1. Field of the Invention
The present invention relates in general to an optical waveguide device and in particular to an optical device that requires the use of an optical waveguide modulator and a polarizing filter for separating a polarization light.
2. Description of Related Art
In the case of an optical waveguide device using a substrate having an electro-optical effect, a portion having a high index of refraction is created in the substrate. In addition, electrodes between which a voltage is applied are created above the wave guide or in close proximity to it. A voltage applied between the electrodes generates an electric field in the substrate, changing the index of refraction of a waveguide. Variations in index of refraction occurring in the waveguide in turn modulate the phase and intensity of a light or switch the path of the light.
First of all, the following description briefly explains the structure and operation of a light-intensity modulating device that utilizes an electro-optical effect closely related to the present invention as an example of an optical waveguide device. In addition, a technology for separating a polarization light is also described briefly as well.
In general, a niobium-acid lithium substrate having a relatively high electro-optical effect among strong dielectrical materials is used as the substrate of the optical waveguide device. In the optical waveguide device using a niobium-acid lithium substrate, a titanium film created on the substrate undergoes a patterning process to form a waveguide pattern. Later on, a wave guide is formed by thermal diffusion for several hours at a high temperature of about 1,000 degrees Celsius. A silicon dioxide buffer film layer which is referred to hereafter as an SiO.sub.2 film is created on the wave guide. Electrodes each made of a metallic film are further created on the SiO.sub.2 film. In some cases, a silicon film which is referred to hereafter as an Si film is created between the SiO2 film and the metallic film.
In the optical waveguide device, it is possible to integrate a function for modulating a light and a function for switching the path of a light on the substrate. In addition, since these functions are high-speed functions, development of the optical waveguide device as an external modulator used in mass optical communication and an optical-path switch employed in an OTDR (Optical Time Domain Reflectometer) is under way.
Depending upon the application of the optical waveguide device, a polarization-light independent type and a low driving voltage type are demanded. It is important that the polarization-light independent type be used in an optical-path switch of an OTDR. On the other hand, it is important that the low driving voltage type can be used for a high-speed optical modulator.
Normally, in an optical waveguide device of the latter type, either a TE mode (that is, a mode in which the electric field component of a waveguide light is parallel to the substrate) or a TM mode (that is, a mode in which the magnetic field component is parallel to the substrate) is selected in order to reduce the driving voltage. When a niobium-acid lithium Z-cut substrate is used, a waveguide light of the TM mode with a driving voltage of about one-third of that of the TE mode is used.
When only one polarization wave is to be applied to the optical waveguide device, or when a polarization wave is to be split into components which are then each applied to the optical waveguide device, it is necessary to increase the polarization-wave quenching ratio (that is, a ratio of a TE component to a TM component) of the light source. In addition, it is also necessary to provide a polarization maintain fiber, which is referred to hereafter as a PMF, for optically connecting the light source to the end surface of a waveguide and, at the same time, to also sustain the main axis of the PMF in a stable state in directions perpendicular and parallel to the waveguide.
In the case of such a configuration, it is theoretically possible to apply only a predetermined polarization component to the waveguide. Because of angle adjustment of the main axis, fiber related factors, or factors external to the fiber such as variations in temperature and a stress applied to the fiber, however, it is quite within the bounds of possibility that the polarization-wave quenching ratio deteriorates. The deterioration of the polarization-wave quenching ratio in turn gives rise to distortion of the modulated waveform, degrading the transmission quality.
In addition, the three conventional polarization-light separating technologies described previously require a special structure of the waveguide and special processes for manufacturing the waveguide in order to obtain the necessary functions. As a result, the conventional optical waveguide device has problems that elements constituting the waveguide increase in size while the structure of the waveguide becomes complex and, on the top of that, the number of manufacturing processes increases and it becomes difficult to maintain a high manufacturing yield.