Note: Descriptions are shown in the official language in which they were submitted.
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OPTICAL-FIBRE MEASURING DEVICE, GYROMETER,
CENTRAL NAVIGATION AND STA9ILIZING SYSTEM
BACKGROUND OF THE INVENTION
The invention relates to an optical-fibre mea-
suring device permitting the measurement of the variation
of a parameter which produces non-reciProcal disturbances
in a SAGNAC ring interferometer.
The SAGNAC interferometer and the Physical phe-
nomena which it utilizes are well known. In such an
interferometer, a beam sPlitter or some other separating
device divides an incident wave. The two oppositelY
propagating waves thus created propagate in opposite
directions along a closed optical Path, recombine and
generate interferences which are dependent uPon the phase
shift of the waves in the course of their recombination.
Originally, the closed oPtical path of the
SAGNAC interferometers was defined by mirrors. It is now
known that it can be formed bY a multi-turn coil of
monomode oPtical fibre.
lt is likewise known that certain physical phe-
nomena are capable of producing disturbances, particu-
larly Phase shifts, which are non-reciprocal, on the
oppositely Propagating waves giving rise to a relative
phase shift of these waves which modify their state of
interference in the course of their recombination.
The measurement of this relative Phase shift
permits the quantification of the physical phenomenon
which has given rise to it.
The principal physical phenomenon capable of
creating these non-reciprocal disturbances i9 the SAGNAC
~ effect produced bY the rotation of the interferometer in
- relation to an axis Perpendicular to the plane of its
closed optical path. The FARADAY effect, or collinear
magneto-optical effect, is likewise known as producing
non-reciprocal effects of this tYpe. This has, for
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example, been described in an article in the journal
Optic Letters (Vol. 7, No. 4, April 1982, pages 1~0-182)
by K. BoHM. Under certain conditions, other effects may
likewise produce a non-reciprocal phase shift.
On the other hand, the variations of numerous
parameters representing the environment which frequently
give rise to disturbances of the measurements have only
reciprocal effects on the SAGNAC interferometer, do not
disturb the relative Phase shift between the oppositely
propagating waves and therefore have no influence on the
measurement of the parameter under investigation. This is
50 in the case of slow variations of temperature, of
indices etc.... which modifY the optical Path traversed
by the waves, but modify it in a reciprocal manner.
lS Numerous Projects have been undertaken for the
purpose of imProving the sensitivitY and the precision of
the measurements which can be made with such a measuring
apparatus. It will be possible, for example, to consult
the document GB 2 152 207 and the publication Electronics
Letters (Vol. 19 No. 23, November 1983 Pages 997-999, an
article bY K. BoHM).
ln Particular, it has first of all been found
that the response provided bY the SAGNAC interferometer
is of the form P = Po(1 + cos. a~ ) and thus that the
sensitivity of this signal close to the phase difference
0 is low. It has been proposed to introduce a
~, phase difference modulation, squared with an amPlitude of
plus or minus ~/2 for examPle, which displaces the
operating Point and produces a periodic signal, the
amplitude of which i~ a sinusoidal function of the
measured Parameter and which it is therefore possible to
use with a greater sensitivity and stability.
It was then shown that the precision of the
measurement is improved bY the use of a zero method which
is likewise referred to as closed-loop oPeration.
According to this method, a suPplement~rY Phase shift
referred to as a feedback phase shift ~-~ is aPplied and
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serves to compensate the phase shift A~ produced by the
measured parameter. The sum of these two phase shifts~-~
and ~ is maintained at zero; this permits the
interferometer to be operated with the maximum Precision~
The measurement is made bY use of the signal required
for the production of the feedback phase shift
Thus, the measurement is stable and linear.
The control may be produced by generating phase
progressions of a height ~ at each time T~ T being the
proPagation time in the coil, the phase modulator or
modulators being Placed at the ends of the coil.
The EurGpean P~tent EP 0,168,292 describes such
a measuring system. Accor,ding to the device which it
proposes, the signal produced by variation of the mea-
sured parameter Produces a variation of the output signal
of the detector. The amplitude of this variation is
extracted by circuits for analog sYnchronous detection
which, after analog processing by a DIP (differential,
integral, proPortional) filter, conventionally ensures
the stability of the control loop. An analog-digital
converter gives the digital value of the progression
~ which has been i~troduced in order to ensure the
compensation, and there are added a control signal
generator, the purpo~e of which is to formulate a digital
ramp in steps, and finallY a digital-analog converter
which generates the drive signal returning from the phase
modulator, on the basis of this ramp.
In seeking a maximum sensitivity and precision
of the measurement, the devices of the prior art exhibit
certain disadvantages:
. ~n analog synchronous detection (also re-
ferred to as demodulation) generally involves a zero
drift at the output which is reflected in an error in the
measurement.
. The analog-digital converter required to con-
struct this device must be able to Process the comPen-
sation phase shift directly. It must include a number of
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bits linked to the maximum amplitude of the signal at its
input; this leads, in practice, in order that it should
not limit the precision of the measurement, to an analog-
digital converter including a number of bits of the order
of 18.
SUMMARY OF THE INVENTION
The object of the Present invention is there-
fore to improve the precision and the sensitivitY of a
measuring device based on a SAGNAC ring interferometer
incorporating a monomode optical fibre,
It is likewise an object in such a measuring
device to overcome any pos~ible zero drifts.
It is a further object of the Present invention
to permit the obtaining of a measurement of quality equal
to or greater than that available by the device of the
Prior art with a less ~ophisticated analog-digital
converter.
In order to achieve these objects, the subiect
of the invention is an oPtical-fibre measuring device of
the type in which the variation of a parameter causes a
difference of phase, comprising a quasi-monochromatic
light source, a SAGNAC ring interferometer incorPorating
a monomode oPtical fibre, a detector, a phase modulator,
a polarizer and a spatial filter which are placed between
the source and the interferometer, electronic means
controlling in feedback the phase modulator as a function
of the signal received from the detector in such a manner
that, on the one hand, the variation of the demodulated
error signal as a function of the difference of Phase
close to the zero is sinusoidal, and that, on the other
hand, this difference of phase i5 maintained at zero, and
supplying, by utilizing the control signal of the
modulator, a signal which is a function of the variation
of the measured parameter.
In ~uch a device, it is proposed that the said
- electronic means comPriSe an analog-digital converter
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intended to digitalize the signal originating from the
detector, a digital processing system utilizing the
signal suPplied bY the analog-digital converter and re-
ducing its component to the frequency of the modulation
around the continuous (dc), a control loop digital filter
supplied by the signal emerging from the digital
processing system, supplying a signal representing the
measured parameter, a register receiving the signal
emerging from the control loop digital filter and
~upplying ~ signal which is a function of the measured
parameter for any desired external use, an ~ccumulator
supPlied by the signal emerging from the regi~ter,
generating a digital ramP~ the sloPe of which i8 a
function of the measured rotation, a digital-analog
converter supplied bY the feedback signal th~t can be a
ramp emanating from the accumulator and controlling the
phase modulator.
The subject of the invention is also a
gyrometer in accordance with the measuring device defined
hereinabove in which the measured parameter i3 the speed
of rotation of the interferometer about its axis.
The subject of the invention is furthermore a
central inertial stabilization or navigation system
comprising at least one gyrometer as defined hereinabove.
~RIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood on
readin~ the description which will follow, which i5 given
with reference to the drawings, in which:
. Figure 1 is a known measuring device based
on a SAGNAC ring interferometer.
. Figure 2 represents the electronic means
for the digital Processins of the signal according to the
invention.
. Figure 3 represents the analog modulation
signal resulting after conversion from the digital
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addition of the feedback signal in the particular case of
a ramP and of the squared modulation.
The optical-fibre measuring device of the in-
vention comprises a quasi-monochromatic light source
which is most frequentlY a laser or a super luminescent
diode, and a SAGNAC ring interferometer incorPoratins a
monomode optical fibre, which interferometer is desig-
nated overall by the reference 2.
This ring interferometer 2 comprises a beam
sPlitter 3 ensuring the separation of the waves at the
entrance of the interferometer, and then their recombi-
nation at the exit, and a closed optical path 4 consis-
ting of a monomode oPtical fibre wound uP on itself.
This measuring device further comprises a de-
tector 5 suPPlying a signal which is a function of the
state of interference of the waves at the exit of the
interferometer itself.
The oPtical signal is supplied to the detector
5 via a beam splitter 6, for examPle consisting of a
semi-transparent Plate.
The interferometer is adiusted in such a manner
that the waves are parallel in the course of their re-
combination, the signal suPPlied by the detector thus
being a function of the phase shift between these two
waves.
On the optical path of the interferometer there
is interposed a modulator 7 which, being controlled on
the basis of an electrical signal, is capable of intro-
ducing a given phase shift of the two waves. The
operation of the interferometer is improved bY inter-
: posing between the light source 1 and the entrance of the
ring, that is to say the beam sPlitter 3, a polarizer 8
and a spatial filter 9. This spatial filter is compo~ed
of a monomode oPtical fibre.
In the text which follows, we shall use without
distinction the terms "phase shift" and "difference of
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phase" to de~ignate the physical effects produced in the
interferometer.
Electronic means 10 control in feedback the
phase modulator 7, as a function of the signal received
from the detector 5.
These electronic means 10 are designed in such
a manner that, on the one hand, the variation of the
demodulated error signal as a function of the difference
of progression produced between the two waves close to
the zero is sinusoidal. This arrangement permits a verY
good sensitivity of the variation of the demodulated
error signal close to the zero of the difference of
progression to be obtain,ed, while it can easi 1Y be
understoo~ that, when the dependency of the signal in
~ 15 relation to the difference of phase is of cosinusoidal
'~ form, the sensitivity close to the zero of the difference
~;, of phase is very low.
v On the other hand, the function of these elec-
tronic means 10 is to maintain the difference of phase at
zero. That is to say that, when the variation of the
- measured Parameter introduces a Phase shift between the
; two waves in the interferometer, this Phase shift
`~ produces a variation of the signal emitted by the
detector S involving, via the electronic means 10 and the
phase modulator 7, an action equal and of opposite
direction to the Phase shift initially Produced in such a
.. manner that the total Phase shift is reduced to the value:~ O.
Finally, these electronic means 10 suPPly, bY
use of the control signal of the phase modulator 7, a
signal which is a function of the variation of the mea-
sured parameter.
According to the invention, the electronic
means 10 comprise an analog-digital converter 11 intended
to digitalize the signal emitted by the detector. The
digital signal emanating from this analog-digital
converter is transmitted to a digital processing system
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12 utilizing this signal and reducing its comPonent to
the frequencY of modulation around the continuous in such
a manner as to extract the genuinelY significant signal.
It is a significant element of the invention to carry out
S the digitalization of the signal at the outPut of the
detector before carrying out its digital processing.
The electronic means 10 comprise a control looP
digital filter 13 which is suPplied bY the low signal
emerging from the digital Processing system which ensures
an operation with low error, low response time and good
stability of the control. This may be a digital
accumulator. This filter 13 supplies a signal
representing the measured parameter.
The electronic means 10 comprise a register 14
receiving the signal emerging from the digital filter 13
supplying a signal which is a function of the measured
parameter for any desired external use.
An accumulator (15) supplied by the signal
emerging from the register (14) generates a feedback
signal which is a function of the measured parameter.
Preferably, the feedback signal is a digital
` ramp 17, the slope of which is a function of the measured
parameter. After addition of a modulation bY the digital
adder 18, a digital-analog converter 16 suPplied bY the
ramp signal 17 emanating from the accumulator 15 controls
the Phase modulator 7.
The operation of the measuring device of the
invention is the following: when the measured parameter
is stable, the electronic control means 10 introduce, via
the phase modulator 7, a constant amPlitude modulation of
- the Phase shift between the oppositely propagating waves
within the ring 4. The detector 5 thus produces a
modulated signal which is digitalized bY the converter 11
and then processed by the digital processing system 12,
which supplies a zero signal to the accumulator 15
maintaining the register 14 at a constant value: this
maintains the ramp 17 in its condition and thus maintains
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the signal supplied to the modulator 7. It is thus
verified that this condition is stable.
In the course of a variation of the measured
parameter, a constant phase shift is superposed on the
periodic phase shift corresponding to the st~ble condi-
tion between the oppositely Propasatins waves at the
location of the ring 4. The signal then supplied by the
detector 5 after digitalization bY the sampler 11 and
processins by the disital processing system 12 thus has a
level different from zero which produces a supPlY at the
location of the accumulator 15 and a variation of the
parameter register 14. This variation involves a
modification of the ramp generated at the location 15 and
thus of the phase shift introduced bY the modulator 7,
thus reducing the phase shift between the oppositely
propagating waves at the output of the ring 4 to a zero
value with the exception of the Periodic modulation
mentioned hereinabove.
It is thus understood that the samPling pro-
posed according to the invention before the digital
; Processins is apPlied to a signal which is a function of
- the variation of the measured parameter and not of the
absolute value of this parameter. This permits the
utilization of a sampler processing a number of bits
which is limited, for example 8, while still maintaining
the precision and the quality of the measurement.
,', According to a Preferred embodiment, the digi-
, tal processing system 12 of the invention cl~ssifies
alternately the digitalized samPles in two classes, of
which it forms the digital mean values, which it then
compares in order to deduce the error signal therefrom.
-` This arrangment permits the avoidance of the effects of a
possible zero drift. Such an arrangement is made Possible
by the digitalization of the signal before its digital
processing.
The neutralization of this drift is all the
more important as in this tYPe of measuring aPParatus
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there is frequently performed an integration of the
measured Parameter over a long period which is very
sensitive to the zero drifts.
Figure 3 shows the digital addition of the ramP
and of the squared modulation 17 which makes uP the ex-
citation signal formulated bY the electronic means 10 in
order to control the phase modulator 7, in a condition in
which the measured parameter is constant. On this graph,
time is rePresented as abscissa and the phase shift as
ordinate. The value of the measured parameter is a
function of the phase shift existing between two
successive Periods of the squared function. The right
hand part of this graph rePresents the phenomena known
per se which take place in the course of overflow of a
; 15 register used in the course of the generation of the ramP,
According to a preferred embodiment, the mea-
suring device of the invention is characterized in that
the sampler oPerates at an aPproximate frequencY of 1 MHz
for a fibre length equal to approximately 200 m.
Under such conditions, the device of the inven-
tion which Permits the reduction of the sampling dynamics
permits the use of a, for examPle, 8-bit analog-digital
converter, while the register 14 containing the value of
-~ 25 the measured parameter ensures a Precision corresPonding
to its size which is of aPProximately 17 to 26 bits.
The construction of a register of this size
rePresents onlY a slight constraint since, in wired
logic, it can be obtained with a plurality of adders, for
example parallel 8-bit adders.
The measuring device of the invention is Par-
ticularly well suited to the construction of a gYrometer.
In this case, the measured parameter is the sPeed of
~; rotation of the interferometer about its axis.
This gyrometer is advantageouslY included in
the construction of inertial stabilization or navigation
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control systems.
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Such an arrangement is likewise verY well
suited to the construction of the device for me~suring
magnetic fields and electric current utilizing the
FARADAY effect.
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