| 摘要 |
PURPOSE:To make obtaining and axial alignment of those of required filter characteristics easy and reduce insertion losses by opposing two focusing type rod lenses of different numerical apertures in performing mode selection. CONSTITUTION:If the numerical aperture of a rod lens 7 is made smller than the numerical aperture of a rod lens 8 and light is entered from an optical fiber 5, then lower roder mode is selectively excited in an optical fiber 6 at low loss. Conversely, if light is entered from the optical fiber 6, higher order mode becomes leakage mode and creates a function as a mode filter, coupling only the lower order mode to the fiber 5. Since the rod lenses 7, 8 are made at 1/4 pitch, the image on the butt end face of one rod lens and fiber forms a real image on the butt ene face of the other rod lens and fiber. This reduces the insertion losses of transmission mode. In addition, the use of the focusing type rod lenses enabled the one of required filter characteristics to be obtained with ease and eables axial alignment to be made easy. |
