FR2863728A1 - Optical switching device, has variable illumination unit with electrodes and voltage source to vary intensity of light beam to modify refractive index of polymer, by which core of channel wave guide is made - Google Patents

Abstract

The device has a channel wave guide whose core is made of polymer very high non-linear coefficients. A variable illumination unit (20) varies Bragg wave length of a Bragg grating. The unit has electrodes (4, 18) and a voltage source (24) to generate a light beam to be absorbed by the polymer and vary the beam intensity to modify refractive index of the polymer, when an electric field is generated and the beam is sent into the core zone.

Description

2863728 1

  INTEGRATED OPTICAL SWITCHING DEVICE WITH WAVELENGTH TUNING

DESCRIPTION 5 TECHNICAL FIELD

  The present invention relates to an integrated optical communication device which is wavelength-tunable.

  The invention relates more specifically to such a device, comprising a Bragg grating (grating grating) which is wavelength tunable, that is to say which is capable of tuning the wavelength of Bragg.

  The invention applies in particular to optical telecommunications.

  STATE OF THE PRIOR ART Optical telecommunications require reconfigurable optical circuits for switching wavelengths, such as optical switches and add-drop circuits.

  A Bragg grating inscribed in an optical guide performs the function of extraction (drop) because it is transparent at all wavelengths, except those which satisfy the following Bragg relation: pA. = 2nmA (1) In this equation (1), A is the pitch (pitch) of the grating, X the wavelength in vacuum (close to 1500nm for optical telecommunications) of the nm effective index mode propagating in the guide and p is an integer greater than or equal to 1 which constitutes the order of reflection.

  The reflection factor of the network (ratio between the reflected intensity and the incident intensity) is well approximated, for the order 1, by the square of the hyperbolic tangent of the product kL: R = tanh2 (kL) (2) In Expression (2), L is the length of the grating and k is a coefficient of efficiency, proportional to An2, where An is the modulation of the index in the grating. The value of R approaches 1 if the length of the network is increased.

  Let us indicate as soon as in the device object of the present invention, a Bragg grating is used which is tunable by varying the value of An by one or more external parameters.

  Thus, by varying the effective index nm between two values no and n1r, the wavelength of Bragg X varies proportionally to An, between two values Ao and X1 where X0 = 2noA and X1 = 2n1A.

  The refractive index of a body depends on a certain number of parameters: temperature (thermo-optical effect), illumination (photochromism and optical Kerr effect) and electric field (Pockels effect). The photochromic effect is slow (several seconds to modify the index). The optical Kerr effect is fast but requires very high light energies to obtain usable index variations (greater than 10-5).

  The temperature affects the refractive index due to expansion (decrease in the concentration of molecules in organic materials) and the settlement of vibronic states. The effect is very different, depending on whether it is mineral or organic bodies.

  In general, mineral bodies have a high heat capacity and heat conduction, which is related to acoustic phonons. It follows that, on the one hand, it is expensive to heat them and that, on the other hand, the temperature is homogenized, which makes it very difficult to achieve, by this means, a network of index (index grating) having a pitch A of the order of 1 m.

  On the other hand, the organic bodies have a low heat capacity and thermal conduction, because of the weak contribution of the acoustic phonons, in relation with the weak interactions between molecules (Van der Waals bonds).

  As a result, the temperature is much less homogenized than in the mineral bodies and it is possible to form thermal networks (and consequently optical networks) with low pitch A in the organic bodies. response is relatively long (of the order of 1ms), mainly because of the heating time of the control electrodes.

  Illumination changes the polarization of the electronic cloud. The answer is fast (less than 10-12s). This is the optical Kerr effect, which is expensive in energy and gives only very small index variations (less than 10-6).

  The electric field polarizes the electronic cloud. So it changes the polarizability and hence the refractive index. The linear variation of the latter with the electric field constitutes the Pockels effect. The response time is very short (less than 10-12s). The variations of index An, for electric fields (of the order of 1MV / cm), which are sufficiently lower than the breakdown field, are between 10-4 and 10-3.

  The state of the art concerning the wavelength-tunable Bragg gratings is described below, these Bragg gratings being formed in optical waveguides.

  Taking into account the Bragg formula X = 2n.A, it is conceivable to vary the tuning wavelength,, either by modifying the pitch A of the Bragg grating or by varying the refractive index n of the core material of the optical waveguide in which the Bragg grating is formed.

  The first solution corresponds to many mechanical techniques playing on the elongation or compression of the core material of the guide to control the wavelength agreement.

  The second solution is based on the variation of the index of refractive index which, as we have seen previously, can be controlled in different ways, namely thermally, electrically or optically.

  Thermal control, through the high thermo-optical index of polymers, is commonly used; its effect is added to the mechanical effect obtained by the thermal expansion of the material. It nevertheless has the double disadvantage of being relatively slow and greedy in energy.

  The electro-optical effect requires high electric fields (greater than 1MV / cm) and only allows variations in the index and thus the wavelength, from 10-4 to 10_3 corresponding to a restricted tuning range, less than 1.5nm, at 1550nm, and therefore can not be used easily beyond.

  PRESENTATION OF THE INVENTION The subject of the present invention is an optical switching device which uses a wavelength-tunable Bragg grating, without presenting the above disadvantages.

  In the present invention, wavelength tuning is faster, requires less power and has a larger range than the known techniques mentioned above.

  Specifically, the present invention relates to an optical switching device comprising: a first optical waveguide (channel optical waveguide) comprising a core surrounded by cladding layers which constitute a sheath (cladding) and within which is included this heart, - a first Bragg grating formed in an area of this heart, this first Bragg grating being able to reflect a first light beam, which propagates in this heart and has a given wavelength, and variation means, capable of varying this given wavelength, this device being characterized in that the core of the first optical ribbon waveguide is made of a non-optical material. linear, noncentrosymmetric and photo-isomerizable, which is transparent to the first light beam, and in that the means of variation comprise: means for creating an electric field in the zone d u core of the first ribbon optical waveguide and - generation and variation means, intended to generate a second light beam, which is adapted to be absorbed by the core material of the first ribbon optical waveguide and to be modified the refractive index of this material, and to vary the intensity of this second light beam, so as to be able to modify this refractive index and therefore the wavelength given when the electric field is created and the second beam light is sent into the zone of the heart of the first ribbon optical waveguide, by varying the intensity of this second light beam.

  According to a preferred embodiment of the device which is the subject of the invention, the generation and variation means are provided for sending the second light beam into the zone of the core of the first optical ribbon waveguide, transversely to this core.

  According to a first particular embodiment of the invention, one of the confinement layers comprises an edge which is able to reflect the second light beam towards the zone of the core of the first ribbon optical waveguide and the means of generation and variation are provided to send the second light beam to the core area of the first ribbon optical waveguide through the edge of this confinement layer.

  The edge of this confinement layer may be coated with a layer that is able to reflect the second light beam.

  According to a second particular embodiment, the device which is the subject of the invention further comprises: a second optical ribbon waveguide comprising a core which is parallel to the core of the first optical ribbon waveguide and close to that the core of this second ribbon optical waveguide being intended to transport the second light beam which is generated by the generation and variation means, and - a second Bragg grating which is blazed (blazed), formed next to the first Bragg grating, in the heart of the second optical waveguide 2863728 8 ribbon, and provided to send this second light beam in the core area of the first ribbon optical waveguide where is formed the first network of Bragg.

  According to a third particular embodiment, the device according to the invention further comprises an optical guiding layer which is intended to transport the second light beam generated by the generation and variation means, this optical guiding layer comprising one end beveled opposite the zone of the core of this first optical ribbon waveguide, where the first Bragg grating is formed, this optical guiding layer being provided for reflecting the second light beam towards this zone, via this beveled end.

  Preferably, the material of which the core of the first ribbon optical waveguide is made is a polymer.

  According to a first particular embodiment of the device which is the subject of the invention, the first Bragg grating is permanently formed in the zone of the core of the first ribbon optical waveguide.

  In this case, the first Bragg grating can be formed by photo-bleaching or photo-inscription through a mask.

  Alternatively, when the core material of the first optical ribbon waveguide is a polymer, which polymer comprises chromophores, the first Bragg grating can be formed by orientation of the chromophores under the effect of a field. electric, this electric field being applied via a comb-shaped electrode having the same pitch as this first Bragg grating.

  According to a second particular embodiment of the device which is the subject of the invention, the first Bragg grating is formed temporarily in the zone of the core of the first ribbon optical waveguide.

  In this case, when the material of which the core of the first ribbon optical waveguide is made is a polymer, this polymer being electro-optical and comprising chromophores, the means for creating the electric field may comprise: first and second electrodes which are placed on either side of the first ribbon optical waveguide, the first electrode being comb-shaped and comprising teeth which extend transversely to the heart of the first ribbon optical waveguide, this comb having the same pitch as the first Bragg grating and a voltage source designed to temporarily bias the first electrode relative to the second electrode and thus temporarily create the electric field and the first Bragg grating.

  In a variant, the device further comprises a mask which is placed opposite the zone of the core of the first ribbon optical waveguide and has a pattern which reproduces the pattern of the first Bragg grating, this first Bragg grating being created when the second light beam is sent into this area through the mask.

BRIEF DESCRIPTION OF THE DRAWINGS

  The present invention will be better understood on reading the description of exemplary embodiments given below, purely by way of indication and in no way limiting, with reference to the appended drawings, in which: FIG. 1 schematically illustrates a manufacturing step; of a device according to the invention, in which a layer of this device is treated, - Figure 2 shows temperature cycles (curve I) and electric field (curve II) which are applied to this layer, - FIGS. 3 and 4 schematically illustrate other manufacturing steps of the device for obtaining the two-dimensional confinement of the guide provided by this device; FIG. 5 is a schematic longitudinal sectional view of a first example of the device of the invention; FIG. 6 is a schematic view of a second example of the device which is the subject of the invention; FIG. 7 is a schematic longitudinal sectional view of a third FIG. 8 is a diagrammatic longitudinal sectional view of a fourth example of the device that is the subject of the invention, FIG. 9 is a schematic top view of the device of FIG. FIG. 10 is a schematic longitudinal sectional view of a fifth example of the device that is the subject of the invention, and FIG. 11 schematically illustrates the operation of the device of FIG. 10.

  DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS Reference is made to the following document: [1] EP 0 966 700 A corresponding to FR 2 760 543 A and to WO 9 840 784 A, Light Switching-Modulating Device, Invention by F. Kajzar , M. Large and P. Raimond.

  The present invention makes use of the teaching of this document [1] and, in particular, materials mentioned therein.

  As explained in this document [1], the refractive index of a non-linear, noncentrosymmetric and photo-isomerizable optical material, which is subjected to an electric polarization field, is modified by the absorption of a beam luminous control parallel to this electric field.

  Under the action of this control beam, the molecules of the material gradually change isomeric conformation and spatial orientation, which results in a change in the refractive index of the material.

  In the presence of an electric field E, the illumination of the material leads to a decrease in the polar order in this material and consequently to a decrease in the second harmonic generation coefficient and the electro-optical coefficient of the material. This effect of decreasing the polar order and the associated change in the refractive index can be used by varying the intensity of the control light beam.

  The decrease in the second harmonic intensity corresponds to a decrease in the electro-optical coefficient of the material and the resulting variation An of the refractive index n of the material, which is preferably a polymer, is given by the following formula: = rn2E (3) If the induced variation of the electro-optical coefficient r is equal to 10 pm / V (corresponding to an initial value of 20 pm / V which is common in such a polymer), for an index n of 1.6 and under an electric field of 2MV / cm, we obtain a refractive index variation An equal to 0.01 and therefore a wavelength variation AX equal to 15 nm, at a central wavelength of 1500 nm.

  The use of polymers having higher nonlinear coefficients proportionally improves the performance of a device according to the present invention.

  As explained above, an optical switching device according to the invention uses a ribbon optical waveguide and a Bragg grating which is formed in this ribbon waveguide and whose we can give the wavelength of Bragg.

  The operation of this device makes use of the variation of the polar order and the associated change in the refractive index of an optical material, preferably a polymer, under illumination, in the presence of an electric polarization field.

  Remember that this material, preferably this polymer, is non-linear, non-centrosymmetric and photoisomerizable.

  Various embodiments of the device that is the subject of the invention will be described in the following.

  We first consider the development of an electro-optical guide which is used in these particular embodiments, with reference to Figure 1.

  On a substrate 2, for example silicon, glass or plastic, are deposited a uniform electrode 4 and a first buffer layer 6, or lower confinement layer, the thickness of which is of the order of 21um.

  In one example, this layer 6 is made of a polymer deposited by centrifugation of a solution, by the technique of spin coating.

  On this buffer layer 6 is deposited a second layer 8, or active layer, which will serve to form the core of the optical guide. Its optical index must not be greater than the optical index nt of the buffer layer 6 so that there is guidance.

  In one example, this layer 8 is made of a polymer in which organic molecules (chromophores) having a strong quadratic optical hyperpolarizability f3 (some 10-48 C.m3.V-2) are dissolved or even grafted. a strong permanent dipole moment (some 10-30 cm).

  Initially, since the chromophore distribution is centrosymmetric, this polymer layer has no electro-optical properties. To give it one, the chromophores are oriented on average in the same direction (polarization phenomenon) by interaction with an electric field E (thanks to the dipolar interaction W = - .E, E being of the order of 108 V cm-1) This electric field is applied, either by means of a needle 10 positioned a few centimeters above the layer 8 and brought to a voltage of the order of a few kilovolts (corona polarization), or by means of an electrode deposited on the layer 8. In the latter case, the polarization can be performed after the deposition of the second buffer layer (see below).

  To allow the chromophores to rotate more freely, the layer 8 is heated in the vicinity of its glass transition, the electric field E is then applied, then the layer 8 is cooled, maintaining the electric field. In this regard, reference is made to the following document: [2] Singer KD, Kuzyk MG and Sohn JE, "Orientationally ordered electro-optic materials", published in "Nonlinear Optical and Electroactive Polymers", PN Prasad and DR Ulrich Editors, Plenum Press, New York 1988, pages 189 to 204.

  Reference is also made to FIG. 2 where the temperature (curve I) and electric field (curve II) cycles are shown for polarizing the active layer, or active film, 8. The temperature T, the electric field E and the time t are respectively expressed in C, in V / cm and in minutes.

  It should be noted that layer 8 is homogeneously polarized so as not to create a static Bragg grating.

  At this stage, the two-dimensional confinement of the heart 12 of the guide can be achieved by bleaching through a mask or by direct photo-inscription by focused beam or by RIE, that is to say by reactive ion etching (reactive ion etching ) in a plasma (Figure 3).

  A second buffer layer 14 (FIG. 4) or upper confinement layer (upper cladding layer) is then deposited on the core 12 of the guide.

  In this way, the Bragg grating can be conventionally formed in the heart of the guide, by bleaching or photo-inscription through a mask.

  But, in the case of a polymer comprising chromophores, this network can also be generated, permanently, during the orientation of the chromophores under the effect of a constant electric field, applied through an electrode in the form of a comb adopting the same pitch as the Bragg grating to be formed, or temporarily, by applying this electric field through this comb-shaped electrode, during the normal operating mode.

We will come back to this later.

  Or again, the transverse illumination used in the invention can be made through a suitable mask, in which case the creation and wavelength of the Bragg grating are done simultaneously by varying the intensity of the illumination beam.

  The last two variants of the invention, therefore lead to the realization of a device that is both tunable and switchable.

  Figure 5 is a schematic longitudinal sectional view of a device according to the invention. As can be seen, this device comprises the optical waveguide of FIG. 4. However, the core 12 of this guide comprises a Bragg grating 16 and an electrode 18 covers the upper confinement layer 14.

  The device of FIG. 5 also comprises variable illumination means 20. These means 20 are intended to generate a light beam 22 which is able to be absorbed by the material of the yard 12 of the guide and to modify the refractive index of this material. These means 20 also make it possible to vary the intensity of the light beam 22.

  Moreover, as can be seen in FIG. 5, these means 20 directly send the light beam 22 into the zone of the core 12 of the guide where the Bragg grating is located, transversely to this core, through the upper confinement layer 14. (Which is chosen to be transparent to the beam 22) and through the electrode 18 (which is also chosen to be transparent to this beam 22).

  The variable illumination means 20 consist, for example, of a laser, for example an argon laser, with the beam of which the whole network 16 is uniformly illuminated and which is provided with control means enabling the intensity of the light to be varied. this beam.

  It is possible to use an electrode 18 made of ITO, that is to say of indium and tin oxide. This electrode 18 may be formed by radio frequency sputtering, for example, on the layer 14.

  In addition, the Bragg grating 16 is formed for example by photoinscription in the core 12 before it is covered by the layer 14 or even through it if the latter is transparent to the wavelength of photo- registration.

  It will be noted that this is a permanent diffraction grating.

  The device of FIG. 5 further comprises a voltage source 24 designed to bias the electrode 18 with respect to the electrode 4 and thus to create a permanent electric field in the zone of the core 12 of the guide where the Bragg grating is located. 16.

  It is specified that the assembly formed by the substrate 2 and the electrode 4 can be replaced by an electrically conductive substrate and the electrode 18 is then polarized with respect to this conductive substrate.

  In Figure 5, we also see incident optical beams 26 and 28 which propagate in the heart 12 of the guide. The diffraction grating 16 decouples the beams thus guided. The beam 26 is not reflected by this network while the beam 28, whose wavelength is equal to the Bragg length of the network, is reflected.

  By modifying the luminous intensity of the beam 22, it is possible to modify this Bragg wavelength and thus to reflect a guided beam having a wavelength different from that of the beam 28, this beam 28 then being transmitted by the guide .

  In another example, the core 12 of the guide is still transversely illuminated by the light beam 22 coupled in the layer 14, but this beam 22, which has become planar, is sent into the zone of the core 12 where the Bragg grating is located. through the edge of the containment layer 14 (see Figure 4 where the Bragg grating is not shown).

  FIG. 6 is a schematic view of another example of the invention, in which the zone of the core of the guide, where the Bragg grating 16 is located, is still illuminated by the edge of the confinement layer 14.

  However, in this case, an edge 30 of the structure forming the guide, and therefore of this confinement layer 14, is beveled to be able to serve as a mirror capable of reflecting the beam 22 towards the heart of the guide, as seen on Figure 6.

  The beam 22 coming from the means 20 is sent into the layer 14, by the edge thereof, via an optical fiber 32, towards the bevelled edge 30, as can be seen.

  During the manufacture of the structure forming the guide, there is provided in the layer 14 a groove 34 for receiving and positioning the end of this optical fiber to send the beam 22 to the edge 30.

  Another optical fiber 36 is also provided to send light beams, such as beams 26 and 28, into the core 12 of the guide. Another reception and positioning groove 38 is seen which is also provided, during the manufacture of the structure, for receiving and positioning the end of this fiber 36.

  We also see another optical fiber 40, 30 which receives the light having passed through the core 12 of the guide, as well as a groove for receiving and positioning 42 of the end of this fiber 40, this groove 42 also being formed during the manufacture of the structure.

  In an alternative embodiment, the beveled edge 30 may be covered with a reflective layer 44, for example a metal layer, capable of reflecting the light beam 22.

  As seen in Figure 6, it is not necessary that the electrodes 4 and 18 extend over the entire device. It suffices that they extend respectively below and above the zone where the Bragg grating 16 is formed.

  Another example of the device according to the invention is schematically shown in longitudinal section in FIG.

  In the device of FIG. 7, the illumination of the core 12 of the guide, in the zone where the Bragg grating 16 is located, is produced transversely to this core 12 via a blazed Bragg grating (blazed Bragg grating) 46, also known as SFBG (for Slanted Fiber Bragg Grating), which is photo-inscribed at the illumination wavelength in the heart 48 of another optical ribbon waveguide, located nearby immediate guide comprising the heart 12 and parallel to the latter.

  The beam 22 is sent by the means 20 into the core 48 of the other guide and the blazed network 46 then sends this beam 22 towards the core 12, in the area where the network 16 is located, as seen on the figure 7.

  To manufacture the device of FIG. 7, when the upper deconfinement layer 14 is formed, an additional layer is formed above the latter, which is capable of transmitting the beam 22 and in which the core 48 is formed parallel to the core 12. .

  Then the blazed network 46 is formed facing the network 16, after which the core 48, including the network 46, is covered by an additional upper confinement layer 50 and the electrode 18 is formed on this layer 50.

  FIG. 8 is a schematic side view of another example of the device forming the subject of the invention and FIG. 9 is a schematic top view of this

another example.

  In this other example, the illumination of the core 12, in the area where the Bragg grating 16 is located, takes place via an auxiliary optical waveguide, which is beveled.

  In this other example, the illumination beam 22 is folded, by a reflective bevel, towards the core 12 of the ribbon optical waveguide which is present in a planar super-structure, elaborated on the substrate 2 on which is still located the electrode 4 (the assembly formed by the substrate 2 and the electrode 4 can be further replaced by an electrically conductive substrate).

  More precisely, to obtain the structure that can be seen in FIGS. 8 and 9, the previously described structure is formed with reference to FIG. 4, and thus comprising the core 12 provided with the Bragg grating 16. On the layer of upper confinement 14, the electrode 18 is then formed, for example in ITO, which is transparent to the light beam 22.

  A layer 52 is then formed on this layer 18 which is transparent to the beam 22 and serves to transport the latter, as can be seen in FIGS. 8 and 9.

  This layer 52 constitutes the auxiliary waveguide and, more specifically, an optical guiding layer: the beam 22 is confined therein, on one side by the air, and on the other side by the layer 18.

  A reflector bevel 54 is then formed above the Bragg grating 16. This bevel 54 is at an angle of 45 with respect to the plane of the electrode 4. This bevel may also be covered with a reflective layer 56, for example metallic, as seen in Figure 8.

  This bevel is formed on the layers going from the guiding layer 52 to the lower confinement layer 6 and stops at the surface of the electrode 4. Moreover, this bevel does not extend on one side to the other of the structure that we see in Figures 8 and 9. As seen in Figure 9, this bevel extends substantially along the Bragg network 16, overflowing a bit of part and other of the latter.

  The beam 22, which is emitted by the means 20, is sent into the guiding layer 52, or illumination layer, then is reflected on the bevel 54 to be directed towards the core 12, in the area where it is located. the Bragg network 16.

  During the manufacture of the structure of FIGS. 8 to 9, a reception and positioning groove 58 is also provided in the illumination layer 52, at the edge of the latter, this groove being perpendicular to the core 12. The beam 22 is sent in an optical fiber 60 whose end is placed in this groove 58 to further illuminate the core 12 transversely thereto.

  Moreover, as can be seen in FIG. 9, two reception and positioning grooves 62 and 64 are still provided in the structure, at the two ends of the core 12, in order to place the respective ends of two optical fibers 66 therein. and 68, one allowing the arrival of the light beams 26 and 28 in the core 12, the other allowing the recovery of the beam 26 which has traveled this heart 12.

  In another example (not shown) of the invention, the Bragg grating 16 is permanently generated during the orientation of the chromophores of the constituent material of the core 12 of the optical waveguide ribbon, under the effect of an electric field. The latter is applied via a comb-shaped electrode that adopts the same pitch as the Bragg grating to be formed.

  This electrode 70 is schematically represented in FIG. 4, where the teeth of the comb have the reference 72.

  In another example of the invention, the Bragg grating of the switching device is generated temporarily in the core 12 of the ribbon optical waveguide, by generating the electric field through this comb-shaped electrode. during the normal operating mode.

  Figure 10 is a schematic longitudinal sectional view of a device according to the invention, corresponding to this other example.

  The comb-shaped electrode 70, constituting a control electrode, is then formed on the upper confinement layer 14. The number of comb teeth that can be seen in this FIG. 10 is obviously much smaller than it is actually: there are typically more than 1000 teeth, corresponding to the network pitch.

  The voltage source 24 is then provided to apply an electrical voltage between this control electrode and the electrode 4 constituting a ground electrode to generate an index grating (index grating) constituting the Bragg grating, in an area 74 of the core 12, which is located under the control electrode.

  In the example of FIG. 10, the source 24 preferably consists of a source of hyper-frequency voltage. The frequency can be several tens of GHz.

  It is specified that the comb-shaped electrode 70 may be formed by evaporation on the upper confinement layer 14. This electrode is then structured into a comb shape with a pitch which is given by equation (1).

  It is further specified that the efficiency of the Bragg grating that can be formed is directly related to the index contrast between the regions where the applied electric field is maximum (E1) and those where it is minimum (E2), according to equation (3).

  The alternation of these values is shown in FIG. 11. It thus shows this electric field just below the teeth 72 of the comb-shaped electrode (maximum field E1) and just below the zones separating these teeth. (minimum field E2).

  In this FIG. 11, the pitch of the teeth, which is also the pitch of the Bragg grating, has the reference P. For purely indicative and in no way limiting, this pitch P is approximately 0.51 μm, the distance H between the electrode 4 and the electrode 70 is greater than or equal to 5 m and the distance h between the core 12 and the electrode 70 is some m, for example 31um.

  The order of magnitude of the alternation mentioned above is obtained by writing that the ratio of the difference E1-E2 to the mean electric field is proportional to the ratio of the pitch of the comb to the transverse thickness, the latter ratio being itself even of the order of 0.1.

  With an electro-optical coefficient of 30 pm / V, a control electric field of 50 V / m and an index equal to 1.5, equation (3) gives a variation of average index of 2.5x10-3 hence, from the previous proportional relations, an index contrast of about 2.5x10-4 in the network. To have a 20 unit of reflection of the order of the unit, it is necessary to have a network having a length close to 1 cm.

  In another example of the invention, the transverse illumination is made through an appropriate mask, in which case the creation and wavelength agreement of the Bragg grating are done simultaneously, by varying the intensity of the beam. bright 22.

  FIG. 5 shows in longitudinal section an example of such a mask 74. This mask is placed on the layer 18 and has openings 76 separated by portions 78 which are opaque to the light beam 22, so that the part the beam 22 passing through the openings 76 makes it possible to form the Bragg grating in the core 12 of the ribbon optical waveguide.

  The mask 74 may be formed by photolithography on the layer 18, for example from an absorbing or reflecting material at the illumination wavelength, such as chromium by

example.

  Examples of materials that can be used to form the core 12 of the optical ribbon waveguide can be found in particular in document [1]. Let us simply indicate, purely indicative and not limiting, that we can use PMMA-DR1, that is to say the polymethyl methacrylate on which the molecules called Disperse Red # 1 were hooked.

Claims (13)

  An optical switching device comprising: - a first optical ribbon waveguide comprising a court (12) surrounded by confinement layers (6, 14) within which this core is comprised; - a first Bragg grating ( 16) formed in an area of this core, this first Bragg grating being able to reflect a first light beam, which propagates in this core and has a given wavelength, and - variation means, able to vary this given wavelength, this device being characterized in that the core of the first ribbon optical waveguide is made of a non-linear, non-centrosymmetric and photo-isomerizable optical material, which is transparent to the first light beam , and in that the means of variation comprise: means (4, 18, 24, 4, 70, 24) for creating an electric field in the zone of the core of the first ribbon optical waveguide and generation and variation means (20), provided to generate a n second light beam (22), which is adapted to be absorbed by the core material of the first ribbon optical waveguide and to modify the refractive index of this material, and to vary the intensity of this second beam light, 2863728 28 so as to modify this refractive index and therefore the wavelength given when the electric field is created and the second light beam is sent into the heart zone of the first ribbon optical waveguide, in varying the intensity of this second light beam.   2. Device according to claim 1, wherein the means of generation and variation (20) are provided to send the second light beam in the heart zone of the first ribbon optical waveguide transversely to this core.   3. Device according to claim 2, wherein one (14) of the confinement layers comprises an edge (30) which is adapted to reflect the second light beam (22) towards the core area of the first optical waveguide ribbon and the generating and varying means (20) are provided for sending the second light beam to the core area of the first optical ribbon waveguide through the edge of this confinement layer (14).   4. Device according to claim 3, wherein the edge of the confinement layer (14) is coated with a layer (44) which is able to reflect the second light beam (22).   The device of claim 2, further comprising: - a second ribbon optical waveguide having a core (48) which is parallel to and adjacent to the core (12) of the first optical ribbon waveguide , the heart of this second optical ribbon waveguide being intended to carry the second light beam (22) which is generated by the generation and variation means (20), and - a second Bragg grating (46) which is blazed, formed opposite the first Bragg grating (16), in the heart of the second ribbon optical waveguide, and provided to send this second light beam into the zone of the heart of the first ribbon optical waveguide where is formed the first Bragg network.   6. Device according to claim 2, further comprising an optical guiding layer (52) which is intended to transport the second light beam, generated by the generation and variation means (20), this optical guiding layer comprising one end beveled (54) facing the region of the core (12) of the first ribbon optical waveguide, where the first Bragg grating (16) is formed, this optical guiding layer being provided for reflecting the second light beam towards this zone, through this beveled end.   7. Device according to any one of claims 1 to 6, wherein the material of which is the core of the first ribbon optical waveguide is a polymer.   8. Device according to any one of claims 1 to 7, wherein the first Bragg grating (16) is formed permanently in the core area (12) of the first ribbon optical waveguide.   9. Device according to claim 8, wherein the first Bragg grating (16) is formed by photo-bleaching or photo-inscription through a mask.   The device of claim 8, wherein the material of which the core (12) of the first ribbon optical waveguide is made is a polymer, this polymer comprises chromophores and the first Bragg grating (16) is formed by orientation. chromophores under the effect of an electric field, this electric field being applied via a electrode (70) in the form of a comb having the same pitch as this first Bragg grating.   11. Device according to any one of claims 1 to 7, wherein the first Bragg grating (16) is formed temporarily in the region of the core (12) of the first ribbon optical waveguide.   12. Device according to claim 11, wherein the polymer is electro-optical and comprises chromophores and the means for creating the electric field comprise: first and second electrodes (4, 70) which are placed on either side of the first ribbon optical waveguide, the first electrode (70) being comb-shaped and having teeth (72) extending transversely to the core (12) of the first ribbon optical waveguide, which comb has the not even the first Bragg grating (16), and - a voltage source (24) provided for temporarily biasing the first electrode (70) relative to the second electrode (4) and thereby temporarily creating the electric field as well as the first Bragg network.   The device according to claim 11, further comprising a mask (74) which is placed opposite the heart area (12) of the first ribbon optical waveguide and has a pattern which reproduces the pattern of the first light waveguide. Bragg (16), this first Bragg grating being created when the second light beam (22) is sent into this area through the mask.