ELECTRO-OPTIC WAVE GUIDE MODULATOR

Abstract: In an optical element having an optical waveguide (2) and flat electrodes (3A, 3B) arranged on a substrate (1) in which an electric field is applied through the flat electrodes (3A, 3B) to the substrate (1) and the optical waveguide (2) so that a refractive index of a portion of the substrate (1) in the vicinity of the optical waveguide (2) is varied to interrupt a light beam propagating through the optical waveguide (2), the flat electrodes (3A, 3B) are disposed asymmetrically about the optical waveguide (2). The optical modulation, optical switching or the like can be carried out by electrically controlling the difference between refractive indices of the optical waveguide (2) and the substrate (1) in the horizontal direction, so that the nonuniformity of the refractive index of the optical waveguide (2) which causes some problems in conventional branching interference type optical modulator and directional optical coupler do not cause any problem. The optical element has a higher light extinction ratio and the output light is by far stabilized against the variation in temperature.

Description: The present invention relates to an optical element in which an optical waveguide and flat electrodes are mounted on a substrate and an electric field is applied through the flat electrodes to the substrate and the optical waveguide so that a refractive index of a portion of the substrate adjacent to the optical waveguide is varied to interrupt a light beam propagating through the optical waveguide, and more particularly to an optical element which is well adapted for use as an optical modulator for converting an electrical signal into a light signal or an optical switch for permitting or interrupting the propagation of a light signal through an optical fiber or an optical waveguide and which ensures a high light extinction ratio (a maximum output light intensity/a minimum output light intensity), a low insertion loss and a high-speed operation. So far an optical modulator and a directional optical coupler utilizing the electro-optical effect have been widely used in order to perform a high-speed operation of an optical modulator or optical switch. Typical optical elements of the type described above are (1) a Mach-Zehnder type thin-film optical modulator (Reference is made to R.G Hunsperger, "Integrated Optics: Theory and Technology", Springer-Verlag, New York, 1982, P.135), employing single-mode optical waveguides formed into a Y branching path and a Y coupling path, in which the phase difference between two light beams is electrically controlled and (2) a thin-film directional optical coupler having two single-mode optical waveguides spaced apart from each other in which optical energy is synchronously transferred from one guide to the other (ibid., p. 129). In either element, it is required that a distribution of an electric field of a light beam, a distribution of a refractive index of an optical guide and a distribution of an applied electric field must be made uniform in the two optical waveguides in order to attain a higher light extinction ratio. However, it has not been easy to increase the light extinction ratios of these optical elements, because of: (1) a complex spatial profile of the refractive index, since an optical waveguide is generally fabricated by the diffusion of impurities; (2) the use of flat electrodes incapable of establishing a uniform electric field within an optical waveguide. Furthermore, in order to maintain a high light extinction ratio, it is essental that an applied voltage must be controlled precisely and the temperature must be kept constant at a high degree of accuracy. This results in a complicated structure of an optical element. In addition to the optical elements described above, there has been devised and demonstrated a cutoff type thin-film optical modulator (Reference is made to A. Neyer and W. Sohler, "Appl. Phys. Lett., Vol. 35, (1979) p. 256") in which a refractive index of a straight optical waveguide is electrically controlled so that an incident light beam is guided or radiated into a substrate. An example of the prior art cutoff type thin-film optical modulators is shown in Fig. 12. In Fig. 12, reference numeral 1 designates a substrate upon which an optical waveguide 2 is formed. Flat electrodes 3A and 3B are placed on both sides of a narrow portion as shown in Fig. 12 and in symmetrical relationship about the axis of the optical waveguide 2. An input prism coupler 5A is placed on the input end of the optical waveguide 2 in order to couple a light beam into the optical waveguide 2, while an output prism coupler 5B is placed on the output end of the optical waveguide 2 in order to receive the light beam propagated to the output end of the optical waveguide 2. In the prior art cutoff type thin-film optical modulator of the type described above, a guided light beam is converted into a radiated light beam by electrically controlling the difference in refractive indices between the optical waveguide 2 and the substrate 1 underneath the waveguide 2 (in other words, the modulator is operated into the cutoff mode), so that it is difficult to achieve a sufficiently high light extinction ratio because of the following reasons: (1) The refractive index of the surface of the optical waveguide 2 is considerably greater than that of the substrate, so that even when the following reason (2) to be described below can be eliminated, a high voltage must be applied in order to decrease the refractive index in order to radiate the guided light beam into the -X-direction. (2) An intensity of an electric field in the substrate 1 generated by applying a voltage to the flat electrodes 3A and 3B is gradually reduced along the -X-direction, so that a considerably high voltage must be applied in order to reduce the difference in refractive indices between the optical waveguide 2 and the substrate 1 underneath the waveguide. A prior art according to the preamble of claim 1 is shown in the EP-A-0 099 282, in the article "Waveguide Electrooptic Modulators" (IEEE Transactions on microwave theory and techniques, Vol. MTT-30, No. 8, August 1982, pages 1121-1137, IEEE, New York) or in the article "High Speed Guided-Wave Optical Modulators" (Journal of optical communications, Vol. 3, No. 2, June 1982, pages 52-58, Fachverlag Schiele & Schon, Berlin). The optical waveguides and electrodes of the optical element shown therein are all designed rectilinear. For the modulation an electric field is generated which has to be relatively strong in order to influence the light beams in the optical waveguides. Furthermore, the electric field distribution is symmetrical, so that the light beams can only be deflected into the substrate with difficulties. It is the object of the invention to produce an optical element according to the preamble of claim 1, in which the optical modulation has been improved by simple means. This object is solved by the features in the characterizing portion of claim 1. According to the invention the optical waveguide and the flat electrodes have a curved portion and each electrode is elongated with parallel sides which are also substantially parallel with the curved sides of said optical waveguide so as to vary the refractive index. By means of these measure, the optical modulation of the optical element can be improved, so that the electric field distribution of a guided light beam becomes asymmetric and the light beam can easily be deflected into the substrate. A first flat electrode may have a portion overlying the optical waveguide and may be disposed on the inner side of the optical waveguide in such a way that one side edge of the flat electrode is aligned with the outer side edge of the curved portion of the optical waveguide or is spaced apart therefrom by a predetermined distance. A second flat electrode may be disposed on the outer side of the curved portion of the optical waveguide and may be spaced apart from the first flat electrode by a predetermined distance. Here, a third flat electrode of the flat electrodes may be disposed on the inner side of the curved portion of the optical waveguide and may be spaced apart from the first flat electrode by a predetermined distance. The above object, features and advantages of the present invention will become more apparent from the following description of preferred embodiments in conjunction with the accompanying drawings. Figs. 1-3 are top views illustrating a first, a second and a third embodiment of an optical element in accordance with the present invention; Figs. 4-6 are top views illustrating yet further embodiments of an optical element in accordance with the present invention; Fig. 7 is an explanatory diagram used to explain the mode of operation of an optical element in accordance with the present invention and illustrates the dependence of a normalized width H of an optical waveguide upon an applied electric field in the cutoff mode; Figs. 8-10 are top views illustrating further embodiments of an optical element in accordance with the present invention; Fig. 11 is an explanatory diagram used to explain the mode of operation of an optical element in accordance with the present invention and illustrates the dependence of a normalized width (H) of an optical waveguide in the cutoff mode and of a normalized width (H₄) of an optical waveguide, whose actual width is 4µm upon an applied electric field; and Fig. 12 is a perspective view illustrating the construction of a prior art cutoff type thin-film optical modulator. Next, referring to Fig. 1, a first embodiment of the present invention will be described. An optical waveguide 12 having a curved portion is placed on a substrate 1 and an outer electrode 13B is placed adjacent to the curved portion of the optical waveguide 12, while an inner electrode 13A is placed on the opposite side of the optical waveguide 12 and is spaced apart therefrom by a predetermined distance. The two electrodes 13A and 13B are substantially in parallel with the optical waveguide 12. In the optical waveguide 12, a light beam propagates straightly, so that the confinement of the light beam becomes weak in the curved portion of the optical waveguide 12. As a result, the optical element of the first embodiment can modulate a light beam at a light extinction ratio higher than that of the optical element as shown in prior art. When a voltage is applied to this optical element so that the cutoff condition is satisfied, the propagating light beam is radiated into the substrate 1 under the electrode 13B as shown by an arrow IV in Fig. 1. Fig. 2 shows a second embodiment of the present invention. In this embodiment, an optical waveguide 12 having a curved portion is placed on a substrate 1. An inner electrode 13A is placed over the optical waveguide 12 in such a way that their outer arcuate sides are aligned with each other while the outer electrode 13B is spaced apart from the optical waveguide 12 by a predetermined distance. The two electrodes 13A and 13B are substantially in parallel with the optical waveguide 12. A buffer layer 16 is interposed between the electrode 13A and the optical waveguide 12. In the second embodiment, as in the case of the first embodiment described above with reference to Fig. 1, the confinement of a light beam within the optical waveguide 12 having the curved portion becomes weak, so that the optical modulation can be carried out at a light extinction ratio higher than that of the optical element as shown in prior art. When a voltage is applied to this optical element of the second embodiment, so that the cutoff condition is satisfied, the propagating light beam is radiated into the substrate 1 between the electrodes 13A and 13B as indicated by an arrow V in Fig. 2. Fig. 3 shows a third embodiment of an optical element in accordance with the present invention which is substantially similar in arrangement to the second embodiment described above with reference to Fig. 2 except that a third flat electrode 13C is placed adjacent to and spaced apart by a predetermined distance from the electrode 13A radially inwardly in such a way that the third electrode 13C and the electrode 13B sandwich the flat electrode 13A. Due to the provision of the third flat electrode 13C, the electric field component in the direction perpendicular to the plane of Fig. 3 under the middle electrode 13A is increased as compared with the electric field component in the direction perpendicular to the plane of the optical element as shown in Fig. 2, so that the third embodiment can carry out the optical modulation at a voltage lower than a voltage which must be applied to the optical element as shown in Fig. 2. Next, referring to Figs. 4-6, further embodiments of the present invention will be described. The optical elements shown in Figs. 4, 5 and 6 are substantially similar in arrangement to those described above with reference to Figs. 1, 2 and 3, respectively. That is, each of them comprises a substrate 1, an optical waveguide 12 and flat electrodes 13A, 13B and 13C, but it further comprises a substrate portion 21 into which no propagating light beam is radiated even in the cutoff mode and into which impurities are diffused so that a refractive index of the substrate 1 is decreased. The modes of operation of these optical elements are substantially similar to those of the corresponding optical elements described above, so that the effect of the diffusion of impurities will be described below only in conjunction with the optical element as shown in Fig. 4. Referring to Fig. 4, the influence of the impurity diffusion upon the cutoff condition, in which the light beam radiates into the substrate 1, may be quantatively analyzed by using the effective index of refraction method. First, the width W of the optical waveguide 12 is infinitely extended so as W=∞ with keeping the depth D of the waveguide 12 unchanged. The effective index of refraction n f of this slab waveguide (The slab waveguide is described in detail by T. Tamir in "Integrated Optics", Springer Verlag, New York, 1975, p. 13) can be calculated by using the effective index of refraction method. In a second step, the depth of the optical waveguide 12 is infinitely extended with keeping the width W unchanged. When a refractive index of the substrate 1 is n s and a wavelength of a light beam propagating in vacuum is λ, the normalized cutoff width H of the second slab waveguide, in which a light wave of the fundamental mode radiates into the substrate 1, is given by The above-described equation is described in detail by T. Tamir in "Integrated Optics", Springer Verlag, New York, 1975, pp. 23-24. In the above-described equation, Δn e represents a decrement in the refractive index of the substrate 1 when an electric field is applied and Δn a represents a decrement in the refractive index of the substrate 1 due to the diffusion of impurities. In the case that the difference Δn d between refractive indices of the surface of the optical waveguide 12 and the substrate 1 is 3x10⁻³, the dependence of normalized cutoff width H of the optical waveguide 12 upon the applied electric field E(X) with Δn a as a parameter is shown by the solid-line curves in Fig. 7. The narrower the width W of the optical waveguide 12 becomes; that is, the smaller the value of H, the greater the quantity of light which leaks into the substrate 1 becomes. Therefore, the cutoff condition is satisfied in the region below each solid-line curve. The greater the quantity of diffused impurities and the higher the applied electric field, the greater the cutoff region is expanded. Therefore, it is seen that it becomes easier to drive the optical element into the cutoff mode. In the case of an optical element whose optical waveguide has an actual width W₄ = 4µm, the effect of the impurity diffusion will be clarified. The normalized width H₄ of the optical waveguide having the width W₄ is given by Since the refractive index of the optical waveguide 12 is decreased in response to the increase in applied electric field E(x), H₄ is also decreased as indicated by the broken-line curve shown in Fig. 7. The value of E(X) at the intersection between H₄ and H represents a minimum field intensity required for cutting off a light beam and it is seen that the intersection is shifted to the side of lower field intensity in response to the increase in Δn a . For instance, when Δn a is increased from 0 to 0.5x10⁻³, it is seen that the field intensity required for driving the optical element into the cutoff mode is 30% of the field intensity required for driving an optical element which is not diffused with impurities into the cutoff mode. The optical elements having impurities thus diffused in accordance with the present invention are different from three types of embodiments described above in that a refractive index profile in the horizontal direction is made asymmetrical so that they can carry out the optical modulation at a voltage by far lower than a voltage required to drive the three embodiments with the same arrangements of the optical waveguide and electrodes. Still further embodiments of the present invention will be described below with reference to Figs. 8-10. The optical elements as shown in Figs. 8-10 are substantially similar to those as shown in Figs. 1, 2 and 3, respectively, in that each of them comprises a substrate 1, an optical waveguide 12 and flat electrodes 13A, 13B and 13C except that each of them is provided with a substrate portion 41 into which the propagating light beam is radiated in the cutoff mode and which is diffused with impurities so that a refractive index of the substrate portion 41 is increased. The modes of operation of these optical elements are substantially similar to those of the corresponding optical elements described above so that the effect of diffused impurities will be described below only with reference to the optical element as shown in Fig. 8. Fig. 11 shows the dependence of the normalized cutoff width H of the optical waveguide where a single-mode beam is radiated into the substrate 1 and the normalized width H₄ of the optical waveguide whose actual width is 4µm upon the applied electric field in the case that the difference Δn d between refractive indices of the surface of the optical waveguide and the substrate is 3x10⁻³. Here, a parameter Δn b indicates an increment in refractive index of the substrate due to the diffusion of impurities. As in the case of Fig. 7, the intersection of curve H and curve H₄ represents a minimum field intensity required for driving the optical element into the cutoff mode and it is seen that the higher the parameter Δn b , the lower the applied electric field at which the optical element is driven into the cutoff mode becomes. The optical elements in accordance with the present invention have a double feature in that a refractive index profile of the substrate 1 in the horizontal direction is made asymmetrical due to the diffusion of impurities into the substrate 1 and that the normalized width of the optical waveguide is decreased. Therefore, as compared with the optical elements having the same arrangement of the optical waveguide and electrodes without impurities diffusion, the optical modulation can be carried out at a lower electric field. As described above, in the optical elements in accordance with the present invention, the optical modulation, optical switching or the like can be carried out by electrically controlling the difference between refractive indices of the optical waveguide and the substrate in the horizontal direction, so that the nonuniformity of the refractive index of the optical waveguide which causes some problems in conventional branching interference type optical modulator and directional optical coupler do not cause any problem in the present invention. In addition, as compared with conventional optical modulator and optical switch in which a phase of the propagating light beam is controlled, an optical element in accordance with the present invention has a higher light extinction ratio and the output light is by far stabilized against the variation in temperature. Furthermore, in accordance with the present invention, light loss is smaller than that of the conventional device. A length of an optical element can be made less than one quarter or one third of those of the component parts in a conventional Mach-Zehnder type optical element and directional optical coupler. The fabrication of the optical element in accordance with the present invention is much facilitated. As it is clear from the first embodiment as shown in Fig. 1, the optical element in accordance with the present invention has an extremely high light extinction ratio, so that it can be used as optical attenuator. Most of conventional optical attenuators have been utilizing the phenomenon that metal absorbs light and there has been devised and demonstrated an optical attenuator in which light is attenuated by varying the thickness of a thin metal film. However, it has some defects that it is too large in size and too heavy in weight and that the degree of attenuation is manually varied so that light cannot be attenuated at a higher rate. The optical element in accordance with the present invention has a ratio in optical intensity between the input and output lights is made higher than 56 dB so that the optical element can be satisfactorily used as high-speed optical attenuator which are compact in size and light in weight.

Claims: An optical element having an optical waveguide (12) and flat electrodes (13A, 13B, 13C) arranged asymmetrically about said optical waveguide (12) on a substrate (1) in which an electric field is applied through said electrodes (13A, 13B, 13C) to said substrate (1) and said optical waveguide (12) so that a refractive index of a portion of said substrate (1) in the vicinity of said optical waveguide (12) is varied to interrupt a light beam propagating through said optical waveguide (12), characterized in that said optical waveguide (12) has a curved portion and each electrode (13A, 13B, 13C) is elongated with parallel sides which are also substantially parallel with the curved sides of said optical waveguide (12). An optical element as claimed in claim 1, characterized in that impurities which alter the magnitude of a refractive index of said substrate (1) are diffused in a portion of said substrate under any one of said electrodes (13A, 13B, 13C) and a portion of said substrate adjacent thereto. An optical element as claimed in claim 1 or 2, characterized in that a first flat electrode (13B) of said flat electrodes (13A, 13B, 13C) is disposed on the outer side of said optical waveguide (12) in such a way that one side edge of said first flat electrode (13B) is aligned with the outer side edge of said curved portion of said optical waveguide (12) or is spaced apart therefrom by a predetermined distance; and that a second flat electrode (13A) of said flat electrodes (13A, 13B, 13C) is disposed on the inner side of said curved portion of said optical waveguide (12) in such a way that said second flat electrode (13A) is spaced apart from the inner side edge of said curved portion of said optical waveguide (12) by a predetermined distance. An optical element as claimed in claim 1 or 2, characterized in that a first flat electrode (13B) of said flat electrodes (13A, 13B, 13C) is disposed on the outer side of said curved portion of said optical waveguide (12) and is spaced apart from a second flat electrode (13A) by a predetermined distance; and that a second flat electrode (13A) of said flat electrodes (13A, 13B, 13C) has a portion overlying said optical waveguide (12) and is disposed on the inner side of said optical waveguide (12) in such a way that one side edge of said first flat electrode (13A) is aligned with the outer side edge of said curved portion of said optical waveguide (12), wherein a buffer layer (16) is interposed between said second electrode (13A) and said optical waveguide (12). An optical element as claimed in claim 1 or 2, characterized in that a first flat electrode (13B) of said flat electrodes (13A, 13B, 13C) is disposed on the outer side of said curved portion of said optical waveguide (12) and is spaced apart from a second flat electrode (13A) by a predetermined distance; and that a second flat electrode (13A) of said flat electrodes (13A, 13B, 13C) has a portion overlying said optical waveguide (12) and is disposed on the inner side of said optical waveguide (12) in such a way that one side edge of said second flat eletrode (13A) is spaced apart from the outer side edge of said curved portion of said optical waveguide (12) by a predetermined distance, wherein a buffer layer (16) is interposed between said second electrode (13A) and said optical waveguide (12). An optical element as claimed in claim 3 to 5, characterized in that a third flat electrode (13C) of said flat electrodes (13A, 13B, 13C) is disposed on the inner side of said curved portion of said optical waveguide (12) and is spaced apart from said second flat electrode (13A) by a predetermined distance. An optical element as claimed in claim 3 to 6, characterized in that impurities which alter the magnitude of a refractive index of said substrate (1) are diffused in a portion (41) of said substrate (1) between a portion adjacent to said one side edge of said optical waveguide (12) and said first flat electrode (13B), a portion under said first flat electrode (13B) and a portion adjacent thereto. An optical element as claimed in claim 3 to 6, characterized in that impurities which alter the magnitude of a refractive index of said substrate (1) are diffused in a portion (41) of said substrate (1) under said first flat electrode (13A) and a portion of said substrate adjacent thereto. An optical element as claimed in claim 3 to 6, characterized in that impurities which alter the magnitude of a refractive index of said substrate (1) are diffused into a portion (21) of said substrate (1) adjacent to said one side edge of said optical waveguide (12), a portion of said substrate (1) under said second flat electrode (13A) and a portion of said substrate (1) adjacent thereto.

Inventors: TAKIZAWA, KUNIHARU NHK TECHNICAL RESEARCH LAB.
Applicants: NIPPON HOSO KYOKAI
Application year: 1986
Main classification: G02F1
IPC codes: G02B6, G02F1