Intertripping can be carried out over PLC (Power Line Carrier) equipment. That is basically the main function of such equipment. However, PLC as its name suggests makes use of the actual power line, either overhead line or underground cable. Information is in some systems transmitted via the power line using the Single Sideband (SSB) method.
Obviously this method of intertripping rules out the use of fibre optic. However, depending on the regional authority and communications infrastructure available, different methods of intertripping are available, some using fibre optic. These methods include intertripping across voice-frequency networks, digital networks, ISDN lines and of course direct via fibre optics.
The only other comment is that typically for this sort of application, as Aakhrinaam suggests, the interface between the relay issuing the trip command and the PLC equipment could very well be fibre optic - if the PLC equipment has the correct connections.