| 摘要 |
PROBLEM TO BE SOLVED: To extend the service life of an optical filter by suppressing deterioration in the mechanism of the optical filter by conducting intermittently the input of a control signal to the optical filter. SOLUTION: Input of an AC control signal 44 to an optical filter 110 is stopped when a center wavelength of the optical filter 110 is discriminated coincident with a wavelength of a transmission light of the optical filter 110. While the input is stopped, a level of the transmitted light of the optical filter 110 is periodically monitored and when the level is discriminated less than a prescribed level, the input of the AC control signal 344 to the optical filter 110 is restarted. |
