An Optical Circulator is a non-reciprocal passive device used in fiber optic communication systems to control the direction of light propagation. Unlike optical isolators that block reflected light, a circulator routes optical signals in a specific order — typically Port 1 → Port 2 and Port 2 →. Faraday circulators (or less specifically optical circulators) are a kind of non-reciprocal optical devices. They are technically related to Faraday isolators, and on a broader scale similar to electronic circulators.
[pdf] An ODN, or Optical Distribution Network, is the passive portion of a fiber-optic network that links the OLT at the central office to ONUs/ONTs at customer premises. Unlike active devices, the ODN requires no external power to function. Instead, the network relies on specific components such as OLT, ONU, ONT. Active optical networks (AON) and passive optical networks (PON) are the two major systems that make FTTH broadband connections possible. In this use, a PON. While the OLT manages the signal and the ONU converts it at the user end, the ODN is the passive infrastructure that carries the optical signal efficiently, reliably, and cost-effectively from the OLT to multiple end-users. These two categories of optical networks differ.
[pdf] Optical communication systems generally include three main active components- a transmitter (Tx), an optical fiber, and a receiver (Rx). A transmitter is a device that enables launching an electrical signal into an optical waveguide. Therefore, we will primarily focus on the basic setup of semiconductor light-emitting diodes (LED) and lasers. Fault Detectability in DWDM provides a treatise on fault mechanisms are detected.
[pdf] 
