Do I need two single-mode optical fibers

You do not always need two single-mode fibers; it depends on whether you use a dual-fiber setup or a single-fiber bidirectional (BiDi) transceiver.Single-Fiber vs Dual-FiberDual-fiber modules use two separate fibers: one for transmitting (TX) and one for receiving (RX). This is the most common setup, widely supported, and generally easier to configure, providing stable full-duplex communication .Single-fiber (BiDi) modules transmit and receive data over a single fiber using two different wavelengths. This saves fiber infrastructure and can reduce costs, but requires compatible BiDi transceivers at both ends .When Two Fibers Are NeededStandard dual-fiber transceivers: If your network devices use conventional dual-fiber SFP/SFP+ modules, you will need two fibers—one for TX and one for RX .Long-distance single-mode links: Single-mode fibers are ideal for long distances (up to tens of kilometers) and high-speed connections. Dual-fiber setups are often preferred for reliability and simplicity .When One Fiber Is SufficientBiDi transceivers: If both ends of the link support BiDi modules, a single fiber can handle both directions of communication, reducing cabling requirements .Shorter or cost-sensitive deployments: Using one fiber can save on installation and material costs, especially in data centers or campus networks where fiber runs are long and expensive .Key ConsiderationsDistance: Single-mode fibers are best for long distances; multimode is better for short runs. BiDi modules are available for single-mode fibers.Equipment compatibility: Ensure your transceivers support either dual-fiber or BiDi operation. Mixing types will not work.Budget and future upgrades: Dual-fiber setups are easier to upgrade and maintain, while single-fiber BiDi can save costs but may limit flexibility.Conclusion: If your devices use standard dual-fiber transceivers, you need two single-mode fibers. If you use BiDi transceivers, one fiber is sufficient. Always check your transceiver type and network requirements before deciding .