Can we connect multimode SFP with Single mode fiber?

Short answer is – No!

An extended answer is – You could try, and on some occasions, it may even work in very short distances, but we definitely do not recommend it. The thing is, a 1000Base-SX multimode transceiver uses an LED as a light source. It’s possible because Multi-mode optical cables have a very wide fiber core – 62.5µm (OM1) or 50 µm (OM2/OM3/OM4/OM5) – so this 1000Base-SX SFP’s transmitting interface is conditioned to connect the LED source to this very wide fiber core. In contrast, the single-mode optical cable core is narrow – 9 µm. When we connect multimode SFP with single-mode fiber, only a fraction of the low-intensity LED emitted optical signal will get into the much narrower fiber core, but sure – some part, which will escape intense attenuation of reflected signal, definitely will get there, but will fade after a meter or less.

So technically speaking, it is possible to connect multimode SFP with single-mode fiber – but the connection will be unreliable, unpredictable, and very short. We don’t need such links in our networks, right?

Sure – some vendors are offering mode conditioning cables, thus allowing interconnect multi-mode transceivers with single-mode cable infrastructure, but this solution should be treated as an instrument of last resort.

We don’t recommend mixing things – it’s always better to use single-mode with single-mode and multi-mode with multi-mode. And let’s remember history – why is there multi-mode in the first place? Because years ago, the production cost of LED transceivers was much lower compared with Lasers used in 1000base-LX SFP. Also, the production cost of multi-mode cables required much lower precision and cost. Nowadays cost of 1000Base-SX SFP multimode short-range transceivers is almost equal to 1000base-LX SFP, so the decision on what to use is mainly related to the existing infrastructure.

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