WO2007033057A2

WO2007033057A2 - Fiber optic current sensor

Info

Publication number: WO2007033057A2
Application number: PCT/US2006/035256
Authority: WO
Inventors: James N. Blake
Original assignee: Nxtphase T & D Corporation
Priority date: 2005-09-12
Filing date: 2006-09-11
Publication date: 2007-03-22
Also published as: WO2007033057A3; CN101351714A; DE102005043322B4; CN101351714B; DE102005043322A1

Abstract

A fiber optic current sensor includes a polarization beam splitter. The polarization beam splitter operates, in the forward path, to polarize light that is transmitted through a reciprocal port toward a Faraday rotator. The polarization beam splitter operates, in a reverse path, to direct light to a photodetector through a non-reciprocal port.

Description

FIBER OPTIC CURRENT SENSOR

BACKGROUND

[0001] The present invention pertains to fiber optic sensors and, particularly, to fiber optic current sensors.

[0002] Fiber optic current sensors operate based on the Faraday effect. Current flowing in a wire induces a magnetic field which, through the Faraday effect, rotates the plane of polarization of the light traveling in the optical fiber wound around the current carrying wire. Faraday's law can be stated as:

I = j HdL (1)

where I is the electrical current, H is the magnetic field and the integral is taken over a closed path around the current. If the sensing fiber is wound around the current carrying wire with an integral number of turns, and each point in the sensing fiber has a constant sensitivity to the magnetic field, then the rotation of the plane of polarization of the light in the fiber depends on the current being carried in the wire and is insensitive to all externally generated magnetic fields such as those caused by currents carried in nearby wires. The angle, ΔΦ, through which the plane of polarization of light rotates in the presence of a magnetic field is given by:

ΔΦ = v J H.dL (2)

where V is the Verdet constant of the fiber glass. [0003] The sensing optical fiber performs the line integral of the magnetic field along its path, which is proportional to the current in the wire, when that path closes on itself. Thus, ΔΦ =VNI where N is the number of turns of sensing fiber wound around the current carrying wire. The rotation of the state of polarization of the light due to the presence of an electrical current can be measured by injecting light with a well-defined linear polarization state into the sensing region, and then analyzing the polarization state of the light after it exits the sensing region. Alternatively, ΔΦ represents the excess phase shift encountered by a circularly polarized light wave propagating in the sensing fiber. , [0004] This technology is related to the in-line optical fiber current sensor as disclosed in U.S. Pat. No. 5,644,397 issued JuI. 1, 1997, to inventor James N. Blake and entitled "Fiber Optic Interferometric Circuit and Magnetic Field Sensor", which is incorporated herein by reference. Optical fiber sensors are also disclosed in U.S. Pat. No. 5,696,858 issued Dec. 9, 1997, to inventor James N. Blake and entitled, "Fiber Optics Apparatus and Method for Accurate Current Sensing" and U.S. Patent No. 6,188,811 to James N. Blake and entitled "Fiber Optic Current Sensor", the disclosures of which are incorporated herein by reference.

[0005] A fiber optic current sensor disclosed in the '811 patent is reproduced herein as Figure 1. Therein, light from source 10 propagates through coupler 11 and polarizer 12 to a 45-degree splice 13, where it divides equally into the two polarization states maintained throughout the rest of the optical circuit. Piezoelectric birefringence modulator 14 differentially modulates the phases of the light in the two polarization states. Modulator 14 is driven by a modulator signal generator 71 that provides an electrical, periodic, alternating signal having either a square or sine wave. The light then propagates through delay line 15, through mode converter 16 which converts the two linear states of polarization into two circular states of polarization, and through optimized sensor coil 17. Optimized sensor coil 17 is wound around current carrying wire 18. The light reflects off reflective termination 19 and retraces its way through the optical circuit, finally arriving at detector 20. Open-loop signal processor 21 converts the detected signal to an output 22 which is indicative of the current flowing in current carrying wire 18. The '811 patent also describes embodiments wherein a Faraday rotator and mode converter can be used instead of the birefringence modulator 14 to passively bias a current sensor.

[0006] Coupler 11 can be implemented as a 3dB optical coupling device and introduces intrinsic optical losses into the system. Intrinsic optical losses refer to losses which will exist in the fiber optic current sensor assuming ideal device characteristics and excluding losses external to the devices themselves, e.g., losses attributable to optical splices. For example, during the forward propagation from the light source 10 to the reflective termination 19, a portion (e.g., approximately half) of the light generated by light source 10 is coupled into the optical fiber 24 by coupler 11 and travels away from detector 20 (as represented by arrow A). This light energy can be dumped or otherwise discarded as it is not used in the current sensing measurement. The remainder of the light (represented by arrow B) travels toward the reflective termination 19 as described above, however about half of that light is lost in polarizer 12. When the light returns from the reflective termination 19, it again experiences losses as it passes through the polarizer 12. Assuming, for illustrative purposes, that the modulator 14 is designed to bias the current sensor to the quadrature point (described below), then the intrinsic optical losses associated with the polarizer 12 on the return path will again be approximately one-half of the light energy passing therethrough. The remaining light is again coupled into optical fiber 24, resulting in a portion of the light (represented by arrow C) traveling to the detector 20 and another portion (represented by arrow D) continuing on toward the source 10. Thus, considering the intrinsic losses, only approximately one-sixteenth of the light energy which was originally generated by light source 10 is actually returned to detector 20.

[0007] Accordingly, it would be desirable to provide optical current sensors which have, among other things, reduced optical losses, lower component count and are more cost efficient to manufacture.

SUMMARY

[0008] According to one exemplary embodiment of the present invention, a method for sensing current using a fiber optic current sensing device includes the steps of polarizing a source light by passing the source light through a polarization beam splitter, introducing a first, biasing phase shift to circularly polarized component waves of light received from the polarization beam splitter, converting the polarization component waves from circularly polarized component waves into linearly polarized component waves, passing the linearly polarized component waves through a polarization maintaining fiber, converting the linearly polarized component waves back into circularly polarized component waves, introducing a second phase shift, associated with the current, to the circularly polarized component waves, returning the light along a reverse path of the fiber optic current sensing device, thereby introducing a third phase shift, associated with the current, and a fourth biasing phase shift to the circularly polarized waves and directing the returned light to a detector via a non- reciprocal port of the polarization beam splitter to sense the current. [0009] According to another exemplary embodiment of the present invention, a fiber optic current sensor includes a light source, a polarization beam splitter connected to the light source having a reciprocal port and a non-reciprocal port, a Faraday rotator connected to the polarization beam splitter, a first quarter-wave plate connected to the Faraday rotator, a polarization maintaining fiber connected to the first quarter-wave plate, a second quarter- wave plate connected to the polarization maintaining fiber, a sensing fiber connected to the second quarter-wave plate, and a detector connected to the polarization beam splitter via the non-reciprocal port and having an output.

BRIEF DESCRIPTION QF THE DRAWINGS

[0010] The accompanying drawings illustrate exemplary embodiments of the present invention, wherein:

[0011] FIG. 1 depicts a conventional fiber optic current sensor;

[0012] FIG. 2 depicts a fiber optic current sensor according to an exemplary embodiment of the present invention;

[0013] FIG. 3 is a graph of intensity as a function of phase for light into a detector for a conventional fiber optic current sensor of FIG. 1;

[0014] FIG. 4 includes a graph of intensity as a function of phase for light into a detector for the fiber optic current sensor of FIG. 2 according to an exemplary embodiment of the present invention; [0015] FIG. 5 is a block diagram illustrating an exemplary closed loop control circuit according to an exemplary embodiment of the present invention; and [0016] FIG. 6 is a flow chart illustrating a method for sensing current using a fiber optic current sensor according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

[0017] The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.

[0018] According to exemplary embodiments of the present invention, optical losses in fiber optic current sensors are reduced by using a polarization beam splitter to guide light toward the reflective termination during forward propagation and toward the detector during its return. Other advantages and benefits are also associated with fiber optic current sensors employing polarization beam splitters as will be described below. [0019] An example is shown in Figure 2. Therein, a light source 50 (e.g., a superluminescent diode (SLED)) emits light having a random polarization state which is coupled into a single mode (SM) optical fiber 51. The light is optionally depolarized by depolarizer 52 such that the polarization state of the light output from depolarizer 52 contains about one-half light having a first linear polarization state and the remainder of the light having a second linear polarization state which is orthogonal to the first linear polarization state. [0020] The depolarized light travels on to the polarization beam splitter (PBS) 54 where it enters through port 55. The polarization beam splitter 54 operates to split the depolarized light into its two orthogonal polarization components. One polarization component (represented by arrow A) is reflected back from a splitting junction within the PBS 54 and is substantially scattered within the device (although some light could be reflected back through the port 55) and the other polarization component (represented by arrow B) is transmitted through port 57 of the PBS. Thus, in the forward propagation direction (i.e., from the source 50 to the reflective termination 56), the PBS 54 operates as a polarizer such that light having a single linear polarization state is conveyed to Faraday rotator 58.

[0021] The linearly polarized light which is incident upon the Faraday rotator 58 can be viewed as two, substantially equal, circularly polarized component waves, e.g., a right- hand circularly (RHC) polarized component wave and a left-hand circularly (LHC) polarized component wave. The Faraday rotator 58 operates to passively introduce a bias between the RHC component wave and the LHC component wave in order to improve the sensitivity of the detector to the amount of current flowing through wire 59, as will be described in more detail below with respect to Figure 3. In this exemplary embodiment of the present invention, the Faraday rotator 58 introduces a phase shift of +22.5 degrees to one of the RHC and LHC component waves and a phase shift of -22.5 degrees to the other of the RHC and LHC component waves. The biased light energy is then output to quarter-wave plate (λ/4) 60 which operates as a mode converter to convert the circularly polarized light to linearly polarized light. [0022] The linearly polarized light is then conveyed via a polarization maintaining

(PM) fiber 61 to another quarter-wave plate 62 which operates as a mode converter to convert the linearly polarized light back into circularly polarized light. The quarter-wave plate 60, PM fiber 61 and quarter- wave plate 62 are provided as a mechanism to aid in maintaining the polarization states (and more significantly the relative phase shift between the polarization components) of the light since the detector 64 operates to detect this phase shift, from which the magnitude of the current flowing through conductor 59 is determined. Depending upon the particular implementation of fiber optic current sensors according to the present invention, the PM fiber 61 may have a length of anywhere from a meter or two to several hundred meters, over which length it is useful to maintain the polarization states of the components and the^" phase shift information. Linear polarization is employed for conveying the light over this part of the system because it is less sensitive to magnetic and stress effects which tend to degrade the purity of the polarization state of the light's component waves. [0023] After the circularly polarized light is output from quarter-wave plate 62, it enters a sensing fiber 66 which encircles the wire 59 whose current being monitored. The detector 64 also achieves its greatest sensitivity when the circular states of polarization are well maintained throughout the sensing fiber 66. As described in the '811 patent, a spun birefringent fiber can preserve a circular state of polarization to some degree. However, for some exemplary embodiments of the present invention, it may be desirable that the circular state of polarization be well maintained so that a very long length (hundreds of meters) of sensing fiber can be used.

[0024] As discussed in the background section, the current running through conductor

59 will introduce an additional phase shift between the RHC and LHC polarization component waves of the light passing through sensing fiber 66 according to ΔΦ =VNI, cumulatively 2VNI. The light will then reach reflective termination 56, e.g., a mirror, where it is reflected back through the sensing fiber 66 to quarter-wave plate 62. During the reverse propagation through sensing fiber 66, the RHC and LHC component waves of the light will acquire a second phase shift therebetween of 2VNI, for a total in the two passes of 4VNI. This second phase shift will be cumulative to the first phase shift (rather than offset it) because the polarization sense of the RHC and LHC component waves reverse upon incidence at the reflective termination and, on the reverse path, the light passes through the magnetic field generated by the current running through conductor 59 in the opposite direction.

[0025] The light will be converted back into linearly polarized light, by quarter-wave plate 62, for the return trip through PM fiber 61, and again back into circularly polarized light by quarter-wave plate 60. The light will be phase shifted again by Faraday rotator 58 such that the cumulative phase shift introduced between the RHC and LHC component waves is 90 degrees + 4VNI. The light output from the Faraday rotator 58 then proceeds on to PBS 54. Some portion of the light output from the Faraday rotator 58 (an amount which depends upon the cumulative phase shift introduced along the forward and reverse paths) will have a polarization that causes it to be reflected from the path axis of the Faraday rotator 58 and directed out through the port 65 of the PBS 54 toward detector 64 (as represented by the arrow C in Figure 2). The remainder of the light will be transmitted back through the port 55 of the PBS 54 toward the source 50 (as represented by the arrow D in Figure 2) and can be isolated or dumped as desired. In this context, port 65 is the "non-reciprocal port" of PBS 54 since the light represented by arrow C is exiting the PBS 54 through a different port on its return path than the port (port 55) through which it entered the PBS 54 along the forward path. Conversely, the portion of the return light represented by arrow D exits the PBS 54 through the reciprocal port 55. Exemplary embodiments of the present invention detect the intensity of light returned through the non-reciprocal port of a polarization beam splitter. [0026] As mentioned above, detector 64 generates intensity data from which the phase shift between the polarization component waves of the light returned to it via the reverse propagation path is determined. This phase shift will be related to the current passing through the conductor 59 and can, therefore, be used to output a current measurement associated therewith. The intensity of light measured by the detector follows a roughly sinusoidal function as illustrated conceptually in Figure 3 for a detector operating in conventional fiber optic current sensor including a coupler 11 and polarizer 12, such as that illustrated in Figure 1. Therein, an intensity of light incident on the detector is plotted as a function 100 of phase. Absent the bias introduced by, e.g., a Faraday rotator or birefringence modulator, the phase change of the light incident upon detector 20 would fluctuate in a small range around zero. As can be seen in Figure 3, the light intensity varies very little in a small phase range around zero. Thus the bias phase shift introduces sufficient bias to move the phase of the light returned to the detector in a range around the quadrature point of the sinusoidal function, where the slope of the function is steeper and the detector is more sensitive to phase changes.

[0027] Consider now the corresponding intensity function 200 for the exemplary embodiment of the Figure 2, shown in Figure 4 overlaid on function 100. Again, like Figure 3, Figure 4 is purely conceptual in the sense that it is not the result of actual circuit measurements, but is instead intended to show the order of magnitude difference in optical losses experienced by the fiber optic current sensor of Figure 1 versus the fiber optic current sensor of Figure 2. Therein, it can be seen that the peak intensities of function 200 are approximately four times the peak intensities of the function 100 for the same light source, as a result of removing the optical coupler 11 and polarizer 12 and instead employing a polarization beam splitter. This provides several advantages for fiber optic current sensors according to exemplary embodiments of the present invention. First, as can be seen in Figure 4, the sensitivity of detector 64 is increased relative to that of detector 20 since the slope of the function 200 in the phase range of feasible values for Φ = 90 degrees + 4VNI is steeper than the corresponding slope for function 100. Alternatively, if the slope of function 100 is deemed to provide sufficient detector sensitivity for a given fiber optic current sensing application, then light source 50 can be implemented as a light source which generates lower intensity light, e.g., a cheaper light source.

[0028] Numerous variations and permutations of the above-described exemplary embodiment are contemplated. For example, detector 64 can be connected to an open-loop signal processor for determining a current associated with the detected phase shift in a manner similar to that illustrated in Figure 1. Alternatively, detector 64 can be connected to a closed-loop signal processor 250 which drives a current generator 260 that produces a phase nulling current as shown in Figure 5, which illustrates only a portion of the fiber optic current sensor according to this exemplary embodiment. The phase nulling current substantially cancels non-reciprocal phase shift produced by the current in current carrying wire 59. Another variation is that the Faraday rotator 58 and quarter- wave plate 60 can be replaced with a birefringence modulator and delay coil. The former combination has the advantage, however, of being a passive device and therefore not requiring power. The optical elements 54, 58 and 60 can be packaged together as a single unit or can be implemented discretely. Additionally, a shield, e.g., a loop of wire, can be placed proximate the Faraday rotator 58 to shield the Faraday rotator against exposure to potentially large magnetic fields, if this portion of the fiber optic current sensor is disposed near the conductor 59. [0029] A corresponding method for sensing current using a fiber optic sensor is illustrated by way of the flowchart of Figure 6. Therein, at step 300, the light is linearly polarized by passing it through a PBS. Circularly polarized component waves are biased by, e.g., a Faraday rotator, at step 302 to introduce a first phase shift between the waves. The circularly polarized waves are converted into linearly polarized waves at step 304, passed through a polarization maintaining fiber (step 306) and then converted back into circularly polarized waves at step 308. A second phase shift is introduced between the circularly polarized waves at step 310 due to the current flowing through wire 59. The light propagates back along a reverse path, thereby introducing third and fourth phase shifts thereto (associated with the current flowing through wire 59 and the biasing device, respectively) at step 312. Then, the light returns to the PBS where it is directed to the detector at step 314 through a non-reciprocal port of the PBS. The light can, optionally, be depolarized prior to step 300.

[0030] As mentioned above, exemplary embodiments of the present invention can employ a birefringence modulator instead of a Faraday rotator to provide the desired bias. In such embodiments, the quarterwave plate 60 can be omitted since the birefringence modulator operates on linearly polarized light.

[0031] The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims. No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article "a" is intended to include one or more items.

Claims

WHAT IS CLAIMED IS;

1. A method for sensing current using a fiber optic current sensing device comprising the steps of: polarizing a source light by passing said source light through a polarization beam splitter; introducing a first, biasing phase shift to circularly polarized component waves of light received from said polarization beam splitter; converting said circularly polarized component waves into linearly polarized component waves; passing said linearly polarized component waves through a polarization maintaining fiber; converting said linearly polarized component waves back into circularly polarized component waves; introducing a second phase shift, associated with said current, to said circularly polarized component waves; returning said light along a reverse path of said fiber optic current sensing device, thereby introducing a third phase shift, associated with said current, and a fourth biasing phase shift to said circularly polarized waves; and directing said returned light to a detector via a non-reciprocal port of said polarization beam splitter to sense said current.

2. The method of claim 1, further comprising the step of: depolarizing said source light prior to said step of polarizing said source light.

3. The method of claim 1, wherein said step of introducing a first, biasing phase shift further comprises the step of: passing said light received from said polarization beam splitter through a Faraday rotator.

4. The method of claim 1, wherein said step of introducing a second phase shift, associated with said current, to said polarization components of said light further comprises the step of: passing said light through a sensing fiber which encircles a conductor carrying said current, wherein a magnetic field associated with said current introduces said second phase shift to said polarization components of said light.

5. A fiber optic current sensor comprising: a light source; a polarization beam splitter connected to said light source having a reciprocal port and a non-reciprocal port; a Faraday rotator connected to said polarization beam splitter; a first quarter-wave plate connected to said Faraday rotator; a polarization maintaining fiber connected to said first quarter-wave plate; a second quarter-wave plate connected to said polarization maintaining fiber; a sensing fiber connected to said second quarter-wave plate; and a detector connected to said polarization beam splitter via said non-reciprocal port and having an output.

6. The fiber optic current sensor of claim 5, wherein said polarization beam splitter operates on light traveling along a forward path toward a reflective termination to polarize light received from said light source and operates on light traveling along a reverse path toward said detector to direct light through said non-reciprocal port to said detector.

7. The fiber optic current sensor of claim 5, further comprising a depolarizer disposed between said light source and said polarization beam splitter.

8. The fiber optic current sensor of claim 5, wherein said polarization beam splitter, said Faraday rotator and said first quarter-wave plate are packaged together.

9. The fiber optic current sensor of claim 5, wherein said light source and said polarization beam splitter are connected via a single mode optical fiber.

10. The fiber optic current sensor of claim 5, wherein said Faraday rotator has a shield proximate thereto to protect said Faraday rotator against magnetic fields.

11. A fiber optic current sensor comprising: a light source; a polarization beam splitter connected to said light source having a reciprocal port and a non-reciprocal port; a birefringence modulator connected to said polarization beam splitter; a polarization maintaining fiber connected to said birefringence modulator; a quarter-wave plate connected to said polarization maintaining fiber; a sensing fiber connected to said quarter-wave plate; and a detector connected to said polarization beam splitter via said non-reciprocal port and having an output.

12. The fiber optic current sensor of claim 11, wherein said polarization beam splitter operates on light traveling along a forward path toward a reflective termination to polarize light received from said light source and operates on light traveling along a reverse path toward said detector to direct light through said non-reciprocal port to said detector.

13. The fiber optic current sensor of claim 11, further comprising a depolarizer disposed between said light source and said polarization beam splitter.

14. The fiber optic current sensor of claim 11, wherein said light source and said polarization beam splitter are connected via a single mode optical fiber.