As an optical network system including three or more stations according to wavelength multiplexing in wavelength multiplexing optical transmission, an optical network system using a wavelength branching apparatus (or diverging device) having optical adding/dropping functions (OADM: Optical Add Drop Multiplexer) has been proposed. Due to the OADM, a signal can be arbitrarily inserted (added) or extracted (dropped) in units of wavelength of light.
In addition, unlike a fiber branching apparatus, in the wavelength branching apparatus having the OADM, in order to perform the wavelength adding/dropping, a signal band in one optical fiber is divided for communication among three stations. In this case, there is a problem in that, if a cable fault occurs in an arbitrary branch, a quality of a signal communicating between remaining two branches is deteriorated. This problem will be described in detail. In a case where a cable fault occurs, a signal which is to be originally inserted is not multiplexed. Therefore, the number of channels of main signals is decreased. Since a power of a relay is almost constant, a decrease in number of channels of main signals leads to an increase in channel power. In addition, there is a problem in that transmission characteristic according to a nonlinear effect of a transmission line is deteriorated due to the increase in channel power.
Various techniques have proposed in order to prevent the deterioration in signal quality according to the cable fault.
For example, in a technique disclosed in PTL 1, a dummy light beam for compensating for optical level is disposed in a transmission band (for example, a band L1 in the case of communication between A and B); an interval of cable disconnection is detected from each of terminal station apparatuses A, B, and C in the entire network system; the dummy light beam which is optimized in terms of signal quality is controlled.
In a technique disclosed in PTL 2, the deterioration in signal quality is prevented by switching an uplink signal and a downlink signal by using a matrix switch during cable disconnection.