| 摘要 |
PROBLEM TO BE SOLVED: To prevent a noise source such as illuminating light in an optical transmission space from impinging on a photodetector and to arrange the photodetector in a space where a light path is hardly blocked. SOLUTION: A photodetector is arranged in a pipe, and the inner surface of the pipe is coated with an antireflection film to serve as an antireflection means, so that light that impinges on the opening of the pipe at an angle of incidence larger than the angle at which the opening is watched from the photodetector never reaches the light receiving plane of the photodetector. A reflecting mirror is arranged at the midway point of a light path so as to set the light path in a space where the light path is never blocked. |
