BIDI vs Duplex Optics: Single-Fiber Bidirectional 25G/100G/400G

Most fiber links use two strands: one to transmit, one to receive. A bidirectional, or BIDI, transceiver puts both directions onto a single strand by sending transmit and receive on different wavelengths and separating them again at the far end. The result is one fiber doing the work of two. This article explains how single-fiber bidirectional optics work, why they must be deployed in matched A and B pairs, and where 25G, 100G and 400G BIDI make sense.

How a Single Fiber Carries Two Directions

A duplex module has a transmitter and a receiver on separate ports, so a pair of fibers keeps the directions apart. A BIDI module instead uses a wavelength-division filter inside the module. The transmitter emits one wavelength and the receiver is tuned to a second wavelength; the filter routes the outgoing light onto the shared strand and directs the incoming light from the far end into the receiver.

Because the two directions occupy different wavelengths, they do not interfere. Light travelling out at 1270nm passes through the filter while light arriving at 1330nm is reflected into the photodetector. The same optical hardware runs in both directions, which is why a single strand can carry full-duplex traffic. The filter adds a small insertion loss, so a BIDI link's usable reach is a little shorter than an equivalent duplex module of the same nominal grade.

A-End and B-End: Why Pairing Is Mandatory

A BIDI link is built from two complementary modules. The A end might transmit on 1270nm and receive on 1330nm; the B end does the reverse. Because the wavelength assignment is fixed inside each module, the two ends are not interchangeable. Two A-end modules on opposite ends of a link both transmit on the same wavelength and both listen on the other one, so no receiver is tuned to what is arriving and the link stays dark.

This is the single most common BIDI deployment error, and it usually happens when two identical modules are pulled from the same tray. Labelling the A and B ends, and pairing them at the warehouse rather than at the patch panel, removes almost all of these faults. Where several BIDI systems share a cable bundle, the wavelength pairs must also be planned so that, for example, 25G fronthaul on 1270/1330nm does not collide with backhaul using 1490/1550nm on the same route.

BIDI and Duplex by Rate

At 1.25G the wavelength pair comes from the original single-fiber standard for Ethernet access. At 10G and 25G, the common pair is 1270/1330nm, which covers most campus and fronthaul runs. At 100G, single-fiber BIDI either uses one 1270/1330nm pair with PAM4 or distributes the load across several coarse wavelengths. At 400G, single-fiber bidirectional operation is still emerging through multi-source agreements rather than a settled IEEE standard, so most 400G links today use parallel or duplex single-mode optics.

When Single-Fiber Saves More Than Fiber

BIDI pays off when fiber itself is the scarce or expensive resource. That includes long metro routes, 5G fronthaul where a single strand reaches a remote radio site, campuses with limited duct space, and any link where existing cabling can be doubled in capacity without pulling new cable. Using one strand instead of two halves the fiber count and the patch-panel space at both ends.

The tradeoffs are operational. You must keep an A and a B population rather than a single stocked part, mispairing is possible, and the filter loss trims reach at the margin. Where those constraints are manageable, single-fiber BIDI is one of the cheapest ways to add capacity on a route that is already full.

Getting Started

Tell us the rate, the distance and whether you are cabling new strands or reusing an installed one, and we will recommend a BIDI or duplex plan complete with the A and B end count. Send your requirement list and we will quote within hours.