Can I use the following combination of fibre optics? We have a remote F/O transmitter/reciever module at each end of a 10 Km fibre optic link. These are single mode devices and are connected together by single mode fibres except for the patch leads to the f/o patch panel. These are approx 30 metres long and are multimode patch leads. Is it possible to use these multimode patch leads instead of single mode? The system appears to be working ok.

 

I have seen this done, but due to the higher number of reflections in multimode cable I would stick with the same type of cable all throughout the network, this way you are sure of no problems due to different types of cable,
for a go tutorial go to:

http://www.arcelect.com/fibercable.htm

has some great links and good information, should be very helpful.

Look at it this way, you would not use a 75ohm patch cable in a 50ohm system right?

matt

 

Connecting single mode to multimode can work but losses are higher when the light crosses the interface from the multimode cable to single mode cable. This is because the smaller single mode cable core has a smaller end surface area with which to catch the light from the multimode cable whos core has a larger end surface area. The light comming out of the multimode cable is spread over its end surface area and so, unless your single mode cable has a high enough numerical aperture to catch some of the light that would otherwise miss its opportunity to enter the single mode cable, you will see substantial losses from multimode to single mode transition.

Things are much better on the trip from single mode to multimode cable because the light emmitted from the single mode cable core is spread over a smaller surface area and the larger core area of the multimode cable is able to contain much of the transmitted light. Often the only losses experienced are due to Fresnel losses.

Julian Nedohin-Macek
Winnipeg, Manitoba

 

You are lucky your multi-mode patch cables work. I assume you are using a rather slow baud rate? 9600 even? Are you testing on the actual 10 km SM fibers, or just in your lab with a sample SM fiber?

As the SM light hits this "wide" MM glass path the beam will start reflecting and spreading it's energy over the 50 or 62.5 micron fiber. This means a lot of things, but I won't go into them all. As the light finally hits the 9 micron SM sensor/receiver, most of the light energy is just lost because it hits outside of that 9 micron target. A similir thing will happen at the transmitter end - most of the SM transmitters power will be lost in the 30m patch cable before your main SM fiber.

I think you can visualize this easily - if you shine a light in a 62.5 mm wide beam at a 9 mm hole, the light that goes through that hole is what remains for your SM fiber. The light which hits the 62.5 mm wide space AROUND the hole is your wasted power (about an 85% loss?). So your design will have a huge power loss - however, it will work just fine as long as *ENOUGH* power remains at the remote receiver. But this means other system problems (slightly crushed cables, weakening companants, etc) have less margin for power loss before your system stops working.

So I'd create a to-do list to eventually replace the MM with SM - starting with the patch cords at the transmitter end.

If this is a new system under factory test, then for sure ignore that "it works" and do it right to start. The cost of the SM patch cables now will be TINY compared to the troubleshooting efforts and loss of productivity a few years from now if performance becomes intermittant because of added line loss.

best regards - LynnL, www.digi.com