ONU (Optical Network Unit) converts optical signals to electrical signals for end users, while optical splitters passively divide a single optical signal from the OLT to multiple ONUs in a PON.

ONU Equipment Overview

An ONU (or ONT, Optical Network Terminal) is the user-side device in a PON system that interfaces with the subscriber's premises. It performs the following functions:

• Signal Conversion: Converts downstream optical signals from the OLT into electrical signals for devices like computers, phones, or set-top boxes, and converts upstream electrical signals back into optical signals for transmission to the OLT .
• Data Aggregation: Collects and organizes upstream data from multiple end-user devices before sending it to the OLT.
• Network Management: Supports functions such as bandwidth allocation, ranging, and signaling control, ensuring each ONU communicates efficiently with the OLT .
• Deployment: ONUs can connect via fiber directly to homes (FTTH), buildings (FTTB), or other access points, and may use additional media like copper or Wi-Fi for internal distribution . The ONU is a powered device, unlike the passive components in the network, and works in a master-slave relationship with the OLT, which manages signaling and monitors ONU status .

Optical Splitter Working Principle

An optical splitter is a passive device in the Optical Distribution Network (ODN) that allows a single optical signal from the OLT to be shared among multiple ONUs . Its key principles include:

• Signal Division: The splitter has one upstream input and multiple downstream outputs. Light from the input fiber is distributed to all output fibers, enabling one OLT port to serve multiple subscribers .
• Passive Operation: No power is required; the splitter relies on optical physics to divide the signal, reducing operational costs and maintenance.
• Signal Coupling: In FBT (Fused Biconical Taper) splitters, light energy partially transfers between closely aligned fiber cores during the tapering process. In PLC (Planar Lightwave Circuit) splitters, the splitting occurs on a semiconductor chip using integrated optical waveguides .
• Architectures:
  • Centralized Splitting: A single-stage splitter connects the OLT directly to multiple ONUs, suitable for dense user areas .
  • Cascaded Splitting: Multiple splitters in series allow flexible distribution over larger areas, balancing cost and signal loss .
• Signal Loss: Each split reduces optical power (e.g., ~3.5 dB for a 1:2 split), which affects transmission distance and bandwidth per user .

Integration in PON

In a typical PON:

1. The OLT at the central office sends downstream optical signals.
2. The optical splitter divides the signal to multiple ONUs.
3. Each ONU converts the optical signal to electrical signals for end-user devices and sends upstream data back through the same splitter to the OLT . This combination of active ONU devices and passive optical splitters enables cost-effective, scalable, and efficient fiber-to-the-home networks.