Abstract:
The present invention relates to an optical device comprising an ionic conductor and a pair of electrodes, the ionic conductor being made of a material that is transparent to light and contains mobile ions and the electrodes being suitable for absorbing and desorbing the ions and being in ionic contact with the ionic conductor. The refractive index in at least a zone of the ionic conductor can be varied under the effect of the voltage applied between the electrodes. The electrodes contain an electrochemically active material selected from an active carbon, a conductive polymer, and an insertion material suitable for inserting ions in its structure.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an optical device for transmitting light through a medium of refractive index that can be modified depending on user requirements. The invention also relates to apparatus using the optical device. The invention also extends to a method enabling the refractive index of such a device to be varied. 
     BACKGROUND OF THE INVENTION 
     An optical device for transmitting light, also known as a waveguide, comprises a light-guiding portion or “core” and a cladding portion for confining light in the core. The device may comprise a waveguide having a core that is circular (optical fibers) or rectangular, and surrounded in cladding of lower refractive index. The device may also be a plane waveguide, likewise comprising a guiding layer and cladding, and deposited on a substrate acting as a support. Waveguides based on silica glasses have been in existence for a long time. Waveguides based on organic polymers constitute a more recent development. 
     The ability to modify the refractive index of a waveguide finds numerous applications in optical components. Thus, wavelength-selective filters are essential components for communications systems. Filters enable spectral separation to be performed by reflecting at certain wavelengths and transmitting at others. The filter function is performed in particular by diffraction gratings known as “Bragg” gratings. A Bragg grating is a wavelength-selective reflector having a periodic structure that establishes index modulation. It is characterized by its passband, i.e. the range of wavelengths that it allows to pass. A given array generally presents a reflection maximum at a certain wavelength λ b  referred to as the “Bragg wavelength”. It is defined by the relationship: λ b =2.n eff .Λ where Λ is the pitch of the Bragg grating and where n eff  is the mean effective refractive index of the guided fundamental mode incident on the grating. When the index can be modified, the Bragg wavelength λ b  can also be modified, so the filter becomes tunable. Bragg gratings can be made by optical, mechanical, or chemical methods that lead to a physical modification of the support. It has recently become possible to transform a small portion of a waveguide into a filter by inducing periodic variation in the refractive index of the core in the form of lines that are regularly spaced apart at a fixed distance; 
     In general, changing the mean effective refractive index n eff  of a simple waveguide enables an optical wavelength in a system to be adjusted. By way of example, this can be used to achieve fine adjustment of the characteristic frequencies of the system. Tunable lasers, tunable filters, and modulators are other examples of components that make use of such variable-index waveguides. 
     By way of example, mention can be made of a two waveguide interferometer of the Mach-Zehnder or Michelson type, which is used to obtain, in particular, a tunable filter or a modulator. Under such circumstances, the refractive index in one of the two waveguides is caused to vary so as to modify the transmission of the interferometer. 
     A Fabry-Perrot filter is a multiple wave interferometer in which a change of index causes resonant frequencies to be shifted and which can be used to obtain a tunable filter. A tunable laser is another particular case of this type of filter where the optical length of the cavity can be adjusted so as to control the emission wavelength very precisely. To do this, a phase section is disposed within the cavity, which phase section is a waveguide of modifiable index. 
     Mention can also be made of arrayed waveguide multiplexers/demultiplexers sometimes referred to as “phasars” or by the initials “AWG” (arrayed waveguide grating). The ability to modify the refractive index of waveguides can be used for tuning the spectral response of the components. 
     Naturally, it is also possible to modify the index of a simple waveguide that does not form part of an interferometer system. For example, in a junction, it can be important to modify the refractive index of one waveguide relative to another. For example, in a Y junction, by changing the index of one of the two outlet arms, it is possible to direct the inlet signal to one outlet or the other. This constitutes a switch known as a digital optical switch. The need for such systems is very important in all switching applications. 
     European patent EP-0 496 278 describes an optical device whose refractive index is controlled by a method based on the principle that the refractive index of a material containing mobile ions, i.e. an ionic conductor, varies reversibly as a function of the electric field applied thereto. The optical device comprises an ionic conductor made of a transparent material of high molecular weight containing mobile ions, and at least one pair of electrodes facing each other across the ionic conductor and in contact with the ionic conductor. When an electric field is applied to the conductor by means of the electrodes, its refractive index is modified in at least one ionic conductor zone, depending on the applied electric field. Under the effect of the potential difference applies across the electrodes, ions contained in the conductor move through the material so that the refractive index of the ionic conductor increases at one of its interfaces with the electrodes and decreases at its other interface. The device operates at low voltage, e.g. 20 volts (V). 
     The capacitance of the electrodes described is very small since ions can accumulate only on their surfaces. As a result, as soon as the voltage is no longer maintained, the change in index at the interface disappears quickly by self-discharge. In addition, the change in refractive index as obtained in that way is restricted to the contact interface between the ionic conductor and the electrodes. As a result the change is difficult to reverse and leads to an aging phenomenon. Finally, that device is not easy to use. It has the drawback of requiring its electrodes to be permanently maintained at a voltage that is relatively high (20 V) in order to conserve the desired refractive index, thereby leading to non-negligible consumption of electricity. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to eliminate the drawbacks of the prior art, and in particular to provide a device in which reversible variation of refractive index is obtained throughout the volume of the material while requiring voltages that are smaller (&lt;20 V) than those previously known. 
     The present invention provides an optical device comprising an ionic conductor constituted by a material that is transparent to light and contains mobile ions, and at least one pair of electrodes, the refractive index in at least one zone of said ionic conductor being variable under the effect of a voltage applied between said electrodes, the device being characterized in that said electrodes are suitable for absorbing and desorbing ions and are in ionic contact with said ionic conductor. 
     The voltage applied to the electrodes varies the concentration of ions in the conductor. This variation of concentration propagates by diffusion within the ionic conductor. The variation in concentration depends on the ability of the electrodes to absorb and desorb ions, and on the quantity of ions present in the conductor. Varying the concentration of ions leads to a variation in the refractive index of the conductor. 
     The electrodes used contain an electrochemically active material whose function is to absorb and retain the ions extracted from the ionic conductor. Each of the electrodes contains an electrochemically active material which is selected from known active materials suitable for performing this function, and in particular from an active carbon, a conductive polymer, and an insertion material suitable for inserting ions in its structure. 
     In a first variant, the electrodes contain an active carbon as the electrochemically active material. It is preferable to use an active carbon having a specific surface area greater than 200 square meters per gram (m 2 /g). Such electrodes are also known as “blocking” electrodes since they store ions using the principle of the electrochemical Helmholtz double layer. The quantity of ions that can be stored is proportional to the specific surface area of the material. In this case, the quantity Q of ions absorbed by the electrodes is proportional to the voltage applied between the two electrodes in application of the relationship Q=C×V where C is the ability of the electrodes to absorb and desorb ions, expressed in units of capacitance (i.e. Farads (F)). 
     In a second variant, a first electrode contains as its electrochemically active material a material suitable for inserting cations in its structure, while the second electrode contains as its electrochemically active material an insertion material suitable for inserting ions in its structure. The material suitable for inserting cations in its structure is preferably selected from an oxide of transition metals, a sulfide, a sulfate, and mixtures thereof. A material suitable for inserting anions in its structure is preferably selected from a graphitic oxide, an oxide, a coke, a carbon black, and a vitreous carbon. Amongst the transition metal oxides that are suitable for use in the present invention, mention can be made of vanadium oxide, lithium-containing oxides of manganese, nickel, and/or cobalt, and mixtures thereof. 
     In a third variant, the electrodes contain a conductive polymer as the electrochemically active material. The conductive polymer is preferably selected from a polypyrrole, a polythiophene, a polyaniline, a polyacetylene, and a polyparaphenylene. Most of these materials can be N or P doped. A P-doped material absorbs anions (negatively charged ions) and an N-doped material absorbs cations (positively charged ions). 
     Naturally, it is possible to combine the electrodes corresponding to the three variants described above with one another. 
     In the present invention, the electrodes are bulk electrodes, i.e. their entire volume is involved in the electrochemical reaction. They therefore possess high capacitance. As a result, as soon as the voltage is no longer applied, the memory effect is large and the resulting variation in index will hardly change. However, in order to conserve the refractive index precisely at the selected value, it is preferable to maintain the applied potential. Thus, all drift is avoided and electricity consumption is practically zero since well below 1 microamp (μA). 
     The device of the present invention comprises an ionic conductor containing mobile ions. The conductor comprises at least one cation and at least one anion. The cation may be selected from Li + , Na + , K + , Ca 2+ , and NH 4 +. The anion may be selected from CF 3 SO 3   − , N(CF 3 SO 2 ) 2   − , N(C 2 F 5 SO 2 ) 2   − , C(CF 3 SO 2 ) 3   − , CF 3 CO 3   − , ClO 4   − , BF 4   − , AsF 6   − , PF 6   − , BH 4   − , SCN − , N 3   − , I − , and Br − . 
     The material constituting the ionic conductor is preferably selected from a polymer containing a dissociated salt and a polymer containing a salt dissolved in a solvent. 
     The term “polymer containing a dissociated salt” is used to mean a polymer which complexes an ionizable salt. Under such circumstances, the polymer plays an active role relative to the salt since it contributes to its dissociation and to salvation of the ions. Such polymers are also referred to as “dry” polymers since they do not contain any solvent. As a polymer containing a dissociated salt, it is possible to select a polyether or a polyoxyethylene. 
     The term “polymer containing a salt dissolved in a solvent” is used to mean a polymer swollen by a solvent which contains a dissociated salt. The polymer is then said to be gelled or plasticized, and the liquid salt solution is often also referred to as a “plasticizer”. Under such circumstances, the polymer plays a passive role relative to the salt since it contributes to a negligible extent to dissociation thereof and to solvation of the ions. As the polymer containing a salt dissolved in a solvent, it is possible to select: a thermoplastic polymer, a fluorine-containing homopolymer, or a fluorine-containing copolymer. When the polymer containing a salt dissolved in a solvent is a thermoplastic polymer, it is preferably selected from a polyacrylonitrile, a polymethyl methacrylate, a polyvinylchloride, and copolymers thereof. 
     Amongst the salts usable in the present invention, it is possible in non-exhaustive manner to select from: a lithium salt, a sodium salt, a potassium salt, a calcium salt, an ammonium salt, and a mixture of such salts. As lithium salt, it is preferable to select lithium perchlorate LiClO 4 , lithium hexafluoroarsenate LiAsF 6 , lithium hexafluorophosphate LiPF 6 , lithium tetrafluoroborate LiBF 4 , lithium trifluoromethanesulfonate LiCF 3 SO 3 , lithium trifluoromethanesulfonimide LiN(CF 3 SO 2 ) (LiTFSI), lithium trifluoromethanesulfonemethide LiC(CF 3 SO 2 ) 3  (LiTFSM), or lithium bis(perfluoroethylsulfonimide) LiN(C 2 F 5 SO 2 ) 2  (BETI). As an ammonium salt, it is possible to use tetralkylammonium chloride. 
     The polymer-based ionic conductor containing mobile ions as described above is included in a waveguide. The waveguide has a guiding portion surrounded by a cladding portion. When it is desired to modify the refractive index of the ionically-conductive cladding portion and when the electrodes are not in direct contact with the guiding portion but are spaced apart therefrom by a layer of the material of the cladding portion, said material must be ionically conductive in order to ensure continuity of ionic contact between the electrodes and the guiding portion. 
     In a first embodiment of the invention, the electrodes are in physical contact with the guiding portion. 
     In a variant, the guiding portion and the cladding portion are ionic conductors. In which case, index variation takes place both in the guiding portion and in the cladding portion. 
     In another variant, the guiding portion is ionically conductive and the cladding portion is not an ionic conductor. In which case, index variation occurs only in the guiding portion. 
     In a second embodiment, the electrodes are disposed in physical contact with the cladding portion, but they are not in contact with the guiding portion. 
     In a variant, the guiding portion and the cladding portion are ionically conductive. The electrodes are then in ionic contact with the guiding portion via the cladding portion. While a voltage is applied between the electrodes, variation in salt concentration propagates by diffusion throughout the volume of the cladding portion and the guiding portion. Given the very small dimensions of an optical fiber or a waveguide, such diffusion occurs almost instantaneously. 
     In another variant, the guiding portion is not ionically conductive and the cladding portion is ionically conductive. The electrodes are then in ionic contact only with the cladding portion. In which case, the variation in salt concentration and thus the variation in refractive index occurs only in the cladding portion which results in a change in the mean effective refractive index n eff  of the waveguide. 
     In a third embodiment, the electrodes are disposed respectively on opposite sides of the guiding portion. 
     In a fourth embodiment, the electrodes are respectively disposed on the same side of the guiding portion. When a voltage is applied between the electrodes, the variation in salt concentration propagates by diffusion throughout the volume of the cladding portion and of the guiding portion regardless of the relative disposition of the electrodes. 
     In a first implementation, the waveguide is an optical fiber comprising a core constituting the guiding portion and cladding constituting the cladding portion. Such a fiber is also known as plastic optical fiber (POF). 
     In a second implementation, the waveguide is a plane waveguide deposited on a substrate, comprising a guiding layer constituting the guiding portion and surrounded by cladding constituting the cladding portion. The waveguide may also be surrounded by a superstrate. 
     Naturally, the optical fibers and the waveguides to which the invention applies may be fabricated using any conventional method. 
     In a particular embodiment of the present invention, the guiding portion contains a Bragg grating. 
     The most remarkable advantage of the present invention lies in the fact that the change of index persists for a relatively long period of time even after the voltage has ceased to be applied, unlike the prior art. It suffices to apply a different voltage in order to cause the index to change again. The user can thus tune the value of the index to requirements on a permanent basis. In addition, index variation applies to the entire volume of the ionic conductor. Reversibility is excellent: 10 million cycles have been performed with blocking electrodes having active carbon as the active material. 
     The invention is more particularly intended for use with “monomode” type waveguides. 
     The present invention also provides a method of modifying the refractive index of an ionic conductor by means of at least one pair of electrodes, the conductor being constituted by a material that is transparent to light and that contains mobile ions, the method being characterized in that it comprises the following steps:
         placing said electrodes in ionic contact with said conductor having an initial refractive index;   applying a voltage of less than 20 V between said electrodes so as to cause a variation in said initial index;   obtaining said ionic conductor with a modified refractive index; and   removing the applied voltage, with said modified refractive index being maintained.       

     The method of the invention has the advantage of applying voltages (&lt;20 V) smaller than required by prior art methods (20 V), and for a short duration. The applied voltage is preferably not more than 5 V (≦5 V). At higher voltages, irreversible reactions might occur which would cause the device to age prematurely. There is no need to apply voltage for a very long duration. Voltage can be applied for a duration lying in the range 10 milliseconds (ms) to 100 ms. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics and advantages of the present invention appear from the following embodiments, which are naturally given by way of non-limiting illustration, and from the accompanying drawings. 
         FIG. 1  is a longitudinal section through a first embodiment of the invention in which the ionic conductor is an optical fiber. 
         FIG. 2  is a cross-section through a second embodiment of the invention in which the ionic conductor is a plane waveguide. 
         FIG. 3  shows a variant of the second embodiment in which the disposition of the electrodes is different. 
         FIG. 4  is a cross-section view of a third embodiment of the invention in which the ionic conductor is a plane waveguide. 
         FIG. 5  shows a variant of the third embodiment in which the disposition of the electrodes is different. 
         FIG. 6  is a cross-section view of a fourth embodiment of the invention in which the ionic conductor is a plane waveguide. 
         FIG. 7  is a longitudinal section view of a fifth embodiment of the invention in which the ionic conductor is a waveguide containing a Bragg grating. 
         FIG. 8  is a view analogous to  FIG. 7  showing a variant of the fifth embodiment in which the disposition of the electrodes is different. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a device  10  of the invention comprising an optical fiber  11  presenting a core  12  surrounded by cladding  13 . The core  12  of refractive index n 12  is constituted by a material comprising a mixture made up of 80% by weight of O′, O′-bis (2-aminopropyl) polyethylene glycol 1900 (sold under the trademark “Jeffamine” by the supplier Huntsman) and 20% by weight of ethylene glycol diglycidyl ether (from the supplier Aldrich), in which a salt has been added constituted by sodium iodide NaI at molar (1M) concentration, together with a cross-linking catalyst constituted by lithium perchlorate LiCiO 4  at a concentration of 0.1 M. 
     The cladding  13  having a refractive index n 13  greater than n 12  is constituted by a material analogous to that of the core but in which a fluorine-containing additive has been added in order to decrease its refractive index. The device  10  also comprises a first electrode  14  and a second electrode  15  disposed on either side of the optical fiber  11 . The electrodes  14  and  15  are in ionic contact with the core  12  via the cladding  13  which is itself an ionic conductor. The two electrodes  14  and  15  are identical, each being constituted by a mixture comprising 50% by weight active carbon BRX manufactured by the supplier Norit and 50% by weight of the material constituting the core  12 . 
     The device  20  of the invention shown in  FIG. 2  comprises a plane waveguide comprising a guiding layer  21  having a refractive index n 21 , constituted by material analogous to that of the core  12  of  FIG. 1 , but in which a sulfur-containing additive has been added in order to increase its refractive index. The guiding layer  21  is surrounded by cladding  22  having a refractive index n 22  less than n 21 , constituted by material analogous to that of the core  12  of  FIG. 1 . The guiding layer  21  surrounded by the cladding  22  is deposited on a substrate  23  which is preferably made of silicon Si or of indium phosphide InP. The device  20  also comprises a first electrode  24  and a second electrode  25  disposed on either side of the waveguide and in physical contact with the guiding layer  21 : they are thus in ionic contact with the guiding layer  21 . Both electrodes  24  and  25  are identical and made of a material analogous to that of the electrodes  14  and  15  of  FIG. 1 . 
     A voltage of 2.2 V is applied between the electrodes  24  and  25  in order to cause the index n 21  of the guiding layer  21  of the waveguide to vary. A variation Δn 21  is obtained of 5×10 −2 . The capacitance of the device  20  is 0.02 F. Once the index variation Δn 21  has been obtained, if it is desired to maintain the voltage between the electrodes  24  and  25 , then the current that is observed to flow is much less than 1 μA. If the applied voltage is removed, then the index variation persists for several hours at least. Subsequently, return to the initial state takes place very slowly. In the present case, the cladding  22  is also constituted by ionically conductive material. By diffusion, variation also occurs simultaneously in the refractive index n 22  of the cladding  22 . 
       FIG. 3  shows another device  30  of the invention comprising a plane waveguide comprising a guiding layer  31  with a refractive index n 31  surrounded by cladding  32  having a refractive index n 32  less than n 31  and constituted by a material analogous to that of the optical cladding  13  of  FIG. 1 . The guiding layer constituted by a material analogous to that of the core  12  of  FIG. 1  but not containing the salt NaI (sodium iodide) is not ionically conductive. The guiding layer could equally well be constituted by silica. The waveguide is deposited on a substrate  33  analogous to that of  FIG. 2 . The device  30  also has a first electrode  34  and a second electrode  35  disposed in the cladding  32  on either side of the guiding portion  31 , but not in physical contact with the guiding portion  31 . The material constituting the cladding  32  is ionically conductive so the electrodes  34  and  35  are thus in ionic contact with the guiding layer  31  via the cladding  32 . The two electrodes  34  and  35  are identical and they are made of a material analogous to that of the electrodes  14  and  15  of  FIG. 1 . 
     As above, a voltage of 2.2 V is applied between the two electrodes  34  and  35  in order to cause the index n 32  of the cladding  32  of the waveguide to vary. A variation Δ n   32 =5×10 −2  is obtained. The capacitance of the device  30  is 0.02 F. Once the variation Δn 32  has been obtained in the index, if it is desired to maintain the voltage between the electrodes  34  and  35 , the current that is observed to flow is much less than 1 μA. If the applied voltage is removed, the change in index persists for several hours at least. Return to the initial state takes place very slowly. In the present case, the material constituting the guiding layer  31  is not ionically conductive since it does not contain any salt (NaI). There is thus no variation in the refractive index n 31  of the guiding layer  31 . The use of such a guiding layer whose constituent material is not ionically conductive has the advantage of enabling the optical characteristics of the material used to be optimized without taking any other consideration into account. 
     In the device  40  of  FIG. 4 , there can be seen a plane waveguide comprising a guiding layer  41  having a refractive index n 41  constituted by material analogous to that of the core  12  of  FIG. 1 . The guiding layer  41  is surrounded by cladding  42  having a refractive index n 42  less than n 41  and constituted by a material analogous to that of the cladding  13  of  FIG. 1 . The waveguide is deposited on a substrate  43  analogous to that of  FIG. 2 . The device  40  also comprises a first electrode  44  and a second electrode  45  that are disposed on the same side of and in physical contact with the guiding layer  41 : they are thus in ionic contact with the guiding layer  41 . The electrode  44  is constituted by a mixture comprising 50% by weight polythiophene and 50% by weight of the material constituting the core  12  of  FIG. 1 . The electrode  45  is constituted by a mixture comprising 50% by weight polypyrrole and 50% by weight of the material constituting the core  12  of  FIG. 1 . 
     In the device  50  of  FIG. 5 , there can be seen a plane waveguide analogous to that of  FIG. 4 , comprising a guiding layer  51  having a refractive index n 51  surrounded by cladding  52  having a refractive index n 52  less than n 51 . The waveguide is deposited on a substrate  53  analogous to that of  FIG. 2 . The device  50  also comprises a first electrode  54  and a second electrode  55  disposed in the cladding  52  on the same side of the guiding layer  51  and in ionic contact therewith via the ionically conductive material of the cladding  52 . The electrodes  54  and  55  are identical and are made of a material analogous to the materials of  FIG. 4 . 
       FIG. 6  shows another device  60  of the invention comprising a plane waveguide comprising a guiding layer  61  having a refractive index n 61 , constituted by a material analogous to that of the core  12  in  FIG. 1 , in which a sulfur-containing additive has been added in order to increase its refractive index. The guiding layer  61  is surrounded by cladding  62  of refractive index n 62  less than n 61 , constituted by a material analogous to that of the core  12  in  FIG. 1 . The waveguide is deposited on a substrate  63  analogous to that of  FIG. 2 . The device  60  also comprises a first electrode  64  and a second electrode  65  disposed in the cladding  62  on the same side of the guiding layer  61  and in ionic contact therewith via the cladding  62 , which is itself ionically conductive. The electrode  64  is constituted by a mixture comprising 50% by weight polythiophene and 50% by weight of the material constituting the core  12  of  FIG. 1 . The electrode  65  is constituted by a mixture comprising 50% by weight polypyrrole and 50% by weight of the material constituting the core  12  of  FIG. 1 . 
       FIG. 7  shows a device  70  of the invention. A waveguide comprises a guiding layer  72  having a refractive index n 72  and is surrounded by cladding  73  having a refractive index n 73  less than n 72 . The guiding layer  72  is constituted by silica and it is not ionically conductive. The cladding  73  is constituted by a material analogous to that of the cladding  13  in  FIG. 1 . The device  70  further comprises a first electrode  74  and a second electrode  75  disposed in the cladding  73  on either side of the guiding layer  71 . The electrodes  74  and  75  are thus in ionic contact with the cladding  73  which is itself an ionic conductor. The electrodes  74  and  75  are identical and are constituted by a material analogous to the material of the electrodes  14  and  15  of  FIG. 1 . 
     A portion of the guiding layer  72  of the fiber  71  is used as an adjustable filter. A Bragg grating  76  has been formed therein which is constituted by periodic variation in the amplitude Δn 72  of the refractive index of the layer  72  about its mean value n 72  with a spatial period Λ (grating pitch) along the propagation axis of the wave. The Bragg grating may also be obtained by variation in the index Δn 73  of the cladding  73  about its mean value n 73 . If a voltage U, e.g. 3 V, is applied between the electrodes  74  and  75 , the variation in the mean effective index of the waveguide n eff  induced by said voltage gives rise to variation in the center wavelength λ b  of the filter by virtue of the relationship: λ b =2.n eff .Λ. 
     The mean effective refractive index n eff  represents a characteristic magnitude of the filter which is a function of n 72  and n 73 . That is why it is possible to obtain variation of n eff  by varying either n 72  or n 73  or indeed both of them. The direction in which the index n eff  varies depends on the respective indices of the salt and of the polymer constituting the ionic conductor: if the salt has an index greater than that of the polymer, the extraction of salt will lower the index of the core, and vice versa. 
     Naturally, a waveguide containing a Bragg grating can be made not only using an optical fiber, but also using any type of waveguide, and in particular plane waveguides of the kind used in integrated optics. 
     In the present case, the Bragg grating may be made in the silica guiding portion by a known optical, mechanical, or chemical method leading to physical modification of the support. It is possible to inscribe a Bragg grating in a polymer, in particular under the effect of irradiation. This can be done providing the polymer is photosensitive. The use of a holographic method has also been mentioned (B. L. Booth et al.: “Polyguide™ polymeric technology for optical interconnect circuits and components”, SPIE 1997). 
     Bragg gratings are in particularly widespread use in devices for wavelength-division multiplexing (WDM) applications. 
       FIG. 8  shows a device  80  of the invention. A waveguide comprises a guiding layer  82  having a refractive index n 72  and surrounded by cladding  83  having a refractive index n 73  less than n 72 . The guiding layer  82  is constituted by a material analogous to that of the core  12  of  FIG. 1 . The cladding  83  is constituted by a material analogous to that of the cladding  13  of  FIG. 1 . The device  80  also comprises a first electrode  84  and a second electrode  85  both disposed on the same side of and in physical contact with the guiding layer  82 : they are thus in ionic contact with the guiding layer  82 . The electrodes  84  and  85  are identical and they are made of a material analogous to the material of the electrodes  14  and  15  of  FIG. 1 .