) means for receiving each of said modulated interferometric signals and splitting each into signals representative of said unique wavelength components;

(b) detection means responsive to each of said split unique wavelength components and producing corresponding sine and cosine signals; and

(c) signal processor means responsive to said modulation signal and also responsive to said sine and cosine signals and producing said demodulator output signals.

7. The system for detecting perturbations according to claim 1, wherein said demodulator comprises:

(a) means for producing a pulse having a predetermined pulse duration and which is applied along with said input light to said sensor;

(b) means for receiving each of said modulated interferometric signals and splitting each into signals representative of said unique wavelength components;

(c) detection means responsive to each of said split unique wavelength components and producing corresponding sine and cosine signals;

(d) gating means for receiving said corresponding sine and cosine signals and being responsive to said pulse to provide said sine and cosine signals as output signals thereof; and

(e) signal processor means responsive to said modulation signal and also being responsive to said output sine and cosine signals of said gating means and producing said demodulator output signals.

8. The system for detecting perturbations according to claim 7, wherein said plurality of reflective Bragg gratings are separated by a fiber delay device into two arrays each of which array has Bragg gratings with unique wavelength components that match each other.

9. A system for detecting perturbations to which a piece of equipment is being subjected thereto, said system comprising:

(a) an optical sensor placed along said piece of equipment and having a plurality of reflective Bragg gratings induced in the sensor and each grating acting as a sensor element, each grating having a unique location along said piece of equipment and a unique wavelength component for reflecting incident light, each of said unique wavelength components of said optical sensor being altered when being subjected to perturbations;

(b) a first light source for providing input light to said sensor at a first operating wavelength λ

having a bandwidth that covers all of said wavelength components so as to reflect incident light from each of said gratings;

(c) a second light source for providing input light to said sensor at a second operating wavelength λ

having a bandwidth that covers all of said wavelength components so as to reflect incident light from each of said gratings;

(d) a first unbalanced interferometer having an operating wavelength corresponding to said wavelength λ and

receiving said reflected incident light from each of said Bragg gratings, said first unbalanced interferometer having first and second arms and generating an interferometric signal for each wavelength component of said incident light reflected by each of said Bragg gratings;

(e) a second unbalanced interferometer having an operating wavelength corresponding to said wavelength λ

'"'"' and receiving reflected light from each of said Bragg gratings, said second unbalanced interferometer having first and second arms and generating an interferometric signal for each wavelength component of said incident light reflected by each of said Bragg gratings;

(f) means for generating a modulation signal;

(g) a first phase modulator for receiving said modulation signal, said first phase modulator being placed in one of said arms of said first unbalanced interferometer, said first phase modulator modulating each of said interferometric signals of said first unbalanced interferometer;

(h) a second phase modulator for receiving said modulation signal, said second phase modulator being placed in one of said arms of said second unbalanced interferometer, said second phase modulator modulating each of said interferometric signals of said second unbalanced interferometer;

(i) a first demodulator for receiving each of said modulated interferometric signals from said first unbalanced interferometer and producing output signals representative of the amount each of said unique wavelength components of said reflected incident light is altered relative to said wavelength λ

when subjected to said perturbations; and

(j) a second demodulator for receiving each of said modulated interferometric signals from said second unbalanced interferometer and producing output signals representative of the amount each of said unique wavelength components of said reflected incident light is altered relative to said wavelength λ

'"'"' when subjected to said perturbations.

10. The system for detecting perturbations according to claim 9, wherein said unique wavelength components of said plurality of Bragg gratings are separated from each other by a wavelength of about 1 nm.

11. The system for detecting perturbations according to claim 9, wherein said sensor is selected from the group consisting of a free-field sensor, a diaphragm sensor, an accelerometer type, a magnetostrictive type, an electrostrictive type, and a piezoelectric type.

12. The system for detecting perturbations according to claim 9, wherein said first and second light sources are each of the type selected from the group comprising an edge-emitting LED (ELED), a superluminescent diode, a superluminescent fiber source, and a fiber Erbium source, and wherein said first and second broadband sources have operating wavelengths of about 1.3 μ

m and 1.55 μ

m, respectively, and each of said first and second broadband sources has a bandwidth of about 50 nm.

13. The system for detecting perturbations according to claim 9, wherein said first demodulator comprises:

(a) first means for receiving each of said modulated interferometric signals from said first unbalanced interferometer and splitting each into signals representative of said unique wavelength components of said Bragg gratings;

(b) first detection means responsive to each of said split unique wavelength components of said first means and producing corresponding sine and cosine signals; and

(c) first signal processor means responsive to said modulation signal and also responsive to said sine and cosine signals of said first detection means and producing said first demodulator output signals.

14. The system for detecting perturbations according to claim 9, wherein said second demodulator comprises:

(a) second means for receiving each of said modulated interferometric signals from said second unbalanced interferometer and splitting each into signals representative of said unique wavelength components of said Bragg gratings;

(b) second detection means responsive to each of said unique wavelength components of said second means and producing corresponding sine and cosine signals; and

(c) second signal processor means responsive to said modulation signal and also responsive to said sine and cosine signals of said second detection means and producing said second demodulator output signals.

15. The system for detecting perturbations according to claim 9, wherein said first demodulator comprises:

(a) means for producing a pulse having a predetermined pulse duration that is applied with said input light of said first and second light sources to said optical sensor;

(b) first means for receiving each of said modulated interferometric signals from said first unbalanced interferometer and splitting each into signals representative of said unique wavelength components;

(c) first detection means responsive to each of said split unique wavelength components of said first means and producing corresponding sine and cosine signals;

(d) first gating means for receiving said corresponding sine and cosine signals of said first detection means and being responsive to said pulse to provide said sine and cosine signals as output signals thereof; and

(e) first signal processor means responsive to said modulation signal and also being responsive to said output sine and cosine signals of said first gating means for producing said first demodulator output signals.

16. The system for detecting perturbations according to claim 9, wherein said second demodulator comprises:

(a) means for producing a pulse having a predetermined pulse duration that is applied with said input light of said first and second light sources to said optical sensor;

(b) second means for receiving each of said modulated interferometric signals from said second unbalanced interferometer and splitting each into signals representative of said unique wavelength components;

(c) second detection means responsive to each of said unique wavelength components of said second means and producing corresponding sine and cosine signals;

(d) second gating means for receiving said corresponding sine and cosine signals of said second detector means and being responsive to said pulse to provide said sine and cosine signals as output signals thereof; and

(e) second signal processor means responsive to said modulation signal and also being responsive to said output sine and cosine signals of said second gating means and producing said second demodulator output signals.

17. The system for detecting perturbations according to claim 9, wherein said plurality of reflective Bragg gratings are separated by a fiber delay device into two arrays each of which array has the same gratings with a unique location and the same unique wavelength components.