GB2151806A

GB2151806A - An optical frequency converter device and a rate gyro containing such a device

Info

Publication number: GB2151806A
Application number: GB08333905A
Authority: GB
Inventors: Herve Arditty; Michel Papuchon
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1982-12-23
Filing date: 1983-12-20
Publication date: 1985-07-24
Also published as: FR2555769B1; NL192466B; NL8304435A; FR2555769A1; DE3346058A1; NL192466C; IT1160140B; DE3346058C2; GB2151806B; CA1255139A; IT8368302A0

Abstract

An optical frequency converter device comprises a flat substrate 3, a pair of wave guides 5 and 6 which are parallel with each other over a predetermined length and which have different characteristics such that an incident wave 23 received wave guide 5 is transferable to the other guide 6, and means 26 for generating an acoustic wave 12 colinear with the parallel wave guides whereby the frequency received in guide 6 varies from the incident wave in guide 5 by the acoustic frequency. The device may be used on a filter, one may be tuned by varying the acoustic frequency. An optical fibre rate gyro (Figure 8) may include two frequency converters according to the invention to introduce a non-reciprocity which compensates for that due to the Sagnac effect. <IMAGE>

Description

SPECIFICATION An optical frequency converter device and a rate gyro containing such a device Background of the invention The invention covers an integrated optical frequency converter device.

Classical optical frequency converters are well known. The frequency converter most usually used is certainly the one based on acousto-optical interaction. In this method, an acoustic network which propagates in a medium produces periodic refractive index variations in the form of a travelling wave. This moving network diffracts the light. If the interaction length is sufficient, a single order may predominate. In the diffracted order (we), the optical wave frequency rn has been modified by a quantity equal to the acoustic wave frequency Q.

Hence, wD = Co + Q.

Fundamental frequency rejection may be excellent because the converted wave and the direct wave (undiffracted) are then separated in space.

It is then possible to study what this gives in integrated optics. Under this name are designed thin layer monilithic structures intended for luminous signal processing which are obtained by the techniques of deposit, diffusion and engraving by masking analogous to those used in the production of electronic integrated circuits. With these techniques, in particular, it is known how to produce linear structures characterized by a refractive index higher than that of the surrounding medium and forming wave-guides along which the light propagates by a series of total reflections or progressive refractions. It is known how to combine two such wave-guides by arranging them in parallel one with respect to the other along part of the path to produce directional couplers.Thanks to the vanishing wave phenomenon, the energy carried in the first wave-guide is passed progressively into the second wave-guide and it is found that the maximum energy is transferred at the end of a certain lenght, called the coupling lenght, which depends on the geometrical and optical parameters of the structure and, in particular, on the value of the refractive indices of the materials forming the two wave-guides and the medium which separates them. Then the energy passes progressively from the second guide into the first and so on.When an electro-optical material used for one of the materials forming the wave-guides or the medium which separates them, it is also known how to cause the index to vary under the effect of an electric field, which makes it possible, by acting on the coupling lenght, to control electrically the part of the energy transfered from one wave-guide to the other. It can be seen that it is also possible to make a light modulator by arranging, in parallel with the wave-guide which carries the luminous wave, a section of wave-guide in which a more or less large part of this energy will be transferred.

Also, there are frequency converters intended to produce, from a guided electromagnetic radiation of frequency co, a guided electromagnetic radiation whose frequency is a multiple of the frequency o. These converters are used, in particular, in the integrated optical field, thus named by analogy with electronic integrated circuits, which are monolithic structures using thin layers.

Converters of the type already described have been produced in integrated optics but they require the use of a flat wave-guide and this is not applicable to microguides. Techniques usable with microguides have already been suggested in which electro-optical modulation can be used. This may then be a serrodyne or balanced modulator system. Such an optical frequency converter contains a wave-guide used as a phase modulator, which is controlled by a signal in the form of a saw-tooth. Such a signal has the same effect as a voltage gradient which allows a variation of the index as a function of time. It may also be an acoustic modulation in which a TE type wave is converted into a TM type with a change in frequency.In this case, the application of a transverse electric field enables the pass-band of a TE-TM mode acousto-optical converter to be modified by the colinear interaction of acoustic surface waves and a guided optical wave.

These two techniques have several disadvantages: - the two waves propagate in the same wave-guide (the frequency translated wave and the fundamental wave) and this can cause problems in separating them; - in certain cases, the effectiveness of the conversion is very closely related to the wave shape (case of the serrodyne translator); - in the case of a TE-TM conversion, one of the problems that may be met with is the extreme sensitivity of the device to wave-length (variation of ss/KTE - ss/KTM) which may, however, allow this type of device to be used as a filter.

The device in the invention enables these disadvantages to be attenuated. In this device, the converted and unconverted waves are separated in space because the diaphony is related to simple geometrical parameters and can be reduced arbitrarily. Also, the two waves keep the same polarizing. The device can be extended and used in the case in which it is required to produce frequency filters.

Brief summary of the invention The purpose of the invention is the production of an optical frequency converter device with a substrate plane made of a first material and at least two wave-guides with different characteristics, one of which receives an incident wave, the wave-guides being arranged on the surface of the substrate and parallel one with respect to the other, the distance between them being such that the radiation of the incident wave is transferable from one wave-guide to the other, the device containing means for generating an acoustic wave colinearwith the incident wave carried by one of the wave-guides, these generation means being arranged between the two wave-guides to produce the said frequency conversion.

The purpose of the invention is also the production of a rate gyro containing such a device.

Brief description of the drawings Other characteristics and advantages will appear in the following description with reference to the Figures attached in which: Figures 1 to 3 show a device of the art known, Figure 4shows the device in accordance with the invention, Figure shows a device of prior art, Figures 6 and 7 show a system containing the device in the invention, Figure 8 shows a variant of the system.

Detailed description of the preferred embodiments Figure 1 and 2 show a sectional view and a view from above respectively of a switch produced in linear optics. The two luminous wave-guides 1 and 2 are inserted in the substrate 3. The material throught which coupling is made is the one forming substrate 3. To insert wave-guides 1 and 2, it is possible, for example, to cause titanium to diffuse in a substrate formed of a monocrystalline sheet of lithium niobate (LiNbO3). The titanium, in the diffusion zone, replaces the niobium in part to give a mixed compound with the formula LITixNb1.xO3 with a refractive index higher than that of the pure niobate. These diffused zones, with an index higher than that of the substrate, form wave-guides 1 and 2.If the diffusion temperature is higher than the Curie point for the material, the cooling phase which follows is used to subject the plate to a uniform electric field in order to polarize the plate uniformly and produce a "single domaine" structure.

When a voltage is applied between the electrodes 10 and 20, a distribution of field lines is produced which is shown as reference 4 on Figure 1. The field component in the direction C perpendicular the substrate surface 23 has the same absolute value but opposite direction in one guide and the other, which gives refractive index variations of the same absolute value and opposite sign. Nonethless, the existence, in a direction perpendicular to the direction of the substrate axis C, the substrate having its extraordinary index, of a non-zero field component, and the fact that the electric field applied also causes the index value to vary in the part of the substrate 22 contained between the two wave-guides cause a certain dissymetry of the phenomenon.The coupling obtained varies in accordance with the polarity of the voltage applied between the electrodes 20 and 21. The polarity of the voltage supplying the maximum coupling can be deduced from the crystallographic orientation of the material forming the substrate. If this orientation is unknown, it is very easy to determine experimentally the optimum polarity by the measurement of the luminous intensity transmitted by one of two wave-guides for two polarities of opposite signs.

If the metallic electrodes are arranged directly on the wave-guide surfaces, the existence of a vanishing wave moving in the relatively absorbent metallic medium can cause energy losses in the coupler. To avoid them it is possible to fit in between as shown in Figure 1, a transparent dielectric layer 11 and 21 between wave-guides 1 and 2 and electrodes 10 and 20. This isolating layer is made of a material with good transmission for the luminous wave-lenght carried by the wave-guide and a refractive index lower than that of the wave-guide. Silicon dioxide (SiO2) is a material perfectly adapted to the case previously described in which the substrate is formed by lithium niobate.

The two wave-guides, as shown in Figure 2, are parallel one with the other on a rectilinear section of lenght L, which is a function of the parameter, called the coupling lenght, which will be defined later. The distance between the rectilinear parallel parts is of a value d which must not exceed a few wave-lengths (calculated in the medium separating the two wave-guides) of the light carried by the wave-guides. The two waves-guides are formed by the same electro-optical material which, when subjected to an electric field, has a refractive index that varies as a function of the applied field value. The refractive index of this material is so chosen that, even in the presence of the applied electric field, it remains higher than the index of the material forming substrate 3.

Because of the electro-optical character of the material forming wave-guides 1 and 2, the distribution of the field lines in the wave-guides produces within them refractive index variations roughly equal in absolute value but of opposite signs.

When a wave is carried by wave-guide, part of the energy propagates outside the wave-guide, in the medium which surrounds it in the form of a vanishing wave. The amplitude of this wave decreases exponentially on leaving the wave-guides walls. If a second wave-guide is arranged parallel to the first one, it picks up progressively, through this vanishing wave, the energy carried in the first wave-guide and, the closer are the wave-guides, the more quickly it does it. After a given distance, called the coupling lenght, which depends on the geometrical and optical parameters of the two wave-guides and of the medium separating them (and of the refractive indices in particular), the maximum of energy has been transferred from the first wave-guide to the second. Beyond this lenght, the reverse phenomenon occurs. The energy transfers progressively from the second wave-guide to the first to leave the minimum value in the second wave-guide. Any modification of the index of one of the media present acts in one direction or the other along the coupling lenght.

In the device shown in Figures 1 and 2, the lenght L can be chosen equal to the coupling lenght in the absence of this applied electric field. Because of the perfect symetry of the two wave-guides in the coupling zone, the energy transfer is complete from the first wave-guide to the second (or from the second to the first).

The application of a voltage between electrodes 20 and 21 reduces the coupling lenght and part of the energy is retransferred from the second wave-guide to the first (or from the first to the second). The final result is then that, as the voltage is increased, the energy transferred from the first wave-guide to the second (or from the second to the first), measured at the end of the coupling zone, is reduced to reach zero value. The coupling between the two wave-guides thus decreases from 100% to 0% when the voltage applied to the electrodes increases. The result would be the same if the lenght Lwas made equal to and odd multiple of the coupling lenght with a zero field.

It is also possible to give the lenght La value equal to an even multiple of the coupling lenght with a zero field. The energy transferred at the output, from one wave-guide to the other, increases from zero when the voltage applied between the electrodes increases from zero.

A device has then made which, when controlled by an electric signal, enables part or all of the energy carried by one wave-guide to be switched to the other associated with it in the coupling zone.

It goes without saying that, if one of the two wave-guides is limited to a section whose minimum lenght is the lenght L of the coupling zone, this device enables the energy carried in the other wave-guide to be modulated 100%.

In the case in which the two wave-guides are different, a periodic structure made between them can make it possible to increase the exchanges between them. When the wave carried in one wave-guide has the same propagation speed as one of the orders diffracted in the other wave-guide, there is then an energy exchange.

To produce this exchange several means can be used, in particular the production of an electric field between two electrodes, for example, of periodic structures 18 and 29 fitted on one side and on the other of the two wave-guides 5 and 6 as shown in Figure 3. A luminous wave 24 propagating in the first wave-guide produces, by coupling due to the presence of a polarization V0, a coupled wave 25 which propagates in the second wave-guide 6. It can also be the production of a network engraved in the substrate between the two wave-guides. In the device of the invention, acoustic waves 12 are produced by the electrodes 13 and 14 in the form of interdigital combs, at whose terminals a generator V is connected, which propagate between the two waves-guides as shown in Figure 4.However, the electrodes can be deposited on a thin layer 26 of a piezoelectric material, a zinx oxide (ZnO) for example, the thin layer being deposited on substrate 3 consisting of another material, silicon dioxide for example. Thin layer 26 can be made of the same material as the substrate, crystalline quartz, gallium arsenide or lithium niobate for example.

The device in accordance with the invention has be advantage of allowing an adjustement of the coupling between wave-guides 5 and 6 which is a function of the acoustic wave frequency. This acousto-optical deflector allows a frequency translation. The luminous waves carried by one of the wave-guides 5 and diffracted by these acousto-optical waves are then converted in frequency and transmitted in the second wave-guide 6. These two wave-guides are not necessarily of the same width.

If a medium 30 is considered in which a beam of elastic waves 31 of frequency f is propagating, as shown in Figure 5, if an incident luminous beam 32 is passed in this medium, a group of diffracted beams 33 is obtained with frequencies F + + kf, in which k is a positive or negative whole number.

The sinusoidal variation of the index, produced by the elastic wave, has an effect on the luminous wave analogous to that of a phase network. The luminous beam 32 penetrating the crystal 30 in parallel with the elastic wave planes is separated into several beams symmetrically inclined with respect to the incident beam by angles 0N:Sin ON = k(A/X) in which A is the wave plane step and A the incident beam wave-lenght.

However, the elastic beam thickness e must be less than a critical value ec. The side waves are produced all long the carrier wave path inside the ultrasonic beam and not only at the output, on the frontier. If, in throught, the elastic beam is divided into thin slices parallel to the propagation direction, for each of these slices parallel to the propagation direction, for each of these slices, the preceding spectral analysis is valid.

The frequencies Q + kco and the direction of propagation 6N of the side waves are the same for the abscissa slices x and x + t. If, for a given order, the contributions of these two slices d apart are added, there is phase opposition for a distance A2 1 {'N = A ' jn2' The interference of the waves emitted by the slices t'N apart may then be destructive. If the width of the beam is greater than eN, the effect of a slice is cancelled by the slice tN away. Under the best conditions, the elastic beam thickness e must not exceed a first order critical value ec = l1 = #/#.

For a Bragg angle incident of the luminous beam 32 with respect to the elastic wave planes, the interaction is the biggest because it enables the interferences to be made constructive for the first order of angular frequency Q - o. Hence, it only supplies a single deviation beam.

The device as in the invention uses a directional coupler whose two wave-guides are not identical. In this case, if ss/K1 and ss/K2 are the propagation constants of the modes in these two wave-guides of the coupler, the relative energy in one of the wave-guides when the other is energized will be written 1 Ap2,4c2 Sin2 1 + 2 4c2 cL in which L is the interaction lenght, c is the coupling constant and 2N #ss=-(ss/K1 - ss/K2) 1# in which X is the wave-lenght in a vacuum.The relative energy present in this wave-guide at the coupler output depends on three parameters, L, c and Ass. If Ass is large with respect to c, it can be sen that, in any case, no matter what the value of L, the maximum energy exchanged can be small. For example, if c = 1.5 10 4 m, # # - #ss = 0.0001, EMAX = 0.0017 and if c = 1.5 10-4 m, -- #ss = 0.01, EMAX = 0.000017 2# These values are very small and can be reduced further arbitrarily by changing the lenght L.

It is known that, if the propagation constants in the two wave-guides are made to vary periodically and the corresponding period is carefully chosen, the exchange between the two wave-guides can be increased by comprising Ass with the network vector K.

The interaction is then written, because of the conservation of the moments 2# 2# ss1 + K = ss2, i.e.: - (ss1 K - ss2/K) = -, # # in which - is the network period.

Hence, if the network is formed, as produced in the device of the invention shown in Figure 4, by an acoustic wave propagating colinearly to the optical wave, there will be a frequency translation of the coupled wave.

The effectiveness of the interaction depends on the value of the index variation induced by the acoustic wave and hence of the power injected. A directional coupler produced in lithium niobate (LiNbO2) by titanium diffusion can be taken as an example. The variation in index corresponding to titanium is usually of the order of: X n 10-3.

It can be seen then that it is possible to produce the two wave-guides with A p/K = 2 10-3. This can be obtained by changing the width andior the thickness of the titanium for these two wave-guides in the coupler. For an interaction lenght of 10 mm, the maximum energy exchanged will be: for = 0.83 Fm,EMAx = 4 10-4.The acoustic wave-length required for compensation will be : 415 am, i.e. in the case of lithium niobate (LlNbO3) a frequency translation of 7.2 MHz. The wave picked up at the output of the second wave-guide (which was not initially energized) will then be obtained with a frequency translation of 7.2MHz and the maximum quantity of fundamental in this wave-guide will be - 33 dB with respect to the total optical energy.

The device as in the invention can also be produced by making one of the wave-guides by proton exchange and the other by titanium diffusion (or the two by proton exchange but different characteristics). In this case, A p/K = 0.1 can be obtained with an interaction lenght of 10 mm, the maximum energy exchanged: - 67 dB of the total energy is obtained with an acoustic wave-lenght of 8.3 m, i.e. an acoustic frequency of the order of 361 MHz.

Hence, in the device of the invention shown in Figure 4, a wave 23 passed in the first wave-guide causes by coupling the existence of a wave 25 in the second wave-guide, this wave then being translated in frequency.

Several wave-guide configurations are possible, with a substrate 3 of lithium niobate, for example. The two wave-guides are obtained by diffusion of titanium in the substrate. The waves guide in the two wave-guides are either two TE waves or two TM waves. ApiK of the order of a few 10 3 is then obtained.

It is also possible to have a crossed interaction, i.e. a TE wave in the first wave-guide and a TM wave in the second or vice versa. ApiK of the order of 0.1 is then obtained.

One of the two wave-guides can be obtained by titanium diffusion and the second one by proton exchange. If an axis C perpendicular to the substrate surface is considered, there is a TM wave in each of the two wave-guides. There could also be two TE waves. Ass/K of the order of 0.1 is then obtained. The two waves-guides can be obtained by proton exchange but their characteristics must then be different. AWK = 0.1 is then obtained.

By changing the acoustic frequency, which can vary from 10 to 300 MHz, a tunable filter can be obtained.

The birefringence of the material varies as function of the frequency.

The pass-band of the device in the invention is a function of the optical wave to acoustic wave interaction lenght. The greater the number of wave planes in the acoustic wave seen during this coupling, the narrower the pass-band.

The device described here can be used then as a filter by using, for example, the variation of the birefringence of a material with the wave-lenght. It can be considered then that it is a TE(TM) wave in the first wave-guide and TM(TE) wave in the second which are coupled by means of the acoustic wave. In this case, for lithium niobate, (Ap1KTM Ass KTE) = 0.1 and again an acoustic wave frequency of about 361 MHz. This filter is adjustable because it is sufficient to change the acoustic wave frequency.

In the device of the invention, electrodes can be deposited, on one side and on the other of the two wave-guides, for example, or on these wave-guides. An insulating buffer layer can also be deposited between the electrodes and the substrate. The electric field produced between these two electrodes enables the device of the invention to be adjusted in its initial or its final state.

The device of the invention has an application in the field of the optical fibre rate gyro.

Figure 6 shows schematically a ring interferometer of known design. A laser source S transmits a beam of parallel rays 41 to a separator device formed by a semi-transparent sheet M.

A certain number of mirrors, M1, M2, M3, define an optical path forming the interferometer ring. This ring can be made, for example, with a monomode optical fibre. The sensitivity of the measurement is increased by the use of a long optical path. This ring is looped back on the separator device M which also acts as a mixer device and determines an output branch 43. Two waves, then, propagating in opposite directions, pass trough the ring, one clockwise (direction 2) and the other anticlockwise (direction 1). These two waves recombine on the separating sheet M. The result of this recombinaison can be seen in the output branch 43 with detector D. Part of the beams is picked up again in the input arm by the separating sheet M' and passes through filter device F again.At the output the two waves recombine on separating sheet M'. The result of this recombination can be seen in the output branch 44. The fact that the filter device F I-as been fitted in the interferometer input arm makes the arm strictly reciprocal. Hence, a wave contained in a single optical mode passes through it. This filter device consists of a mode filter followed by a polarizer. The incident beam 41 passes through this filter and the fraction which comes out is in a single mode. It is possible then to consider either the emerging beam 43 corresponding to the interference of the two beams which have not passed through the mode filter device or the part of the beams which is picked up again in the input arm by semi-transparent sheet M. This part of the beams passes through filter device F again.At its output, the two beams passed through arm 44 by means of semi-transparent sheet M' are contained in the same mode and this makes the interferometer insensitive to 'reciprocal' disturbances.

If AE is the difference in phase between the two waves which propagate in opposite directions in the ring and Ps is the optical power which can be measured in output branch 44, then, when there is 'non reciprocal' disturbances; A 4 is zero.

If a rate gyro using this ring interferometer is considered, a 'non reciprocal' disturbance will be produced when the rate gyro is spun. The phase difference 6 is not zero and A = af in which Q is the speed of rotation and a = k(UAC) in which k is a constant depending on the rate gyro geometry, L the lenght of the optical path, X the length of the light wave emitted by the laser source S and C the speed of the light in ring 42. When the speed of rotation Q increases, the phase difference A increases in the same proportion because coefficient a remains constant. The optical power Ps changes according to a cosine law.

in which Pls corresponds to direction 1 and P25 to direction 2. The measurement sensitivity for a given value a is given by the derivative of Ps.

The interferometer sensitivity is very low if the phase difference is little different from zero. This is the case in a rate gyro if it is required to measure small rotational speeds Q. The variation in optical power in the output branch is shown in the diagram in Figure 7.

It may be considered that the terms P15 and P25 are equal. It follows that, for a phase difference , = the power detected is a minimum. It passes through a maximum Psmax when A = 0; 27r and so on.

To increase the interferometer sensitivity, a constant 'non reciprocal' bias can be introduced in the phase of the two waves circulating in opposite directions in order to move the interferometer operating point.

In the case of a function varying in accordance with a cosine law, the greatest sensitivity point is obtained for angles (2k + 1) 1r/2 in which k is a whole number. A bias can then be chosen which introduces a phase variation for each wave with an absolute value of 1r/4 but with opposite signs. In the absence of 'non reciprocal' disturbance, the phase difference then becomes: m' = Xdzo in which AXo = it12. This is point Ain Figure 7.

As shown in Figure 6, it is possible to add in the wave path in ring 42 a phase modula or 45 which uses a rexiprocal effect to obtain better sensitivity with the device. This modulator is so energied as to produce a phase variation in the wave passing through it. This variation is periodic, the period bei 1g 2. in which T is the time for a wave to pass in the ring.

The difference then becomes: ' = A (t- . ) in which each of the waves circulating g in the opposite direction undergoes this phase shift when it passes through the modulator with d)(t) = r ,(t - 2-).

The operating point then describes the curve Ps = ffAd) in Figure 7 symmetrically between a pair of end points.

The device (reciprocal phase modulator) which enables the disturbance (t) to be introduced can, with advantage, be divided into two devices, 45 and 46, one at each end of the path as shown in Figure 6, one giving the phase shift , It) and the other the phaseshiftd,2(t). These phase modulator devices, symmetrically placed at the two ends of the optical path may be in opposition. This arrangement adds additional symetry to the phenomena and reduces the second order errors due to possible non-linearity in the modulators.

The ideal isto work at and B on the curve shown in Figure 7 to start with. To work atA, dX1(t)=w/4 and d,2(t)=- I,4 and then to work at point B, 61(t)= -1r 4 and (b2(t)= i "'4.

This result can be obtained by using two square-shaped signals with two levels, -n; and sus4.

If the phase modulation signals are frequency F, if the gyroscope is not rotating, a rectified signal at frequency 2F is obtained after detection. However, if the gyroscope is rotating, frequencies F and 2F are obtained. This device has the disadvantage of not including a zero technique. Also, the -neasurement is not linear.

If a zero method is required, a non reciprocal effect compensating the rotational effect must be considered.

A component at frequency F in the signal detect which is zero must then be obtained. The modified parameter enabling the rotational speed to be found is then measured.

The field applied to the modulator electrode terminals can be altered if it is electro-optical. The difference of frequency in the modes which are propagating can be altered and this results in a phase shift at the detector output.

The device in the invention finds its application in this optical fibre rate gyro field in which two frequency converters of the invention can be arranged in the two arms working at frequencies such that the non reciprocity, introduced by the fact that the two waves in the interferometer are not at the same frequency, compensates for that due to the Sagnac effect.

Two converters, 62 and 63, arranged beside the modulators, 45 and 46, as shown in Figure 6 can be considered.

The device in the invention then makes possible digital adjustment. If two frequency converters are placed beside the two modulators, it is possible to compensate for the component of frequency F which is due to the Sagnac effect, when there is rotation. There are then two frequencies, F1 and F2, in the two converters.

In the standby condition, F1 = F2 should be obtained. When the gyroscope rotates at constant speed, frequencies F1 and F2 beat the number of beats can be counted.

The progress made in the production of low loss optical fibres makes it possible to use optical fibres to produce these ring interferometers as it has already been said. An example of the production of a ring interferometer complying with the invention is shown in Figure 8. The fibre 52 wound round itself forms ring 42 of the interferometer. The various branches of the interferometer are made of integrated optics. The wave-guides are made by integration in a substrate. The substrate can be chosen, for example, from among the following materials : lithium niobate or tantalum niobate in which, to make the wave-guides, titanium or niobium respectively have been diffused.

The frequency converter is broken down in two converters, 54 and 55, placed at the ends of the fibre. These converters, are the devices already described in the invention which make it possible to compensate for the Sagnac effect, when the two frequencies of the two acoustic waves (58, 59), generated by the electrodes (56, 57) are altered. The phase modulators, 60 and 61, shown by the electrodes placed on one side and the other of each of the wave-guides are in the loop to make it possible to find out the instants at which the gyroscope rotates. In this case, a component of the signal at frequency F is detected as it has already been explained.

The optical radiation separators consist of monomode wave-guides connected between themselves to form Ys, these Ys being connected between themselves by one of their branches acting the role previously played by the semi-transparent sheets in Figure 6. The wave-guide 48 actrs the role of the monomode filter in Figure 1, a polarizer being made, for example, by metallizing 49 on the substrate surface over wave-guide 48.

The device in the invention also finds applications in optical telecommunications to multiplexidemultiplex optical waves in wave-length.

Claims

1. An optical frequency converter device with a flat substrate made of a first material and at least two wave-guides with different characteristics, one of which receives an incident wave, arranged on the surface of the substrate, the wave-guides being parallel one with the other over a predetermined length and such a distance apart that the incident wave radiation is transferable from one wave-guide to another, the device containing means for generating an acoustic wave colinear with the incident wave passed in one of the wave-guides, these generation means being arranged between the two wave-guides to produce the said frequency conversion.

2. A device as in claim 1, in wich the acoustic wave generation means contain a thin layer of a second piezoelectric material arranged on the substrate surface and in which two electrodes in the form of interdigital combs are deposited on the surface of this second material.

3. A device as in claim 1, in which the acoustic wave generation means contain two electrodes in the form of interdigital combs deposited on the substrate surface.

4. A device as in claim 1, in wich the first material is made of lithium niobate.

5. A devise as in claim 4, in wich at least one of the two wave-guides is made as a bar inserted in the substrate, in which titanium is introduced into the said lithium niobate.

6. A device as claim 4, in which at least one of the two wave-guides is made as a bar inserted in the substrate, H ions being substituted for lithium ions in the said lithium niobate.

7. A device as in claim 1, containing means to apply a modulating field to at least one of the wave-guides, these means being produced by electrodes arranged on one side and on the other of this wave-guide.

8. A device as in claim 1, containing means to apply a modulating field to at least one of the wave-guides, these means being produced by electrodes arranged on this wave-guide.

9. A rate gyro containing an optical interferometer device intended to measure a non reciprocal phase shift undergone by two radiations circulating in opposite directions in a wave-guide in the form of a ring, which contains a monochromatic luminous source, means for the photodetection of the interference of these radiations and optical separator and mixer means connecting the ends of this wave-guide directly to this luminous source and to these photodetector means and electrically controlled optical phase shift means, characterized by the fact that it contains, inclued in the ring wave-guide, at least one device such as that claimed in claim 1.

10. A rate gyro whose ring is formed by an optical fibre and in which the energy source, the wave separation and mixing means and the detection means are entirely made in the solid medium by integration on a substrate, on which have been made two wave-guides coupled and connected at their first end to the energy source and to the detection means respectively and at their second end to the ends of the optical fibre and which contains, integrated on the substrate, at least one pair of electrodes arranged on one side and on the other of one of the two wave-guides to form an electro-optical effect phase modulator, these wave separation and mixing means being made by integration of wave-guides on a substrate, these wave-guides being in the shape of the two Ys connected one to the other by one their branches, this rate gyro containing at least one device such as that claimed in claim 1 arranged on the substrate at the second two ends of the wave-guides.

11. An optical frequency converter substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.