Optical sensing

• US 20060256344A1
• Filed: 04/13/2006
• Published: 11/16/2006
• Est. Priority Date: 09/30/2003
• Status: Active Grant

• Current Assignee: British Telecommunications PLC (BT Group PLC)
• Original Assignee: British Telecommunications PLC (BT Group PLC)
• Inventors: Healey, Peter, Sikora, Edmund S.R.
• Granted Patent: US 7,667,849 B2
• Time in Patent Office: Days
• US Class Current: 356/477
• CPC Class Codes: H04B 10/2587 using a single light source for multiple stations

Abstract

An optical sensing method transmits radiation from a source onto an optical link, the radiation being passed through an unbalanced interferometer connected between the source and the link. The radiation which is reflected back and has travelled through a portion of the link which is subject to an external disturbance is received and passed through the same unbalanced interferometer. A detector is used to detect the radiation that has returned through the unbalanced interferometer. To improve sensitivity, the unbalanced interferometer includes a coupler for coupling radiation to the detector, the coupler being an N x N coupler where N is an integer greater or equal to 3.

Claims

1. A method of sensing an external effect interferometrically, said method comprising:

transmitting pairs of signal copies onto an optical link, the signal copies being transmitted in the same direction along the link and with a temporal offset relative to one another, the optical link being arranged such that the external effect causes at least a phase modulation of the transmitted signal copies;

receiving modulated signal copies previously transmitted onto the link; and

for received pairs of signal copies, causing one signal copy of a pair to interfere with the other signal copy of that pair so as to produce an interferometric signal representative of the external effect,

wherein the first and second signals are caused to interfere with a phase offset relative to one another, the phase offset being a non integer multiple of 180 degrees.

2. A method as in claim 1, wherein the phase offset is constant.

3. A method as in claim 1, wherein the first and second signal copies are caused to interfere at a coupling stage having:

a first waveguide at which the first signal copies are received, a second waveguide at which the second signal copies are received, and a third waveguide, each one of the waveguides being coupled to each of the other two waveguides such that the first and second signal copies interfere with the phase offset.

4. A method as in claim 3, wherein the first and second signal copies are passed through an interferometer arrangement having a first path and a second path, the first and second paths being coupled at the coupling stage.

5. A method as in claim 1, wherein the first and second signals are formed using signals from a light source having a coherence time associated therewith, which coherence time is less than the temporal offset.

6. An optical sensing system for sensing an external effect on a waveguide, including a light source and an unbalanced interferometer arrangement arranged such that radiation from the light source can be transmitted through the interferometer arrangement and onto the optical waveguide, the interferometer arrangement comprising:

a first coupling stage coupled to the light source;

a second coupling stage for coupling to the waveguide;

a first path; and

,

a second path,

the first and second paths each extending between the first and the second coupling stage, the first coupling stage having at least a first, a second, and a third coupled waveguides,

the first waveguide being arranged to receive radiation from the first path and to couple the received radiation onto at least the second and the third waveguides,

the second waveguide being arranged to receive radiation from the second path and to couple the radiation onto at least the first and the third waveguides.

7. An optical sensing system as in claim 6, wherein the first coupling stage is a 3×

3 coupler, having a first, a second and a third output, the first output being connected to the optical source.

8. An optical sensing system as in claim 7, wherein the optical sensing system includes a photo detector for converting optical signals into electrical signals, the photo detector being connected to the second output of the first coupling stage.

9. An interferometric sensing method of sensing an external effect on an optical link, said method comprising:

generating pairs of signal copies, which signal copies of a given pair are temporally displaced relative to one another;

transmitting the time-displaced signal copies in the same direction as one another onto the optical link;

receiving previously transmitted signal copies;

using an interferometer arrangement to temporally realign the received signal copies from a given pair; and

interferometrically combining the realigned signals of a given pair so as to generate an interferometer signal representative of an external effect oh the optical link,

wherein the first and second signal copies of a given pair are combined with a predetermined phase shift relative to one another, the phase shift being a non integer multiple of 180 degrees.

10. A method as in claim 9, wherein the interferometer arrangement is used to generate the time-displaced signal copies for transmission onto the optical link.

11. A method as in claim 9, wherein the interferometer arrangement includes an N×

N coupler that is used to combine the signal copies of a given pair with the predetermined phase shift, N being an integer equal to or greater than three.

12. A method as in claim 9, wherein the interferometer arrangement is arranged to channel radiation such that it is shared between a first path and a second path and to recombine the radiation from the first and second paths, the first and second paths of the interferometer arrangement having a differential delay associated therewith.

13. A method as in claim 9, wherein the signal copies of a given pair are transmitted over a common waveguide of the optical link.

14. A method as in claim 9, wherein the signals are carried by the optical link in a single mode fashion.

15. A method of communicating between a first location and a second location, the method comprising the steps of:

at the first location, generating output signals having an irregular phase component, and copying the output signals at least in part such that for each output signal, there is a pair of signal copies, the irregular phase component being common at least in part to each of the signal copies of a pair;

transmitting, from the first location, each signal copy of a pair over a common communications link to a second location, the signal copies being transmitted in the same direction as one another along the link;

at the second location, mixing data onto the irregular phase component of a signal copy for at least some of the pairs of signal copies; and

,

at the first location, receiving signal copies from the second location and, for pairs of received signal copies, interferometrically combining the respective irregular components of the signal copies of a pair in order to extract therefrom data mixed at the second location,

wherein the signal copies are interferometrically combined using an N by N coupler where N is an integer greater or equal to 3.

16. A method of sensing an optical link, said method comprising:

transmitting radiation from a source onto the optical link, the radiation being passed through an unbalanced interferometer connected between the source and the link;

receiving radiation transmitted over the link, which radiation has travelled through a portion of the link which is subject to an external disturbance;

passing the received the radiation through the unbalanced interferometer; and

,

using a detector to detect the radiation that has passed through the unbalanced interferometer,

wherein the unbalanced interferometer includes a coupler for coupling radiation to the detector, the coupler being an N×

N coupler where N is an integer greater or equal to 3.

17. A method as in claim 16, wherein the radiation is returned to the interferometer by a process of reflection or distributed backscattering.