1. Field of the Invention
The present invention relates to a Wavelength Division Multiplexing (WDM: Wavelength Division Multiplexing) optical communication network, and more particularly to an optical add-drop multiplexer which adds and drops optical signals having specific wavelengths of WDM optical signals bi-directionally transmitted through a single transmission line at each remote node in a WDM ring network.
2. Description of the Related Art
As of late, metro/access networks for connecting central office nodes and subscribers are attracting more attention as home communication traffic increases due to spread of the Internet. Methodology must be available for easily increasing the capacity of the networks to match the increase in demand for ultra high speed services, and the methodology must be economical in order to accept the many new subscribers. The metro/access networks, if implemented to operate in a WDM system, can transfer WDM optical signals by a plurality of wavelengths regardless of transmission methods or speed. Thus the networks can efficiently transfer the traffic at a high speed and on wide bandwidths.
Optical WDM bidirectional hubbed ring networks can be employed as metro/access networks useful in this regard. An optical WDM bidirectional hubbed ring network has the shape of a ring, and is formed by a single line which connects the central office and a plurality of remote nodes with each other. WDM optical signals having two WDM channels are transmitted through the single line in directions opposite to each other. For instance, WDM optical signals having odd wavelengths and even channels are transmitted in one direction and in the reverse direction, respectively. The central node is connected not only to the remote nodes but also to other networks. The remote nodes are established nearby areas of high subscribers concentration, and serve to connect the central office to subscribers. Therefore, each remote node must be able to drop, for its own use, specific signals transmitted from the central office and add specific signals to the optical transmission line for conveyance to the networks. Accordingly, each remote node comprises optical add-drop multiplexers which add or drop optical signals having specific wavelength among the bidirectional WDM optical signals in the optical WDM bidirectional hubbed ring networks.
As shown in FIG. 1, the bidirectional optical add-drop multiplexer constituting a remote node in an optical WDM bidirectional hubbed ring network separates two WDM optical signals by using optical circulators 104, 118 each having three ports. Two WDM optical signals are respectively inputted through optical transmission lines 100, 102 connecting both sides of the remote note to two neighbor nodes (not depicted), in the shape of a ring network. That is, the lines 100, 102 are each portions of the same single transmission line connecting the nodes of the ring network. The bidirectional optical add-drop multiplexer adds and drops optical signals having specific wavelengths by means of optical circulators 106, 114 and an optical channel selector 110 located in a first side and optical circulators 108, 116 and an optical channel selector 112 located in a second side. Here, the optical signals added or dropped in the same direction of the multiplexer have the same wavelength. Namely, if optical signals inputted in the first direction of the multiplexer are dropped, optical signals having the same wavelength are added and transmitted in the same direction.
When the bidirectional optical add-drop multiplexer shown in FIG. 1 receives optical signals having wavelengths λ2, λ4, λ6 corresponding to three channels 2, 4, 6 from the optical transmission line 100 connected with a first of two neighbor nodes, an optical signal of wavelength λ2 is dropped and a signal of the same wavelength may be added. Likewise, when optical signals having wavelengths λ1, λ3, λ5 corresponding to three channels 1, 3, 5 are received from the optical transmission line 102 connected with the second neighbor node, an optical signal of wavelength λ1 is dropped and a signal of the same wavelength may be added.
In particular, the optical signals having wavelengths λ2, λ4, λ6 are inputted to a port 104a of the optical circulator 104 through the optical line 100 and those having wavelengths λ1, λ3 and λ5 are inputted to a port 118a of the optical circulator 118 through the optical line 102. The optical circulators 104 to 108 and 114 to 118 are 3-port optical circulators having three ports circularly arrayed. As already known in the art, each port of the optical circulators 104 to 108 and 114 to 118 outputs the optical signals to the next port according to the sequence arrayed in clockwise or counterclockwise direction in FIG. 1.
The optical signals having wavelengths λ2, λ4, λ6 inputted the port 104a of the circulator 104 are outputted to a port 104b and progress along an upper route shown in FIG. 1, which extends from an optical circulator 106 through an optical wavelength selector 110 to an optical circulator 114. In the same manner, the optical signals having wavelengths λ1, λ3, λ5 inputted the port 118a of the circulator 118 are outputted a next port 118b and progress along a lower route shown in FIG. 1, which extends from an optical circulator 116 through an optical wavelength selector 112 to an optical circulator 108. The optical wavelength selectors 110, 112 select the wavelengths λ2, λ1 as reflection wavelengths, respectively. Thus, the optical signals having the wavelengths λ2, λ1 are reflected by the optical wavelength selectors 110, 112, respectively and the other signals not being reflected pass through the optical wavelength selectors 110, 112
As described above, the optical signals having the wavelengths λ2, λ4, λ6 inputted to a port 106a of the optical circulator 106 from the port 104b of the optical circulator 104 are outputted to a next port 106b and then inputted to the optical wavelength selector 110. The optical signal having the wavelength λ2 is reflected by the optical wavelength selector 110, again inputted to the port 106b of the optical circulator 106 and dropped by outputting to a port 106c of the optical circulator 106. The other optical signals having the other wavelengths λ4, λ6 pass through the optical wavelength selector 110 and are inputted to a port 114b of the optical circulator 114. Concurrently, an optical signal having the wavelength λ2 to be added is inputted to a port 114a of the optical circulator 114. Thus, the optical signal of wavelength λ2 to be added is outputted to the port 114b of the optical circulator 114, reflected by the optical wavelength selector 110, and inputted to the port 114b of the optical circulator 114 with the optical signal of the wavelengths λ4, λ6. Accordingly, the optical signals of the wavelengths λ2, λ4, λ6 from the port 114c of the optical circulator 114 are inputted to a port 118c of the optical circulator 118 and transmitted through the optical line 102 by outputting to the port 118a of the optical circulator 118.
Analogously, the optical signals of the wavelengths λ1, λ3, λ5 inputted to a port 116a of the optical circulator 116 from a port 118b of the optical circulator 118 are outputted to a port 116b and inputted to the optical wavelength selector 112. The optical signal of the wavelength λ1 is reflected by the optical wavelength selector 112, inputted to the port 116b and dropped by outputting to a port 116c. The other optical signals of the other wavelengths λ3, λ5 pass through the optical wavelength selector 112 and are inputted to a port 108b of the optical circulator 108. An optical signal of the wavelength λ1 is added by means of port 108a. The optical signal of the wavelength λ1 to be added is outputted to the port 108b, after reflection by the optical wavelength selector 112, returns to the port 108b with the optical signals of the wavelengths λ3, λ5. Therefore, the optical signals of the wavelengths λ1, λ3, λ5 from the port 108c are transmitted through the optical line 100 by being outputted to the port 104a. 
As described above, when the bidirectional optical add-drop multiplexer drops optical signals inputted in one direction, the bidirectional optical add-drop multiplexer adds an optical signal having the same wavelength as that of the dropped signals to the other optical signals and transmits them in the same direction.
Disadvantageously, since both added and dropped signals of the same wavelength are reflected by the same optical wavelength selector, the cross talk of the added optical signal degrades the quality of the dropped optical signal.
To prevent the above problems, the optical wavelength selectors must have an isolation of above 30 dB. However, the optical wavelength selectors having high isolation are very expensive, so that the cost of the multiplexer increases.
Another technique used to resolve the above problem is disclosed in U.S. Pat. No. 5,926,300, entitled “OPTICAL ADD-DROP MULTIPLEXER”, filed on Jul. 20, 1999. This multiplexer not only uses respectively different optical wavelength selectors for the added optical signal and the dropped optical signal but also uses optical isolators between two optical wavelength selectors so that it can prevent the declination of the transmission characteristics resulting from leak components, which are not reflected but transmitted.
However, according to the U.S. Pat. No. 5,926,300, although the add-drop multiplexer can prevent the declination of the transmission characteristics resulting from such leak components, the number of the optical wavelength selectors increases and the optical isolators are supplemented. Accordingly, the structure of the multiplexer becomes complex and therefore the cost of the multiplexer also increases.
As described above, the typical bidirectional optical add-drop multiplexer has the same wavelengths for both the added and the dropped optical signal, and both the added and the dropped optical signal are reflected by the same optical wavelength selector. Thus, the resulting cross-talk between the optical signals added and dropped degrades optical signal quality characteristics. Accordingly, preventing the above problems requires a multiplexer having a complex structure and higher cost.