1. Field of the Invention
The present invention relates to a method of producing an optical waveguide to be widely used for optical communications, optical information processing, a position sensor and other general optics.
2. Description of the Related Art
An optical waveguide is typically produced by forming a core (light passage) in a predetermined pattern on a surface of an under-cladding layer, and forming an over-cladding layer to cover the core. Particularly, where the over-cladding layer is formed as having a desired shape (e.g., as having a lens portion at its end), the formation of the over-cladding layer is achieved with the use of a mold which includes a cavity having a mold surface conformable to the desired shape (see, for example, JP-A-2008-281654).
Where a photosensitive resin is used as a material for the over-cladding layer, the mold should transmit illumination radiation such as ultraviolet radiation and, therefore, a light-transmissive quartz mold is used as the mold. The cavity is provided in a lower surface of the quartz mold as having the mold surface conformable to the shape of the over-cladding layer. After the core is formed on the surface of the under-cladding layer, the lower surface of the mold is brought into intimate contact with the surface of the under-cladding layer with the core being properly positioned in the cavity of the mold. Subsequently, the photosensitive resin as the material for the over-cladding layer is injected into a molding space defined by the mold surface of the cavity, the surface of the under-cladding layer and a surface of the core, and then is exposed to the illumination radiation (e.g., the ultraviolet radiation) through the mold, whereby the photosensitive resin is cured. Then, the resulting product is demolded. Thus, the optical waveguide is produced, which includes the under-cladding layer, the core and the over-cladding layer.
When the quartz mold is produced as having the desired shape, a plurality of mold pieces for the mold are prepared, and then are bonded together with a bonding resin. This is based on the following ground. If cutting of a quartz block material is started or stopped at a middle portion of the block material, the block material is liable to crack. Therefore, the quartz block material should be cut from one side to the other side. For this reason, the quartz mold pieces are each produced by cutting the quartz block material from one side to the other side, and then are bonded together with the bonding resin, whereby the quartz mold is produced as having the desired shape.
However, the mold produced by bonding the mold pieces together has lower dimensional accuracy because of bonding offset. If the over-cladding layer is formed by using the mold having lower dimensional accuracy, it is impossible to position the core and the over-cladding layer in proper positional relation. As a result, light outputted from a distal end of the core, for example, cannot be properly converged by the lens portion provided at the end of the over-cladding layer, so that the light is outputted from the lens portion in a diverged state. Therefore, the intensity of the light received from the lens portion is reduced, resulting in poorer optical transmission characteristics.