Dual slope fiber optic array interrogator

• US 6,778,720 B1
• Filed: 04/02/2002
• Issued: 08/17/2004
• Est. Priority Date: 04/02/2002
• Status: Expired due to Term

• Current Assignee: Bushlavits, LLC
• Original Assignee: Optiphase, Inc. (Halliburton Company)
• Inventors: Bush, Ira Jeffrey, Cekorich, Allen Curtis
• Primary Examiner(s): Frech, Karl D.
• Assistant Examiner(s): Hess, Daniel A.
• Application Number:
  US10/113,613
• Time in Patent Office: 868 Days
• Field of Search: 385/15, 356/463, 356/464, 356/345, 356/359, 359/193
• US Class Current: 385/15
• CPC Class Codes: G01D 5/35383 using multiple sensor devices using multiplexing techniques

Abstract

An apparatus and method is presented presented to provide wide dynamic range measurements of the input phase to a time domain multiplex interferometer array using a dual slope modulation method for interrogation. Features of the present invention include a topology capable of using only single mode fiber components and self-correcting processing approaches that reduce measurement errors providing high accuracy measurements and significantly reducing the cost of interrogation.

Claims

1. An apparatus for determining the phase of signal pulses which comprises:

(a) a compensator;

said compensator comprising a coupler, a delaying path and a modulating path;

said delaying path comprising a delaying means and a reflector;

said modulating path comprising a phase modulator and a reflector;

whereby light pulses from a pulse light source are bifurcated and directed simultaneously by said coupler into said delay path and said modulating path and reflections from said delay path and said modulating path are combined by said coupler as pulse pairs;

(b) one or more interferometers;

(c) one or more optical receivers for converting the incoming light signals into electrical signals

(d) a bi-directional coupling device for directing pulse pairs from said coupler to said one or more interferometers and for directing reflections from said one or more interferometers to said one or more optical receivers;

(e) a digitizer for sampling the electrical signals exiting said one or more optical receivers in a fashion synchronous with the pulse light source;

(f) a modulation signal generator;

said modulation signal generator outputting a dual slope waveform comprised of alternating positive and negative slopes of equal magnitude for commanding said phase modulator in a fashion synchronous with the digitizer;

(g) a demodulation means for cycle quadrant determination and output phase determination using the digitized electrical signals from both positive and negative modulation slopes;

whereby output phase of signal pulses are determined by the demodulation of the digitized electrical signals.

2. The apparatus for determining the phase of signal pulses as claimed in claim 1 wherein said phase modulator is a piezoelectric transducer-driven winding of fiber.

3. The apparatus for determining the phase of signal pulses as claimed in claim 1 wherein said reflector of the modulating path is a Faraday Reflector mirror.

4. The apparatus for determining the phase of signal pulses as claimed in claim 1 wherein said bi-directional coupling device is a bi-directional coupler.

5. The apparatus for determining the phase of signal pulses as claimed in claim 1 wherein said bi-directional coupling device is an optical circulator.

6. The apparatus for determining the phase of signal pulses as claimed in claim 1 wherein said one or more interferometers comprise a linear array of interferometers.

7. The apparatus for determining the phase of signal pulses as claimed in claim 1 wherein said optical receiver is a polarization diversity receiver.

8. The apparatus for determining the phase of signal pulses as claimed in claim 7 wherein said polarization diversity receiver comprises an optical splitter and a plurality of receivers.

9. The apparatus for determining the phase of signal pulses as claimed in claim 1 wherein the modulation signal generator comprises a waveform generator, a digital-to-analog converter and a modulation driver.

10. The apparatus for determining the phase of signal pulses as claimed in claim 1 wherein the modulation signal generator creates a dual slope waveform of adjustable phase and amplitude.

11. The apparatus for determining the phase of signal pulses as claimed in claim 10 wherein the dual slope waveform of adjustable phase and amplitude is a sine wave of adjustable phase and amplitude.

12. The apparatus for determining the phase of signal pulses as claimed in claim 10 wherein the dual slope waveform of adjustable phase and amplitude is a triangle wave of adjustable phase and amplitude.

13. The apparatus for determining the phase of signal pulses as claimed in claim 1 further comprising a fringe generator;

said fringe generator operably maintaining and updating fringe values by reading successive values from the demodulation means for cycle quadrant determination thereby determining fringe crossings, whereby said fringe generator adds or subtracts 2π

radians to a preceding fringe value depending on the respective direction of fringe crossing.

14. The apparatus for determining the phase of signal pulses as claimed in claim 13 further comprising a modulation signal phase correction means for converting,samples of five or more points about the positive slope of the zero crossing of the electrical signals and samples of five or more points about the negative slope of the zero crossing of the electrical signals thereby determining the phase error from one-half the difference of the demodulated signal phase angle of the negative slope samples as augmented by said fringe generator and demodulated signal phase angle of the positive samples as augmented by said fringe generator, respectively.

15. The apparatus for determining the phase of signal pulses as claimed in claim 14 wherein said modulation signal amplitude correction means converts samples of five or more points about a positive slope of a zero crossing of the electrical signals and samples of five or more points about a negative slope of successive zero crossings of the electrical signals thereby determining amplitude error per slope sign and per cycle quadrant by summing positive slope amplitude error and negative slope amplitude error, each respectively determined from the positive slope samples and the negative slope samples respectively, where quadrant determination is performed by logically inspecting the location within the unit circle of the demodulated positive slope phase angle for the positive slope amplitude error and the demodulated negative slope phase angle for the negative slope amplitude error.

16. The apparatus for determining the phase of signal pulses as claimed in claim 15 wherein the demodulation means for output phase determination provides one-half the sum of the demodulated positive slope phase angle, as augmented by the fringe generator, and the demodulated negative slope phase angle, as augmented by the fringe generator.

17. The apparatus for determining the phase of signal pulses as claimed in claim 15 wherein the demodulation means selects alternately from the demodulated positive slope phase angle and the demodulated negative slope phase angle based on five or more samples of the electrical signals about the positive slope of a zero crossing and five or more samples of the electrical signals about the negative slope respectively of a successive zero crossing alternately, each adjusted by the concatenation of fringe,

whereby measurements of R(t) are output at twice the rate over an average of the two relationships.

18. The apparatus for determining the phase of signal pulses as claimed in claim 15 wherein the demodulation means for output phase determination provides alternately the demodulated positive slope phase angle followed by the demodulated negative slope phase angle based on five or more samples of the electrical signals about the positive slope of a zero crossing and five of more samples of the electrical signals about the negative slope of a successive zero crossing, each respectively concatenated with fringe, whereby the pulse rate of the source light pulse and the output phase rate of the apparatus are the same.

19. The apparatus for determining the phase of signal pulses as claimed in claim 13 further comprising a modulation signal phase correction means for converting samples of five points about the positive slope of the zero crossing of the electrical signals and samples of five points about the negative slope of the zero crossing of the electrical signals thereby determining the phase error according to the following relationship:

20. The apparatus for determining the-phase of signal pulses as claimed in claim 19 wherein said modulation signal amplitude correction means converts samples of five points about a positive slope of a zero crossing of the electrical signals and samples of five points about a negative slope of successive zero crossings of the electrical signals thereby determining the amplitude error per slope sign and per cycle quadrant according to the following eight relationships:

21. The apparatus for determining the phase of signal pulses as claimed in claim 20 wherein the demodulation means for output phase determination provides alternately the demodulated positive slope phase angle, as augmented by the fringe generator, followed by the demodulated negative slope phase angle, as augmented by the fringe generator, based on five samples of the electrical signals about the positive slope of a zero crossing as follows:

22. The apparatus for determining the phase of signal pulses as claimed in claim 20 wherein the demodulation means for output phase determination provides one-half the sum of the demodulated positive slope phase angle, as augmented by the fringe generator, and the demodulated negative slope phase angle, augmented by the fringe generator, based on five samples of the electrical signals about the positive slope of a zero crossing and five sample of the electrical signals about the negative slope of a successive zero crossing as follows:

23. A method of determining the phase of signal pulses which comprises the steps of:

(a) dividing each light pulse from a pulse light source into first and second divided pulses;

(b) modulating each first divided pulse with at least one slope via an initial and reflected pass through a fiber path producing a modulated path length;

said modulated path being effected by a phase modulator;

(c) delaying in time each second divided pulse relative to the first pulse via an initial and reflected pass through a length of fiber;

(d) combining each reflected modulated first pulse and each reflected delayed second pulse to form a pulse pair;

(e) transmitting each pulse pair to one or more interferometers;

(f) transmitting return signals of each pulse pair from the one or more interferometers;

(g) collecting the returned pulse pairs;

(h) converting the collected pulse pairs into receiver-generated electrical signals;

(i) digitally sampling the receiver-generated electrical signals;

(j) demodulating the sampled electrical signals for cycle quadrant determination and phase determination to the unit circle;

(k) determining the cycle quadrant;

(l) tracking fringe crossings using successively determined cycle quadrants and determining fringe count;

(m) determining the fringe by multiplying the fringe count by 2π

; and

(n) determining the output phase by adding the unit circle determined phase to the determined fringe.

24. The method for determining the phase of signal pulses as claimed in claim 23, the step of collecting the returned pulse pairs further comprises the steps of splitting the collected light signals into a plurality of optical paths, filtering each resulting split optical path with polarizers of differing polarization orientations, such that at least one of the filtered paths has a high interferometric visibility.

25. The method of determining the phase of signal pulses as claimed in claim 23 after the step of determining the cycle quadrant, the method further comprising the step of adjusting modulation phase of the phase modulator using five positive and five negative slope sampled electrical signals respectively.

26. The method of determining the phase of signal pulses as claimed in claim 25, wherein the step of adjusting modulation phase of the phase modulator comprises the steps of:

(a) generating a phase error from the sampled electrical signals according to the relationship;

27. The method of determining the phase of signal pulses as claimed in claim 23 after the step of determining the cycle quadrant, the method further comprising the step of adjusting modulation amplitude of the phase modulator using the determined cycle quadrant and the sampled electrical signals.

28. The method of determining the phase of signal pulses as claimed in claim 27, wherein the step of adjusting modulation amplitude of the phase modulator comprises the steps of:

(a) determining, as function of cycle quadrant, a first part of the amplitude error from the five or more samples taken about the zero crossing of the positively-sloped portion of the modulated signal according to following relationship;

29. The method of determining the phase of signal pulses as claimed in claim 23, the method further comprising the step of determining the output phase according to the following relationship:

30. The method of determining the phase of signal pulses as claimed in claim 23, the method further comprising the step of determining the output phase according to the following alternating relationships:

31. The method of determining the phase of signal pulses as claimed in claim 23 wherein the step of digitally sampling the receiver-generated electrical signals comprises sampling five or more samples corresponding to a zero crossing of a positively sloping portion of the modulated signal, sampling five or more signals corresponding to a successive zero crossing of a negatively sloping portion of the modulated signal.

32. The method of determining the phase of signal pulses as claimed in claim 31, after the step of determining the cycle quadrant, the method further comprising the step of adjusting modulation phase of the phase modulator using five or more positive and five or more negative slope sampled electrical signals respectively.

33. The method of determining the phase of signal pulses as claimed in claim 32, the step of adjusting modulation phase of the phase modulator comprises the steps of:

(a) generating a phase error from the sampled electrical signals by taking one-half of the difference between the demodulated positive slope phase angle, as augmented by the determined fringe, and the demodulated negative slope phase angle, as augmented by the determined fringe;

(b) filtering the phase error; and

(c) blending the filtered phase error with the most current prior value of the modulation phase of the phase modulator;

whereby pulse samples are substantially centered about the zero crossings of the positively and negatively sloped portions of the modulated signal.

34. The method of determining the phase of signal pulses as claimed in claim 31, after the step of determining the cycle quadrant;

the method further comprising the step of adjusting modulation amplitude of the phase modulator using the determined cycle quadrant and the sampled electrical signals.

35. The method of determining the phase of signal pulses as claimed in claim 34, the step of adjusting modulation amplitude of the phase modulator comprises the steps of:

(a) converting samples of five or more points about a positive slope of a zero crossing of the electrical signals and samples of five or more points about a negative slope of successive zero crossings of the electrical signals thereby determining amplitude error per slope sign and per cycle quadrant by summing positive slope amplitude error and negative slope amplitude error, each respectively determined from the positive slope samples and the negative slope samples respectively, thereby producing the amplitude error for one modulation cycle where quadrant determination is performed by logically inspecting the location within the unit circle of the demodulated positive slope phase angle for the positive slope amplitude error and the demodulated negative slope phase angle for the negative slope amplitude error;

(b) filtering the phase error; and

(c) blending the filtered phase error with the most current prior value of the modulation amplitude of the phase modulator;

whereby amplitudes of the modulated signal are maintained and with the amplitude maintenance, positive and negative values of the modulated signal zero crossings are determined.

36. The method of determining the phase of signal pulses as claimed in claim 31, the method further comprising the step of determining the output phase using five or more positive and five or more negative slope sampled electrical signals respectively by taking one-half of the sum of the demodulated positive slope phase angle, as augmented by the determined fringe, and the demodulated negative slope phase angle, as augmented by the determined fringe.

37. The method of determining the phase of signal pulses as claimed in claim 31, the method further comprising the step of determining the output phase by alternately:

(a) demodulating five or more positive slope sampled electrical signals, concatenating the demodulated negative slope with fringe, and providing the demodulated positive slope phase angle with concatenated fringe and

(b) demodulating five or more negative slope sampled electrical signals, concatenating the demodulated negative slope with fringe, and then providing the demodulated negative slope phase angle with concatenated fringe,

whereby the pulse rate of the source light pulse and the output phase rate of the apparatus are substantially the same.