Assessing A Network

• US 20080123085A1
• Filed: 12/15/2005
• Published: 05/29/2008
• Est. Priority Date: 12/17/2004
• Status: Active Grant

• Current Assignee: British Telecommunications PLC (BT Group PLC)
• Original Assignee: British Telecommunications PLC (BT Group PLC)
• Inventors: Sikora, Edmund SR, Healey, Peter
• Granted Patent: US 8,045,174 B2
• Time in Patent Office: Days
• US Class Current: 356/73.1
• CPC Class Codes: H04B 10/071 using a reflected signal, e.g. using optical time domain reflectometers [OTDR]

Abstract

The present invention relates to a method of assessing a network, in particular a network having a main line and a plurality of branch lines. The method includes the steps of: (i) introducing test signals into the main line, the main line being coupled to the branch lines; (ii) imposing a modulation on test signals which propagate along the branch line and, (iii) monitoring test signals returned along the main line. The imposed phase modulation is detected interferometrically such that the test signals from the designated branch line can be distinguished with respect to the signals returned from other branch lines.

Claims

1. A method of assessing an optical network having a main line and a plurality of branch lines, the method including the steps of:

(i) introducing test signals into the main line, the main line being coupled to the branch lines in a branching fashion such that the test signals propagate along the branch lines;

(ii) imposing a modulation on test signals which propagate along a designated branch line; and

, (iii) monitoring test signals returned along the main line, the imposed phase modulation being detected interferometrically such that the test signals from the designated branch line can be distinguished with respect to the signals returned from other branch lines.

2. A method as claimed in claim 1, wherein the test signals introduced into the main line are formed as pairs of signals, which signals of a pair are at least in part copies of one another.

3. A method as claimed in claim 2, wherein the copies of a given pair are introduced into the main line with a temporal offset relative to one another.

4. A method as claimed in claim 3, wherein the modulation occurs on a time scale that is shorter than the temporal offset.

5. A method as claimed in claim 3 or claim 4, wherein the relative delay is undone, at least for signals returned from the designated branch line.

6. A method as claimed in any of claim 3, wherein the temporal offset is caused at an interferometer arrangement having a first path and a second path, the transit time of the first path being longer than that of the second path, copies of a pair being caused to travel along a different respective path to one another.

7. A method as claimed in claim 6, wherein the interferometer arrangement has a first coupling stage that is coupled to an optical source, the coupling stage being arranged to channel one portion of the incoming radiation intensity from the source along one path, and another portion of the incoming radiation intensity along the other path, so as to form a first and a second copy.

8. A method as claimed in claim 7, wherein the interferometer arrangement has a second coupling stage for combining radiation from the first and second paths, and for coupling the combined radiation onto the main line.

9. A method as claimed in claim 8, wherein the returned signals are each channeled along the first and second paths by the second coupling stage, and wherein the so channeled signals are subsequently combined at the first coupling stage.

10. A method as claimed in claim 1, wherein the test signals are returned from the branch lines by a process of distributed backscattering.

11. A method as claimed in claim 1, wherein the test signals are formed as optical pulses.

12. A method as claimed in claim 3, wherein the test signals are formed as optical pulses;

and

wherein the optical pulses are formed from a waveform with a phase coherence time associated therewith, the phase coherence time for a given pulse being less than the pulse duration.

13. A method as claimed in claim 3, wherein the test signals are optical signals having a waveform with a phase coherence time associated therewith, the phase coherence time being less than the temporal offset.

14. A method as claimed in claim 13, wherein the phase coherence time is less than the temporal offset by at least a factor of 2, preferably by at least a factor of 5, yet more preferably by at least a factor of 10.

15. A method as claimed in claim 1, wherein the test signals are generated by a semiconductor diode device.

16. A method as claimed in claim 1 wherein the modulation is imposed using a modulation device having a waveguide material through which the test signals are guided, the waveguide material having a refractive index that is electric-field-sensitive, which sensitivity is used to impose the modulation with a time varying electric field.

17. A method as claimed in claim 16, wherein the modulation device is a semiconductor device.

18. A method as claimed in claim 1, wherein the modulation has a cyclical form having a frequency associated therewith.

19. A method as claimed in claim 18, wherein the cyclical form is sinusoidal.

20. A method as claimed in claim 18, wherein returned signals have a frequency component corresponding to the modulation frequency.

21. A method as claimed in claim 1, wherein a variation in the returned signals with time is monitored.

22. A method as claimed in claim 1, wherein a plurality of branch lines are designated, a modulation being imposed on the test signals of each designated branch line such that the signals returned from the designated branch lines can be distinguished from one another and/or from test signals returned from other branch lines.

23. A method as claimed in claim 22, wherein test signals on different designated branch lines are modulated a different respective frequencies.

24. A method as claimed in claim 1, including the step of selecting which of the branch lines is a designated branch lines.

25. A method as claimed in claim 24, wherein a plurality of branch lines are each selected sequentially.

26. A method as claimed in claim 23, wherein activating means are provided to activate means for applying a modulation to signals on a selected branch line in response to the selection of that branch line.

27. A method as claimed in claim 1, wherein test signals are returned from 35 the branch lines by a process of Rayleigh backscattering.

28. A method as claimed in claim 1, wherein the branch lines are formed from optical fibre.

29. A method as claimed in claim 1, wherein the modulation is imposed at a location such that the test signals can propagate in a downstream direction beyond said location.

30. A method as claimed in claim 1, wherein the modulation is an acoustic modulation.

31. A method as claimed in claim 30, wherein the acoustic modulation is provided by a modulator having a piezo electric element for generating acoustic vibrations.

32. A method as claimed in claim 30, wherein the acoustic modulation frequency is less or equal to 10 kHz, preferably less or equal to 1 kH.

33. A method as claimed in claim 30, wherein the acoustic modulation frequency is within the bandwidth of the test signals.

34. A method as claimed in claim 30, wherein the test signals are introduced into the main line in a repetitive fashion with a repetition frequency F.

35. A method as claimed in claim 34, wherein the modulation frequency is substantially equal to F/N, where N is an integer greater or equal to 2.

36. Apparatus for assessing an optical network, the network having a main line and a plurality of branch lines, the apparatus including:

(i) means for introducing test signals into the main line, the main line being coupled to the branch lines in a branching fashion such that the test signals propagate along each of the branch lines;

(ii) means for imposing a phase modulation on the test signals which propagate along a designated branch line; and

, (iii) monitoring means for monitoring the main line to detect the test signals returned from the designated branch line, the monitoring means being configured such that, in use, the imposed phase modulation is detected interferometrically, thereby allowing the test signals from the designated branch line to be distinguished with respect to the signals returned from other branch lines.

37. Apparatus for assessing an optical network, the network having a main line and a plurality of branch lines, the apparatus including:

generating means for generating pairs of test pulses, which pulses of a pair are, in respect of at least one component thereof, copies of one another;

means for imposing a temporal offset between the pulses of a pair;

means for introducing the temporary offset pulses onto the main line;

means for combining signals arising from the distributed backscattering of the test pulses along the branch line; and

, means for recording as a function of time a signal resulting from said combination.

38. Apparatus as claimed in claim 37, wherein the respective backscattered signals arising from a pair of pulse copies are combined.

39. Apparatus as claimed in claim 37, wherein the generating means is configured to generate pulses having at least one frequency within a transmission range, and filter means are provided for removing frequencies within the transmission range, such that the combination signal arising from signals returned from a designated branch can be distinguished.

40. A method of assessing an optical network having a main line and a plurality of branch lines, the method including the steps of:

(i) introducing test signals into the main line, the main line being coupled to the branch lines in a branching fashion such that the test signals propagate along the branch lines;

(ii) imposing a modulation on test signals which propagate along a designated branch line; and

, (iii) monitoring the main line to detect returned test signals, the modulation being such that the test signals from the designated branch line can be distinguished with respect to the signals returned from other branch lines.