US 4,904,940 A · Fiber-optic multicomponent magnetic field gradiometer for first, second and higher order derivatives

Abstract

A fiber-optic magnetic field gradiometer uses a plurality of magnetic transducers to simultaneously determine multicomponents of the gradient and field strength of an external magnetic field so as to permit accurate determination of the location of a ferromagnetic object located in an array of objects. The magnetic transducers are made of several layers of magnetostrictive glass which are wrapped with an optical fiber and immersed in an applied magnetic field to null out material differences and the earth's magnetic field. The null conditions for each adjacent pair are accomplished witout disturbing the null conditions of the other adjacent pair. The nulling technique may be accomplished in real time and does not require cutting of the DC bias coils of adjacent coils. Thus the nulling may be accomplished simultaneously for all coils as the balancing of each coil is independent of its neighbors. A magnetic field to be detected along the axes of the magnetic transducers causes an optical path length change in the fibers. By employing eight magnetic transducers, all gradients and fields may be determined at the same time. Further by employing additional transducers, for a total of thirteen, the second derivatives of the fields may also be determined.

Claims27

Dependent claims

A multicomponent magnetic field gradiometer comprising:

(a) first through fourth magnetic transducers, each having a magnetically sensitive axis oriented along a first, x direction, wherein said first and third magnetic transducers are colinear and said second and fourth magnetic transducers are displaced along a second, y direction and a third, z direction respectively relative to said first and third magnetic transducers, said x, y and z directions being mutually perpendicular;
(b) first through fourth optical fibers, one fiber wrapped around each of said magnetic transducers wherein said fibers define first through fourth optical paths;
(c) laser means coupled to said fibers for providing laser light within said fibers, wherein the laser light travels along the optical paths defined by said fibers;
(d) an applied magnetic field generating means for immersing each of said magnetic transducers in an applied magnetic field having an AC and a DC component;
and
(e) first means operative when said fibers are to an external magnetic field to be measured for detecting a change in the length .DELTA.L.sub.1,j of the optical paths defined by said fibers between said first and jth magnetic transducers, where j takes on integral values 2, 3 and 4, said changes in length proportional to the gradient of Bx in the x, y and z directions.

3 more elements

A method of detecting multicomponent magnetic field gradients of an external magnetic field comprising the steps of:

(a) orienting a magnetically sensitive axis of a first through fourth magnetic transducer along a first given direction, defined as the x direction, along which the gradient of the external magnetic field is desired to be measured, said magnetic transducers wrapped with a first through fourth optical fiber, respectively;
(b) transmitting a coherent radiation beam through each of said first through fourth optical fibers, said first through fourth optical fibers defining, respectively, first through fourth optical paths;
(c) applying a bias magnetic field having AC and DC components to immerse each of said magnetic transducers in said bias magnetic field;
(d) in the absence of said external magnetic field, detecting an optical path length difference between adjacent pairs of said first through fourth optical paths and adjusting said AC and DC components of said bias magnetic field to establish a bias field;
and
(e) then, in the presence of said external field, detecting optical path length differences between adjacent pairs of said first through fourth magnetic transduces to thereby detect said field gradients of said external magnetic field.

3 more elements

Dependencies are inferred from the claim text.

Fiber-optic multicomponent magnetic field gradiometer for first, second and higher order derivatives

Life of this patent

Fiber-optic multicomponent magnetic field gradiometer for first, second and higher order derivatives

Life of this patent

Abstract

Claims27