BiDi Optical Modules: Doubling Capacity on the Same Fiber

How BiDi works

A standard duplex module uses one fiber for transmit and one for receive. A BiDi module puts both directions on a single fiber by using two different wavelengths — for example, a module transmitting at 1310 nm while receiving at 1550 nm.

This works because the two ends of the link are paired in opposite configurations:

• End A: transmits 1310 nm, receives 1550 nm
• End B: transmits 1550 nm, receives 1310 nm

Because they emit on different wavelengths, the two ends can share one fiber without interfering.

Why this matters operationally

Halving fiber count is rarely about the cost of the fiber itself. It matters because of:

• Fiber scarcity in existing ducts. Upgrading a 5G fronthaul ring from 10G to 25G often requires more links than the installed fiber can carry. BiDi doubles capacity without new trenching.
• Installation cost. In long or difficult runs, the labour and permitting cost of a second fiber far exceeds the module cost.
• Port economics. In some access and fronthaul scenarios, BiDi optics let a single-fiber infrastructure serve twice as many endpoints.

Pairing rules you must not get wrong

1. Always order in matched pairs. A BiDi link needs one A-side and one B-side module at the two ends. Two A-sides will not work.
2. Check the wavelength pair, not just the speed. 1310/1550 and 1270/1330 are both common and are not interchangeable with each other.
3. Confirm distance class. A 20 km pair and an 80 km pair are not substitutes.
4. Label both ends at installation. This is the single most effective way to avoid a future mis-pairing during maintenance.

Typical deployment

| Application | Typical BiDi pairing | Reach | |---|---|---| | 5G fronthaul / C-RAN | 1.25G or 10G, 1270/1330 nm | 10–20 km | | Access / FTTx | 1.25G, 1310/1550 nm | 20–40 km | | Metro short links | 10G, 1270/1330 nm | 10–40 km | | Long access spans | 1.25G, 1490/1550 nm | 80–120 km |

What to ask a supplier

Ask for the exact wavelength pair, the Tx/Rx specification pair for both A and B sides, and confirmation that the two sides are supplied as a matched set. A BiDi quote that only lists speed and distance is incomplete.

Canjin Technology supplies BiDi optics from 1.25G SFP through 10G SFP+ and 25G SFP28, in 1270/1330, 1310/1550 and 1490/1550 wavelength pairs, with matched A/B part numbers.