PURPOSE:To couple a photodetecting and light emitting element with an optical fiber terminal efficiently by specifying the length of a distributed index lens which has a specific refractive index distribution and connecting an end part of an optical transmission fiber to an end surface of the lens. CONSTITUTION:The distributed index lens is a columnar body made of transparent glass while having both end surfaces which are flat, parallel to each other, and at right angles to the optical axis, and its refractive index distribution is as shown by an equation I , where N(r) is a refractive index at radial distance (r) from the center axis 4, NO is a refractive index on the center axis 4, and A is a positive constant. When the lens length Z is selected as shown by an equation II, a light beam 6 from a spot light source 5 placed in front of one lens end surface at distance l is focused on the other end 2A of the lens on the condition shown by an equation III. In this case, alpha is set between 0.05-0.5P to eliminate the need to bring the photodetecting and light emitting element 5 in tight contact with the lens end surface, and the photodetecting and light emitting element 5 and optical fiber terminal 1 are coupled together optically. Thus, they are coupled efficiently.
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