Optical waveguide device

Abstract: An optical waveguide device includes an optical waveguide (2, 2a, 2b) formed on a surface of a substrate (1) having an electrooptical effect, and a pair of electrodes (5, 6) formed on corresponding regions above the optical waveguide and receiving a driving electric signal thereacross. By applying the electric signal to the electrodes, an electric field is formed in a vicinity of the optical waveguide. By changing a distribution of the electric field in accordance with a change in a frequency of the electric signal, it is possible to suppress a DC drift of the device to effectively compensate a fluctuation in operational characteristics due to the DC drift, stress, or the like.

Description: The present invention relates to an optical waveguide device, and more particularly to an optical waveguide device such as a modulator, switch, distributor, or the like, used in optical communication apparatus. A typical optical waveguide device used in an optical switch, an optical modulator, or the like, is constituted such that an electric field is applied to an optical waveguide formed on a surface of a substrate consisting of electrooptical crystal such as lithium niobate (LiNbO 3 ), or the like, so as to change a refractive index, and thus switching or phase modulation of a light signal traveling through the optical waveguide can be carried out. As an example of the optical waveguide device, a Mach-Zehnder type modulator is well known. A modulator of this type includes at least an optical waveguide formed on a surface of a Z-cut LiNbO 3 substrate and a pair of asymmetrical electrodes formed on corresponding regions above the optical waveguide. In such a structure, charges collected on the surface of the LiNbO 3 substrate by the pyroelectric effect act on the asymmetrical structure of the electrodes and thus a disadvantage occurs in that the distribution of charges is made uneven and an adverse influence is exerted on characteristics of the device. Also, since the distribution of resistance, dielectric constant and capacitance of each portion of the structure or partial fine structure is delicately changed in accordance with the process conditions, the electrical equivalent circuit accordingly becomes complicated. This results in a problem in that a direct current (DC) component of an electric signal applied across the electrodes greatly changes an application manner of the electric field with a long-term time constant and the optical response is also accordingly changed. This phenomenon is called a DC drift. In an optical waveguide device formed on an electrooptical crystal substrate having a pyroelectric effect, it is preferable that a semiconductive layer of, e.g., silicon (Si) is formed on a buffer layer so as to make uniform a distribution of charges occurring in the vicinity of the surface of the optical waveguide. In this case, in a structure having a narrow signal line electrode and a wide grounding electrode, a potential on the semiconductive (Si) region on the outside of the signal line electrode (i.e. on the opposite side of the grounding electrode) is uniformly made equal because the silicon (Si) layer acts as a conductor in a low frequency band. As a result, a drawback arises in that the electric field to be concentrated on the vicinity of the electrode for signal line is dispersed into the outside semiconductive (Si) region and thus it is not effectively applied to the optical waveguide. To cope with this drawback, a third electrode may be provided on the outside of the signal line electrode, i.e. on the opposite side of the grounding electrode. By this structure, it is possible to cause a voltage drop in proportion to the distance between the signal line electrode and the third electrode and thus concentrate the dispersed electric field on the vicinity of the signal line electrode. Also, by suitably changing the thickness of the silicon (Si) layer between the signal line electrode and the third electrode, it is possible to change a distribution of resistance therebetween to regulate a distribution of the voltage drop and thus concentrate the electric field more effectively. On the other hand, when a high frequency electric signal is applied to the pair of driving electrodes, charges in the silicon (Si) layer cannot follow a change in the high frequency electric signal and thus the semiconductive layer functions as a dielectric. Also, by disposing the third electrode on a region spaced from the signal line electrode by a sufficient distance, it is possible to reduce the influence exerted on the characteristic impedance of the traveling wave electrode. JP-62-073207 discloses an optical waveguide device in which three aluminium electrodes are formed on a buffer layer overlying a LiNbO 3 -substrate. A similar device, without a buffer layer, is disclosed in "Phase Tuning in Optical Directional Coupler", O. Mikami et al, Japanese Journal of Applied Physics, Volume 16 (1977) Supplement 16-1, pp 321-324. However, neither document is actually concerned with the same problem as is addressed here. It is therefore desirable to provide an optical waveguide device capable of suppressing a DC drift thereof to effectively compensate for a fluctuation in operational characteristics due to the DC drift, stress, or the like. According to the present invention there is provided an optical waveguide device comprising: an optical waveguide formed on a surface of a substrate having an electro-optical effect; a pair of electrodes operatively connected and arranged such that a change in frequency of a driving electric signal applied therebetween when the device is in use brings about a corresponding change in a distribution of an electric field which occurs in the vicinity of the optical waveguide, and a third electrode arranged so as to be spaced by a predetermined distance from the said pair of electrodes and connected so as to receive when in use a direct current voltage, a low frequency voltage or to be grounded, characterised in that the said third electrode comprises a semiconductive layer. By employing a semiconductive layer having a relatively low resistance in place of a conductor as the third electrode, it is possible to reduce the influence exerted on the characteristic impedance. Although the above structure premises that the third electrode is grounded, the third electrode may receive a voltage signal. In this case, more effective advantages can be obtained. Namely, by applying a DC or low frequency voltage to the third electrode, it is possible to compensate for a fluctuation in operational characteristics occurring due to a DC drift, mechanical stress, or the like. In this case, it is preferable that a semiconductor layer consisting of material having a relatively high resistance, e.g. silicon (Si), is formed close to the signal line electrode. The semiconductor layer may receive a voltage directly from an external power source, or may receive the voltage via a conductor layer, which is formed on the semiconductor layer at a position spaced from the signal line electrode by a sufficient distance such that its presence does not greatly affect a characteristic impedance of the traveling wave electrode. By applying the voltage directly or indirectly to the semiconductor layer, it is possible to change a refractive index of the optical waveguide beneath the signal line electrode and thus compensate for a fluctuation in operational characteristics. In this case, the semiconductive layer functions as a conductor with respect to a low frequency voltage and functions as a dielectric with respect to a high frequency voltage. Accordingly, the presence of the semiconductive layer has less infuence on the characteristic impedance in the high frequency band of the applied voltage. Also, another semiconductive layer may be formed all over the buffer layer so as to improve temperature characteristics. In this case, by decreasing the thickness of the semiconductive layer in the vicinity of the signal line electrode and concentrating the electric field on the thinly formed portion, it is possible to realise the same effect as above. Reference will now be made, by way of example, to the accompanying drawings, in which: Figs. 1A and 1B show respective plan and sectional views of a structure of a prior art Mach-Zehnder type modulator; Figs. 2A and 2B show respective plan and sectional views of a structure of a prior art optical waveguide device (optical modulator); Figs. 3A and 3B show plan and sectional views, respectively, of a structure of a first optical modulator embodying the present invention; Figs. 4A and 4B show plan and sectional views, respectively, of a first modification of the embodiment shown in Figs. 3A and 3B; Figs 5A and 5B show plan and sectional views, respectively, of another optical modulator embodying the present invention; Figs. 6A and 6B show plan and sectional views, respectively, of a structure of a proposed optical waveguide device (optical modulator) which does not embody the present invention; Fig. 7 shows a sectional view of a first modification of the structure shown in Figs. 6A and 6B; Fig. 8 shows a sectional view of a second modification of the structure shown in Figs. 6A and 6B; Fig. 9 shows a sectional view of a third modification of the structure shown in Figs. 6A and 6B; Fig. 10 shows a sectional view of a fourth modification of the structure shown in Figs. 6A and 6B; and Fig. 11 shows a sectional view of a fifth modification of the structure shown in Figs. 6A and 6B. For a better understanding of the preferred embodiments of the present invention, the problems in the prior art will be explained with reference to Figs. 1A and 1B. Figures 1A and 1B show a structure of a prior art Mach-Zehnder type modulator. Note, Fig. 1B shows a sectional structure along the line A-A' in Fig. 1A. The illustrated structure shows an example using asymmetrical electrodes for a traveling wave. In the drawings, reference 1 denotes a Z-cut LiNbO 3 substrate and reference 2 denotes an optical waveguide including two-forked optical waveguides 2a and 2b. The optical waveguide 2 (2a,2b) is formed by effecting a striplike patterning of a Titanium (Ti)-deposited layer formed on a surface of the substrate 1 and then effecting a thermal diffusion of the Titanium into the substrate 1. Thus, the optical waveguide 2 (2a,2b) has a larger refractive index than that of the substrate 1. Also, to prevent light propagating through the optical waveguide from being absorbed in driving electrodes (a signal line electrode 5 and a grounding electrode 6), a dielectric buffer layer 3 is formed between the electrodes 5, 6 and the optical waveguide 2. The buffer layer 3 is translucent and has a smaller refractive index than that of the optical waveguide 2. The buffer layer 3 is comprised of, for example, silicon dioxide (SiO 2 ). The dielectric layer 3 electrically functions as a capacitance containing relatively high resistance material. In an optical waveguide device using a substrate of Z-cut LiNbO 3 , a disadvantage occurs in that charges collected on the surface of the substrate by the pyroelectric effect act on the asymmetrical structure of the electrodes formed on the buffer layer and thus an uneven distribution of charges is formed, which has an adverse influence on characteristics of the optical waveguide device. In view of this, a semiconductive layer 4 comprised of, for example, silicon (Si) is formed between the buffer layer 3 and the electrodes 5, 6 and all over the buffer layer 3 (see Fig. 1B). By this structure, it is possible to make uniform the distribution of surface charges occurring due to a change in temperature or the like and stabilize the characteristics of the optical waveguide device. Furthermore, since the optical waveguide 2 is formed by diffusing the Titanium with high temperature into the LiNbO 3 substrate 1, its resistance, dielectric constant and capacitance are different from those of the bulk portion of the substrate 1. Additionally, a resistance of the surface of the LiNbO 3 substrate 1 is different from that of the bulk portion thereof due to the diffusing process. In view of the complicated distribution of the resistance, dielectric constant and capacitance of the above layers and partial fine structure, the electrical equivalent circuit also becomes complicated. As a result, a problem occurs in that a DC component of an electric signal applied across the electrodes 5, 6 greatly changes an application manner of the electric field with a long-term time constant in accordance with the process conditions and the optical response characteristics are also accordingly changed. Namely, a DC drift is caused. To cope with the problem, there have been taken measures to connect the electrodes 5, 6 of the modulator to an external power source by way of capacitance coupling and thus prevent a DC component of the power source from being directly applied to the modulator, or measures to connect a biasing DC power source to the electrode for signal line 5 by means of capacitance coupling and thus compensate a DC drift occurring due to some causes. According to the measures to employ an external power source by way of capacitance coupling, however, a drawback occurs in that the device is brought to an electrically open state seen from the power source and thus an adverse influence is exerted on the operation of the power source. A disadvantage also occurs in that an available frequency band of the power source is limited in dependence on the capacity thereof. On the other hand, according to the measures to employ a biasing DC power source by way of capacitance coupling, a drawback occurs in that a characteristic impedance of the traveling wave signal line electrode fluctuates and thus it is impossible to satisfactorily carry out the modulation operation. Also, where a stress is applied to the optical waveguides for some reason, the refractive index of the optical waveguides normally changes, resulting in a fluctuation in operational characteristics of the modulator. To cope with this disadvantage, there has been heretofore taken measures to add a DC voltage to the application voltage so as to cancel a change in the refractive index occurring due to the stress. However, to take the measures to add a DC voltage to the application voltage is often difficult from a viewpoint of the limit of the dielectric strength of the DC power source. In view of the above problems, a structure or means other than the electrodes provided for switching or modulation needs to be provided for suppressing a DC drift and compensating for a fluctuation in operational characteristics due to the DC drift, stress, or the like. Another previously-considered optical modulator is shown in Figs. 2A and 2B, in which reference 1 denotes a substrate consisting of lithium niobate (LiNbO 3 ), i.e. a Z-cut LiNbO 3 substrate. On the surface of the substrate 1, a striplike optical waveguide 2 having a width of approximately 7 µm is formed. The optical waveguide 2 includes two forked optical waveguides 2a and 2b, which are formed such that the forking angle is 1 and the distance therebetween is 15 µm. The optical waveguide 2 (2a,2b) is formed by effecting a patterning of a Titanium (Ti)-deposited layer formed on the surface of the substrate 1 and then giving a heating of 10 hours with a temperature of 1050 °C to effect a thermal diffusion of the Titanium into the substrate 1. Reference 3 denotes a buffer layer 3, which is formed all over the substrate 1 including the optical waveguide 2 (2a,2b). The buffer layer 3 is comprised of silicon dioxide (SiO 2 ) and has a thickness of approximately 0.6 µm. Reference 5 denotes a signal line electrode having a width of 7 µm and a thickness of approximately 10 µm, and reference 6 denotes a grounding electrode having a thickness of approximately 10 µm. The electrodes 5 and 6 are comprised of gold (Au) and formed by plating on corresponding regions (buffer layer 3) above the optical waveguides 2a and 2b, respectively. The grounding electrode 6 is formed so as to be surrounded by the signal line electrode 5. Also, reference 7 denotes a source for applying a driving electric signal across the electrodes 5 and 6, and reference 8 denotes a terminal resistor connected between the electrodes 5 and 6. Reference 10 denotes a third electrode having a thickness of approximately 10 µm and comprised of gold (Au). The third electrode 10 is formed by plating on the outside of the signal line electrode 5, i.e. on the opposite side of the grounding electrode 6, and receives a DC or low frequency voltage fed from a power source 20. In the present example, the third electrode 10 is disposed on a region (buffer layer 3) spaced with a distance of approximately 50 µm from the electrode for signal line 5. By this constitution, it is possible to decrease the influence exerted on the characteristic impedance of the traveling wave electrode 5, 6. Namely, by applying a DC voltage or a low frequency voltage below 5 to 6 kHz to the electrode 10, it is possible to change a refractive index of the optical waveguide 2a beneath the signal line electrode 5. Note, the power source 20 is independent from the electric signal source 7 and thus mutual interference is not substantially present. Therefore, it is possible to easily compensate for a fluctuation in operational characteristics due to a DC drift, stress, or the like, by applicaton of the DC or low frequency voltage to the electrode 10. Figures 3A and 3B show a structure of a first optical modulator embodying the present invention. In the present example, an electrode 10a comprised of a semiconductive layer of silicon (Si) is provided in place of the metal electrode 10 shown in Figs. 2A and 2B. The semiconductive electrode 10a is disposed on a region (buffer layer 3) spaced with a distance of approximately 15 µm from the signal line electrode 5. In this case, by selecting a specific resistance of the semiconductive electrode 10a to be 100 Ωm and a thickness thereof to be 0.2 µm, the electrode 10a can function as a conductor or a dielectric. Namely, when the electrode 10a receives a DC voltage or a low frequency voltage below 500 to 600 Hz, it functions as an ordinary metal electrode. On the other hand, when the electrode 10a receives a high frequency voltage above 5 to 6MHz, it rather functions as a dielectric because its equivalent resistance becomes relatively high and thus charges cannot follow a change in the electric field. In this case, the presence of the semiconductive electrode 10a does not substantially have an influence on the characteristic impedance of the traveling wave electrode 5, 6. Also, since the semiconductive electrode 10a is disposed fully close to the signal line electrode 5, it is possible to effectively change the refractive index of the optical waveguide 2a beneath the siqnal line electrode 5. Therefore, it is possible to more effectively compensate for a fluctuation in operational characteristics due to the DC drift, stress, or the like, by applicaton of the DC or low frequency voltage to the electrode 10a. The device of Figs. 3A and 3B is produced according to the process conditions below. First, Titanium (Ti) is deposited on the Z-cut LiNbO 3 substrate 1 to have a thickness of approximately 90 nm and, using a photoresist, a waveguide pattern is formed on the Ti-deposited layer. The formed pattern acts as a mask. Then, a chemical etching is carried out to form a metal (Ti) pattern. The metal (Ti) pattern has a width of approximately 7 µm and includes a forked waveguide portion having a forking angle of 1° and a parallel waveguide portion having a length of 20 mm and spaced with a distance of 15 µm from each other. Then, the metal (Ti) pattern is diffused into the substrate 1 with a heat of 1050 °C and a time of 10 hours in a wet O 2 atmosphere, so as to form a single mode optical waveguide 2 (2a,2b) having an optical wavelength of 1.5 µm. Next, the buffer layer (SiO 2 ) 3 having a thickness of 0.6 µm is formed by depositing on the substrate 1 including the optical waveguide. Then, the semiconductive layer (Si) 10a having a thickness of 0.1 µm and a length of 20 mm is formed by sputtering on a region (buffer layer 3) spaced with a distance of 15 µm from the optical waveguide 2a. The semiconductive layer (Si) 10a has a specific resistance of approximately 10 kΩ · cm. Finally, the electrodes 5 and 6 comprised of gold (Au) and having a length of 20 mm and a thickness of 10 µm are formed by plating on corresponding regions (buffer layer 3) above the optical waveguide 2a, 2b. The metal (Au) electrodes 5 and 6 are spaced with a distance of 15 µm from each other and constitute asymmetrical electrodes for a traveling wave. Note, the signal line electrode 5 has a width of 7 µm. The operational voltage of the formed modulator is 5 V and the characteristic impedance of the electrodes for a traveling wave is 50 Ω. The value of the characteristic impedance is equal to that in the case where the semiconductive layer (Si) 10a is not provided. This means that the presence of the semiconductive layer 10a does not have an influence on the characteristic impedance. Also, when phase control corresponding to the operational voltage is carried out by means of the power source 20, a voltage of the power source 20 is 6 V and a frequency thereof is 10 Hz. These values are the same as those in Figs. 2A and 2B in which the metal electrode 10 is provided in place of the semiconductive electrode 10a. Note, the characteristic impedance of the electrodes for a traveling wave employed in Figs. 2A and 2B is 36 Ω. Figures 4A and 4B show a structure of a modification of the device of Figs. 3A and 3B. In the present example, a semiconductive layer 4 comprised of silicon (Si) is added to the structure shown in Figs. 3A and 3B. The semiconductive layer 4 has a thickness of 0.1 to 0.2 µm and is formed between the buffer layer 3 and the electrodes 5, 6 and 10a and all over the buffer layer 3. In this case, by selecting a specific resistance of the semiconductive electrode 10a to be two figures smaller than that of the semiconductive layer 4, it is possible to concentrate an electric field generated by a voltage applied to the electrode 10a on the region between the electrodes 10a and 5. As a result, it is possible to effectively change the refractive index of the optical waveguide 2a beneath the electrode for signal line 5. Although the thickness of the semiconductive layer 4 is selected to be constant, it is not restrictive. For example, by partially changing the thickness of the semiconductive layer 4 and decreasing the thickness of the layer 4 at a desired portion on which an electric field should be concentrated, it is possible to realize the same effect as above. Figures 5A and 5B show a structure of another optical modulator embodying the present invention. The illustrated structure is intended to prevent a deterioration in modulation characteristics caused by the semiconductive (Si) layer 4 provided for an improvement in temperature characteristics of the device formed on the LiNbO 3 substrate. The feature and effect caused thereby are explained below in contrast with the prior art modulator shown in Figs. 1A and 1B. Referring to Figs. 1A and 1B, when a DC voltage is applied between the electrodes 5 and 6, a voltage drop is caused linearly between the electrodes after a considerably long time has elapsed. Accordingly, a potential on the semiconductive region 4 on the outside of the signal line electrode 5 (i.e., on the opposite side of the grounding electrode 6) is equalised with that on the signal line electrode 5. As a result, the lines of electric force caused by the voltage applied between the electrodes 5 and 6 are not concentrated on the vicinity of the electrode 5, but are dispersed into the semiconductive region 4 on the outside of the electrode 5. Therefore, it is impossible to effectively change the refractive index of the optical waveguide 2a beneath the electrode 5. To cope with this disadvantage, the third electrode 10 is provided on the outside of the signal line electrode 5 (see Figs. 7A and 7B). In this case, the third electrode 10 is grounded and disposed on a region spaced from the electrode 5 with a sufficient distance such that its presence does not substantially affect the characteristic impedance of the electrode 5. By this structure, a voltage drop is caused linearly between the electrodes 5 and 10 and thus the lines of electric force are concentrated on the vicinity of the electrode 5. Although the thickness of the semiconductive layer 4 is selected to be constant, it is not restrictive. For example, by changing the thickness of the semiconductive layer 4 between the electrodes 5 and 10 and decreasing the thickness of the layer 4 in the vicinity of the electrode 5, it is possible to concentrate the voltage drop on the thinly formed portion and more effectively concentrate the lines of electric force on the vicinity of the electrode 5. In this respect, Figs. 8 to 13 show structures of further optical modulators which do not have a third electrode and hence do not embody the present invention, but which are useful for understanding how the thickness of the layer 4 might be adjusted. The structure of Figs. 6A and 6B is characterized by a semiconductive layer 4a, 4b formed between the buffer layer 3 and the electrodes 5 and 6. The semiconductive layer is comprised of silicon (Si) and has a thickness of approximately 0.2 µm, and is separated into two regions 4a and 4b in the center of the corresponding region thereof between the electrodes 5 and 6. The gap between the separated portions 4a, 4b, indicated by reference P, is selected to be 1 to 4 µm. Note, each semiconductive layer 4a, 4b has a broader width than that of the corresponding electrode 5, 6. In this case, a specific resistance of the semiconductive layer 4a, 4b can be designed and selected over a wide range from 0.001 to 100000 Ωm. Where the present optical modulator is used in a frequency of 5 to 6 MHz or more, the specific resistance is preferably selected to be 100 Ωm. Of course, the specific resistance is altered in accordance with a thickness of each layer, a layer-to-layer distance, a distance between each electrode, and the like. By adopting the above structure, components of a DC voltage or a low frequency voltage of 50 to 60 Hz applied across the electrodes 5, 6 are concentrated on the separated portion P and thus do not substantially affect the characteristic impedance of the optical waveguide 2 (2a, 2b). On the other hand, where a high frequency voltage of approximately 5 to 6 MHz is applied across the electrodes 5, 6, charges cannot easily follow a change in the voltage and thus the semiconductive layer (4a, 4b) functions as a dielectric. Namely, since the high frequency voltage is applied between the electrodes 5 and 6, it is possible to effectively change the refractive index of the optical waveguide 2 (2a, 2b). Therefore, according to the present example, it is possible to disregard a DC component of the applied voltage and thus suppress the influence of DC drift. Figure 7 shows a first modification of the structure shown in Figs. 6A and 6B. In the present example, the semiconductive layer 4a is formed only in the vicinity of the electrode for signal line 5. Since the width of the semiconductive layer 4a formed under the electrode 5 can be arbitrarily designed and selected, it is possible to realize better frequency characteristics. Figure 8 shows a structure of a second modification. In the present example, in place of the separated semiconductive layers 4a, 4b shown in Figs. 8A and 6B, a semiconductive layer 4c comprised of silicon (Si) is formed between the buffer layer 3 and the electrodes 5 and 6 and all over the buffer layer 3. The semiconductive layer 4c has a thinly formed portion Q in the center of the corresponding region thereof between the electrodes 5 and 6. By this structure, a DC component or a low frequency component of the voltage applied across the electrodes 5, 6 is concentrated on the thinly formed portion Q and thus it is possible to gain the same effect as in Figs. 6A and 6B. Figure 9 shows a structure of a third modification, which is a combination of the example of Fig. 7 and the example of Fig. 8. In the present example, since the semiconductive layer 4c is formed only in the vicinity of the signal line electrode 5, it is possible to realize better frequency characteristics as in the example of Fig. 7. Figure 10 shows a structure of a fourth modification. In the present example, a semiconductive layer 4 and a second semiconductive layer 9a are provided in place of the separated semiconductive layers 4a, 4b shown in Figs. 6A and 6B. The semiconductive layer 4 is comprised of silicon (Si) and formed between the buffer layer 3 and the electrode 6 and all over the buffer layer 3. The semiconductive layer 9a is comprised of silicon (Si) and formed between the semiconductive layer 4 and the electrode 5. Also, the semiconductive layer 9a has a specific resistance different from that of the semiconductive layer 4 and has a broader width than that of the corresponding electrode 5. In the device using the asymmetrical electrodes 5, 6 for a traveling wave, the electric field in the vicinity of the optical waveguide 2b beneath the grounding electrode 6 is relatively weak. Therefore, even if a DC electric field is concentrated on the region between the electrodes 5 and 6, it is possible to suppress the influence exerted on the refractive index of the optical waveguide 2b beneath the electrode 6 and thus gain the same effect as above. Figure 11 shows a structure of a fifth modification, which is an improvement of the example of Fig. 10. In the present example, a semiconductive layer 9b comprised of silicon (Si) is added to the structure shown in Fig. 10. The semiconductive layer 9b is formed between the semiconductive layer 4 and the electrode 6. Also, the semiconductive layer 9b has a specific resistance different from that of the semiconductive layers 4, 9a and has a broader width than that of the corresponding electrode 6. Since the present device has a number of designing parameters, it is possible to design and optimize the device more effectively and thus gain a better effect. Although, in the above embodiments and the associated modifications, the explanation is given by way of the Mach-Zehnder type modulator, the present invention is applicable to other optical waveguide devices such as optical switches, or the like. Also, the present invention can be applied to an optical waveguide device to which a low frequency signal and a high frequency signal are fed independently. Although the present invention has been disclosed and described by way of selected embodiments and the associated modifications, it is apparent to those skilled in the art that other-embodiments and modifications of the present invention are possible.

Claims: An optical waveguide device comprising: an optical waveguide (2, 2a, 2b) formed on a surface of a substrate (1) having an electro-optical effect; a pair of electrodes (5, 6) operatively connected and arranged such that a change in frequency of a driving electric signal applied therebetween when the device is in use brings about a corresponding change in a distribution of an electric field which occurs in the vicinity of the optical waveguide (2, 2a, 2b), and a third electrode (10, 10a) arranged so as to be spaced by a predetermined distance from the said pair of electrodes (5, 6) and connected so as to receive when in use a direct current voltage, a low frequency voltage or to be grounded, characterised in that the said third electrode comprises a semiconductive layer (10, 10a). An optical waveguide device as claimed in claim 1, further comprising a buffer layer (3) formed so as to overlie all the substrate (1) including the optical waveguide (2, 2a, 2b). An optical waveguide device as claimed in claim 2, wherein the buffer layer (3) comprises dielectric material and has a smaller refractive index than that of the optical waveguide (2). An optical waveguide device as claimed in claim 3, wherein the dielectric material comprises silicon dioxide. An optical waveguide device as claimed in any preceding claim, further comprising a second semiconductive layer (4) formed between the buffer layer (3) on the one hand and the pair of electrodes (5, 6) and the third electrode (10, 10a) on the other and all over the buffer layer (3), wherein the third electrode (10, 10a) has a specific resistance different from that of the second semiconductive layer (4). An optical waveguide device as claimed in claim 5, wherein the said third electrode (10) is grounded and is spaced from the said pair of electrodes (5, 6) by a distance sufficient to ensure that the presence thereof does not have a substantial influence on a characteristic impedance of the said pair of electrodes (5, 6). An optical waveguide device as claimed in any one of claims 1 to 4, further comprising a second semiconductive layer (4a, 4b) formed between the buffer layer (3) on the one hand and the said pair of electrodes (5, 6) and the said third electrode (10) on the other, the said second semiconductive layer (4a, 4b) comprising two regions separated from one another at a part of said layer corresponding to the gap between the said pair of electrodes (5, 6), each separated region of said second semiconductive layer (4a, 4b) having a broader width than that one of the said pair of electrodes under which it lies, thereby to ensure that, when a low frequency electric signal is applied to the pair of electrodes (5, 6), the electric field is concentrated in the gap (P) between the separated regions of the said second semiconductive layer (4a, 4b). An optical waveguide device as claimed in claim 7, wherein that one (4a) of the separated regions (4a, 4b) of the second semiconductive layer (4a, 4b) which underlies a signal line electrode (5) of the pair of electrodes (5, 6) is formed only in the vicinity of that signal line electrode (5). An optical waveguide device as claimed in any one of claims 1 to 4, further comprising a second semiconductive layer (4c), formed between the buffer layer (3) on the one hand and the pair of electrodes (5, 6) and the third electrode (10) on the other, having a thinly formed portion (Q) at a part of the said second semiconductive layer (4c) which corresponds to the gap between the pair of electrodes (5, 6), thereby to ensure that, when a low frequency electric signal is applied to the pair of electrodes (5, 6), a greater part of a voltage drop caused by the application of the electric signal is applied to the thinly formed portion (Q) of the second semiconductive layer (4c) and thus the electric field is concentrated on the thinly formed portion (Q). An optical waveguide device as claimed in claim 9, wherein the second semiconductive layer (4c) including the thinly formed portion (Q) is formed only in the vicinity of the pair of electrodes (5, 6). An optical waveguide device as claimed in claim 5, further comprising a third semiconductive layer (9a, 9b) formed between the second semiconductive layer (4) and a signal line electrode (5) of the pair of electrodes (5, 6), the third semiconductive layer (9a, 9b) having a specific resistance different from that of the second semiconductive layer (4) and having a broader width than that of the signal line electrode (5). An optical waveguide device as claimed in claim 11, wherein the said third semiconductive layer (9a, 9b) comprises two regions (9a, 9b) separated from one another at a part of said layer corresponding to the gap between the said pair of electrodes (5, 6), each separated region (9a, 9b) of the said third semiconductive layer having a broader width than that one of the said pair of electrodes (5, 6) under which it lies. An optical waveguide device as claimed in any preceding claim, wherein the semiconductive layer of claim 1 comprises silicon.

Inventors: DOI MASAHARU, KUBOTA YOSHINOBU, MEKADA NAOYUKI, NAKAZAWA TADAO, SEINO MINORU, DOI, MASAHARU, KUBOTA, YOSHINOBU, MEKADA, NAOYUKI, NAKAZAWA, TADAO, SEINO, MINORU
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1991
Main classification: G02F1
IPC codes: G02B6, G02F1