CA1154955A

CA1154955A - Process and device for modulating the phase of the waves traveling in a ring interferometer

Info

Publication number: CA1154955A
Application number: CA000366426A
Authority: CA
Inventors: Herve Arditty; Claude Puech; Michel Papuchon
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1979-12-14
Filing date: 1980-12-09
Publication date: 1983-10-11
Also published as: FR2471583A1; JPH0130317B2; EP0030891A1; EP0030891B1; FR2471583B1; JPS5694680A; DE3066113D1

Abstract

PROCESS AND DEVICE FOR MODULATING THE PHASE OF THE WAVES TRAVELING IN A
RING INTERFEROMETER

ABSTRACT OF THE DISCLOSURE

Procedure consisting in inserting along the ring of a ring interfero-meter at least one reciprocal-effect phase modulator. The .PHI.(t) function, representing the modulation of the phase of the two waves traveling in opposite directions in the ring of the interferometer, is a periodic function confirming the relationship .PHI.(t) = .PHI.(t + 2?), where ? is the time which electromagnetic waves take to travel round the ring. The effect of this modulation is to shift the measuring point on the curve representing the optical power depending on the variation of the phase difference .DELTA.? between the two waves and to obtain a maximum sensitivity of the measurement about the point (.DELTA.?) = 0 by making the power component detected, representing the magnitude to be measured, proportional to sin (.DELTA.?). A control signal generator supplies simultaneously these signals to the modulator and to a synchronous detector.

Description

BACKGROUND OF' T~IE INV~NTION
_ _ The present invention relates to a process and a device for modulating the phase of the waves traveling in a ring interferometer, also called Sagnac interferometer.
Such an interferometer comprises principally a source of light energy formed generally by a laser; an opti-cal device formed either by a number of mirrors, or by an optical fiber wound on itself, th:Ls device forming a wave-guide; a device for separating and mixing the light and a device for detecting and processing the detected signal.
It is known that there exist in these interfero-meters two waves coming from the separating device and traveling in opposite directions over the same optical path.
A fundamental property of ring interferometers is the reciprocity which may be expressed as follows: any dis-turbance of the optical path affects similarly both waves despite that these two waves are not subjected to it exactly at the same time nor in the same direction.
There exist however two types of disturbances which .20 affect reciprocity.
These are, on the one hand, disturbances which vary : :
in:time, within a period of time comparable to the time which the waves take to propagate along the optical path of the interferometer; and on the other hand the so-called "non-reciprocal" disturbances, i.e. disturbances not having the same effect on the waves according as to whether they propa-gate in one direction or in another along the optical path.
It is a question of physical effects which destroy the symmetry of the medium in which the waves are propagated.
Two known effects preseni this latter type of dis-turbance:

- the Faraday effect, or colinear magneto-optic , .

9~

effect, in which a ma~netic field creates a preferential ori-entation of the spin of the electrons of an optical material;
~ and the Sagnac effect, or relativistic inertial ef~ect, in which the rotation of the interferometer with respect to a Gallilean reference destroys the symmetry of the propagation time. This effect is used for constructing gyro-meters in particular.
If no "nonreciprocal" disturbances appear, the phase difference (which will be called hereafter (Q(~)) be-tween the two waves which are recombined in the separatingand mixin~ device after having traveled over the optical path is zero. The detection and processing device detects signals representing the optical power of the composite wave obtained after recombination. This power may be broken down in inter-ferometers of the prior art into two components: a constantcomponent and a component proportional to Cos (Q~), this component only existing on appearance of "nonreciprocal"

: distur:bances.
If it is desired to measure low-amplitude dis-turbancesj for example in the case of gyrometers, low ratesof~rotation, the component containing the Cos (Q~) term varies little, since the phase shift Q~ is close to zero.
It is then necessary to introduce artificially a fixed additional phase shift or "nonreciprocal bias" so as to 25 ~ ncrease the sensitivit~ of the measurement. A particularly ~ interes~ting case is th~at where the new phase shift measured ;~ ; Q~' is such that Q~' = Q~ +;~2 In this case, the sensitivity is maximum since the term to be measured is proportional to Cos (~ + 2)' i.e. to sln (Q~).
: ~
3~0 Although attractive, this process has met up to present with difficulties in its accomplishment and particu-larly the possibility of constructing a device introducing a ~ - 2 -:

suEficie:ntly stable "nonreciprocal bias" to be usable. rrhe instability of the devices of khe prior art is in general o:~
the same order of size as~ the variations of the magnitude to be measured.
Methods for obtaining greater stability of these devices have also been proposed, but the improvement in the sensitivity of the measurement is however much lower than could have been hoped for, since the theoretic maximum sensi-tivity is determined by calculation of the limit due to quantum noise.
SUMMARY OF THE INVENq'ION
To palliate these disadvantages, the invention provides a process for shifting the operating point of a ring interferometer so as to obtain a gain in sensitivity in the measurement of a physical effect intxoducing low amplitude "nonreciprocal" disturbances, this being achieved without requiring either a "nonreciprocal bias" or a great stability of the phenomena brought into play.
The invention provides then a process for modu-20 ~ lating the phase of waves traveling in a ring interferometercomprising means forming a ring waveguide in which two electromagnetic waves travel in opposite directions, a source of electromagnetic energy, means for separating and mixing the electromagnetic waves traveling in the ring and means for : 25 detecting the interference of the waves traveling in the ring and responsive to the phase difference hetween these two : waves; said process comprising the following steps:
- periodic and symmetric modulation of the phase of the waves traveling in the ring in accordance with a periodic functlon ~(t1 confirming the relationship ~(t) = ~(t ~ 2T) where T iS. the time which each of the waves takes to travel : ; over the path defined by the ring;

: ~ 3 -- and dekection at the frequency l/(2~) of the function ~(t) of the phase difference of the two waves traveling in opposite directions ln the ring.
The invention provides further a device for implementing such a process.

DESCRIPTION OF THE DRAWINGS
The invention will be better understood and other advantages will appear from the following description and the accompanying ~igures.

Figure 1 is a ring interferometer of the prior art.
Figure 2 is a diagrarn representing the variation of the optical power detected with respect to the phase differ-ence between the two waves traveling in the ring of the interferometer.
Figure 3 illustrates a process providing a gain in sensitivity in accordance with the prior art.
~: ~ Figures 4 to 8 are diagrams illustrating particular aspects of the process of the invention.
~: Figure 9 illustrates schematically a first vari-20 ~ ation of a device using the process of the invention.
Figure 10 illustrates schematically a second vari-ation of~the device.
Figure ll illustrates schematically a third vari.-~ation of the device in which optical fibers are used.

25~ ~ Figures 12-15 are embodiments of phase modulators used by the invention.

~ DESCRIPTION OF THE PREFERRED EMBODIMENTS
:~ : : Figure l shows schematically a ring interferometer of the prior art. A laser source SO produces a parallel beam :30 of rays l in the direction of a separating device ~ormed by a : plate or semitransparent mirror M.
::
: A number of mirrors (three in Figure 1 : Ml to M3) ~:

9~

define an optical path which forms the riny of the interfero-meter and is looped back to the separating device M which also plays the role of a mixing device and thus defines an output branch 3. The emerging beam is thus directed to a device D for detecting and processing the detected signals.
As is known, two waves propagated in opposite directions travel rGund the ring: one in the clockwise direction (or direction 2)j the other in the anticlockwise direction (or direction 1). These two waves are recombined at separating plate M. The result of this recombination may be observed in the output branch 3.
In the input branch 1 the following equation is con~irmed:

Pe Ple + Ple + 2 V PleP2e Cos (~) (1) In the output branch 3, the following equation is confirmed:

s Pls + P2s 2 V PlsP2s Cos (~) (2) in whlch Pe and Ps~are~the optical powers which may be mea-sured respective}y in the input 1 and output 3 branches and Pni being~the optical power which would be measured in branch ` i if only the wave being propagated in direction n were con-sidered. ~ is the phase difference between the two waves being propagated ~n opposite directions in ring 2 at the moment of recombination. In the absence of a "nonreciprocal"
disturbance, we have seen that this phase difference ~ is :
zero.

If we consider by way of nonlimiting example the partlcular case of a gyrometer using a ring interferometer, a "nonreciprocal" disturbance will be created by setting the gyrometer in rotation. In this case the phase difference : :~
~ ~ is no longer zero. The relationship giving ~ is:
~ ,~

~ ~ - 5 -, : . ' . ; ' -~ , (3) where Q is the speed of rotation and a is a variable given by the relationship:
~ = k- ~c (4) where:
- k is a constant dependent on the geometry oE the gyrometer;
- L the lenyth of the optical path;
- ~ the wavelength oE the light produced by the laser source SO;
- and c the speed of the light in ring 2.
When the speed of rotation Q increases the phase difference Q(~ increases in the same proportions, the coef-ficient ~ of relationship (3) remaining constant. The opti-cal power Ps in output branch 3 develops in accordance with a ~cosinusoidal law from the third term of relationship (2).
The sensitivity of the measurement for a given value Q~ is expressed by the derivative of the function Ps:
.
d (~ 2 V PlS P2S sin (A~) (5) It can be seen that the sensitivity of the inter-ferometer is very low if the phase difference Q~ is not very dlfferent from æero~ This is the case in a gyrometer if it is desired to measure low rotational speeds Q.
The variation of the optical power in output branch

2 is illustrated by the diagram of Figure 2.
It may be assumed that terms PlS and P2S are equal.
It follows~ that for a phase d1fference Q~ = o, the detected power is zero. It passes through a maximum PSmax for Q~ =
;~ and agaln through zero for 2~ and so on.
In order to increase the sensitivity of the inter-30 ~ ferometer, a constant "nonreciprocal bias" may be introduced , ~ in~to the phase of the two waves traveling in opposite -directions so as to shift the operating point of the inter-ferometer.
In the case of a function varying in accordance with a cosinusoidal function, the point of highest sensi-tivity is obtained for angles of t2 k + 1) ~2 ~ with k beinga whole number. A "bias" may then be chosen introducing a phase variation into each wave with an absolute value of ~/4 but of the opposite sign. In the absence of "nonreciprocal"
disturbance, the phase difference Q(~ then becomes Q~' = Q(~ O (6) with ~O = l2 .
The position is then that shown at point Pso on the curve of Figure 2.
A first process for obtaining such a bias is illus-trated by Figure 3.
Beams of slightly divergent light rays are used.
It follows that even in the absence of "nonreciprocal" dis-turbances there appear ring interference fringes which may be observed ln the output branch 3. On the optical axis O of this branch a minimum is observed for the phase difference of 20~ the two recombined~waves remains equal to zero. If we move away~from~the optical axis O, we can observe a series of maxima ml, m2, m3 etc., corresponding to phase variations of ~, 2~, 3~ etc.. due to the divergence. If we take the position at Or where the phase difference is equal to ~/2, the operating point of the interferometer is accordingly shifted. ~
A second~process of the prior art, able to be used ~ in the case of a gyrometer, consists in imparting to the ::
gyrometer an initial speed ~O corresponding to a phase dif~ference Q~O of ~/2, Q~O being given as a function of ~O by the relationship (3).

.

Although attractive, these processes do not however allow all the ~ain in sensitivity expected to be obtained.
In fact, a constant value for the artificially introduced phase variation ~O must be able to be provided. The sta-bility to be attained is of the order of size of 10 5 radianper hour. Typically/ in the case of a gyrometer, the initial speed of rotatlon QO should be about 10+3 radians per hour, with the above-mentioned stability for obtaining the maximum theoretical sensitivity, i.e. limited only by the quantum noises. It follows that the processes of the prior art using a device introducing a "nonreciprocal bias" do not allow a high degree of sensitivity to be reached.
The process of the invention gets over these disad-vantages. In accordance with the invention, there is intro-duced into the path of waves in ring 2 a phase modulatorbringing lnto play a reciprocal effect: elasto-optic or ;electro-optic for example. This modulator is energized so ~; as to introduce a phase variation in the wave which passes therethrough and which exits periodically, this period must be equal to a value 2T, T being the time taken by a wave to travel in the ring.
Because of the periodicity, the following relation-shlp is confirmed:
~(t) = ~(t + 2T) (7) : 25 The periodicity condition is of course fulfilled - for any periodic function whose frequency is a whole multiple ~k of the frequency 21l~

: ` :
Each of the two waves traveling in opposite dlrections is subjected also to this phase shift since it 30~ passes through the modulator. It follows that the phase difference (~ in the absence of this phase shifter) becomes:

~ - 8 -::
~::

~S~63'~

Q~ (t) - ~(t -~ ~) (8) If we call ~(r) the followiny function:
~ (t) = ~(T) - ~(t + r) (9) because of the periodicity of ~ , the function ~ ) is symmetrical, which means that:
I~(t) = - ~(t -~ T) (10) The result is that the optical power detected in each branch:
Pl + P2 - 2 ~ Cos (~ (t) (11) presents a frequency spectrum illustrated in Figure 4 repre senting the components of the optical power detected as a Eunction of the frequency.
This spectrum breaks down into:
- a continuous component PO;
~ a component P' of frequency 1/2~ proportional to lS sin (~) or useful component;
a component P" of frequency 1/~ proportional to Cos (,~
different components at harmonics of upper orders, the harmonic depending on the exact form of the function ~ (T) .
This latter function, representing the phase modu-lation of the waves traveling i.n opposite directions in ring 2, may have any form whatsoever. However, functions such as those representing square or sinusoidal signals for example 25~ may present certain advantages. These advantages may be of : ~ :
several orders: facility in generating these functions, decomposition .into harmonics in accordance with a known spectrum or facillty of being synchronized on these signaIs.

If we come back to the example of the gyrometer, Figures 6 and 7 represent the effect of a phase modulation by - a symmetrical function ~(~). The operating point describes : ~
~ - 9 _ ,

3 ~ 5 the curve P = f(~(~) of Figure 2 .symmetrically between a couple of extreme points: the :Eirst couple in Figure 6, representing the case where the speed of rotation to be measured is zero, is reduced to two values ~ 2 ' + 2 ; the second couple in Figure 7, representing the case where the speed to be measured is no longer zero and is expressed by a value ~O of the phase dif:Eerence, is represented by the value (2 ~ O) and (+2 ~ o) If, by way of nonlimiting example, the control 10. signal of the phase modulator is an essentially rectangular signal (except Eor the rise times) as illustrated by curve Vc, varying between the values ~ V2, -Vl of the time diagram t of Figure 8, according as to whether the position is as in Figure 6 or in Figure 7, the power signals detected by a quadratic type detector are illustrated respectively by curves PsO and Psl. Apart from the gaps due to the rise times, curve PsO has a constant value PO. If the first harmonic is extracted, we obtain a sinusoidal curve P'So of frequency (1/l). On the other hand, in the presence of a nonreciprocal disturbance, i.e. for a phase difference ~O ~ o (nonzero speed of rotation ~O), the function PSl varies between two maxima : P' and P". If the first harmonic P
~: is extracted therefrom, a sinusoidal function of frequency 1/2l is obtained. The amplitude of these signals is pro-por~ional to the amplitude of the magnitude to be measured and their phase with respect to the control signals indicates the direction of rotation.
From these observations it can be seen that the : process of the invention, besides the fundamental advantage : 30. of avoiding a "nonreciprocal bias" presenting the difficul-: ties which have been called to mind, presents numerous ::
-- 10 -- , ~7L$~l~i;;;~

advantages detailed below.
A synchronous detection will be carried out by transmitting also to the detection means, which will be described further on with reference to Figures 9, 10 and l:L, the control signal Vc. A very stable frequency of these signals may be obtained by using an oscillator controlled by a quartz with thermostatic temperature control. These devices are well-known to the man skil]ed in the art and do not represent particular difficulties. The control signal Vc is at frequency 1/21. The signal detected at this ~requency is proportional to sin (A(~). The sensitivity of the inter-ferometer is then maximum about the phase difference (~) = o.
A variation or drift of the form or of the amplitude of signal Vc, i.e. correlatively with the resulting function ~(t), only affects the "scale factor" of the measurement.
The observation of the slow variations of the "nonreciprocal"
magnitude forming the subject matter of the measurement is ~; then no longer limited but by the noise of the detection and ;~ amplification chain of the device using the process of the 20 invention.
. ~ ~
Moreo~er, as illustrated by Figure 4, since the measured signal has a frequency 1/2T, we are detached from the problems of very low frequency detection and in particu-lar of the noise whose amplitude varies proportionally ~::
inversely to the frequency. The curve of noise PB with respect;to -the frequency is illustrated in Figure 5, bringing out this additional advantage of the invention. Since in practice, in a ring interferometer, the losses are small and : ~ :
the optical powers brought into play relatively high, the ~uantum noise of light (photonic noise) dominates the o-ther sources of noise and particularly the noise due to the :
:: : ~ :
~ : - 11 -~L~ILr~ 3~ 5 detection chain. It follows that the purpose oE the ln-vention is reached, we are close to the attainable sensi-tivity limit. It may also be noted that the process of the invention, very different from the heterodyne detection methods used in the prior art, also presents advantages while allowing a gain in sensitivity not attained by these methods.
Three variations of devices using the process of the invention will now be described in detail. The elements common with those of Figure 1 will not be described again.
The first variation is illustrated in Figure 9.
The prime difference with the interferometers of the prior art is formed by the insertion of a reciprocal-effect ~ modu-lator. This modulator receives at one of its inputs signal Vc of frequency 1/2T elaborated by a generator OSC comprising lS a quartz-controlled oscillator of great stability. The signal is also transmitted to a synchronous detector D. This detector is of the quadratic type and its role is to detect the component of the optical power proportional to sin (~
i.e. the component and frequency 1/2~ and only the component at this frequency. The active element or opto-electric sensor may be formed by a photodiode or photomultiplier tube.
Such detectors are well-known and do not need to be further described. The output signals VD of this detector are fed to a s.ignal elaboration clrcuit symbolized by an amplifier A.
Its role is multiple. It must amplify and adapt them for subsequent use by circuits not shown. The output signals Vs may be of the type shown in the diagram of Figure 8 by signals P'Sl proportional to the amplitude of the component containing the term "sin (~
In a second variation two reciprocal-effect phase modulators ~1 and ~2 may also be disposed at each end of the ring 2 of the lnterferometer, as shown in Figure lO. Each modulator ~1 or ~2 introduces a periodic phase shift as previously, at fre~uency 1/2T~ but so that we have the relationship:
(t) = ~ ~2(t) (12) (t) being the phase shift provided by modulator ~1 and ~2 (t) the phase shift provided by modulator ~2 . If the relationship ( 12 ) is substantially confirmed whatever the value of t, this particular arrangement provides an addition-al gain in stability due to the symmetry.
Although they are in accordance with the invention,the devices which have just been described with reference to E'igures 9 and 10 may present certain difficulties in con-struction. It is particularly difficult to obtain a long op-tical path, i.e. a fairly long transit time ~. Processes have been proposed for elongating this optical path. This is particularly the case of the process described in French patent application No. 75 26 520, published under the number . 22 83 424. This process consists in causing the lasex beam 20 to travel several times round ring 2, while angularly off-setting change-of-direction mirrors similar to those (M, Ml to M3) of Figures 1, 9 or 10.
However, the progress achieved in obtai`ning low-~ loss optical fibers justifies the use of these optical fibers for constructing ring interferometers whose optical path 2 isvery long. In fact, by way of example, an optical fiber wound a thousancl times about a cylinder of radius 15 cm has a length of 900 m If the fiber has an attenuation of 2 db/Km, 60o of the light transmitted is retained in the fiber. More-over, these fibers also allow greater miniaturization of theelectro-optical elemen-ts and particularly of the periodical phase-shifted modulators by lntegration. One embodiment of a ring interferometer in accordance with the invention uslng an optical fiber is illustrated in Fi~ure 11. Fiber 2 wound on itself forms the ring of the interferometer. The periodic modulator is placed at one of the ends of fiber 2.
More precisely, the periodic modulators used in the invention may take advantage of different known electro-optical effects, such as Pockels effect, Kerr effect, this by way of nonlimiting example.
A first example of a ~ phase modulator is illus-trated by Figure 12. This is a parallelopipedic crystal bar capable of presenting a linear electro-optical effect, called Pockels effect. Two electrodes El and E2 formed by a metal film deposited on two opposite faces receive the control signal Vcl at frequency 1/2~. The laser beam in ring 2 is modulated symmetrically in phase, this at the frequency 1/2l.
In Figure 12, it is a question of the longitudinal effect, the transverse effect which would be obtained by placing the electrodes on the input or output faces of the laser beam 2 may also be used. In this case, these elec-trodes must be very little absorbent at the frequency of the light of the laser used. These electrodes may be formed by a metal deposit of very small thickness. The material usable for the bar may be, by way of nonlimiting example, KDP, ADP
or Niobates such as Lithium Niobate.
A second example of a ~ phase modulator intended more particularly for optical fiber interferometers is illus-trated by Figure 13. This modulator is formed by a hollow ~
cylinder made from a piezoelectric material energized by two electrodes El-E2, deposited respectively on the external and internal faces of the ~ cylinder, and receiving control signal Vc at frequency 1/2T. One of the ends of the fiber forming the ring 2 o~ the interferometer has been wound in tight spirals. The deformations of the piezoelectric material induced by the periodic signal Vc are transmitted to the optical fiber and result in the periodic modulation of the phase of the waves traveling in opposite directions in the fiber.
A third example of a particularly interesting ~
phase modulator for a fiber interferometer and permitting an advanced level of integration is illustrated in Figure 14.
It is in fact an embodiment similar to the one described in relation to Figure 2 of French patent application No.
77 35 039 and published under the No. 24 09 512, relating to a laser interferometric gyrometer. This gyrometer comprises a laser source sO; this source may be a gas laser, helium-neon for example, a semiconductor GaAs laser, or a solid ` laser. Preferably the laser chosen has a wavelength corre-sponding to an absorption minimum for the optical fibers, ; 20 i.e. about 0.85 ~m or about 1.2 ~m. The emissive face of this source sO is coupled to the end of a solid light guide 1 constructed on a substrate Sb. The optical circuit inte-grated on this substrate comprises in thls embodiment a directional coupler controlled by two electrodes El and E3 between which an electric field may be applied.
This directional coupler is formed from guide 1 ;~ and guide 3 parallel over a part of thelr paths at a prede-termined distance. When an electric field is applied between the electrodes, the index of the electro?optical material varies and the control by the electric field allows the light guided by one guide to be totally transferred to the other guide. The other end of guide l and one end of guide 3 are coupled to both ends of a wound optical fiber 2, the second end of guide 3 being coupled to the receiving face of light detector D. Such a device operates in the following way:
the coupler is used as a light separator and mixer. A
control signal not shown provldes for light emission by the laser. The field applied between electrodes El-E3 of the coupler is such that half of the light is transferred from guide l to guide 3. The two ends of fiber 2 receive then half of the energy and the light travels through the fiber (simultaneously in both directions). The coupling between the two guides is such that the two return waves interfere.
Detector D placed at the end of guide 3 receives then a signal Ps depending on the speed of rotation of the gyro-meter. Such a device operates continuously but it is possi-ble to cause it to operate by pulses, the laser only emitting for a period less than the propagation time in the fiber.
Detection is then made by pulses.
Electrode El is extended to be placed opposite an ; 20 add1tiona1 electrode E2. The controI signal Vc previously described lS applied between these electrodes. The resulting field modulates the phase 2 of the waves traveling in oppo-s1te directions in the end of fiber 2 to provide the ~ phase modulator of Figure ll. Two separate sets of electrodes may also be~used by splitting up electrode El. The material ` ~ forming the substrate is a material chosen from those already mentioned in relation to Figure 12, provided that waveguides l and 2 may be provided therein: by way of example, Lithium Niobate in which metal ions, for example of titanium, have been diffused. The Lithium Tantalate-Niobium couple may also be used. Finally other materials may be used such as Gallium :
~ ~ - 16 "

~L~ $~

Arsenide in which the waveguides 1 and 3 are formed by ionic or protonic implantation.
As in the case of the d~vice of Figure lO, two phase modulators may be used. Figure 15 illustrates such an arrangement. In this arrangement, the separator-mixer modu-lator comprising electrodes El-E3 is entirely separate Erom ~1 and ~2 phase modulators comprising respectively theelectro-couples ELl-E12 and E2l-E22. These modulators receive control signals Vcl and Vc2 so that re:lationship (12) is confirmed.
To give an idea, it may be indicated that the propagation time oE light in an optical fiber is of the order of: 5 ~s/Km, or for 2T : 10 ~s/Km.
~ f the fiber has a length of l Km, the frequency of the control slgnal must be lO0 KHz, which is compatible with 15 the response time of the phase modulators used. In inte-grated optics, wlth a phase modulator such as the one shown in Figure 14, fre~uencies of the order of lO MHz may be used.
The process of the invention has been tested on a ~; ring interferometer gyrometer whose optical fiber had a length of 1 Km. A sensitivity of the order of lO 5 rd/~
was obtained which is only removed by an order of size lO
from the maximum sensitivity which one may hope to obtain by calculation. This result should be compared with the sensi-~; tivity of ring interferometer gyrometers of the prior art 25 which is about 106 times less than this final sensitivity.
The invention is not limited to the embodimentswhich have just been described, by way of illustration.
The light waves may be replaced more generally by electro-magnetic waves. The practical construction means, particu-larly of the phase modulators, must then be adapted to the ; ~ ~ speciflc characteristics of the waves brought into play.

- 17 ~

Claims

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A process for modulating the phase of the waves traveling in a ring interferometer comprising means forming a ring waveguide in which two electromagnetic waves travel in opposite directions, a source of electromagnetic energy, means for separating and mixing the electromagnetic waves traveling in the ring and means for detecting the interference of the waves traveling in the ring and responsive to the phase difference between these two waves; said process comprising the following steps:
- periodic and symmetric modulation of the phase of the waves traveling in the ring in accordance with a period .PHI.(t) function confirming the relationship .PHI.(t) = .PHI.(t + 2?) where ? is the time which each of the waves takes to travel over the path defined by the ring;
- and detection at frequency 1/(2?) of the .PHI.(t) function of the phase difference of the two waves traveling in opposite directions in the ring.

2. The process as claimed in Claim 1, wherein the waves traveling in opposite directions in the ring are phase modulated a first time in accordance with a first periodic .PHI.(t) function and a second time according to a second periodic .PHI.2(t) function and wherein the two functions .PHI.1(t) and .PHI.2(t) confirm simultaneously the relationships:
.PHI.1(t) = - .PHI.2(t) .PHI.1(t) = .PHI.1(t + 2?) .PHI.2(t) = .PHI.2(t + 2?).

3. The process as claimed in Claim 2, wherein the periodic functions modulating the phase of the waves repre-sent square-wave signals whose amplitude varies between two given states and of frequency 1/(2?).

4. The process as claimed in Claim 2, wherein the periodic functions modulating the phase of the waves repre-sent sinusoidal or cosinusoidal signals of frequency 1/(2?).

5. The process as claimed in Claim 2, wherein the amplitude of the variations of the periodic functions is such that the variation of the periodic phase difference between the two waves traveling in opposite directions is ? radians.

6. A device for modulating the phase of the waves traveling in a ring interferometer in accordance with the process claimed in Claim 1, comprising, the waves being light waves, at least one reciprocal-effect phase modulator con-trolled by a periodic signal of frequency 1/(2?), placed in the optical path covered by the waves traveling in opposite directions.

7. The device as claimed in Claim 6, comprising two reciprocal-effect phase modulators placed at given positions in the optical path, introducing phase shifts in accordance with the two periodic functions .PHI.1(t) and .PHI.2(t).

8. The device as claimed in Claim 7, wherein the reciprocal-effect phase modulators bring into play an electro-optical effect.

9. The device as claimed in Claim 8, wherein the electro-optical effect is a linear effect and wherein each phase modulator is formed by a Pockels cell.

10. The device as claimed in Claim 8, wherein the electro-optical effect is a quadratic effect and wherein each phase modulator is formed by a Kerr cell.

11. The device as claimed in Claim 7, wherein, with the ring of the interferometer formed by an optical fiber, each modulator brings into play a mechano-optical effect and wherein the modulator is formed by an elongated, hollow body made from piezoelectric material provided with electrodes placed respectively on the principal internal and external faces, an optical fiber being wound with edge-to-edge turns on said body over a given length; the piezoelectric body, energized by the control signal, vibrating at the frequency of this signal and introducing phase variations in the waves traveling in this fiber.

12. The device as claimed in Claim 7 for an inter-ferometer whose ring is formed by an optical fiber and the energy source, the means for separating and mixing the waves and the detection means are formed entirely in a solid medium by integration on a substrate, on which two coupled waveguides are formed connected at one of their ends, respectively to the energy source and to the detection means and at their other ends to the ends of the optical fiber; said device com-prising, integrated on the substrate, at least one pair of electrodes disposed on each side of one of the two waveguides so as to form an electro-optical effect phase modulator; the electrodes receiving the periodic control signal.

13. The device as claimed in Claim 12, comprising two pairs of electrodes, integrated on the substrate, dis-posed on each side respectively of each waveguide so as to form two phase modulators.

14. The device as claimed in Claim 13, wherein the detection means are quadratic and comprise a synchronous detector synchronized to the frequency of the control signal and wherein this control signal transmitted simultaneously to a synchronous detector and to each phase modulator is elabo-rated by a local oscillator.