One 4-core single-mode fiber is paired with two

A single 4-core single-mode fiber can support two devices using either duplex connections or BiDi (bidirectional) technology, with proper wavelength pairing and core allocation.

Core Allocation and Duplex Connections

A 4-core single-mode fiber contains four individual optical cores, each capable of transmitting a single light signal. When connecting two devices, you typically use two cores per device: one for transmitting (TX) and one for receiving (RX) to form a duplex link . This allows each device to send and receive data simultaneously. The remaining two cores can be reserved for redundancy or future expansion.

BiDi (Bidirectional) Option

Instead of using two cores per device, you can use BiDi transceivers, which allow one core to carry signals in both directions by using different wavelengths for TX and RX . In this case, a single core per device is sufficient, and the 4-core fiber can support two BiDi links, with each device transmitting and receiving on opposite wavelengths (e.g., 1310 nm TX / 1550 nm RX for one end, reversed at the other end), . Proper A/B wavelength pairing is essential; mismatched ends (AA or BB) will not work.

Simplex vs Duplex Cables

• Simplex cables use a single fiber per connection, suitable for BiDi links.
• Duplex cables use two fibers per connection, one for TX and one for RX, which is standard for traditional single-mode links . For a 4-core fiber, you can either create two duplex links (using all four cores) or two BiDi links (using two cores, leaving two for redundancy).

Practical Considerations

• Distance: Single-mode fibers are ideal for long-distance transmission, often exceeding 10 km .
• Connector types: Ensure both ends use compatible connectors (LC, SC, etc.) and maintain clean terminations.
• Redundancy: Reserve extra cores for backup or future devices.
• Wavelength matching: For BiDi, each end must use complementary wavelengths to avoid signal collision . By carefully planning core usage and selecting the appropriate transceivers, a 4-core single-mode fiber can efficiently support two devices, either through traditional duplex links or modern BiDi technology.