OPTICAL SWITCHING/MODULATION DEVICE

Abstract: An optical switching/modulation device (1) comprises a substrate (2) made from lithium niobate, a first, a second and a third waveguide (3-5) for the light, formed on the substrate and forming a Y structure, a central electrode (6) lying between the second and third waveguide (4, 5) and a pair of outer electrodes (7, 8) adjacent to the second and third waveguide (4, 5) respectively on opposite sides from the central electrode (6); the waveguides (3-5) are connected by means of a connecting waveguide (10) having a longitudinal axis (9) and including a multimodal transmission region (14); the central electrode (6) has a longitudinal extension (6b) extending within the multimodal region (14) and having an end (6c) terminating within the multimodal region (14), in such a way that a particularly high value of the extinction ratio of the device can be provided.

Description: OPTICAL SWITCHING/MODULATION DEVICE DESCRIPTION The present invention relates to an optical switching/modulation device. There are known digital optical switches which can be used to select alternative optical connections between light waveguides. These include the known digital optical switches, or "DOSs", which have a stepped response to the operating voltage. By comparison with other types of optical switching device, DOSs are particularly useful because their particular response characteristic enables them to operate without a strict control of the drive voltage and also permits the simultaneous driving of a large number of switches by using a single voltage source. DOSs can also be used to switch both light polarizations (TE and TM) simultaneously, are essentially unaffected by the wavelength of the transmitted light, and make it possible to avoid optical-electrical and electrical-optical conversions. DOSs are used increasingly in the optical telecommunications sector, mainly as devices for the selective direction of the signals transmitted in telecommunications lines and networks. For example, DOSs can be used to carry out protective functions in telecommunications networks, or switching functions at nodes of a network accessed by a plurality of users "Access Switch", or other functions of known types ("Main Distribution Frame", "Optical Cross Connect", etc.). DOSs usually comprise a specified number of waveguides formed on a common substrate, for example a substrate of lithium niobate (LiNb0 3 ) . The number and arrangement of the waveguides can vary according to requirements. In particular, there are known switches of the "a X" or "2x2" type, having two inputs and two outputs, and switches of the "a Y" type which can be used either as "1x2" switches, in other words with one input and two outputs, or as "2x1" switches, in other words with two inputs and one output. A DOS can also be used as an elementary unit for building more complex switching structures, for example switching matrixes comprising a plurality of DOSs integrated on a single chip. A DOS generally comprises a connecting waveguide region interposed between the input waveguide (s) and the output waveguide (s) . The input and output waveguides are usually connected to optical fibres suitable for carrying the transmitted signals, for example optical fibres of the monomodal type. Switches constructed in this way typically have a longitudinal axis of virtual symmetry. X (or 2x2) switches have a first operating condition in which a first input is optically connected to a first output while a second input is optically connected to a second output, and a second operating condition in which the first input is optically connected to the second output and the second input is optically connected to the first output . Y switches of the 1x2 type have a first operating condition in which the single input is optically connected to a first output, and a second operating condition in which the input is optically connected to a second output. The two outputs then operate alternatively to receive the optical signal from the input . Y switches of the 2x1 type have a first operating condition in which a first input is optically connected to the single output, and a second operating condition in which a second input is optically connected to the output. The two inputs then operate alternatively to supply optical signals to the output . DOSs formed on lithium niobate substrates typically comprise waveguides made from diffused titanium, and use the electro-optic effect to switch the light between the different waveguides. The electro-optic effect is associated with a variation of the refractive index in a waveguide due to a variation of the electrical field present in the region occupied by the waveguide . The transmission characteristics of the waveguide vary with its refractive index. It is therefore possible, by applying variations of refractive index of opposite sign to the alternative waveguides of the switch (such as those forming the two input branches or the two output branches of a Y switch) , to control the optical power of the light rays transmitted in these guides. For example, in the case of a Y switch of the 1x2 type, in the aforesaid first operating condition the refractive indices of the two output waveguides are such that practically all of the light passes from the input to the first output, while in the second operating condition the refractive indices of the two output waveguides are such that practically all of the light passes from the input to the second output. The change from the first to the second operating condition and vice versa is made by varying the operating voltage of the switch in such a way as to cause the desired variations of refractive index. In order for the switch to operate correctly, it is necessary to satisfy a condition of adiabaticity, corresponding to an absence of coupling between the local normal modes of the optical structure. This condition is met by designing the waveguides in such a way that there are no abrupt changes of direction in the propagation of the signal. In particular, the angles at the branches (in other words between two alternative waveguides for switching) must be particularly small. The condition of adiabaticity is a necessary condition for achieving a low level of crosstalk between the different waveguides. This crosstalk (measurable in dB) causes the undesirable presence of a non-zero optical power at the output of a waveguide when the electrical field has been applied in such a way that all the optical power is carried in the alternative waveguide. As an alternative to crosstalk, a parameter commonly used to describe the quality of a switch is the "extinction ratio" (abbreviated to E.R.). The extinction ratio provides a measure of the maximum obtainable ratio between the optical powers in two alternative branches in one of the two switching conditions. In the simplest case of a 1x2 switch, in which the light enters through the input waveguide and leaves alternatively through one or the other of the two output waveguides, then, for a minimum and maximum optical power P L and P H which can be extracted through one of the two waveguides in the two operating conditions described above, the extinction ratio is defined thus : E.R. = 10 log P H /P L (1) The behaviour of the switch improves as the value of the extinction ratio increases, since there is a decrease in the optical power carried in the undesired branch. The required value of the extinction ratio depends on the type of application. The applicant has observed that switches of the known type generally have an extinction ratio smaller than that usually required, and therefore it may sometimes be necessary to connect two or more switches in cascade in order to obtain the required value of the extinction ratio. There are various known types of digital optical switch, which differ from each other in the shape of the electrodes and/or in the shape of the waveguides . The article "Digital Optical Switch", by Y. Silberberg, P. Perlmutter and J.E. Baran, Appl . Phys . Lett., vol. 51, no. 16, pp. 1230-1232, July 1978, describes a digital optical switch made from Ti:LiNb0 3 with an X structure. The structure is based on an asymmetric waveguide junction, consisting of two unequal waveguides, a bimodal central region and a symmetrical output branching. The symmetry of the output branching can be broken by applying an external electrical field. This field can be applied by providing a non-zero voltage between the central electrode and the lateral electrodes. Care must be taken to ensure that the applied field is not such that the condition of adiabaticity of propagation is infringed. For this purpose, as shown in Fig. 1 of the article, the electrodes are shaped in such a way that the electrical field is incremented gradually towards the junction area. In practice, the central electrode has a longitudinal extension which passes completely through the bimodal central region and the outer electrodes (having the same length as the central electrode) are such that they progressively approach the longitudinal extension. The article by .K. Burns, "Voltage-Length Product for Modal Evolution-Type Digital Switches", Journal of Lightwave Technology, Vol. 8, No. 6, June 1990, describes the operating principles of digital switches of both the X and the Y type. In particular, the dependence of the crosstalk (measured in dB) between the guides of the switch on the variation of the product V-L is studied, where V is the voltage applied between the electrodes and L is the length of the electrodes. In order to study this dependence, a switch with a symmetrical Y configuration (Figure 2) is considered, in which the input waveguide and the two output waveguides (a and b) are monomodal. The switch in question has an electrode placed between the two output waveguides and two outer electrodes, each placed on an opposite side of an output waveguide from the central electrode. The three electrodes have a length of L and are positioned between a point B and a point C, where the point B is located below the point A of branching of the output waveguide along the direction of propagation of the light. The configuration of the electrodes is such that Δβ (ΔV) =β a - β b is constant along the branches, where β a and β b are the usual constants of propagation in the presence of a large channel separation. The applicant has observed that digital optical switches of the known type have a connection region between the waveguides typically comprising a region of multimodality of the light, in other words a region within which the propagation of the light takes place in at least a bimodal way. The applicant has also observed that the electrodes usually all have the same length (measured parallel to the axis of virtual symmetry of the switch) , and therefore it is generally possible to define a first end and a second end of these electrodes. For example, in the case of a Y structure of the 1x2 type or an X structure, the first end can be considered to be that which faces the input branch or branches, and the second end can be considered to be that which faces the output branches, while in the case of a Y structure of the 2x1 type the first end can be considered to be that which faces the output branch and the second end can be -considered to be that which faces the input branches. The applicant has also observed that, in the case in which the electrodes, for any reason, are made with different lengths, the aforesaid first and second end can be considered to correspond to the useful ends of the electrodes for the purposes of the generation of the electrical field (in other words, to the ends of the shorter electrode or electrodes) ; this is because, if one of the electrodes is shorter than the others, the electrical field beyond the end of this electrode is essentially zero. The applicant has tackled the problem of making a digital optical switch (DOS) which has a high extinction ratio. The applicant has found that, if the electrodes are shaped in such a way that the first end, defined as above, is positioned in the multimodality region of the switch, particularly in one of a plurality of specified longitudinal positions of this multimodality region, the digital optical switch (DOS) may have a particularly high extinction ratio. In practice, when the variation of the extinction ratio as a function of the position of the first end within the multimodality region is observed, it is possible to observe the presence of a plurality of peaks. The applicant has observed that these peaks are absent if the first end is outside the multimodality region, as is the case in the switches described in the aforesaid articles by Silberberg et al . and Burns. Since the position of the second end is usually predetermined, because it is generally set at a point at which the mutual distance between the output waveguides is such that there is a considerable optical decoupling between them, the aforesaid condition concerning the position of the first end can also be expressed as a condition concerning the length of the electrodes. The applicant has found that, in the case of a switch formed on a substrate of the "X-cut" and "Y-propagation" type (with a cut along the X axis and propagation along the Y axis) , for example, a particularly suitable configuration for obtaining the aforesaid effect includes a central electrode, located between the waveguides between which the signals are to be switched, and two outer (or lateral) electrodes, in which the central electrode is provided with a longitudinal extension penetrating into the multimodality region and terminating at a specified longitudinal position in the multimodality region. In particular, the applicant has found that this specified longitudinal position can be chosen from a plurality of longitudinal positions at which the extinction ratio that can be obtained is particularly high. The applicant has also found that a preferred configuration for the outer electrodes is that in which their distance from the extension is essentially constant within the multimodality region. This is because this particular configuration of the outer electrodes makes it possible to have an essentially uniform electrical field of relatively high intensity in the area occupied by the electrodes within the multimodality region, and the applicant considers that this condition is advantageous for obtaining particularly high values of the extinction ratio. The applicant has also noted that this configuration of the outer electrodes does not create any particular disturbance in the condition of adiabaticity of the switch. In a first aspect, the present invention relates to an optical switching/modulation device, comprising: a substrate; at least a first, a second and a third waveguide formed on the said substrate; a connecting waveguide which optically connects the said first, second and third waveguides, the said connecting waveguide having a longitudinal axis and comprising a multimodal transmission region confined between a first and a second longitudinal position, the said second and third waveguides being associated with, respectively, a first and a second refractive index, and forming two alternative branches for carrying the light from, or to, the said connecting waveguide; - electrodes associated with at least one of the said second and third waveguides and capable of generating an electrical field region to vary at least one of the said first and second refractive indices; in which the said electrodes comprise at least a first electrode having one longitudinal end located between the said first and second longitudinal positions. The said electrodes preferably comprise a pair of electrodes having an essentially constant distance between them in the said multimodal region. In a possible embodiment, the said first electrode forms a central electrode interposed between the said second waveguide and the said third waveguide, and the said electrodes comprise a second and a third electrode forming outer electrodes located on the sides of the said second waveguide and the said third waveguide respectively opposite the said central electrode. The said central electrode may have an essentially rectilinear extension extending along the said longitudinal axis in the said multimodal region. In another possible embodiment, the said first electrode is partially superimposed on the said second waveguide and the said electrodes comprise a second electrode partially superimposed on the said third waveguide. The said substrate is preferably made from lithium niobate. The said first, second and third waveguides can be made by diffusing titanium into the substrate. Preferably, the said second waveguide and the said third waveguide form a first angle of less than 2° between them. The device may comprise a fourth waveguide forming, with the said first waveguide, a second angle of less than 2°. The said fourth waveguide and the said first waveguide are preferably of different widths. Preferably, the said first, second and third waveguides are essentially rectilinear. In a further aspect, the present invention relates to an optical transmission system, comprising at least one transmitter capable of transmitting an optical signal, at least one receiver capable of receiving the said optical signal, and an optical connection which connects the said transmitter to the said receiver and which is capable of carrying the said optical signal, and also comprising an optical switching/modulation device as defined above, connected optically in series with the said optical connection to switch/modulate the said optical signal. Further details can be found in the following description, which refers to the attached figures listed below: - Figure 1 relates to a device with a Y structure formed according to the present invention; Figure 2 shows the response curve of the device of Figure 1 ; Figure 3 shows an optical ring network in which the device of Figure 1 can advantageously be used as a switch; Figure 4 shows a possible structure of one of the nodes of the network of Figure 3, comprising devices of the type shown in Figure 1; Figure 5 shows the variation of the extinction ratio of the device of Figure 1 as a function of the length of the electrodes of the device, for different values of the potential difference applied to its electrodes, in the case of TE polarization; Figure 6 shows the variation of the extinction ratio of the device of Figure 1 as a function of the potential difference applied to its electrodes, for different values of the length of the electrodes, in the case of TE polarization; Figure 7 is a graph similar to that of Figure 5, but relating to the case of TM polarization; Figure 8 is a graph similar to that of Figure 6, but relating to the case of TM polarization; Figure 9 shows the result of experimental measurements on a switching/modulation device made according to the invention; Figure 10 shows a WDM telecommunications line in which the device of Figure 1 can advantageously be used as a variable attenuator; Figure 11 shows a device with an X structure made according to the principles of the present invention; Figure 12 shows a possible structure of a node of the network of Figure 3, comprising devices of the type shown in Figure 11; Figure 13 relates to a device with a Y structure, made according to the invention on a substrate with a cut along the z axis; and - Figure 14 shows an apparatus for experimental measurements on a device made according to the invention. Figure 1 shows a device 1 for digital switching/modulation of optical signals, made according to the present invention. The device 1 comprises a substrate 2, preferably of electro-optic material, a first, a second and a third waveguide 3-5 for carrying the light, and electrodes 6, 7 and 8 for the electrical control of the device 1. The device 1 has a plane of virtual symmetry orthogonal to the plane of the figure, and forming an axis 9 in the plane of the figure. The substrate 2 can be made from materials with different optical properties. Preferably, the substrate 2 is made from lithium niobate (LiNb0 3 ) or from another material which, like lithium niobate, has an electro-optic effect, such as lithium tantalate (LiTa0 3 ) . Alternatively, the substrate 2 can be made from a polymeric material. If a substrate 2 made from lithium niobate is used, this structure is advantageously orientated with the cut perpendicular to the x axis ("x-cut") and the direction of propagation of the light is preferably chosen to coincide with the y axis ("y-propagation" ) . Alternatively, the structure may comprise a substrate with a cut perpendicular to the y axis ("y-cut") and with propagation of the light essentially along the x axis ( "x-propagation" ) . This structure shows small thermal drift phenomena (in other words small variations of the operating point due to variations of temperature) and requires relatively small values of potential difference necessary for switching or attenuating the light. As a further alternative, the substrate can be of the type with a cut along the z axis ("z-cut") and with the direction of propagation along the x axis ( "x-propagation") or along the y axis ("y- propagation" ) , as will be described subsequently with reference to Figure 13. In all the cases considered, the structure of the device is such that the optical signals have effective directions of propagation, defined essentially by the directions of extension of the waveguides in which the signals are propagated, which form angles of preferably less than 2° with the principal axis of the crystal which determines, as described above, the direction of propagation. The waveguides 3-5 are formed by depositing a layer of titanium having a thickness of less than 500 nm, more preferably in the range from 50 nm to 150 nm, on the substrate 2; their profiles are then formed by photolithographic techniques, and finally the titanium remaining within the underlying substrate 2 is thermally diffused. Preferably, the waveguides 3-5 are essentially rectilinear and have an essentially constant width, permitting the propagation of a single mode. The device of Figure 1 is a Y switch which can operate either as a 1x2 switch (if the light enters from the first waveguide 3 and alternately leaves either from the second waveguide 4 or from the third waveguide 5) or as a 2x1 switch (if the light entering from either the second or the third waveguide 4, 5 leaves from the first waveguide 3) . The invention described below is equally applicable to an X-shaped switch, as will be explained below with reference to Figure 11. The first waveguide 3 extends essentially along the axis 9, while the second and third waveguides 4, 5, which form the two arms of the Y, are symmetrical with each other about the axis 9 and are separated by a specified angle θ from a bifurcation point P (located on the axis 9) . The angle θ, which is preferably smaller than 2°, must be as small as possible, compatible with the dimensions of the device 1, in order to meet the aforesaid condition of adiabaticity. Alternatively, the second and third waveguides 4, 5 can be positioned asymmetrically with respect to the axis 9, can have different widths, or can have a non-rectilinear extension (for example with a curvature towards the axis 9, as described in US Patent 5,123,069). The waveguides 3-5 are connected by means of a connecting waveguide 10, delimited in an approximate way in Figure 1 by the segments of broken lines a and c orthogonal to the axis 9. The connecting waveguide 10 widens progressively as it passes from the area communicating with the first waveguide 3 to the area communicating with the second and third waveguides 4, 5. The connecting waveguide 10 comprises a multimodal waveguide region 14 (for example a bimodal region) , essentially confined between two longitudinal positions indicated (in an approximate way) by the segments of broken lines b and c (orthogonal to the axis 9) . In the multimodal region 14, the width of the connecting waveguide is such as to permit the transmission of at least one higher order mode in addition to the fundamental mode . The electrodes 6-8 include a central electrode 6 located between the second and the third waveguide 4, 5 and a first and a second outer electrode 7, 8 located on the sides of the second waveguide 4 and of the third waveguide 5, respectively, opposite the central electrode 6. The electrodes 6-8 are capable of generating an electrical field region to vary the refractive index of at least one of the waveguides 4 and 5, as described below. Preferably, the electrodes 6-8 have the same length L (measured along a direction parallel to the axis 9) and form, in combination, an essentially rectangular structure. The electrodes 6-8 can be made by depositing a layer of conductive material, for example titanium, on the surface of the substrate 2 which has previously been covered with a layer of insulating material, for example silicon dioxide Si0 2 , and then using a photolithographic technique of a known type to provide the electrodes with the desired shape. If the electrodes 6-8 are made from titanium, their thickness is preferably less than approximately 500 nm, or more preferably is within the range from approximately 50 nm to approximately 150 nm. The central electrode 6 comprises a principal portion 6a which is preferably essentially triangular in shape, with two sides which are symmetrical about the axis 9, one being adjacent to the second waveguide 4 and the other being adjacent to the third waveguide 5, and with the vertex between these sides located near the point P of bifurcation of the second and third waveguides 4, 5. Advantageously, the central electrode 6 has an essentially rectilinear extension 6b of specified length 1, which extends along the axis 9 and into the multimodal region from the vertex of the principal portion 6a. Figure 1 shows, for ease of description, a broken line 13 orthogonal to the axis 9, which forms the point from which the extension 6b extends. According to the present invention, the length of the extension 6b is such that a first end 6c of it is located inside the multimodal region. The applicant has observed that the extinction ratio of the device 1, as defined by the relation (1) (E.R. = 10 log P H / P ) is a function of the length of the extension 6b, particularly of the position within the multimodal region of the end 6c of the extension 6b. In particular, the applicant has found that, for some values of the length of the extension 6b, the extinction ratio is particularly high; in other words, as will be demonstrated subsequently, the curve representing the extinction ratio as a function of the length of the extension 6b has relatively high peaks at the aforesaid values of length. This behaviour is observable for both polarizations of the light, namely TE and TM. Advantageously, the length of the extension 6b (and consequently the position of the end 6c) is chosen in such a way as to obtain the highest possible value of the extinction ratio. If necessary, the length of the central electrode 6 could be different from that of the outer electrodes 7 and 8. In this case, the present invention requires that at least the central electrode 6 or the outer electrodes 7, 8 terminate in the multimodal region, in other words at a longitudinal position lying between the segments b and c. In this case also, the first end of the electrodes (as defined above) would be located inside the multimodal region. Preferably, the outer electrodes 7 and 8 are symmetrical with each other about the axis 9 and are essentially trapezoidal in shape. Each of the outer electrodes 7, 8 has an oblique side adjacent to a corresponding waveguide 4 or 5 on the side opposite the central electrode 6. Preferably, the distance between the outer electrodes 7, 8 is essentially constant in the multimodal region. Additionally, the distance between each of the outer electrodes 7, 8 and the extension 6b is preferably the same, at least in the multimodal region. Thus, it is possible to have an essentially constant electrical field with a relatively high value in the area occupied by the electrodes in the multimodal region 14. As shown in Figure 1, the portions of the outer electrodes 7 and 8 with an essentially constant distance between them extend preferably to the outside of the multimodal region 14, and more preferably to the start of the extension 6b (in other words, up to the broken line 13) . The portions of the electrodes 6-8 adjacent to the second and third waveguide 4 and 5 extend to a longitudinal position at which the mode coupling between the second and third waveguide 4, 5 is essentially zero. This end of the electrodes 6-8 forms a second longitudinal end opposite the first. If the electrodes 6-8 are of different lengths, the second end is defined (in a similar way to the first end) by the end of the shorter electrode. The second and third waveguides 4, 5 preferably terminate at one end of the substrate 2, and can be coupled to planar optical waveguide or optical fibre structures (not shown in the figure) . The outer electrodes 7 and 8 are preferably connected electrically to each other through a conducting bridge 11, formed on the substrate 2 above the first waveguide 3, which keeps them at the same potential. Additionally, one of the two outer electrodes 7, 8 (that indicated by 7 in the case in question) and the central electrode 6 are electrically connected to the poles of a voltage generator 12. Thus an electrical potential difference of ΔV can be established between the central electrode 6 and the two outer electrodes 7, 8, which induces a controllable electrical field into the region occupied by the waveguides 4 and 5 and in the connection region 10. The device 1 operates in the following way. In the case of operation as a 1x2 switch, if the potential difference ΔV applied to the electrodes 6-8 is zero, the light entering the device 1 through the first waveguide 3 leaves from the device 1 which is equally divided between the second and the third waveguides 4, 5. However, if a non-zero potential difference ΔV is applied between the electrodes 6 and 7, the electrical field which is thus generated induces, by the electro-optic effect, an increase +Δn of the refractive index in one of the waveguides 4, 5 and an equivalent decrease -Δn of the index of refraction in the other waveguide 5, 4. Consequently there is an increase in the optical power guided by the waveguide with a higher refractive index and, at the same time, a reduction of the optical power guided by the other waveguide. In practice, the power of the monomodal signal supplied to the connecting region 10 from the first waveguide 3 is distributed between the waveguides 4, 5 in a way which is correlated with the aforesaid potential difference. In particular, if the applied potential difference Δv is sufficient to provide complete switching, the monomodal signal leaving the waveguide with a higher refractive index will have an optical power which is essentially equal to that of the signal entering the device 1, while the optical power at the output from the waveguide with a lower refractive index will be essentially zero. Similarly, when used as a 2x1 switch, the device 1 is capable of selecting among the monomodal signals entering through the waveguides 4 and 5 the one to be sent to the output through the first waveguide 3. In practice, when a potential difference ΔV sufficient to provide complete switching is applied, the monomodal signal entering through the waveguide with the lower refractive index will be radiated into the substrate 2, and the first waveguide 3 will receive only the monomodal signal arriving from the waveguide with the higher refractive index. Figure 2 shows the variation of the optical potential difference ΔP between the two waveguides 4, 5 as a function of the potential difference ΔV applied between the two electrodes 6 and 7. The characteristic ΔP/ΔV is essentially symmetrical about a central point at which ΔV = 0 and ΔP = 0 (balanced distribution of the optical power in the case of zero potential difference) and comprises: a first region of essential saturation, indicated as region I, distinguished by values of potential difference which are smaller than a specified threshold value -ΔV TH , in which the optical potential difference ΔP takes an essentially constant minimum value; an essentially linear region, indicated as region II, distinguished by values of potential difference in the range from the first threshold value -ΔV TH and a second threshold value +ΔV TH whose absolute value is equal to the first, in which the characteristic is essentially linear and rising; and a second region of essential saturation, indicated as region III, distinguished by values of potential difference which are greater than the second threshold value +ΔV TH , in which the optical potential difference ΔP takes an essentially constant maximum value. The threshold potential differences -ΔV TH and +ΔV TH are selected according to the value of some parameters, such as the angle θ between the waveguides 4, 5, the width of the waveguide 3, 4, 5 and the distance between the electrodes in the connection region 10. Typical values for the threshold potential differences -ΔV TH and +ΔV TH are, for example, -40 V and +40 V. The device 1 is typically used in the regions of essential saturation I and III to provide digital switching of the light between the waveguides 4 and 5, while the essentially linear region II forms a transition region for the change from one switching condition to the other. In practice, when there are rapid variations of the potential difference ΔV from one threshold value (-ΔV TH or +ΔV TH ) to the other, it is possible to vary the refractive indices of the waveguide 4 and 5 in such a way that the optical power entering the device 1 is supplied to only one of the waveguides 4 , 5. A possible application of the device 1 as a digital switch is to a self-protected optical ring network, in other words to an optical network configured essentially in the form of a ring in which a suitable protection method enables the correct flow of data to be ensured even in the presence of a fault. A network of this type is shown in Figure 3 and is indicated there by the number 30. In a network such as the network 30, the device 1 can be used, for example, at a signal add/drop node to carry out the optical redirection of the signals in case of a fault. In detail, the network 30 comprises a first and a second optical fibre ring 31, 32 (the outer ring and the inner ring respectively) , capable of carrying optical signals in opposite directions of transmission (anti-clockwise and clockwise respectively) , and a plurality of optically reconfigurable nodes 33 which are optically connected along the first and second rings 31, 32. The nodes 33 are designed for the add and drop of the signals at the locations of users connected to the network or of interconnections with further optical networks. The network 30 can be used either for short -distance communications (e.g. the LAN, or "local area network" type) or for long-distance communications (e.g. transoceanic communications) . In the latter case, the network is usually provided with optical amplifiers to re-establish the correct power level of the signals after transmission through a long section of optical fibre (usually of the order of hundreds of kilometres) . The network 30 is suitable for the transmission of wavelength multiplexed signals (or "WDM" signals) , in other words signals distributed among a specified number of channels at different wavelengths. In particular, a set of N wavelengths λ 1; λ 2 , ..., λj, in a specified waveband in which the optical fibres and optical components of the network 30 can operate (between 1520 nm and 1610 nm, for example) is considered. Owing to a modular structure which is described subsequently, each of the nodes 33 is configurable in such a way that it can control a corresponding subset of wavelengths selected from the set λ 1( λ 2 , ..., λ N and can provide, by using these wavelengths, bi-directional communications with one or more of the other nodes of the network. A generic pair of communicating nodes has a corresponding pair of wavelengths λ x , λ y (indicated below as the first and second wavelengths) associated with it for the exchange of signals. The following description will refer principally to this generic pair of communicating nodes, but each consideration can be extended to any number of such pairs . In conditions of correct operation of the network 30, the two nodes of the pair in question are capable of exchanging signals in a single arc of the network (in other words in a single arc-shaped path along the two rings 31, 32), using the first wavelength λ x in the first ring 31 and the second wavelength λ y in the second ring 32. The first wavelength λ x in the second ring 32 and the second wavelength λ y in the first ring 31 are reserved for protection and are not used in normal working conditions (in other words in the absence of faults) . In Figure 3, the wavelengths used in normal working conditions have the suffix "w" , while the wavelengths reserved for protection have the suffix "p" . The pair of wavelengths λ x , λ y used previously by one pair of nodes in the network 30 can be used by a further pair of nodes in the network 30, provided that the arcs of the network in which the corresponding communications take place in normal working conditions are not superimposed. The wavelengths λ x p , λ y p reserved for protection are shared between all the pairs of nodes which use the frequencies λχ ,w / λ y,w i n normal working conditions. Each node 33 of the network 30 is structured in such a way that, in addition to the normal functions of extraction, insertion and transmission without modification (or with amplification if necessary) , it is also capable of carrying out a function of redirecting the signals in case of fault. In a ring network of this type, a fault may occur as a result of damage to the optical fibres which form the rings, or as a result of the malfunction of one of the devices present in the nodes of the network. In particular, in case of a fault at one point of the network 30, each node 33 which communicates with other nodes through arcs of the network comprising this point is capable of redirecting each of these communications along the corresponding complementary arc, using for this purpose the wavelengths previously reserved for protection. For a clearer understanding of the structure and operation of the network 30, reference should be made to Figure 4, which shows in detail one of the nodes 33. The node 33 includes: a first and a second OADM (optical add/drop multiplexer) device 35, 36 for the add/drop of optical signals into/from the first ring 31 and into/from the second ring 32 respectively, at the wavelengths at which the node 33 is designed to operate (in the specific case, λ x and λ y ) ; the OADMs 35, 36 do not operate on the other wavelengths used in the network 30, and transmit these wavelengths unchanged (or with amplification if necessary) ; the OADMs 35, 36 are, for example, of the Pirelli Optical Systems OADM/P4-R1 type (of the WaveMux6400 group of products) ; and at least one transmission/reception module 34 for the transmission of optical signals into the network 30 at the wavelengths λ x , λ y and the reception of optical signals from the network 30 to the wavelengths λ x , λ y ; in practice, there will be one module 34 for each pair of wavelengths which the node 33 has to control, in other words for each different connection which this node has to form within the network 30. In turn, the transmission/reception module 34 includes: a first and a second transmitter Tx 1# Tx 2 for transmitting signals at the first wavelength λ x and at the second wavelength λ y respectively; in normal working conditions, the first and the second transmitter Tx 1# Tx 2 are connected to the first ring 31 and to the second ring 32 respectively; a first and a second receiver Rx. l t Rx 2 , for receiving signals at the second wavelength λ y and at the first wavelength λ x respectively; in normal working conditions, the first and the second receiver Rx x , Rx 2 are connected to the second ring 32 and to the first ring 31 respectively; a first, a second, a third and a fourth transmission signal regenerator ("transmitting transponder") TxT 1 (λ x ), TxT 1 (λ y ), TxT 2 (λ x ), TxT 2 (λ y ) to vary the wavelengths of the transmitted signals in such a way as to make them suitable for transmission into the network 30; a first, a second, a third and a fourth reception signal regenerator ("receiving transponder") RxT 1 (λ x ), RxT 1 (λ y ), RxT 2 (λ x ), RxT 2 (λ y ) to vary the wavelengths of the signals received from the network 30 in such a way as to make them suitable for reception by the receivers Rx 1; Rx 2 ; a switch unit 37 for optically connecting the transmitters Tx 1# Tx 2 and the receivers Rx 1( Rx 2 to the OADMs 35, 36 in a selective way; and - a central processing unit (CPU) 38 for controlling the operation of the switch unit 37 and for controlling the operations of the transmitting regenerators TxT and the receiving regenerators RxT. The transmitters Tx l r Tx 2 and the receivers Rx x , Rx 2 represent the points of access of external users to the network 30 for the insertion and extraction of data. This insertion and extraction of data can be carried out at wavelengths different from the wavelengths used for the transmission of signals in the network 30 (in other words, different from the wavelengths λ 1# λ 2 , ..., λ N ) , since the transmission signal regenerators TxT and reception signal regenerators RxT are designed to vary the wavelengths of the transmitted and received signals, respectively, in a specified way. The transmitters Tx x , Tx 2 and the receivers Rx 1; Rx 2 can be, for example, terminals of the Sonet OC- 48/SDH STM-16 type (of the type produced by the Nortel company, for example) . The first and second transmission signal regenerators TxT 1 (λ x ), TxT 1 (λ y ) are optically connected to the first ring 31 by means of the first OADM 35 and are capable of supplying signals at the first wavelength λ x and at the second wavelength λ y respectively to the first ring 31. The third and fourth transmission signal regenerators TxT 2 (λ x ), TxT 2 (λ y ) are optically connected to the second ring 32 by means of the second OADM 36 and are capable of supply signals at the first wavelength λ x and at the second wavelength λ y respectively to the second ring 32. Each transmission signal regenerator TXT- L (λ x ) , TxT x (λ y ) , TxT 2 (λ x ) , TxT 2 (λ y ) is capable of receiving optical signals from the switch unit 37, of converting them to electrical signals to enable them to be processed by the processing unit 38 and of re-converting them to optical signals having, as stated above, an associated wavelength suitable for transmission. The processing of the signals, in this case, includes the supplying of data for network protection purposes (for example channel identifiers, information for transmission efficiency monitoring, protection protocol, etc.) to the transmitted signals (in the part of the signal known as the "channel overhead" ) . The first and second reception signal regenerators RxT x (λ x ), RxT 1 (λ y ) are optically connected to the first ring 31 by means of the first OADM 35 and are capable of receiving from the first ring 31 signals at the first wavelength λ x and at the second wavelength λ y respectively. The third and fourth reception signal regenerators RxT 2 (λ x ) , RxT 2 (λ y ) are optically connected to the second ring 32 by means of the second OADM 36 and are capable of receiving from the second ring 32 signals at the first wavelength λ x and at the second wavelength λ y respectively. Each reception signal regenerator RxT- L (λ x ), RxT 1 (λ y ), RxT 2 (λ x ) , RxT 2 (λ y ) is capable of receiving optical signals from the rings 31, 32, of converting them to electrical signals to enable them to be processed by the processing unit 38, and of re-converting them to optical signals having, as stated above, an associated wavelength suitable for reception by a corresponding receiver Rx x , Rx 2 . The processing of the signals, in this case, includes the extraction from the received signal of the previously inserted information (in the "channel overhead" of the signal) for the network protection functions. The transmission signal regenerators TxT and reception signal regenerators RxT can be, for example, of the Pirelli Optical Systems WCM/F-xxx type (of the WaveMux6400 group of products, where xxx is the output wavelength code) . Alternatively, the signal regenerators can be wholly optical devices, for example of the S.O.A. (abbreviation of "semiconductor optical amplifier") type, capable of controlling information associated with the signal, for example a pilot tone which overmodulates the wavelength carrying the signal . The processing unit 38 is capable of communicating with the transmission signal regenerators TxT to supply them with information to be added to the signals, and with the reception signal regenerators RxT to receive information extracted from received signals to be processed, and is also capable of carrying out checks on the conditions of the various connections, and of monitoring the state of the switch unit 37. In Figure 4, the logical connections between the processing unit 38 and the other units controlled by it are represented by a broken lines. The switch unit 37 provides the suitable switching functionality for network protection, by means of its capacity for selectively connecting the transmitters Tx and the receivers Rx to the transmission and reception signal regenerators TxT, RxT. As mentioned above, the architecture of Figure 4 is modular, in the sense that each node 33 can include a plurality of transmission/reception modules 34 to control the same number of wavelengths, and, consequently, the same number of connections. In this case, the various pairs of wavelengths which the generic node 33 has to control are separated from each other by the OADMs 35, 36 and directed to the corresponding transmission/reception modules 34. The switch unit 37 comprises eight switching devices la-lh made according to the present invention, commanded by the CPU 38 by means of an appropriate control logic. The switching devices la-lh form selective connections between the transmitters Tx x , Tx 2 , the receivers Rx. l r Rx 2 , the transmission signal regenerators TXT- L (λ x ) , TxT 1 (λ y ), TxT 2 (λ x ), TxT 2 (λ y ) and the reception signal regenerators RxT 1 (λ x ), RxT 1 (λ y ), RxT 2 (λ x ), RxT 2 (λ y ). In detail, - a first switching device la has its first waveguide 3a connected to the first transmitter TX- L and its third waveguide 5a connected to the first transmission signal regenerator TXT- L (λ x ) ; a second switching device lb has its first waveguide 3b connected to the second receiver Rx 2 and its second waveguide 4b connected to the first reception signal regenerator RxT 1 (λ x ); a third switching device lc has its first waveguide 3c connected to the second reception signal regenerator RxT*, (λ y ) ; a fourth switching device Id has its first waveguide 3d connected to the third transmission signal regenerator TxT 2 (λ x ) and its third waveguide 5d connected to the second waveguide 4a of the first switching device la; - a fifth switching device le has its first waveguide 3e connected to the second transmitter Tx 2 and its third waveguide 5e connected to the fourth transmission signal regenerator TxT 2 (λ y ) ; a sixth switching device If has its first waveguide 3f connected to the first receiver Rx 1 , its second waveguide 4f connected to the fourth reception signal regenerator RxT 2 (λ y ) and its third waveguide 5f connected to the second waveguide 4c of the third switching device lc; a seventh switching device lg has its first waveguide 3g connected to the third reception signal regenerator RxT 2 (λ x ) , its second waveguide 4g connected to the third waveguide 5b of the second switching device lb, and its third waveguide 5g connected to the second waveguide 4d of the fourth switching device Id; and an eighth switching device lh has its first waveguide 3h connected to the second transmission signal regenerator TxT 1 (λ y ), its second waveguide 4h connected to the third waveguide 5c of the third switching device lc, and its third waveguide 5h connected to the second waveguide 4e of the fifth switching device le. The switching devices la-lh can be integrated on a single substrate . In Figure 4, the connections within the switch unit 37 relating to normal working conditions are shown in continuous lines, while the connections usable in case of a fault are shown in broken lines. In operation, the connection between two nodes of the network 30 of Figure 3 (for example between the nodes C and F) is established by using a first wavelength λ x in the first ring 31 in such a way as to form a first working channel λ x w , and using a second wavelength λ y in the second ring 32 in such a way as to form a second working channel λ y W . The same wavelengths can be used, in the same way, for further connections not superimposed on the previously defined connection. In the first ring 31, the second wavelength λ y can be used to form a protection channel λ y p and can be shared among all the connections operating at λ x , λ y . In the same way, the first wavelength λ x can be used in the second ring 32 to form a further protection channel λ x p and can be shared among all the connections operating at λ x , λ y . Conversely, connections superimposed on that previously defined must use pairs of wavelengths other than λ x , λ y . In case of a fault between a first and a second node communicating with each other (for example, a fault located between the nodes 33a and 33b, if the communicating nodes are 33c and 33f) , both communicating nodes are reconfigured in such a way as to invert the connections between the corresponding transmitters Tx and receivers Rx and the rings 31, 32. This reconfiguration is commanded by the processing unit 38 and takes place by means of the switching of some of the switches 1 present in the switch unit 37. In practice, the first transmitter Tx x of the first node and the second receiver Rx 2 of the second node, which previously communicated with each other via the working channel λ x w in the first ring 31, are optically connected to the second ring 32 in such a way as to occupy the protection channel λ x p . This takes place by the switching of the devices la and Id of the first node and the devices lb and lg of the second node. In the same way, the second transmitter Tx 2 of the second node and the first receiver RX- L of the first node, which previously communicated with each other via the working channel λ y w in the second ring 32, are connected optically to the first ring 31 in such a way as to occupy the protection channel λ y _ p . This takes place by the switching of the devices le and lh of the second node and of the devices lc and If of the first node. In this way, the transmission between the two nodes is switched to the part of the network 30 not affected by the fault. Any other connections passing through the point of the network at which the fault is present are modified in a similar way to that of the first and second node. In general, for each connection affected by a fault, only the terminal nodes of the connection are reconfigured, while the intermediate nodes of the connection remain unchanged. This condition is maintained until the fault is identified and repaired. During this fault condition, the protection channel used cannot be used simultaneously by other connections which may request it, for example because of the presence of further faults. After the fault has been repaired, the original situation is restored. With reference to Figure 11, the number 40 indicates an alternative configuration of the device according to the invention. The device 40 differs from the device 1 in that the waveguides are arranged in an X configuration instead of in a Y configuration. The device 40 comprises a first, a second, a third and a fourth waveguide 43-46, of which the third and the fourth (45, 46) are equivalent to the second and third waveguides 4, 5 of the device 1. The other parts of the device 40 are essentially equivalent to the corresponding parts of the device 1 and are therefore indicated by the same reference numbers. The substrate 2 of the device 40 may have a larger area than the substrate 2 of the device 1, owing to the presence of the first and second waveguides 43, 44. The first and second waveguides 43 and 44 are inclined with respect to each other at an angle of θ' , which is preferably equal to the angle θ between the third and fourth waveguides 45, 46. Additionally, the first and second waveguides 43 and 44 are preferably symmetrical about the axis 9 and are preferably rectilinear. Unlike the third and fourth waveguides 45, 46, which preferably have the same width, the first and second waveguides 43, 44 preferably have different widths from each other. For example, the first waveguide 43 may have a width equal to the width of the third and the fourth waveguide 45, 46, while the second waveguide 44 may have a width smaller than that of the first waveguide 43. The difference in width between the first and second waveguides 43, 44 reduces the optical coupling between them, since it causes a difference in the refractive index. The fundamental modes of propagation of the waveguide 43, 44 therefore have different propagation constants and are therefore "asynchronous" . This condition of asynchrony can alternatively be provided by making one of the waveguides 43, 44 curved. The value of the angles θ and θ' is advantageously as small as possible, compatible with the dimensions of the device 40, in such a way as to preserve the condition of adiabaticity. Preferably, the angles θ and θ' are smaller than 2°. The device 40 comprises a connecting waveguide 10' connecting the waveguides 43-46 and delimited by the segments of broken lines a ' and c ' which are orthogonal to the axis 9. The connecting waveguide 10 has a larger area than the connecting waveguide 10 of the device 1, since it additionally comprises a portion containing the connection with the first and second waveguides 43, 44. The connecting waveguide 10' has a continuously variable width (with a minimum in a position intermediate between the segments of broken lines a ' and c') and comprises a multimodal transmission region whose limits are essentially the same as those of the connecting waveguide 10' . The electrodes 6-8, if made from titanium, have a thickness which is preferably less than approximately 500 nm, more preferably in the range from approximately 50 nm to 150 nm, and are formed as described above with reference to the device 1. The operation of the device 40 is similar to the operation described above for the device 1, except for the fact that, since two input guides and two output guides are present, the device 40 is preferably used as a 2x2 switch. The operating principle of an X switching device is described, for example, in the aforesaid article by Silberberg et al . In practice, the mode carried by the first waveguide 43 is converted, in the connection region 10', into the normal first-order local mode (fundamental mode) , while the mode carried by the second waveguide 44 is converted, in the connection region 10', into the normal second-order local mode (first higher-order mode) . The first-order mode is converted into the fundamental mode of the output guide with a higher index and the second-order mode is converted into the fundamental mode of the output guide with a lower index. If no voltage is applied to the electrodes, the signals from the first and second waveguides 43, 44 are separated equally ("3 dB splitting") between the third and fourth waveguides 45, 46. As for the device 1, the applicant has observed that the device 40 has an extinction ratio (E.R.) which is a function of the longitudinal dimensions of the extension 6b of the central electrode 6, and more particularly of the longitudinal position of the end 6c of the central electrode 6 within the multimodal region. In particular, the applicant has observed that the value of the extinction ratio is particularly high for some values of the longitudinal dimensions of the extension 6b. The applicant has observed that this behaviour is demonstrable for both polarizations of the light, namely TE and TM, as for the device 1. The device 40 can be used, for example, to switch the optical signals in a ring network such as that described previously with reference to Figure 3. Figure 12 shows a node 33' which can be used in the network of Figure 3, comprising a switch unit 37' in which a plurality of devices 40 is present instead of a plurality of devices 1 as in the switch unit 37 of Figure 4. The switch unit 37' comprises a group of devices 40a-40d according to the present invention, commanded by the CPU 38 by means of an appropriate control logic (not shown) . The remaining parts of the node 33' are similar to the corresponding parts of the node 33 of Figure 4 and are therefore indicated by the same names and reference numbers . In detail, the group of devices 40a-40d comprises: a first device 40a having its second waveguide 44a connected to the first transmitter Tx 1# its third waveguide 45a connected to the third transmission signal regenerator TxT 2 (λ x ) and its fourth waveguide 46a connected to the first transmission signal regenerator TxT 1 (λ x ); a second device 40b having its first waveguide 43b connected to the third reception signal regenerator RxT 2 (λ x ) , its second waveguide 44b connected to the first reception signal regenerator R T- L (λ x ) , its third waveguide 45b connected to the first waveguide 43a of the first device 40a, and its fourth waveguide 46b connected to the second receiver Rx 2 ; a third device 40c having its second waveguide 44c connected to the second transmitter Tx 2 , its third waveguide 45c connected to the second transmission signal regenerator TxT x (λ y ) and its fourth waveguide 46c connected to the fourth transmission signal regenerator TxT 2 (λ y ) ,- and a fourth device 40d having its first waveguide 43d connected to the second reception signal regenerator RxT 1 (λ y ), its second waveguide 44d connected to the fourth reception signal regenerator RxT 2 (λ y ) , its third waveguide 45d connected to the first waveguide 43c of the third device 40c, and its fourth waveguide 46d connected to the first receiver RX- L . The operation of the node 33' is similar to that of the node 33. By comparison with the node 33, the node 33' has the advantage of requiring a smaller number of switching devices (4 instead of 8) , but has the disadvantages of having a bulkier switch unit (since X switches are bulkier than Y switches) and of forming superfluous connections, not used for signal transmission, during its operation. With reference to Figure 13, the number 50 indicates a device made according to the invention, comprising a LiNb0 3 substrate of the "z-cut" type, with propagation along the x axis or along the y axis. The device 50 has a guide structure of the Y type, similar to that of the device 1 described previously. In particular, the device 50 comprises a first, a second and a third waveguide 54, 55, 56, corresponding to the waveguides 3, 4 and 5 of the device 1, formed by diffusing titanium on a lithium niobate substrate 51 with a cut along the z axis. The first waveguide 54 has a longitudinal axis 57 which forms an axis of substantial symmetry for the device 50. In a similar way to the case of the device 1, the waveguides 54-56 are connected by means of a connecting waveguide comprising a multimodal region. The device 50 could alternatively comprise a waveguide structure of the X type, similar to the structure of the device 40 described above. The device 50 also comprises a first and a second electrode 52 and 53, identical to each other and formed on top of the substrate 51, preferably in positions symmetrical about the axis 57. The electrodes 52 and 53, unlike the electrodes of the device 1, extend on top of the waveguides 55, 56. The electrodes 52 and 53 are capable of establishing an electrical field within the waveguides 55, 56, and are both connected to a voltage generator (not shown) by means of corresponding contact pads 58, 59 of conductive material, preferably gold. The applicant has observed that the device 50, like the device 1, has an extinction ratio (E.R.) which is a function of the longitudinal dimensions (measured parallel to the axis 57) of the electrodes 52, 53. In particular, the extinction ratio of the device 50 has particularly high values when one end of the electrodes 52, 53 is placed in specified longitudinal positions of the multimodal region. In this case also, the applicant has observed that this behaviour is demonstrable for both polarizations of the light, namely TE and TM. The longitudinal dimensions (measured parallel to the axis 57) of the electrodes 52, 53 are therefore selected in such a way as to optimize the extinction ratio (E.R.) . The operation and any applications of the device 50 are entirely similar to the applications described above for the device 1 and will not be described further. Returning to the characteristic ΔP/ΔV of Figure 2, the applicant has observed that the device 1 (or one of its possible variants which have been described) can act as a variable attenuator, if it is used in the aforesaid essentially linear region (region II) of the characteristic. This is because the optical power in the waveguides 4, 5 can be controlled continuously by varying the potential difference ΔV in the range between the threshold values -ΔV TH and +ΔV TH . The device can be used as an attenuator either with light entering from the first waveguide 3 and leaving from the second and third waveguides 4, 5, or with light entering from the second or third waveguides 4, 5 and leaving from the first waveguide 3. In particular, if the light enters the device 1 from the first waveguide 3, it is possible, by varying the refractive index in a controlled way in the second and third waveguides 4, 5, to control the optical power of the light leaving the device 1 through the waveguides 4, 5, without operating in switching conditions (in other words, without the optical power being transferred completely into one or other of the waveguides 4, 5) . On the other hand, if the light enters the device 1 from the second or from the third waveguide 4, 5, then it is possible, by varying the refractive index in this waveguide in a controlled way, to control the optical power of the light which passes from this waveguide into the connecting waveguide 10, and thus it is possible to control the power of the light leaving the device 1 through the first waveguide 3. When operating as an attenuator, the device 1 can be associated with a parameter known as "attenuation" , which expresses the correlation between the optical power present in one of the two waveguides 4, 5 and the potential difference ΔV applied by means of the voltage generator 12. The relation between the attenuation, indicated by A, and the potential difference ΔV, is expressed by the following f ormula : A (ΔV) = 10 log ( P (ΔV) /P max ) ( 2 ) where P(ΔV) is the optical power passing through the waveguide in question and P max is the power passing through this waveguide in saturation conditions, in other words with a value of ΔV having a higher absolute value than the threshold value |ΔV TH [ . The applicant has noted that, when the device is used as an attenuator, the selection of an electrode length such that one end of the electrodes lies in the multimodal region provides a particularly high power dynamic. An example of an application of the device 1 as a variable attenuator is found in multiple wavelength (WDM) optical fibre telecommunications systems. Recent technological developments in the fabrication of erbium doped fibre optical amplifiers have yielded a rapid increase in the transmission capacities of these systems. However, owing to the non-uniformity of the spectral profile of the gain and of the saturation conditions of erbium doped fibre optical amplifiers, the channels transmitted undergo different degrees of amplification- For some channels, this leads to a greater probability of errors in the reception of the signals. Moreover, the gain profile of erbium doped fibre optical amplifiers is subject to variation with time (for example as a result of temperature variations) and the optical power of the received signals is therefore difficult to predict with accuracy. In an optical telecommunications system of this type, the device 1 can be used as an attenuator to equalize the power of the signals transmitted in the different channels. In practice, the device 1 can be used to reduce the power of those channels which, owing to the non-uniformity of the spectral profile of gain of the erbium doped amplifiers, have an optical power which is greater than that of the other channels. Figure 10 shows a telecommunications system 16 of the WDM suitable for the transmission of optical signals in N different channels associated with corresponding transmission wavelengths λ 1# λ 2 , ..., λ j , in a specified waveband. The telecommunications system 16 comprises: a plurality of transmitters 17, each capable of transmitting an optical signal to a corresponding wavelength selected from the set λ 1# λ 2 , ..., λ jj ,- a plurality of receivers 18, each capable of receiving an optical signal at a corresponding wavelength from the an optical fibre line 19 capable of guiding optical signals at the wavelengths λ 1# λ 2 , ..., λ N ; a wavelength multiplier 20 having N inputs, each connected to a corresponding transmitter 17, and one output connected to the optical fibre line, and capable of grouping the optical signals arriving from the transmitters 17 into a single WDM signal to be supplied to the optical fibre line 19; and - a wavelength demultiplexer 21 having one input connected to the optical fibre line 19 and N outputs, each of which is connected to a corresponding receiver 18, capable of separating the WDM signal arriving from the optical fibre line 19 into the different component signals (at an individual wavelength) and of supplying each of these signals to a corresponding receiver 18. The optical fibre line 19 comprises: a power amplifier 24 located immediately after the wavelength multiplexer 20, to provide the WDM signal with an adequate initial power; a plurality of sections of optical fibre 25 of specified length (for example a hundred kilometres or so) to guide the WDM signal; a plurality of line amplifiers 26 (only one of which is shown) spaced apart by sections of optical fibre 25, to periodically provide the WDM signal with a specified degree of amplification; and a preamplifier 27, located immediately before the wavelength demultiplexer 21, to provide the WDM signal with a degree of amplification such that the component signals can be correctly received after their separation by the demultiplexer 21. The telecommunications system 16 also comprises N devices 1 made according to the invention, used as power attenuators, each having its input connected to a corresponding transmitter 17 and its output connected to the wavelength multiplexer 20. More precisely, each device 1 has its first waveguide 3 connected to the corresponding transmitter 17 and its second waveguide 4 connected to the wavelength multiplexer 20. The third waveguide 5 is not connected to any device and the light guided by it is dispersed. Finally, the telecommunications system 16 comprises a control unit 28 having N inputs, each connected to a corresponding transmitter 17 and N outputs, each connected to a corresponding device 1. The control unit 28 is capable of receiving from each transmitter 17 a signal S indicating the value of the power of the optical signal transmitted at the corresponding wavelength and, according to the information contained in the signals S, of sending to each device 1 a command signal C for adjusting its operating point . In use, each transmitter 17 generates an optical signal at a corresponding wavelength selected from the set λ l t λ 2 , ..., λ N , and simultaneously sends the signal S to the control unit 28. Each of the optical signals generated in this way passes through the corresponding device 1, entering from the first waveguide 3 and leaving from the second waveguide 4, and is consequently attenuated according to the preceding relation (2) . The signals leaving the devices 1 through the second waveguides 4 are received by the multiplexer 20 and wavelength multiplexed in such a way as to generate the WDM signal. The WDM signal undergoes a first amplification by the power amplifier 24, passes through the sections of optical fibre 25, undergoing periodic amplification by the power amplifiers 26, and finally reaches the preamplifier 27, where it is further amplified to a power level sufficient for the reception of the individual signals. The signals are then separated in the demultiplexer 21 and sent to the corresponding receivers 18. The control unit 28, having access to the information of the signals S, in other words the information relating to the power level generated by each transmitter 17, sends to each device 1 the corresponding command signal C to establish the operating point of the device 1 along the characteristic ΔP (ΔV) . The operating points of the different devices 1 are selected in such a way as to adjust the power of each signal according to a specified value P(λi) corresponding to the wavelength λ t of the channel. RESULTS OF NUMERICAL SIMULATIONS AND EXPERIMENTAL MEASUREMENTS To evaluate the dependence of the extinction ratio of the device 1 on the length of the extension 6b, the applicant used a numerical model of the device 1, of the type described in the article by D. Sciancalepore, F. Dell'Orto and I. Montrosset, "Novel theoretical approach for Y digital optical switch", in Proc . ECIO '99, pp. 413-415, Apr. 1999. To demonstrate the effect of the extension 6b, the device 1 was considered in a 2x1 configuration; in other words, it was assumed that the signal was fed to the second or third waveguide 4, 5, and the optical power leaving from the first waveguide 3 was measured. To implement this model, the following characteristic values were chosen: length of waveguides 4 and 5 equal to 7 mm; width of waveguides 3, 4 and 5 equal to 6 μm; - length (along the axis 9) of the connection region 10 equal to 3 mm; angle θ equal to 0.15°; potential difference ΔV variable between -100 V to + 100 V in steps of 2.5 V; distance between the outer electrodes 7 and 8 in the multimodality region 14 equal to 8.5 μm; width of the extension 6b equal to 2 μm; and wavelength of the optical signal supplied to the device 1 equal to 1550 nm. Figure 5 shows, for the case of TE polarization only, the variation of the extinction ratio as a function of the longitudinal position of the end 6c of the extension 6b, in other words of the length 1 of the extension 6b. More particularly, the extinction ratio is measured, for ease of processing of the model, as a function of a longitudinal coordinate z measured, as shown in Figure 1, from the end of the electrodes 6-8 opposite the connecting waveguide 10 and towards the connecting waveguide 10. The different curves correspond to different values of the electrical potential difference ΔV applied to the electrodes 6, 7. As the results in Figure 5 demonstrate, the extinction ratio has particularly high values (in absolute terms) at certain longitudinal positions z (in other words for certain values of the length 1) which are within the multimodal region 14 (whose limit is indicated by the segment of broken line c corresponding to that of Figure 1) and for relatively high values of the potential difference ΔV (in absolute terms) . In particular, if the potential difference ΔV is such that the device 1 operates in switching or near-switching conditions, the curves representing the extinction ratio have peaks, in the case in question five peaks for z equal to 7515 μm, 8220 μm, 8732 μm, 9240 μm and 9775 μm (corresponding to lengths 1 of the extension 6b equal to approximately 518 μm, 1223 μm, 1735 μm, 2243 μm and 2778 μm respectively) . The curves corresponding to values of potential difference greater than or equal to the threshold potential difference |ΔV TH | are those in which the presence of the peaks is more marked. Figure 6 shows the variation of the extinction ratio as a function of the potential difference ΔV applied to the electrodes 6 and 7. The different curves, indicated by the letters m-u, relate to different values of the position of the end 6c, in other words of the length 1 of the extension 6b, and in particular to the values of the length 1 corresponding to the aforesaid peaks (curves m, o, q, s, u) and to the values intermediate between these (curves n, p, r, t) . As shown in both Figure 5 and Figure 6, the extinction ratio has an absolute value of more than 40 dB at certain values of the longitudinal coordinate z and therefore of the position of the end 6c (in other words of the length 1) . In the case of the curve m, the value of the extinction ratio exceeds 45 dB at the peak. The dependence of the extinction ratio on the spatial coordinate z was also determined in the case of TM polarization. The results are shown in Figures 7 and 8. In particular, Figure 7 shows the variation of the extinction ratio as a function of the spatial coordinate z (in other words of the position of the end 6c) , at different values of the potential difference ΔV, and Figure 8 shows the variation of the extinction ratio as a function of the potential difference ΔV, for the two values of the z coordinate (in other words of the position of the end 6c) at which the curve of Figure 7 has its peaks (curves v and y) and the value intermediate between these two values of the z coordinate (curve w) . In the case of TM polarization also, the curves representing the extinction ratio have, in the case of sufficiently high values of ΔV, peaks at specified values of the z coordinate, in the case in question for z equal to 7367 μm and 8754 μm (corresponding to lengths 1 of the extension 6b equal to 370 μm and 1575 μm respectively) . At these peaks, the device 1 has an extinction ratio of more than 45 dB, as shown in Figure 7. The applicant also conducted an experiment to confirm the results previously obtained by numerical simulation. Figure 14 shows a measuring apparatus used to carry out an experimental measurement on the device 1. This apparatus comprises : a laser source 61 capable of emitting at 1550 nm, - an optical fibre 62 forming a "polarization controller" connected to the laser source 61; the optical fibre 62 is a standard optical fibre wound in such a way as to form three loops and having a twist controlled in such a way as to have a desired polarization of the electromagnetic field at the output; a first lens 63 for shaping the optical beam leaving the fibre 62; a polarizer 64 positioned facing the first lens 63 and capable of allowing only one polarization of the optical beam to pass; a second lens 65 positioned facing the polarizer 64 on the opposite side from the first lens 63 and capable of focusing the optical beam on the first waveguide 3 of the device 1; - a third lens 66 positioned facing the second waveguide 4 of the device 1 and capable of shaping the optical beam leaving the device 1; a fourth lens 67 facing the third lens 66 and capable of receiving the optical beam shaped by the third lens 66; - a photodiode 68, for receiving the optical beam and consequently generating an electrical signal; a voltage generator 69 for driving the device 1; and a processor 70 connected to the voltage generator 69 and to the photodiode 68. The measurements were made on two prototype devices having a structure similar to that of the device 1, with extensions 6b of different lengths. In particular, the first device has an extension 6b with a length 1 of 400 μm and the second device has an extension 6b with a length 1 of 1600 μm. In both cases, the end 6c of the extension 6b is inside the multimodal region. The other characteristic parameters of the two devices used are identical to those listed previously with reference to the numerical simulations, except for the angle θ between the second and third waveguides 4, 5, which is 0.26° instead of 0.2°. The measurements were made with an optical power of approximately 0 dBm supplied to the device 1. The results of the measurements are shown in Figure 9, in which the variation of the extinction ratio is shown as a function of the potential difference applied to the electrodes. As shown by the graph, the curve for ' 1 = 400 μm has a peak of approximately 34 dB, while the curve for 1 = 1600 μm does not have any peaks. The applicant observes that the difference . in height between the peak obtained in the experimental measurement and the peaks obtained in the numerical simulations may be due to the fact that the value 1= 400 μm corresponds to a value of z which, probably, although being in the vicinity of a value of z for which the E.R./z characteristic has a peak, does not coincide with the latter value.

Claims: CLAI S 1. Optical switching/modulation device, comprising: a substrate (2; 51); at least a first, a second and a third waveguide (3-5; 44-46; 54-56) formed on the said substrate; a connecting waveguide (10; 10') which optically connects the said first, second and third waveguides, the said connecting waveguide having a longitudinal axis (9; 57) and comprising a multimodal transmission region (14) confined between a first and a second longitudinal position (a, c) , the said second and third waveguides being associated with, respectively, a first and a second refractive index, and forming two alternative branches for carrying the light from, or to, the said connecting waveguide; electrodes (6, 7, 8; 52, 53) associated with at least one of the said second waveguide (4; 45; 55) and third waveguide (6; 46; 56) and capable of generating an electrical field region to vary at least one of the said first and second refractive indices; characterized in that the said electrodes comprise at least a first electrode having one longitudinal end (6c) located between the said first and second longitudinal positions (a, c) . 2. Device according to Claim 1, characterized in that the said electrodes comprise a pair of electrodes (7, 8; 52, 53) having an essentially constant distance between them in the said multimodal region (14) . 3. Device according to Claim 1, characterized in that the said first electrode (6) forms a central electrode interposed between the said second waveguide (4; 45; 55) and the said third waveguide (6; 46; 56), and in that the said electrodes (6, 7, 8; 52, 53) comprise a second and a third electrode (7, 8) forming outer electrodes located on the sides of the said second waveguide and the said third waveguide respectively opposite the said central electrode. 4. Device according to Claim 3, characterized in that the said central electrode (6) has an essentially rectilinear extension (6b) extending along the said longitudinal axis (9) in the said multimodal region. 5. Device according to Claim 1, characterized in that the said first electrode is partially superimposed on the said second waveguide (4; 45; 55), and in that the said electrodes comprise a second electrode (7, 8) partially superimposed on the said third waveguide (5; 46; 56) . 6. Device according to Claim 1, characterized in that the said substrate is made from lithium niobate. 7. Device according to Claim 1, characterized in that the said first, second and third waveguides are made by diffusing titanium into the substrate. 8. Device according to Claim 1, characterized in that the said second waveguide (4; 45; 55) and the said third waveguide (5; 46; 56) form a first angle of less than 2° between them. 9. Device according to Claim 8, characterized in that it comprises a fourth waveguide (43) forming, with the said first waveguide (44), a second angle of less than 2°. 10. Device according to Claim 9, characterized in that the said fourth waveguide (43) and the said first waveguide (44) are of different widths. 11. Device according to Claim 1, characterized in that the said first, second and third waveguides (3-5; 44-46; 54-56) are essentially rectilinear. 12. Optical transmission system, comprising at least one transmitter (17; Txl, Tx2) capable of transmitting an optical signal, at least one receiver (18; Rxl , Rx2) capable of receiving the said optical signal, and an optical connection (19; 31, 32) which connects the said transmitter to the said receiver and which is capable of carrying the said optical signal, characterized in that it comprises at least one optical switching/modulation device (1; 50) according to Claim 1, connected optically in series with the said optical connection (19; 31, 32) to switch and/or modulate the said optical signal.

Inventors: DELL ORTO FLAVIO, SCIANCALEPORE DAVIDE, DELL'ORTO, FLAVIO, SCIANCALEPORE, DAVIDE
Applicants: DELL ORTO FLAVIO, OPTICAL TECHNOLOGIES ITALIA S, SCIANCALEPORE DAVIDE, DELL'ORTO, FLAVIO, OPTICAL TECHNOLOGIES ITALIA S.P.A., SCIANCALEPORE, DAVIDE
Application year: 2000
Main classification: G02F1
IPC codes: G02F1