Connection of ONU and two optical splitters

In a PON, an ONU can be connected through a two-stage optical splitter arrangement, where the OLT feeds the first splitter, which cascades to a second splitter before reaching the ONU.

Two-Stage Splitting Topology

In a two-stage optical splitting setup, the connection from the OLT to the ONU involves:

1. Feeder Fiber: The optical signal from the OLT is transmitted via a feeder fiber to the first-stage splitter. This splitter is typically located at a central junction or intersection of the fiber network .
2. First-Stage Splitter: This splitter divides the optical signal into multiple outputs, often with a split ratio like 1:8 or 1:16, depending on network design and coverage requirements .
3. Branch Fiber: Each output from the first-stage splitter is connected via branch fibers to a second-stage splitter, usually located closer to the end-user, such as in a fiber distribution box (FDB), .
4. Second-Stage Splitter: The second splitter further divides the signal, typically with a smaller split ratio like 1:4 or 1:8, to serve individual ONUs .
5. ONU Connection: Each output of the second-stage splitter is connected to an ONU using a branch fiber. The ONU then provides service to the end-user via Ethernet or other interfaces .

Key Considerations

• Split Ratios: The combination of first- and second-stage split ratios determines the total number of ONUs served by a single OLT port. For example, a 1:8 first-stage splitter followed by a 1:4 second-stage splitter can serve 32 ONUs .
• Signal Isolation: There is no direct optical channel between ONUs on the splitter; communication between ONUs is managed by the OLT or higher-layer network devices .
• Deployment Flexibility: Two-stage splitting is often used in rural or wide-area deployments to reduce fiber usage and optimize cost, while single-stage splitting is more common in dense urban areas .
• Passive Operation: Both splitters are passive devices, requiring no power, which simplifies maintenance and reduces operational costs .

Summary

Connecting an ONU through two optical splitters involves a cascaded splitter architecture: the OLT sends signals to a first-stage splitter, which distributes them to second-stage splitters, and finally to individual ONUs. This design balances cost, coverage, and bandwidth allocation, while maintaining the point-to-multipoint nature of PON networks and ensuring that all ONUs receive controlled downstream and upstream communication via the OLT .