1. Field of the Invention
The present invention relates to an optical transmission apparatus from and to which a signal beam is added and dropped, whereby the number of signal wavelengths is arbitrarily changed.
2. Description of the Related Art
With advancements of multimedia networks, demands for communication traffics have shown a big leap, and a WDM (Wavelength Division Multiplexing) transmission system using optical amplifiers that multi-relay-amplify optical signals, is performing a significant role in terms of economizing the communication systems in a multimedia society.
Over the recent years, the WDM system has positively been introduced into an intra-urban metro core network that puts an emphasis on cost and size. Along with this, introduction of an optical add drop multiplexer (OADM) into each of stations configuring the WDM system is advancing.
FIG. 8 shows a conventional configuration of the OADM apparatus (node) serving as an optical transmission apparatus. A configuration of an optical circuit is such that an SV (Supervisory) filter 1 demultiplexes (splits) only an SV optical beam from a WDM signal beam and the SV optical beam (Supervisory signal beam) transmitted from a transmission path, and an O/E (optical/electrical) converter 2 converts the optical beam into an electric signal.
The WDM signal beam is demultiplexed into wavelengths corresponding to channels (ch) by use of an AWG (Arrayed Waveguide Grating) 3. Further, with respect to each of the channels, an optical switch 4 adds and drops the signal beam. The added or transmitted-through signal beams are again multiplexed by an AWG 5.
The multiplexed WDM signal beam is, after being amplified by an EDFA (Erbium-Doped Fiber-Optical Amplifier) 9 including a demultiplexer (optical branching filter) 7 for monitoring input power to an amplifier 6 and also a photo detector (e.g., photo diode) 8, multiplexed by a multiplexer (optical branching filter) 11 with the SV optical beam generated by an E/O (electrical/optical) converter 10, and is thereafter transmitted to a transmission path.
As for a control circuit, a node number information receiving circuit 12 obtains information (drop node number), indicating a node that dropped a signal beam in a channel per channel, the drop node numbers are obtained from a supervisory (SV) signal acquired through the SV optical beam O/E conversion conducted by the O/E converter 2. The drop node number is set in the SV signal in a node disposed on upper side when dropping a signal beam. The node number information receiving circuit 12 calculates, based on the drop node number, how many nodes an ASE (Amplified Spontaneous Emission) beam reaches a node itself via (the number of nodes via which the ASE beam travels: ASE transmission node count) on the basis of the drop node number with respect to every channel.
An ASE power calculation circuit 15 calculates ASE power of each channel from the ASE information described above and from a relational expression (a block 14 in FIG. 8) between the ASE transmission node count and the ASE power, which is stored beforehand as information in the node, and further calculates a sum of values of the ASE power of the respective channels. This item of ASE power information is sent to the EDFA 9. Moreover, a wavelength count information receiving circuit 13 calculates the number of wavelengths (wavelength count) including the signal from the supervisory (SV) signal obtained by the O/E converter 2, and the wavelength count information is sent to the EDFA 9. These items of information are inputted to an ALC (Automatic Level Control) target value determination circuit 16 provided in the EDFA 9.
The ALC target value determination circuit 16 calculates and determines, based on ASE power information, the wavelength count information and input total power (S (signal)+ASE) information obtained by a photo detector 8 for input power of the EDFA 9, such an ALC (Automatic Level Control: output fixing control) target value of output total power (S+ASE) that a signal output of each channel from the amplifier 6 becomes fixed or a constant.
The amplifier 6 includes an ALC control circuit, and the ALC control circuit performs the automatic level control (ALC) of controlling a gain of the amplifier 6 so that the power (output total power) of the signal beam outputted from the amplifier 6 becomes the ALC target value.
Note that the drop node number obtained by the node number information receiving circuit 12 and the information (the node number of the self-node, which is defined as the drop node number) showing that the signal is dropped at a SW 4, are sent to a node number information transmitting circuit 17 and are transmitted as node number information to the E/O converter 10. Further, the wavelength count information acquired by the wavelength count information receiving circuit 13 and the information (the information on the wavelength (of the signal) added from the self-node) representing that the signal is added at the SW 4, are given to a wavelength count information transmitting circuit 18. The wavelength count information transmitting circuit 18 inputs the wavelength count information to the E/O converter 10. The E/O converter 10 generates the SV optical beam containing the node number information and the wavelength count information, and transmits this SV optical beam to the multiplexer 11.
A technology disclosed in, e.g., the following Patent document 1 is given as the prior art related to the present invention.
[Patent document 1] Japanese Patent Application Laid-Open Publication No. 2000-4213
In the prior art illustrated in FIG. 8, however, acquisition of the information on the ASE power defined as a parameter for determining the ALC target value involves obtaining the node number information and the wavelength count information by use of an SV channel. Required therefore are the O/E converter 2 and the E/O converter 10 for the SV channel, and the electric circuits for transmitting and receiving the node number information and the wavelength count information between the neighboring nodes, such as the node number information receiving circuit 12, the wavelength count information receiving circuit 13, the node number transmitting circuit 17 and the wavelength count information transmitting circuit 18. Moreover, the system gets complicated enough to need to multiplex and demultiplex the SV beam with respect to the WDM signal in order to perform the communications (transmission and reception of the SV beam) between the nodes.
Moreover, the SV-beam-based information is transferred sequentially via an SV beam transmitting/receiving circuit provided in each node from the most uplink node. In terms of such a characteristic, a length of transfer time on the order of several hundreds of milliseconds [ms] is required, and the ALC control might be delayed corresponding to this transfer time.
Further, generally, the ASE occurs within an amplification bandwidth of the optical amplifier and can be monitored with a wavelength off a signal bandwidth including the channels, however, according to the prior art illustrated in FIG. 8, the WDM signal beam is demultiplexed into the respective channels and again multiplexed by the AWG 5, in which process the optical beam off the signal band including the channels is not outputted, and hence the ASE can not be monitored with the wavelength off the signal band including the channels.