Co-axial copper cables have long been used as conduits of analog RF signals but there is now a medium for electromagnetic energy that is starting to compete with coax: optical fiber. A recent article in Microwaves & RF, 'Why More RF Engineers Are Choosing Fiber Connectors', explains the whys and hows of using fiber for both short distance as well as in-building longer runs, in small-cell applications and distributed-antenna systems (DAS).
The much-lower attenuation of fiber (around 1 dB/km) compared to coax (roughly tens of dB/km) makes it especially attractive in the latter cases. Fiber's immunity to EMI/RFI is a big plus, too, especially in wide-area installations that have to deal with building motors and equipment, well-known and notorious interference sources.
Key to using fiber is the RF-to-optical (aka electro-optical, or E/O) converter at the source end and the complementary optical-to-RF converter at the receiver. For the transmitter, a distributed-feedback (DFB) semiconductor laser is used when wide dynamic range and low noise are critical, while for applications with lower-performance requirements, a Fabry-Perot (FP) laser is generally chosen. At the receiver, a PIN diode captures the photons and coverts them into electrical signals.
Avionics is a place where RF-over-fiber is essential in a system that needs light weight and to be immune to EMI.