Optical fiber magnetometer for measuring D.C. and low frequency fields

Claim 2

In a magnetometer having an optical fiber interferometer with first and second arms and a laser beam input coupled to said interferometer for travel through each of said arms, a system for measuring a D.C. to a low frequency ambient magnetic field comprising: a pair of matched magnetostrictive elements, each of said elements being coupled to a different arm of said interferometer; means for applying a bias magnetic field to each of said magnetostrictive elements 180° out of phase to provide balanced oscillations; and means for comparing the laser beam outputs of each of said interferometer arms to develop a signal representative of the ambient magnetic field.

Claim 3

The magnetometer of claim 1 wherein said signal developed by the comparing means includes a component, the intensity of which is proportional to the low frequency magnetic field and a component which is proportional to the unbalance of the system and further including means for eliminating the unbalance component from said signal.

Claim 4

The magnetometer of claim 1 wherein each of said magnetostrictive elements are coupled to an equal length of optical fiber in each of said interferometer arms.

Claim 5

The magnetometer of claim 3 wherein the length of optical fiber to which each of the magnetostrictive elements is coupled is at least 4 meters.

Claim 6

The magnetometer of claim 1 wherein said magnetostrictive elements are made of nickel.

Claim 7

The magnetometer of claim 1 wherein said magnetostrictive elements are made of a metallic glass.

Claim 8

The magnetometer of claim 1 wherein said magnetostrictive elements are made of a rare earth material.

Claim 9

The magnetometer of claim 1 wherein each of said magnetostrictive elements is a hollow rod.

Claim 10

The magnetometer of claim 1 wherein each of said magnetostrictive elements is a rod with the optical fiber of each interferometer arm being secured to the outer surface of the rod.

Claim 11

The magnetometer of claim 1 wherein each of said magnetostrictive elements is a rod having a diameter of at least one inch wherein the optical fiber of each interferometer arm is wound about the circumference of the respective magnetostrictive rod to which it is coupled.

Claim 12

In a magnetometer having an optical fiber interferometer with first and second arms and a laser beam input coupled to said interferometer for travel through each of said arms, a system for measuring a D.C. to a low frequency ambient magnetic field comprising: a first magnetostrictive element being coupled to a length of the optical fiber in said first interferometer arm; a second magnetostrictive element being coupled to a length of the optical fiber in said second interferometer arm; means for driving said first and second rods 180° out of phase to apply respective oscillating bias magnetic fields to each of said rods, the ambient magnetic field providing an offset which is combined which each of said oscillating bias magnetic fields to induce a change in path length of each of said interferometer arms; means responsive to the output of said interferometer for developing a signal representative of the difference in the change in path length of each of said interferometer arms; and means responsive to the difference signal for providing an output signal, the intensity of which is proportional to the ambient magnetic field.

Claim 13

In a magnetometer having an optical fiber interferometer with first and second arms and a laser beam input coupled to said interferometer for travel through each of said arms, a system for measuring a D.C. to a low frequency ambient magnetic field comprising: a first magnetostrictive element being coupled to a length of the optical fiber in said first interferometer arm; a second magnetostrictive element being coupled to a length of the optical fiber in said second interferometer arm; means for driving said first and second rods 180° out of phase to apply respective oscillating bias magnetic fields to each of said rods, the ambient magnetic field providing an offset which is combined which each of said oscillating bias magnetic fields to induce a change in path length of each of said interferometer arms; means responsive to the output of said interferometer for developing a signal representative of the difference in the change in path length of each of said interferometer arms; and means responsive to the difference signal for providing an output signal, the intensity of which is proportional to the ambient magnetic field.

Claim 14

The magnetometer of claim 11 wherein said difference signal includes components which are proportional to the unbalance of the system and said means for providing the output signal includes means for eliminating said unbalance components.

Claim 15

The magnetometer of claim 12 wherein said driving means provides a reference signal and said eliminating means includes an amplifier locked in frequency and in phase to said reference signal.

Claim 16

The magnetometer of claim 11 wherein said means for providing the output signal includes a bandpass filter for eliminating low frequency components from the difference signal due to temperature changes in the optical fibers.

Claim 17

The magnetometer of claim 11 wherein said means for developing the difference signal includes: a light detector responsive to the output of said interferometer to form an interference pattern, the output of said light detector being in terms of the light in the interference pattern and nonlinear; and means responsive to the output of the light detector for providing a signal which is linear in the difference in the change in path length of each of the interferometer arms.

Claim 18

The magnetometer of claim 15 wherein the means for providing a linear signal includes a continuous null feedback controller; and compensation means responsive to the output of the feedback controller for changing the path length of one of said interferometer arms.

Claim 19

The magnetometer of claim 16 wherein said means for changing the path length includes a piezoelectric transducer having the optical fiber of said one of said interferometer arms coupled thereto.

Claim 20

The magnetometer of claim 11 further including means for modulating the path length of one of said interferometer arms in response to said means for developing said difference signal to maintain the interferometer arms balanced.

Claim 21

In a magnetometer having an optical fiber interferometer with first and second arms and a laser beam input coupled to said interferometer for travel through each of said arms, a gradiometer for measuring the gradient between first and second low frequency magnetic fields comprising: first and second magnetostrictive elements forming a first matched pair, each of said first and second elements being coupled to a different arm of said interferometer and responsive to said first low frequency magnetic field; third and fourth magnetostrictive elements forming a second matched pair, each of said third and fourth elements being coupled to a different arm of said interferometer and responsive to said second low frequency magnetic field; means for applying a bias magnetic field to each of said magnetostrictive elements wherein the bias field applied to said second element is 180° out of phase with said first element, the bias field applied to said fourth element is 180° out of phase with said third element and the bias field applied to said third element is 180° out of phase with said first element; and means for comparing the laser beam outputs of each of said interferometer arms to develop a signal representative of the gradient between said first and second low frequency magnetic fields.

Claim 22

In a magnetometer having an optical fiber interferometer with first and second arms and a laser beam input coupled to said interferometer for travel through each of said arms, a gradiometer for measuring the gradient between first and second low frequency magnetic fields comprising: first and second magnetostrictive elements forming a first matched pair, each of said first and second elements being coupled to a different arm of said interferometer and responsive to said first low frequency magnetic field; third and fourth magnetostrictive elements forming a second matched pair, each of said third and fourth elements being coupled to a different arm of said interferometer and responsive to said second low frequency magnetic field; means for applying a bias magnetic field to each of said magnetostrictive elements wherein the bias field applied to said second element is 180° out of phase with said first element, the bias field applied to said fourth element is 180° out of phase with said third element and the bias field applied to said third element is 180° out of phase with said first element; and means for comparing the laser beam outputs of each of said interferometer arms to develop a signal representative of the gradient between said first and second low frequency magnetic fields.

Claim 23

The magnetometer of claim 19 wherein said first and third magnetostrictive elements are coupled to the optical fiber of the first interferometer arm, and separated by a distance and said second and fourth magnetostrictive elements are coupled to the optical fiber of the second interferometer arm and separated by an equal distance.

Claim 24

The magnetometer of claim 19 wherein said signal developed by the comparing means includes a component, the intensity of which is proportional to the gradient and a component which is proportional to the unbalance of the system and further including means for eliminating the unbalance component from said signal.

Claim 25

In a magnetometer having an optical fiber interferometer with first and second arms and a laser beam input coupled to said interferometer for travel through each of said arms, a system for measuring a D.C. to a low frequency ambient magnetic field comprising: a pair of matched magnetostrictive elements, each of said elements being coupled to a different arm of said interferometer; means for driving said pair of elements 180° out of phase to apply respective oscillating bias magnetic fields to each of said elements, the ambient magnetic field providing an offset which is combined with each of said oscillating bias magnetic fields to induce a change in path length of each of said interferometer arms; means responsive to the output of said interferometer for developing a signal which varies as a function of the changes in path length due to said magnetic fields and due to temperature fluctuations; means responsive to said signal for holding the changes in path length due to temperature to a fixed value to prevent said signal from drifting with temperature; and means responsive to said signal for providing an output proportional to the ambient magnetic field.

Claim 26

In a magnetometer having an optical fiber interferometer with first and second arms and a laser beam input coupled to said interferometer for travel through each of said arms, a system for measuring a D.C. to a low frequency ambient magnetic field comprising: a pair of matched magnetostrictive elements, each of said elements being coupled to a different arm of said interferometer; means for driving said pair of elements 180° out of phase to apply respective oscillating bias magnetic fields to each of said elements, the ambient magnetic field providing an offset which is combined with each of said oscillating bias magnetic fields to induce a change in path length of each of said interferometer arms; means responsive to the output of said interferometer for developing a signal which varies as a function of the changes in path length due to said magnetic fields and due to temperature fluctuations; means responsive to said signal for holding the changes in path length due to temperature to a fixed value to prevent said signal from drifting with temperature; and means responsive to said signal for providing an output proportional to the ambient magnetic field.

Claim 27

The magnetometer of claim 22 wherein said holding means includes: a sample and hold feedback controller; and compensation means coupled to one arm of said interferometer and responsive to the output of said feedback controller for changing the path length of said one interferometer arm.

Claim 28

The magnetometer of claim 22 wherein said holding means includes: means for sampling said signal at a predetermined period of time with respect to the output of said driving means to derive a temperature compensation signal; and means responsive to said temperature compensation signal for changing the path length of an arm of said interferometer to hold the change in path length due to temperature to a fixed value.

Claim 29

The magnetometer of claim 24 wherein said drive means generates a periodic drive signal having a plurality of turning points therein, said sampling means sampling said signal at a selected turning point of said drive to derive the temperature compensation signal, the path length changing means being responsive to said temperature compensation signal to hold the change in path length due to temperature to a fixed value such that said signal is held to a relative zero value at the selected turning point.

Claim 30

The magnetometer of claim 25 wherein said signal, at the turning points other than said selected turning point, varies with said ambient magnetic field and said output providing means includes means for sampling said signal at one of said other turning points.