some thru the air use of lasers but mostly is optical fibers.
pulses of light in fiber of glass or plastic/polymer.
very high bandwidth, very low attenuation, EMI/noise/crosstalk immunity, small and lightweight.
does not radiate and is difficult to tap (without detection?), so security is good.
medium of choice for fixed high-speed digital communication:
feeder and trunk (longhaul) of telephone and CATV (HFC), LANs (Ethernet and FDDI), FTTH.
OC-n n*56.8Mbps optical carrier.
eg. OC-48 optical cable: 2.4Gbps per fiber. up to Tbps
electro-optical transmitter: LED (cheaper, shortwave 850nm, short distance) and semiconductor laser (longwave 1300 and 1550nm, higher speed and longer distances).
optical fiber: transparent core surrounded by reflective cladding of lower refractive index thus making tubular mirror confining most of the light in the core (total internal reflection). diameter 125um
singlemode SM fiber is narrow 8.3um, single wavelgnth laser light goes straight-thru for long distance.
multimode fiber: 50um or 62.5um, light bounces a lot, smearing signal, limiting bandwidth. stepped-index SI. graded-index GI reduces multimode dispersion. LED light source, multiple wavelengths, short distances, cheaper than SM, smaller bend radius.
"core/cladding": 50/125 62/125 etc.
fiber surrounded by plastic spacer then toughener like kevlar, then sheath.
Fiber modes

SC (squarish, push-pull) and ST (round, twsit-on twist-off), LC (flange on top), FC, MTRJ etc connectors. pair of fibers, one for Tx, one for Rx: full-duplex

OTDR: detect splices and problems.

disadvantages: fiber is many times more expensive than UTP. splicing together and connecting to other devices needs special equipment and training. fiber must be clean, polished, rounded, capped when not in use, don't bend it too much, don't touch it (get into body?), don't look at it. cannot carry power
submarine Fiber cable
PC-1 cable capacity: 640Gbps. lit: 180Gbps. length:20890km, started Jan 2001

Coax cable: center thick copper conductor D, insulator C, braided metal foil shield/shell/mesh/sleeve B, cable PVC/teflon sheath A.

Compared to twisted pair: higher bandwith (eg. 100 TV channels or 10,000 voice channels), lower attenuation, more EMI and crosstalk resistant. Was used for long distance telephone. Was used in 10Base5, 10Base2 but heavy, inflexible and costly compared to twisted pair. CATV and cable modem Internet access use.

limited to indoors one-room line-of-sight short-range. PAN uses.
IrDA standards.

narrow beam. weather-dependent.