Optical
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
OS: singlemode SM fiber is narrow 8.3um, single wavelength laser light goes straight-thru for long distance.
OM: 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": 8/125 50/125 62/125 etc.
Fiber surrounded by plastic spacer then toughener like kevlar, then jacket/sheath.
Fiber modes

Connectors: SC (squarish, push-pull) and ST (round, twist-on twist-off), LC (flange on top), FC, MT-RJ, MU, CS, etc
MPO, MTP multi fibers (e.g. 12) connector.

pair of fibers, one for Tx, one for Rx: full-duplex
288-fiber cable breakout to 24 MPI/MPO cables each breakout to 12 LC connectors.
Light loss at connector. Reflection backward.

OM1: orange, 1Gbps, 62.5um, 300m .
OM3: aqua, 10Gbps, 50um, 300m, (40 or 100Gbps w/MPO connector &100m)
OM4: 550m, (100Gbps w/MPO &150m)
Above have 3.5 dB loss in 1 km.
OM4+: (100Gbps w/MPO &300m)
OM5: lime green, 40 and 100Gbps, SWDM

OS1: yellow, indoor, 2km, $, 1dB/km attenutation
OS2: yellow, outdoor, 10km, $$$, .4dB/km attenutation

Different ferrule (end) polish types: UPC, APC, PC. (PC and UPC: 0 angle, APC: 8°). Must match.
Cleaning the ends: fluids and wipes, pen type. Microscope to inspect.
Tool to measure the amount of light loss in the cable. Distortions (dispersion and jitter).

Transceiver: SFP+ (10Gbps), QSFP+ (4x=40Gbps). Modular. converts between optical and electronic at the port.
Cross over cable.

BiDi: traffic in both directions using different wavelengths

OTDR: detect splices and problems.

Disadvantages: fiber is 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 (bend radius), 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
Broadcom Stingray 100Gbps

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

Laser
narrow beam. weather-dependent.
laser