Detecting a Disturbance in the Propagation of Light in an Optical Waveguide

• US 20080297772A1
• Filed: 11/03/2005
• Published: 12/04/2008
• Est. Priority Date: 11/03/2004
• Status: Active Grant

• Current Assignee: Viavi Solutions, Inc. (Lumentum Holdings Inc.)
• Original Assignee: Polarmetric Limited
• Inventors: Shatalin, Sergey Vladimir, Rogers, Alan John, Kanellopoulos, Sotiris Emil
• Granted Patent: US 7,872,736 B2
• Time in Patent Office: Days
• US Class Current: 356/73.1
• CPC Class Codes: G01M 11/3118 using coded light-pulse sequences
• CPC Class Codes: G01M 11/3127 using multiple or wavelength variable input source

Abstract

An optical time domain reflectometry apparatus has a laser and light modulator for producing coherent light pulses, each having two sections of higher intensity separated by a gap of lower or substantially zero intensity. As the light pulses propagate along the optical fibre, light is continuously Rayleigh backscattered by inhomogeneities of the optical fibre. A photodetector generates backscatter signals representing the intensity of light Rayleigh backscattered in the optical fibre as each light pulse travels along the optical fibre. The PC uses these backscatter signals to derive a difference signal representing a change dI in intensity between signals generated from two successive pulses. The PC then calculates the Root Mean Square (RMS) of the difference signal averaged over the interval between the two sections of the light pulses. Next, the PC averages the backscatter signal generated from the first of the pulses over the same interval and normalises the RMS difference signal using the averaged signal to obtain a compensated difference signal that depends only on differences in the rate of change of phase of light of the light pulses as they travelled along the waveguide. This is repeated at different wavelengths to allow the compensated difference signal to be adjusted to represent the magnitude of the differences.

Claims

1. An apparatus for detecting a disturbance in the propagation of light in an optical waveguide, the apparatus having a light source for sending a light pulse along the waveguide, the light pulse being substantially coherent and varying in intensity over its duration to provide two sections of higher intensity separated from one another by a section of lower or substantially zero intensity.

2. The apparatus of claim 1, wherein the comprising means for selecting sensitivity to the disturbance by varying the length of the section of lower or substantially zero intensity.

3. The apparatus of claim 1, wherein the intensity of the light pulse varies with a substantially square waveform.

4. The apparatus of claim 1, having a photodetector for generating a signal representing a time distributed intensity of light backscattered in the waveguide as the light pulse travels along the waveguide.

5. The apparatus of claim 4, wherein the photodetector has a response time approximately equal to the duration of either of the two sections of higher intensity of the light pulse.

6. The apparatus of claim 4, wherein the light source sends two such light pulses along the waveguide and the photodetector generates first and second such signals, one for each of the light pulses.

7. The apparatus of claim 6, having processing means for deriving a difference signal indicative of differences between the first and second signals.

8. The apparatus of claim 7, having processing means for:

averaging the first signal over a period approximately equal to the duration of the light pulses;

calculating a root mean square of the difference signal and averaging this over a period approximately the same as the period of averaging of the first signal; and

normalising the averaged root mean square of the difference signal using the averaged first signal to derive a compensated difference signal indicative of a change in the propagation of light in the waveguide.

9. The apparatus of claim 1, wherein the light source sends light pulses having different wavelengths along the waveguide.

10. The apparatus of claim 6, wherein the light source sends a further two such light pulses along the waveguide having a different wavelength from the two such light pulses and the photodetector generates further first and second such signals, one for each of the further light pulses.

11. The apparatus of claim 10, having processing means for deriving a further difference signal indicative of differences between the further first and second signals.

12. The apparatus of claim 10, having processing means for comparing the difference signals to derive an adjusted difference signal indicating the magnitude of the change in the propagation of light in the waveguide.

13. The apparatus of claim 11, having processing means for:

averaging the further first signal over a period approximately equal the duration of the further two such light pulses;

calculating a root mean square of the further difference signal and averaging this over a period approximately the same as the period of averaging of the further first signal;

normalising the averaged root mean square of the further difference signal using the averaged further first signal to derive a further compensated difference signal indicative of a change in the propagation of light in the waveguide; and

comparing the compensated difference signals at the two wavelengths to generate an adjusted compensated difference signal indicative of the magnitude of the change in the propagation of light in the waveguide.

14. An apparatus for optical time domain reflectometry comprising the apparatus of claim 1.

15. An apparatus for detecting a change in the effective refractive index or optical path length of an optical waveguide comprising the apparatus of claim 1.

16. The apparatus of claim 1, wherein the light pulse(s) is/are sufficiently coherent that light backscattered as it/they travel(s) along the waveguide has at least a component of intensity detectable by a/the photodetector arising from interference between light backscattered from each of the two sections of higher intensity of the (respective) light pulse(s).

17. An apparatus for optical time domain reflectometry in a monomode optical waveguide using pairs of substantially mutually coherent light pulses.

18. A method of detecting a disturbance in the propagation of light in an optical waveguide, the method comprising sending a light pulse along the waveguide, the light pulse being substantially coherent and varying in intensity over its duration to provide two sections of higher intensity separated from one another by a section of lower or substantially zero intensity.

19. The method of claim 18, comprising selecting sensitivity to the disturbance by varying the length of the section of lower or substantially zero intensity.

20. The method of claim 18, wherein the intensity of the light pulse varies with a substantially square waveform.

21. The method of claim 18, comprising generating a signal representing a time distributed intensity of light backscattered in the waveguide as the light pulse travels along the waveguide.

22. The method of claim 21, wherein a signal is generated with a response time approximately equal to the duration of each of the sections of higher intensity of the light pulse.

23. The method of claim 21, comprising sending two such light pulses along the waveguide and generating first and second such signals, one for each of the light pulses.

24. The method of claim 23, comprising deriving a difference signal indicative of differences between the first and second signals.

25. The method of claim 24, comprising:

averaging the first signal over a period approximately equal to the duration of the light pulses;

calculating a root mean square of the difference signal and averaging this over a period approximately the same as the period of averaging of the first signal; and

normalising the averaged root mean square of the difference signal using the averaged first signal to derive a compensated difference signal indicative of a change in the propagation of light in the waveguide.

26. The method of claim 18, comprising sending light pulses having different wavelengths along the waveguide.

27. The method of claim 23, comprising sending a further two such light pulses along the waveguide having a different wavelength to the two such light pulses and generating further first and second such signals, one for each of the further light pulses.

28. The method of claim 27, comprising deriving a further difference signal indicative of differences between the further first and second signals.

29. The method of claim 28, comprising comparing the difference signals to derive an adjusted difference signal indicating the magnitude of the change in the propagation of light in the waveguide.

30. The method of claim 28, comprising:

averaging the further first signal over a period approximately equal the duration of the further two such light pulses;

calculating a root mean square of the further difference signal over a period approximately the same as the period of averaging of the further first signal;

normalising the averaged root mean square of the further difference signal using the averaged further first signal to derive a difference signal indicative of a change in the propagation of light in the waveguide; and

comparing the difference signals at the two wavelengths to indicate the magnitude of the change in the propagation of light in the waveguide.

31. A method of optical time domain reflectometry comprising the method of claim 18.

32. A method of detecting a change in the effective refractive index or optical path length of an optical waveguide comprising the method of claim 18.

33. The method of claim 18, wherein the light pulse(s) is/are sufficiently coherent that light backscattered as it/they travel(s) along the waveguide has at least a component of intensity detectable by a/the photodetector arising from interference between light backscattered from each of the two sections of higher intensity of the (respective) light pulse(s).

34. A method of optical time domain reflectometry in monomode optical waveguide using pairs of substantially mutually coherent light pulses.

35. A method of phase disturbance location comprising:

causing a coherent optical pulse to propagate within a monomode optical fibre;

detecting a Rayleigh-backscattered, interference, difference signal, from the optical pulse;

time-resolving the signal to yield a distribution of any optical-phase disturbances imposed on the fibre by external influences; and

enhancing the signal in respect to noise due to variations in backscatter amplitudes by normalising to undisturbed values.

36. The method of claim 35, wherein the signal is enhanced in respect of noise due to variations in the amplitude and phase of the backscatter by employing a coherent-pulse shape which allows detection and resolution of the scatter structure within the pulse, and a suitable averaging and normalisation of the resulting signal structure.

37. (canceled)

38. (canceled)