| 摘要 |
PURPOSE:To hold the S/N ratio constant and high by displacing the center wavelength of a variable wavelength type optical filter when signal light varies in wavelength. CONSTITUTION:The variable wavelength type optical filter 21 is, for example, a Fabri-Perot interferometer type and provided instead of a fixed wavelength type optical filter. Then a control circuit 22 controls the center wavelength f0 of the optical filter 21 to the wavelength of signal light 8 following up variation in the wavelength of the signal light 8 according to signal light (1st signal light) before it is transmitted through the optical fiber 21 and signal light 23 C 2nd signal light) after the transmission. This control circuit 22 consists of a displacement direction determining circuit 24 and a displacement quantity determining circuit 25. The displacement direction determining circuit 24 determines the direction where the center wavelength f0 of the optical filter 21 is displaced according to the signal light 8 and signal light 23, and the displacement quantity determining circuit 25 determines a quantity by which the center wavelength f0 of the optical filter 21 is displaced according to the signal light 23 and a reference voltage signal Vref. |
