A communication light visualization module includes multiple communication light visualization adapters into which an optical connector is fitted and to which the optical connector is optically connected and a housing that houses the communication light visualization adapters in an arrangement manner. Each of the communication light visualization adapters includes a light extraction hole from which part of communication light transmitted through an optical communication path is extracted. The housing includes a common protection cover that collectively covers the light extraction holes of the plurality of communication light visualization adapters or collectively renders the light extraction holes uncovered.

The circuit monitors operation of an optoelectronic transceiver that includes a laser transmitter and a photodiode receiver. The circuit includes analog to digital conversion circuitry configured to convert a first analog signal corresponding to a first operating condition of said optoelectronic transceiver into a first digital value, and convert a second analog signal corresponding to a second operating condition of said optoelectronic transceiver into a second digital value corresponding to a second operating condition. The circuit also includes a memory configured to store the first digital value in a first memory location that is mapped to a predefined and unique first address and to store the second digital value in a second memory location that is mapped to a predefined and unique second address. The circuit includes an interface configured to enable a host external to the optoelectronic transceiver to access the first digital value using the first address and to access the second digital value using the second address.

In a wavelength division multiplexing transmission system, a controller of a relay node of a post-stage notifies level decrease information to a transmission node, upon detecting a decrease in a level of a wavelength in a wavelength multiplex signal received from the transmission node of a pre-stage by the optical channel monitor of the transmission node. The controller of the transmission node of the pre-stage determines that a failure occurs in the wavelength of the transponder of the transmission node, upon detecting a decrease in the level of the wavelength in the wavelength multiplex signal transmitted to the relay node by the optical channel monitor of the transmission node and upon receiving the level decrease information for the wavelength from the relay node. Therefore, a wavelength channel failure in the transponder can be determined in a preferable manner.