1G and 10G SFP BiDi Transceivers on Multimode fiber

Bidirectional optical transceivers, by their definition, allow full-duplex optical transmission through one optical fiber. This is achieved with two independent signals which differ from each other in their wavelength, 1310nm/1550nm, or 1310nm/1490nm. BiDi transceivers are designed to be used with single-mode fiber, in distances from 10km to 120km, utilizing a laser as a light source.

As we all know signal is transmitted as a light impulse which travels in the optical fiber core, side to side, bouncing off the cladding. Single-mode fiber (OS2) has a 9um diameter core, which is much smaller than the multimode fiber core diameter of 50um (OM2). A smaller core minimizes this impulse bouncing and therefore allows for achieving greater distances.

Nowadays, there are places where legacy multimodal infrastructure still exists. Mostly, this infrastructure is observable on the business premises as short-distance connections, for example, to connect nearby buildings. As the cabling is still usable, why not use it wisely?

Classically, multimode fiber is used together with multimode dual fiber transceivers. These transceivers usually have simple and cheap LED or lasers as light sources, which produce signals at 850nm and cover 550 meters max distance. Natively BIDI transceivers specifically for multimode fiber do not exist. So the big question today is whether single-mode 1G BiDi LX transceivers are capable of achieving working conditions when connected with multimode fiber?

• •1G BiDi LR (EDGE PN: BIDI-1.25G-SFP-10-AD, 1310nm/1550nm) transceiver connected with OM2 grade multimode cable assured no CRC error-free transmission till 300 meters;
• •1G BiDi LR (EDGE PN: BIDI-1.25G-SFP-10-AD, 1310nm/1550nm) transceiver connected with OM3 grade multimode cable assured no CRC error-free transmission till 500 meters;

Additionally 10G BiDi transceiver (EDGE PN: BIDI-10G-SFP-10) connected with OM3 grade multimode cable assured no CRC error-free transmission till 200 meters. (OM1 and OM2 should work for much smaller distances, because of the fact that these fibers are not laser optimized)

So what is happening when we use transceivers destined for “long range” communication in legacy multimode fiber?

The long-range transceiver has a 9um laser output diameter. It matches the single-mode fiber core diameter, therefore its signal can be precisely inputted into single-mode fiber. But if we connect this transceiver to multimode fiber, the multimode fiber core (50um) is much larger than the single-mode fiber core (9um). At the signal transmission direction, there will be no problem because a 9um-wide signal will be inserted precisely in the center of the 50um fiber core. But problems happen at the receiving end. The optical signal, when it propagates fiber, scatters and bounces through the 50um core. And at the receiver end, a small fraction of the signal will be detected by the 9um transceiver detector. The largest signal part will be lost and can be defined as “huge insertion loss”.

Our tests concluded that multimode fiber can be used with BiDi single-mode transceivers, but distance is limited due to multimode fiber construction and the number how many bends the cable has. Each cable bend will cause the signal to propagate more alongside the cladding, therefore increasing insertion loss at the receiving end (decreasing transmission distance).

If you have any other questions, please do not hesitate to contact us!