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Claims  |
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What is claimed is:
1. A rotation sensor capable of sensing rotation about an axis of a coiled
optical fiber, said sensing based on electromagnetic waves propagating in
opposite directions in said coiled optical fiber to impinge on a
photodetector with a phase relationship which has an effective maximum
offset error associated with a specified maximum rotation rate offset
error, said rotation sensing comprising:
a bias optical phase modulator means having an input and being positioned
in an optical path portion selected from among those optical path portions
taken by electromagnetic waves to reach or leave said coiled optical fiber
en route on an optical path to said photodetector, said bias optical phase
modulator means for phase modulating any such electromagnetic waves
passing therethrough and propagating along said optical path so as to
provide a varying phase difference between such electromagnetic waves
propagating through said coiled optical fiber in opposing directions in
response to a corresponding electrical signal on said input thereof which,
if substantially periodic at a selected fundamental frequency, causes said
varying phase at a frequency twice that of said fundamental frequency to
have an amplitude that is first fraction of that amplitude that said
varying phase has at said fundamental frequency.
a phase modulation generator means having an output electrically connected
to said bias optical phase modulator means input, said phase modulation
generator means for providing a substantially periodic electrical output
signal at said output thereof having a fundamental component at said
selected fundamental frequency and a selected amplitude, and having a
harmonic component at a frequency twice that of said fundamental frequency
with an amplitude that is a second fraction of said fundamental component
amplitude; and
a signal component selection means having a detection input, electrically
connected to said photodetector to receive an output therefrom
representative of any phase differences occurring between pairs of
electromagnetic waves impinging thereon, and having an output, said signal
component selection means for causing signals to appear at said output
thereof based on said fundamental frequency, said bias optical phase
modulator means providing a varying phase difference at said fundamental
frequency between electromagnetic waves propagating through said coiled
optical fiber in opposing directions of a fundamental phase difference
amplitude in response to said phase modulation generator means providing
said phase modulation generator means output signal at said output
thereof, said phase modulation generator means including means for
controlling said second fraction such that said second fraction is of a
value less than a ratio of said effective maximum offset error to said
fundamental phase difference amplitude, said bias optical phase modulator
for controlling said first fraction such that said first fraction is of a
value less than a ratio of said effective maximum offset error to said
fundamental phase difference amplitude.
2. The apparatus of claim 1, wherein said control means of said phase
modulating generator means through which said phase modulation generator
means output signal is provided, includes filter means for reducing
amplitudes of signal components provided therethrough at twice said
fundamental frequency by a selected attenuation factor below amplitudes of
signal components provided therethrough at said fundamental frequency to
achieve said value for said second fraction.
3. The apparatus of claim 1 wherein said bias optical phase modulator
includes a piezoelectric material structure having an input and having a
surface of revolution with a perimeter which can vary in extent in
response to an electrical signal provided to said input, a length of
optical fiber insertable in said optical path and forming a coil wrapped
about said surface of revolution, a pair of interface layers which are
compressible but with nonlinear stiffness in resisting compression such
that resistance to compression increases substantially with sufficient
increases in compression, and clamping means or mounting said
piezoelectric material structure between said pair of interface layers.
4. The apparatus of claim 1 wherein said phase modulation generator means
output signal approximates a rectangular waveform, and said second
fraction is approximately equal to that difference occurring between said
rise times and said fall times therein divided by that time duration
required for electromagnetic waves to propagate from said optical phase
modulator means through said coiled fiber to a symmetric point in an
optical path therebetween on an opposite side of said coiled fiber.
5. The apparatus of claim 1 wherein said signal component selection means
has a demodulation input electrically connected to said phase modulation
generator means output to receive said output signal therefrom, said
signal component selection means for using signals supplied at said
demodulation input thereof, having a substantial demodulation signal
component therein at a selected demodulation frequency, to cause signals
to appear at said output thereof representing amplitudes of components of
signals occurring at said detection input thereof based on said
demodulation frequency.
6. The apparatus of claim 3 wherein said length of optical fiber has a
thin, adherent jacket thereabout.
7. The apparatus of claim 3 wherein said length of optical fiber is bonded
both to itself and to said piezoelectric material structure.
8. The apparatus of claim 3 wherein said phase modulation generator means
output is electrically connected to said bias optical phase modulator
means input with an interconnection hat is a flexible wire in a portion
close to that said input.
9. The apparatus of claim 7 wherein said length of optical fiber is bonded
by wrapping it about said piezoelectric material structure with an uncured
bonding agent thereon which is cured after such wrapping.
10. The apparatus of claim 7 wherein said length of optical fiber has
substantially an integral number of turns about said piezoelectric
material structure in being wrapped therearound.
11. A rotation sensor capable of sensing rotation about an axis of a coiled
optical fiber, said sensing based on electromagnetic waves propagating in
opposite directions in said coiled optical fiber to impinge on a
photodetector with a phase relationship which has an effective maximum
offset error associated with a specified maximum rotation rate offset
error, said rotation sensor comprising:
a bias optical phase modulator means having an input and being positioned
in an optical path portions selected from among those optical path
portions taken by electromagnetic waves to reach or leave said coiled
optical fiber en route on an optical path to said photodetector, said bias
optical phase modulator means for phase modulating any such
electromagnetic waves passing therethrough and propagating along said
optical path so as to provide a varying phase difference between such
electromagnetic waves propagating through said coiled optical fiber in
opposing directions in response to a corresponding electrical signal on
said input thereof which, if substantially periodic at a selected
fundamental frequency, causes said varying phase at a frequency twice that
of said fundamental frequency to have an amplitude that is a first
fraction of that amplitude that said varying phase has at said fundamental
frequency;
a phase modulation generator means having an output electrically connected
to said bias optical phase modulator means input, said phase modulation
generator means for providing a substantially periodic electrical output
signal at said output thereof having a fundamental component at said
selected fundamental frequency and a selected amplitude, and having a
harmonic component at a frequency twice that of said fundamental frequency
with an amplitude that is a second fraction of said fundamental component
amplitude; and
a signal component selection means having a detection input, electrically
connected to said photodetector to receive an output therefrom
representative of any phase differences occurring between pairs of
electromagnetic waves impinging thereon, and having an output, said signal
component selection means for causing signals to appear at said output
thereof representing amplitudes of components of signals occurring at said
detection input thereof based on said fundamental frequency, said bias
optical phase modulator means providing a varying phase difference at said
fundamental frequency between electromagnetic waves propagating through
said coiled optical fiber in opposing directions of a fundamental phase
difference amplitude in response to said phase modulation generator means
providing said phase modulation generator means output signal at said
output thereof, said bias optical phase modulator means and said phase
modulation generator means having means for establishing parameter values
therein so as to provide a weighting factor multiplying at least one of
said first fraction and said second fraction to form a product therewith
such that the remaining one of said first and second fractions and said
product are both of values less than an output ratio of said effective
maximum offset error to said fundamental phase difference amplitude with
that one of said first and second fractions used in forming said product
being greater than said output ratio.
12. The apparatus of claim 11 wherein both of said first and second
fraction form corresponding first and second products through being
multiplied by said weighting factor, and both of said first and second
products are of values less than said output ratio, with both of said
first and second fractions being greater than said output ratio.
13. The apparatus of claim 11 wherein said weighting factor is based on a
response ratio, said bias optical phase modulator means providing a
varying phase at said fundamental frequency in electromagnetic waves
propagating through said coiled optical fiber of a fundamental phase
amplitude in response to said phase modulation generator providing said
phase modulation generator means output signal at said output thereof, and
further providing a varying phase at a frequency twice that of said
fundamental frequency in electromagnetic waves propagating through said
coiled optical fiber of a harmonic phase amplitude in response to said
phase modulation generator means providing a periodic signal at said
output thereof at a frequency twice that of said fundamental frequency,
said response ratio being equal to a ratio of said harmonic phase
amplitude to said fundamental frequency amplitude.
14. The apparatus of claim 11 wherein said signal component selection means
has a demodulation input electrically connected to said phase modulation
generator means output to receive said output signal therefrom, said
signal component selection means being capable of using signals supplied
at said demodulation input thereof, having a substantial demodulation
signal component therein at a selected demodulation frequency, to cause
signals to appear at said output thereof representing amplitudes of
components of signals occurring at said detection input thereof based on
said demodulation frequency.
15. The apparatus of claim 12 wherein said weighting factor is based on a
sinusoid of (.omega..sub.g .tau./2) with .omega..sub.g being said
fundamental frequency and kept within a selected range about .pi./.tau.
over a selected temperature range with .tau. being that time duration
required for electromagnetic waves to propagate from said optical phase
modulator means through said coiled fiber to a symmetric point in an
optical path therebetween on an opposite side of said coiled fiber.
16. The apparatus of claim 12 wherein said weighting factor is based on
[J.sub.1 (.phi.)-J.sub.3 (.phi.)] with .phi. being said fundamental phase
difference amplitude and kept within a selected range about 3.05 over a
selected temperature range.
17. The apparatus of claim 13 wherein said means for establishing of said
bias optical phase modulator includes a piezoelectric material structure
having an input and having a surface of revolution with a perimeter which
can vary in extent in response to an electrical signal provided to said
input, a length of optical fiber insertable in said optical path and
forming a coil wrapped about said surface of revolution, a pair of
interface layers which are compressible but with nonlinear stiffness in
resisting compression such that resistance to compression increases
substantially with sufficient increases in compression, and clamping means
for mounting said piezoelectric material structure between said pair of
interface layers.
18. The apparatus of claim 15 wherein said phase modulation generator means
output signal approximates a rectangular waveform, and said second
fraction is approximately equal to that difference occurring between said
rise times and said fall times therein divided by that time duration
required for electromagnetic waves to propagate from said optical phase
modulator means through aid coiled fiber to a symmetric point in an
optical path therebetween on an opposite side of said coiled fiber.
19. The apparatus of claim 17 wherein said length of optical fiber has a
thin, adherent jacket thereabout.
20. The apparatus of claim 17 wherein said length of optical fiber is
bonded both to itself and to said piezoelectric material structure.
21. The apparatus of claim 17 wherein said phase modulation generator means
output is electrically connected to said bias optical phase modulator
means input with an interconnection that is a flexible wire in a portion
close to that said input.
22. The apparatus of claim 20 wherein said length of optical fiber is
bonded by wrapping it about said piezoelectric material structure with an
uncured bonding agent thereon which is cured after such wrapping.
23. The apparatus of claim 20 wherein said length of optical fiber has
substantially an integral number of turns about said piezoelectric
material structure in being wrapped therearound.
24. A bias optical phase modulator for positioning in an optical path and
capable of phase modulating electromagnetic waves passing therethrough
propagating along said optical path, said bias optical phase modulator
comprising:
a piezoelectric material structure having an input and having a surface of
revolution with a perimeter which can vary in extent in response to an
electrical signal provided to said input;
a length of optical fiber insertable in said optical path and forming a
coil wrapped about sad surface of revolution;
a pair of interface layers which are compressible but with nonlinear
stiffness in resisting compression such that resistance to compression
increases substantially with sufficient increases in compression; and
clamping means for mounting said piezoelectric material structure between
said pair of interface layers.
25. The apparatus of claim 24 wherein said pair of interface layers are
each formed of a soft foam material.
26. The apparatus of claim 24 wherein said piezoelectric material structure
is configured as a ring structure and further comprises a centering ring
which is compressible but with nonlinear stiffness in resisting
compression such that resistance to compression increases substantially
with sufficient increases in compression, said centering ring being
positioned inside said piezoelectric material ring structure about a rigid
locating core.
27. The apparatus of claim 24 wherein said length of optical fiber has a
thin, adherent jacket thereabout.
28. The apparatus of claim 24 wherein said length of optical fiber is
bonded both to itself and to said piezoelectric material structure.
29. The apparatus of claim 24 wherein said phase modulation generator means
output is electrically connected to said bias optical phase modulator
means input with an interconnection that is a flexible wire in a portion
close to that said input.
30. The apparatus of claim 25 wherein said foam is open cell foam.
31. The apparatus of claim 26 wherein said centering ring is formed of a
soft foam material.
32. The apparatus of claim 28 wherein said length of optical fiber is
bonded by wrapping it about said piezoelectric material structure with an
uncured bonding agent thereon which is cured after such wrapping.
33. The apparatus of claim 28 wherein said length of fiber has
substantially an integral number of turns about said piezoelectric
material structure in being wrapped therearound.
34. The apparatus of claim 31 wherein said foam is open cell foam. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention concerns fiber optic system phase modulators and,
more particularly, arrangements for accommodating such phase modulation of
electromagnetic waves traveling therein in changing conditions.
Fiber optic gyroscopes are an attractive means with which to sense rotation
of an object supporting such a gyroscope. Such gyroscopes can be made
quite small and can be constructed to withstand considerable mechanical
shock, temperature change, and other environmental extremes. Due to the
absence of moving parts, they can be nearly maintenance free, and they
have the potential of becoming economical in cost. They can also be
sensitive to low rotation rates that can be a problem in other kinds of
optical gyroscopes.
A fiber optic gyroscope has a coiled optical fiber wound on a core and
about the axis thereof around which rotation is to be sensed. The optical
fiber is typical of a length of 100 to 2,000 meters, or so, and is part of
a closed optical path in which an electromagnetic wave, or light wave, is
introduced and split into a pair of such waves to propagate in opposite
directions through the coil to both ultimately impinge on a photodetector.
Rotation about the sensing axis of the core, or the coiled optical fiber,
provides an effective optical path length increase in one rotational
direction and an optical path length decrease in the other rotational
direction for one of these waves. The opposite result occurs for rotation
in the other direction. Such path length differences between the waves
introduce a phase shift between these waves for either rotation direction,
i.e. the well-known Sagnac effect. The use of a coiled optical fiber is
desirable because the amount of phase difference shift due to rotation,
and so the output signal, depends on the length of the entire optical path
through the coil traversed by the two electromagnetic waves travelling in
opposed direction, and so a large phase difference can be obtained in the
long optical fiber but in the relatively small volume taken by it as a
result of being coiled.
The output current from the photodetector system photodiode, in response to
the opposite direction traveling electromagnetic waves impinging thereon
after passing through the coiled optical fiber, follows a raised cosine
function. That is, the output current depends on the cosine of the phase
difference between these two waves. Since a cosine function is an even
function, such an output function gives no indication as to the relative
directions of the phase difference shift, and so no indication as to the
direction of the rotation about the coil axis. In addition, the rate of
change of a cosine function near zero phase is very small, and so such an
output function provides very low sensitivity for low rotation rates.
Because of these unsatisfactory characteristics, the phase difference
between the two opposite direction traveling electromagnetic waves is
usually modulated by placing an optical phase modulator, or what is
sometimes referred to as a bias modulator, in the optical path on one side
of the coiled optical fiber. As a result, one of these opposite direction
propagating waves passes through the modulator on the way into the coil
while the other wave, traversing the coil in the opposite direction,
passes through the modulator upon exiting the coil.
In addition, a phase-sensitive detector serving as part of a demodulator
system is provided to receive a signal representing the photodetector
output current. Both the phase modulator and the phase-sensitive detector
can be operated by a sinusoidal signal generator at the so-called "proper"
frequency to reduce or eliminate modulator induced amplitude modulation,
but other waveform types of the same fundamental frequency can be used.
Other frequencies can be used, and often are, to reduce the frequency to a
more manageable value.
The resulting signal output of the phase-sensitive detector follows a sine
function, i.e. the output signal depends on the sine of the phase
difference between the two electromagnetic waves impinging on the
photodiode, primarily the phase shift due to rotation about the axis of
the coil in the absence of occurrence of other significant but unwanted
phase shifts. A sine function is an odd function having its maximum rate
of change at zero phase shift, and so changes algebraic sine on either
side of zero phase shift. Hence, the phase-sensitive detector signal can
provide an indication of which direction a rotation is occurring about the
axis of the coil, and can provide the maximum rate of change of signal
value as a function of the rotation rate near a zero rotation rate, i.e.
the detector has its maximum sensitivity for phase shifts near zero so
that its output signal is quite sensitive to low rotation rates. This is
possible, of course, only if phase shifts due to other sources, that is,
errors, are sufficiently small. In addition, this output signal in these
circumstances is very close to being linear at relatively low rotation
rates. Such characteristics for the output signal of the phase-sensitive
detector are a substantial improvement over the characteristics of the
output current of the photodetector without optical phase modulation.
An example of such a system from the prior art is shown in FIG. 1. The
optical portion of the system contains several features along the optical
paths to assure that this system is reciprocal, i.e. that substantially
identical optical paths occur for each of the opposite direction
propagating electromagnetic waves except for the specific introductions of
non-reciprocal phase difference shifts, as will be described below. The
coiled optical fiber forms a coil, 10, about a core or spool using a
single mode optical fiber wrapped about the axis around which rotation is
to be sensed. The use of a single mode fiber allows the paths of the
electromagnetic or light waves to be defined uniquely, and further allows
the phase fronts of such a guided wave to also be defined uniquely. This
greatly aids maintaining reciprocity.
In addition, the optical fiber can be so-called polarization-maintaining
fiber in that a very significant birefringence is constructed in the fiber
so that polarization fluctuations introduced by unavoidable mechanical
stresses, by the Faraday effect in magnetic fields, or from other sources,
which could lead to varying phase difference shifts between the
counter-propagating waves, become relatively insignificant. Thus, either
the high refractive index axis, i.e. the slower propagation axis, or the
low index axis is chosen for propagating the electromagnetic waves
depending on the other optical components in the system. In the present
system, the slow axis has been chosen in view of the optical components
used therein.
The electromagnetic waves which propagate in opposite directions through
coil 10 are provided from an electromagnetic wave source, or light source,
11, in FIG. 1. This source is typically a laser diode which provides
electromagnetic waves, typically int he near-infrared part of the
spectrum, with a typical wavelength of 830 nm. Source 11 must have a short
coherence length for emitted light to reduce the phase shift difference
errors between these waves due to Rayleigh and Fresnel scattering at
scattering sites in coil 10. Because of the nonlinear Kerr effect in coil
10, different intensities in the two counter propagating waves can lead to
different phase shifts therebetween. This situation can be overcome also
by use of a short coherence length source for source 11 which leads to
modal phase shift canceling.
Between laser diode 11 and fiber optic coil 10 there is shown an optical
path arrangement in FIG. 1 formed by the extension of the ends of the
optical fiber forming coil 10 to some optical coupling components which
separate the overall optical path into several optical path portions. A
portion of polarization-maintaining optical fiber is positioned against
laser diode 11 at a point of optimum light emission therefrom, a point
from which it extends to a first optical directional coupler, 12.
Optical directional coupler 12 has light transmission media therein which
extend between four ports, two on each end of that media, and which are
shown on each end of coupler 12 in FIG. 1. One of these ports has the
optical fiber extending from laser diode 11 positioned thereagainst. At
the other port on the sense end of the optical directional coupler 12
there is shown a further optical fiber positioned thereagainst which
extends to be positioned against a photodiode, 13, which is electrically
connected to a photodetection system, 14.
Photodiode 13 detects electromagnetic waves, or light waves, impinging
thereon from the portion of the optical fiber positioned thereagainst and
provides a photo current in response. This photocurrent, as indicated
above, in the case of two nearly coherent light waves impinging thereon,
follows a cosine function in providing a photocurrent output which depends
on the cosine of the phase difference between such a pair of substantially
coherent light waves. This photovoltaic device will operate into a very
low impedance to provide the photo current which is a linear function of
the impinging radiation, and may typically be a p-i-n photodiode.
Optical directional coupler 12 has another optical fiber against a port at
the other end thereof which extends to a polarizer, 15. At the other port
on that same side of coupler 12 there is a non-reflective termination
arrangement, 16, involving another portion of an optical fiber.
Optical directional coupler 12, in receiving electromagnetic waves, or
light, at any port thereof, transmits such light so that approximately
half thereof appears at each of the two ports of coupler 12 on the end
thereof opposite that end having the incoming port. On the other hand, no
such waves or light is transmitted to the port which is on the same end of
coupler 12 as is the incoming light port.
Polarizer 15 is used because, even in a single spatial mode fiber, two
polarization modes are possible in electromagnetic waves passing through
the fiber. Thus, polarizer 15 is provided for the purpose of passing one
of these polarization modes through the optical fiber, along the slow axis
thereof as indicated above, while blocking the other. Polarizer 15,
however, does not entirely block light in the one state of polarization
that it is intended to block. Again, this leads to a small non-reciprocity
between two opposite direction traveling electromagnetic waves passing
therethrough and so a small non-reciprocal phase shift difference occurs
between them which can vary with the conditions of the environment in
which the polarizer is placed. In this regard, the high birefringence in
the optical fiber used again aids in reducing this resulting phase
difference, as indicated above.
Polarizer 15 has a port on either end thereof with the electromagnetic wave
transmission medium contained therein positioned therebetween. Positioned
against the port on the end thereof opposite that connected to optical
directional coupler 12 is another optical fiber portion which extends to a
further optical bidirectional coupler, 17, which has the same wave
transmission properties as does coupler 12.
The port on the same end of coupler 17 from which a port is coupled to
polarizer 15 again is connected to a non-reflective termination
arrangement, 18, using a further optical fiber portion. Considering the
ports on the other end of coupler 17, one is connected to further optical
components in the optical path portions extending thereto from one end of
the optical fiber in coil 10. The other port in coupler 17 is directly
coupled to the remaining end of optical fiber 10. Between coil 10 and
coupler 17, on the side of coil 10 opposite the directly connected side
thereof, is provided an optical phase modulator, 19. Optical phase
modulator 19 has two ports on either end of the transmission media
contained therein shown on the opposite ends thereof in FIG. 1. The
optical fiber from coil 10 is positioned against a port of modulator 19.
The optical fiber extending from coupler 17 is positioned against the
other port of modulator 19.
Optical modulator 19 is capable of receiving electrical signals to cause it
to introduce a phase difference in electromagnetic waves transmitted
therethrough by changing the index of refraction of the transmission
medium, or transmission media, therein to thereby change the optical path
length. Such electrical signals are supplied to modulator 19 by a bias
modulation signal generator, 20, providing a sinusoidal voltage output
signal at a modulation frequency f.sub.g that is intended to be equal to
C.sub.1 sin(.omega..sub.g t) where .omega..sub.g is the radian frequency
equivalent of the modulation frequency f.sub.g. Other suitable periodic
waveforms could alternatively be used.
This completes the description of the optical portion of the system of FIG.
1 formed along the optical path followed by the electromagnetic waves, or
light waves, emitted by source 11. Such electromagnetic waves are coupled
from that source through the optical fiber portion to optical directional
coupler 12. Some of such wave entering coupler 12 from source 11 is lost
in non-reflecting terminating arrangement 16 coupled to a port on the
opposite end thereof, but the rest of that wave is transmitted through
polarizer 15 to optical directional coupler 17.
Coupler 17 serves as a beam-splitting apparatus in which electromagnetic
waves entering the port thereof, received from polarizer 15, split
approximately in half with one portion thereof passing out of each of the
two ports on the opposite ends thereof. Out of one port on the opposite
end of coupler 17 an electromagnetic wave passes through optical fiber
coil 10, modulator 19, and back to coupler 17. There, a portion of this
returning wave is lost in non-reflective arrangement 18 connected to the
other port on the polarizer 15 connection end of coupler 17, but the rest
of that wave passes through the other port of coupler 17 to polarizer 15
and to coupler 12 where a portion of it is transmitted to photodiode 13.
The other part of the wave passed from polarizer 15 to coil 10 leaves the
other port on the coil 10 end of coupler 17, passes through modulator 19,
and optical fiber coil 10 to re-enter coupler 17 and, again, with a
portion thereof following the same path as the other portion to finally
impinge on photodiode 13.
As indicated above, photodiode 13 provides an output photocurrent,
i.sub.PD.sbsb.13, proportional to the intensity of the two electromagnetic
waves or light waves impinging thereon, and is therefore expected to
follow the cosine of the phase difference between these two waves
impinging on that diode as given by the following equation:
##EQU1##
This is because the current depends on the resulting optical intensity of
the two substantially coherent waves incident on photodiode 13, an
intensity which will vary from a peak value of I.sub.o to a smaller value
depending on how much constructive or destructive interference occurs
between the two waves. This interference of waves will change with
rotation of the coiled optical fiber forming coil 10 about its axis as
such rotation introduces a phase difference shift of .phi..sub.R between
the waves. Further, there is an additional variable phase shift introduced
in this photodiode output current by modulator 19 with an amplitude value
of .phi..sub.m and which is intended to vary as cos(.omega..sub.g t).
Optical phase modulator 19 is of the kind described above and is used in
conjunction with a phase-sensitive detector as part of a demodulation
system for converting the output signal of photodetection system 14,
following a cosine function as indicated above, to a signal following a
sine function. Following such a sine function provides in that output
signal, as indicated above, information both as to the rate of rotation
and the direction of that rotation about the axis of coil 10.
Thus, the output signal from photodetection system 14, including photodiode
13, is converted to a voltage and provided through an amplifier, 21, where
it is amplified and passed through a filter, 22, to such a phase sensitive
detector means, 23. Phase-sensitive detector 23, serving as part of a
phase demodulation system, is a well known device. Such a phase-sensitive
detector extracts the amplitude of the first harmonic of the filtered
photodiode system output signal, or the fundamental frequency of
modulation signal generator 20, to provide an indication of the relative
phase of the electromagnetic waves impinging on photodiode 13. This
information is provided by phase-sensitive detector 23 in an output signal
following a sine function, that is, this output signal follows the sine of
the phase difference between the two electromagnetic waves impinging on
photodiode 13.
Bias modulator signal generator 20, in modulating the light in the optical
path at the frequency f.sub.g described above, also leads to harmonic
components being generated by the recombined electromagnetic waves in
photodetection system 14. Filter 22 is a bandpass filter which is to pass
the modulation frequency component of the output signal of photodetector
14, i.e. the first harmonic, after its amplification by amplifier 21.
In operation, the phase difference changes in the two opposite direction
propagating electromagnetic waves passing through coil 10 in the optical
path, because of rotation, will vary relatively slowly compared with the
phase difference changes due to modulator 19. Any phase differences due to
rotation, or the Sagnac effect, will merely shift the phase differences
between the two electromagnetic waves. The amplitude scaling factor of the
modulation frequency component of the output signal of photodetection
system 14, appearing at the output of filter 22, is expected to be set by
the sine of this phase difference modified further only by the factors of
a) the amplitude value of the phase modulation of these waves due to
modulator 19 and generator 20, and b) a constant representing the various
gains through the system. Then, the periodic effects of this sinusoidal
modulation due to generator 20 and modulator 19 in this signal component
are expected to be removed by demodulation in the system containing
phase-sensitive detector 23 leaving a demodulator system (detector) output
signal depending on just the amplitude scaling factor thereof.
Thus, the voltage at the output of amplifier 21 will typically appear as:
V.sub.21-out =k{1+cos [.phi..sub.R+.phi.m cos(.omega..sub.g t+.theta.]}
The constant k represents the gains through the system to the output of
amplifier 21. The symbol, .theta., represents additional phase delay in
the output signal of amplifier 21 with respect to the phase of the signal
provided by generator 20. Some of this phase shift will be introduced in
photodetection system 14, and some will be due from other sources such as
a phase shift across modulator 19 between the phase of the signals
supplied by generator 20 and the response of modulator 19 in having the
index of refraction of the media therein, and/or its length,
correspondingly change. The other symbols used in the preceding equation
have the same meaning as they did in the first equation above.
The foregoing equation can be expanded in a Bessel series expansion to give
the following:
##EQU2##
This signal at the output of amplifier 21 is applied to the input of
filter 22.
Filter 22, as indicated above, passes primarily the first harmonic from the
last equation, i.e. the modulation frequency component. As a result, the
output signal of filter 22 can be written as follows:
V.sub.22-out =-2kJ.sub.1 (.phi..sub.m)sin.phi..sub.R cos(.omega..sub.g
t+.theta.+.PSI..sub.1)
The further phase delay term appearing, .PSI..sub.1, is the additional
phase shift in the first harmonic term added as a result of passing
through filter 22. This added phase shift is expected to be substantially
constant and a known characteristic of filter 22.
The signal from filter 22 is then applied to phase-sensitive detector 23,
as is the signal from bias modulator generator 20, the latter again
intended to be equal to C.sub.1 sin(.omega..sub.g t) where .omega..sub.g
is the radian frequency equivalent of the modulation frequency f.sub.g.
Assuming that a phase shift equal to .theta.+.omega..sub.1 can be added by
phase-sensitive detector 23 to its output signal, the output of that
detector with such a generator 20 output signal will then be the
following:
V.sub.23-out =k'J.sub.1 (.phi..sub.m)sin.phi..sub.R
The constant k' accounts for the system gains through phase-sensitive
detector 23.
However, these expected results may not be achieved in the system of FIG.
1. One reason for failing to achieve the expected results is that bias
modulation signal generator 20, in modulating the light in the optical
path at frequency f.sub.g as described above through phase modulator 19,
not only results in harmonic components being generated in photodetection
system 14 by the recombined electromagnetic waves, but also directly
supplies some harmonic components in the varying optical path phase
because of nonlinearities occurring both in generator 20 and modulator 19.
That is, as a first possibility, the output signal supplied by modulation
generator 20 at its output may contain not only a fundamental signal at
frequency f.sub.g, but also significant harmonics thereof. Even if a
signal free of such harmonics could be provided, nonlinear component
characteristics and hysteresis in phase modulator 19 can result in
introducing such harmonics into the varying phase provided thereby in the
optical path. Such harmonics can lead to significant rate bias errors in
the output signal of the fiber optic gyroscope. Thus, there is desired an
interferometric fiber optic gyroscope in which such errors due to the
modulation system are reduced or eliminated.
SUMMARY OF THE INVENTION
The present invention provides an error control arrangement for an optical
fiber rotation sensor based on electromagnetic waves propagating in
opposite directions in said coiled optical fiber to impinge on a
photodetector with a phase relationship. These electromagnetic waves
propagating in opposite directions both pass through a bias optical phase
modulator operated by a phase modulation generator both of which can
contribute second harmonic distortion resulting in errors in the sensor
output signal. Direct limitation of both contributions to keep them less
than the equivalent output error limitation, or indirect limitation by a
limiting factor to keep the combination of the factor and the
contributions less than the equivalent output error limitation can be used
to provide an acceptable sensor. Control of the bias optical phase
modulator contribution for a modulator having a piezoelectric body wrapped
with an optical fiber portion is accomplished by mounting the body
utilizing layers having nonlinear stiffness.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a system schematic diagram of the present invention combining
a signal processing arrangement and an optical transmission path and
device arrangement; and
FIGS. 2A and 2B show a modulator system of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Typically, just the next harmonic after the fundamental in the time varying
phase imposed on the optical path to and from coil 10 by phase modulator
19 and modulation generator 20 of the bias modulation subsystem has an
amplitude significant enough to cause significant errors. Hence, only the
second harmonic need be considered. Thus, modulation signal generator can
be considered, in the absence of measures being taken to eliminate same,
to provide an output signal, particularly at higher output voltage
amplitudes, that varies as:
V.sub.20 =C.sub.2 [cos.omega..sub.g t+.delta..sub.e cos(2.omega..sub.g
t+.PSI.'.sub.e)]
rather than as C.sub.1 sin(.omega..sub.g t) where the change from a sine
function representation to a cosine function representation is an
arbitrary choice.
In this representation of the output signal of generator 20, .delta..sub.e
is the relative amplitude of the unwanted second harmonic signal
distorting the desired output, relative to the amplitude of the
fundamental component, and C.sub.2 is a general gain constant for
generator 20 which is set at a value sufficient to provide the fundamental
output signal component therefrom at a desired amplitude. The phase,
.PSI.'.sub.e, developed in the generation of the second harmonic
component, has been arbitrarily chosen relative to the zero phase value
arbitrarily selected for the fundamental signal.
Phase modulator 19 may be a ceramic material body exhibiting piezoelectric
effects, and have a portion of the optical fiber leading to coil 10 from
loop coupler 17 wrapped in several turns therearound. This ceramic body is
typically a truncated portion of a hollow cylinder (a ring) formed of a
material such as lead zirconate titanate (PZT), and the electrical leads
interconnecting this ring and leading to interconnections with generator
20 are typically placed one each on the outside and the inside curved
surfaces of the truncated cylindrical body. Under electrical energization,
the ring exhibits behavior as an electrical circuit component whi | | |
