With the advent of modern microprocessor relaying, much of the communication between relays has been shifting from power-line-carrier apparatus or microwave to fiber.
Since power utilities own the easements for the high voltage transmission lines, it is only logical to install fiber up there. One ingenious location is the lightning shield wire. It is the top most wire on the tower. It is not a current carrying wire, so no line sag issues (due to heat). Also the wire is smaller gauge, so there’s less impact to the tower design.
The shield wire constructed with fiber inside it is called the Optical Ground Wire (OPGW). The one shown in the GIF image comes with up to 144 count fiber. Additional specifications on its construction can be found here.
From relaying standpoint only 2 fibers are needed (1-TX, 1-RX) for each relay pair. Typical connection looks like below.
The fiber coming in from outside and the one coming in from the relay gets spliced inside a fiber distribution panel. See video below on how fiber gets spliced.
Distances between substations could be a few miles to sometimes hundreds of miles. The laser light traveling inside the fiber needs to traverse these distances. To minimize light reflections, which impacts how far the light travels, the core is constructed with a diameter as small as possible. A 9micron core fiber (a.k.a. single mode) is used for communication between substations. A 62.5micron fiber (a.k.a. multimode) is used for communication within the substation. Note the core to cladding ratio for this fiber in the image below.
Another item to consider is the wavelength of the light. Light inside a 9micron fiber at 1300nm wavelength can travel 80km. Whereas, in the same 9micron fiber, a 1550nm wavelength light can travel 120km.
An excellent value proposition for power utilities is to lease few fibers to telecom or internet service providers. Considering how much high voltage transmission lines cover the length and breadth of USA (see below image), imagine if most wires had fiber. Gigabit internet service for most folks will be within reach.