| 摘要 |
PURPOSE:To obtain stable operation against disturbance in a light source or the like by setting the length L of a single mode optical fiber to satisfy an inequality L>Co/2n.DELTAnu wherein Co is light velocity, (n) the refractive index of the single mode optical fiber and DELTAnu the spectral line width of the light source. CONSTITUTION:Light from a light source in a controller 4 is led via a single mode optical fiber 1 to a sensor head 2, wherein physical quantity is optically detected. Resultant signal light is led again via the optical fiber 1 to a signal detector in the controller 4 and electrically converted to be detected. Since, when the length of the optical fiber bundle 1 is L, light velocity Co, the refractive index of the optical fiber 1 (n) and the spectral line width of the light source DELTAnu and relationship L>Co/2n.DELTAnu is set, the mode interval of a Fabry- Perot resonator is narrowed, change in a transmittance is averaged. Accordingly, noise can be prevented from being strengthened by the Fabry-Perot resonator and operation is stabilized against disturbance in the light source or the single mode optical fiber.
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