Assessing a network

• US 8,045,174 B2
• Filed: 12/15/2005
• Issued: 10/25/2011
• 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 S R, Healey, Peter
• Primary Examiner(s): LEE, HWA S
• Application Number:
  US11/791,927
• Publication Number:
  US 20080123085A1
• Time in Patent Office: 2,140 Days
• Field of Search: 356/73.1, 356/477, 356/478
• US Class Current: 356/478
• CPC Class Codes: H04B 10/071 using a reflected signal, e.g. using optical time domain reflectometers [OTDR]

Abstract

A network is assessed, 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 comprising:

introducing test pairs of signals into the main line, wherein test signals of a pair are at least in part copies of one another, the main line being coupled to the branch lines in a branching fashion such that the test pairs of signals propagate along the branch lines;

imposing a phase modulation using a modulator positioned on a designated branch line on a test pair of signals which propagate along the designated branch line;

monitoring test signals returned along the main line at a monitoring location; and

distinguishing the monitored test pair of signals returned from the designated branch line with respect to the monitored test pairs of signals returned from other branch lines by interferometrically detecting the phase modulation imposed when the distinguished test pair of signals were propagating along the designated branch line.

2. A method as claimed in claim 1, wherein the copies of a given pair are introduced into the main line with a temporal offset relative to one another.

3. A method as claimed in claim 2, wherein the modulation occurs on a time scale that is shorter than the temporal offset.

4. A method as claimed in claim 2, wherein the temporal offset is undone, at least for signals returned from the designated branch line.

5. A method as claimed in claim 2, 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.

6. A method as claimed in claim 5, wherein the interferometer arrangement uses 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.

7. A method as claimed in claim 6, wherein the interferometer arrangement uses second coupling stage for combining radiation from the first and second paths, and for coupling the combined radiation onto the main line.

8. A method as claimed in claim 7, wherein the returned signals of said at least one test pair are each channelled along the first and second paths by the second coupling stage, and wherein the so channelled signals are subsequently combined at the first coupling stage.

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

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

11. A method as claimed in claim 2, 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.

12. A method as claimed in claim 2, 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.

13. A method as claimed in claim 12, wherein the phase coherence time is less than the temporal offset by at least a factor of 2.

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

15. A method as claimed in claim 1 wherein the modulator has 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.

16. A method as claimed in claim 15, wherein the modulator uses a semiconductor device.

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

18. A method as claimed in claim 17, wherein the cyclical form is sinusoidal.

19. A method as claimed in claim 17, wherein returned signals have a frequency component corresponding to the modulation frequency.

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

21. 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.

22. A method as claimed in claim 21, wherein test signals on different designated branch lines are modulated a different respective frequencies.

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

24. A method as claimed in claim 23, wherein a plurality of branch lines are each selected sequentially.

25. A method as claimed in claim 22, 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.

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

27. A method as claimed in claim 1, wherein the branch lines use optical fiber.

28. 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.

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

30. A method as claimed in claim 29, wherein the acoustic modulation is provided by a modulator having a piezo electric element for generating acoustic vibrations.

31. A method as claimed in claim 29, wherein the acoustic modulation frequency is less or equal to 10 kHz.

32. A method as claimed in claim 29, wherein the acoustic modulation frequency is within the bandwidth of the test signals.

33. A method as claimed in claim 29, wherein the test signals are introduced into the main line in a repetitive fashion with a repetition frequency F.

34. A method as claimed in claim 33, wherein the modulation frequency is substantially equal to F/N, where N is an integer greater or equal to 2.

39. A method as claimed in claim 1, wherein the phase modulation is detected by combining signals of a given pair.

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

a device for introducing test pairs of signals into the main line, wherein test signals of a pair are at least in part copies of one another, 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;

at least one modulator, each said at least one modulator being positioned on a designated branch line for imposing a phase modulation on a test pair of signals which propagate along the designated branch line; and

a monitoring apparatus arranged to monitor the main line to detect the test signals returned from a plurality of branch lines, the monitoring apparatus being configured such that, in use, the phase modulation imposed using said at least one modulator on test signals propagating along a designated branch line is detected interferometrically, wherein the modulation detected in the test pair of signals on the main line indicates the branch line on which the modulation was imposed on the test pair of signals, thereby allowing the test pair of signals returned from the designated branch line to be distinguished with respect to the test pairs of signals returned from other branch lines.

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

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 temporally offset pulses onto 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;

means for applying a phase modulation to a test pair of pulses on one of said branch lines;

means for combining respective backscatter signals from said test pair of pulses arising from the distributed backscattering of the phase modulated test pulses along the branch lines; and

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

37. Apparatus as claimed in claim 36, 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.

38. A method of assessing an optical network having a main line and a plurality of branch lines, the method comprising

introducing test pairs of signals into the main line, wherein test signals of a pair are at least in part copies of one another, the main line being coupled to the branch lines in a branching fashion such that the test signals propagate along the branch lines;

imposing a phase modulation on a test pair of signals which propagate along a designated branch line; and

monitoring the main line to detect returned back-scattered test signals, wherein a test pair of respective back-scattered signals from the designated branch line have a phase modulation imposed which distinguishes said test pair of respective back-scattered signals with respect to the test pairs of signals returned from other branch lines.

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

introducing a plurality of test pairs of signals into the main line, wherein signals of a test pair are at least in part copies of one another, the main line being coupled to the branch lines in a branching fashion such that the test pairs of signals propagate along the branch lines;

imposing a modulation on a test pair of signals which propagate along a designated branch line; and

monitoring said test pair of signals returned along the main line, the imposed phase modulation being detected interferometrically such that the test pair of signals from the designated branch line is distinguished with respect to the test pairs of signals returned from other branch lines,

wherein said signals of a given test pair are introduced into the main line with a temporal offset relative to one another, and

wherein the temporal offset is undone at least for the copies of a given test pair of signals returned from the designated branch line.