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United States Patent |
6,208,775
|
Dyott
|
March 27, 2001
|
Current sensor
Abstract
A method for fabricating a transformer of linearly polarized light to
elliptically polarized light is presented. The method involves twisting a
birefringent fiber through angles that depend on the polarization desired.
This technique obviates the need to splice fibers, as in common
approaches. In the final step of the method, the polarization can be fine
tuned by heating the fiber to cause the core of the fiber to diffuse into
the cladding. Using this transformer of polarized light, a current sensor
is presented that exploits the Faraday Effect in a Sagnac interferometer.
Inventors:
|
Dyott; Richard B. (Oak Lawn, IL)
|
Assignee:
|
KVH Industries, Inc. (Middletown, RI)
|
Appl. No.:
|
337231 |
Filed:
|
June 22, 1999 |
Intern'l Class: |
G02B 6/0/0 |
Field of Search: |
385/5-12
|
References Cited
U.S. Patent Documents
4603941 | Aug., 1986 | Yoshimasa et al.
| |
5701376 | Dec., 1997 | Masataka.
| |
6023331 | Feb., 2000 | Blake et al.
| |
Foreign Patent Documents |
2 535 463 | May., 1984 | FR.
| |
WO 98 58268 | Dec., 1998 | WO.
| |
Primary Examiner: Kim; Robert
Attorney, Agent or Firm: Foley, Hoag & Eliot LLP
Parent Case Text
CROSS-REFERENCED TO RELATED APPLICATIONS
This application is based upon and claims priority to the following U.S.
patent applications. U.S. provisional patent application, Ser. application
No. 60/119,999, filed on Feb. 11, 1999; U.S. provisional patent
application, Ser. application No. 60/120,000, filed on Feb. 11, 1999; U.S.
provisional patent application, Ser. application No. 60/133,357, filed on
May 10, 1999; and U.S. provisional patent application, Ser. application
No. 60/134,154, filed on May 14, 1999. This application is also based upon
U.S. application Polarization Transformer, invented by Richard Dyott,
which has been filed concurrently with the present application on Jun. 22,
1999. All of the aforementioned applications are hereby incorporated by
reference.
Claims
What is claimed is:
1. A current sensor comprising
a) a source of linearly polarized light;
b) a transformer of polarized light for transforming the linearly polarized
light to circularly polarized light, said transformer including a
birefringent fiber which is twisted through an angle into a corkscrew
shape at an appropriate distance from an end of the fiber, the angle and
distance so chosen that linearly polarized light entering an end of the
fiber farthest from the corkscrew shape exits the fiber circularly
polarized;
c) a coil of optical fiber having multiple turns;
d) a directional coupler for optically coupling the circularly polarized
light from the transformer of polarized light to the coil to create
counter-propagating light beams within the coil; and
e) an optical detector for receiving said counter-propagated light beams
for producing an output signal indicative of a magnetic field produced by
an electric current.
2. A current sensor comprising
a) a source of linearly polarized light;
b) a transformer of polarized light for transforming the linearly polarized
light to circularly polarized light, said transformer including a
birefringent fiber which is twisted through an angle of approximately
.pi./4 radians into a corkscrew shape at an approximate distance from an
end of the fiber of one quarter of a beatlength;
c) a coil of optical fiber having multiple turns;
d) a directional coupler for optically coupling the circularly polarized
light from the transformer of polarized light to the coil to create
counter-propagating light beams within the coil; and
e) an optical detector for receiving said counter-propagated light beams
for producing an output signal indicative of a magnetic field produced by
an electric current.
3. A current sensor as in claim 2 wherein the source of linearly polarized
light is a diode laser.
4. A method of detecting the current in a conductor comprising
a) providing a source of linearly polarized light;
b) transforming the linearly polarized light into circularly polarized
light using a transformer of polarized light, said transformer including a
birefringent fiber which is twisted through an angle into a corkscrew
shape at an appropriate distance from an end of the fiber, the angle and
distance so chosen that linearly polarized light entering an end of the
fiber farthest from the corkscrew shape exits the fiber circularly
polarized;
c) providing a coil of optical fiber having multiple turns;
d) with a directional coupler, coupling the circularly polarized light from
the transformer of polarized light to the coil to create
counter-propagating light beams within the coil; and
e) receiving said counter-propagated light beams with an optical detector
for producing an output signal indicative of a magnetic field produced by
an electric current.
5. A method of detecting the current in a conductor comprising
a) providing a source of linearly polarized light;
b) transforming the linearly polarized light into circularly polarized
light using a transformer of polarized light, said transformer including a
birefringent fiber which is twisted through an angle of approximately
.pi./4 radians into a corkscrew shape at an approximate distance from an
end of the fiber of one quarter of a beatlength;
c) providing a coil of optical fiber having multiple turns;
d) with a directional coupler, coupling the circularly polarized light from
the transformer of polarized light to the coil to create
counter-propagating light beams within the coil; and
e) receiving said counter-propagated light beams with an optical detector
for producing an output signal indicative of a magnetic field produced by
an electric current.
6. A current sensor comprising
a) a source of linearly polarized light;
b) a transformer of polarized light for transforming the linearly polarized
light to elliptically polarized light, said transformer including a
birefringent fiber which is twisted through an angle into a corkscrew
shape at an appropriate distance from an end of the fiber, the angle and
distance so chosen that linearly polarized light entering an end of the
fiber farthest from the corkscrew shape exits the fiber elliptically
polarized;
c) a coil of optical fiber having multiple turns;
d) a directional coupler for optically coupling the elliptically polarized
light from the transformer of polarized light to the coil to create
counter-propagating light beams within the coil; and
e) an optical detector for receiving said counter-propagated light beams
for producing an output signal indicative of a magnetic field produced by
an electric current.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This application relates to optical devices that transform linearly
polarized light into elliptically polarized light and their use in current
sensors.
2. Description of Related Art
Devices that transform linearly polarized light to circularly polarized
light are known in the literature. To make such optical devices, one may
use one birefringent fiber with two beams of light of equal frequency and
amplitude (or, equivalently, one beam that is the vector sum of these two
beams). If the two beams are propagated perpendicular to the optic axis,
circularly polarized light may result. Alternatively, linearly polarized
light may be transformed to circularly polarized light by using one beam
and two fibers.
In practice, constructing a single-beam transformer of linearly to
circularly polarized light involves first starting with a length of
transforming fiber greater than a predetermined length, and performing
several iterations of cutting and measuring polarization until the
polarization is deemed to be circular to within some specification.
Needless to say, this is a tedious and lengthy procedure requiring lots of
guesswork.
SUMMARY OF THE INVENTION
A current sensor is presented including a source of linearly polarized
light; a transformer of polarized light as in claim 1 for transforming the
linearly polarized light to circularly polarized light; a coil of optical
fiber having multiple turns; a directional coupler for optically coupling
the circularly polarized light from the transformer of polarized light to
the coil to create counter-propagating light beams within the coil; and an
optical detector for receiving said counter-propagated light beams for
producing an output signal indicative of a magnetic field produced by an
electric current.
A current sensor is presented including a source of linearly polarized
light; a transformer of polarized light as described above for
transforming the linearly polarized light to circularly polarized light; a
coil of optical fiber having multiple turns; a directional coupler for
optically coupling the circularly polarized light from the transformer of
polarized light to the coil to create counter-propagating light beams
within the coil; and an optical detector for receiving said
counter-propagated light beams for producing an output signal indicative
of a magnetic field produced by an electric current. The source of
linearly polarized light may be a diode laser.
A method of detecting the current in a conductor is also presented
including providing a source of linearly polarized light; transforming the
linearly polarized light into circularly polarized light using a
transformer of polarized light as in claim 1; providing a coil of optical
fiber having multiple turns; with a directional coupler, coupling the
circularly polarized light from the transformer of polarized light to the
coil to create counter-propagating light beams within the coil; and
receiving said counter-propagated light beams with an optical detector for
producing an output signal indicative of a magnetic field produced by an
electric current.
A method of detecting the current in a conductor is also presented
including providing a source of linearly polarized light; transforming the
linearly polarized light into circularly polarized light using a
transformer of polarized light as in claim 3; providing a coil of optical
fiber having multiple turns; with a directional coupler, coupling the
circularly polarized light from the transformer of polarized light to the
coil to create counter-propagating light beams within the coil; and
receiving the counter-propagated light beams with an optical detector for
producing an output signal indicative of a magnetic field produced by an
electric current.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 illustrates the conventional method of fabricating a transformer of
linearly to circularly polarized light by splicing two fibers that are
properly oriented.
FIG. 2 is a schematic of a twisted fiber of the present invention that
obviates the need to splice fibers together.
FIG. 3 illustrates how fine tuning of the polarization can be achieved by
heating the fiber to cause diffusion of the core into the cladding.
FIG. 4 illustrates a current sensor that includes a polarization
transformer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
It is often desirable to transform the polarization of a beam of light from
one state to another. For this purpose optical devices have been
fabricated that input linearly polarized light and output elliptically
polarized light. These devices typically function by causing one of two
incident linearly polarized light beams to lag behind the other by a
pre-selected phase difference. Altering the relative phase of the two
incident beams has the effect of changing the state of polarization of the
light that exits the optical device. Before considering how these devices
of the prior art perform the transformation of linearly to elliptically
polarized light and before presenting the detailed description of the
preferred embodiment of the present invention, it will be useful to first
recall how elliptically polarized light arises.
Two orthogonal electric fields, E.sub.x and E.sub.y, both propagating in
the z direction can be described by the following two equations
E.sub.x =iE.sub.0x cos(kz-.omega.t) (1)
and
E.sub.y =jE.sub.0y cos(kz-.omega.t+.delta.), (2)
where i and j are unit vectors in the x and y directions, k is the
propagation number, .omega. is the angular frequency, and .delta. is the
relative phase difference between the two modes. The total electric field
E is just given by the vector sum E.sub.x +E.sub.y. An observer standing
at a fixed point on the z-axis and measuring the components E.sub.x and
E.sub.y of the total electric field simultaneously would find that these
components would fall on the curve
(E.sub.x /E.sub.0x).sup.2 +(E.sub.y/E.sub.0y).sup.2
-2(E.sub.x/E.sub.0x)(E.sub.y/E.sub.0y) cos .delta.=sin.sup.2.delta.. (3)
Equation (3) is the well known equation of an ellipse making an angle
.alpha. with the (E.sub.x, E.sub.y)-coordinate system, where
tan 2.alpha.=(2E.sub.0x E.sub.0y cos .delta.).div.(E.sub.0x.sup.2
-E.sub.0y.sup.2). (4)
Hence, E corresponds to elliptically polarized light. From Equation (3) can
be seen that the phase difference .delta. dictates some of the
characteristics of the ellipse. For example, if .delta. were equal to an
even multiple of 2.pi. (i.e., if E.sub.x and E.sub.y are in phase), then
Equation (3) reduces to E.sub.y =(E.sub.0y /E.sub.0x)E.sub.x, which is the
equation of a straight line; in that case, E is linearly polarized. On the
other hand, if .delta. is equal to .+-..pi./2, .+-.3.pi./2, .+-.5.pi./2, .
. . , and assuming E.sub.0x =E.sub.0y =E.sub.0, Equation (3) reduces to
E.sub.0x.sup.2 +E.sub.0y.sup.2 =E.sub.0.sup.2, which is the equation of a
circle. In that case, E is circularly polarized. Of course, linearly and
circularly polarized light are just special cases of elliptically
polarized light, a line and a circle being special types of ellipses.
From the above considerations, it is clear that if two perpendicular modes
of light with equal amplitudes, such as that described by Equations (1)
and (2) with E.sub.0x =E.sub.0y, enter an optical device, and proceed to
exit the device with a phase shift of .pi./2, the result would be
circularly polarized light. Typical optical devices that serve to
transform linearly polarized light to circularly polarized light work on
this principle.
For example, birefringent light fibers are anisotropic meaning that they
don't have the same optical properties in all directions. Such fibers have
an optic axis, which is arbitrarily taken here to be the z axis, with the
following properties: Two linearly polarized light beams traveling along
the optic axis have the same speed v even if their polarization directions
differ; however, if, instead, two linearly polarized light beams are
traveling perpendicular to the optic axis, say along the x axis, and
furthermore one beam is polarized along the y axis and the other along the
z axis, then, while the beam polarized along the y axis will travel at the
previously mentioned speed v, the other beam that is polarized along the z
axis will have a different speed. Such two beams moving perpendicular to
the optic axis may enter the fiber in phase, but because of their
disparate speeds will exit with a non-zero phase difference .delta.. The
result, as was seen above, is elliptically polarized light.
In the time, .DELTA.t, that it takes the faster moving beam to traverse the
birefringent fiber, the faster moving beam, with speed v.sub.fast, will
outpace the slower moving beam, with speed v.sub.slow, by a distance
(v.sub.fast -v.sub.slow).DELTA.t. This last mentioned distance contains
(v.sub.fast -v.sub.slow).DELTA.t/.lambda..sub.slow waves of the slower
moving beam having wavelength .lambda..sub.slow. Noting that
.DELTA.t=L/v.sub.fast, where L is the fiber length, the phase difference
between the two beams is given by
.delta.=2.pi.(v.sub.fast -v.sub.slow)L/(.lambda..sub.slow v.sub.fast). (5)
This last equation can be rewritten by substituting
v.sub.fast =.lambda..sub.fast.nu., (6)
and
v.sub.slow =.lambda..sub.slow.nu., (7)
where .nu. is the common frequency of the slow and fast beams, to yield
L=(.delta./2.pi.)(1/.lambda..sub.slow -1/.lambda..sub.fast).sup.-1 (8)
This last equation makes clear that one can tailor a birefringent fiber to
act as a transformer of linearly polarized light into elliptically
polarized light simply by choosing the correct length, L, of fiber,
although this length depends on the frequency of the light through
Equations (7) and (8). The length of fiber that results in a phase
difference of 2.pi. and that therefore leaves the polarization unchanged
is known as a beatlength, denoted by L.sub.b, and will play a role in the
discussion below.
To make optical devices that transform linearly polarized light into
elliptically polarized light, one may use one birefringent fiber with two
beams of light of equal frequency and amplitude (or, equivalently, one
beam that is the vector sum of these two beams). As was discussed above,
if the two beams are propagated perpendicular to the z (i.e., the optic)
axis, and their polarizations are along the z and y axes, elliptically
polarized light results. Alternatively, linearly polarized light may be
transformed to circularly polarized light by using one beam and two
fibers, one of which is birefringent and of length L.sub.b /4.
Referring to FIG. 1, such a single-beam transformer of linearly polarized
light to circularly polarized light may be constructed by fusing two
silica or glass fibers. One of these fibers is the transmitting fiber 2
that delivers light to a second birefringent fiber known as the
transforming fiber 4. The transforming fiber 4 is cut to a length of
L.sub.b /4. In addition, the relative orientation of the two fibers is
chosen so that the direction of polarization of a light beam traveling in
the transmitting fiber 2 is rotated .pi./4 radians with respect to the
optic axis of the transforming fiber's optic axis, as indicated by the
transmitting fiber cross section 6 and the transforming fiber cross
section 8. Such an operation may be done with a standard commercially
available fusion splicer. However, any misalignment of the fibers results
in some light being lost at the splice 10. Moreover, as Equation 5 makes
clear, errors in the phase difference .delta. grow linearly with errors in
the fiber length L. In practice, constructing a single-beam transformer of
linearly to circularly polarized light involves first starting with a
length of transforming fiber 4 greater than L.sub.b /4, and performing
several iterations of cutting and measuring polarization until the
polarization is deemed to be circular to within some specification.
Needless to say, this is a tedious and lengthy procedure requiring lots of
guesswork.
The present invention resolves some of the aforementioned problems by
presenting an alternate method of fabricating a single-beam transformer of
polarized light. Referring to FIG. 2, instead of splicing two fibers
offset by .pi./4 radians, in the method of the present invention a single
birefringent fiber 12 is twisted by this angle. The twist 14 in the fiber
can be accomplished by heating the birefringent fiber 12 using arc
electrodes 16.
Referring to FIG. 3, in lieu of the tedious iterations of cutting and
monitoring, in the method of the present invention, fine tuning is
achieved by heating with a diffusing arc 26 produced by arc electrodes 22
to cause diffusion of the fiber core into the cladding. The heating can
continue until a polarization monitor 24 indicates that the right
polarization state is achieved. The effect of the diffusion is to expand
the fields of the fiber modes and so reduce the effective difference
v.sub.fast -v.sub.slow.
The steps of twisting and diffusing are conceptually independent, and each
can be used profitably to make transformers of linearly to elliptically
polarized light. Varying the angle through which the birefringent fiber 12
is twisted is tantamount to varying the amplitudes E.sub.0x and E.sub.0y
of Equation (3) and results in different states of elliptically polarized
light. The step of diffusing, on the other hand, can be used any time some
fine tuning of the polarization is required. For example, after splicing
two fibers of appropriate length according to conventional methods, the
state of polarization can be fine tuned by causing the core to diffuse
into the cladding.
One can also fabricate a transformer using one birefringent fiber and two
beams of linearly polarized light. If the two beams are propagated
perpendicular to the z (i.e., the optic) axis, and their polarizations are
along the z and y axes, elliptically polarized light results. After
cutting the single fiber to an appropriate length, fine tuning of the
sought-after polarization can be achieved by heating the fiber to cause
diffusion of the core into the cladding as mentioned above.
The present invention presents a more convenient method to fabricate a
transformer of polarized light. The first step of the method obviates the
need to splice a transmitting fiber 2 to a transforming birefringent fiber
4 of length L.sub.b /4 with the aim of producing a transformer of linearly
to circularly polarized light. Instead, a convenient length of a
birefringent fiber 12 is heated to the softening point of the glass and
then twisted through an angle of approximately .pi./4 radians, the
direction of the twist 14 (i.e. clockwise or anticlockwise) determining
whether the emitted light is right or left circularly polarized. In a
preferred embodiment, the twisting should occur over as short a length as
possible. Twisting a single fiber by .pi./4 radians instead of splicing
two fibers offset by this angle keeps optical losses low. What losses do
occur are scarcely measurable in practice.
In the next step of the invention, fine tuning is performed in the
following manner. First, the birefringent fiber 12 is cut so that its
length from the twist 14 to the end of the fiber is slightly larger than
L.sub.b /4. The twisted birefringent fiber 12 is positioned between the
arc electrodes 22 of a fiber fusion splicer where the arc electrodes 22
are retracted further from the fiber than their position in a normal
splicing operation. A diffusing arc 26 is struck at a current lower than
that used for splicing in order to raise the temperature of the
birefringent fiber 12 to a point below its melting point but where the
fiber core begins to diffuse into the cladding. The effect of the
diffusion is to expand the fields of the fiber modes and so reduce the
effective index of propagation. The light emerging from the transformer is
monitored during this operation with the use of a polarization monitor 24,
and diffusion is stopped when the light is circularly polarized. FIG. 3
shows the arrangement.
Although what was described above is a preferred method for fabricating a
single-beam transformer of linearly to circularly polarized light by the
steps of twisting and diffusing, it should be understood that these two
steps are independent and each may be profitably used individually. For
example, to form a single-beam transformer of linearly to circularly
polarized light, a single birefringent fiber can be twisted as described
above, and then fine tuned not by the preferred method of diffusing, but
by a conventional method of iterations of cutting the fiber to an
appropriate length and monitoring the polarization.
Alternatively, two fibers may be spliced together as in usual approaches.
The transforming fiber would then be cut to a length of approximately
L.sub.b /4. However, unlike the usual methods that then fine tune by
iterations of cutting and monitoring, the tuning could proceed by causing
the core to diffuse into the cladding, as described above.
Finally, instead of twisting a birefringent fiber through an angle of
.pi./4 radians, which corresponds to choosing E.sub.0x =E.sub.0y in
Equation (3), the fiber could be twisted through varying angles. This
would be effectively equivalent to varying the amplitudes E.sub.0x and
E.sub.0y. As can be seen from this equation, even if the length of the
fiber would lead to a phase difference of .pi./2 radians, the result would
generally be elliptically polarized light that is non-circular.
The above methods have involved fabricating a single-beam transformer of
linearly to circularly, or in the case where the twisting angle is not
.pi./2 radians, elliptically polarized light. As mentioned above, one can
also build a transformer using one birefringent fiber and two beams of
linearly polarized light (of course, two beams of superposed light is
equivalent to a single beam equal to the vector sum of the two constituent
beams). If the two beams are propagated perpendicular to the z (i.e., the
optic) axis, and their polarizations are along the z and y axes,
elliptically polarized light results. According to Equations 3, 4, and 5,
the type of elliptically polarized light that results depends on the
length of the fiber, L. After cutting a birefringent fiber to an
appropriate length, fine tuning of the polarization can proceed by
diffusing the core into the cladding, as described above.
The transformer of linearly to circularly polarized light described above
can be used in a current sensor exploiting the Faraday Effect in a Sagnac
interferometer. A main feature of a Sagnac interferometer is a splitting
of a beam of light into two beams. By using mirrors or optical fibers,
both beams of light are made to traverse at least one loop, but in
opposite directions. At the end of the trip around the loop, both beams
are recombined thus allowing interference to occur. Any disturbance that
affects one or both beams as they are traversing the loop has the
potential to alter the interference pattern observed when the beams
recombine. Rotating the device is the traditional disturbance associated
with Sagnac's name. Another disturbance, giving rise to the Faraday
Effect, involves applying an external magnetic field to the medium that
forms the loop through which the light travels. Under the influence of
such a field, the properties of the light-transmitting medium forming the
loop are altered so as to cause a change in the direction of polarization
of the light. In turn, this change in the direction of polarization
results in a change in the interference pattern observed. These types of
disturbances that give rise to a modification in the observed interference
pattern are known as non-reciprocal disturbances. They are so-called
because, unlike reciprocal effects in which the change produced in one
beam cancels with that produced in the other, the changes produced in the
two beams reinforce to yield a modification in the resultant interference
pattern.
In FIG. 4 is shown a schematic of a Sagnac interferometer current sensor of
the present invention. The light beam 31 emerges from a laser source 32
which is preferably a diode laser oscillating predominantly in a single
transverse mode and having a broad and Gaussian-shaped optical spectrum so
that back-scatter noise and Kerr-effect problems are reduced. The light
beam 31 passes through a first directional coupler 33 that isolates the
optical detector 34, and then a transformer 35 of linearly to circularly
polarized light to ensure a single polarization state, which in a
preferred embodiment is circular polarization. The light beam is then
split in two by the second directional coupler 36. One beam is directed
into one end of a sensing coil 41 comprising loops of polarization
maintaining fiber 37; this polarization maintaining fiber 37 is not
birefringent. The other light beam from the directional coupler 36 is
directed through a phase modulator 40 into the other end of the sensing
coil comprising loops of polarization maintaining fiber 37. Light emerging
from the two fiber ends is recombined by the directional coupler 36 and
detected by an optical detector 34.
A current carrying wire 38, with its accompanying magnetic field 39, runs
out of the page. The magnetic field 39 changes the physical properties of
the sensing coil of polarization maintaining fiber 37. The circular
polarization of both beams traveling around the sensing coil of
polarization maintaining fiber 37 is thus modified. In particular, the
magnetic field causes a phase shift (which should not be confused with the
phase difference .delta. from Equation (2)) corresponding to a rotation of
the direction of the electric field. This phase shift results in a change
in the interference when both light beams are reunited at the directional
coupler 36 before passing through the transformer 35 of linearly to
circularly polarized light to the optical photodetector 34.
As mentioned above, in a preferred embodiment the state of polarization of
the light beams entering the sensing coil of polarization maintaining
fiber 37, after leaving the transformer 35 of linearly to circularly
polarized light, is circular. Correspondingly, the coil's polarization
maintaining fiber 37 is circularly cored. However, when the fiber 37 is
bent into a coil, stresses occur that give rise to anisotropic effects.
For this reason it is advantageous to prepare the fiber 37 for the
transmission of light by annealing the fiber 37 while it is in a coil. It
is desirable to keep the fiber as symmetrical as possible; in the absence
of an external magnetic field, one aims to not change the phase of the
transmitted light appreciably over the length of the sensing coil, which
is about six meters long. Ideally, the beatlength should not be less than
six meters; however, in the case at hand, the beatlength is approximately
3 millimeters. Thus one should start with a fiber that is as symmetrical
as possible. One may draw the fiber from a pre-form as the pre-form is
spun around with the goal of producing a symmetrical fiber. As mentioned
above, after the fiber is wound into a coil, annealing can help eliminate
any stresses.
The transformer 35 of linearly to circularly polarized light is that
transformer discussed above wherein a birefringent fiber is twisted
through 45 degrees after which it is cut at approximately one quarter of a
beatlength. Fine tuning may then proceed as described above with one
addition: the end closest to the twist is first spliced to the circular
cored fiber that is wound into the sensing coil of polarization
maintaining fiber 37. Only then does the fine tuning proceed by heating.
In measuring the phase shift .alpha. arising from the Faraday Effect, it is
helpful to remember that the measured optical power is proportional to the
square of the absolute value of the detected electric field. Ignoring the
non-reciprocal power difference, which is negligible for the typically
used coil lengths, the detected power turns out to be proportional to
(1+cos .alpha.). This factor presents somewhat of a difficulty when trying
to measure the typically small phase shifts .alpha.. In particular, the
rate of change of 1+cos .alpha. is asymptotic to -.alpha., as .alpha.
approaches zero, making it difficult to experimentally measure changes in
the phase shift. It therefore becomes necessary to add a biasing phase
difference to shift the sensed signal so as to avoid both the maxima and
minima of the sinusoid. The phase modulator 40 in FIG. 4 performs this
function by creating the desired amount of phase difference modulation.
Since the phase modulator 40 is positioned at one end of the polarization
maintaining fiber 37, the two counter-propagating light beams both receive
the same phase modulation but at different times, thereby realizing a
non-reciprocal phase difference modulation between the interfering beams.
Since the sensed signal becomes biased on a high-frequency carrier, (i.e.,
the phase modulation signal,) electronic noise is substantially eliminated
while measurement sensitivity is increased.
For the current sensor of FIG. 4, a unitary length of optical fiber is used
for the polarization maintaining fiber 37, with a segment of fiber
extending from one end of the coil being used to establish a light path
between the optical source 32, the directional coupler 33, the transformer
35 of linearly to circularly polarized light, and the coupler 36. A
segment of fiber extending from the other end of the polarization
maintaining fiber 37 establishes a light path between the corresponding
coil end, the phase modulator 40 and the directional coupler 36.
For optimizing the performance of the current sensor of FIG. 4, magnetic
field sensitivity must be maximized and noise sensitivity must be
minimized. To accomplish this, it is desirable to match the transit time t
required for the counter-propagating light beams to traverse the length of
the fiber coil with the phase modulation frequency f.sub.m according to
the following relationship:
.omega..sub.m t=.pi. (9)
where .omega..sub.m is the radian frequency of the modulation source and is
equal to 2.pi.f.sub.m. In terms of the group velocity v.sub.g of the
optical wave guided by the fiber, the transit time t is defined as
t=L.sub.f /v.sub.g (10)
where L.sub.f is the length of the polarization maintaining fiber 37.
Substituting Eq. (10) into Eq. (9) yields the following expression for the
modulation frequency:
f.sub.m =v.sub.g /2L.sub.f. (11)
Since the group velocity v.sub.g is approximately equal to c/n, where c is
the speed of light in vacuum, and n is the average refractive index of the
fiber core and cladding, the quantity v.sub.g represents a constant.
Accordingly, the modulation frequency f.sub.m is inversely proportional to
the length of the polarization maintaining fiber L.sub.f.
There is therefore in place a technique for measuring the current through a
conductor: as a consequence of the Biot-Savart Law, an infinitely long
conducting wire, for example, carrying a current i, gives rise to a
magnetic field whose magnitude at a distance R from the wire is .mu..sub.0
i.div.(2.pi.R), where .mu..sub.0 is the permeability of free space. If the
Sagnac interferometer described above is immersed in this magnetic field,
the properties of the polarization maintaining fiber 37 that composes the
coil will change so as to affect the interference pattern observed. Thus,
from the change in this pattern, the current i can be inferred. Similar
current sensors are known in the prior art, e.g., Interferometer device
for measurement of magnetic fields and electric current pickup comprising
a device, United States utility patent application, filed May 14, 1985,
application Ser. No. 4,560,867, naming Papuchon; Michel; Arditty; Herve;
Puech; Claude as inventors, which is incorporated by reference herein. The
design of current sensors is similar to that of fiber optic rotation
sensors of the type that appears in Fiber Optic Rotation Sensor or
Gyroscope with Improved Sensing Coil, United States utility patent
application, filed Apr. 7, 1995, application Ser. No. 5,552,887, naming
Dyott, Richard B. as inventor, which is incorporated by reference herein.
The aforementioned current sensor has several attractive features. It has
no moving parts, resulting in enhanced reliability. There are no
cross-axis sensitivities to vibration, acceleration or shock. The current
sensor is stable with respect to temperature fluctuations and has a long
operational life, making it useful in a wide variety of applications,
including land navigation, positioning, robotics and instrumentation.
One application of the current sensor is for the measurement of high
voltages (>0.1 MV) in conductors present in voltage transformers. About
six meters of polarization maintaining fiber is wound into a multi-turn
loop, annealed in situ and then the conductor is threaded through the
sensing coil. The current sensor can also be used as a trip-out device
that would very quickly detect a short-circuit.
It will be understood by those of ordinary skill in the art, that perfectly
linearly or circularly polarized light may be an idealization that can not
be realized. I.e., in practice, there may exist uncontrollable factors
that give rise to some deviations from perfectly linearly or circularly
polarized light. Therefore, it should be understood that when reference is
made to linearly or circularly polarized light the meaning of these terms
should be taken to mean effectively or approximately linearly or
circularly polarized light.
While the invention has been disclosed in connection with the preferred
embodiments shown and described in detail, various modifications and
improvements thereon will become readily apparent to those skilled in the
art. Accordingly, the spirit and scope of the present invention is to be
limited only by the following claims.
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