Chapter 41: Fiber Optics and Global Telephony

Era span: 1966 Kao proposal → present · Difficulty: high
Requires: Ch 17 glass, Ch 34/35 photonics/electronics, Ch 26 power ·
Unlocks: Ch 42's physical layer, global real-time civilization

Copper carries bandwidth; glass carries BANDWIDTH. Fiber moved humanity from scarce, expensive communication to ambient, effectively-free communication — the substrate the internet assumes. This chapter builds light into a utility.

41.1 Why Glass

Total internal reflection traps light inside a core whose index exceeds its cladding — the principle of optical fiber. But ordinary glass absorbs catastrophically (~dB per METER): every meter steals signal until nothing arrives. Kao & Hockham (1966) computed that IF attenuation fell below ~20 dB/km, communication-grade fiber was possible; impurities (iron, copper ions, water's OH) were the enemy, not physics.

Corning delivered <20 dB/km in 1970 via vapor-deposition purity; modern transmission fiber runs ~0.15–0.2 dB/kM near 1550 nm — half the light survives ~15–20 km. Purity culture (Ch 35's discipline applied to silica) is the whole ballgame.

41.2 Fiber Types

• Step-index multimode: fat core, simple/cheap sources, modal dispersion smears pulses over distance — buildings/campus only.
• Graded-index: parabolic index profile equalizes path speeds — legacy LAN backbone.
• Single-mode: tiny core (~9 μm); one path = no modal dispersion; chromatic dispersion managed by dispersion-shifted designs/compensation. THE long-haul standard.
• Windows: 1310 nm (zero dispersion) and 1550 nm (lowest loss) — erbium-doped fiber amplifiers live at 1550.

41.3 Sources and Amplifiers

• Laser diodes (edge-emitting, then DFB single-frequency) inject light efficiently into single-mode cores; direct modulation gave way to external modulators at speed.
• EDFA (1987): pump a spool of erbium-doped fiber with 980/1480 nm lasers and it amplifies passing 1550-nm signals OPTICALLY — no conversion to electricity mid-ocean. Transoceanic systems became economic overnight.
• WDM: dozens-to-hundreds of closely-spaced wavelengths share one fiber (prism-like mux/demux splitting colors at endpoints); coherent detection + DSP later packed terabits per fiber pair. Capacity growth outpaced Moore's law for stretches — spectrum is the new acreage.

41.4 Submarine Cables

~500+ cables carry >95 % of intercontinental data (satellites are rounding errors for bulk traffic):

• Repeatered design: amplifier housings every ~60–80 km fed by constant-current copper in the cable itself.
• Armor where anchors/sharks bite (yes — shark bites are a logged maintenance category), burial near shore, deep-water sections nearly bare.
• Cable ships splice repairs at sea; fault location via OTDR backscattering math. Landing stations are chokepoints — route diversity is resilience doctrine (Ch 47).

41.5 Terrestrial Build-Out and Access

Backbone rings → metro rings → access. The last mile history: dial-up modems (audio-band cleverness) → DSL (reusing phone copper at MHz) → cable DOCSIS → FTTH (GPON sharing one fiber across 32–128 homes passively). Each generation re-litigated the same economics: construction labor dominates; dig once, pull more fiber later.

41.6 Telephony's Switching Evolution

The circuit side matured in parallel:

• Manual boards → Strowger step-by-step automatic switching (Almon Strowger, patents 1889–91; the popular story attributes the invention to an undertaker who suspected operators were diverting his calls to a rival — likely embellished, but the patents and their automation consequences are solid).
• Digital PCM: voice sampled 8 kHz, encoded 64 kbps; T-carrier/E-carrier trunks multiplexed conversations; SS7 separated signaling from speech (call setup as data).
• Mobile generations rode semiconductor progress (Ch 35): 1G analog FM → 2G GSM digital + SMS → 3G packet data → 4G LTE all-IP → 5G massive MIMO/beamforming. Cell concept: reuse frequencies spatially; handoff keeps calls alive at walking/driving speed.
• VoIP collapsed circuit/packet distinction entirely — voice became an application.

Dead end avoided: building parallel dedicated networks per service (voice/video/data). Converged IP infrastructure won on economics; plan convergence from the start.

Key threshold: when international bandwidth costs fall below the price of postage-equivalent value exchange, global real-time collaboration becomes default — remote work, telemedicine, distributed science all become ordinary rather than exotic. That threshold crossed circa the 2000s fiber glut; the overbuild was civilization's bargain of the century (Ch 42 inherits it directly).

41.7 The Fiber Papers

• Light-guiding-by-water-jet demos trace to Colladon (Geneva, 1840s) with Tyndall's Royal Institution version (1870) making it famous — total internal reflection entertained audiences for a century before communication used it.
• Maiman's ruby laser (Hughes, May 16, 1960) supplied the carrier source; glass remained the bottleneck until Kao & Hockham's 1966 calculation reframed attenuation as impurity removal. Corning's team (Maurer, Keck, Schultz) delivered ~16.8 dB/km titanium-doped silica fiber in April 1970, then pushed lower — the purity-culture chapter promise kept within four years.
• First operational systems: AT&T's Chicago downtown trial (1976) ran 45 Mbps under city streets; TAT-8 (1988) crossed the Atlantic at 280 Mbps using electronic repeaters; erbium-doped amplifiers (Southampton and Bell Labs papers, 1987) removed repeaters' electronics from the following cable generations — capacity then compounded via wavelength division.
• Submarine maintenance reality, from repair logs: anchors and trawls account for most faults; shark bites are documented but marginal; fault location runs on OTDR backscatter arithmetic (§41.4).
• Historians of bandwidth note the pattern: each overbuild (fiber glut circa 2001, dark fiber resold cheaply) preceded the next usage explosion — video streaming consumed the glut the bust left behind (Ch 42 inherits the lesson: capacity arrives before demand proves it).