Optical signal transmission system, particularly optical cable television system, with monitoring and service-channel facilities

System for optically transmitting a useful signal (S.sub.1) from a transmitter (1) over a transmission link including an optical waveguide (3) or a fiber-optic network to a receiver (2), characterized in that

• the transmitter (1) transmits the useful signal (S.sub.1) and an auxiliary signal (S.sub.2) over the transmission link toward the receiver (2) at different wavelengths;
• means (4a, 4b, 5, 6, 7) are provided at a desired point in the transmission link, for receiving the auxiliary signal (S.sub.2) and for providing a response signal (S.sub.3) containing information for monitoring the useful signal (S.sub.1) over the same optical waveguide (3) or the same fiber-optic network to the transmitter (1);
• the transmitter (1) receives the response signal (S.sub.3) and controls the further transmission of the useful signal (S.sub.1) and the auxiliary signal (S.sub.2) in accordance with the information in the response signal (S.sub.3);
• a fiber-optic amplifier (9) is coupled before the desired point in the transmission link and passes the useful signal (S.sub.1), the fiber-optic amplifier (9) having a pump source (7) for receiving the auxiliary signal (S.sub.2) and transmitting the response signal (S.sub.3);
• the pump source and monitoring means (4a, 4b, 5, 6, 7) includes a laser (10) and a photodiode (5) for receiving both the backface emission of the laser (10) and the auxiliary signal (S.sub.2);
• and
• the wavelength at which the auxiliary signal (S.sub.2) is transmitted and the laser (10) of the pump source (7) are assigned so that the auxiliary signal (S.sub.2) can be passed through the laser (10) without being changed and without interfering with the operation of the laser (10).

System for optically transmitting a useful signal (S.sub.1) from a transmitter (1) over a transmission link including an optical waveguide (3) or a fiber-optic network to a receiver (2), characterized in that

• the transmitter (1) transmits the useful signal (S.sub.1) and an auxiliary signal (S.sub.2) over the same optical waveguide (3) or the same fiber-optic network toward the receiver (2) at different wavelengths;
• means (4a, 4b, 5, 6, 7) are provided at a desired point in the transmission link, for receiving the auxiliary signal (S.sub.2) and for providing a response signal (S.sub.3) containing information for monitoring the useful signal (S.sub.1) over the same optical waveguide (3) or the same fiber-optic network to the transmitter (1);
• the transmitter (1) receives the response signal (S.sub.3) and controls the further transmission of the useful signal (S.sub.1) and the auxiliary signal (S.sub.2) in accordance with the information in the response signal (S.sub.3);
• and
• the means (4a, 4b, 5, 6, 7) for transmitting the response signal (S.sub.3) include a light source (7) with a laser (10) and a photodiode (5) for receiving both the backface emission of the laser (10) and the auxiliary signal (S.sub.2).