Few-moded fiber for sensing current

Abstract

A few-moded fiber is doped such that spatial modes of a signal exhibit different magnetic-field-dependent effects. Based on these magnetic-field-dependent effects, one can determine the electric current that induced a magnetic field that caused these effects.

Claims

1. A system, comprising: a first single-moded fiber; a few-moded fiber comprising an input end, the few-moded fiber further comprising an output end, the input end being spliced to the first single-moded fiber, the few-moded fiber to propagate an optical signal, the optical signal comprising a first spatial mode, the optical signal further comprising a second spatial mode, the first spatial mode exhibiting a first magnetic-field-dependent birefringence, the second spatial mode exhibiting a second magnetic-field-dependent birefringence, the second magnetic-field-dependent birefringence being different from the first magnetic-field-dependent birefringence; and a second single-moded fiber spliced to the output end of the few-moded fiber.

2. The system of claim 1, the few-moded fiber being a two-moded doped fiber.

3. The system of claim 1, the few-moded fiber comprising a bend.

4. The system of claim 3, the bend inducing a linear birefringence (.sub.Linear) in the few-moded fiber.

5. The system of claim 4, the few-moded fiber exhibiting a circular birefringence (.sub.Circular), the .sub.Circular being induced by a magnetic field.

6. The system of claim 5, the few-moded fiber exhibiting a total birefringence (), such that: $=\sqrt{{}_{\mathrm{Linear}}^2+{}_{\mathrm{Circular}}^2}.$

7. The system of claim 5, the few-moded fiber exhibiting a total birefringence (), such that: ${}_{\mathrm{Linear}}+\frac{{}_{\mathrm{Circular}}^2}{{}_{\mathrm{Linear}}}.$

8. A system, comprising: a few-moded fiber; and a doped region within the few-moded fiber, the doped region to propagate a signal having a first spatial mode and a second spatial mode, the first spatial mode exhibiting a first magnetic-field-dependent birefringence, the second spatial mode exhibiting a second magnetic-field-dependent birefringence, the first magnetic-field-dependent birefringence being different from the second magnetic-field dependent birefringence.

9. The system of claim 8, further comprising an input fiber spliced to the few-moded fiber.

10. The system of claim 8, further comprising an output fiber spliced to the few-moded fiber.

11. The system of claim 8, the few-moded fiber being a two-moded fiber.

12. The system of claim 8, the few-moded fiber comprising a bend.

13. The system of claim 12, the few-moded fiber exhibiting a circular birefringence (.sub.Circular), the .sub.Circular being induced by a magnetic field.

14. The system of claim 13, the few-moded fiber exhibiting a total birefringence (), such that:
={square root over (.sub.Linear.sup.2+.sub.Circular.sup.2)}, where .sub.Linear represents a linear birefringence.

15. The system of claim 13, the few-moded fiber exhibiting a total birefringence (), such that: ${}_{\mathrm{Linear}}+\frac{{}_{\mathrm{Circular}}^2}{{}_{\mathrm{Linear}}},$ where .sub.Linear represents a linear birefringence.

Description

DETAILED DESCRIPTION OF THE EMBODIMENTS

(1) According to Ampere's law, an electric current generates a magnetic field that is proportional to the electric current. More specifically, according to Maxwell's equations, when a current flows through a conductor, it induces a magnetic field that is perpendicular to the conductor. For some materials, like quartz, the Faraday effect causes a circular birefringence, which can be exploited to sense current. In particular, these current sensors are based on the effect that the induced birefringence has on a degree of polarization of light that is propagating parallel to the direction of the magnetic field lines. The rotation of the polarization can be measured by using a polarizing element (e.g., optics polarizer, polarizing fiber, etc.), or by causing an interference between the rotated field and a reference field and measuring the interference.

(2) The present disclosure seeks to exploit these magneto-optical properties to measure current. Specifically, for some embodiments, a few-moded fiber is doped so that different spatial modes of a signal exhibit different magnetic-field-dependent effects. Thus, in the presence of current-induced magnetic field, the magnetic-field-dependent effects on the different spatial modes can be determined. And, the electrical current can be calculated from these magnetic-field-dependent effects.

(3) With this in mind, reference is now made in detail to the description of the embodiments. While several embodiments are described, there is no intent to limit the disclosure to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications, and equivalents.

(4) Broadly, one embodiment of the invention is a system comprising a few-moded fiber with a doped region. One end of the few-moded fiber is spliced to an input fiber (e.g., a single-moded fiber), while the other end of the few-moded fiber is spliced to an output fiber (e.g., another single-moded fiber). The few-moded fiber is configured to propagate a signal that has multiple spatial modes. For simplicity, this is illustrated herein using a two-moded fiber that carries two spatial modes (a first spatial mode and a second spatial mode). The few-moded fiber is doped such that each of the spatial modes exhibits a different magnetic-field-dependent birefringence. So, for the two-moded fiber, the first spatial mode will experience a magnetic-field-dependent birefringence that is different from the second spatial mode. As explained above, the magnetic-field-dependent birefringence is a circular birefringence (.sub.Circular).

(5) When there is a bend in the few-moded fiber, the bend induces a linear birefringence (.sub.Linear) in the few-moded fiber, where .sub.Linear is perpendicular to .sub.Circular. In other words, the bend-induced birefringence is perpendicular to the magnetic-field-induced birefringence. The total birefringence () that the few-moded fiber exhibits is:
={square root over (.sub.Linear.sup.2+.sub.Circular.sup.2)}[Eq. 1].

(6) If the linear, bend-induced birefringence is greater than the circular, magnetic-field-induced birefringence, then the total birefringence can be approximated according to:

(7) $\begin{array}{cc}{}_{\mathrm{Linear}}+\frac{{}_{\mathrm{Circular}}^2}{{}_{\mathrm{Linear}}}& \left[\mathrm{Eq}.2\right].\end{array}$

(8) Shown from Eqs. 1 and 2, the linear, bend-induced birefringence is affected as a function of the circular, magnetic-field-induced birefringence.

(9) As shown above, the few-moded fiber for current sensing can greatly simplify design of an all-optical current sensing device. Furthermore, given the known relationship between a current and its induced magnetic field, this type of few-moded fiber produces a high-accuracy measurement of current based on measurable birefringence. Additionally, this type of few-moded fiber-based sensor reduces complexity and, consequently, system-level costs. The reduced complexity also results in greater reliability. It should also be appreciated that this type of few-moded fiber-based sensor provides a mechanism for measuring currents in electrical conductors without completely encircling the electrical conductor. As such, this type of few-moded fiber for sensing current can be employed in numerous contexts, such as, for example, industrial applications, energy-related applications, medicine, transportation, aerospace-related applications, defense-related applications, government laboratories, telecommunications environments, and fiber lasers, to name a few.

(10) Although exemplary embodiments have been shown and described, it will be clear to those of ordinary skill in the art that a number of changes, modifications, or alterations to the disclosure as described may be made. All such changes, modifications, and alterations should therefore be seen as within the scope of the disclosure.