Modern telecom fibre networks are based on Wavelength Division Multiplexing (WDM) technology, where data is sent using multiple optical wavelengths that each carry independent traffic. The flow of this information through the core network is controlled using Reconfigurable Optical Add/Drop De-multiplexers (ROADMs). Figure 1 shows a schematic of a ROADM that routes traffic in three directions (east, west, and north). In the express part of the ROADM, optical wavelengths can either routed to any of the output directions, and at the add/drop part of the ROADM, data can be either added or dropped from the core network to a local network.
The key building blocks of a ROADM are the 1×N WSSs. These allow the paths that the optical wavelengths take to be remotely reconfigured, thereby enabling the performance of the telecom network to be optimised in real-time from a central office. LCoS WSSs have become the technology of choice for this application due their ability to operate with newer flexible spectrum data formats that utilise the data capacity of the fibre network more efficiently.
Figure 1 – Example of a ROADM that routes traffic in three directions
(east, west and north), and adds or drops traffic to the local network.
The ability of the ROADMap WSS module to simultaneously support multiple 1×N WSSs and N×N WSSs has several advantages in terms of a ROADM design:
1) In the transit part of the ROADM, the multiple independent 1×N WSSs can be implemented in a single module. This results in significant cost and operating power savings. Alternatively, using the same technology platform, an N×N WSS can be configured that provides the same functionality.
2) In current Colourless, Directionless, and Contentionless (CDC) ROADM architectures, the add/drop side uses either a broadcast and select approach with tuneable flexible spectrum filters (Fig. 2(a)), or an array of independent 1×N WSSs and an array of N×1 space switches (SS) (Fig. 2(b). The stacked WSS module allows all the individual switching functions of the CDC ROADM add/drop side to be integrated into a single, low loss, low cost module (Fig. 2(c)). Alternatively, the WSS module can be reconfigured to provide a single M×N WSS solution (Fig. 2(d)).
These two applications illustrate that a stacked WSS module can offer cost and performance benefits by reducing the number of components in a ROADM. The cost reduction per port associated with integrating multiple WSSs into a single module will also benefit the METRO and ACCESS networks as higher performance WDM data links are deployed in these markets
Figure 2(a) – Current CDC add/drop solution based on the multicasting switches and flexible spectrum tunable filters.
Figure 2(b) – Current CDC add/drop solution using an array of 1xN WSSs.
Figure 2(c) – CDC add/drop solution using a stacked WSS module to replace the independent WSSs.
Figure 2(d) – CDC add/drop solution using a stacked WSS module configured in an NxM configuration.