Interferometric signal analysis with modulation switching

In a fiber optic rotation sensor, two phase modulators are symmetrically offset with respect to the center of an optical fiber loop and alternately excited by a waveform whose frequency alters the differential phase between counter-propagating light waves in the loop to maintain a fixed intensity signal at a detector upon loop rotation. The difference between the waveform frequencies is proportional to the speed of sensor rotation while the mean value of the waveform frequencies equals the frequency value in the absence of sensor rotation. In the alternative, a single modulator is excited by a waveform whose polarity is reversed at intervals of time.

TECHNICAL FIELD 
This invention relates to optical interferometers, and more particularly to 
a Sagnac interferometer having modulation switching in a feedback loop 
thereof to improve rotation rate measurement accuracy. 
BACKGROUND ART 
Interferometric signal analysis involves the determination of signal 
wavelength, wave velocities, distances and directions using interference 
phenomena between, e.g., two coherent optical signals. Particular 
application may include optical signal analysis in a fiber optic gyroscope 
(FOG). 
A FOG typically includes a light source, e.g., a laser diode, which 
provides coherent light split into two substantially equal waves by a 
beamsplitter. The beamsplitter is coupled to the ends of a length of 
optical fiber wound in a coil. The light waves are launched into each end 
of the coil, and are recombined interferometrically at the coil output 
such that the light intensity seen by a detector depends on the relative 
phases of these waves. 
When the coil is subject to rotation about an axis normal to the coil, the 
counter-propagating waves take different times to traverse the coil. This 
non-reciprocal phenomenon, known as the Sagnac effect, causes a change 
(shift) in the relative phase between the waves reaching the detector and, 
therefore, a change in the light intensity signal at the detector. 
Depending on the initial phase difference, which can be controlled, for 
example, by application of suitable phase modulation at one end of the 
coil, the magnitude and sense of the change in the light intensity signal 
depend, respectively, on the rate and direction of the rotation applied to 
the coil about the axis. 
The rotation-induced change can be compensated for, i.e., nulled, by 
imposing between the waves a further phase difference, equal and opposite 
to the Sagnac phase difference. Various methods are known in the art of 
imposing such a phase difference. For example, a lithium niobate 
integrated optical phase modulator at one end of the coil can be driven by 
an appropriate waveform, such as an analog or stepped digital periodic 
ramp, i.e., a serrodyne waveform. Control of an appropriate waveform 
parameter, e.g., frequency, in response to rotation alters the 
modulator-induced phase difference such that it nulls the rotation-induced 
difference. The value of the parameter that produces the desired null 
serves as a measure of the rotation rate. For example, in an appropriately 
initialized FOG employing a fixed-amplitude serrodyne modulation waveform, 
the serrodyne (ramp) frequency change needed to null the Sagnac phase is 
proportional to the rotation rate; and the sign, i.e., the direction, of 
this change indicates the rotation direction. 
More specifically, when a serrodyne FOG is not subject to rotation, there 
are various ramp frequency values, including zero, for which the gyro 
output is nulled. Frequently, FOG initialization selects "zero" as the 
operating "setpoint". This value has the advantaqe that it is independent 
of physical parameters of the FOG. However, the ramp polarity must then 
reverse whenever the rotation direction reverses, to avoid the need for 
"negative" frequencies. If the ramp reversal is imperfect, large 
scale-factor errors may adversely affect low-rate measurements. An 
alternative to this approach is to select one of the non-zero 
output-nulling frequencies f.sub.0 as the setpoint. If the FOG is then 
subject to rotation about an axis normal to the coil, and the ramp 
frequency is controlled to null the Sagnac phase shift, the difference 
between the new frequency and the setpoint f.sub.0 serves as a measure of 
the rotation rate and represents the output of this closed-loop FOG. If 
the selected value f.sub.0 is sufficiently large, the ramp frequency 
remains positive even at the largest rotation rates, and rate-dependent 
ramp polarity reversals are avoided. 
However, the non-zero setpoint values typically depend on physical 
parameters of the FOG, which may be subject to drift, thereby degrading 
the accuracy in measuring FOG rotation rate. Setpoint drift may be caused 
by changes in the environment in which the FOG must operate, e.g., changes 
in temperature, and usually can not be precisely predicted. Unpredictable 
errors produced by such drift in a FOG operated as described above make it 
unsuitable for use in systems requiring highly accurate rotation sensing. 
Similar arguments are applicable to other closed-loop interferometric 
sensors in which the phase shift response to a measurand can be nulled by 
controlling one or more phase modulation parameters in proportion to the 
sense and magnitude of the measurand. 
DISCLOSURE OF INVENTION 
Objects of the present invention include provision of an interferometric 
optical sensor employing improved closed-loop signal analysis which 
increases sensor sensitivity to measurands such as sensor rotation, 
reduces sensitivity to variations of certain sensor parameters which cause 
setpoint drift, and provides, while measurements are being made, an 
accurate indication of what the sensor output would be in the absence of a 
measurand, such indication being helpful in evaluating certain parameters 
of the sensor itself. 
According to a first aspect of the present invention, two matched phase 
modulators are symmetrically offset with respect to the center of a 
sensing loop of optical waveguide in a Sagnac interferometer sensor, the 
modulators are alternately excited by a waveform, specific parameters 
(e.g., frequency) of which are controlled by a detector feedback signal to 
alter the differential phase between counter-propagating waves which pass 
through the modulators and the loop, to maintain thereby a fixed intensity 
signal at the detector when the sensor is subjected to a non-reciprocal 
perturbation, such as an arbitrary rotation rate, the difference between 
the waveform parameters applied to the two modulators being, preferably, 
proportional to the applied perturbation, the mean value of the controlled 
waveform parameters applied to the two modulators equalling approximately 
the parameter value which would be assumed in the absence of the 
perturbation. 
According to a second aspect of the present invention, a phase modulator is 
offset from the center of a sensing loop of optical waveguide in a Sagnac 
interferometer sensor, the modulator is alternately excited by a signal 
with specific waveform parameters (e.g., frequency), then by a signal of 
reverse polarity with changed values of these parameters, the values being 
controlled by a detector feedback signal to control the differential phase 
between counter-propagating waves which pass through the modulator and the 
loop, to maintain thereby a fixed intensity signal at the detector when 
the sensor is subjected to a non-reciprocal perturbation, such as an 
arbitrary rotation rate, the difference between the controlled waveform 
parameters applied alternately to the modulator being, preferably, 
proportional to applied perturbation, the mean value of these waveform 
parameters equalling approximately the parameter value which would be 
assumed in the absence of the perturbation. 
The present invention represents a significant advancement over previous 
methods of determining sensor rotation. Modulator induced phase 
differences produced between counter-propagating light waves in a loop of 
optical waveguide are an odd-symmetric function of both modulator offset 
with respect to loop center and polarity of the modulation. Thus, in a 
closed loop Sagnac interferometer which maintains a fixed optical output 
intensity signal, the difference between the waveform parameters applied 
alternately to two modulators symmetrically offset with respect to the 
sensing loop in accordance with a first aspect of the invention, and 
similarly the difference between the waveform parameters applied to a 
single modulator alternately operated at reverse polarities in accordance 
with a second aspect of the invention, provides a sensor output which is 
doubly sensitive to non-reciprocal perturbations such as sensor rotation 
rate and is insensitive to setpoint drift resulting from variations of 
certain sensor parameters. 
Other objects, features and advantages of the present invention will become 
more apparent in the light of the following detailed description of an 
exemplary embodiment thereof, as illustrated in the accompanying drawing.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to FIG. 1, a closed loop interferometric optical sensor; more 
specifically, a fiber optic gyroscope (FOG) 10, includes a light source 
12, e.g., a laser diode or superluminescent diode. Light from the source 
12 is coupled by known means, e.g., through a first optical fiber 14, to a 
first port 16 of an optical assembly 18. 
Assembly 18 comprises known optical components such as beamsplitters (e.g., 
four-port fused-fiber star couplers or integrated optical Y-couplers), 
polarizers, and single-mode elements (e.g., single-mode fibers or 
waveguides). The components are arranged such that light which has entered 
port 16 is split by known means, e.g., a Y-shaped integrated optical (IO) 
waveguide, preferably after having passed through a single-mode 
single-polarization filter component, which may be, e.g., a known 
metallized optical waveguide segment, into two waves which may be of 
approximately equal intensities and which exit from the assembly through 
second and third ports 20,22. 
Ports 20,22 are optically coupled by known means, e.g., through second and 
third optical fibers 24,26, to first and second ports 28,30, respectively, 
of a modulator assembly 32, which may alternatively be an integral part of 
optical assembly 18. Modulator assembly 32 may be fabricated on an 
integrated optical device. Or, beamsplitter 80 and modulator assembly 32 
may be fabricated on an integrated optical device. Assembly 18 and 
modulator assembly 32 of FIGS. 1 and 2 may represent integrated optical 
devices. Waves entering ports 28,30 are directed, e.g., through optical 
fibers or waveguides, to a pair of optical phase modulators 34,36, 
respectively. Each modulator 34,36 may comprise, e.g., an optical 
waveguide sandwiched between a pair of parallel electrodes according to 
known techniques. The modulators are selectively driven by a modulation 
signal on lines 38,40 via a switch 42, the signal being provided on a line 
44 from a modulation control circuit 46. After passing through the 
modulators the waves exit the modulator assembly 32 through third and 
fourth ports 48,50. Integrated optics device 82 incorporates assemblies 18 
and 32. 
A fourth optical fiber 52, is wound into a Sagnac sensing loop 54. The 
fiber 52 typically comprises a single-mode fiber, but may be a multi-mode 
fiber if desired. The ends 56,58 of the fiber are coupled by known optical 
means to the modulator assembly ports 48,50. The two waves exiting the 
modulator assembly via ports 48,50 counter-propagate in the loop 54. The 
loop may comprise a plurality of turns of optical fiber which may be wound 
on a cylindrical spool 60. 
After traversing the loop, the waves re-enter the modulator assembly via 
ports 48,50, pass again through the modulators 34,36, and re-enter optical 
assembly 18 via ports 20,22. In the optical assembly the waves are 
recombined by known means, e.g., the aforementioned Y-shaped IO waveguide. 
A portion of the recombined light is split off by known means, e.g., a 
fused-fiber coupler, preferably after again passing through the 
aforementioned single-mode single-polarization filter component to ensure, 
as is known, reciprocal optical paths for the counter-propagating waves in 
the FOG in the absence of rotation. The split-off portion of light exits 
the optical assembly via a fourth port 62 and is directed by known optical 
means, e.g., an optical fiber 64, to a photodetector 66, e.g., a 
photodiode. The remainder of the recombined light typically exits the 
optical assembly via first port 16, and may pass through or be absorbed in 
the source 12. 
The photodetector 66 provides on a line 68, to the modulation control 
circuit 46, an electrical signal proportional to the intensity of the 
optical signal at the fourth port 62 of the optical assembly, this optical 
signal representing the optical output signal of the FOG. 
During FOG operation, the modulation control circuit alternately drives the 
modulators through switch 42 with a modulation signal on the lines 38,40. 
The output of the modulation control circuit is also provided on a line 70 
and, as described hereinafter, represents the output of FOG 10. 
As is well known in the art, the modulation signal in a FOG may be a 
stepped ramp or linear ramp serrodyne signal having a fixed amplitude of 
2.pi. radians and an essentially instantaneous flyback time. The operating 
modulator induces a phase difference between the counter-propagating waves 
because it acts on the waves at different times. 
If the serrodyne frequency is an integer multiple of the loop 
eigenfrequency, f.sub.e, the induced phase difference is zero. Flyback 
discontinuities in the serrodyne waveform make evaluation of the phase 
difference somewhat more complex for other frequencies. However, it is 
known that the effective phase difference, with regard to the behavior of 
the light intensity signal at the photodetector, is substantially 
proportional to small deviations of the serrodyne frequency from an 
integer multiple of the eigenfrequency, the eigenfrequency being given by: 
EQU f.sub.e =1/2.pi.=c/2nL (eq. 1) 
where: c is the speed of light in a vacuum; n is the index of refraction of 
the loop waveguide and L is the length of the loop. The total phase 
difference between counter-propagating waves comprises the sum of this 
modulation induced difference and the Sagnac phase shift. 
The modulation control circuit is typically designed to recognize, as a 
reference, an electrical signal, on the line 68, which is characteristic 
of the reference light intensity signal produced at the photodetector 66 
when the total phase difference is zero, e.g., the signal corresponding to 
a gyro at rest with serrodyne modulation at a multiple of the loop 
eigenfrequency. The modulation control circuit responds to deviations from 
this condition by altering a modulation signal parameter, e.g., the 
serrodyne frequency, to cancel the deviations and maintain the reference 
light intensity signal. 
Thus, when the gyro is at rest, the serrodyne frequency is driven to a 
setpoint value, f.sub.0, which equals a multiple of the loop 
eigenfrequency, f.sub.e. If the gyro rotates at an arbitrary rate, R, the 
serrodyne frequency, in order to null the Sagnac phase shift and maintain 
the reference light intensity signal, changes to a first value, f.sub.1, 
which differs from the setpoint frequency in proportion to the rate. The 
rotation rate is then given by: 
EQU R=K (f.sub.1 -f.sub.0) (eq. 2) 
where K is a proportionality factor. 
If the rate, R, remains constant and the modulation signal is switched to 
the other modulator, located at the opposite end of the loop, the 
effective polarity of the modulation-induced phase difference between 
waves is reversed and the serrodyne frequency, in order to again null the 
Sagnac phase shift, changes to a second value, f.sub.2, such that : 
EQU -R=K (f.sub.2 -f.sub.0) (eq. 3) 
Combining equations 2 and 3 yields: 
EQU f.sub.1 -f.sub.2 =2R/K (eq. 4) 
which shows that the frequency difference (f.sub.1 -f.sub.2) is doubly 
sensitive to the FOG rotation rate, R, and is independent of the setpoint 
frequency, f.sub.0. This is advantageous because f.sub.0 may sometimes, 
for practical reasons, be set at a large non-zero value which may drift 
with variations of the sensor parameters appearing in Eq. 1. 
An additional aspect of the present invention is that the setpoint 
frequency, f.sub.0, can be easily determined by summing the modulator 
excitation frequencies, as given by: 
EQU f.sub.1 +f.sub.2 =2f.sub.0 (eq. 5) 
If the FOG is initialized, as described above, such that the modulation 
frequency f.sub.0 is equal, in the absence of rotation, to a multiple of 
the loop eigenfrequency, f.sub.e, equation 5 may be used to determine the 
eigenfrequency, and thus the relevant sensing loop parameter combination 
given by equation 1, even when the FOG is subjected to rotation and even 
if the eigenfrequency drifts due to environmental influences or other 
disturbances. 
Equation 1 illustrates the relationship between the eigenfrequency and the 
length, L, and index of refraction, n, of the loop. If the loop 
temperature is measured, permitting temperature-compensated determination 
of L, the resulting value of L, along with the eigenfrequency, as 
determined in equation 5, can be used in equation 1 to calculate n. If the 
temperature and wavelength dependencies of n are known, and if the latter 
dependence is sufficiently strong, it is possible to deduce the wavelength 
corresponding to the calculated value of n at the measured temperature. 
This wavelength can be used in determining the FOG scale factor. 
The frequencies f.sub.1 and f.sub.2, which appear in Eqs. 4 and 5 are 
provided by the modulation control circuit at its output on the line 70. 
This output may, e.g., further provide these frequencies to known signal 
processing apparatus (not shown), which is synchronized with the switch 
42, for evaluation of the rotation rate and setpoint frequency based on 
Eqs. 4 and 5. 
Although the invention is illustrated as being implemented with two 
modulators 34,36 symmetrically offset with respect to the center of the 
loop 54, the invention would work equally as well with just the single 
modulator 36, as illustrated in FIG. 2. The use of the single modulator in 
this alternative embodiment eliminates the modulator 34, switch 42 and 
signal lines 38,40 from the embodiment of FIG. 1. Now, the output of the 
modulation control circuit 46 on the line 44 is input directly to the 
single modulator 36. 
By using a single modulator, when the gyro is at rest, the serrodyne 
frequency is driven to a setpoint value, f.sub.0, which equals an integral 
multiple of the loop eigenfrequency, f.sub.e. If the gyro rotates at an 
arbitrary rate, R, the serrodyne frequency, in order to null the Sagnac 
phase shift and maintain the reference light intensity signal, changes to 
a first value, f.sub.1', which differs from the setpoint frequency in 
proportion to the rotation rate. The rate is then given by: 
EQU R=K (f.sub.1' -f.sub.0 ) (eq. 6) 
where K is a proportionality factor. 
If the rate, R, remains constant and the polarity of the modulation signal 
on the line 44 is switched (reversed), the polarity of the 
modulation-induced phase difference between waves is reversed and the 
serrodyne frequency, in order to again null the Sagnac phase shift, 
changes to a second value, f.sub.2', such that: 
EQU -R=K (f.sub.2' -f.sub.0 ) (eq. 7) 
Combining equations 2 and 3 yields: 
EQU f.sub.1' -f.sub.2' =2R/K (eq. 8) 
EQU f.sub.1' +f.sub.2' =2f.sub.0 (eq. 9) 
Equation 8, which is similar to eq. 4, shows that the frequency difference 
(f.sub.1 -f.sub.2) is doubly sensitive to the FOG rotation rate, R, and is 
independent of setpoint variations, as in eq. 4. Also, eq. 9, which is 
similar to eq. 5, may be used to determine the loop eigenfrequency when 
the setpoint is initialized as an integral multiple of this frequency. 
Knowledge of this eigenfrequency is useful in a FOG which may, as is well 
known, employ additional dithered phase modulation at one end of the loop, 
yielding an easily measured detector signal component at the dither 
frequency, to enhance the FOG sensitivity to rotation. It is known, e.g., 
that setting the dither frequency equal to the loop eigenfrequency reduces 
FOG errors such as those resulting from optical backscatter and from 
spurious intensity modulations. 
Although the invention is illustrated primarily as applied to a rotation 
sensor, it would work equally well in applications where optical phase 
differences between counter-propagating waves in the loop are induced by 
alternative perturbations, e.g., non-reciprocal measurands such as 
magnetic fields or time-varying phase disturbances, rather than rotation. 
Furthermore, although the controlled modulation signal parameter is 
illustrated as being the frequency of a serrodyne ramp signal, parameters 
of alternative known types of modulation, related in similar ways to the 
measurand of interest, may be used. 
In addition, although the invention is illustrated as comprising in part an 
optical assembly which may provide optical splitting, filtering and 
combining operations, along with a separate modulator assembly, the 
invention would work equally as well when implemented with individual 
components to provide these functions, or with some or all of the 
components integrally combined in one or more IO devices, in a manner 
which should be apparent to one skilled in the art in light of the 
teachings herein. 
It suffices for the present invention that, in accordance with a first 
aspect thereof, two matched phase modulators are oppositely, and 
preferably symmetrically, offset with respect to the center of an optical 
waveguide loop in a Sagnac interferometer sensor, the modulators are 
alternately excited by a waveform, specific parameters (e.g., frequency) 
of which are controlled by a detector feedback signal to alter the 
differential phase between counter-propagating waves which pass through 
the loop and the modulators, maintaining thereby a fixed intensity signal 
at a detector when the sensor is subjected to a non-reciprocal 
perturbation, such as rotation, the difference between the waveform 
parameters applied to the two modulators depending in a known, preferably 
linear, way on the magnitude and sense of the perturbation, the mean value 
of the controlled waveform parameters applied to the two modulators 
equalling the parameter value which would be assumed in the absence of the 
perturbation. 
It also suffices that, in accordance with a second aspect of the present 
invention, a phase modulator is offset from the center of the loop, the 
modulator is alternately excited by a signal with specific waveform 
parameters (e.g., frequency), then by a signal of reverse polarity with 
changed values of these parameters, the values being controlled by a 
detector feedback signal to control the differential phase between the 
counter-propagating waves, maintaining thereby a fixed intensity signal at 
the detector when the sensor is subjected to a non-reciprocal 
perturbation, such as rotation, the difference between the controlled 
waveform parameters applied alternately to the modulator depending in a 
known, preferably linear, way on the magnitude and sense of the 
perturbation, the mean value of these waveform parameters equalling the 
parameter value which would be assumed in the absence of rotation. 
Although the invention has been described and illustrated with respect to 
certain exemplary embodiments thereof, it should be understood by those 
skilled in the art that various changes, omissions and additions may be 
made without departing from the spirit and the scope of the invention.