Patent ID: 7619813

Claim:
A wavelength division multiplexing (WDM) device, comprising: a level adjusting unit receiving an optical signal, adjusting an optical power level of the received optical signal, and outputting the level-adjusted optical signal; and a controlling unit analyzing the optical power level of the optical signal output from the level adjusting unit, controlling the level adjusting unit so that the optical power level of the output optical signal stays at a constant level, determining whether the WDM device is the furthest upstream WDM device, transitioning to an “off-state” when the optical power level of the output optical signal becomes less than a predetermined disconnection-detection threshold or when a voltage rate of the output optical signal drops below a predetermined threshold rate, so that the level adjusting unit gives a predetermined attenuation to the input optical signal, and transitioning from the “off-state” when the optical power level of the output optical signal is greater than a predetermined recovery threshold, if the WDM device is the furthest upstream WDM device, or when the optical power level of the output optical signal is greater than the predetermined recovery threshold and an upstream wavelength division multiplexing device is not in the “off-state”, if the WDM device is not the furthest upstream WDM device.