Optical network and optical add/drop apparatus

An optical network comprising a plurality of optical add drop apparatuses connected to a transmission path, the optical add drop apparatus comprising:

• a plurality of fixed wavelength drop filters connected in series to a transmission path and each demultiplexing an optical signal having a fixed wavelength from wavelength division multiplexed light flowing along the transmission path;
• a plurality of fixed wavelength add filters connected in series to the transmission path and to the fixed wavelength drop filters, and each adding a optical signal having a fixed wavelength to be transmitted to a transmission source of each of the demultiplexed optical signals to the wavelength division multiplexed light flowing along the transmission path;
• a first optical branching unit connected in series to the transmission path and to the fixed wavelength drop filter and the fixed wavelength add filter, and branching part of the wavelength division multiplexed light flowing along the transmission path;
• and
• a variable wavelength drop filter demultiplexing the optical signal having the wavelength corresponding to a specified value from the wavelength division multiplexed light branched by the first optical branching unit.