A protected optical single-fiber WDM system, characterized in (a) that a first and a second head-end terminal and at least one optical add/drop filter device are connected to form a chain-like transmission path, each of the first and second head-end terminals (3, 5) being connected, at a WDM port (3a, 5a), through a single optical fiber to a western or an eastern WDM port of an adjacent one of the optical add/drop filter devices, respectively, and each of the optical add/drop filter devices being connected, at an eastern or western WDM port (11d, 11a), to a western or eastern WDM port (11a, 11d) of an adjacent one of the optical add/drop filter devices, respectively, (b) that at least one tail-end terminal is connected to each optical add/drop filter device, wherein a first and a second channel port of the respective add/drop tail-end terminal is connected to a dedicated first (11 b) and second (11 c) channel port of the optical add/drop filter device through a respective optical fiber (15,17), and (c) that the head-end terminals (3, 5), the optical add/drop filter devices and the tail-end terminals are adapted to selectively or simultaneously establish a bidirectional communication between each tail-end terminal and the first and second head-end terminal (3, 5), wherein each tail-end terminal bidirectionally communicates through its first channel port with the first head-end terminal and bidirectionally communicates through its second channel port with the second head-end terminal, one of these communication paths serving as a working communication path and the other of these communication paths serving as a protection path, (d) wherein for the communication between each of the tail-end terminals and the first and second head-end terminal (3, 5), respectively, dedicated optical channel signals lying in a first wavelength band and a second wavelength band are used for each transmission direction, the first and second wavelength band revealing no overlap and the optical channel signals in each wavelength band forming, in each transmission direction in the chain-like transmission path, respective optical WDM signals. (a) that a first and a second head-end terminal and at least one optical add/drop filter device are connected to form a chain-like transmission path, each of the first and second head-end terminals (3, 5) being connected, at a WDM port (3a, 5a), through a single optical fiber to a western or an eastern WDM port of an adjacent one of the optical add/drop filter devices, respectively, and each of the optical add/drop filter devices being connected, at an eastern or western WDM port (11d, 11a), to a western or eastern WDM port (11a, 11d) of an adjacent one of the optical add/drop filter devices, respectively, (b) that at least one tail-end terminal is connected to each optical add/drop filter device, wherein a first and a second channel port of the respective add/drop tail-end terminal is connected to a dedicated first (11 b) and second (11 c) channel port of the optical add/drop filter device through a respective optical fiber (15,17), and (c) that the head-end terminals (3, 5), the optical add/drop filter devices and the tail-end terminals are adapted to selectively or simultaneously establish a bidirectional communication between each tail-end terminal and the first and second head-end terminal (3, 5), wherein each tail-end terminal bidirectionally communicates through its first channel port with the first head-end terminal and bidirectionally communicates through its second channel port with the second head-end terminal, one of these communication paths serving as a working communication path and the other of these communication paths serving as a protection path, (d) wherein for the communication between each of the tail-end terminals and the first and second head-end terminal (3, 5), respectively, dedicated optical channel signals lying in a first wavelength band and a second wavelength band are used for each transmission direction, the first and second wavelength band revealing no overlap and the optical channel signals in each wavelength band forming, in each transmission direction in the chain-like transmission path, respective optical WDM signals.