| 摘要 |
PURPOSE:To improve the signals to noise ratio and to surely detect a fault point by preventing excitation light from being unnecessarily injected to an erbium doped fiber. CONSTITUTION:When the signals of a prescribed frequency for detecting the fault point whose frequency is different from the one of optical signals are superimposed on a transmission line, a corresponding level detection circuit 28U or 28D detects the signals together when there is no discontinuity in the transmission lines 11U and 11D and the corresponding level detection circuit does not detect the signals when the there is discontinuity. The output of the level detection circuits 28U and 28D is supplied to respective LD control circuits 24U and 24D after prescribed logic is obtained with logic circuits 29, 31 and 32. As a result, the operation of a laser diode 14U (14D) to the erbium doped fiber at a discontinued part is stopped and the generation of ASE noise is prevented. |
