Abstract:
An optical device includes a waveguiding configuration and an electrical capacitive configuration. The capacitive configuration includes glass electrode region(s) and glass dielectric region(s) with electrical leads, e.g., metallic conductors, for applying electrical control signals to the electrode region(s). The preferred metals for the conductors are Ni, Ti and Au. The waveguiding structure is configured so that the fields associated with optical signals propagating therein interact with dielectric region(s). During the use of the device control signals are applied to the dielectric region(s) by the electrical leads. The control signals have the effect that temporary electric fields are applied to the dielectric region(s). The fields change the optical properties of the dielectric region(s) and this affects the propagation of the optical signals. The device can either be produced in planar or fibre configurations.

Description:
RELATED APPLICATION 
     This application is related to co-pending commonly assigned U.S. application Ser. No. 09/068,084 now U.S. Pat. No. 6,044,190 filed May 4, 1998, entitled OPTICAL WAVEGUIDE STRUCTURE naming Raman Kashyap as inventor. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to optical devices which include a waveguiding configuration. 
     2. Related Art 
     SUMMARY OF THE INVENTION 
     It has been recognised that the application of an electrical field to a glass can change the optical properties of the glass. In particular the electric field can change the refractive index of the glass. Since the refractive index affects the velocity of light in the medium changing the refractive index changes the velocity of light and this can be used to produce phase modulation. Phase modulation can be used to produce other effects, e.g. a Mach Zehnder configuration converts phase modulation into on/off modulation. 
     According to this invention an optical device includes a waveguiding configuration and an electrical capacitive configuration wherein the capacitive configuration comprises glass electrode regions and glass dielectric regions with electrical leads, e.g. metallic conductors, for applying electrical control signals to the electrode regions. The preferred metals for the conductors are Ni, Ti and Au. The waveguiding structure is configured so that the fields associated with optical signals propagating therein interact with dielectric regions. During the use of the device control signals are applied to the dielectric regions by means of the electrical leads. The control signals have the effect that temporary electric fields are applied to the dielectric regions. The fields change the optical properties of the dielectric regions and this affects the propagation of the optical signals. 
     Optical devices according to the invention can be considered as glass structures which comprise a waveguiding configuration superimposed upon an electrical capacitive configuration. Both configurations are implemented as a base glass composition, e.g. silica, with various additives to produce the different regions of the waveguiding configuration and the electrical capacitive configuration. 
     As is conventional, the waveguiding configuration may either be in fibre or, preferably, planar format. In either case the waveguiding configuration comprises a core formed of one or more glass compositions and a cladding comprising one or more glass compositions. In a conventional waveguide, it is desirable that the core has a uniform refractive index and the cladding has a uniform (lower) refractive index. In other words, for a conventional waveguide, all the glasses comprised in the core should have the same refractive index and all the glasses comprised in the cladding should have the same (lower) refractive index. This conventional waveguiding structure may be modified in devices according to the invention. For example, as will be discussed in greater detail below, it may be convenient for electrode regions and/or dielectric regions to extend into both the core and cladding. Since it is clearly convenient for any one region to have a constant composition it follows that any one region will, conveniently, have a constant refractive index. This further implies that there may be places where the refractive index of the core will be equal to the refractive index of the cladding. This is unconventional in waveguide design but, at least for small regions, the waveguiding properties are not substantially degraded and this is acceptable to achieve better electrical properties. 
     The electrical capacitive configuration comprises one or more glass regions having a very high electrical resistivity to constitute the dielectric region or regions and glasses having a lower electrical resistivity to constitute the electrode region or regions. Since the changes in optical properties occur primarily in the dielectric regions it is necessary that these regions be located where they will interact with the fields of optical signals propagating in the waveguide. The dielectric regions may either be placed in the claddings to interact with the evanescent fields of signals travelling in the core or a more direct interaction can be achieved when the dielectric regions are located in the core. 
     Planar devices according to the invention are made using a combination of deposition techniques and photolithography. There are several well known techniques for depositing glasses onto substrates in order to make planar devices, e.g. flame hydrolysis, chemical vapour deposition and sol-gel deposition. A complicated structure will require several stages of deposition interspersed with several stages of photolithography. It is also necessary to incorporate the electrical leads at a suitable stage of the process. For fibre configurations, it is appropriate to prepare a preform, e.g. by vapour phase deposition, which is then pulled into a fibre. In this case it is usually more convenient to attach the leads after the pulling. 
     The preparation of devices according to this invention includes, as well as the stages mentioned above, polling to enhance the electrical effects. The purpose of polling is to introduce permanent electric fields into the structure. The fields are “permanent” in that they remain for the lifetime of the device. Polling comprises applying high voltages, e.g. 10-100 kv, to the dielectric regions of the device. It is convenient to apply the polling voltage by means of the leads connected to the electrode regions. 
     It is believed that the high voltages applied during polling cause the migration of electrons within the glass structure. This creates electron depletion regions which effectively have a positive electrical charge and electron excess regions which effectively have a negative electric charge. The permanent fields exist between the depletion region and the excess region. 
     During the polling the structure is preferably irradiated, e.g. with ultraviolet light (e.g. within the wavelength range 190 nm to 270 nm). The irradiation enhances the electrical conductivity of the glasses and it is convenient to define the effect of the irradiation as “photo-conductivity”. The polling and irradiation may be carried out at ambient temperature but, in some cases, higher temperatures, e.g. 100° C. to 400° C., enhance the effect. It is also possible to achieve polling without irradiation, e.g. at high temperatures such as 100° C. to 400° C., but irradiation constitutes the preferred technique. 
     The polling applies high fields at a time when the electrical conductivity is enhanced and therefore electric charges (electrons) are more mobile. The result is that electric fields are produced in or near the dielectric regions. For example the fields are produced at or across the interfaces between the electrode regions and dielectric regions. When the enhancement is removed these fields remain and they become “permanent” as described above. In the use of the device control signals of 5-100 v are applied to the leads. The temporary fields produced by the control signals interact with the permanent fields to modify the optical properties of the dielectric region. 
     It is emphasised that the electrical conductivity is enhanced during the polling and the enhancement facilitates the movement of electric charges. When the enhancement is removed the electrical conductivities return to normal and the charges become trapped. It will be recognised that it is desirable to avoid conditions which will enhance the electrical conductivity after the polling has taken place. Unintended enhancement of the conductivity could allow the charges to leak and degrade the operational performance of the device. Thus it is advisable to avoid irradiation or heating of the device after polling. In other words, any shaping of the device which requires heating to high temperatures should be carried out before the polling. Temperatures which could result in charge leakage are higher than those to be expected in the normal use of telecommunications devices. 
     It is now convenient to discuss the chemical constitution of the preferred glasses which constitute the devices described above. 
     The major component of all the regions is silica, SiO 2 . More specifically all of the regions contain at least 70% of weight by SiO 2 . The dielectric regions which, as stated above, have the highest electrical resistance in the systems are preferably constituted of pure SiO 2  or SiO 2  lightly doped to adjust its refractive index in accordance with waveguiding requirements. Other regions are constituted by SiO 2  with various dopants to modify the properties and to produce the structure of the device. More specifically four dopants are particularly useful and these will be identified and their effects described. It is convenient to specify the dopants as elements although it will be appreciated that the elements are present in the glass in combined form. The four (elementary) dopants are germanium, phosphorus, boron and fluorine. The effects of each will now be described. 
     Germanium 
     Germanium is present in the silica glass compositions as GeO 2  and it has the effect of increasing the refractive index of the silica. In addition, germanium has the effect of increasing both the conductivity and photo-conductivity of the glass so that it is convenient for forming an electrode region. Where vapour phase deposition is used to prepare the devices the germanium can be introduced into the reactions as GeCl 4 . Germanium may be present in the glass at concentrations of up to 20% by weight, e.g. 2-20% by weight, based on the element Ge and the total glass composition. 
     Phosphorus 
     Phosphorus may be present in the glasses in various forms but is believed that most of the phosphorus is present in the forms of phosphates of the other elements in the glass. In any case, phosphorus increases the refractive index of the glass and it also increases the electrical conductivity. The presence of phosphorus tends to soften the glass during processing and it is, therefore, sometimes convenient to use phosphorus as a processing aid. In vapour phase depositions phosphorus is usually fed into the reaction as POCl 3  or POCl 3 . Phosphorus may be present at concentrations up to 15% by weight, e.g. 2-15% by weight, based on the element and the total glass composition. 
     Boron 
     Boron is probably present in the glass as B 2 O 3  and it can be introduced into vapour phase deposition in the form of BCl 3 . Boron has the effect of reducing the refractive index. Boron may be present at up to 15% by weight, e.g. 2-15% by weight, based on the element and the total glass composition. 
     Fluorine 
     Fluorine is probably present in the glasses as fluorides and it is introduced into vapour phase depositions in the form of CCl 2 F 2  or SiF 4 . Fluorine is similar to boron in that it reduces the refractive index and has little or no effect on the electrical properties. Fluorine may be present at up to 5% by weight based on the element and the total of glass composition. 
     The invention, which is more fully defined in the claims, includes devices comprising glass waveguiding and glass capacitive regions. It also includes methods of making these devices and more complicated devices which include them. Examples of more complicated devices include optical switches and modulators such as Mach Zehender modulators, 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention will now be described by way of example with reference to the accompanying drawings in which; 
     FIG. 1 is a longitudinal, vertical cross section through the core of a planar device, 
     FIG. 2 is a horizontal cross section through the core shown in FIG. 1, 
     FIG. 3 is a horizontal cross section through a terminal, 
     FIG. 4 is vertical cross section through a terminal as shown in FIG. 3, 
     FIG. 5 is a vertical, longitudinal cross section through a layered core of a planar device. 
     FIG. 6 is a horizontal view of the core and terminals of a device as shown in FIG.  5 . 
     FIG. 7 is a vertical cross section through the terminal  26  of FIG.  6 . 
     FIG. 8 is a vertical cross section through terminal  27  of FIG.  6 . 
     FIG. 9 is a vertical cross section through terminal  28  of FIG.  6 . 
     FIG. 10 is a vertical cross section through terminal  29  of FIG.  6 . 
     FIG. 11 is an longitudinal vertical cross section through the core and a terminal of an embodiment in which the silicon substrate is utilised as an electrical connector. 
     FIG. 12 is a transverse cross section through the core and terminal shown in FIG.  11 . 
     FIG. 13 is a transverse cross section through the core of the device shown in FIG. 11 (but not at the terminal). 
     FIG. 14 is a transverse cross section through a fibre device. 
     FIG. 15 is an longitudinal cross section through the device illustrated in FIG.  14 . 
     FIG. 16 is a longitudinal cross section at right angles to FIG.  15 . 
     FIG. 17 is a transverse cross section through a preform for making a fibre device with one electrode region and one dielectric region. 
     FIG. 18 is a transverse cross section through fibre prepared from the preform of FIG.  17 . 
     FIG. 19 is a longitudinal cross section of the fibre shown in FIG.  18 . 
     FIG. 20 is a transverse cross section of partially prepared perform for a fibre embodiment having a single dielectric region and two electrode regions. 
     FIG. 21 is a cross section through the completed preform of FIG.  20 . 
     FIG. 22 is a transverse cross section through fibre prepared from the preform of FIG. 21; and 
     FIG. 23 is a longitudinal cross section through the fibre illustrated in FIG.  22 . 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The device shown in FIG. 1 has a silicon substrate  10  having a buffer layer  11  of silica. The buffer layer  11  is part of the cladding of the device. 
     The core, generally indicated by the numeral  21 , is located on the buffer layer  11 . The core comprises end portions  12  lightly doped with germanium to increase the refractive index. The core has a segmented construction comprising (as shown) six electrode segments each indicated by the numeral  13  and separated by dielectric segments each indicated by the numeral  14 . It will be appreciated that the application of a control voltage to the electrode segments  13  applies electric fields to the dielectric segments  14 . The electrode segments  13  are formed of silica highly doped with germanium and phosphorus with boron to reduce the refractive index. The presence of the germanium and the phosphorus raise the electrical conductivity and the photo-conductivity and relatively large concentrations are needed to achieve this. This tends to make the refractive index too high and boron is added to reduce the refractive index to the same value as the dielectric segments  14 . The dielectric segments  14  have the same chemical composition and refractive index as the core ends  12 . 
     The core is covered, and surrounded on both sides, with cladding  15  which has the same refractive index as the buffer layer  11 . The cladding is doped with phosphorus and boron to facilitate processing in the preparation of the device. 
     The horizontal cross section of the FIG. 2 shows that each of the electrode segments  13  has its own terminal generally indicated by the numeral  16 . Because the electrode segments  13  are separated it is necessary that each of the electrode segments has its own terminal  16 . As can also be seen from FIG. 2 half of the electrode segments  13  are connected to a metallic connector  19  and the other half are connected to a metallic connector  20 . These connectors provide control voltages in the use of the device. (As will be explained later the connectors also provide polling voltages during the last stages of preparation.) 
     All of the terminals  16  have the same configuration and this configuration is shown in FIGS. 3 and 4. 
     FIG. 3 is a plan view and it includes one of the electrode segments  13 . The terminal, indicated by the numeral  16  in FIG. 2, comprises a head  18  and a tail  17 . All of the regions  13 ,  17  and  18  are formed of the same glass composition and head  18  provides a convenient (large) structure for the attachment of the connectors  19  or  20 . The tail  17  links the head  18  to the electrode segment  13 . The terminals (i.e. the tails  17  and the heads  18 ) are located in the cladding of the waveguiding structure but they have the same refractive index as the core. Thus they represent a possible defect in the waveguiding function. The use of a narrow tail  17  minimises this disadvantage. As can be seen from the vertical cross section of FIG. 4 the terminal has the same height as the waveguide. 
     The structure illustrated in FIGS.  1 - 4  can be prepared using the conventional deposition and photolithography. 
     In the first stages the buffer layer  11  is produced on the substrate  10 . This can be achieved by either oxidising the surface of the silicon substrate  10  or by depositing (pure) silica using flame hydrolysis. The preparation of the buffer layer  11  is followed by the deposition of a complete layer having the constitution of the electrode segments  13 . This layer is the precursor of the electrode regions  13  and the terminals  16 . After deposition, photolithography removes the unwanted material to leave behind the electrode segments  13  and terminals  16 . 
     After etching, another complete layer is deposited. This layer has the composition of the dielectric segments  14  and it covers the whole of the surface and, therefore, fills in the gaps. After deposition the excess is removed to leave only the core  21  and terminals  16 . The final cladding region  15  is deposited and selectively etched to expose the terminals  16  which are metalised and the metalic connectors  19  and  20  are attached. 
     At this stage the device is illuminated with UV light within the wavelength range 190 nm to 270 nm while applying a polling voltage of 1-30 kV. This causes the movement of charges within the device so that permanent fields are applied to the electrode segments  14 . The illumination and the polling voltages are removed and these charges become trapped. 
     After polling, the device is complete and it can be connected into optical systems. The application of signal voltages, e.g. 5-150V, to the metallic connectors  19  and  20  changes the refractive index of the dielectric segments  14  whereby the phase of optical signals travelling in the core  21  is modified. 
     In an alternative, not shown in any drawing, the dielectric segments  14  have the same composition as the cladding  15 . This has the effect that part of the core  21  has the same refractive index as the cladding  15  but, provided the segments  14  are small enough, the waveguiding properties are acceptable. This modification simplifies the preparation described above. 
     FIGS.  5 - 10  illustrate a modification in which the core  21  has a layered configuration instead of a segmented configuration. The core still has end portions  12  but it comprises four electrode layers  23  (specifically  23 . 1 ,  23 . 2 ,  23 . 3  and  23 . 4 ) separated by three dielectric layers  24  (specifically  24 . 1 ,  24 , 2  and  24 . 3 ). 
     FIG. 6 shows a plan view of the device of FIG.  5  and it can be seen that there are four terminals  26 ,  27 ,  28  and  29  associated with the core. Specifically, there is one terminal for each of the dielectric layers  23 . In plan view the four terminals have the same configuration as is shown in FIG.  3 . However the vertical cross sections are different and these are shown in FIGS.  7 - 10 . 
     Terminal  26 , illustrated in FIG. 7, has the same height as the core  21  and the connector  19  is indirectly attached to the top electrode layer  23 . 4 . 
     Terminal  27 , illustrated in FIG. 8, is similar but the electrode layer  23 . 4  and the dielectric layer  24 . 3  are removed so that the connector  20  is attached to the electrode layer  23 . 3 . 
     The terminal  28 , illustrated in FIG. 9, has two dielectric and two electrode layers and two dielectric layers (namely  23 . 4 ,  24 . 3 ,  23 . 3  and  24 . 2 ) removed so that the connector  19  is connected to lower electrode layer  23 . 2 . 
     Finally connector  29 , illustrated in FIG. 10, has three electrode layers and all three dielectric layers removed so that the connector  20  is attached to the electrode layer  23 . 1 . 
     The embodiment illustrated in FIGS.  5 - 10  is prepared in a similar manner to the previous embodiment. The regions  11 ,  23  and  24  are deposited, in sequence, as complete layers. The configuration is produced by four stages of etching. In the first stage the core  21  and the terminal  26  are masked (leaving terminals  27 ,  28  and  29  unmasked) and the layers  23 . 4  and  24 . 3  are removed. In the second stage the core  20  and the terminals  26  and  27  are all masked and layers  23 . 3  and  24 . 2  are removed. In the third stage the core  20  and terminals  26 ,  27  and  28  are all masked leaving terminal  29  unmasked and layers  23 . 2  and  24 . 1  are removed. In the last stage of etching the core  20  all of the terminals are masked and the etching is completed down to the buffer layer  11 . It will be apparent that this produces the configuration shown in FIGS.  5 - 10 . The cladding layer  15  is deposited and selective etched to expose the terminals  26 ,  27 ,  28  and  29 . The exposed terminals are metalised and the connectors  19  and  20  are attached. At this stage the device is illuminated and polling carried out. 
     In the use of the device, control signals are applied to the connectors  19  and  20  and this applies electric fields to the dielectric regions  23 . Thus the refractive index of a substantial length of the path is affected and hence phase modulation can be achieved. 
     The embodiment shown in FIGS. 11,  12  and  13  is supported on a silicon substrate  10  which forms part of the electrical system of the device. As can be seen from the longitudinal cross section FIG. 11, the device has a silicon substrate  10  and a longitudinally extending core at  21 . Along part of its length the core  21  is supported on a lower electrode region  30  which makes electrical contact with the silica substrate  10 . Along the remainder of its length the core  21  is supported on the buffer region  11 . The buffer region  11  consists of pure silica, e.g. obtained by oxidising the substrate  10 , and the lower electrode region  30  is doped with germanium and phosphorus to enhance the conductivity and photo conductivity. Boron and/or fluorine are present to match the refractive index to that of the buffer  11 . 
     The core  21  is partly covered by the cladding  15  and an upper electrode region  23 . The cladding  15  is formed of silica doped with phosphorus as a processing aid and with boron to match its refractive index to that of the buffer  11 . The upper electrode region  23  is located opposite the lower electrode region  30  and both electrode region  23  and  30  have the same chemical composition. The electrode region  23  is metalised for the attachment of an external conductor  19  and the substrate  10  is metalised for the connection of an external connector  20 . After attaching the connectors, the device is irradiated and polled as described above. 
     FIG. 12 shows a transverse cross section through the core  21  and the electrode regions  23  and  30 . All of the regions  23 ,  15 ,  11  and  30  have the same refractive index which is lower than the refractive index of the core  21 . FIG. 13 is a transverse cross section which does not pass through the regions  23  and  30  and, therefore, the configuration is simpler. The core  23  is surrounded by buffer  11  and cladding  15 . Both of FIGS. 12 and 13 show that there is a conventional waveguiding structure in which a core  21  is surrounded by a cladding of lower refractive index. 
     The device shown in FIGS.  11 - 13  is relatively simple to make. A complete buffer layer  11  is provided on the silica substrate  10 , the complete, uniform layer. This layer is etched to provide a groove for the lower electrode region  30  which is deposited after the etching. The excess material is removed by etching to leave a structure comprising regions  10 ,  11  and  30  of FIG.  11 . 
     A complete layer of core material is deposited over the whole structure and etched to leave the core  23 . After the core  23  has been formed, cladding  15  is deposited over the whole structure and a groove is removed by selective etching and the groove is filled with upper electrode region  23 . The device is now metalised, external contacts  19  and  20  are provided and polling is carried out as described above. 
     In the arrangement of FIGS.  11 - 13 , substantially the whole of the path  21  constitutes the dielectric region of the electrical system so that changes in the electrical properties of the core  21  have a maximum effect upon the transmission of optical signals. There are two electrode regions ( 23  and  30 ) and a single dielectric region namely the core  21 . 
     It should be noted that, in arrangement of FIGS.  11 - 13 , the silicon substrate  10  is part of the electrical system. The lower electrode region  30  makes electrical contact with the substrate and the connector  20  is connected to the substrate  10  to complete the circuit. It should also be noted that, in this embodiment, that the whole of the path  21  constitutes the dielectric region of the electrical system so that changes in the optical properties of core  21  have a maximum effect upon the transmission of optical signals. It should also be noted that there are two electrode regions ( 23  and  30 ) and only a single dielectric region, namely the core  21 . 
     The embodiments described with respect to FIGS.  1 - 13  are all planar devices supported on a silicon substrate. In the remaining figures the devices have a fibre format. In these examples the fibre devices were prepared using the well-known inside deposition process. The inside deposition process begins with a silica substrate tube and gaseous reactants are passed through the bore of the tube. The tube is heated so that the various regions are deposited on the inside of the tube and an annular configuration is built up from the outside inwards. When the last layer has been deposited the tube is collapsed into a rod. In a modification, usually known as “rod-in-tube”, a rod is inserted into the bore of the tube, usually after one or more layers has been deposited on the inside surface of the bore. The tube is collapsed to consolidate with the rod into a single unit. Furthermore, the rod may have a composite (annular) structure which can be prepared by the inside deposition process. Thus it is well understood that rod-like preforms can be prepared. 
     After the preform has been prepared it is transferred to a drawing-machine in which the length is stretched many times. Although the pulling changes the sizes it does not change the fundamental configuration. 
     To prepare fibre devices according to the invention two flat, parallel surfaces are ground on the preform to expose inner regions of the preforms. The end portions of the preforms are provided with circular cross sections so that the preform may be fitted into the pulling machine. After pulling the fibre is cut into device lengths, e.g. 1 mm-100 mm, and the segments are metallised on the flat surfaces, exposed to radiation and polled to produced a finished fibre device. 
     A first fibre configuration is illustrated in FIGS.  14 - 16 . 
     FIG. 14 shows the annular cross section (which is derived from the annular structure of the preform). The, normally circular, cross section of the fibre has been modified by the parallel, flat surfaces  46 ,  47  which were originally ground onto the preform. Especially in FIG. 14, it can be seen that the fibre comprises a core region  43  which is highly doped with germanium and phosphorus plus boron to adjust to the refractive index to provide an electrode region which is part of the core. The electrode region  43  is surround by a silica region  42 . The region  42  constitutes the dielectric region of the device and it is also part of the core. The dielectric region  42  it surround by an outer cladding electrode  41  which is doped with germanium and phosphorus to give the electrical conductivity plus boron and/or fluorine to reduce the refractive index to that of the core. Finally, the device comprises region  40  which represents the remnants of the substrate tube which was used at the beginning the preparation. The flats  46 ,  47  expose the inner core  43  for the attachment of the connector  44  and the outer electrode  42  for the provision of the connector  45 . 
     FIG. 15 is a longitudinal cross section parallel to the flat surfaces  46 ,  47  of FIG.  14 . This cross section shows the longitudinal extending core comprising the regions  42  and  43  and it also show the residues of the substrate  40 . FIG. 15 emphasises that, in this cross section, electrode regions are not exposed and, therefore there are no connectors in this plane. 
     FIG. 16 is a longitudinal cross section at right-angle to the flat surfaces  46 ,  47  of FIG.  14 . This cross section also shows the longitudinal extent of the core comprised by regions  42  and  43  and it also shows the that region  43  is exposed for the attachment of an electrode. Region  41  is also similarly exposed. It will be appreciated that, in order to expose the electrode regions  41  and  43  the core is partially exposed. However, air has a lower refractive index than the glass of which the fibre is composed and waveguiding is still effective. Also, as mentioned above, it is possible to encapsulate the device in order to enhance the waveguiding property. 
     During polling the device has to be illuminated and it is appropriate to utilise a beam which is parallel to the flat surfaces so that the radiation does not have to penetrate any metallisation used to form of the contacts. 
     FIG. 17,  18  and  19  illustrate a fibre embodiment of the invention which has only one dielectric region located in the core and one electrode region located in the cladding. The preparation begins with a substrate tube  40  which is made of pure silica. An annulus  48  is deposited onto the inner surface of the substrate tube  40 . The annulus  48  is formed of the silica highly doped with germanium and phosphorus to increase the electrical conductivity and the enhanced electrical conductivity. The annulus  48  is also doped with boron and/or fluorine to reduce the refractive index below that of the substrate tube  40 . A rod  49  of pure silica is provided into the bore by the rod-in-tube process. Preparation of the perform is completed by the grinding flats  46  and  47  onto the longitudinally central portion of the perform. It will be noted that this grinding exposes two flats on the annulus  48 . 
     The perform is pulled into fibre which is metallised on the flat surfaces  63 ,  64  and connectors  44  and  45  are provided. FIG. 18 also shows that the core  49  is completely surrounded by the electrode region  48  and remnants of the substrate tube  40  are also illustrated. 
     The longitudinal cross section of FIG. 19 shows that there is a longitudinally extending core  49  of pure silica which is contact with a single region  48  doped for electrical conductivity. The connectors  44  and  45  can also be seen. In all of the embodiments previously described the illumination during polling is uniform so that the whole of the glass doped for conduction is given enhanced conductivity. In the embodiment shown in FIGS.  17 - 19  the illumination is non-uniform. More specifically the illumination consists of longitudinally extending segments of bright and dark (zero) illumination. The polling voltage is applied while this banded illumination is applied. 
     It will be noted that the two connectors  44  and  45  are both applied to the same conductive region  48  but they are spaced apart (in fact they are diametrically opposite). The result is that electrical charges move within the illuminated bands of the structure and, when the illumination is removed, these charges become locked so that there is a permanent electric field. In the use of the device small voltages are applied to the connectors  44  and  45  and the (normal) conductivity of the single electrode region  48  is such that control voltages are applied to the core  49 . 
     It is emphasised that, in this embodiment, there is only one dielectric region which constitutes the whole cross section of the core  49 . Furthermore there is only one electrode region  48  although this single region has two connectors namely  44  and  45 . The configuration has the result that the optical effects associated with the electrical field are applied over the whole of the cross section of the core over a substantial length. 
     The first stage in the preparation of the embodiment shown in FIGS.  20 - 23  requires the deposition of an annulus of silica highly doped with germanium and phosphorus to give electrical conductivity plus boron and/or fluorine to reduce the refractive index. This layer is deposited, as is usual, on the inner surface of the substrate tube formed of pure silica. The ratio of the dopants in the deposit is such that the deposit has the same refractive index as the substrate tube  40 . 
     After deposition of the annulus an etchant gas, e.g. SF 6  or CCl 2 F 2 , is flowed through the bore of the tube and diametrically opposite segments of the tube, indicated by numeral  61 , are heated to activate the etchant. The result is that two segments of the deposit are removed give the cross section shown in FIG.  20 . The result is a silica substrate tube  40  with segments  61 . 1  and  61 . 2  of doped silica deposited thereon. 
     The cross section shown in FIG. 20 is collapsed and, part way through the collapse, a silica rod  62  is inserted into the bore and the preparation the perform is completed by the rod-in-tube process to give the cross section shown in FIG.  21 . At this stage, as was described for previous embodiments, the perform is ground. This leaves end segments of circular cross section to engage with the fibre pulling machine and the centre section is ground flat along the lines  63  and  64  of FIG.  21 . 
     The perform is now pulled into fibre which is cut into suitable lengths for devices and the flat surfaces  63  and  64  are metallised for the attachment of connectors  65  and  66 . The transverse cross section of the fibre is shown in FIG.  22  and longitudinal cross section is shown in FIG.  23 . 
     The preparation of the device is completed by illumination and polling as described above. It can be seen that the device has a single dielectric region  62  which is in contact with two electrode regions  60 . 1  and  60 . 2 . The two electrode regions are diametrically opposite one another and the substrate tube  40  also contacts the core  62  between the dielectric regions  60 . 1  and  60 . 2 . When control voltages are applied to the connectors  65  and  66  the optical properties of a substantial length of core are affected. It is also emphasised that, because there is only one dielectric region, the optical properties are affected over the full cross section of the core. 
     It is convenient to attach fibre tails to all of the embodiments described above. These fibre tails are not illustrated in any drawings but they are convenient for incorporating the devices into optical systems. It is appropriate to attach the fibre tails before polling so that the optical properties of the device can be monitored during polling. Also, the heating needed to attach the tails is applied before poling so that the possibility of the leakage of charge (caused by the enhanced conductivity at high temperarures) is reduced.