Abstract: An add/drop node of an optical WDN-network which has two fiber paths for light of a plurality of channels propagating in opposite directions comprises two add/drop modules (231, 23r) for each of the channels. All the modules are identically constructed. Each module comprises an add device (251, 25r) for adding light to one of the paths and a drop device (27r, 271) for deflecting a portion of light from a second one of the paths. A module comprises a house (41) enclosing the add device and the drop device. A first fixed connector (53, 73) is attached to the house for connection in the first path and to a an optical fiber (45, 65) which extends freely from the house and has a first free connector (43, 63) at its free end to be attached to the fixed connector of a neighboring add/drop module for continuing the first path through the considered add/drop module to the neighboring module.

Abstract: Wavelength drift in an optical WDM system between wavelengths launched by lasers (110) and received at a demultiplexer (50) can lead to disturbance of traffic signals if not identified and rectified rapidly. It is proposed to identify wavelength drift by determining a difference in temperature of the demultiplexer (50) for each channel between an actual temperature and a temperature for optimal transmission of the channel. The mean of these temperature differences for all channels is indicative of wavelength drift, while each difference is utilised to determine whether an individual channel is drifting in wavelength and to correct the drift. Wavelength drift can be detected and corrected whether sourced at a laser (110) or due to a temperature variation at the demultiplexer (50). Measurement and control can be performed during normal operation without impeding or degrading traffic flow. Wavelength influencing parameters other than temperature can be utilized.

Abstract: An optical communication line having optical fiber amplifiers including a pump laser can be configured to suppress the transmission of optical signals by reducing the power of the pump laser. In order that the portion of the line downstream of the amplifier can be monitored for fiber breaks causing only negligible crosstalk with a parallel working line, and with the elimination of the risk of mistaking reflections from a working line for light emitted by the standby amplifier, the light emitted by the standby amplifier is modulated by a low frequency control tone. The modulated laser light causes modulation of the amplified spontaneous emission (ASE) generated in the amplifier to create a control tone. Monitoring means disposed at the end of the optical line detect this control tone and use this as the criteria for a functional connection downstream the amplifier.