| 摘要 |
PROBLEM TO BE SOLVED: To reliably detect an optical path length difference even when there is a difference in optical power between a test signal light that transmits a main line and a test signal light that transmits a bypass.SOLUTION: Different wavelengths are used for a test signal light L21 that transmits a main line 102 and a test signal light L22 that transmits a bypass 103. By adjusting an optical power difference before multiplexing two test signal lights L21 and L22 in an upper optical branch coupler 202, reliable detection of an optical path length difference is made possible even when there is an optical power difference between the test signal light L21 that transmits the main line 102 and the test signal light L22 that transmits the bypass 103. |
