Fiber-optic magnetic field gradiometer for first, second and higher order derivatives
First Claim
1. A magnetic field gradiometer comprising:
- at least a first, second and third magnetic transducer;
at least three optical fibers, one fiber wrapped around each of said magnetic transducers wherein said fibers define at least three optical paths;
a laser means coupled to said fibers to provide laser light within said fibers wherein the laser light travels along the optical paths defined by said fibers;
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
first means operative when said fibers are subjected to an external magnetic field to be measured for detecting a change in the length Δ
L1,2 of the optical paths defined by said fibers between said first and second magnetic transducers and the change in length Δ
L2,3 of the optical paths defined by the fibers between the second and third magnetic transducers, and second means for detecting a total length change Δ
LT proportional to |Δ
L1,2 - Δ
L2,3 | of said fibers wherein said total change in length Δ
LT of the optical paths provides an indication of the second derivative of said external magnetic field.
1 Assignment
0 Petitions
Accused Products
Abstract
A fiber-optic magnetic field gradiometer uses the second derivative of a magnetic field to determine the location of a ferromagnetic object located in an array of objects. Three adjacent magnetic transducers made of several layers of magnetostrictive glass 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 without disturbing the null conditions of the other adjacent pair. A magnetic field to be detected along the axes of the magnetic transducers causes an optical path length change in the fibers. The overall path length change is proportional to the second derivative of the magnetic field.
-
Citations
22 Claims
-
1. A magnetic field gradiometer comprising:
-
at least a first, second and third magnetic transducer; at least three optical fibers, one fiber wrapped around each of said magnetic transducers wherein said fibers define at least three optical paths; a laser means coupled to said fibers to provide laser light within said fibers wherein the laser light travels along the optical paths defined by said fibers; 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 first means operative when said fibers are subjected to an external magnetic field to be measured for detecting a change in the length Δ
L1,2 of the optical paths defined by said fibers between said first and second magnetic transducers and the change in length Δ
L2,3 of the optical paths defined by the fibers between the second and third magnetic transducers, and second means for detecting a total length change Δ
LT proportional to |Δ
L1,2 - Δ
L2,3 | of said fibers wherein said total change in length Δ
LT of the optical paths provides an indication of the second derivative of said external magnetic field. - View Dependent Claims (3, 4, 5, 6, 7, 8, 9, 10, 12, 13, 14, 15)
-
- 2. A magnetic field gradiometer as claimed in claim wherein said first, second and third magnetic transducers comprises a first, second and third cylinder, and first, second and third fibers wrapped on said first, second and third cylinders, respectively, said first and second cylinders being adjacent one another and said second and third cylinders being adjacent one another.
-
17. A method of detecting a second derivative of an external magnetic field comprising the steps of:
-
(a) orienting a magnetically sensitive axis of a first second and third magnetic transducer along a given field direction for which the second derivative of the external magnetic field is desired to be measured, said magnetic transducers wrapped with a first, second and third optical fiber, respectively; (b) transmitting a coherent radiation beam through each of said first, second and third optical fibers, said first, second and third optical fibers defining, respectively, first, second and third optical paths; (c) applying a bias magnetic field having AC and DC components to immerse each of said magnetic tranducers in said bias magnetic field; (d) in the absence of said magnetic external field, detecting an optical path length difference between each adjacent pair of said first, second and third 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 magnetic field, detecting a total optical path length difference of optical path length differences between each adjacent pair of said first, second and third magnetic transducers to thereby detect said second derivative of said external magnetic field. - View Dependent Claims (18, 19, 20, 21, 22)
-
Specification