# RF to Fiber Optics Conversion

I have a RF signal at L-Band, 950-2150 MHz. with bandwidth up to 100 MHz. this signal needs to be converted to optics.

Does single mode or multi mode fiber have frequency/bandwidth limitations after conversion?

• How long is the optical fiber? – stark Dec 7 '15 at 13:02
• It may have a bandwidth of 100 MHz but what is the data rate (bearing in mind that you do need to convert to baseband and then use an amplitude modulation scheme in optics). We still find it hard to phase modulate carriers with wavelengths circa 1000 nm!! – Andy aka Dec 7 '15 at 13:48

Any limitation in your fibre will be inconsequential compared to those in your optical modulation/demodulation processes.

You will have to look critically at the characteristics of your incoming signal to decide what sort of optical modulation scheme will best preserve the parameters you want to keep, and which you can allow to degrade.

If it's a data signal, you may want to throw away weak carriers. If it's a spectrum scan, you may want to keep weak ones, and ensure that strong nearby signals generate don't obscure them with intermodulation.

The most conceptually straightforward way to encode an RF signal onto optics is to analogue AM modulate a subcarrier, which has a frequency of several times your bandwidth. However, the linearity of this could vary between bad and very bad, which would affect your EVM slightly, and your co-channel hugely, if such things are of consequence in your original signal.

An ideal route would be to fully demodulate your signal, assuming it to be data, and ship the data along the fibre, using a standard data format and end-point chipsets.

There is not enough use-case information in your question to answer more than very generally at the moment. Depending on your field of employment, you may not be able to give out use case information!

Bandwidth on singlemode is theoretically unlimited, practically only limited by the electronics connected to it.

Bandwidth on multimode depends strongly on distance; the differing grades of 50 micron core have less dispersion (and more bandwidth for a given length) as you progress from OM2, to OM3, to OM4 (OM1 is the 62.5 micron core with a considerably worse BW, or BW * distance product.)

So, @stark's question in the comments matters; if the distance required is more than a few hundred meters, it's probably safest to say that singlemode is the way to go.

• Even OM2 fiber is 500 MHz $\cdot$ km, so if OP's distance need is less than 5 km (assuming we can trust the 100 MHz bandwidth spec), multimode should still be under consideration. – The Photon Dec 7 '15 at 17:00
• The price-premium (yes, premium - go shop bulk cable) of multimode cable rather quickly eliminates any savings on optics for links which are not fairly short. Likewise, 4-5 Km is going to be hard to manage with the typical loss budget of multimode optics, where it's trivial with SM optics. – Ecnerwal Dec 7 '15 at 17:48
• Don't worry, I'm well aware that MMF is pricier than SMF (The cost savings of MMF comes in the connectors, and yes, obviously that swings the cost consideration toward SMF for longer lengths) But OP hasn't said what distance s/he needs. Maybe s/he only wants a 100 m link. – The Photon Dec 7 '15 at 17:52