Apparatus for reducing magnetic field-induced bias errors in a fiber optic gyroscope

Apparatus for suppressing the bias errors induced by the Faraday effect in the output of a sensor coil exposed to a magnetic field. Arrangements are formed at two leads of the sensor coil for compensating the bias shifts. One of such arrangements comprises at least one loop of optical fiber for compensating the effect induced by the magnetic field component oriented transverse to the axis of the sensor coil while the other comprises at least one loop oriented at a predetermined pitch angle for compensating the effect induced by a magnetic field component along the axis. In each case, a predetermined degree of twist of a preselected fiber twist mode is imposed upon the compensator loop for creating a counteracting, corrective Faraday effect. Cross-coupling does not occur between the two compensators as their twist rate perodicities are unequal.

BACKGROUND 
1. Field of the Invention 
The present invention relates to fiber optic gyroscopes. More particularly, 
this invention pertains to apparatus for suppressing bias errors induced 
by magnetic fields oriented both transverse and axially with respect to 
the gyroscope sensor coil. 
2. Description of the Prior Art 
Fiber optic rotation sensing devices, such as gyroscopes, comprise two main 
components, (1) a front end including a light source and detector and (2) 
a fiber optic interferometer, including sensor coil, coupler and polarizer 
that are mounted to a system. Light from the source is split by the 
coupler into two beams, each of which is coupled into an opposed lead of 
the sensing coil. The interferometer and associated electronics process 
the phase relationship between the two interfering, counter-propagating 
beams of light when they emerge from opposite leads of the coil and are 
combined. A phase shift difference between the two beams results from (1) 
coil rotation and (2) so-called "environmental" factors. 
Environmental factors include such variables as temperature, vibration 
(both acoustical and mechanical) and magnetic fields (Faraday effects). 
These factors can induce phase shifts between the counter-propagating 
beams that are indistinguishable from those induced by rotation. In the 
event that the sensing loop is of ideal single mode fiber, the Faraday 
effect is cancelled when the light travels through the fiber coil and a 
phase difference between the counterpropagating beams is not generated. 
The phase difference is observed, due to the nonreciprocity of the Faraday 
effect, when retarders are located asymmetrically within the fiber loop. 
Fiber twist, occurring naturally during manufacture or induced during the 
winding of the coil, acts as an actual and inevitable retarder that leads 
to bias drift in the presence of a magnetic field. One common method for 
avoiding the influence of magnetic fields is to place the sensor coil in a 
.mu.-metal housing. This solution is affected at the cost of an increase 
in both the weight and cost of the fiber optic gyro. 
The Faraday effect in fiber loops is discussed in articles by Kazuo Hotate 
and Kunio Tabe ("Drift of an Optical Fiber Gyroscope Caused by the Faraday 
Effect: Influence of the Earth's Magnetic Field," Applied Optics, Vol. 25 
No. 7 (Apr. 1, 1987) pp. 1086-1092 and "Drift of an Optical Fiber 
Gyroscope Caused by the Faraday Effect: Experiment," Journal of Lightwave 
Technology, Vol. LT-5, No. 7 (July 1987) pp. 997-1001). Hotate and Tabe 
discuss a relationship between the bias and drift of the fiber optic gyro 
(FOG) due to transversely-directed magnetic fields (i.e. fields 
substantially in the plane of the loops that, in combination, constitute 
the sensor coil) and the twisting of the optical fiber. Twisting of the 
polarization maintaining (PM) optical fiber is unavoidable as mentioned 
earlier, occurring during various stages of coil construction. Fiber 
fabrication inevitably imparts some twists. When the spool is then wound 
from the fiber, the nearly impossible-to-avoid misalignment of the coil 
winder and the gyro spool axis will produce further twisting. When the 
axis of the winding machine is at a tilt with respect to the axis of the 
fiber coil, twist is induced in the coil which is periodic with a twist 
rate that varies as a sinusoid as the fiber is wrapped about the 
circumference of the spool Angular misalignments on the order of 
milliradians can produce magnetic sensitivities on the order of 
degrees/hour-Gauss. While a large number of twist modes will be generated 
and randomly distributed within a resulting sensor coil, Hotate and Tabe 
have found and experimentally verified that (only) the twist component 
whose twist rate period is equal in fiber length to a loop of the sensor 
coil is responsible for the sensitivity to transverse magnetic fields. 
The above-cited articles are confined to the effect of transverse magnetic 
fields and, thus, the authors' insights are of limited practical 
significance. In the real world, both transverse and axial magnetic field 
components are generally encountered. Hotate et al. suggest that one 
employ a polarization-maintaining fiber (PM fiber) sensor coil to suppress 
magnetic field sensitivity. In practice, however, the birefringence of 
currently-available PM fiber is not sufficiently large to suppress the 
bias error due to the Faraday effects completely. Bias errors of between 1 
and 5 degree/hour-Gauss are normally detected in the output of a FOG 
having a PM fiber coil. 
SUMMARY OF THE INVENTION 
The present invention provides twist compensation arrangements by which the 
bias and drift caused by both axial and transverse magnetic fields may be 
significantly suppressed. 
It addresses the preceding and other shortcomings of the prior art by 
providing a compensated sensor coil for a fiber optic gyroscope. The coil 
includes a single mode optical fiber. Such fiber is arranged into a 
plurality of adjacent coaxial turns. The turns are arranged into a sensor 
coil that comprises a plurality of adjacent layers. Each of such layers 
comprises a plurality of adjacent turns, the sensor coil being 
characterized by a plurality of randomly distributed fiber twist modes. 
The fiber is additionally arranged into a compensator adjacent the sensor 
coil. Such compensator comprises at least one turn of the fiber. The twist 
rate of a predetermined twist mode of such turn is selected to offset the 
Faraday effect due to an applied magnetic field having a known orientation 
with respect to the axis of the sensor coil. 
The preceding and other features and advantages of this invention will 
become further apparent from the detailed description that follows. Such 
description is accompanied by a set of drawing figures. Numerals of the 
drawing figures, corresponding to those of the written description, point 
to the various features of the invention. Like numerals refer to like 
features throughout both the written description and the drawing figures.

DETAILED DESCRIPTION 
FIG. 1 is a perspective view of a sensor coil 10 in accordance with the 
invention with fiber coil windings removed for purposes of clarity. The 
coil 10 includes arrangements for compensating optical phase shifts 
otherwise induced by the presence of external magnetic fields. As will be 
seen, compensation can provided for the effects of magnetic field 
components aligned both with and transverse to the axis of rotation 12 of 
the generally-cylindrical coil 10. In the latter instance, the magnetic 
field component also lies substantially within the plane of the loops of 
optical fiber of the coil 10. (These magnetic field components are 
labelled H.sub.A --with flux directed as at 14-- and H.sub.T --with flux 
as at line 16-- respectively.) 
The coil 10 comprises a single continuous PM optical fiber wound into the 
generally-cylindrical configuration of FIG. 1. Both symmetrical and 
non-symmetrical winding patterns may be employed. Symmetrical windings, in 
which the coil 10 is formed by winding matching patterns in opposite 
directions from the midpoint of the continuous fiber, minimize the impact 
of such environmental factors as temperature change upon the output, a 
source of potential bias in an asymmetrically wound coil. The coil winding 
process involves the generation of layers of adjacent turns begun from an 
innermost layer 18 and proceeding in an outwardly radial direction as 
indicated by 20 until the sensor portion of the coil is completed with the 
winding of an outermost layer 22. Regular winding patterns for creating 
the sensor coil 10 may comprise layers of helical turns or such symmetric 
arrangements as those disclosed in U.S. Pat. No. 4,793,708 of Bednarz 
covering "Fiber Optic Sensing Coil" and in U.S. Pat. No. 4,856,900 of 
Ivancevic covering "Quadrupole-Wound Fiber Optic Sensing Coil and Method 
of Manufacture Thereof". Each of such patents is the property of the 
Assignee herein. 
The sensor coil 10 provides the large number of fiber loops required for 
creation of a phase difference, measurable through a resultant 
interference pattern, that is indicative of rotation rate. The coil 10 is 
generally mounted upon a spool (not shown in FIG. 1) that provides a 
central core. As an alternative, it may be free-standing with its shape 
maintained by potting or encapsulating the coil windings with an 
appropriate adhesive material. 
In accordance with the invention, means are provided for compensating the 
undesired magnetic field-induced optical phase shifts between light beams 
counterpropagating within the coil 10, such means being either formed from 
the continuous optical fiber of the sensor coil or from another optical 
fiber (of, perhaps, different optical characteristics) that has been 
spliced or otherwise joined to the sensor coil fiber. In order to achieve 
compensation, the leads of the optical fiber of the sensing coil may be 
formed into one or two specific bias compensating geometries. Both of such 
compensators are shown in FIG. 1. These include a transverse field 
compensator 24 consisting of at least one turn of one lead and an axial 
field compensator 26 comprising at least one turn of the other lead wound 
on the sensor coil. The axial compensator 26 has a predetermined angular 
pitch .gamma..sub.c. It will be seen later that the relationship of 
.gamma..sub.c to the pitch of the sensor coil fiber serves as a design 
criterion of the present invention. As will become apparent from the 
following description of the invention, the transverse field compensator 
24 compensates or nulls the Faraday effect-induced phase shift that would 
otherwise be observed in the signal output of the coil sensor 10 when a 
transversely-oriented magnetic field component H.sub.T is applied while 
the axial field compensator 26 overcomes the Faraday effect-induced phase 
shift that would be observed in the output when an axial magnetic field 
component, H.sub.A, is present. 
As already mentioned, the invention incorporates one or more compensators 
in conjunction with an otherwise-conventional sensor coil 10 comprising a 
plurality of layers of turns of a continuous optical fiber. It will become 
apparent from the discussion that critical design parameters, such as 
number and inclination of turns and the fiber twist rate of the 
compensator, must be preserved to maintain compensator effectiveness. It 
is, therefore, to be understood that the compensator configurations 
discussed in detail below are fixed during and throughout the winding 
process and are maintained by appropriate application of conventional 
adhesive means such as EPOXY or the like. 
FIG. 2 is a partial schematic diagram of a fiber optic gyroscope that 
includes a representative fiber loop 30. A coil coordinate system is 
superimposed thereon to facilitate analysis of the design of the 
already-illustrated transverse field compensator 24. In this figure, the 
radius R of the loop 30 represents an average of the radii of the turns of 
the sensor coil taken from the plurality of concentric layers, each 
successive layer being characterized by a larger value of R, beginning 
with the innermost layer 18 and continuing to the outermost layer 22. 
Representative fiber optic gyro sensor coils may comprise, for example, 
ten (10) to thirty-six (36) layers of windings, each comprising of about 
fifty (50) turns of optical fiber. Such figures are intended to be 
representative only and by no means exhaustive of reasonable sensor coil 
designs and specific designs will reflect intended applications which 
will, of course, be affected by accuracy, cost and like requirements. 
The design of a compensator in accordance with the invention depends in 
part upon the twist rate of the selected fiber twist modes. The measured 
phase shift .DELTA..psi..sub.t between clockwise and counterclockwise 
light beams propagating within the loop 30 in the absence of rotation is 
initially measured for the purpose of evaluating the twist rate of the 
relevant fiber mode. In the case of a transverse magnetic field effect, it 
is the twist rate .phi..sub.t (.theta.) of the mode of periodicity 2.pi.R 
of the sensing coil that is significant. This is known from the 
above-referenced findings of Hotate and Tabe. As discussed, the phase 
shift due to a magnetic field H.sub.T oriented transverse to the axis of 
rotation 12 of the sensing coil (of which the loop 30 of FIG. 2 is 
representative) results from the interaction between this specific twist 
mode and H.sub.T. The magnitude, or the rate of twist of such mode 
.phi..sub.t (.theta.) is obtained by analyzing the effect of H.sub.T on 
.DELTA..psi..sub.t in the absence of rotation. It is known that the 
Faraday rotation .zeta. is equal to the-product of the strength of the 
magnetic field applied, H.sub.T, and V, the well-known Verdet constant of 
the optical fiber. The transverse magnetic field will produce phase shifts 
as follows: 
##EQU1## 
EQU .zeta.=.zeta..sub.o sin (.theta.-.theta..sub.o) (2) 
Where .DELTA..beta. is the birefringence of the optical fiber, .zeta..sub.o 
is the Faraday rotation of the fiber and .DELTA..psi..sub.t is, as 
mentioned, the transverse magnetic field-induced phase shift. 
.theta..sub.o is the direction angle of magnetic field H.sub.T as 
illustrated in FIG. 2. 
Solving each of the above equations for the contributions of the orthogonal 
components of the transverse magnetic field H.sub.T to the measured phase 
shift yields: 
##EQU2## 
The above equations may be solved to demonstrate that .phi..sub.t (.theta.) 
is proportional to the birefringence .DELTA..beta. and inversely 
proportional to the product of the radius R and the Verdet constant 
.zeta..sub.o. 
A discussion of the design criteria of axial and transverse field 
compensators will follow. Throughout the discussion it is assumed that the 
compensator is formed of the same continuous optical fiber as the sensor 
coil. However, it will be appreciated by those skilled in the art that the 
teachings provided with reference to such "single fiber" designs may be 
readily extended to a compensator formed of an optical fiber that has been 
spliced onto an end of the sensor coil fiber by correction for disparities 
between the fiber parameters affecting optical properties such as fiber 
birefringence, Verdet constant and the like along with measurable 
anamolies introduced by the presence of the optical splices. 
Keeping the foregoing caveat in mind, in order to design the transverse 
field compensator 24, the following equation must be solved: 
EQU .DELTA..psi..sub.t (.theta..sub.o)+.DELTA..psi..sub.tc 
(.theta..sub.o)=0(all .theta..sub.o) (5) 
Where .DELTA..psi..sub.tc is the phase shift generated by the compensator 
and required to counteract that induced by the transverse magnetic field 
component H.sub.T. It is known that the unavoidable presence of a known 
twist mode in the optical fiber serves as the source of a transverse 
field-induced bias error. Further, it is known, from the analysis provided 
by Hotate and Tabe, that such responsible twist mode has twist rate 
periodicity of 2.pi.R, i.e., equal to the average length of a fiber loop 
30 of the sensor coil 10. 
Solving equation 5 for the compensator twist rate of the above-identified 
twist mode leads to the following design criterion for the transverse 
field compensator 24: 
##EQU3## 
Where m is the total number of turns of the optical fiber of the sensor 
coil and n is the number of turns of the transverse field compensator 24. 
Referring to the prior discussion, .phi..sub.t (.theta.) may be measured 
and determined in a straightforward manner by solving equations 1 to 2 in 
view of the known relationship .zeta..sub.o =VH. 
Thus, in accordance with the invention, an m-turn sensor coil output that 
is insensitive to the presence of a transverse magnetic field component 
(i.e. a component oriented transverse to the axis of rotation 12), is 
obtained by the addition of a compensator at a fiber lead of the coil 
windings comprising n fiber turns characterized by a twist mode of twist 
rate periodicity equal to the average length of the fiber loop. Further, 
the direction of twist of the compensator coil fiber is opposite to that 
of the sensor coil fiber. As mentioned earlier, the value of .phi..sub.t 
is obtained by reference to equations 1 and 2 above after measuring the 
phase shift .DELTA..psi..sub.t induced in the output of the sensor coil 
(absent rotation) in the presence of a varying transverse magnetic field 
H.sub.T. 
FIG. 3 is a graph of the relationship between the twist rate of the twist 
modes of the sensor coil and the transverse compensator whose periodicity 
is equal to the fiber length of a loop. An m:n ratio of 5:1 is assumed. 
For convenience, a one-turn compensator with a square-function type twist 
rate can be created by twisting one fiber lead at a positive constant rate 
during the first half turn and at a negative constant rate during the 
second half turn. The twist rate required to compensate the transverse 
sensitivity in such a case is: 
EQU .phi..sub.tc =m.pi..phi..sub.t.sup..multidot. /4 (6B) 
Where .phi..sub.t.sup..multidot. is the maximum of the measured twist rate 
.phi..sub.t (.theta.) of the sensing coil. 
The inventors have expanded the design of the magnetic field bias 
compensator beyond one based upon the theory of Hotate and Tabe. In 
addressing the phase shift due to an axially-directed magnetic field 
component, H.sub.A, they have realized a further theoretical insight upon 
which design of the axial compensator is based. As before, the inventors 
have found that it is the presence of twist modes in the continuous PM 
optical fiber that serves as the source of the bias. Unlike the phase 
shift occasioned by the presence of a transversely-directed magnetic field 
component H.sub.T, they have found that it is the twist mode of twist rate 
period equal to twice the fiber length of a wound layer that is 
responsible for the bias error observed in the presence of an 
axially-directed magnetic field. 
In the case of an axial field, the direction of the field is nearly 
perpendicular to the plane of the sensing loop. While it would seem that 
such a magnetic field should produce a Faraday effect without effect on 
the gyro output, the inventors have found that an axial magnetic fields 
do, in fact, affect the gyro output considerably. 
For a practical FOG comprising many turns or loops of PM fiber, the 
approximation that .DELTA..beta. &gt;&gt; .phi., .zeta. holds, allowing one to 
approximate the equation for a sensing coil as: 
##EQU4## 
Where .phi..sub.a is the twist rate responsible for axial field 
sensitivity, L is the fiber length of the sensing loop, and .gamma.(z) is 
the pitch angle of the sensing coil. The pitch angle is constant for the 
fiber within a layer but of opposite sign for adjacent layers. A constant 
twist will result in zero axial magnetic field-induced phase shift as the 
sign of the pitch angle changes: As a consequence, any phase "picked up" 
at one layer will be cancelled by the next layer since the integral in 
equation 7 will equal zero. The only factor that can produce a significant 
degree of axial magnetic field sensitivity is the fiber twist component 
having a period equal to the total fiber length of two layers. 
The above analysis of the inventors may be employed to design twist 
compensators for offsetting bias error due to axial magnetic fields, 
thereby reducing overall magnetic sensitivity. Such a compensator consists 
of a twisted section of fiber lead comprising at least one loop the coil 
of twist and spatial periodicity equal to the twist spectrum in the coil. 
Typically the twist in the sensing coil is not known and, therefore, the 
axial sensitivity must be measured. The twist rate of the coil may be 
evaluated by applying the following equation: 
EQU .phi..sub.a 
=(.DELTA..psi..sub.a).multidot..DELTA..beta./(8R.zeta..vertline..gamma..ve 
rtline.m.pi.) (8) 
Where .DELTA..psi..sub.a is the measured phase shift of the sensing coil, R 
is the radius of the fiber loop and m is the number of turns of the coil. 
The twist rate of the compensator required to suppress the axial field 
sensitivity is: 
EQU .phi..sub.ac =-m.vertline..gamma..vertline.R.phi..sub.a /(n.sub.c R.sub.c 
tan .vertline..gamma..sub.c .vertline.) (9) 
Where n.sub.c is the number of turns of the compensator, R.sub.c is the 
radius of the compensator loop and .gamma..sub.c is the pitch angle of the 
compensator fiber. .gamma..sub.c should be made as large as possible to 
reduce the required number of turns and twist rate. 
FIG. 4 is a side elevation view of the sensor coil of FIG. 1 that 
demonstrates the relationship between the windings of the sensing coil 10 
and the compensator loop of the axial field compensator 26. The spool 10 
is wound in a helical pattern with a pitch angle .gamma. defining the 
inclination of each turn from a line 36 drawn orthogonal to the axis of 
rotation 12. The axial compensator loop is inclined at a different angle 
.gamma..sub.c. In contrast to the transverse compensator, the twist rate 
.phi..sub.ac relates to the twist mode of periodicity 2L .sub.layer that 
is, the compensator twist rate is related to the sensor coil twist rate 
whose period is equal in length to the fiber employed in winding two 
layers of turns, while .phi..sub.tc (.theta.) relates to the mode of twist 
rate periodicity 2.pi.R. Thus the twist rate of the fiber comprising an 
axial compensator may be reduced by increasing the compensator pitch angle 
.gamma..sub.c and/or by increasing the number of compensator turns. 
FIG. 5 is an illustration of an axial field compensator, unlike the axial 
field compensator illustrated in FIG. 1 and 4, that comprises two 
compensator turns or layers as each layer of the compensator comprises a 
single turn. One may compare the single-turn (or layer) compensator of 
prior figures with that of FIG. 5, referring the design equation 9 above 
to see that the varying configurations indicate alternative approaches to 
the desirable goal of minimizing the compensator twist rate .phi..sub.ac. 
While the axial compensator reduces the required twist rate by increasing 
n.sub.c, the number of compensator turns, both the single and multiple 
turn compensators as illustrated further attempt to minimize .phi..sub.ac 
by orienting the turn(s) at the maximum .gamma..sub.c permitted by the 
dimensions of the sensor coil. 
Data has been obtained with regard to both axial and transverse field 
compensation in accordance with the invention. Such data has been 
generated by means of a test bed that included a 200 m fiber gyro with a 
broadband light source and an MIOC. The gyro was operated in open-loop 
fashion with a lock-in amplifier providing demodulation of the gyro 
signal. The MIOC leads to the fiber coil were oriented perpendicular to 
the magnetic field (generated by a pseudo-Helmholtz coil capable of 
selectively generating fields in both the transverse and axial 
directions.) Compensators were integrated into the test bed by fabrication 
onto a spool similar to that of the fiber gyro and then splicing the fiber 
leads to one of the gyro's input leads. The test compensator was then 
stacked onto the sensor coil to assure that both sensor coil and 
compensator saw the same uniform magnetic field. 
Axial Magnetic Field Compensator 
A quadrupole sensor coil winding structure was employed with twist spectrum 
comprising a twist in one direction (due either to the winding process or 
intrinsic to the fiber) for one layer followed by a twist in the reverse 
direction for the adjacent layer. Thus a twist component in the axial 
direction was assured. 
The twist was applied at a constant rate for one compensator loop and then 
reversed over succeeding loops in creating the multiple loop 
configurations. The fiber was attached to the spool to prevent relaxation. 
Data was obtained by measuring the sensitivity of the sensor coil to an 
axial magnetic field, integrating a compensator configuration and then 
measuring the effect upon sensitivity. Different twist rates were employed 
to establish a baseline. A sensitivity of about 1 deg/hr-Gauss to axial 
magnetic fields was measured in the uncompensated sensor coil. FIG. 6 is a 
table of data that summarizes the combined sensitivity of a sensor coil 
incorporating different compensator designs. It is apparent from such data 
that the compensators successfully reduced axial sensitivity and, in one 
case, actually over-compensated. Such data clearly demonstrates the 
utility of an axial compensator in accordance with the invention. 
Transverse Magnetic Field Compensator 
Due to the difficulty of fabricating a sinusoidally-varying twist rate into 
a transverse field compensator, a square wave twist rate was employed, the 
twist rate was being held constant for half the circumference of the spool 
and then reversed (same twist rate in the opposite direction) over the 
remaining half of the circumference. As mentioned earlier, the compensator 
twist rate can be achieved over more than a single fiber loop as long as 
the twist components each of loop are in phase. (Additional loops may be 
added to fine tune the compensator to eliminate residual sensitivity. To 
determine the twist rate needed for this, sensitivity is measured by 
placing the coil in a transverse magnetic field and monitoring the bias 
change that occurs as the coil is rotated through 360 degrees in the plane 
of the field.) 
FIG. 7 is a graph of the response of a 200 m fiber as it is rotated in a 14 
Gauss field. Maximum bias change was used to determine the maximum twist 
rate for nulling out field sensitivity. For the particular coil measured, 
a compensator twist of 1.25 turns over the first half of the circumference 
and -1.25 turns over the second half of the circumference was employed. 
The compensator was fabricated on a separate spool and then spliced into 
the interferometer. The measured compensator response is illustrated in 
FIG. 8. It can be observed from the graph of FIG. 8 that the response of 
the square wave twist spectrum approximates the sinusoidal response of the 
fiber coil as shown in FIG. 7. The coil and the compensator were aligned 
and then placed in a transverse magnetic field so that their respective 
responses were 180 degrees out-of-phase. 
FIG. 9 is a graph of the response of the combined system to a 14 Gauss 
transverse field as a function of field orientation. As can be seen, the 
net magnetic sensitivity was reduced to less than 0.05 deg/hr-Gauss 
without magnetic shielding. This, of course, can result in significant 
weigh savings. Additional experiments were conducted to measure 
cross-coupling among the axial and transverse compensators and the effect 
on gyro sensitivity. It was found that the transverse compensator did not 
affect the axial sensitivity of the gyro. Conversely, the observed gyro 
sensitivity to transverse fields was not affected by the axial 
compensator. 
Thus, the above experiments demonstrate that effective compensators for 
both axial and transverse magnetic fields can be achieved. The axial 
sensitivity of a compensated gyro has been shown to be reduced by an order 
of magnitude over that of the gyro without a compensator. The level of 
performance could be increased, of course, by improved control over the 
compensator twist rate. The transverse sensitivity of the gyro with a 
compensator was less than 0.05 deg/hr-Gauss. This represents almost two 
orders of magnitude improvement and is equivalent to performance achieved 
with magnetic shields. 
Thus it is shown that the present invention provides apparatus for 
minimizing the magnetic field-generated bias otherwise observed in the 
output of a fiber optic gyro. Such bias results from Faraday effect 
interaction between the magnetic field and the light beams propagating 
within the sensor coil. By employing the teachings of the invention, the 
effects of magnetic fields oriented both transversely and axially with 
respect to the coil geometry are overcome. 
While this invention has been illustrated with respect to its 
presently-preferred embodiment, it is not limited thereto. Rather, this 
invention is limited only insofar as defined by the following set of 
patent claims and includes within its scope all equivalents thereof.