Fiber optic DC magnetometer without hysteresis ambiguity

Claim 2

A fiber optic magnetometer for detecting DC magnetic fields comprising: a length of optical fiber having an optical length L in an unstrained condition; a light source means for transmitting a beam of light through said optical fiber, said light beam having a phase; magnetostrictive means closely associated with said fiber such that said magnetostrictive means undergoes a dimensional change in the presence of a magnetic field so as to strain said fiber causing a change in the optical length of said fiber and thus a shift in phase of said light beam; an AC field generating means for generating a first AC magnetic field having a frequency ω o and a second AC magnetic field having a frequency ω s different from ω o in the vicinity of said magnetostrictive means so as to cause time varying dimensional changes in said magnetostrictive means, so that said phase shift is time varying, said phase shift being a sum of several frequency components including an ω o component, an (ω o +ω s ) component, and an (ω o -ω s ) component; detection means for detecting said phase shift, wherein said phase shift changes in magnitude upon exposure of said magnetostrictive means to a DC magnetic field, the change in magnitude indicating the presence of a DC magnetic field.

Claim 3

A magnetometer as recited in claim 1 wherein said phase detection means further comprises: a signal producing means, said signal being proportional to said phase shift such that the signal also has several frequency components including ω o , (ω o +ω s ) and (ω o -ω s ) components; and a filter means for receiving the phase detector signal, said filter means filtering out all frequency components of the signal other than the ω o , (ω o +ω s ), and (ω o -ω s ) components so as to pass to its output only the ω o , (ω o +ω s ), and (ω o -ω s ) components.

Claim 4

A magnetometer as recited in claim 2 wherein said filter means is a lock-in amplifier set to amplify at ω o , and wherein ω s is substantially less than ω o such that said lock-in amplifier passes (ω o +ω s ) and (ω o -ω s ) components as well as the ω o component.

Claim 5

A magnetometer as recited in claim 3 wherein said AC magnetic field generating means includes a wire coil in proximity to said magnetostrictive means, and a first AC current source for supplying an AC current of frequency ω o to said coil, and a second AC current source for supplying an AC current of frequency ω s to said coil.

Claim 6

A magnetometer as recited in claim 4, further comprising a display means for receiving a signal from said second AC current source, and also receiving an output signal from said lock-in amplifier, said display means displaying a plot of the lock-in amplifier output signal as a function of the second magnetic field, whereby a closed loop curve is displayed by said dsiplay means which shifts in position upon exposure of said magnetometer to a DC magnetic field.

Claim 7

A magnetometer as recited in claim 5 wherein ω o is between 10 Hz and 1 KHz.

Claim 8

A magnetometer as recited in claim 5, wherein said phase detector means includes a second optical fiber equal in length to the other optical fiber, said second optical fiber also receiving a light beam from said light source means, wherein said phase detector means interferometrically detects the phase shift by comparing the phase of the beams transmitted through said optical fiber.

Claim 9

A magnetometer as recited in claim 7, wherein said display means is an oscilloscope.

Claim 12

A fiber optic magnetometer for detecting DC magnetic fields comprising: a reference arm which includes a length of optical fiber; a sensing arm which includes a length of optical fiber equal in length to that of said reference arm, said sensing arm fiber having a magnetostrictive means closely associated therewith which undergoes a dimensional change in the presence of a magnetic field so as to change the optical length of said sensing arm; a light source means for transmitting a light beam through each said arm, whereby in the presence of a magnetic field the change in length of said sensing arm induces a phase shift in the beam transmitted therethrough; an AC magnetic field source which includes a wire coil in proximity to said magnetostrictive means, first AC current source for supplying an AC current of frequency ω o to said coil, and a second AC current source for supplying an AC current of frequency ω s , ω s being at least an order of magnitude less than ω o , whereby the AC magnetic fields produce a time-varying phase shift having several frequency components, including ω o , (ω o +ω s ), and (ω o -ω s ) components; interferometric phase detector means for detecting the time varying phase shift by comparing the phase of the beams transmitted through each fiber, said phase detector means producing a signal proportional to the detected phase shift such that the signal also has ω o , (ω o +ω s ) and (ω o -ω s ) components; a lock-in amplifier for receiving and amplifying the signal from said phase detector means at the frequency ω o wherein frequency ω s is sufficiently small such that said lock-in amplifier produces an output signal having (ω o +ω s ) and (ω o -ω s ) components as well as an ω o component; a display means for receiving a signal from said second AC current source, and also receiving the output signal from said lock-in amplifier, said display means displaying a closed loop curve which is a plot of the lock-in amplifier output signal as a function of the second magnetic field, said closed loop curve shifts in position upon exposure of said magnetometer to a DC magnetic field.

Claim 13

A fiber optic magnetometer for detecting DC magnetic fields comprising: a reference arm which includes a length of optical fiber; a sensing arm which includes a length of optical fiber equal in length to that of said reference arm, said sensing arm fiber having a magnetostrictive means closely associated therewith which undergoes a dimensional change in the presence of a magnetic field so as to change the optical length of said sensing arm; a light source means for transmitting a light beam through each said arm, whereby in the presence of a magnetic field the change in length of said sensing arm induces a phase shift in the beam transmitted therethrough; an AC magnetic field source which includes a wire coil in proximity to said magnetostrictive means, first AC current source for supplying an AC current of frequency ω o to said coil, and a second AC current source for supplying an AC current of frequency ω s , ω s being at least an order of magnitude less than ω o , whereby the AC magnetic fields produce a time-varying phase shift having several frequency components, including ω o , (ω o +ω s ), and (ω o -ω s ) components; interferometric phase detector means for detecting the time varying phase shift by comparing the phase of the beams transmitted through each fiber, said phase detector means producing a signal proportional to the detected phase shift such that the signal also has ω o , (ω o +ω s ) and (ω o -ω s ) components; a lock-in amplifier for receiving and amplifying the signal from said phase detector means at the frequency ω o wherein frequency ω s is sufficiently small such that said lock-in amplifier produces an output signal having (ω o +ω s ) and (ω o -ω s ) components as well as an ω o component; a display means for receiving a signal from said second AC current source, and also receiving the output signal from said lock-in amplifier, said display means displaying a closed loop curve which is a plot of the lock-in amplifier output signal as a function of the second magnetic field, said closed loop curve shifts in position upon exposure of said magnetometer to a DC magnetic field.