| 摘要 |
PURPOSE:To change over an optical lie with a slight added cost by compensating the phase difference caused by an optical path length difference between a switching object section line section and a switching optical line and switching the switching object section line section into the switching optical line. CONSTITUTION:When an optical filter 27 transmits a light having a wavelength for phase difference measuring, the difference DELTAtau between a delay time at a line section 21 and a delay time in a switching optical line 29 is measured by monitoring the pulse of the wavelength. Based on the result of measurement, a variable delay device 28 is adjusted to compensate the phase difference. Then a delay time difference measuring system such as a photoelectric converter 30 is removed from a beam splitter 22 and the attenuation of a variable light attenuator 26 in the switching optical line 29 is increased sufficiently to switch the wavelength characteristic of the optical filter 27 into the state transmitting the active wavelength, the line section 21 is removed to use the switching optical line 29 only. |
