Patent ID: 6937820

Claim:
An optical level control method of handling up-direction light including up-direction main signal light and up-direction sub-signal light different in wavelength from the up-direction main signal light and down-direction light including down-direction main signal light and down-direction sub-signal light different in wavelength from the down-direction main signal light in first and second stations existing on both sides of a trouble occurrence zone in an optical transmission line, said method comprising: a second-station up-direction light reception step in which said second station receives an output of said up-direction sub-signal light; a feedback step in which said second station adjusts said down-direction sub-signal light to output the adjusted down-direction sub-signal light; a first-station restoration step in which said first station detects restoration on the basis of a control signal included in said down-direction sub-signal light, and an output power of said down-direction sub-signal light adjusted in said feedback step to start an output of said up-direction main signal light, by a logic computation in accordance with both the down-direction sub-signal light and the control signal; a second-station restoration determination step in which said second station determines on restoration of said up-direction main signal light on the basis of said up-direction sub-signal light and said up-direction main signal light obtained in said first-station restoration step; and a second-station restoration step in which said second station brings said down-direction main signal light back to a predetermined level when the decision in said second-station restoration determination step shows the restoration and outputs said down-direction main signal light brought back to said predetermined level, and continues a stop of an output of said down-direction main signal light when the decision in said second-station restoration determination step shows no restoration.