(1) Field of the Invention
The present invention relates to optical waveguide devices, travelling-wave light modulators, and a process for producing the optical waveguide devices.
(2) Related Art Statement
In the optical communication field, it is presumed that since the communication capacity will drastically increase, the capacity of the light transmitting system needs to be enlarged. At present, the light transmitting speed of 1.6 GB/sec. has been practically employed. However, as compared with the frequency band (about 200 THz) in which transmission can be effected through optical fibers, this level is merely one hundred thousandth. It is important in drastically increasing the transmission capacity to develop the light modulation technology.
There is the possibility that a traveling-wave light modulator using lithium niobate (LiNbO.sub.3), lithium tantalate (LiTaO.sub.3) or gallium-arsenide (GaAs) for the optical waveguide, can realize a broad bandwidth at a high efficiency. Lithium niobate and lithium tantalate have materials as a excellent materials as a ferroelectric properties have large electro-optical coefficients and can control light within a short optical path. Factors which suppress the modulation frequency of the traveling-wave light modulator, are velocity mismatch, dispersion and electrode loss. Since the velocity mismatch and the dispersion are determined by the structure of the traveling-wave light modulator, it is important to analyze the structure and make appropriate design thereof. On the other hand, the conductivity and a surface skin effect of the material is important for the electrode loss.
The concept of velocity mismatch will now be further explained. In the traveling-wave light modulator, the velocity of the light propagating through the optical waveguide largely differs from that of the modulating wave (microwave) propagating along this electrode. Assume that the light and the modulation wave propagating through the crystal have different velocity Vo and Vm, respectively. For example, in the case of the LiNbO.sub.3 optical modulator having planar type electrodes, the refractive index of the LiNbO.sub.3 single crystal is 2.14, and the velocity of the light propagating through the optical waveguide is inversely proportional to the refractive index. On the other hand, the effective index for modulating wave is given by a square root of the dielectric constant near a conductor. LiNbO.sub.3 is uniaxial crystal, and the dielectric constant in the Z-axis direction is 28 and that in the X-axis and Y-axis directions is 43. Therefore, even if an influence of air having the dielectric constant of 1, the effective index for modulating wave in the LiNbO.sub.3 modulator having a conventional structure is about 4 which is about 1.9.times.2.14. Thus, the velocity of the light wave is large about 1.9 times as much as that of the modulating wave.
The upper fm bandwidth of the light modulator or the modulating velocity is inversely proportional to a difference in velocity between the light wave and the modulating wave. That is, fm=1/(Vo-Vm). Therefore, assuming that the power loss by electrode is zero, a limit is a fm bandwidth X electrode length 1=9.2 GHz.cm. Actually, it is reported that in a light modulator having an electrode length 1=2.5 mm, fm=40 GHz. The effect due to the limit of operation speed becomes more conspicuous as the electrodes become longer. Therefore, a light modulator having a broad bandwidth and high efficiency has been earnestly demanded.
Recently, it is proposed in the case of the optical waveguide devices such as the optical waveguide-type high speed modulators and the high speed switches that the phase matching frequency between the light propagated through the optical waveguide and the modulating wave applied from outside voltage is shifted to a higher side by tens of GHz through designing the configuration of an upper electrode on a substrate in a special shape or forming a layer of glass ("EO devices using LN" in "O plus E", May 1995, pp 91-97).
According to this literature, since the speed of the modulating wave is determined by the average value of the dielectric constant of an area through which electric forces pass between a thin signal electrode and an earth electrode, the modulating speed is increased by thickening the electrode and a buffer layer composed of SiO.sub.2. Further, since the traveling-wave type electrode constitutes a traveling passage, its characteristic impedance needs to be around 50 .OMEGA.. In order to satisfy the above requirements, it is proposed that the electrode and the buffer layer be designed in a protruded shape, a hang-over shape, a grooved shape, sealed shape or the like.
However, since the buffer layer and the electrodes having complicated configurations need be formed on the substrate in the traveling-wave light modulator the production process is complicated since a lot of producing steps are needed, and the production cost is high. In addition, the optical waveguide must be kept in alignment with the buffer layer and the electrodes having the complicated configurations at high accuracy. Furthermore, characteristics such as refractive index are likely to be changed by the formation of a work damaged layer due to damage during the production process. According to a simulation result of an optical waveguide device, the characteristics are degraded and a light absorption characteristic and an extinction ratio characteristic become insufficient.
In addition, although the above difficult problems resulting from the production process are solved, it is still difficult to realize high speed modulation of greater than 10 GHz.cm.