1. Field of the Invention
The present invention relates to a flexible polymeric optical waveguide-forming master plate, a method for producing a polymeric optical waveguide, and an aperture changeable polymeric optical waveguide (a polymeric optical waveguide with a changeable aperture).
2. Description of the Related Art
In producing a polymeric optical waveguide, the following methods have been proposed: (1) a method in which a film is impregnated with a monomer, and a core portion is selectively exposed to light so as to change the refractive index in the core portion, and the film is then laminated on a substrate (selective polymerization method), (2) a method in which a core layer and a clad layer are applied to a substrate, and then a clad portion is formed by using reactive ion etching (RIE method), (3) a method using a photolithographic method in which an ultraviolet ray-curable resin obtained by adding a photosensitive material to a polymer material is used, exposed to UV light and developed (direct exposure method), (4) a method using an injection molding and, (5) a method in which a core layer and a clad layer are applied to a substrate and then a core portion is exposed to light so as to change the refractive index of the core portion (photo-bleaching method). However, the selective polymerization method (1) has a problem in lamination of the films. Methods (2) and (3) are expensive since a photolithographic method is used. Method (4) has a problem in accuracy of a core diameter. Method (5) has a problem in that a sufficient refractive index difference cannot be obtained. At present, only methods (2) and (3) are practical methods for providing waveguides with high performance. However, none of these methods are suitable for the formation of a polymeric optical waveguide on a flexible substrate having a large area.
David Hart of Sharp Corp. has proposed a method for producing a polymeric optical waveguide in which a pattern substrate with a groove pattern which is to be a capillary is brought into close contact with a plane substrate by using a clamping jig, and the capillary is filled with a monomer solution under a reduced pressure (see Japanese Patent No. 3151364). However, this method has a drawback in that, unless the clamp is used to bring the pattern substrate into close contact with the plane substrate, the monomer solution also enters portions other than the core and therefore a precise waveguide structure cannot be formed. This method has another drawback in that the volume of the monomer solution changes when undergoing polymerization to form a macromolecule (solidification), leading to change in a core shape. Moreover, still another drawback is that the core shape collapses at the time of removal of the capillary because a polymer obtained by the polymerization of the monomer solution partially adheres to the capillary.
Recently, George M. Whitesides et al. of Harvard University has proposed a method called “capillary micro-mold” as one of soft liqhographic methods in new technologies for making a nano-structure. In this method, a master substrate is produced by using photolithography, the nano-structure of the master substrate is exactly copied on a mold of a polydimethylsiloxane (PDMS) by utilizing the adhesiveness and easy separability of the PDMS, and a liquid polymer is infused into the mold by utilizing capillarity and solidified. The detail thereof is described in SCIENTIFIC AMERICAN September 2001 (Nikkei Science, December 2001 issue). Moreover, a patent about the capillary micro-mold method was granted to Kim Enoch et al., from the group of George M. Whitesides, of Harvard University (see U.S. Pat. No. 6,355,198). However, in the case of a concave portion to be filled having a small sectional area such as the core of an optical waveguide, the production process described in this patent is unsuitable for mass-production since a long period of time is required to fill the concave portion (to form a core). This process also has a drawback in that the volume of a monomer solution changes when the monomer solution is reacted and solidified into a polymer, causing change in a core shape.
Meanwhile, if the aperture of an optical waveguide can be changed in accordance with the size of the various optical fibers, light-emitting elements or light-receiving elements, coupling loss can be reduced. However, in conventional methods, a typical example of which is the direct exposure method using photolithography, it is difficult to change the thickness of the resultant film continuously. Accordingly, the aperture of the core cannot be controlled at will to reduce coupling loss. Moreover, a method of finely processing a silicon substrate by FIB or the like is known. However, the method has a problem in that a huge number of steps are required to produce an optical waveguide having a large aperture and a large area, such as a multimode optical waveguide, and such steps are substantially impossible to conduct.
As an attempt for overcoming the above-mentioned problem, a method for pouring a UV curable resin into a mold to form a an aperture changeable polymeric optical waveguide has been proposed (see Japanese Patent Application Laid-Open (JP-A) No. 10-253845). In this method, a polymer liquid for forming a core is put into a very shallow tank whose depths continuously change from one end thereof to the other end thereof, and a polymeric waveguide with apertures which change in the longitudinal direction of the waveguide is formed by making use of the depth difference of the tank. It is therefore necessary to dispose the tank just parallel to a gravitational direction. Moreover, this process is susceptible to vibration. Thus, the process is not practical as a process for mass production. Furthermore, this publication never describes a specific process for making a groove having a satisfactory surface roughness and shape precision which an optical waveguide is required to have and having thicknesses and widths which change in the longitudinal direction thereof.
Accordingly, there is a demand for a process for inexpensively and simply producing a polymeric optical waveguide-forming master plate.
There is also a demand for a process for inexpensively and simply producing a polymeric optical waveguide.
There is also a need for an aperture changeable polymeric optical waveguide having apertures which change at the both ends thereof in accordance with the sizes of various optical elements.