Method and apparatus for continuous readout of fabry-perot fiber optic sensor

• US 7,940,400 B2
• Filed: 02/16/2009
• Issued: 05/10/2011
• Est. Priority Date: 04/15/2004
• Status: Active Grant

• Current Assignee: Halliburton Energy Services Incorporated (Halliburton Company)
• Original Assignee: Halliburton Energy Services Incorporated (Halliburton Company)
• Inventors: Lopushansky, Richard L., Berthold, John W.
• Primary Examiner(s): Connolly; Patrick J
• Application Number:
  US12/371,817
• Publication Number:
  US 20090225325A1
• Time in Patent Office: 813 Days
• Field of Search: 356/478, 356/480, 356/506, 356/519, 356/454
• US Class Current: 356/519
• CPC Class Codes: G01B 2290/25 Fabry-Perot in interferometer, e.g. etalon, cavity
• CPC Class Codes: G01B 9/02004 using frequency scans
• CPC Class Codes: G01B 9/02023 Indirect probing of object, e.g. via influence on cavity or fibre
• CPC Class Codes: G01D 5/268 using optical fibres G01D5/28 - G01D5/38 take precedence
• CPC Class Codes: G01D 5/35303 using a reference fibre, e.g. interferometric devices
• CPC Class Codes: G01L 9/0079 with Fabry-Perot arrangements

Abstract

A pressure measurement system and method are described. The system uses a tunable laser and a Fabry-Perot sensor with integrated transducer. A detector senses the light modulated by the Fabry-Perot sensor. A signal conditioner, which can be located up to 15 km away, then uses the detector signal to determine the displacement of the diaphragm, which is indicative of pressure exerted against the diaphragm. Use of a temperature sensor to generate a signal, fed to the signal conditioner, to compensate for temperature is also contemplated.

Claims

1. A system for monitoring and measuring a displacement of a Fabry-Perot sensor, the system comprising:

a gap of unknown variable length spanning a first reflective surface and a second reflective surface of the Fabry-Perot sensor;

a laser light that is tunable at a fast scan rate over a range of frequencies, where the light is provided to the Fabry-Perot sensor, and the Fabry-Perot sensor modulates the light as the light is fast scanned over the range of frequencies;

a detector for generating a signal from a normalized intensity ratio of the modulated light; and

a signal conditioner for identifying at least two frequency minima in the normalized intensity ratio and generating an output characteristic of the gap length between the first and second reflective surfaces based on the at least two identified frequency minima.

2. The system of claim 1, where the Fabry-Perot sensor comprises a high-finesse Fabry-Perot sensor.

3. The system of claim 1, where the detector includes an InGaAs element.

4. The system of claim 1, further comprising a pulsing device for pulsing the light.

5. The system of claim 4, where the pulsing devise is selected from a group comprising:

a fast shutter, an electro-optic modulator, and an on/off circuit for controlling current provided to the laser light.

6. The system of claim 1, where the range of frequencies is tunable between approximately 187.5 THz and approximately 200 THz.

7. The system of claim 1, where at least one of the first and second reflective surfaces is within a pressure transducer.

8. A method for monitoring and measuring a displacement of two reflective surfaces in a Fabry-Perot sensor spanning an unknown and variable gap comprising:

tuning a laser light source over a range of frequencies to produce light;

fast scanning the Fabry-Perot sensor with the light over the range of frequencies;

generating a signal from a normalized intensity ratio of light modulated by the Fabry-Perot sensor;

identifying at least two frequency minima in the normalized intensity ratio;

generating an output characteristic of the displacement between the two reflective surfaces based on the at least two identified frequency minima; and

calculating an absolute value for the unknown variable gap distance during the fast scanning of the Fabry-Perot sensor.

9. The method of claim 8, where the absolute value for the unknown variable gap is used to calculate an environmental parameter.

10. The method of claim 9, further comprising:

monitoring a temperature sensor to generate a temperature signal; and

applying a temperature correction factor in calculating the environmental parameter of the Fabry-Perot sensor.

11. The method of claim 10, further comprising:

monitoring the light modulated by the Fabry-Perot sensor during the fast scanning to detect changes in the environmental parameter.

12. The method of claim 11, further comprising pulsing the light fast scanning the Fabry-Perot sensor.

13. The method of claim 8, further comprising pulsing the light fast scanning the Fabry-Perot sensor.

14. A system for monitoring and measuring a displacement of reflective surfaces in a Fabry-Perot sensor, the system comprising:

a laser providing light tunable at a fast scan rate over a range of frequencies;

a gap of unknown variable length spanning a first reflective surface and a second reflective surface of the Fabry-Perot sensor, the Fabry-Perot sensor receiving and modulating the fast scanned light;

a temperature sensor generating a temperature signal characteristic of an environmental temperature at the Fabry-Perot sensor;

a detector for generating a signal from a normalized intensity ratio of the modulated light; and

a signal conditioner for identifying at least two frequency minima in the normalized intensity ratio and generating an output characteristic of the displacement between the two reflective surfaces based on the at least two identified frequency minima.

15. The system of claim 14, further comprising a pulsing device for pulsing the light.

16. The system according to claim 15, where the pulsing device is selected from a group comprising:

a fast shutter, an electro-optic modulator, and an on/off circuit for controlling current provided to the laser.

17. The system of claim 14, where the range of frequencies is between approximately 187.5 THz and approximately 200 THz.

18. The system of claim 14, where at least one of the first and second reflective surfaces is included within a pressure transducer.

19. The system of claim 14, where the Fabry-Perot sensor comprises a high-finesse Fabry-Perot sensor.

20. The system of claim 14, where the detector includes an InGaAs element.