Passive optical Sagnac interferometer for current sensing.

Abstract

Passive Optical Sagnac Interferometer, Methods Using and Manufacturing Same In one embodiment, a passive optical Sagnac interferometer for current sensing includes an NxN fiber coupler, where N>3, a fiber coil disposed on a first side of the NxN fiber coupler, a first port of the NxN fiber coupler coupled to a first end of the fiber coil via a first linear polarizing element, and a second port of the NxN fiber coupler coupled to a second end of the fiber coil via a second linear polarizing element, the fiber coil configured to support only elliptical polarization states in counter-propagating optical signals in the fiber coil.

Description

[Technical field]

[0001] The present invention relates generally to a passive optical Sagnac interferometer for current sensing. [Background technology]

[0002] Any mention and / or discussion of prior art throughout this specification should in no way be considered as an admission that said prior art is well known or forms part of the common general knowledge in the field.

[0003] Many types of optical fiber based current sensors have been presented and developed by many research teams around the world. Most of these sensors are based on fiber optic gyroscope technology, featuring active elements to maximize the interferometric phase sensitivity to the current. The Faraday effect is used wherever a non-reciprocal phase shift is induced in an optical fiber coiled around a conductor. For light traveling in opposite directions around the coil, the phase shift is equal and opposite.

[0004] US5,677,622 describes a passive optical current sensor using a Sagnac interferometer with a simple 2x2 optical beam splitter to counter-propagate light beams with two elliptical polarization modes each in a spun, single-mode birefringent optical fiber coil to detect current flow via the Faraday effect. The Sagnac interferometer is an ideal choice since it generally produces only non-reciprocal phase shifts due to either the Faraday effect or gyroscope rotation. In general, slow environmental changes can be considered as "common modes" and are equal in both directions, such as effects induced by ambient temperature.

[0005] However, passive Sagnac interferometry using a simple 2x2 optical beam splitter results in an interferometer with zero static phase bias. In effect, this results in all light returning from the input port, and zero intensity returning (with zero current) at the second port on the input side of the 2x2 optical beam splitter, i.e., both the input port and the second port on the input side of the 2x2 optical beam splitter are points of low sensitivity. This property gives rise to the name "loop mirror" due to all light returning through the input port.

[0006] The use of a 3x3 single mode fiber coupler / combiner has also been proposed for passive Sagnac interferometry in US 5,677,622. To detect current via the Faraday effect, light beams with two elliptical polarization modes each from two of the three ports at the output side of the 3x3 single mode fiber coupler / combiner are counter-propagated in a spun, single mode birefringent optical fiber coil. The optical input port of the 3x3 single mode fiber coupler / combiner operates similarly to a loop mirror, but the other two ports at the input side of the 3x3 single mode fiber coupler / combiner are no longer at a stationary point of low sensitivity (zero current) but at a relatively high sensitivity point, i.e., + / 2π / 3 radians.

[0007] However, achieving a stable passive Sagnac interferometer for current sensing with both good sensitivity to current and low zero current error remains challenging.

[0008] SUMMARY OF THE PRESENT EMBODIMENTS Embodiments of the present invention seek to address at least one of the above problems. Summary of the Invention

[0009] According to a first aspect of the present invention, there is provided a passive optical Sagnac interferometer for current sensing, comprising: an NxN fiber coupler, where N>3; a fiber coil disposed on a first side of the NxN fiber coupler, a first port of the NxN fiber coupler coupled to a first end of the fiber coil via a first linear polarizing element, and a second port of the NxN fiber coupler coupled to a second end of the fiber coil via a second linear polarizing element; Equipped with The fiber coil is configured to support only elliptical polarization states in counter-propagating signals within the fiber coil. An interferometer is provided.

[0010] According to a second aspect of the present invention, there is provided a passive optical Sagnac interferometer for current sensing, comprising: An NxN fiber coupler; a fiber cable coupled to the NxN fiber coupler, a first output port of the NxN fiber coupler coupled to a first end of a first fiber of the fiber cable, and a second output port of the NxN fiber coupler coupled to a first end of a second fiber of the fiber cable; at least first and second substantially circular polarizing elements, the first circular polarizing element being coupled between the first output port of the NxN fiber coupler and the first end of the first fiber, the second circular polarizing element being coupled between the second output port of the NxN fiber coupler and the first end of the second fiber, and second ends of the first and second fibers being coupled to each other; The fiber cable is wound in a coil shape, The first fiber comprises a non-polarizing, spun birefringent optical fiber, and the second fiber comprises a non-spun, highly birefringent optical fiber. An interferometer is provided.

[0011] According to a third aspect of the present invention there is provided a passive optical Sagnac interferometer for current sensing comprising: an NxN polarization-maintaining fiber coupler; a fiber cable coupled to the NxN polarization-maintaining fiber coupler; a first output port of the NxN polarization-maintaining fiber coupler coupled to a first end of a first fiber of the fiber cable; and a second output port of the NxN polarization-maintaining fiber coupler coupled to a first end of a second fiber of the fiber cable. a first linear polarizing element coupled between the first output port of the NxN polarization-maintaining fiber coupler and the first end of the first fiber, a second linear polarizing element coupled between the second output port of the NxN polarization-maintaining fiber coupler and the first end of the second fiber, and second ends of the first fiber and the second fiber coupled to each other; The fiber cable is wound in a coil shape, The first fiber comprises a spun elliptically birefringent optical fiber and the second fiber comprises a non-spun highly birefringent optical fiber. An interferometer is provided.

[0012] According to a fourth aspect of the present invention there is provided a passive optical Sagnac interferometer for current sensing comprising: An NxN fiber coupler; a fiber cable coupled to the NxN fiber coupler, a first output port of the NxN fiber coupler coupled to a first end of a first fiber of the fiber cable, and a second output port of the NxN fiber coupler coupled to a first end of a second fiber of the fiber cable; at least first and second substantially circular polarizing elements, where the first circular polarizing element is coupled between the first output port of the NxN fiber coupler and the first end of the first fiber, and the second circular polarizing element is coupled between the second output port of the NxN fiber coupler and the first end of the second fiber; a first half-wave plate coupled between a second end of the first fiber and a first end of a third fiber of the fiber cable; a second half-wave plate coupled between the second end of the second fiber and the second end of the third fiber; The fiber cable is wound in a coil shape, Each of the first, second and third fibers comprises a non-polarizing spun birefringent optical fiber. An interferometer is provided.

[0013] According to a fifth aspect of the present invention there is provided a method of sensing a current in a conductor using an interferometer of any one of the first to fourth aspects.

[0014] According to a sixth aspect of the present invention there is provided a method of manufacturing an interferometer according to any one of the first to fourth aspects.

[0015] Embodiments of the present invention, by way of example only, will be readily apparent to those skilled in the art when taken in conjunction with the following written description and drawings. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic diagram illustrating a 3×3 Sagnac current sensor featuring two in-line polarizers and using a coil of SPOF to measure hermetic current, according to one embodiment. [Diagram 2] FIG. 2 is a schematic diagram showing a 3x3 Sagnac current sensor featuring two circular polarizers and using a spun HiBi coil to measure hermetic current, according to one embodiment. [Diagram 3] FIG. 3 is a schematic diagram illustrating a 3x3 Sagnac current sensor featuring two in-line polarizers and using a figure-of-eight SPOF coil to measure hermetic current and eliminate rotation, according to one embodiment. [Figure 4] FIG. 4 is a schematic diagram showing a 3x3 Sagnac current sensor featuring two circular polarizers and using a figure-of-eight spun HiBi coil to measure hermetic current and eliminate rotation, according to one embodiment. [Diagram 5]Figure 5(a) is a schematic diagram showing a 3x3 Sagnac current sensor featuring two circular polarizers and using a cable containing one spun HiBi fiber and one non-spun HiBi fiber to measure hermetic current and eliminate rotation, according to one embodiment. Figures 5(b) and (c) are schematic diagrams showing the geometry of the loop-back configuration of the interferometer of Figure 5(a). [Figure 6] Figure 6(a) is a schematic diagram showing a 3x3 Sagnac current sensor featuring two circular polarizers and using a cable containing three spun HiBi fibers to measure hermetic current and eliminate rotation, according to one embodiment. Figures 6(b) and (c) show plots of fringe intensity without fringe fading and with 75% fringe fading, respectively. [Figure 7] FIG. 7 is a schematic diagram illustrating a 3×3 polarization-maintaining Sagnac current sensor featuring two in-line polarizers and using a coil of SPOF to measure hermetic current, according to one embodiment. [Figure 8] FIG. 8 is a schematic diagram illustrating a 3×3 polarization-maintaining Sagnac current sensor featuring two circular polarizers and using a spun HiBi coil to measure hermetic current, according to one embodiment. [Figure 9] FIG. 9 is a schematic diagram illustrating a 3×3 polarization-maintaining Sagnac current sensor featuring two in-line polarizers and using the coil of the SPOF shown in FIG. 8 to measure hermetic current and eliminate rotation, according to one embodiment. [Figure 10] FIG. 10 is a schematic diagram showing a 3x3 polarization-maintaining Sagnac current sensor featuring two circular polarizers and using a figure-of-eight spun HiBi coil to measure hermetic current and eliminate rotation, according to one embodiment. [Figure 11] FIG. 11 is a schematic diagram showing a 3x3 polarization-maintaining Sagnac current sensor featuring two in-line polarizers and using a cable containing one SPOF fiber and one non-spun HiBi fiber to measure hermetic current and eliminate rotation, according to one embodiment. [Figure 12]FIG. 12 is a schematic diagram showing a 3x3 polarization-maintaining Sagnac current sensor featuring two circular polarizers and using a cable containing one spun HiBi fiber and one non-spun HiBi fiber to measure hermetic current and eliminate rotation, according to one embodiment. [Figure 13] FIG. 13 is a schematic diagram showing a 3x3 polarization-maintaining Sagnac current sensor featuring two circular polarizers and using a cable containing three spun HiBi fibers to measure hermetic current and eliminate rotation, according to one embodiment. [Figure 14] FIG. 14 is a schematic diagram illustrating a method and apparatus for performing passive interferometric measurements using a 3×3 optical coupler in conjunction with an optical multiplexing network. [Figure 15] FIG. 15 shows a schematic diagram illustrating details of an exemplary circular polarizer for use in the described exemplary embodiments. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Embodiments of the present invention provide methods and systems for achieving and maintaining minimal fringe fading in a passive optical Sagnac interferometer for current sensing that can be used in a number of configurations. In various exemplary embodiments, sensitivity to current is preferably maximized for circularly polarized light and minimized to zero for linearly polarized light.

[0018] Simply put, the Faraday effect is the name given to the response of the polarization of light when it traverses a medium subjected to a magnetic field aligned parallel to its direction of propagation. Michael Faraday first observed that a magnetic field rotates the polarization state of light passing through the medium, leading him to conclude that light and magnetic fields are linked.

[0019] This rotation of the linear polarization occurs due to circular birefringence in the medium, which will rotate the linear polarization state, or equivalently change the relative phase velocities of the left and right circular polarization states.

[0020] Circularly polarized light in normal telecommunication fiber undergoes a phase shift due to a magnetic field, but any birefringence (e.g., bend-induced) changes the polarization state, and if light is periodically or randomly swapped from right-handed circularly polarized light to left-handed circularly polarized light, the magnetic-field-induced phase shift is also periodically or randomly swapped. As a result, the linear birefringence in the sensing fiber suppresses the Faraday-induced rotation, effectively reducing sensitivity to magnetic fields. The bending-induced linear birefringence in normal telecommunication fiber alone would suppress sensitivity to magnetic fields. Spinning a preform of highly birefringent fiber (HiBi fiber) during the fiber drawing process adds circular birefringence to a structure that would otherwise have a very large linear birefringence. The resulting elliptical birefringence is large compared to any packaging-induced bending birefringence that occurs in fibers with elliptical birefringence that is relatively unaffected by packaging.

[0021] In one group of exemplary embodiments, this can be accomplished using spun elliptically birefringent, effectively polarizing, optical fiber in a fiber coil surrounding a current carrying conductor or a fiber cable wound around a current carrying conductor in a loop-back configuration, as described in more detail below with reference to certain exemplary embodiments. Such fibers are also referred to herein as spun polarizing optical fiber (SPOF). In one example, the SPOF is drawn from a HiBi fiber preform that is spun during a draw process with a predetermined ratio between the spin pitch and the fiber beat length at the designed operating wavelength. Careful fiber design and control of fiber parameters and manufacturing conditions results in a fiber that supports two fundamental elliptically polarized eigenmodes, i.e., elliptically birefringent fiber. In addition, the fiber is designed to have a wavelength region where the slow elliptically polarized mode is transmitted and the fast mode is non-reciprocal. This creates a single polarization operating region that can be described as a single polarization window to realize the SPOF. The two modes are typically described as a transmission mode and a non-reciprocal mode, respectively. A SPOF, which typically has a differential loss of only 1 dB per meter, will provide 30 dB of extinction over 30 meters of fiber. For comparison, the best bulk optic polarizers currently made using birefringent crystals typically provide an extinction ratio of 20-30 dB. Unlike bulk optic crystal polarizers, SPOFs are distributed polarizers, meaning that they continuously polarize light as it passes through them, as explained in more detail below.

[0022] In this group of exemplary embodiments, broadband depolarized light can be launched into the fiber coil, and as the light travels along the coil, the SPOF ensures that only one elliptical, and preferably at least approximately circular, polarization state emerges at the other end of the coil. This polarization effect is reciprocal, so that the SPOF effectively supports only one polarization state in each direction with the same handedness, i.e., either the polarization state is right-handed elliptical / circular for both counterpropagating beams, or the polarization state is left-handed elliptical / circular for both counterpropagating beams.

[0023] In another group of exemplary embodiments, only one elliptical, preferably at least approximately circular, polarization state is excited into an unpolarized spun elliptically birefringent optical fiber (also referred to herein as spun HiBi fiber). The inclusion of equivalents, approximately circular polarizers, at the input and output ends of the sensing coil ensures that light polarized in the same handedness is excited in each direction. Referring to the above description of SPOF, spun HiBi fiber is drawn from a HiBi fiber preform that is spun during the draw process at a predetermined ratio between the spin pitch and fiber beat length at the designed operating wavelength. Again, careful fiber design and control of fiber parameters and manufacturing conditions results in a fiber that supports two fundamental elliptically polarized eigenmodes, i.e., an elliptically birefringent fiber. However, in contrast to SPOF, spun HiBi fiber is designed to operate in a region where both elliptical modes are guided, as described in more detail below.

[0024] In this group of exemplary embodiments, a near-circular polarizer can be achieved by combining a linear polarizer with a quarter wave plate. In fiber, this can be done by combining / splicing a linear polarizer with a short section of non-spun highly birefringent optical fiber (also referred to herein as non-spun HiBi fiber) (quarter beat length in length) with careful alignment of the polarization-maintaining axis, in a preferred embodiment, the polarization axis aligned at 45 degrees for example.

[0025] In another group of exemplary embodiments, the 3x3 beam coupler is preferably constructed using polarization-maintaining (PM) fiber, specifically, the non-spun HiBi fiber, aligned with the axis of the non-spun HiBi fiber pigtail of the linear polarizer to achieve optimal and stable fringe contrast. This can ensure that fringe fading is minimized without stringent packing requirements, and thus can ensure maximum signal-to-noise performance and minimum static phase error, preferably to achieve minimized DC value calculation error. It should be noted that, as described in more detail below, when using the PM 3x3 beam coupler, AC current measurement accuracy also remains very high and potentially improved.

[0026] Another group of exemplary embodiments uses a particular loopback configuration in which one section of the fiber cable is sensitive to electrical currents while the other section has zero sensitivity to electrical currents while propagating in both directions through the cable, thus providing excellent rejection of gyroscopic effects, as described in more detail below.

[0027] In another group of exemplary embodiments, a specific loopback configuration is provided that can triple the sensitivity to current by swapping the handedness of the elliptical (preferably circular) polarization modes in a current sensing cable having three non-polarized spun elliptically birefringent optical fibers. This can be achieved by using two half-wave plates at the splices between each pair of spun HiBi fibers at one end of the current sensing cable in the loopback configuration, as described in more detail below.

[0028] It should be noted that the above groups of exemplary embodiments are not mutually exclusive and instead can be combined into various exemplary embodiments each having one or more features of the above groups of exemplary embodiments.

[0029] In the following, some specific exemplary embodiments are described in detail by way of example and not by way of limitation.

[0030] FIG. 1 shows a schematic diagram illustrating an exemplary embodiment of a 3x3 Sagnac current sensor 100 comprising a coil of SPOF 102 for measuring current in a hermetically sealed conductor 104. The coil 102 has one or more loops formed around the conductor 104. Broadband depolarized light can be launched into the coil of SPOF 102, for example, via one of the single mode low birefringence input pigtails 112, of a 3x3 single mode fiber coupler 106. Two single mode low birefringence output pigtails (designated 109, 111) of the 3x3 single mode fiber coupler 106 are spliced ​​to non-spun HiBi pigtails of linear polarizers 114, 116. The other non-spun HiBi pigtails of linear polarizers 114, 116 are spliced ​​to each end of the coil of SPOF 102 (designated 113, 115). At all splices, the polarization-maintaining axes at each end face are preferably aligned. In practice, as the ellipticity approaches a circle, alignment becomes less important.

[0031] As light travels along the SPOF, the SPOF ensures that only one elliptical polarization state emerges at the other end of the coil 102. This polarization effect is reciprocal, so that the SPOF coil 102 effectively supports only one polarization state in each direction with identical handedness, i.e., the polarization state is right-handed elliptical / circular for both counter-propagating beams, or the polarization state is left-handed elliptical / circular for both counter-propagating beams. In this embodiment, the single polarization of the light interfering in the 3x3 single mode fiber coupler 106 is further enhanced using linear polarizers 114, 116, which are in the form of in-line polarizers.

[0032] 2 shows a schematic diagram illustrating a 3x3 Sagnac current sensor 200 according to one example embodiment, comprising a 3x3 single mode fiber coupler 205, (at least approximately) two circular polarizers 202, 204, and a coil 206 of spun HiBi fiber to measure the current in a sealed conductor 208. The coil 206 comprises one or more loops formed around the conductor 208.

[0033] A single circular polarization state is excited into each end of the coil 206 of spun HiBi fiber. The inclusion of identical, approximately circular polarizers 202, 204 at the input and output ends of the sensing coil 206 ensures that the same handedness of polarization is excited in each direction of the spun HiBi fiber sensing coil 206. The circular polarizers 202, 204 in this exemplary embodiment include at least approximately linear polarizers 210, 212 in combination with a 1 / 4 beat length section of non-spun HiBi fiber (i.e., having a length of 1 / 4 beat length). Specifically, two single mode low birefringence output pigtails of a 3x3 single mode fiber coupler 205 are spliced ​​(denoted by 213, 215) to the non-spun HiBi pigtails of the linear polarizers 210, 212, respectively. The other non-spun HiBi pigtails of the linear polarizers 210, 212 are spliced ​​with their polarization axes at 45 degrees at the end faces to the 1 / 4 beat length sections of non-spun HiBi fiber spliced ​​to each end of the coil of spun HiBi fiber 206 (joint faces jointly designated 209, 211). At splices other than the splice between the non-spun HiBi pigtails of the linear polarizers 210, 212 and the 1 / 4 beat length sections of non-spun HiBi fiber, the polarization-maintaining axes at each end face are preferably aligned. In practice, as the ellipticity approaches a circle, alignment becomes less important.

[0034] As a result, only a single polarization state propagates in each direction, i.e., for both counter-propagating beams interfering in the 3x3 single mode fiber coupler 205. The linear polarizers 210, 212 are in the form of in-line polarizers in this exemplary embodiment.

[0035] 3 shows a schematic diagram illustrating a 3x3 Sagnac current sensor 300 according to one exemplary embodiment comprising a coil of eight SPOF 302 to measure the current in a sealed conductor 304 while eliminating rotation that may occur due to gyroscopic effects. The coil 302 comprises one or more figure eight loops formed around the conductor 104. Broadband depolarized light can be launched into the coil of SPOF 302 via one of the single mode low birefringence input pigtails, e.g., 312 of a 3x3 single mode fiber coupler 306. The two single mode low birefringence output pigtails of the 3x3 single mode fiber coupler 306 are spliced ​​to the non-spun HiBi pigtails of linear polarizers 314, 316 (denoted by 309, 311). Other non-spun HiBi pigtails of linear polarizers 314, 316 are spliced ​​(designated 313, 315) onto each end of the SPOF coil 302. At all splices, the polarization-maintaining axes at each end are preferably aligned. In practice, as the ellipticity approaches a circle, alignment becomes less important.

[0036] As light travels along the SPOF, the SPOF ensures that only one elliptical state emerges at the other end of the coil 302. This polarization effect is reciprocal, so that the SPOF coil 302 effectively supports only one polarization state in each direction with the same handedness, i.e., the polarization state is right-handed elliptical / circular for both counterpropagating beams, or the polarization state is left-handed elliptical / circular for both counterpropagating beams. In addition, as each counterpropagating beam changes its direction around the conductor 304 in the SPOF figure-of-eight coil 302, any gyroscopic effect is substantially cancelled. The single polarization of the light interfering in the 3x3 single mode fiber coupler 306 is further enhanced using linear polarizers 312, 314, which in this exemplary embodiment are in the form of in-line polarizers.

[0037] 4 shows a schematic diagram illustrating a 3x3 Sagnac current sensor 400 according to one exemplary embodiment comprising a 3x3 single mode fiber coupler 405, two (at least approximately) circular polarizers 402, 404, and a coil 406 of spun HiBi fiber as shown in FIG 8 to measure the current in a sealed conductor 408 while eliminating rotation that may occur due to gyroscopic effects. The coil 406 comprises one or more figure-of-eight loops formed around the conductor 408.

[0038] A single circular polarization state is excited into each end of the spun HiBi fiber coil 406. The inclusion of identical, nearly circular polarizers 402, 404 at the input and output ends of the sensing coil 406 ensures that the same handedness of polarization is excited in each direction of the spun HiBi fiber sensing coil 406. The circular polarizers 402, 404 in this exemplary embodiment include a combination of linear polarizers 410, 412 and a 1 / 4 beat length section of non-spun HiBi fiber (i.e., having a length of 1 / 4 beat length). Specifically, two single mode low birefringence output pigtails of a 3x3 single mode fiber coupler 405 are spliced ​​(denoted by 413, 415) to the non-spun HiBi pigtails of linear polarizers 410, 412, respectively. The other spun HiBi pigtails of linear polarizers 410, 412 are spliced ​​to 1 / 4 beat length sections of non-spun HiBi fiber that are spliced ​​to each end of the coil of spun HiBi fiber 406 (the splices are designated 409, 411) with the polarization axes at 45 degrees at the end faces. At splices other than those between the non-spun HiBi pigtails of linear polarizers 410, 412 and the 1 / 4 beat length sections of non-spun HiBi fiber, the polarization-maintaining axes at each end face are preferably aligned. In practice, as the ellipticity approaches a circle, alignment becomes less important.

[0039] As a result, only a single polarization state propagates in each direction, i.e., for both counterpropagating beams. In addition, as each counterpropagating beam changes its direction around the conductor 408 in each loop of the spun HiBi fiber figure-of-eight coil 406, any gyroscopic effect is substantially canceled. The linear polarizers 410, 412 are in the form of in-line polarizers in this exemplary embodiment.

[0040] 5A shows a schematic diagram illustrating a 3x3 Sagnac current sensor 500 according to one exemplary embodiment, comprising (at least approximately) two circular polarizers 502, 504 and a cable 506 including one spun HiBi fiber 508 and one non-spun HiBi fiber 510. The cable 506 is wrapped in one or more loops around a conductor 512. The circular polarizers 502, 504 in this exemplary embodiment also include a combination of linear polarizers 524, 526 and a ¼ beat length section of non-spun HiBi fiber (i.e., having a length of ¼ beat length).

[0041] In this embodiment, the spun HiBi fiber 508 senses the current in the sealed conductor 512, whereas the non-spun HiBi fiber 510 does not. To effectively form a closed loop (also referred to herein as a "loopback configuration") with the cable 506 surrounding the conductor 512, two optimized splices are placed as close as possible to each other at one end of the loop, indicated at 516.

[0042] Referring to FIG. 5B, a quarter-beat-length section of HiBi fiber that is part of the circular polarizer 502 is spliced ​​to one end 510a of a non-spun HiBi fiber 510, and the other end 510b of the non-spun HiBi fiber 510 is spliced ​​to one end 508b of a spun HiBi fiber 508 (indicated as 522). The other end 508a of the spun HiBi fiber 508 is spliced ​​to a quarter-wavelength section of HiBi fiber that is part of the circular polarizer 504 (indicated as 520). Placing the splices 520, 522 as close as possible preferably ensures that in the loop-back configuration, the path followed by the current-sensing spun HiBi fiber 508 is a good approximation to a closed loop. At all splices, the polarization-maintaining axes at each end face are preferably aligned. In practice, as the ellipticity approaches a circular shape, alignment becomes less important.

[0043] Returning to FIG. 5A, by bringing the splices 520, 522 (collectively designated 516 in FIG. 5A) closer together to form a loop-back configuration, the path followed by the current-sensitive spun HiBi fiber 508 is a good approximation to the closed loop designated 514. Thus, light from each of the two output ports of the 3x3 single mode fiber coupler 518 travels in one section of the cable 506 that has sensitivity to current, i.e., the spun HiBi fiber 508, and in the other section that has zero sensitivity to current, i.e., the non-spun HiBi fiber 510, while traveling in both directions through the cable 506 in a loop-back configuration. Thus, excellent rejection of the gyroscopic effect can be provided. Single polarization of the interfering light in the 3x3 single mode fiber coupler 518 is achieved using linear polarizers 524, 526 (part of each of the circular polarizers 502, 504), which in this embodiment are in the form of in-line polarizers.

[0044] 6A shows a schematic diagram illustrating a 3x3 Sagnac current sensor 600 according to one exemplary embodiment, comprising a 3x3 single mode fiber coupler 605, (at least approximately) two circular polarizers 602, 604, and a cable 606 housing three spun HiBi fibers 608, 609, and 610. A 1 / 2 wave plate 611 is placed / spliced ​​between the end 608a of the spun HiBi fiber 608 and the end 609a of the spun HiBi fiber 609, effectively swapping the handedness of the polarization modes. Similarly, another 1 / 2 wave plate 613 is placed / spliced ​​between the end 609b of the spun HiBi fiber 609 and the end 610b of the spun HiBi fiber 610. Placing the splices for the wave plates 611, 613 and the splices between the circular polarizers 602, 604 and the spun HiBi fibers 608, 610 as close as possible preferably ensures that each path followed by the current-sensing spun HiBi fibers 608, 609, 610 is well approximated to a closed loop. This effectively triples the sensitivity of the loop / coil 614 to current and also provides better rotation rejection. In FIG. 6A, the splices between the wave plates 611, 613 and the splices between the circular polarizers 602, 604 and the spun HiBi fibers 608, 610 are not shown for clarity of the drawing, but are placed in close proximity.

[0045] As will be appreciated by those skilled in the art, in the 3x3 single mode coupler according to the above embodiment, optimal interference occurs when the polarization states of the two recombined light beams are perfectly aligned. On the other hand, orthogonal polarization states do not interfere and only the average intensity is seen at the output. When the polarization states start to diverge or move out of perfect alignment, fringe fading occurs. This has been recognized by the inventors as being possible in the optical fiber leads of the optical beam coupler in a Sagnac current sensing interferometer. Fringe fading can not only effectively reduce the signal-to-noise performance, but also result in an error in the calculated phase that effectively appears as a static phase error. For illustration, FIG. 6B shows a graph of fringe intensity without fringe fading, while FIG. 6C shows a graph of fringe intensity with 75% fringe fading.

[0046] Although careful packaging can reduce fringe fading to a minimum in various exemplary embodiments, in another group of exemplary embodiments, the 3x3 beam coupler is also constructed using polarization-maintaining (PM) fiber, specifically, non-spun HiBi fiber, and the axis of the non-spun HiBi fiber of the 3x3 beam coupler is preferably aligned with the axis of the non-spun HiBi fiber pigtail of the linear polarizer in various examples to achieve optimal and stable fringe contrast. This is also referred to herein as a 3x3 PM coupler. This can ensure that fringe fading is minimized without stringent packaging requirements, and therefore can ensure maximum signal-to-noise performance and minimum static phase error, preferably to achieve minimized DC value calculation error.

[0047] 7 is a schematic diagram of an exemplary embodiment of a 3x3 polarization-maintaining Sagnac current sensor 700 comprising two in-line polarizers 702, 704 and a coil of SPOF 706 to measure the current in a sealed conductor 708. The 3x3 polarization-maintaining Sagnac current sensor 700 is similar to the exemplary embodiment described above with reference to FIG. 1, except that this exemplary embodiment preferably minimizes fringe fading and provides improved stability and reduced zero current error. More specifically, to preferably achieve optimal and stable fringe contrast, the 3x3 PM coupler 710 is constructed using polarized non-spun HiBi fiber, and the axis at the end face of the non-spun HiBi fiber output pigtail of the 3x3 PM coupler 710 is aligned with the axis at the end face of the non-spun HiBi fiber pigtail of the linear polarizers 702, 704, indicated by reference numerals 709, 711, during splicing. This can ensure that fringe fading is minimized, thereby ensuring maximum signal-to-noise performance and minimum static phase error, preferably to achieve minimized DC value calculation error.

[0048] 8 is a schematic diagram illustrating a 3x3 polarization-maintaining Sagnac current sensor 800 according to one exemplary embodiment, comprising two circular polarizers 802, 804 and a coil of spun HiBi 806 to measure the current in a sealed conductor 808. The 3x3 polarization-maintaining Sagnac current sensor 800 is similar to the exemplary embodiment described above with reference to FIG. 2, except that this exemplary embodiment preferably minimizes fringe fading and provides improved stability and reduced zero current error. More specifically, in this exemplary embodiment, the 3x3 PM coupler 810 is constructed using non-spun HiBi fiber, and the end face axis of the non-spun HiBi fiber output pigtail of the 3x3 beam coupler 810 is aligned with the end face axis of the non-spun HiBi fiber pigtail of the linear polarizers 812, 814 (part of the circular polarizers 802, 804, respectively) during splicing, preferably to achieve optimal and stable fringe contrast. This can ensure that fringe fading is minimized, which can ensure maximum signal-to-noise performance and minimum static phase error, preferably to achieve minimized DC value calculation error.

[0049] 9 is a schematic diagram of a 3x3 polarization-maintaining Sagnac current sensor 900 according to one exemplary embodiment, comprising two in-line linear polarizers 902, 904 and a coil of SPOF 906 arranged in a figure of eight to measure the current in a sealed conductor 908 and eliminate rotation. The 3x3 polarization-maintaining Sagnac current sensor 900 is similar to the exemplary embodiment described above with reference to FIG. 3, except that this exemplary embodiment preferably minimizes fringe fading and provides improved stability and reduced zero current error. More specifically, in this exemplary embodiment, the 3x3 PM coupler 910 is constructed using non-spun HiBi fiber, and the axis at the end face of the non-spun HiBi fiber output pigtail of the 3x3 beam PM coupler 910 is aligned with the axis at the end face of the non-spun HiBi fiber pigtail of the linear polarizers 902, 904, indicated by reference numerals 909, 911, during splicing, to preferably achieve optimal and stable fringe contrast. This can ensure that fringe fading is minimized, thereby ensuring maximum signal-to-noise performance and minimum static phase error, preferably to achieve minimized DC value calculation error.

[0050] 10 is a schematic diagram illustrating a 3x3 polarization-maintaining Sagnac current sensor 1000 according to one exemplary embodiment that includes (at least approximately) two circular polarizers 1002, 1004 and a coil of spun HiBi 1006 arranged in a figure of eight to measure current and eliminate rotation in a sealed conductor 1008. The 3x3 polarization-maintaining Sagnac current sensor 1000 is similar to the exemplary embodiment described above with reference to FIG. 4, except that this exemplary embodiment preferably minimizes fringe fading and provides improved stability and reduced zero current error. More specifically, in this exemplary embodiment, the 3x3 beam PM coupler 1010 is constructed using non-spun HiBi fiber, and the axis of the non-spun HiBi fiber output pigtail end face of the 3x3 PM coupler 1010 is aligned with the axis of the non-spun HiBi fiber pigtail end face of the linear polarizers 1012, 1014 (part of the circular polarizers 1002, 1004, respectively) during splicing, preferably to achieve optimal and stable fringe contrast. This can ensure that fringe fading is minimized, and therefore can ensure maximum signal-to-noise performance and minimum static phase error, preferably to achieve minimized DC value calculation error.

[0051] 11 shows a schematic diagram illustrating a 3x3 polarization-maintaining Sagnac current sensor 1100 according to one exemplary embodiment, comprising two in-line linear polarizers 1102, 1104 in a loop-back configuration and a cable 1106 including one SPOF fiber 1108 and one non-spun HiBi fiber 1110. The cable 1106 is wrapped in one or more loops around a conductor 1112. In this exemplary embodiment, the SPOF fiber 1108 senses the current in the sealed conductor 1112, but the non-spun HiBi fiber 1110 does not.

[0052] This exemplary embodiment preferably minimizes fringe fading and provides improved stability and reduced zero current error. More specifically, in this exemplary embodiment, to preferably achieve optimal and stable fringe contrast, the 3x3PM coupler 1111 is constructed with non-spun HiBi fiber, and the axis of the non-spun HiBi fiber output pigtail of the 3x3PM coupler 1111 is aligned with the axis of the non-spun HiBi fiber pigtail of the linear polarizers 1102, 1104 during splicing, as indicated by reference numerals 1107, 1109. This can ensure that fringe fading is minimized, and thus, can ensure maximum signal-to-noise performance and minimum static phase error, preferably to achieve minimized DC value calculation error. To effectively form a closed loop (also referred to herein as a "loopback configuration") with the cable 1106 surrounding the conductor 1112, two optimized splices, one between the end 1108a of the SPOF fiber 1108 and the non-spun HiBi fiber tail of the linear polarizer 1104, and the other end 1108b of the SPOF fiber 1108 and one end 110b of the non-spun HiBi fiber 1110 in the cable 1106, are placed as close as possible to each other, indicated by the reference numeral 1116, preferably at one end of the loop, ensuring that the path followed by the current-sensing spun HiBi fiber 1108 in the loopback configuration is a good approximation to a closed loop (compare also Fig. 5A, B). In the optimized splice, the polarization-maintaining axes at the end faces are preferably aligned. In practice, as the ellipticity approaches a circular shape, the alignment becomes less important.

[0053] Thus, light from each of the two output ports of the 3x3 PM coupler 1111 travels in one section of the cable 1106 that has sensitivity to electric current, i.e., in the SPOF fiber 1108, and in the other section that has zero sensitivity to electric current, i.e., in the non-spun HiBi fiber 1110, while traveling in both directions through the cable 1106 in a loop-back configuration. Thus, excellent rejection of the gyroscopic effect can also be provided. Single polarization of the interfering light in the 3x3 PM coupler 1111 is achieved using linear polarizers 1102, 1104, which in this exemplary embodiment are in the form of in-line polarizers.

[0054] 12 is a schematic diagram illustrating a 3x3 polarization-maintaining Sagnac current sensor 1200 according to one exemplary embodiment, comprising (at least approximately) two circular polarizers 1202, 1204 and a cable 1206 housing one spun HiBi fiber 1208 and one non-spun HiBi fiber 1210. The 3x3 polarization-maintaining Sagnac current sensor 1200 is similar to the exemplary embodiment described above with reference to FIGS. 5A and B, except that this exemplary embodiment preferably minimizes fringe fading and provides improved stability and reduced zero current error. More specifically, in this exemplary embodiment, to preferably achieve optimal and stable fringe contrast, the 3x3 beam PM coupler 1210 is constructed using HiBi fiber, and the axis of the non-spun HiBi fiber end face of the 3x3 PM coupler 1210 is aligned with the axis of the non-spun HiBi fiber pigtail end face of the linear polarizers 1212, 1214 (part of the circular polarizers 1202, 1204, respectively) during splicing, as indicated by reference numerals 1207, 1209. This can ensure that fringe fading is minimized, and thus can ensure maximum signal-to-noise performance and minimum static phase error, preferably to achieve minimized DC value calculation error.

[0055] 13 shows a schematic diagram illustrating a 3x3 polarization-maintaining Sagnac current sensor 1300 according to one exemplary embodiment, comprising two circular polarizers 1302, 1304 and a cable 1306 including three spun HiBi fibers 1308, 1309 and 1310. Compared to the exemplary embodiment described above with reference to FIG. 6(a), in this exemplary embodiment, a 1 / 2 wave plate 1317 is placed between the end 1308a of the spun HiBi fiber 1308 and the end 1309a of the spun HiBi fiber 1309, effectively swapping the handedness of the polarization modes. Similarly, another 1 / 2 wave plate 1319 is placed / spliced ​​between the end 1309b of the spun HiBi fiber 1309 and the end 1310b of the spun HiBi fiber 1310. Placing the splices for the wave plates 1317, 1319 and the splices between the circular polarizers 1302, 1304 and the spun HiBi fibers 1308, 13010 as close as possible preferably ensures that each path followed by the current-sensing spun HiBi fibers 1308, 1309 and 1310 is well approximated to a closed loop. In FIG. 13, the splices between the wave plates 1317, 1319 and the splices between the circular polarizers 1302, 1304 and the spun HiBi fibers 1308, 1310 are not shown for clarity of the drawing, but are placed close together.

[0056] This exemplary embodiment effectively triples the sensitivity of the loop / coil 1316 to current while also providing superior rotation rejection. Additionally, this exemplary embodiment preferably minimizes fringe fading and provides improved stability and reduced zero current error. More specifically, in this exemplary embodiment, to preferably achieve optimal and stable fringe contrast, the 3x3 beam coupler 1311 is constructed using HiBi fiber, and the axis of the non-spun HiBi fiber of the 3x3 beam coupler 1311 is aligned with the axis of the non-spun HiBi fiber pigtail of the linear polarizers 1313, 1315 (part of the circular polarizers 1302, 1404, respectively) during splicing. This can ensure that fringe fading is minimized, which can in turn ensure maximum signal-to-noise performance and minimum static phase error to achieve minimized DC value calculation error with preferably minimized zero current error.

[0057] The exemplary embodiment described above can provide useful measurements for signal processing for passive interferometry, preferably without the need for calibration procedures. This can be achieved by implementing techniques described in WO 03 / 002932. With reference to FIG. 14, briefly, a method and apparatus is implemented to implement a passive interferometer utilizing a 3×3 optical coupler 1400, together with an optical multiplexing network 1402, an optical input unit 1404 for inserting an optical input signal via the optical multiplexing network 1402 into each of the ports 1411-1413, and a detection unit 1406 for measuring the optical output corresponding to each of the ports 1411-1413 via the optical multiplexing network 1402 when an optical input is inserted into one of the ports 1411-1413.

[0058] On the other side of the 3x3 coupler is a passive Sagnac interferometer 1420 and a sealed conductor 1422. The Sagnac interferometer 1420 may take the form of any one of the exemplary embodiments described above.

[0059] The processing unit 1408 is configured to generate results, each result being one of three 2 , where each result includes the same optical multiplexing network 1402 dependent factors. In this manner, the processing unit 1408 can advantageously remove the optical multiplexing network 1408 dependent factors based on processing the formed results to obtain a substantially network independent measurement.

[0060] Also, apart from the small wavelength-dependent (secondarily small) effect of the optical multiplexing network 1402 dependent factors across the source spectrum of the optical input unit 1404, the zero error of the interferometer (for zero current in conductor 1422) is zero.

[0061] Advantageously, the light input unit 1404 comprises a substantially monochromatic light source.

[0062] If the optical input unit 1404 includes a broadband optical source, the optical multiplexing network 1402 preferably includes a wavelength-flattening optical coupler.

[0063] The processing unit 1408 may be configured such that removing the optical multiplexing network 1402 dependent factors includes forming an independent ratio of results.

[0064] The processing unit 1408 may be configured such that forming each result includes forming a geometric mean of the factors. In other embodiments, a computational means may be used.

[0065] FIG. 15 shows a schematic diagram showing details of an exemplary circular polarizer 1500 for use in the described exemplary embodiment. The circular polarizer 1500 includes a combination of an at least approximately linear polarizer 1502 and a 1 / 4 beat length section 1504 of non-spun HiBi fiber (i.e., having a length of 1 / 4 beat length). Specifically, one of the output pigtails 1505 of a fiber coupler (not shown) is spliced ​​(indicated by reference number 1506) to the non-spun HiBi pigtail 1508 of the linear polarizer 1502. The other non-spun HiBi pigtail 1510 of the linear polarizer 1502 is spliced ​​(indicated by reference number 1512) with the polarization axis at the end face at 45 degrees to the 1 / 4 beat length section 1504 of non-spun HiBi fiber (indicated by reference number 1514), which is in turn spliced ​​to one end 1516 of a sensing coil or cable (not shown). At the splices indicated at 1506, 1514, the polarization maintaining axes at each end face are preferably aligned. In practice, as the ellipticity approaches a circle, alignment becomes less important.

[0066] In one embodiment, a passive optical Sagnac interferometer for current sensing is provided, comprising: an NxN fiber coupler, where N>3; a fiber coil disposed on a first side of the NxN fiber coupler; a first port of the NxN fiber coupler coupled to a first end of the fiber coil via a first linear polarizing element; and a second port of the NxN fiber coupler coupled to a second end of the fiber coil via a second linear polarizing element, wherein the fiber coil is configured to support only elliptical polarization states in counter-propagating signals in the fiber coil.

[0067] The fiber coil may be configured to support only at least one approximately circular polarization state in a counter-propagating signal within the fiber coil.

[0068] The fiber coil may include a spun elliptically birefringent polarizing optical fiber.

[0069] The fiber coil may include a non-polarizing spun elliptically birefringent optical fiber, and the interferometer comprises at least first and second substantially circular polarizing elements, the first circular polarizing element including the first linear polarizing element and a first quarter wavelength fiber section coupled between the first port of the NxN fiber coupler and the first end of the fiber coil, and the second circular polarizing element including the second linear polarizing element and a second quarter wavelength fiber section coupled between the second port of the NxN fiber coupler and the first end of the fiber coil.

[0070] The NxN fiber coupler may include an NxN single mode optical fiber coupler.

[0071] The NxN fiber coupler may include an NxN polarization-maintaining optical fiber coupler.

[0072] The fibre coil may have the configuration of FIG.

[0073] an optical multiplexing network disposed on a second side of the NxN fiber coupler; an optical input unit configured to insert an optical input signal through the optical multiplexing network into each port of the second side of the NxN fiber coupler; and a detection unit configured to measure a corresponding optical output through the optical multiplexing network at each port of the second side of the NxN fiber coupler when an optical input is inserted into one of a plurality of ports of the second side of the NxN fiber coupler; 2 and a processing unit for forming measurement results including at least some of the measurable optical outputs of the optical fibers, each measurement result including an identical optical multiplexing network dependent factor, the processing unit removing the optical multiplexing network dependent factor to obtain a substantially optical multiplexing network independent measurement.

[0074] In one embodiment, a passive optical Sagnac interferometer for current sensing is provided, the interferometer comprising: an NxN fiber coupler; a fiber cable coupled to the NxN fiber coupler; a first output port of the NxN fiber coupler coupled to a first end of a first fiber of the fiber cable; a second output port of the NxN fiber coupler coupled to a first end of a second fiber of the fiber cable; and at least first and second substantially circular polarizing elements, the first circular polarizing element being coupled between the first output port of the NxN fiber coupler and the first end of the first fiber, the second circular polarizing element being coupled between the second output port of the NxN fiber coupler and the first end of the second fiber, second ends of the first and second fibers being coupled to each other, the fiber cable being configured in a coiled configuration, the first fiber comprising a non-polarizing spun birefringent optical fiber, and the second fiber comprising a non-spun highly birefringent optical fiber.

[0075] The NxN fiber coupler may include an NxN single mode optical fiber coupler.

[0076] The NxN fiber coupler may include an NxN polarization-maintaining optical fiber coupler.

[0077] an optical multiplexing network disposed on a second side of the NxN fiber coupler; an optical input unit configured to insert an optical input signal through the optical multiplexing network into each port of the second side of the NxN fiber coupler; and a detection unit configured to measure a corresponding optical output through the optical multiplexing network at each port of the second side of the NxN fiber coupler when an optical input is inserted into one of a plurality of ports of the second side of the NxN fiber coupler; 2and a processing unit for forming measurement results including at least some of the measurable optical outputs of the optical fibers, each measurement result including an identical optical multiplexing network dependent factor, the processing unit removing the optical multiplexing network dependent factor to obtain a substantially optical multiplexing network independent measurement.

[0078] In one embodiment, a passive optical Sagnac interferometer for current sensing comprises: an NxN polarization-maintaining fiber coupler; a fiber cable coupled to the NxN polarization-maintaining fiber coupler; a first output port of the NxN polarization-maintaining fiber coupler coupled to a first end of a first fiber of the fiber cable; a second output port of the NxN polarization-maintaining fiber coupler coupled to a first end of a second fiber of the fiber cable; and first and second linear polarizing elements, the first linear polarizing element being coupled to the NxN polarization-maintaining fiber coupler. the second linear polarizing element is coupled between the second output port of the NxN polarization-maintaining fiber coupler and the first end of the first fiber, the second linear polarizing element is coupled between the second output port of the NxN polarization-maintaining fiber coupler and the first end of the second fiber, second ends of the first and second fibers are coupled to each other, the fiber cable is configured in a coiled configuration, the first fiber comprises a spun elliptically birefringent optical fiber, and the second fiber comprises a non-spun highly birefringent optical fiber.

[0079] an optical multiplexing network disposed on a second side of the NxN polarization-maintaining fiber coupler; an optical input unit configured to insert an optical input signal through the optical multiplexing network into each port of the second side of the NxN polarization-maintaining fiber coupler; and a detection unit configured to measure a corresponding optical output at each port of the second side of the NxN polarization-maintaining fiber coupler through the optical multiplexing network when an optical input is inserted into one of a plurality of ports of the second side of the NxN polarization-maintaining fiber coupler; 2and a processing unit for forming measurement results including at least some of the measurable optical outputs of the optical multiplexing units, each measurement result including the same optical multiplexing network dependent factors, the processing unit removing the optical multiplexing network dependent factors to obtain substantially optical multiplexing unit independent measurements.

[0080] In one embodiment, a passive optical Sagnac interferometer for current sensing includes an NxN fiber coupler; a fiber cable coupled to the NxN fiber coupler; a first output port of the NxN fiber coupler coupled to a first end of a first fiber of the fiber cable; a second output port of the NxN fiber coupler coupled to a first end of a second fiber of the fiber cable; and at least first and second substantially circular polarizing elements, where the first circular polarizing element is coupled between the first output port of the NxN fiber coupler and the first end of the first fiber. the second circular polarizing element is coupled between the second output port of the NxN fiber coupler and the first end of the second fiber; a first half-wave plate coupled between the second end of the first fiber and a first end of a third fiber of the fiber cable; and a second half-wave plate coupled between the second end of the second fiber and a second end of the third fiber, the fiber cable being configured in a coiled configuration; and each of the first, second, and third fibers comprising a non-polarizing spun birefringent optical fiber.

[0081] The NxN fiber coupler may include an NxN single mode optical fiber coupler.

[0082] The NxN fiber coupler may include an NxN polarization-maintaining optical fiber coupler.

[0083] an optical multiplexing network disposed on a second side of the NxN fiber coupler; an optical input unit configured to insert an optical input signal through the optical multiplexing network into each port of the second side of the NxN fiber coupler; and a detection unit configured to measure a corresponding optical output through the optical multiplexing network at each port of the second side of the NxN fiber coupler when an optical input is inserted into one of a plurality of ports of the second side of the NxN fiber coupler; 2 and a processing unit for forming measurement results including at least some of the measurable optical outputs of the optical fibers, each measurement result including an identical optical multiplexing network dependent factor, the processing unit removing the optical multiplexing network dependent factor to obtain a substantially optical multiplexing network independent measurement.

[0084] In the above embodiment, N may be equal to three.

[0085] In one embodiment, there is provided a method of sensing a current in a conductor using the interferometer of any one of the above embodiments.

[0086] In one embodiment, there is provided a method of manufacturing an interferometer according to any one of the above embodiments.

[0087] The above description of illustrated embodiments of the systems and methods is not intended to be exhaustive or to limit the systems and methods to the precise forms disclosed. Specific embodiments and examples of system components and methods are described herein for illustrative purposes, but various equivalent modifications are possible within the scope of the systems, components and methods, as those skilled in the art will recognize. The teachings of the systems and methods provided herein may be applied to other processing systems and methods, as well as the systems and methods described above.

[0088] Those skilled in the art will appreciate that numerous variations and / or modifications may be made to the present invention as illustrated in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive. Moreover, the present invention includes any combination of the features described in the different embodiments contained in the summary section, even if that feature or combination of features is not expressly specified in the claims or the detailed description of the embodiments.

[0089] For example, although a 3x3 coupler is described in the exemplary embodiment, an NxN coupler (N>=3) may be used in different embodiments.

[0090] In general, in the following claims, the terms used should not be construed to limit the systems and methods to the specific embodiments disclosed in the specification and claims, but should be construed to include all processing systems that operate within the scope of the claims. Thus, the systems and methods are not limited by this disclosure; instead, the scope of the systems and methods should be determined entirely by the claims.

[0091] Unless the context clearly requires otherwise, throughout this specification and claims, the words "comprise," "comprising," and the like are to be construed in an inclusive sense, i.e., "including, but not limited to," as opposed to an exclusive or exhaustive sense. Words using the singular or plural number also include the plural or singular, respectively. Additionally, the terms "herein," "below," "above," "below," and words of similar import refer to this application as a whole and not to any particular portion of this application. When the word "or" is used in connection with a list of two or more items, the word includes all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list.

Claims

1. A passive optical Sagnac interferometer for current sensing, an N×N fiber coupler, where N≥3, a fiber coil disposed on a first side of the N×N fiber coupler, a first port of the N×N fiber coupler coupled to a first end of the fiber coil via a first linear polarizer, a second port of the N×N fiber coupler coupled to a second end of the fiber coil via a second linear polarizer, comprising: the fiber coil is configured to support only an elliptical polarization state in a backward-propagating optical signal in the fiber coil, a first optical signal propagating in a first direction in the fiber coil has the same left-right symmetry as a second optical signal propagating in a second direction opposite to the first direction in the fiber coil interferometer.

2. The fiber coil is configured to support only at least one approximately circular polarization state in a backward-propagating optical signal in the fiber coil The interferometer according to claim 1.

3. The fiber coil includes a spun elliptical birefringent polarization optical fiber The interferometer according to claim 1 or 2.

4. The fiber coil includes a non-polarized spun elliptical birefringent optical fiber, the interferometer comprises at least first and second substantially circular polarization elements, the first circular polarization element includes a first 1 / 4 wavelength fiber portion coupled between the first linear polarizer, the first port of the N×N fiber coupler, and the first end of the fiber coil, the second circular polarization element includes a second 1 / 4 wavelength fiber portion coupled between the second linear polarizer, the second port of the N×N fiber coupler, and the first end of the fiber coil The interferometer according to claim 1 or 2.

5. The N×N fiber coupler includes an N×N single-mode optical fiber coupler The interferometer according to any one of claims 1 to 4.

6. The N×N fiber coupler includes an N×N polarization-maintaining optical fiber coupler The interferometer according to any one of claims 1 to 4.

7. The fiber coil has an 8-shaped configuration The interferometer according to any one of claims 1 to 6.

8. an optical multiplexing network disposed on a second side of the N×N fiber coupler, An optical input unit that inserts an optical input signal into each port on the second side of the N×N fiber coupler via the optical multiplexing network; A detection unit that measures a corresponding optical output at each port on the second side of the N×N fiber coupler via the optical multiplexing network when an optical input is inserted into one of a plurality of ports on the second side of the N×N fiber coupler; A processing unit that forms a measurement result including at least some of 32 measurable optical outputs; Comprising; Each measurement result includes the same optical multiplexing network-dependent factor; The processing unit substantially removes the optical multiplexing network-dependent factor in order to obtain an optical multiplexing network-independent measurement. The interferometer according to any one of claims 1 to 7.

9. A passive optical Sagnac interferometer for current sensing, An N×N fiber coupler; A fiber cable coupled to the N×N fiber coupler; A first output port of the N×N fiber coupler coupled to a first end of a first fiber of the fiber cable; A second output port of the N×N fiber coupler coupled to a first end of a second fiber of the fiber cable; At least first and second substantially circular polarization elements; Comprising; The first circular polarization element is coupled between the first output port of the N×N fiber coupler and the first end of the first fiber; The second circular polarization element is coupled between the second output port of the N×N fiber coupler and the first end of the second fiber; Second ends of each of the first fiber and the second fiber are coupled to each other; The fiber cable is configured to be wound in a coil shape; The first fiber includes a non-polarized spun birefringent optical fiber; The second fiber includes a non-spun highly birefringent optical fiber. Interferometer.

10. The N×N fiber coupler includes an N×N single-mode optical fiber coupler. The interferometer according to claim 9.

11. The N×N fiber coupler includes an N×N polarization-maintaining optical fiber coupler. The interferometer according to claim 9.

12. An optical multiplexing network disposed on a second side of the N×N fiber coupler; An optical input unit that inserts an optical input signal into each port on the second side of the N×N fiber coupler via the optical multiplexing network; When an optical input is inserted into one of the plurality of ports on the second side of the N×N fiber coupler, a detection unit that measures corresponding optical outputs at each port on the second side of the N×N fiber coupler via the optical multiplexing network; A processing unit that forms a measurement result including at least some of 32 measurable optical outputs; Comprising: Each measurement result includes the same optical multiplexing network-dependent factor; The processing unit substantially removes the optical multiplexing network-dependent factor in order to obtain an optical multiplexing network-independent measurement The interferometer according to any one of claims 9 to 11.

13. A passive optical Sagnac interferometer for current sensing, An N×N polarization-maintaining fiber coupler; A fiber cable coupled to the N×N polarization-maintaining fiber coupler; A first output port of the N×N polarization-maintaining fiber coupler coupled to a first end of a first fiber of the fiber cable; A second output port of the N×N polarization-maintaining fiber coupler coupled to a first end of a second fiber of the fiber cable; First and second linear polarization elements; Comprising: The first linear polarization element is coupled between the first output port of the N×N polarization-maintaining fiber coupler and the first end of the first fiber; The second linear polarization element is coupled between the second output port of the N×N polarization-maintaining fiber coupler and the first end of the second fiber; Second ends of each of the first fiber and the second fiber are coupled to each other; The fiber cable is configured to be wound in a coil shape; The first fiber includes a spun elliptical birefringent fiber; The second fiber includes a non-spun highly birefringent fiber Interferometer.

14. An optical multiplexing network disposed on a second side of the N×N polarization-maintaining fiber coupler; An optical input unit that inserts an optical input signal into each port on the second side of the N×N polarization-maintaining fiber coupler via the optical multiplexing network; When an optical input is inserted into one of the plurality of ports on the second side of the N×N polarization-maintaining fiber coupler, a detection unit that measures corresponding optical outputs at each port on the second side of the N×N polarization-maintaining fiber coupler via the optical multiplexing network; A processing unit that forms a measurement result including at least some of 32 measurable optical outputs; Comprising: Each measurement result includes the same optical multiplexing network-dependent factor, The processing unit substantially removes the optical multiplexing network-dependent factor in order to obtain an optical multiplexing unit-independent measurement The interferometer according to claim 13.

15. N = 3 The interferometer according to any one of claims 1 to 14.

16. A method for sensing a current in a conductor using the interferometer according to any one of claims 1 to 15.

17. A method for manufacturing the interferometer according to any one of claims 1 to 15.

Images