Waveguide-type optical device

Abstract: A waveguide-type optical device comprising a substrate (1) with an electro-optical effect, two adjacent optical waveguides (2a,2b), a signal electrode (4a) covering either of the optical waveguides and an earth electrode (4b) covering the other optical waveguide and having a segment (4d) of the same form as the signal electrode which lies close to the other optical waveguide.

Description: The present invention relates to a waveguide-type optical device, and particularly to a waveguide-type optical device which is used for modulation of light waves, switching between optical paths, etc. using electric signals in the microwave band. Waveguide-type optical devices, particularly waveguide-type optical devices with waveguides fabricated on a substrate of ferroelectric crystals such as LiNbO 3 crystals, have the excellent characteristics of low optical absorption, low loss and high performance, and a wide variety of waveguide-type optical devices have been reported including directional coupler type modulator-optical switches, Mach-Zehnder type optical modulators, balanced bridge type light modulator-optical switches, total reflection type switches, etc. Characteristics required of such waveguide-type optical devices include a broadband, low moduration voltage, low crosstalk, low loss, and operational stability against changes in environmental conditions such as temperature variations and impact. Among them, it is particularly important to ensure operational stability over the broadband against changes in environmental conditions such as temperature variations and impact. Fig. 1A and Fig. 1B are a plan view and a cross-sectional view of a conventional Mach-Zehnder modulator, respectively. This conventional waveguide-type optical device is described in Nakajima, "HIGH SPEED LiNbO 3 MODULATORS AND APPLICATION", Second Optoelectronics Conference (OEC '88) Technical Digest, pp. 162-163, 3C1-1, October, 1988. The conventional waveguide-type optical device described therein comprises a z-plate (z-cut) LiNbO 3 substrate 1; two Y-branch optical waveguides 6a and 6b fabricated in the surface of the substrate; and two arm optical waveguides 2a and 2b fabricated in the surface of the substrate which connect the two Y-branch waveguides. In addition, a buffer layer 3 is placed overlaying the substrate 1, the Y-branch optical waveguides 6a and 6b and the arm optical waveguides 2a and 2b. Furthermore, placed on the buffer layer are a signal electrode 4a which covers the arm optical waveguide 2a via the buffer layer; and an earth electrode 4b which covers the arm optical waveguide 2b via the buffer layer. The two Y-branch optical waveguides and the two arm optical waveguides make up a Mach-Zehnder type optical circuit. The buffer layer 3 is placed to prevent absorption of light by the metal electrodes 4a and 4b. Gold is employed for the fabrication of the electrodes 4a and 4b. Here it is notable that the electrode arrangement of the conventional waveguide-type optical device is not symmetric with respect to the two arm optical waveguides 2a and 2b. More specifically, the signal electrode 4a is fabricated on one of the arm optical waveguides 2a, with a width roughly identical to that of the arm optical waveguide 2a; that is, the two edges of the signal electrode 4a are located in the vicinity of the arm optical waveguide 2a. On the other hand, the other arm optical waveguide 2b is fabricated in such a manner that only one edge thereof lies near the arm optical waveguide 2b. Suppose the device shown in Fig. 1A and Fig. 1B undergoes distortion due to temperature variation or extraneous impact. The distortion will be focused on the edges of the signal electrode 4a and earth electrode 4b. The distortion produces photoelastic and other effects which cause changes in the refractive index of the part of the electro-optic crystal substrate which is close to the edges of the signal electrode 4a and earth electrode 4b under distortion, that is, in the vicinity of the two arm optical waveguides 2a and 2b. As a result, the propagation constants of the two optical waveguides 2a and 2b also change. Since the electrodes 4a and 4b fabricated over the two optical waveguides 2a and 2b have different structures along the waveguides, more specifically, since the two arm optical waveguides 2a and 2b have a different positional relationship with the edges of their respective counterpart electrodes 4a and 4b, the two arm optical waveguides 2a and 2b undergo different changes in their propagation constants due to distortion. The different propagation constants of the two arm optical waveguides 2a and 2b provide applied voltage-dependent optical output characteristics which are equivalently parallel along the axis of applied voltages, resulting in changes in the extinction ratio and in shifting of the operating point of the high-speed optical modulator. Thus, with the electrode arrangement of conventional high-speed optical modulators which are driven by electric signals in the microwave band, problems occur such as changes in the extinction ratio and shifts in the operating point in response to temperature variation, extraneous impact, etc. JP-A-3 200 924 discloses a Mach-Zehnder interferometer. The signal electrode covering the first optical waveguide has the same width as the grounding electrode covering the second optical waveguide. The article "Modeling and Design of Ti:LiNbO 3 Optical Modulator Electrodes with a Buffer Layer" by Tsutomu Kitoh and Kenji Kawano, Electronics and Communications in Japan, Part II: Electronics 76(1993), No. 1, New York, US, pp. 25-33 discloses optical modulators having a symmetric coplanar strip line, i.e. the with of the two electrodes is the same, an asymmetric coplanar strip line, i.e. the width of the two electrodes is different, and a coplanar waveguide. In the case of the coplanar waveguide the width of the two electrodes covering the optical waveguides is different. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a waveguide-type optical device which operates in a stable manner against environmental changes including temperature variations and impact. According to the present invention, there is provided a waveguide-type optical device comprising as defined in claim 1. Preferred embodiments of the invention are defined in the dependent claims. With waveguide-type optical devices according to the present invention, operational stability is achieved against changes in environmental conditions such as temperature variations, impact, etc. while maintaining the broadband characteristics. More specifically, the stability in operation according to the present invention results from the fact that, a non-conventional structure is employed; part of which entrails a modification of the conventional ground electrode which is adapted to provide the two arm optical waveguides with identical distortions, and the rest of which is a duplicate of the conventional electrode structure in order not to deteriorate the broadband characteristics. BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantages of the present invention will become more apparent when the following description is read in conjunction with the accompanying drawings, wherein: Figs. 1A and 1B are views which show the construction of a conventional waveguide-type optical device, wherein Fig. 1A is a plan view and Fig. 1B is a cross-sectional view; Fig. 2 is a plan view which shows a first comparative example of a waveguide-type optical device; Fig. 3A is a cross-sectional view of the first comparative example which is taken on line B-B' in Fig. 2; Fig. 3B is another cross-sectional view of the first comparative example which is taken on line A-A' in Fig. 2; Fig. 4 is a cross-sectional view of a second comparative example and Fig. 5 is a plan view of a third comparative example Fig. 6 is a plan view of a first embodiment of the present invention. Fig. 7 is a plan view of a second embodiment of the present invention. In these figures, the same reference characters depict the same parts, respectively. DESCRIPTION OF THE PREFERRED EMBODIMENTS An explanation will now be given regarding a first comparative example with reference to Fig. 2, Fig. 3A and Fig, 3B. Fig. 2 is a plan view which shows the first example, while Fig. 3A is a cross-sectional view taken on line A-A' in Fig. 2, and Fig. 3B is a cross-sectional view taken on line B- B' in Fig. 2. As shown in these drawings, the first example comprises: a z-cut LiNbO 3 substrate 1; two Y-branch optical waveguides 6a and 6b fabricated in the surface of the substrate; two arm optical waveguide 2a and 2b fabricated in the surface of the substrate which connects between the two Y-branch waveguides; a buffer layer 3 which lies overlaying the two Y-branch optical waveguides 6a and 6b, the two arm optical waveguides 2a and 2b, and the surface of the substrate 1; a signal electrode 4a which covers the arm optical waveguide 2a via the buffer layer 3 and supplies electric signals in the microwave band; an earth electrode 4b which covers the arm optical waveguide 2b via the buffer layer 3, and part of which is hollowed out in the vicinity of the arm optical waveguide 2b in such a manner that the resulting segment 4d of the earth electrode 4b which lies immediately over the waveguide 2b (hereunder referred to as the "smaller earth electrode") is provided with the same form as of the signal electrode 4a; and an earth connector section 8 which is received in the resulting through-hole and connects between the smaller earth electrode 4d over the arm optical waveguide 2b and the rest of the earth electrode 4b. Here, the buffer layer 3 consists mainly of SiO 2 or a congener thereof, nevertheless, comprises Al 2 O 3 , MgF 2 , SiON, Si 3 N 4 , etc. as well, with a thickness on the order of 0.2-2 µm. The electrodes 4a and 4b and the earth connector section 8 consist mainly of Au which, however, may be replaced by another materials such as Al, Mo, ITO or ZnO, a conductive polymer, etc., from about 1 µm to 30 µm in thickness. A thick metal film is usually deposited by plating, but may also be applied by vapor deposition, sputtering or the like. Referring to Fig. 2, a region is formed wherein the electrode arrangement is symmetric with respect to the two arm optical waveguides 2a and 2b. Concretely, the earth. electrode 4b constructed over the optical waveguide 2b has a structure wherein a through-hole is made in a part thereof so that a part of the ground electrode 4b, or the smaller earth electrode 4d, has the same width as of the signal electrode 4a. The cross-sectional view of the device taken on line B-B' in Fig. 2 is shown in Fig. 3A. As shown in Fig.3A, a gap width W 1 between the signal electrode 4a and the smaller earth electrode 4d is substantially the same as a gap width W 2 between the smaller earth electrode 4d and the earth electrode 4b. In the embodiments, W 1 and W 2 may be in the range of 5-40µm. In the region defined by the electrodes 4a and 4d shown in Fig. 2, there is provided a site 7 in which the electrode arrangement is symmetric with respect to the two optical waveguides (hereunder referred to as the "electrode-symmetric site"), in which site identical degrees of distortion are applied to the two optical waveguide 2a and 2b even when distortion occurs due to temperature variations, extraneous impact or the like, and thus no difference occurs between the propagation constants of the two optical waveguides 2a and 2b. As a result, optical waveguide-type devices which comprise even a trace of such electrode-symmetric structure operate in a more stable manner with fewer changes in extinction ratios and fewer shifts in operating points than the conventional optical control devices. In addition, regarding high-frequency microwaves, since the smaller earth electrode 4d is connected with the adjacent earth electrode 4b, the characteristics of the frequency band are less impaired. Fig. 3B is a cross-sectional view of the first comparative example taken on line A-A' in Fig. 2, or a cross-sectional view of the first example without the electrode-symmetric site 7. In this example, the earth electrode 4b and smaller earth electrode 4d are connected through the earth connector section 8. This earth connector section may be formed by lithographic techniques simultaneously with the other electrodes. The section may also be formed by optical CVD after the formation of the other electrodes. All that is needed to establish more stable operation is to increase the area of the electrode-symmetric site 7. In this connection, however, the present inventors have found that the earth electrode 4d close to the signal electrode 4a cannot function as the earth for microwaves in cases where the length P of the electrode-symmetric site 7 is longer than half the wavelength of the microwave propagating through the signal electrode 4a which drives the optical control device. Therefore, in such cases a plurality of electrode-symmetric sites 7 are provided to make the length P each of the electrode-symmetric sites 7 shorter than half the wavelength of the microwave when it propagates through the signal electrode 4a. If this requirement is satisfied, then the microwave propagating the signal electrode cannot be induced to the smaller earth electrode; actually no impairment of the characteristics of the frequency band was observed when the requirement was met. Thus, there is provided a high-speed optical control device which operates in a stable manner against changes in environmental conditions such as temperature variations or impact, while maintaining high-speed operation over the frequency band. Next, reference is made to Fig. 4 which is a cross-sectional view showing the earth connector section 8 of a second comparative example. The plan view of the second embodiment is identical to Fig. 2. Also, the cross-sectional view of the electrode-symmetric site 7 is identical to Fig. 3A. In this example, an aerial wire is used as the earth connector section 8, through which the earth electrode 4b and smaller earth electrode 4d are connected to each other. A gold ribbon connector or the like may be employed for the formation of the earth connector section 8. According to the present example, since the distortions applied to the two optical waveguides 2a and 2b are symmetric over the entire electrode region, there is provided a high-speed waveguide-type optical control device which operates in a more stable manner than the first example, against changes in environmental conditions such as temperature variations or impact, while maintaining high-speed operation over the frequency band. Fig. 5 is a plan view which shows a third comparative example. Also in the third example, the earth connector section 8 is realized by aerial wiring in the same manner as in the second example. The only difference between the second and third examples is that the earth connector section 8 is formed with a gold ribbon in the second example, whereas the means is a bonding wire in the third example. Since a bonding wire is lighter than a gold ribbon as is well known, dynamic boundary conditions of the edge section of the signal electrode 2a and those of the edge section of the earth electrode 2b may be made closer in the third example than in the second example. This means that the present example provides a waveguide-type optical device which operates in a more stable manner than the second example against changes in environmental conditions. In the above examples, a micro-strip line type structure is adopted as an electrode arrangement. However, in the optical modulators according to the invention a co-planar arrangement is adopted as the electrode arrangement as shown in Figs. 6 and 7, which show a first and second embodiment of the invention. In Figs. 6 and 7, an additional earth electrode 4c is added to the first and second comparative examples, respectively. The additional earth electrode is placed near the electrode 4a with a gap width W 1 which equals or nearly equals to W l and W 2 . Since the first and second embodiments provide a more symmetrical electrode arrangement than the first-to-third comparative examples, the first and second embodiments provide more stable devices against changes in environmental condition. In the foregoing explanation, the detailed explanation of the present invention was given with reference to a Mach-Zehnder type optical modulator as an example; nevertheless, the present invention may undoubtedly be readily applied to other types of waveguide-type optical devices, including directional coupler type and balanced bridge type ones, for example.

Claims: A waveguide-type optical device comprising: a substrate which has an electro-optical effect; an optical circuit comprising two adjacent optical waveguides (2a, 2b) which are formed in a surface of said substrate; a first electrode which covers the first optical waveguide, and in use supplies an electric signal; and a second electrode, a segment of which covers the second optical waveguide, wherein the cross section of the segment of said second electrode which covers said second optical waveguide is identical to the cross section of said first electrode, characterized in that said device further comprises an additional earth electrode (4c) formed on said substrate near said first electrode (4a), wherein the gap between said additional electrode and said first electrode has the same width (W 3 ) as the gap (W 1 ) between said first electrode (4a) and said second electrode (4d). The waveguide-type optical device as claimed in claim 1, wherein said second electrode (4d) has a through-hole in the vicinity of said second optical waveguide (2b) in such a manner that the segment of said second electrode (4d) which has resulted from the formation of the through-hole and is close to said second optical waveguide, has the same form as said first electrode (4a). The waveguide-type optical device as claimed in claim 2, wherein a plurality of connector sections (8) are formed in said through-hole for the connection of said segment and the rest of said second electrode. The waveguide-type optical device as claimed in claim 3, wherein the spaces between the adjacent pairs of said plurality of the connector sections (8) are smaller than half the wavelength of an electric signal propagating through said first electrode (4a) for which the device has been designed. The waveguide-type optical device as claimed in claim 4, wherein said connector section (8) connects between said segment and said second electrode (4d) without contacting with said through-hole. The waveguide-type optical device as claimed in claim 4, wherein said connector section is a bonding wire.

Inventors: NISHIMOTO HIROSHI, SUZAKI TETSUYUKI, NISHIMOTO, HIROSHI, SUZAKI, TETSUYUKI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1995
Main classification: G02F1
IPC codes: G02F1