U.S. patent application number 11/311009 was filed with the patent office on 2006-07-27 for apparatus and method for using a counter-propagating signal method for locating events.
This patent application is currently assigned to Future Fibre Technologies Pty Ltd. Invention is credited to Jim Katsifolis, Lee J. McIntosh, Edward E. Tapanes.
Application Number | 20060163457 11/311009 |
Document ID | / |
Family ID | 36677293 |
Filed Date | 2006-07-27 |
United States Patent
Application |
20060163457 |
Kind Code |
A1 |
Katsifolis; Jim ; et
al. |
July 27, 2006 |
Apparatus and method for using a counter-propagating signal method
for locating events
Abstract
An apparatus and method for using a counter-propagating signal
method for locating events is disclosed. The apparatus and method
uses a Mach Zehnder interferometer through which
counter-propagating signals can be launched. If the sensing zone of
the Mach Zehnder interferometer is disturbed, modified
counter-propagating signals are produced and the time difference
between receipt of those signals is used to determine the location
of the event. Polarisation controllers (43, 44) receive feedback
signals so that the polarisation states of the counter-propagating
signals can be controlled to match the amplitude and/or phase of
the signals. Detectors are provided for detecting the modified
signals.
Inventors: |
Katsifolis; Jim; (Northcote,
AU) ; Tapanes; Edward E.; (Wheelers Hill, AU)
; McIntosh; Lee J.; (Stockholm, SE) |
Correspondence
Address: |
David R. Price;Michael Best & Friedrich LLP
Suite 3300
100 East Wisconsin Avenue
Milwaukee
WI
53202-4108
US
|
Assignee: |
Future Fibre Technologies Pty
Ltd
Mulgrave
AU
|
Family ID: |
36677293 |
Appl. No.: |
11/311009 |
Filed: |
December 19, 2005 |
Current U.S.
Class: |
250/227.14 ;
250/227.17; 250/227.19 |
Current CPC
Class: |
G08B 13/186 20130101;
G02B 6/2793 20130101; G01M 11/39 20130101; G02B 6/29352
20130101 |
Class at
Publication: |
250/227.14 ;
250/227.17; 250/227.19 |
International
Class: |
G01J 1/04 20060101
G01J001/04; G01J 4/00 20060101 G01J004/00; G01J 1/42 20060101
G01J001/42 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 11, 2005 |
AU |
2005900109 |
Claims
1. An apparatus for detecting and locating disturbances,
comprising: at least one light source; an optical system with at
least one optical waveguide, the optical waveguide having at least
one detection zone at which a disturbance can occur and affect
optical signals from the light source when traveling through the
waveguide, in the detection zone, along counter-propagating optical
channels; at least one beam separator between the light source and
the optical system, wherein the beam separator couples at least two
beams into the optical waveguide for each of the at least two
counter-propagating optical channels; at least one polarization
controller operable to manage optical properties of said
counter-propagating optical channels, the polarization controller
adjusting optical properties for at least one of the optical
signals when propagating toward the detection zone; at least one
detector coupled to the optical waveguide and responsive to the
optical signals after traversing the detection zone; a data
processing unit coupled to the detector, the data processing unit
being operable to localize a place of the disturbance in the
detection zone from a difference between times at which effects of
the disturbance appear at the detector; and, a feedback control
coupled to the optical system and to at least one said polarization
controller, wherein the feedback control and the polarization
controller are configured to maximize a signal-to-noise ratio and
to minimize a polarization contribution to said difference between
times, by at least one of: seeking a predetermined relationship
between polarization phase transformations along the
counter-propagating optical channels, maximizing a peak swing in
intensity at a point of interference of the beams, and varying an
input state of polarization for one of testing and adjusting a
balance between said polarization transformations for the
counter-propagating channels.
2. The apparatus of claim 1, wherein the light source comprises a
laser.
3. The apparatus of claim 1, wherein the light source is wavelength
tunable.
4. The apparatus of claim 1, wherein the light source comprises a
single beam source coupled to the at least one beam separator,
wherein the beam separator couples a portion of light energy from
the single beam source separately into each of the
counter-propagating optical channels, respectively.
5. The apparatus of claim 1, wherein the light source comprises at
least two beam sources that are coupled respectively to said
counter-propagating channels.
6. The apparatus of claim 1, wherein the optical waveguide
comprises at least one optical fiber in the detection zone, and the
counter-propagating beams are passed through said at least one
optical fiber in the detection zone.
7. The apparatus of claim 6, wherein the at least one optical fiber
in the detection zone comprises a single mode optical fiber.
8. The apparatus of claim 1, wherein the optical waveguide
comprises at least two optical fibers that are coextensive at least
in the detection zone, and wherein both of said at least two
optical fibers are subject to the disturbance in the detection
zone.
9. The apparatus of claim 1, wherein optical waveguide comprises at
least one optical fiber extending along the detection zone, and
wherein said optical fiber is at least one of configured and
controlled such that the optical properties are substantially the
same for said counter propagating channels with respect to optical
phase.
10. The apparatus of claim 1, wherein optical waveguide comprises
at least two optical fibers, at least one of which extends along
the detection zone, and wherein said two optical fibers are at
least one of configured and controlled such that the optical
properties are substantially the same for the said counter
propagating channels with respect to optical phase.
11. The apparatus of claim 1, wherein said at least one
polarization controller is placed between said light source and at
least one of the counter-propagating optical channels.
12. The apparatus of claim 1, wherein the said optical waveguide is
coupled to define at least one path in an interferometer.
13. The apparatus of claim 12, wherein the said interferometer is
configured as a Mach-Zehnder interferometer.
14. The apparatus of claim 1, wherein said beam separator is
polarization insensitive.
15. The apparatus of claim 1, wherein said beam separator is
polarization sensitive.
16. The apparatus of claim 1, wherein the polarization controller
is operable to transform the optical properties of at least one
beam of the counter propagating optical channels from a first
arbitrary state of polarization to a second arbitrary state of
polarization.
17. The apparatus of claim 1, wherein said at least one optical
detector is operable to sense at least one aspect of a light signal
from said counter propagating channels.
18. The apparatus of claim 17, wherein said optical detector is
operable to sense an intensity aspect of the light signal.
19. The apparatus of claim 1, wherein-said at least one optical
detector is operable individually to sense at least one aspect of
light signals emerging respectively from said counter propagating
channels.
20. The apparatus of claim 1, wherein the feedback control to the
polarization controller is configured to maintain a signal to noise
ratio for a signal resulting from the disturbance.
21. The apparatus of claim 1, wherein the feedback control and the
polarization controller are configured to minimize at least one of
polarization induced signal fading and polarization induced phase
shift.
22. The apparatus of claim 21, wherein the polarization controller
is configured at least in one mode substantially to scramble a
polarization state of at least one the beams, to obtain a
substantially random input state of polarization
23. The apparatus of claim 21, wherein the polarization controller
and feedback control are coupled to maximize a peak-to-peak swing
of interference intensity of the light signals.
24. The apparatus of claim 23, wherein the polarization controller
and feedback control are configured to maximize said peak-to-peak
swing of interference intensity at a fixed phase difference between
the two beams when caused to interfere.
25. The apparatus of claim 23, wherein the polarization controller
and feedback control are configured to maximize said peak-to-peak
swing of interference intensity at an arbitrary phase difference
between the two beams when caused to interfere.
26. The apparatus of claim 23, wherein the polarization controller
and feedback control are configured to maximize said peak-to-peak
swing of interference intensity by adjusting a polarization state
relation of the beams for one of the counter-propagating light
signals while scrambling a polarization state relationship of the
beams for another of the counter-propagating light signals.
27. The apparatus of claim 1, wherein the polarization controller
is placed between the light source and said optical waveguide, such
that the polarization controller simultaneously affects both the
counter-propagating optical signals.
28. The apparatus of claim 27, further comprising at least one
additional polarization controller, wherein said polarization
controllers are operable to vary polarization properties for one of
the two optical signals, by varying a polarization transformation
for at least one of the counter-propagating optical signals while
polarization transformations for both the counter-propagating
optical signals are matched, at least relative to one another.
29. The apparatus of claim 28, wherein the said polarization
controllers are operable to vary said polarization properties by
scrambling the polarization transformation for said at least one of
the counter-propagating optical signals.
30. The apparatus of claim 1, wherein the data processing unit is
programmed to resolve a location of the disturbance in the
detection zone from signals received at the detector.
31. The apparatus of claim 30, wherein the said data processing
unit comprises at least one of a programmable gate array and a
digital signal processor.
32. The apparatus of claim 1, wherein the at least one polarization
controller is operable to hold a balance of polarization states in
the counter-propagating optical channels, whereby said polarization
states are made more likely to correspond during the
disturbance.
33. The apparatus of claim 1, wherein the at least one polarization
controller is operable to scramble the polarization in the
counter-propagating optical channels.
34. The apparatus of claim 1, wherein the at least one polarization
controller is operable to hold a relation in polarization states
between the counter-propagating optical channels, for maintaining a
state of interference between the optical channels.
35. The apparatus of claim 1, further comprising a data
transmission path traversing the optical waveguide, supporting at
least one optical data transmission signal.
36. The apparatus of claim 35, wherein an operating wavelength of
the counter propagating optical beam is different from an operating
wavelength of the optical data transmission signal.
37. The apparatus of claim 35, wherein the optical data
transmission signal is carried over at least one same channel as
the counter-propagating optical channels.
38. The apparatus of claim 1, further comprising a communication
device operable to report information regarding the
disturbance.
39. The apparatus of claim 38, wherein the communication device
comprises one of a wired and wireless reporting link to a remote
location.
40. A method for detecting and locating disturbances, comprising:
establishing an optical system including at least one optical
waveguide extending along at least one detection zone at which a
disturbance can occur, so as to affect optical signals propagating
along counter-propagating optical channels from at least one light
source to a detector; separating from the at least one light
source, and coupling into each of the counter-propagating optical
channels, at least two beams; managing optical properties in the
counter-propagating optical channels using a polarization
controller to vary optical properties for at least one of the
optical signals while propagating toward the detection zone;
detecting the optical signals after traversing the detection zone
and determining a difference between times at which effects of the
disturbance appear in the respective counter-propagating channels
after traversing said detection zone; calculating from said
difference between times and localizing in the detection zone a
place where the disturbance occurred; wherein said managing of the
optical properties comprises providing a control signal to the
polarization controller that maintains a signal-to-noise ratio and
minimizes a contribution to said difference between times caused by
polarization effects, including at least one of: seeking a
predetermined relationship between polarization phase
transformations along the counter-propagating optical channels,
maximizing a peak swing in intensity at a point of interference of
the beams, and varying an input state of polarization for one of
testing and adjusting a balance between said polarization
transformations for the counter-propagating channels.
41. The method of claim 40, further comprising tuning a wavelength
of the light source.
42. The method of claim 40, wherein said separating comprises
dividing a portion of light energy from a single beam source
separately into each of the counter-propagating optical channels,
respectively.
43. The method of claim 40, wherein the counter-propagating
channels are established through at least one optical fiber
extending through the detection zone.
44. The method of claim 40, wherein the counter-propagating
channels are established through at least two optical fibers
extending through the detection zone.
45. The method of claim 40, comprising managing said optical
properties to obtain substantially equal optical phase
transformations through said counter propagating channels.
46. The method of claim 40, wherein said detection zone defines a
portion of an interferometer and further comprising developing said
intensity signal at a point of interference of said beams.
47. The method of claim 40, comprising applying the polarization
controller to transform the optical properties of at least one beam
of the counter propagating optical channels from a first arbitrary
state of polarization to a second arbitrary state of
polarization.
48. The method of claim 40, wherein varying the input state of
polarization comprises producing a substantially random input state
of polarization.
49. The method of claim 48, further comprising successively varying
said input state.
50. The method of claim 40, comprising adjusting a polarization
state relation of the beams for one of the counter-propagating
light signals while scrambling a polarization state relationship of
the beams for another of the counter-propagating light signals.
51. The method of claim 40, comprising placing at least one said
polarization controller between the light source and said optical
waveguide, such that the polarization controller simultaneously
affects both the counter-propagating optical signals.
52. The method of claim 51, further comprising placing at least one
additional said polarization controller so as to vary polarization
properties for one of the two optical signals.
53. The method of claim 40, comprising varying a polarization
transformation for at least one of the counter-propagating optical
signals while polarization transformations for both the
counter-propagating optical signals are matched, at least relative
to one another.
54. An improved method for detecting and locating disturbances
affecting an optical system including at least one optical
waveguide extending along at least one detection zone at which a
disturbance can occur, thereby affecting optical signals
propagating along counter-propagating optical channels from at
least one light source to a detector, wherein at least two beams
are separated from the at least one light source and coupled into
each of the counter-propagating optical channels, and an effect of
the disturbance is detected after the beams have traversed the
detection zone and a time difference is determined for calculating
a location of the disturbance in the detection zone, wherein the
improvement comprises: managing optical properties in the
counter-propagating optical channels using a polarization
controller to vary optical properties for at least one of the
optical signals while propagating toward the detection zone,
wherein said managing includes providing a control signal to the
polarization controller that maintains a signal-to-noise ratio and
minimizes a contribution to said difference between times caused by
polarization effects, and comprises at least one of seeking a
predetermined relationship between polarization phase
transformations along the counter-propagating optical channels,
maximizing a peak swing in intensity at a point of interference of
the beams, and varying an input state of polarization for one of
testing and adjusting a balance between said polarization
transformations for the counter-propagating channels.
55. An apparatus for locating the position of an event, comprising:
a light source; a waveguide for receiving light from the light
source so that the light is caused to propagate in both directions
along the waveguide to thereby provide counter-propagating optical
signals in the waveguide, the waveguide being capable of having the
counter-propagating optical signals or some characteristic of the
signals modified or affected by an external parameter caused by or
indicative of the event to provide modified counter-propagating
optical signals which continue to propagate along the waveguide;
detector means for detecting the modified counter-propagating
optical signals affected by the parameter and for determining the
time difference between the receipt of the modified
counter-propagating optical signals in order to determine the
location of the event; a controller for controlling polarisation
states of the counter-propagating optical signals so that the
signals are amplitude and phase matched; and wherein the waveguide
comprises a first arm for receiving the counter-propagating
signals, and a second arm for receiving the counter-propagating
signals, the first and second arms forming a Mach Zehnder
interferometer.
56. The apparatus of claim 55 wherein the input polarisation states
of the counter-propagating signals are controlled to achieve
maximum output fringes. However, in other embodiments, polarisation
states which lead to amplitude and phase matched outputs, but with
sub-maximum fringe visibilities can also be utilised.
57. The apparatus of claim 55 wherein the control unit comprises
the detector means, a polarisation controller for each of the
counter-propagating signals and the light source.
58. The apparatus of claim 55 wherein the detector means comprises
a first detector for one of the counter-propagating signals and a
second detector for the other of the counter-propagating
signals.
59. The apparatus of claim 55 wherein the light source comprises a
laser light source having bragg gratings and an adjuster for
controlling the bragg gratings and/or laser cavity of the laser
light source to thereby alter the wavelength of the light signal
output from the laser for producing the counter-propagating
signals.
60. The apparatus of claim 55 wherein the control unit includes a
processor for receiving outputs from the detectors and for
processing the outputs to indicate an event and to determine the
location of the event.
61. The apparatus of claim 55 wherein the processor is coupled to a
polarisation control driver and the polarisation control driver is
coupled to the polarisation controllers for controlling the
controllers to thereby set the polarisation of the signals supplied
from the light source to the first and second arms of the Mach
Zehnder interferometer to in turn set the polarisation of the
counter-propagating signals.
62. The apparatus of claim 55 wherein the detectors are connected
to a Mach Zehnder output monitor for monitoring the
counter-propagating signals detected by the detectors so that when
the modified counter-propagating detectors are detected by the
detector, the MZ output determines detection of those signals by
the detectors for processing by the processor.
63. The apparatus of claim 55 wherein invention the first arm of
the Mach Zehnder interferometer is of different length than the
second arm of the Mach Zehnder interferometer so that the first and
second arms have a length mismatch, the control unit further
comprising a dither signal producing element for controlling the
light source to wavelength dither the output from the light source
to produce a dither in the phase difference between the MZ arms, in
turn which produces artificial fringes at the drifting output of
the MZ.
64. The apparatus of claim 63 wherein the dither signal element
dithers the phase difference between the MZ arms by at least
360.degree., to produce artificial fringes, so that the drifting
output of the Mach Zehnder's operating point always displays its
true fringe visibility.
65. A method of locating an event comprising the steps of:
launching light into a waveguide so that the light is caused to
propagate in both directions along the waveguide to thereby provide
counter-propagating optical signals in the waveguide, the waveguide
being capable of having the counter-propagating optical signals or
some characteristic of the signals modified or affected by an
external parameter caused by the event, to provide modified
counter-propagating optical signals which continue to propagate
along the waveguide; substantially continuously and simultaneously
monitoring the modified counter-propagating optical signals, so
that when an event occurs, both of the modified counter-propagating
optical signals affected by the external parameter are detected;
determining the time difference between the detection of the
modified signals in order to determine the location of the event;
forming the waveguide as a Mach Zehnder interferometer having a
first arm through which the counter-propagating optical signals
travel, and a second arm through which the counter-propagating
optical signals travel; and controlling the polarisation states of
the counter-propagating optical signals input into the waveguide to
provide amplitude and phase matched counter-propagating signals
from the waveguide.
66. The method of claim 65 wherein the polarisation states of the
counter-propagating signals provide amplitude and phase-matched
counter-propagating signals from the waveguide which achieve
maximum output fringes. However, in other embodiments, the control
of the polarisation states may be such that phase matched
sub-maximum fringes are provided.
67. The method of claim 65 wherein the step of controlling the
polarisation states comprises randomly changing the input
polarisation states of the counter-propagating signals whilst
monitoring the counter-propagating optical signals output from the
Mach Zehnder interferometer to detect a substantially zero state of
intensities, or maximum state of intensities of the
counter-propagating signals, and selecting the input polarisations
which provide the substantially zero or substantially maximum
intensities.
68. The method of claim 65 wherein fringes for determining the
polarisation states are artificially created.
69. The method of claim 68 wherein the artificially created fringes
are created by dithering or modulating the wavelength of the light
source and providing a path length mismatch between the first and
second arms of the Mach Zehnder interferometer.
70. The method of claim 65 wherein the step of controlling the
polarisation states comprises controlling the polarisation
controllers to thereby set the input polarisation state of the
signals supplied from the light source to each input of the
bidirectional Mach Zehnder interferometer to provide phase matched
counter-propagating output signals.
71. The method of claim 65 wherein the laser source wavelength is
dithered by an amount which leads to the dithering of the phase
difference between the MZ arms by 360.degree., to produce
artificial fringes, so that with a drifting operating point, the
Mach Zehnder's counter-propagating outputs always display their
true fringe visibility.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a method and apparatus for
locating events, such as intrusions into a secured premises, or
breakdown or other events associated with structures, so that the
location of the event can be determined.
BACKGROUND OF THE INVENTION
[0002] Apparatus and method for locating events are disclosed in
our U.S. Pat. Nos. 6,621,947 and 6,778,717. The contents of these
two patents are incorporated into this specification by this
reference.
[0003] The system used in the above-identified U.S. patents
utilises a Mach Zehnder (MZ) interferometer in which
counter-propagating signals are provided. By measuring the time
difference of perturbed signals caused by an event, the location of
the event along the sensing device formed by the Mach Zehnder
interferometer can be determined.
[0004] Thus, when an event perturbs the MZ sensor portion of the
system, the difference in the arrival time of the
counter-propagating signals at the detectors can be used to
calculate the exact location of the perturbation on the MZ sensor.
This type of sensor can be applied to perimeter or infrastructure
security applications, with typical sensing lengths exceeding 50
km.
SUMMARY OF THE INVENTION
[0005] The object of the invention is to improve the system and
method disclosed in the above-identified patents so that a more
exact location of the event can be provided.
[0006] The invention provides an apparatus for detecting and
locating disturbances, comprising: [0007] at least one light
source; [0008] an optical system with at least one optical
waveguide, the optical waveguide having at least one detection zone
at which a disturbance can occur and affect optical signals from
the light source when traveling through the waveguide, in the
detection zone, along counter-propagating optical channels; [0009]
at least one beam separator between the light source and the
optical system, wherein the beam separator couples at least two
beams into the optical waveguide for each of the at least two
counter-propagating optical channels; [0010] at least one
polarization controller operable to manage optical properties of
said counter-propagating optical channels, the polarization
controller adjusting optical properties for at least one of the
optical signals when propagating toward the detection zone; [0011]
at least one detector coupled to the optical waveguide and
responsive to the optical signals after traversing the detection
zone; [0012] a data processing unit coupled to the detector, the
data processing unit being operable to localize a place of the
disturbance in the detection zone from a difference between times
at which effects of the disturbance appear at the detector; and,
[0013] a feedback control coupled to the optical system and to at
least one said polarization controller, wherein the feedback
control and the polarization controller are configured to maximize
a signal-to-noise ratio and to minimize a polarization contribution
to said difference between times, by at least one of: seeking a
predetermined relationship between polarization phase
transformations along the counter-propagating optical channels,
maximizing a peak swing in intensity at a point of interference of
the beams, and varying an input state of polarization for one of
testing and adjusting a balance between said polarization
transformations for the counter-propagating channels.
[0014] The invention also provides a method for detecting and
locating disturbances, comprising: [0015] establishing an optical
system including at least one optical waveguide extending along at
least one detection zone at which a disturbance can occur, so as to
affect optical signals propagating along counter-propagating
optical channels from at least one light source to a detector;
[0016] separating from the at least one light source, and coupling
into each of the counter-propagating optical channels, at least two
beams; [0017] managing optical properties in the
counter-propagating optical channels using a polarization
controller to vary optical properties for at least one of the
optical signals while propagating toward the detection zone; [0018]
detecting the optical signals after traversing the detection zone
and determining a difference between times at which effects of the
disturbance appear in the respective counter-propagating channels
after traversing said detection zone; [0019] calculating from said
difference between times and localizing in the detection zone a
place where the disturbance occurred; [0020] wherein said managing
of the optical properties comprises providing a control signal to
the polarization controller that maintains a signal-to-noise ratio
and minimizes a contribution to said difference between times
caused by polarization effects, including at least one of: [0021]
seeking a predetermined relationship between polarization phase
transformations along the counter-propagating optical channels,
[0022] maximizing a peak swing in intensity at a point of
interference of the beams, and [0023] varying an input state of
polarization for one of testing and adjusting a balance between
said polarization transformations for the counter-propagating
channels.
[0024] The invention also provides an improved method for detecting
and locating disturbances affecting an optical system including at
least one optical waveguide extending along at least one detection
zone at which a disturbance can occur, thereby affecting optical
signals propagating along counter-propagating optical channels from
at least one light source to a detector, wherein at least two beams
are separated from the at least one light source and coupled into
each of the counter-propagating optical channels, and an effect of
the disturbance is detected after the beams have traversed the
detection zone and a time difference is determined for calculating
a location of the disturbance in the detection zone, wherein the
improvement comprises: [0025] managing optical properties in the
counter-propagating optical channels using a polarization
controller to vary optical properties for at least one of the
optical signals while propagating toward the detection zone,
wherein said managing includes providing a control signal to the
polarization controller that maintains a signal-to-noise ratio and
minimizes a contribution to said difference between times caused by
polarization effects, and comprises at least one of [0026] seeking
a predetermined relationship between polarization phase
transformations along the counter-propagating optical channels,
[0027] maximizing a peak swing in intensity at a point of
interference of the beams, and [0028] varying an input state of
polarization for one of testing and adjusting a balance between
said polarization transformations for the counter-propagating
channels.
[0029] The invention still further provides an apparatus for
locating the position of an event, comprising: [0030] a light
source; [0031] a waveguide for receiving light from the light
source so that the light is caused to propagate in both directions
along the waveguide to thereby provide counter-propagating optical
signals in the waveguide, the waveguide being capable of having the
counter-propagating optical signals or some characteristic of the
signals modified or affected by an external parameter caused by or
indicative of the event to provide modified counter-propagating
optical signals which continue to propagate along the waveguide;
[0032] detector means for detecting the modified
counter-propagating optical signals affected by the parameter and
for determining the time difference between the receipt of the
modified counter-propagating optical signals in order to determine
the location of the event; [0033] a controller for controlling
polarisation states of the counter-propagating optical signals so
that the signals are amplitude and phase matched; and [0034]
wherein the waveguide comprises a first arm for receiving the
counter-propagating signals, and a second arm for receiving the
counter-propagating signals, the first and second arms forming a
Mach Zehnder interferometer.
[0035] By matching the amplitude and phase of the
counter-propagating signals, output fringes at the detector are
produced which are easily detected and therefore the time
difference between receipt of the two modified counter-propagating
detectors can be accurately recorded to thereby accurately
determine the location of the event. This also improves the
sensitivity of the system and method.
[0036] In the preferred embodiment of the invention, the input
polarisation states of the counter-propagating signals are
controlled to achieve maximum output fringes. However, in other
embodiments, polarisation states which lead to amplitude and phase
matched outputs, but with sub-maximum fringe visibilities can also
be utilised.
[0037] Preferably the control unit comprises the detector means, a
polarisation controller for each of the counter-propagating signals
and the light source.
[0038] Preferably the detector means comprises a first detector for
one of the counter-propagating signals and a second detector for
the other of the counter-propagating signals.
[0039] Preferably the light source comprises a laser light source
having bragg gratings and an adjuster for controlling the bragg
gratings to thereby alter the wavelength of the light signal output
from the laser for producing the counter-propagating signals.
[0040] Preferably the control unit includes a processor for
receiving outputs from the detectors and for processing the outputs
to indicate an event and to determine the location of the
event.
[0041] In one embodiment the processor is coupled to a polarisation
control driver and the polarisation control driver is coupled to
the polarisation controllers for controlling the controllers to
thereby set the polarisation of the signals supplied from the light
source to the first and second arms of the Mach Zehnder
interferometer to in turn set the polarisation of the
counter-propagating signals.
[0042] Preferably the detectors are connected to a Mach Zehnder
output monitor for monitoring the counter-propagating signals
detected by the detectors so that when the modified
counter-propagating detectors are detected by the detector, the MZ
output determines detection of those signals by the detectors for
processing by the processor.
[0043] In one embodiment of the invention the first arm of the Mach
Zehnder interferometer is of different length than the second arm
of the Mach Zehnder interferometer so that the first and second
arms have a length mismatch, the control unit further comprising a
dither signal producing element for controlling the light source to
wavelength dither the output from the light source to produce a
dither in the phase difference between the MZ arms, in turn which
produces artificial fringes at the drifting output of the MZ.
[0044] Preferably the dither signal element dithers the phase
difference between the MZ arms by at least 360.degree., to produce
artificial fringes, so that the drifting output of the Mach
Zehnder's operating point always displays its true fringe
visibility.
[0045] The invention also provides a method of locating an event
comprising the steps of: [0046] launching light into a waveguide so
that the light is caused to propagate in both directions along the
waveguide to thereby provide counter-propagating optical signals in
the waveguide, the waveguide being capable of having the
counter-propagating optical signals or some characteristic of the
signals modified or affected by an external parameter caused by the
event, to provide modified counter-propagating optical signals
which continue to propagate along the waveguide; [0047]
substantially continuously and simultaneously monitoring the
modified counter-propagating optical signals, so that when an event
occurs, both of the modified counter-propagating optical signals
affected by the external parameter are detected; [0048] determining
the time difference between the detection of the modified signals
in order to determine the location of the event; [0049] forming the
waveguide as a Mach Zehnder interferometer having a first arm
through which the counter-propagating optical signals travel, and a
second arm through which the counter-propagating optical signals
travel; and [0050] controlling the polarisation states of the
counter-propagating optical signals input into the waveguide to
provide amplitude and phase matched counter-propagating signals
from the waveguide.
[0051] Preferably the polarisation states of the
counter-propagating signals provide amplitude and phase-matched
counter-propagating signals from the waveguide which achieve
maximum output fringes. However, in other embodiments, the control
of the polarisation states may be such that phase matched
sub-maximum fringes are provided.
[0052] Preferably the step of controlling the polarisation states
comprises randomly changing the input polarisation states of the
counter-propagating signals whilst monitoring the
counter-propagating optical signals output from the Mach Zehnder
interferometer to detect a substantially zero state of intensities,
or maximum state of intensities of the counter-propagating signals,
and selecting the input polarisations which provide the
substantially zero or substantially maximum intensities.
[0053] In one embodiment fringes for determining the polarisation
states are artificially created.
[0054] Preferably the artificially created fringes are created by
dithering or modulating the wavelength of the light source and
providing a path length mismatch between the first and second arms
of the Mach Zehnder interferometer.
[0055] In one embodiment the step of controlling the polarisation
states comprises controlling the polarisation controllers to
thereby set the input polarisation state of the signals supplied
from the light source to each input of the bidirectional Mach
Zehnder interferometer to provide phase matched counter-propagating
output signals.
[0056] Preferably the wavelength of the laser source is dithered by
an amount which leads to the dithering of the phase difference
between the MZ arms by 360.degree., to produce artificial fringes,
so that with a drifting operating point, the Mach Zehnder's
counterpropagating outputs always display their true fringe
visibility.
BRIEF DESCRIPTION OF THE DRAWINGS
[0057] Preferred embodiments will be described, by way of example,
with reference to the accompanying drawings, in which:
[0058] FIG. 1 is a diagram of a basic layout of a conventional
uni-directional MZ system;
[0059] FIG. 2 is a Poincare sphere illustrating polarisation states
which achieve maximum fringe visibility for a conventional
unidirectional MZ;
[0060] FIG. 3 is a diagram showing the outputs in FIG. 1 caused by
the phase differences between the MZ arms;
[0061] FIG. 4 is a diagram of a system according to preferred
embodiments of the invention;
[0062] FIG. 5 is a Poincare sphere diagram relating to the
embodiment of FIG. 4;
[0063] FIG. 6A and FIG. 6B are graphs showing maximum fringe
visibility outputs according to the embodiment of FIG. 4;
[0064] FIG. 7A and FIG. 7B are illustrative diagrams showing
non-phase matched maximum fringe outputs;
[0065] FIG. 8 is a Poincare sphere diagram illustrating various
polarisation states of one counter-propagating signal to provide
various fringe visibilities according to embodiments of the
invention;
[0066] FIG. 9 is a diagram similar to FIG. 8 relating to the other
counter-propagating signal;
[0067] FIG. 10 is a graph showing the effect of wavelength dither
of the light source on the output of the MZ used in one embodiment
of the invention;
[0068] FIG. 11 is a graph showing the effect of a drifting
operating point in the MZ output, which drifts in and out of
quadrature, on the stimulated fringes produced by the Mach Zehnder
interferometer;
[0069] FIG. 12 is a diagram similar to FIG. 11 but showing a
360.degree. dither of the phase difference between the MZ arms for
a drifting MZ output;
[0070] FIG. 13 is a block diagram of a first embodiment of the
invention;
[0071] FIG. 14 is a diagram of a typical Mach Zehnder
interferometer used in the preferred embodiments;
[0072] FIG. 15 is a diagram showing the controller of the
embodiment of FIG. 13;
[0073] FIG. 16 is a schematic view of a second embodiment of the
invention;
[0074] FIG. 17 is a block diagram of the controller of the
embodiment of FIG. 16; and
[0075] FIG. 18 is a block diagram of a third embodiment of the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0076] With reference to FIG. 1, both outputs of a conventional
unidirectional MZ (shown in FIG. 3), which are complementary, will
drift and vary in an approximately sinusoidal fashion with time due
to environmental and other effects. The maximum possible amplitude,
or fringe visibility, of the intensity of both MZ outputs can be
related to the alignment of the polarisation states of the
interfering signals in the MZ interferometer arms. It is possible
to control the alignment of polarisation states of the interfering
signals, and therefore the fringe visibility of a MZ, by
controlling the polarisation of the light signal in the input lead
fibre. In fact, there are two possible polarisation states at the
input lead of the fibre MZ for which the fringe visibility of the
outputs of a unidirectional MZ is at a maximum approaching unity.
This is shown in FIG. 2 for a unidirectional MZ using a Poincare
sphere to illustrate the maximum fringe input polarisation
states.
[0077] As shown in FIG. 2, there exist two unique polarisation
states at the input of the MZ 10, for which either output of the
conventional unidirectional MZ will have a maximum fringe
visibility. When plotted on the Poincare sphere, these two
polarisation states are diametrically opposed.
[0078] With reference to FIG. 4, the inventors have shown
experimentally that the bidirectional MZ 10 can be treated as two
separate unidirectional MZs, one for each direction of propagation.
However, the two counter-propagating MZs are not completely
independent, and do share an important polarisation related
behaviour.
[0079] The counterpropagating outputs of the bi-directional MZ will
also drift and vary in the same fashion. For each direction, there
will also be two input polarisation states for which the MZ outputs
will achieve a maximum output fringe. Although the choice of either
one of these input polarisation states achieves a maximum output
fringe, and thus a maximum sensitivity for a conventional MZ, in
the case of a bi-directional MZ used to locate events, the choice
of input polarisation state for each direction carries an important
significance. For this discussion, it will be assumed that only one
MZ output for each direction is used (CW.sub.out and
CCW.sub.out).
[0080] Given that there are two possible input polarisation states
for each direction which achieves maximum fringe visibility, then
there are four possible pairings of counter-propagating input
polarisation states which will simultaneously achieve maximum
output fringe visibilities for both directions.
[0081] The bi-directional MZ 10 shown in FIG. 4 includes a coupler
C4 to incorporate a fibre lead-in length 12 to the MZ sensor,
L.sub.lead2. This is one practical way to set up the system as it
allows for encapsulation of the optoelectronics and associated
optical components in one controller unit 20. Also included are two
polarisation controllers, PC.sub.cw 43 and PC.sub.ccw 44, which can
be used to control the input polarisation state to the MZ 10 for
the CW (clockwise) and CCW (counter-clockwise) directions,
respectively. Controlling the input polarisation state in the lead
fibre of a MZ can achieve maximum output fringes. This can be
applied independently to both directions on the bi-directional MZ
10 in order to simultaneously achieve maximum output fringes for
both directions. Various multiple-plate, voltage controlled
polarisation controllers can be used to control the input
polarisation state, and can include liquid crystal based
polarisation controllers or piezo-based polarisation
controllers.
[0082] For the CW propagation direction there are the two possible
input polarisation states which give maximum output
fringes--SOP.sub.1a and SOP.sub.1b. Equally, for the CCW
propagation direction, the two possible input polarisation states
which give maximum output fringes are SOP.sub.2a and SOP.sub.2b.
These polarisation states can be represented on a Poincare sphere
as shown in FIG. 5.
[0083] Although there are 4 possible pairings which will
simultaneously lead to maximum fringes at both counterpropagating
outputs CW.sub.out and CCW.sub.out of the bidirectional MZ
(SOP.sub.1a and SOP.sub.2a, or SOP.sub.1a and SOP.sub.2b, or
SOP.sub.1b and SOP.sub.2a, or SOP.sub.1b and SOP.sub.2b,), only two
of these pairings will lead to the outputs that have both maximum
fringe visibility and are exactly matched in phase.
[0084] For the example shown in FIG. 4, the phase matched maximum
fringe counter-propagating input polarisation states are:
(SOP.sub.1a and SOP.sub.2a), and (SOP.sub.1b and SOP.sub.2b). This
is shown in the FIGS. 6A and 6B.
[0085] FIGS. 6A and 6B show two waves which are completely
overlapped, namely SOP.sub.1a and SOP.sub.2a in FIG. 6A, and
SOP.sub.1b and SOP.sub.2b in FIG. 6B.
[0086] This phase and amplitude matching condition is important for
the Locator system, as it will allow for the most accurate location
of events on the sensing cable to be determined. This means that it
is essential that there is no time difference between the
counter-propagating drifting MZ output signals when the MZ sensor
is in the rest state (no disturbance). If the counter-propagating
outputs are not matched in phase, then this will lead to the
introduction of an error in the time difference calculation and
thus the calculation of the location.
[0087] FIGS. 7A and 7B show the counter-propagating MZ outputs for
the non phase matched maximum fringe counter-propagating input
polarisation states, namely SOP.sub.1a and SOP.sub.2b in FIG. 7A,
and SOP.sub.1b and SOP.sub.2a in FIG. 7B.
[0088] The achievement of counter-propagating, phase-matched
maximum fringe outputs leads to two important results with respect
to the system. It allows for accurate locating of events, as well
as maximum sensitivity of the bi-directional MZ.
[0089] However, input polarisation states which lead to amplitude
and phase matched counter-propagating outputs are not limited only
to the input polarisation states which achieve maximum output
fringes. There is also a plurality of other input polarisation
state pairs which also lead to amplitude and phase matched outputs,
but with sub-maximum fringe visibilities. For example, it is
possible to adjust both polarisation controllers 43 and 44 such
that the fringe visibility of both outputs is identical and less
than the theoretical maximum of 100%, but phase matched. Although a
reduction in fringe visibility will lead to a reduction in
sensitivity of the bidirectional MZ 10, as long as the fringe
visibility is kept relatively high (for example >75%), it is
still possible for the system to calculate accurate locations
whilst maintaining an acceptable level of sensitivity.
[0090] The variation in fringe visibility of the MZ output for each
direction in the bi-directional MZ can be plotted on a Poincare
sphere to show the relationship between input polarisation states
and MZ output fringe visibility. A typical response is shown in
FIGS. 8 and 9.
[0091] The two unique input polarisation states which lead to a
maximum fringe visibility form two opposite `poles` on the sphere,
SOP.sub.CW1 and SOP.sub.CW2 (FIG. 8). For non-maximum fringe
visibilities, polarisation states of equal fringe visibility form
latitudinal belts, with the equatorial belt E representing the
polarisation states of minimum fringe visibility. Moving away from
the poles, towards the equatorial belt, say B.sub.1 and B.sub.2,
which is midway between the two poles, the fringe visibility
decreases and becomes a minimum at the equatorial belt E. Both
"hemispheres" are essentially mirror images of each other.
[0092] The position of opposing maximum fringe visibility poles,
and therefore the latitudinal and equatorial belts, will vary
according to the birefringence of the bi-directional MZ system,
namely the input lead 12 and MZ arms 14 and 15 for the CW
direction. This can be thought of as a rotation of the fringe
visibility poles and latitudinal belts around the sphere. The
minimum fringe visibility is not necessarily always zero, as would
be expected in an ideal MZ 10, but can be non-zero. The actual
value of the minimum fringe visibility will also vary with the
birefringence of the MZ system 10 for that direction. So in
summary, a change in birefringence in the MZ system 10, which for
the CW direction can include a change in the birefringence in the
input lead length 12, and/or MZ sensor arms 14 and 15, can cause
not only the fringe visibility poles and latitudinal belts to
rotate for each direction, but can also change the range of fringe
visibilities possible. Importantly, though, the maximum fringe
visibility always approaches unity, irrespective of the
birefringence of the MZ system.
[0093] Looking at the CCW direction of propagation (shown in FIG.
9) in the bi-directional MZ 10, a similar relationship between the
fringe visibility of the MZ output 1, 2 and input polarisation
states can be seen. The position of opposing maximum fringe
visibility poles, and therefore the latitudinal and equatorial
belts, will vary according to the birefringence of the
bi-directional MZ system, which includes the input lead
(L.sub.lead1) and MZ arms 14 and 15. In fact, the minimum and
maximum fringe visibility values are the same as for the CW
direction for any given time, only the absolute position of the
maximum fringe visibility states, SOP.sub.CCW1 and SOP.sub.CCW2 is
different when compared with the CW direction.
[0094] The optical fibre cables that make up the apparatus of the
preferred embodiments are actually installed in a variety of
environments where they will be subjected to fluctuating and random
conditions such as wind, rain, mechanical vibrations, stress and
strain, and temperature variations. As mentioned earlier, these
effects can vary the birefringence of the optical fibre in the
cables, which in turn can change the fringe visibilities of both
Locator MZ outputs through the polarisation induced fringe fading
(PIFF) effect. So, in a realistic installation, where environmental
factors will cause random birefringence changes along the fibres of
the Locator system, the fringe visibilities of the respective MZ
output intensities can vary randomly with time.
[0095] In the apparatus of the preferred embodiments, it is
necessary to search and find the input polarisation states for the
CW and CCW directions of the bidirectional MZ 10 which correspond
to both MZ outputs having the same fringe visibility and being
phase matched. One way this could be done is by monitoring the two
Locator MZ outputs whilst scrambling the polarisation controllers.
A number of scrambling algorithms could be used as long as they
achieve the coverage of most of the possible input polarisation
states in a relatively short time.
[0096] Once these input polarisation states are found, they need to
be set to achieve amplitude and phase matched MZ output
intensities. To keep the MZ outputs in the amplitude and phase
matched condition, it is also necessary to continue adjusting the
input polarisation states to compensate for any PIFF that may lead
to non-matching counter propagating fringe visibilities, and
therefore non-phase matched MZ outputs. This requires knowledge of
the actual fringe visibility of the counter-propagating outputs of
the bidirectional MZ.
[0097] For an apparatus which is using a CW laser as its source, it
is not possible to continuously monitor the fringe visibilities of
the two MZ outputs, especially in the absence of disturbances. This
is because the time taken for the MZ output intensities to go
through a full fringe amplitude excursion will vary with time and
will be a function of the random phase fluctuations in both arms 14
and 15 of the MZ 10, as well as the PIFF due to the random
birefringence changes in the fibres along the length of the
bi-directional MZ system.
[0098] However, it is possible to determine that a maximum fringe
state exists for either of the MZ output intensities if they move
through or very close to a zero or maximum intensity level. This is
because the zero or maximum level intensities are unique to a
maximum fringe visibility. So, for a bidirectional MZ system with
polarisation controllers at the inputs of the bi-directional MZ, as
shown in FIG. 4, one method for determining the required input
polarisation states, which are associated with the phase matched
maximum MZ output fringe visibilities, is to use a polarisation
scrambling technique to randomly change the input polarisation
state whilst simultaneously monitoring the MZ outputs. When the MZ
output intensities reach a zero (or near-zero), or maximum level,
then the corresponding input polarisations can be used to set the
MZ output intensities to maximum fringe visibilities.
[0099] Given that there are 2 possible input polarisation states
that simultaneously give a maximum fringe visibility for each
direction in the bidirectional MZ, and that only two out of the 4
possible pairings of counter-propagating input polarisation states
will give phase matched MZ outputs, it is necessary to check that
the chosen two polarisation states yield phase matched MZ outputs.
This can be done by simply monitoring the MZ outputs for a
predetermined time. If they are not in phase, then polarisation
scrambling can be used to find two input polarisation states, and
their corresponding maximum fringe outputs, to continue to search
for phase matching.
[0100] Once the phase matched maximum fringe states are found and
set, a tracking algorithm can be used to continue to keep the MZ
outputs in a phase matched condition by adjusting the voltage
drives to the individual plates of both polarisation controllers
accordingly.
[0101] This technique will be described in detail with reference to
FIGS. 14 and 15. One of the drawbacks of using this technique is
that even though we are continuously monitoring the MZ outputs, we
are not continuously monitoring the output fringe visibilities of
the MZ outputs. To detect a maximum fringe, it is necessary to wait
until the MZ output intensity goes to, or very near, a zero or
maximum intensity level. Since the MZ outputs will randomly vary in
speed and amplitude, the time taken for which a maximum output
fringe can be detected for either of the MZ outputs will vary.
Another draw back is that if the MZ output is at a maximum fringe
state but is not at a zero or maximum intensity level, it is not
possible to detect this situation using only this technique.
[0102] A more direct technique would involve continuously
monitoring the fringe visibilities of the MZ outputs. This requires
that fringes are created artificially in the system.
[0103] Fringes can be artificially created in the MZ 10 by using a
transducer in one of the sensing arms to modulate the phase of the
light propagating through the fibre. However, for an event location
system where it is preferable that the sensing cables are totally
passive, this is not a practical solution.
[0104] Another technique for stimulating fringes in a fibre MZ is
to modulate or dither the wavelength of the laser source 16. As
long as there is a path length mismatch between the MZ arms 14 and
15, then the modulation in optical wavelength (which can also be
expressed as an optical frequency) will lead to the creation of
fringes. This comes about due to the wavelength dependent phase
difference between the MZ arms caused by the path length mismatch.
For a Mach Zehnder 10 with a path length mismatch .DELTA.L, the
phase difference .DELTA..phi. between the arms can be expressed by:
.DELTA. .times. .times. .PHI. = 2 .times. .pi. n co .DELTA. .times.
.times. L .DELTA. .times. .times. v c ##EQU1## where n.sub.co is
the refractive index of the fibre core, c is the speed of light in
a vacuum, and .DELTA.v is the laser's optical frequency change. In
the case of a bidirectional MZ, as is described in FIG. 4, the
phase difference Ac between the arms of the MZ will be the same for
each direction of propagation. It should be noted that the phase
difference referred to here is additional to the phase difference
between the arms which is induced by a perturbation event on the MZ
sensor.
[0105] For a MZ whose operating point is at quadrature, a full
fringe excursion can be achieved for a given path length mismatch
by modulating the laser source's frequency/wavelength by an amount
which results in .DELTA..phi.=.pi..sup.c. For a typical fibre core
index n.sub.co=1.46, a path length mismatch .DELTA.L=1 m, and a
full fringe .DELTA..phi.=.pi..sup.c, will give an optical frequency
dither of .DELTA. .times. .times. v = c .DELTA. .times. .times.
.PHI. 2 .times. .pi. n co .DELTA. .times. .times. L = 3 10 8 .pi. 2
.times. .pi. 1.46 1 = 102.75 .times. .times. MHz ##EQU2##
[0106] For a centre wavelength of 1550 nm, this corresponds to a
wavelength dither of -0.8 .mu.m.
[0107] One of the simplest ways to modulate the wavelength of a
standard laser diode is to modulate the drive current to the laser.
These types of lasers however do not normally have a high enough
coherence to be suitable for the applications discussed herein.
[0108] The pumped fibre laser source 16 requires a mechanical
modulation of the fibre laser's cavity, or fibre Bragg gratings to
achieve wavelength modulation. This can be achieved by using either
a temperature tuning approach, or a mechanical piezo tuning
approach using a piezo transducer (PZT). Since temperature tuning
is very slow, the piezo tuning method is more suited to such a
laser in order to achieve the wavelength dithering or modulation.
In order to use the dithering of the laser wavelength to
continuously monitor the fringe visibility of the MZ, it is
necessary to create at least 2 full artificial fringes per cycle of
PZT modulation. This requirement is determined by the fact that, as
mentioned earlier, the operating point of the MZ drifts in and out
of quadrature with time, and the creation of only one full fringe,
that is, .DELTA..phi.=180.degree., would not be sufficient to
continuously show the true fringe visibility. This is illustrated
in FIG.
[0109] FIG. 10 shows that for a static MZ output operating point
which is exactly at quadrature, applying a sinusoidal phase
(.DELTA..phi.) modulation with an 180.degree. excursion will lead
to a full fringe per cycle of phase modulation. In other words, the
fringe visibility can be continuously monitored. It should be noted
that since the transfer function of the MZ is a raised cosine and
it is being modulated about the quadrature point with a sinusoidal
signal, the resulting stimulated fringes will additionally contain
harmonics of the fundamental modulating frequency.
[0110] However, in a real MZ 10, the MZ's output operating point
drifts in and out of quadrature. This is illustrated in FIG.
11.
[0111] If, however, the dithering is used to achieve at least
360.degree. of phase modulation at all times, the true fringe
visibility of the stimulated fringes can be continuously monitored,
irrespective of the drift of the MZ output's operating point. This
is illustrated in FIG. 12.
[0112] If a 360.degree. phase modulation (or more) is used, that is
2 fringes per cycle of modulation are stimulated, this will ensure
that the true fringe visibility will always be measurable,
irrespective of the drift in the MZ output. This will essentially
produce higher harmonics in the stimulated fringes. As the MZ
output operating point drifts to the left or the right of
quadrature, it will cause higher harmonics of the dither frequency
(2.sup.nd, 3.sup.rd, 4.sup.th, etc.) together with the fundamental
dither frequency to be present in the stimulated fringes.
[0113] To make sure that the stimulated fringes do not interfere
with the fringes created by the events which are to be sensed by
the apparatus, it is important for the frequency of the stimulated
fringes to be in a frequency range well outside that of the event
signals detected by the apparatus. For example, in a typical
installation, where the frequency range of interest may be 0-20
kHz, the fundamental frequency of the stimulated fringes should be
higher, eg. 50 kHz. FIG. 11 shows that with the drifting MZ output
operating point 21 and the phase dither 22 shown in FIG. 11, a
stimulated full range fringe 23 will not be achieved due to the
drifting. FIG. 12 shows that by using a full 360.degree. dither 25,
true fringe visibility 26 is always present in stimulated fringes,
irrespective of the drifting operating point 21. There will also be
frequency doubling for the 360.degree. phase modulation for this
case. In FIG. 10, the static operating point 15 which is at
quadrature and the phase dither 25 always produces the stimulated
full fringe 26 shown in FIG. 10. In FIG. 10 to 12, the phase dither
is at a frequency of about 40 kHz. The frequency of the fringes 26
in FIG. 10 is 40 kHz. In FIG. 11 the onset of frequency doubling
can be seen in 23, whilst the output fringes in FIG. 12 will
include even harmanics of the fundamental frequency, not excluding
the fundamental dither frequency. Generally speaking, the output
fringes will include a proportion of odd and even harmonics of the
fundamental dither frequency. The amplitude of the odd and even
harmonic frequencies at any given time will depend on exactly where
the drifting MZ output's operating point is at any given time.
[0114] FIG. 13 is a schematic block diagram of a first embodiment
of the invention in which the controller 20 is separated from the
Mach Zehnder interferometer which forms the sensing system of the
various embodiments. FIG. 14 shows the bi-directional MZ 10 and
includes the lead-in fibre 12 as previously described. The lead-in
fibre connects with a coupler C4 so that a first signal is launched
into arm 14 of the Mach Zehnder 10 and a second signal is launched
into arm 15 of the Mach Zehnder 10. The Mach Zehnder interferometer
10 has a sensing length of Ls which may typically be several or
more kilometres. The arms 14 and 15 are connected to a further
coupler C5 so that the signal launched into the arms 14 and 15,
whose signals recombine at C5, is received in fibre 31 as an output
signal CW out. Simultaneously, a counter-propagating signal is
received in the fibre 31 which in turn travels to coupler C5 and
then launches into arms 14 and 15 so that the two signals recombine
at C4 and exit coupler C4 at lead-in line 12. Thus,
counter-propagating signals are received in both arms 14 and 15.
The output propagating signal received in fibre 12 passes through
coupler C2 to a first detector 40 and the other counter-propagating
signal passes through the fibre 31 and through a coupler C3 to a
second detector 50. Thus, if there is a perturbation, as shown in
FIG. 14, at part of the sensing length L.sub.s of the Mach Zehnder
10, then modified counter-propagating signals continue to propagate
through the sensing arms 14 and 15 back to the respective detectors
40 and 50. The time difference between receipt of the modified
counter-propagating signals are provided from the detectors 40 and
50 to output monitor 60 and then to processor 62 which form the
detecting unit of the preferred embodiment so that the event can be
recognised and the event location determined by the time difference
between receipt of the modified counter-propagating signals
detected by the detectors 40 and 50.
[0115] As is shown in FIG. 15, the controller 20 includes the
wholly coherent laser 16 which produces a light output signal which
is split into two by coupler Cl. The split signals are then passed
to polarisation controllers PC.sub.cw and PC.sub.ccw via fibres 37
and 38. The polarisation controllers are respectively connected to
couplers C2 and C3 so that the counter-propagating signals are
launched into the Mach Zehnder interferometer 10 as previously
described. The polarisation controllers control the input
polarisation states of the signals which are input into the arms 14
and 15.
[0116] To find the input polarisation states which produce matched
outputs at the detectors 40 and 50, the polarisation controllers 43
and 44 can be scrambled to randomly change the input polarisation
states such that as many different states as possible are covered
in as short a time as possible. By continuously monitoring the
outputs at the detectors 40 and 50, the polarisation states
corresponding to the outputs passing a zero or near zero level, or
maximum are stored in the microprocessor 62. When the output
reaches a zero or maximum level, the corresponding input
polarisation state is considered to be such that it achieves a
maximum fringe visibility for that output. When a suitable number
of input polarisation states are stored, the scrambling is stopped.
The outputs from the arms 14 and 15 and received by the detectors
40 and 50 are then compared for combinations of input polarisation
states and the degree of phase matching between the
counter-propagating outputs is determined. When the degree of phase
matching is above a predetermined acceptable level, the
corresponding input polarisation states for which the degree of
phase matching was acceptable are set to maintain the phase matched
outputs detected by the detectors 40 and 50. If an acceptable
degree of phase matching is not reached, the scrambling and
comparing procedure described above is repeated again until
acceptable degrees of phase matching is achieved.
[0117] The polarisation controllers 43 and 44 are driven by PC
driver 60 so as to continuously change the polarisation of the
signals in the fibres 37 and 38 and therefore provided to the
couplers C2 and C3 as the counter-propagating inputs to the arms 14
and 15.
[0118] When the required input polarisation states which achieve
phase matched maximum fringe visibility at the outputs detected by
the detectors 40 and 50 are found, these required input
polarisation states are set and the outputs detected by the
detectors 40 and 50 are continuously monitored and the
micro-processor 62 adjusts the polarisation controllers via the PC
driver 63 to maintain the phase matched condition.
[0119] The output monitor 60 determines an event by passing the
signals detected by the detectors 40 and 50 through a band pass
filter having for example a bandwidth of from 1 kHz to 20 kHz
(which is the expected frequency of an actual event or perturbation
to the apparatus which needs to be detected). The arrival of
modified propagating signals within this bandwidth and the time
difference between receipt of the counter-propagating signals
enables the event to be recognised and also the location of the
event to be determined.
[0120] Thus, the band pass filtered signals are provided from the
monitor 60 to the processor 62 for determining the location of the
event.
[0121] In order to set the polarisation states, the complete output
signal from the detectors 40 and 50 relating to both
counter-propagating signals is received at the monitor 60. This is
essentially the raw signal from both detectors 40 and 50 and that
signal is low pass filtered and used to search for maximum fringes
during the polarisation scrambling by detecting zero or maximum
intensity levels. When the maximum fringes have been located, the
processor 62 also checks for phase alignment. When the desired
polarisation states are controlled, these are continuously fed to
the PC driver and in turn, the PC driver drives the polarisation
controllers 43 and 44 to maintain those polarisation states during
use of the system. Monitoring can be performed continuously or
intermittently to ensure that the required polarisation states are
maintained.
[0122] FIG. 16 shows a second embodiment of the invention. This
embodiment is similar to that shown in FIG. 14, except the Mach
Zehnder interferometer 10 has a path length mismatch of .DELTA.L
between the arms 14 and 15 so that one of the arms 14 has a length
L.sub.s-.DELTA.L and the arm 15 has a length L.sub.s. The path
length mismatch is required to achieve stimulation of artificial
fringes by dithering the wavelength of the laser source 16. One
input of the coupler C4 is used as the clockwise input to the Mach
Zehnder 10 and the other input of the coupler C4 is not used as in
the earlier embodiment. Similarly, one of the inputs of the coupler
C5 is connected to fibre 13 and provides the input for the
counter-propagating signal and the other arm of the coupler C5 is
also not used.
[0123] FIG. 17 is a view of the controller 20 according to this
embodiment of the invention in which like reference numerals
indicate like components to those described with reference to FIG.
15.
[0124] In this embodiment the laser 16 is a diode pumped bragg
grating base doped fibre laser. To dither the wavelength of the
laser 16, a piezoelectric transducer (not shown) is used, for
example, on the internal bragg gratings in the fibre laser to
modulate the output wavelength of the laser 16.
[0125] To create the artificial fringes, a dither signal which has
a frequency above the event frequency of the perturbations which
are expected to be provided to the Mach Zehnder 10 and sensed by
the Mach Zehnder 10 is applied to the laser 16 from dither signal
source 70. This dithers the wavelength of the laser and effectively
creates fringes whose frequency consists of the dither frequency
and harmonics of the dither frequency (as has been described in
detail with reference to FIGS. 10 to 12).
[0126] By using the suitable path length mismatch .DELTA.L
previously described and adjusting the amplitude of the dithering,
continuous fringes are created at the outputs of the Mach Zehnder
10 and which are supplied to the detectors 40 and 50. The outputs
which are received by the detectors 40 and 50 will be composed of
the dither frequency as well as harmonics of the dither frequency.
A fringe visibility monitor 80 is connected to the detectors 40 and
50 for detecting the artificial fringes and determining the fringe
visibility for each direction. The frequency range of the
artificial fringes is above the event signal frequency range caused
by a perturbation. Microprocessor 62 uses a suitable control
algorithm, such as a simulated annealing control algorithm, to
search and adjust the input polarisation controllers PC.sub.cw and
PC.sub.ccw via driver 60 so that the stimulated artificial fringes
are at a maximum visibility. The phase matching between the
stimulated fringes is also detected by the microprocessor 62 and
again, once a suitable input polarisation state from each of the
controllers is achieved, that polarisation state is set. A control
algorithm is used to adjust the input polarisation controllers
PC.sub.cw and PC.sub.ccw to counteract any PIFF, so that the phase
matched maximum fringe visibility condition is maintained.
[0127] FIG. 18 shows a still further embodiment of the invention.
Once again, like reference numerals indicate like parts to those
previously described. The embodiment of FIG. 18 is a modification
to the embodiment of FIG. 17 previously described. Once again, the
output wavelength of the laser 16 is dithered in the manner
described above. Once again, the input polarisation states of the
counter-propagating signals supplied to the fibres 12 and 31 are
controlled by polarisation controllers 43 and 44 respectively. The
controllers 43 and 44 are connected to polarimeters 46 and 47 which
respectively measure the polarisation state of the outputs from the
polarisation controllers 43 and 44. The input light signals having
the polarisation states are then supplied to the fibres 12 and 31
via the couplers C2 and C3 in the same manner as previously
described. The polarimeters 46 and 47 allow the fringe visibility
of the outputs from the controllers 43 and 44 to be related to
their respective input polarisation states. The polarimeters 46 and
47 also allow for the capability of moving from one input
polarisation state to another for both directions of propagation in
a deterministic way, should that be desired. This essentially
allows the implementation of a polarisation control strategy which
can quickly identify where on the sphere the current input
polarisation state is with respect to the two maximum fringe
visibility SOP states for each direction of propagation, as well as
how to change the polarisation controllers such that it is possible
to find and maintain the input polarisation states SOP.sub.cw and
SOP.sub.ccw which produce phase-matched bi-directional outputs from
the Mach Zehnder 10 labelled CW.sub.out and CCW.sub.out in the
drawings. Since modifications within the spirit and scope of the
invention may readily be effected by persons skilled within the
art, it is to be understood that this invention is not limited to
the particular embodiment described by way of example
hereinabove.
[0128] In the claims which follow and in the preceding description
of the invention, except where the context requires otherwise due
to express language or necessary implication, the word "comprise",
or variations such as "comprises" or "comprising", is used in an
inclusive sense, i.e. to specify the presence of the stated
features but not to preclude the presence or addition of further
features in various embodiments of the invention.
* * * * *