Recently, the technique for reducing mounting costs is studied and developed by use of a coupling system called a direct optical coupling (butt joint) system. In the coupling system, an optical fiber and optical semiconductor element are arranged in close positions to face each other and optically coupled without using a lens. When the direct optical coupling system is used, it is important to arrange an optical semiconductor element and optical fiber in close positions and prevent light from reaching an unwanted portion. Therefore, there is proposed a method for directly forming an electrical interconnection on the main surface of an optical transmission line holding member (a so-called optical fiber ferrule) that holds the optical fiber and the like and mounting an optical semiconductor element on the main surface to set the light receiving/transmitting region thereof in opposition to the optical fiber (Jpn. Pat. Appln. KOKAI Publication No. 2001-159724).
In this type of method, it is desired to fill resin in between the optical semiconductor element and the optical transmission line so as to suppress reflection light caused by discontinuity of the refractive index. Specifically, resin having a role as a refractive index matching material and an under-filling function for protection of the electrical connecting portion of the optical element is filled in between the optical semiconductor element and the end face of the optical transmission line. Further, resin used to mechanically fix the optical transmission line on a ferrule is filled.
As a method for forming the above resins, a method for performing an under-filling process before fixing the optical transmission line and a method for fixing the optical transmission line and then performing an under-filling process are considered. When the optical transmission line is first fixed, there occurs a possibility that fixing resin flows into holding holes for the optical transmission line to reach the end face on the optical element side. Further, since the fixing resin fills in the holding holes, the holes via which air escapes when under-fill resin is injected are closed or blocked. This means that the holes via which a solvent produced at the curing time of the under-fill resin is extracted are closed. Therefore, there occurs a possibility that bubbles remain in the resin at the curing time of the under-fill resin and this may prevent the optical coupling. When the under-filling process is first performed, there occurs a problem that high mechanical precision cannot be attained.