Hi Ulrich, nice to hear from you.

SFP modules are being used since a long time in computer / enterprise networks, data centres.

The advantages of SFP modules: they are quick and easy to replace and hot swappable. Network devices can be easily converted to other transmission technologies and media (fiber, copper, different bandwidths). Defective modules are inexpensive to replace and do not require repair or replacement of the complete device.

For RME devices the supported default media is multimode fiber. Cable standards are OM3 and OM4.
If you do not need OM4 then use OM3 and best stick to this one cable type, as they should not be mixed on one connection.
Then it will also fit to the coupler in the RME offering.

With single mode (laser based) the cables between devices may even be 10km, but older devices needed special order to be equipped with single mode modules. So I would concentrate on multimode with OM3 cables, because this is usually what everybody has and then you can also use the coupler in the RME offering.
Multimode is also more secure, because single mode is laser light which may hurt your eyes.

RME SFP: it's sometimes a little bit difficult to pick the proper SFP with suitable wavelength and a firmware that is not dedicated to certain manufacturers. Therefore the easiest way is to get the modules which are tested to fully support RME even if it is a little more expensive, this can save time and frustration.

Instead of using couplers you can also pick the proper OM3 cables with the following combination of plugs: SC-SC, LC-LC, SC-LC. Simply look whats required between your devices and think about that so called duplex cables contain two fibers.

Patch cables are usually available from 0.5m up to 50m length. In house cabling can have even longer cable length and can even go across several patch fields in larger buildings (which should be the exception). I think with the normal patch cables of up to 50m length you will easily be able to chain even multiple preamps between different recording rooms.

> Mini GBIC, 155 Mbps, 2km, Multimode, 1310nm, LC duplex connector.
> Probably it should be compatible according "multi-source agreement (MSA)".

Yes, someting like this with 1310nm should do:
https://www.amazon.de/DIGITUS-Ethernet- … B2CTS?th=1 €21,99

> I strongly hope there are no problems with these passive SC-SC couplers, I need to connect a 50m SC cable
> in order to reach my MADIface Pro.

If you stick with the same cable type, OM3 or OM4, then absolutely no problems.
In computer networks, you sometimes also have to go through several patch panels using patch cables.

I didn't find SC/LC as OM3 cable, but as OM4 cable, for example this here: https://www.all4fiber.de/LWL-Patchkabel … amp;p=2319 €48,90

Sometimes I have the feeling that more variations of plugs and cable lengths are available for OM4 now.

So with ~€80 you have all incl. cable.

> I still wonder: Why ths SFP port? Just for optical MADI sticking with SC connectors would have been perfect -
> will this SFP port be ready for future options different from MADI?

I told you  reasons above, additionally it's better for customers not having to pay for MADI if you connect the preamp using ADAT. This is maybe a bad example as this module contains also additional functions, I only want to mention that MADI modules also have their price .. https://www.thomann.de/de/rme_madi_card_i64.htm

One of the main advantages is definitely the lower price, because SFPs are manufactured for a worldwide high demand for networks and servers and can be offered much cheaper just because of the much higher quantity.

And as I mentioned already, you have the freedom to order MM or SM and you can replace them  easily.