Fibre Optic Basics

It rather depends on the bandwidth of those signals coming in over the copper pairs. I agree for a short length you could have 20 pairs, and the same applies to fibre, but in both cases that is really wasteful, and normally  even over wire, you'd multiplex the signals in some way, perhaps in an analog manner with modulation on  a sub-carrier tone per signal, or in a digital way, either time division or digitisation and then as streams. 

So are these pairs carrying gigabit ethernet, or perhaps video or audio, or maybe really slowly changing voltages representing signals like temperatures  of a factory process?

Once you have a figure for the bandwidth needed for the combined signal, that can be worked back into a number of fibres.   Again, the carrying capacity  is length dependant, and you can shine more than one wavelength (think colour, but infrared) down a fibre and separate them with filters at the other end.

As a rough start, OM3 will take data  at a rate of perhaps  10gigabits per second for about quarter of a km and 40gigabits about 100m, if you want a lot more capacity or distance then single mode fibre OSxx  is the way to go. more on that here

 First realise that the ‘simple’ copper is not so simple, only when you know what the signals are, then translate to fibre.

Mike

Hi Jones,

Well honestly speaking that comparision was back in days, might be 30 years ago. Currently each have own appliance.

For the instance you mentioned , it is not feasible to replace a copper with fiber unless you want to gain some bandwidth. at the end of the day it will be and end-to-end connection from each equipment to main frame.

Let me give you a different example:

Lets say that you need to install 50 new equipment at site and all of them needed to wire back to control room which is 20 km away. also each equipment uses 10 pairs of cable.

With fiber beauty, you can install an equipment rack nearby this equipment and install 1 pair fiber to each equipment where the field equipment will multiple all the signals to single fiber and send to main frame. imagine that we did it all 50 field equipment and now it is time to deliver this 50 equipment's 10 pair signal to 20 km away. Now another multiple will happen in main frame and we will transport all this 50x10=500 signals to 20 km away with 1 pair fiber. 

This concept is called MUX-DEMUX , it was used for coppers back in days but signal loss and noise was a huge impediment. As nature of fiber practically eliminates all noises

let me know if you need any help

it is huge concept but simple

Again about OM3, it is a multi-mode cable with 50/125 , the distance will vary from the bandwidth. 

coming back, you need to know first what you need exactly. what is the expected bandwidth (I assume for field equipment it will be very less) and to what distance you want to transmit.