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BACKGROUND OF THE INVENTION
1. Field of the Invention:
The present invention relates to magnetometers, and more particularly, to
magnetometers based on fiber optic interferometry.
2. Art Background:
As man ventures deeper into space and further explores his planet, the need
to measure various physical parameters places increasing demands on state
of the art measurement techniques. For example, the present plans to
measure planetary, interplanetary and even intergalactic magnetic fields
will challenge the capabilities of present day space magnetometers. A
variety of methods are known for measuring magnetic fields, including
magnetometers based on moving and stationary coils, Hall effect, thin
films, flux gates, magnetic resonances, and super conducting devices. It
is also known to use light carrying optical fibers for detecting a
magnetic field. One method of detection involves passing a beam of
polarized light through an optical fiber from one end to the other in the
presence of a longitudinal magnetic field, and measuring the extent of
rotation (twist) of the plane polarized light. The extent of rotation is
dependent upon the prevailing magnetic field. (See for example, U.S. Pat.
No. 3,936,742.) Direction of rotation depends upon the direction of the
applied field. Using this "Faraday Effect" approach, only large currents
and magnetic fields can be detected since the Verdet constant of most
doped silica fiber is small. In addition, this approach requires special
materials (i.e. silica fiber doped with rare earth ions to enhance the
effect), and sophisticated fiber drawing techniques to provide reasonable
magnetic field detection sensitivity.
Another approach which has been used in the past employs a Mach-Zehnder
interferometer with one of the arms referred to as a sensor arm encoded or
wound on a magnetostrictive material (MSM). When exposed to a magnetic
field, the MSM undergoes dimensional change thereby altering the path of
the beam traversing that fiber. The resulting phase difference between the
two beams in the interferometer is directly related to the applied
magnetic field. Using this technique, measurement sensitivities on the
order of 10.sup.-[ 5.times.10.sup.-9 G/m of fiber has been reported. (See
for example, U.S. Pat. No. 4,371,838.) However, due to hysteresis effects
the response of the MSM to a magnetic field will depend on its previous
magnetic history.
In another approach, a multimode optical fiber is used to detect electrical
currents or magnetic fields from a remote source. The optical fiber is
composited with metal capable of conducting electricity. Optical radiation
is introduced into the fiber from a source which may either be coherent or
incoherent. An electrical current is applied to the portion of the
electrically conducting optical fiber, and the magnetic field is applied
to the current carrying optical fiber. The stretching of the fiber in the
presence of a magnetic field induces differential phase shifts in the
light between the fiber modes. These phase shifts or losses are detected
by a detector and the magnetic field strength thereby determined. (See
U.S. Pat. No. 4,348,587.) However, this method does not permit direction
or gradient measurement of the magnetic field.
As will be described, the present invention provides a fiber optic
magnetometer which overcomes the above-referenced limitations in prior art
magnetometers. The present invention employs a Mach-Zhender interferometer
wherein one of the arms of the interferometer includes a metallic
conductor attached to the fiber. The presence of a magnetic field is
detected by the bowing of the fiber attached to the conductor through
which a current is applied. The magnetic field direction may be determined
from the current direction and fiber bend. The present invention provides
a magnetometer which has been calculated to have sensitivity on the order
of 10.sup.-18 Tesla/m. In addition, inasmuch as no ferromagnetic materials
are used by the present invention, problems associated with hysteresis
effects are avoided.
SUMMARY OF THE INVENTION
The present invention provides an improved fiber optic magnetometer having
particular application for use in environments having very small magnetic
field ranges and gradients, such as space based applications. The
magnetometer includes a laser which generates a coherent beam of light
which is split by a beam splitter into first and second beams. The first
beam is passed through a single mode fiber optic on which a short metallic
conductor is attached. This fiber is referred to as the "sensor arm" of
the magnetometer. Current I is passed through the conductor. If the fiber
is in the vicinity of a magnetic field, displacement of the current
carrying conductor will cause the fiber to bow. The resulting stretching
of the fiber increases the pathlength for the light beam traversing
through the sensor arm. The change in path length is related to the force
exerted by the magnetic field on the conductor. The direction in which the
fiber bows is dependent upon the magnetic field direction. The second beam
is passed through a single mode fiber wrapped around a modulator that is
coupled to a feedback circuit which adjusts the magnetometer such that
ambient system noise is filtered, and the device is maintained at maximum
sensitivity. The output of the two fibers is then combined thereby forming
an interference pattern whereby phase changes in the pattern due to
displacement of sensor arm are mathematically converted into field
strength and direction data. The present invention also discloses an
improved modulator for maintaining the present invention at the point of
optimum sensitivity.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of the present invention disclosing the
sensor and reference arms of the magnetometer.
FIG. 2 is a schematic illustration of the present invention using prior art
piezo-electric modulators to maintain the system at a quadrature point.
FIG. 3 is a schematic illustration of another embodiment of the present
invention in which a conductor is attached perpendicular to the fiber and
an improved modulator is employed.
FIG. 4 is a graph illustrating the forces acting upon the sensor arm when
the fiber is displaced in the presence of a magnetic field.
FIG. 5 is a graph illustrating the quadrature point where small phase
difference changes induce the greatest intensity changes in the present
invention's output.
DETAILED DESCRIPTION OF THE INVENTION
An improved fiber optic magnetometer is disclosed having particular
application for use in detecting wide ranges of magnetic field intensities
as well as magnetic field gradients. In the following description for
purposes of explanation, particular fibers, system configurations,
magnetic field strengths, detectors, etc. are set forth in order to
provide a thorough description of the present invention. However, it will
be apparent to one skilled in the art that these specific details are not
required in order to realize the teachings of the present invention. In
addition, various known optical and electrical circuits and components are
not set forth in this description in order not to obscure the present
invention unnecessarily.
Referring now to FIG. 1, the basic configuration of the present invention's
fiber optic magnetometer is disclosed. A laser 10 generates a coherent
beam of light 12 which is passed onto a beam splitter 14 thereby
generating a first beam 16 and a second reference beam 18, as shown in the
diagram. Beam 16 is coupled through an optic coupler 22 such that it
passes through a single mode fiber optic 24 comprising the sensing arm of
the present invention. As shown, a short metallic (for example aluminum or
copper) conductor 26 having length "l" is provided over a portion of fiber
24. The conductor may comprise a metallic coating over the fiber, or the
simple mechanical attachment of a metal wire to fiber 24. Metallic
conductor 26 is coupled to a circuit which includes a voltage source 28
and a variable resistor 30, to permit a selectable current I to be passed
through the conductor. A mechanical stopper 32 is disposed on one side of
the fiber to preclude bowing of the fiber in one direction. As will be
described, bowing of the fiber 24 is induced by an applied magnetic field,
thereby permitting the strength of the field to be determined. The basic
components of the sensor arm of the present invention are identified by
dashed lines in FIG. 1 and referred to generally by the numeral 29. At the
termination of fiber 24, an optic coupler 34 permits beam 16 to exit fiber
24 and incident upon a beam splitter 36.
As illustrated, beam 18 is coupled through an optic coupler 38 to a
reference fiber 40. Fiber 40 is attached to a modulator 42 which, as will
be described, maintains the present invention at a point known as "phase
quadrature" for optimum performance. Beam 18 exits fiber 40 through optic
couple 44 and is incident upon beam splitter 36 where it is combined with
beam 16. The combined beams are then directed onto a photodiode detector
46 which is coupled to a signal processing system 50. The combining of
beams 16 and 18 generates an interference pattern, as is well known, from
which using standard electronic technology phase differences between the
two beams may be detected.
The present invention determines the strength and direction of a magnetic
field based upon the displacement of conductor 26 attached to fiber 24.
The direction in which fiber 24 bows is dependent upon a magnetic field
direction; provided the current I is unidirectional. Due to the presence
of stopper 32, the phase change measurement between beam 16 and 18 may be
made only when the fiber bows away from the stopper 32. Accordingly, the
magnetic field direction may be inferred once the current flow direction
is known.
If I is defined as the current passing through conductor 26 (referred to as
"C" ) having length ("l" ), then the force exerted on it by the unknown
magnetic field B is
F=Il.times.B=IB.sub..perp. (A)
Where B.sub..perp. is the magnetic field that is perpendicular to the
conductor 26. The force F displaces the conductor in a direction
determined by the above equation (A), resulting in the bowing and
stretching of the fiber 24. The resulting phase difference between beam 16
and 18 is
##EQU1##
Where .delta.L is the extra path induced in fiber 24. For small
displacement y, the graph illustrated in FIG. 4 may be approximated to a
triangle. If 2L is the length of the sensor arm then
##EQU2##
Under equilibrium conditions, F=T'=2Tsin .theta., where T is the tension in
the fiber. The Young's modulus of the stretched fiber
##EQU3##
where r is the fiber core radius. From the above equations (B) and (D) we
have
##EQU4##
using the relations T=F/2sin.theta. and sin.theta.=y/.sqroot.y.sup.2
+L.sup.2, we have for the phase difference
##EQU5##
combining equations (A), (D), and (G), we get
##EQU6##
Where it is assumed that L.apprxeq.L' for small values of y. The units of
the above equation (H) is Tesla amp in the MKS system.
It will be appreciated by one skilled in the art that the sensitivity of
the present invention may be varied by varying the magnitude of the
current I. For weak magnetic fields the quantity IB.sub..perp. may be
enlarged by increasing the value of current I. Similarly, in large
magnetic fields (for example in the vicinity of a large planet such as
Jupiter), the value of IB.sub..perp., may be reduced by lowering the value
of current I. It will be noted that the present invention is capable of
determining the direction of the applied magnetic field by simply changing
the direction of current I and rotating the sensor arm. For example, if in
the illustration of FIG. 1 the magnetic field direction is reversed, the
present invention in its illustrated orientation may detect the field and
note its direction despite the presence of stopper 32 simply by reversing
the direction of current I. Magnetic fields in other directions may be
detected by appropriately rotating the sensor arm (for example on a
rotatable external spacecraft platform) such that magnetic field
directions in all XYZ directions may be determined.
Assuming electronic phase difference detectability of 10.sup.-6 radians,
the smallest field which may be sensed by the present invention may be
estimated. For a 1 centimeter long (l) conductor 26 attached to a silica
fiber having a core diameter of 4 microns, length 2L =1 m and
Y=0.55.times.10.sup.11 N/m.sup.2, and an operating wavelength of 0.63
microns, the minimum detectable field will be 6.2.times.10.sup.-18 Tesla
or 6.2.times.10.sup.-14 Gauss for 1 ampere current through the conductor
26. If the 1 amp current provides thermal problems, the magnetometer may
be operated in a pulsed mode. Problems associated with Ohmic heating may
also be reduced or eliminated if the value of I is lowered. However, the
lowering of I would affect the detection sensitivity. Thus, for I=1 mA,
the minimum detectable field is of the order of 10.sup.-11 Gauss/meter of
the fiber. In the event that phase detectability of the present invention
drops to a milliradian, the field detectability accordingly drops to
2.times.10.sup.-9 Gauss/m/A; which at the present time is approximately an
order of magnitude better than most prior art systems.
Inasmuch as the parameter detected by the present invention is the extra
phase induced in the sensor arm by a magnetic field, measurement of the
phase changes is rendered more difficult by phase noises induced in the
sensor and reference arms by external perturbations. For maximum
sensitivity whereby differences in phase induce the greatest intensity
changes in the interference pattern generated by combined beams 16 and 18,
the present invention should be retained at the "quadrature" point
illustrated in FIG. 5. Fibers 24 and 40 comprise single mode polarization
insensitive fibers, which are commercially available. The electric field
of the light beam in the sensor arm just prior to optic coupler 34 may be
described as
E.sub.s =E.sub.s.sup..degree. exp i {.omega.t+S(t) .theta..sub.s }
and the electric field at the corresponding point in the reference arm,
just prior to optic coupler 44 will be
E.sub.r =E.sub.r.sup..degree. exp i {.omega.t+.sub.100 }
typically, the signal S(t) <<.pi.. The arbitrary phase factors .theta.s and
.theta.r will be constant under ideal conditions. However, they may drift
in a random manner and the amplitude of this drift may be of the order of
2.pi. or greater. To extract information about S(t) from this phase noise,
modulator terms (the variables A and B) are included in the phase of the
reference beam.
E.sub.r =E.sub.r.sup..degree. exp i {.omega.t+A(t)+B(t) +.theta..sub.r }
These terms are generated by stretching sections of fiber wound on a
piezoelectric element, as illustrated in FIG. 2, and is known in the art.
The piezoelectric element is generally in the shape of a small drum which
expands and contracts in accordance with the signal applied to it. The
expansion and contraction of the piezoelectric element stretches and
contracts that section of fiber 40 wrapped around the element. Phase A
represents a phase produced by a feedback circuit to hold the
interferometer at the quadrature condition. The phase B is always of the
form .theta..sub.m sin.omega..sub.m t. Depending upon the type of
detection system actually used, either A, B, or both, or neither may be
required in a particular application in order to exclude noise from the
system. The signal to be detected can be obtained from the output of the
feedback circuit which keeps the interferometer in quadrature. The
electronic circuitry and archiecture nee needed for the feedback controls
is well known and hence will not be discussed herein. The following
references provide the necessary reading required to develop such
circuits: "Control Action", Van Nostrand's Scientific Encyclopedia, 5th
Ed., pgs. 662-669, 1976, "Control Algorithm", McGraw-Hill Encyclopedia of
Electronics and Computers, pgs. 223-224, 1984; "Position Sensing
Photodetectors", United Detector Technology, Techanical Brochurre, 1984.
Referring now to FIG. 3, the present invention provides an alternate method
than that illustrated in FIG. 2 to generate the phase factors A(t) and
B(t). The modulators 55 and 57 utilized by the present invention comprise
a portion of fiber 40 coated with metal within each modulator and wound
onto drums formed out of a non-metallic material. By keeping the drum axis
parallel to the magnetic field, A(t) may be generated by passing an
appropriate DC current through modulator 55, and in modulator 57 [for
B(t)]by passing a sinusoidal current of appropriate frequency through the
metallic coating over the fiber within modulator 57. Accordingly, the
modulators utilized by the present invention are significantly less
expensive than those illustrated in FIG. 2.
In addition, in FIG. 3 an alternate embodiment of the present invention is
illustrated. A conductor 60 may be mechanically bonded to fiber 24 such
that the conductor lies perpendicular to fiber. A current I passing
through the conductor 60 will in the presence of a magnetic field, force
the conductor to displace the fiber in a direction dependent upon the
magnetic field and current directions, as is well known. The mathematical
description previously set forth relative to the embodiment of FIG. 1 is
equally applicable to the embodiment of FIG. 3, and appropriate stoppers
and circuitry may be provided as in the preceeding Figures to permit the
magnitude and direction of the applied magnetic field to be determined.
The present invention may also be used to detect gradients within magnetic
fields. Unlike some prior art systems which could not be "turned off" such
that the sensor and reference arms responded identically (by simply
eliminating the current I passing through the conductor 26 or 60), the
present invention has such capability. By providing a similar conductor 26
and appropriate circuitry on fiber 40 comprising a reference arm, one arm
may be used as a reference arm, or alternatively, as a sensor arm.
Balancing of the two arms to nullify the effect of time varying spatially
coherent signals and/or magnetic noises may be done by adjusting the
length of conductors on each arm. It will be appreciated that in the
aforementioned embodiment of the present invention permitting the
measurement of magnetic gradients either arm of the invention may be used
as a reference, or alternatively, a sensor arm by providing or
eliminating, as may be required, the current I passing through the
conductor 26.
Accordingly, an improved fiber optic magnetometer has been described.
Although the invention has been described with particular reference to
FIGS. 1 through 5, it will be appreciated that the Figures are for
illustration only and do not limit the invention. For example, the present
invention may be used to measure current through a conductor as well as
magnetic field strength. Since it is well known that a current passing
through a conductor generates a magnetic field, by disposing the present
invention adjacent to a conductor passing a current, the current may be
determined based on the measured magnetic field strength induced around
the conductor. Moreover, the present invention may be used as a
temperature sensor by eliminating the current I passing through conductor
26 or 60, inasmuch as the metal comprising conductors 26 or 60 will expand
when heated and thereby stretch fiber 24. The temperature may be
determined based on induced phase change between beams 16 and 18 which is
a function of the amount of expansion of the conductor in the embodiment
of
FIG. 1. Using the present invention for purposes of determining
temperature, permits sensitivities of 10.sup.-8 Centigrade up to the
melting temperature of the fiber which is approximately 1,000 Centigrade.
* * * * *
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