PURPOSE:To correctly measure the temperature distribution along an optical fiber by holding the difference of wavelengths of two input lights lambda1, lambda2 at all times at the Raman shift wavelength, thereby eliminating the dependency on the wavelength of the transmission loss of the optical fiber under the high temperatures or radioactive environment, etc. CONSTITUTION:An exciting light of the wavelength lambda0 from a light source 1 is input to an optical fiber 2 thereby to generate an induced Raman scattering light of the wavelength lambda1. The light of the wavelength lambda1 enters an optical multiplexer/demultiplexer 30 along with the light of the wavelength lambda0. The demultiplexer 30 separaes the light of the wavelength lambda0 from the light of the wavelength lambda1 and sends them to an optical switch 4. The lights are alternately put into a sensor optical fiber 5. When the light of the wavelength lambda0 is introduced, the back scattering light including the Stokes light of the wavelength lambda1 is brought into an optical multiplexer/demultiplexer 31. The demultiplexer 31 sends the light of the wavelength lambda1 generated in the optical fiber 5 to an operating/processing system. If the light of the wavelength lambda1 is input, the back scattering light including the anti-Stokes light of the wavelength lambda0 because of the exciting light of the wavelength lambda1 is sent to the operating/ processing system. Accordingly, the temperature distribution along the optical fiber 5 is obtained according to the OTDR method.
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