1. Field of the Invention
The present invention relates to an optical multiplex communication system and an optical transmission apparatus having a transmission part and a receiving part, and a control method of optical multiplex communication by which multiplexed optical signals are controlled to be transmitted stably.
2. Description of the Related Art
A wavelength division multiplexing (WDM) transmission method is known as a transmission means for transmitting a large amount of information at a high speed. In the WDM transmission method, the optical signals having different wavelengths are respectively assigned to a plurality of channels, so that the optical signals are multiplexed and transmitted to one optical transmission path. In this case, at a transmission part, the optical signals having different wavelengths are multiplexed, amplified by an optical amplifier such as an Er doped optical fiber, and then transmitted to the optical transmission path. A relay amplifier or an end amplifier consists of an optical amplifier similar to the above mentioned amplifier so that the optical signal is amplified. The optical signals that are multiplexed are de-multiplexed for respective channels so that the receiving process is implemented at the receiving part.
The optical amplifier amplifying the optical signal that is multiplexed generally has an ALC structure in which a feedback control is implemented so that an amplified output optical level has a designated value. In this case, it is necessary to demultiplex the optical signal level for respective channels having designated levels. Hence, the optical amplifier is controlled for the number of channel. In a case where the optical signal that is multiplexed is amplified and transmitted, an optical signal level of other channel changes based on an increase/decrease of the number of channels or a change of the optical signal level of one channel.
With respect to a received optical signal of the receiving part, which is influenced by the transmission property of the optical signal that is multiplexed, including the optical amplifier and the optical transmission path, a means for adjusting the optical signal power of respective channels at a receiving side so that the respective optical signal power of the respective channels become the same is known. See the Japanese Laid-Open Patent Application No. 6-69891.
In addition, a multiplex optical communication system, in which an optical amplifier amplifying the optical signal that is multiplexed is controlled based on a change of the number of channels so that the respective optical signal power of the respective channels at the receiving part becomes the same and the control process is made smooth, is known. See the Japanese Laid-Open,Patent Application No. 8-195733.
Furthermore, a system, in which variations in the number of channels are detected and the time needed to control an optical amplifier and the time needed to input a multiplexed optical signals having the channel number varied to the optical amplifier are adjusted so as to reduce variations of optical signal power of respective channels at a receiving part, is known. See the Japanese Laid-Open Patent Application No. 2001-53682.
At the receiving part of the optical multiplex communication system in which the optical signals having different wavelengths are respectively assigned to a plurality of channels and multiplexed so as to be amplified and transmitted, it is necessary that the optical signal levels of respective channels have more than designated values in order to reduce an error of the receiving process. Hence, transmission quality information such as the optical signal level for respective channels at the receiving part or an error ratio is provided to the receiving part. Furthermore, a means for controlling the optical signal levels that are multiplexed for respective channels based on the transmission quality information from the receiving part can be applied to the receiving part.
However, in a case where the transmission quality information for respective channels is provided from the receiving part to the transmission part, there is a problem. That is, a control of the optical signal levels for respective channels at the receiving part based on the above mentioned transmission quality information may be delayed at a path for the above mentioned control. Because of this, the optical signal levels that are multiplexed may change greatly so that the optical signal levels of other channels are influenced and the whole of the transmission quality may decline. Furthermore, in a case where the optical signal levels for respective channels becomes large wholly and the input power of the optitcal amplifier of the transmission part increases, if the optical signal levels for respective channels are reduced evenly, the transmission qualities of all of the channels are reduced. Because of this, level controls against the optical signal of the respective signals based on the transmission quality occur frequently so that an automatic level control at the optical amplifier becomes unstable and time for convergence becomes long. As a result of this, the change of the qualities of respective channels may occur.