Passive optical Sagnac interferometer for current sensing

1 . A passive optical Sagnac interferometer for current sensing comprising:

an N×N fibre coupler, wherein N≥3; and

a fibre coil disposed on a first side of the N×N fibre coupler, a first port of the N×N fibre coupler coupled to a first end of the fibre coil via a first linear polariser and a second port of the N×N fibre coupler coupled to a second end of the fibre coil via a second linear polariser;

wherein the fibre coil is configured to support only one elliptical polarisation state in counter-propagating light signals in the fibre coil, such that a first light signal propagating in a first direction through the fibre coil has a same handedness as a second light signal propagating through the fibre coil in a second direction opposite to the first direction.

2 . The interferometer of claim 1 , wherein the fibre coil is configured to support only one at least approximate circular polarisation state in the counter-propagating light signals in the fibre coil.

3 . The interferometer of claim 1 , wherein the fibre coil comprises spun elliptically birefringent polarising optical fibre.

4 . The interferometer of claim 1 , wherein the fibre coil comprises non-polarising spun elliptically birefringent optical fibre and the interferometer comprises first and second at least approximately circular polarisers, wherein the first circular polariser comprises the first linear polariser and a first ¼ wave fibre section is coupled between the first port of the N×N fibre coupler and the first end of the fibre coil and the second circular polariser comprises the second linear polariser and a second ¼ wave fibre section is coupled between the second port of the N×N fibre coupler and the first end of the fibre coil.

5 . The interferometer of claim 1 , wherein the N×N fibre coupler comprises a N×N single mode optical fibre coupler.

6 . The interferometer of claim 1 , wherein the N×N fibre coupler comprises a N×N polarisation maintaining optical fibre coupler.

7 . The interferometer of claim 1 , wherein the fibre coil comprises a “figure 8” configuration.

8 . The interferometer of claim 1 , comprising:

an optical multiplexing network disposed on a second side of the N×N fibre coupler;

an optical input unit for inserting an optical input signal into each port on the second side of the N×N fibre coupler via the optical multiplexing network;

a detector unit for measuring corresponding optical outputs at each of the ports on the second side of the N×N fibre coupler via the optical multiplexing network when an optical input is inserted into one of the ports on the second side of the N×N fibre coupler; and

a processing unit for forming measurement products, each measurement product containing at least some of 3 2 possible measured optical outputs, wherein the respective measurement products comprise the same optical multiplexing network dependent factor, and for eliminating the optical multiplexing network dependent factor to obtain substantially optical multiplexing network independent measurements.

9 . The interferometer of claim 1 , wherein N=3.

10 . A method of sensing current in a conductor using the interferometer of claim 1 .

11 . A passive optical Sagnac interferometer for current sensing comprising:

an N×N fibre coupler;

a fibre cable coupled to the N×N fibre coupler, a first output port of the N×N fibre coupler coupled to a first end of a first fibre of the fibre cable and a second output port of the N×N fibre coupler coupled to a first end of a second fibre of the fibre cable; and

first and second at least approximately circular polarisers, wherein the first circular polariser is coupled between the first port of the N×N fibre coupler and the first end of the first fibre and the second circular polariser is coupled between the second port of the N×N fibre coupler and the first end of the second fibre, and wherein respective second ends of the first fibre and the second fibre are coupled to each other;

wherein the fibre cable is configured to be wound into a coil; and

wherein the first fibre comprises non-polarising spun elliptically birefringent optical fibre and the second fibre comprises unspun highly birefringent optical fibre.

12 . The interferometer of claim 11 , wherein the N×N fibre coupler comprises a N×N single mode optical fibre coupler.

13 . The interferometer of claim 11 , wherein the N×N fibre coupler comprises a N×N polarisation maintaining optical fibre coupler.

14 . The interferometer of any one of claim 11 , comprising:

an optical multiplexing network disposed on a second side of the N×N fibre coupler;

an optical input unit for inserting an optical input signal into each port on the second side of the N×N fibre coupler via the optical multiplexing network;

a detector unit for measuring corresponding optical outputs at each of the ports on the second side of the N×N fibre coupler via the optical multiplexing network when an optical input is inserted into one of the ports on the second side of the N×N fibre coupler; and

a processing unit for forming measurement products, each measurement product containing at least some of 3 2 possible measured optical outputs, wherein the respective measurement products comprise the same optical multiplexing network dependent factor, and for eliminating the optical multiplexing network dependent factor to obtain substantially optical multiplexing network independent measurements.

15 . The interferometer of claim 11 , wherein N=3.

16 . A method of sensing current in a conductor using the interferometer of claim 11 .

17 . A passive optical Sagnac interferometer for current sensing comprising:

an N×N polarisation maintaining fibre coupler;

a fibre cable coupled to the N×N fibre coupler, a first output port of the N×N fibre coupler coupled to a first end of a first fibre of the fibre cable and a second output port of the N×N fibre coupler coupled to a first end of a second fibre of the fibre cable; and

first and second linear polarisers, wherein the first linear polariser is coupled between the first port of the N×N polarisation maintaining fibre coupler and the first end of the first fibre and the second linear polariser is coupled between the second port of the N×N polarisation maintaining fibre coupler and the first end of the second fibre, and wherein respective second ends of the first fibre and the second fibre are coupled to each other;

wherein the fibre cable is configured to be wound into a coil; and

wherein the first fibre comprises spun elliptically birefringent polarising optical fibre and the second fibre comprises unspun highly birefringent optical fibre.

18 . The interferometer of claim 17 , comprising:

an optical multiplexing network disposed on a second side of the N×N fibre coupler;

an optical input unit for inserting an optical input signal into each port on the second side of the N×N fibre coupler via the optical multiplexing network;

a detector unit for measuring corresponding optical outputs at each of the ports on the second side of the N×N fibre coupler via the optical multiplexing network when an optical input is inserted into one of the ports on the second side of the N×N fibre coupler; and

a processing unit for forming measurement products, each measurement product containing at least some of 3 2 possible measured optical outputs, wherein the respective measurement products comprise the same optical multiplexing network dependent factor, and for eliminating the optical multiplexing network dependent factor to obtain substantially optical multiplexing network independent measurements.

19 . The interferometer of claim 17 , wherein N=3.

20 . A method of sensing current in a conductor using the interferometer of claim 17 .