How Fiber Optic Cables Transmit Data at Near-Light Speed

A Single Fiber Strand Thinner Than Human Hair Can Carry 1.84 Petabits Per Second

In 2022, researchers at Japan's National Institute of Information and Communications Technology transmitted 1.84 petabits per second — equivalent to streaming 460 million HD videos simultaneously — through a single optical fiber strand 0.125 mm in diameter. Commercial fiber networks operate at a fraction of this theoretical maximum, but the principle holds: glass fibers carrying pulses of light have made the modern internet possible in a way copper wire never could.

Optical fiber is a hair-thin strand of ultra-pure glass (or plastic) that guides light along its length through a physical phenomenon called total internal reflection. The concept is simple. The engineering required to manufacture fibers pure enough and thin enough to maintain signal integrity across 10,000 kilometers of ocean floor is not.

Total Internal Reflection: Why Light Stays Inside the Glass

Light changes speed when it crosses between materials. Snell's Law describes this: n₁ sin θ₁ = n₂ sin θ₂, where n₁ and n₂ are the refractive indices of the two materials and θ₁ and θ₂ are the angles of incidence and refraction.

When light travels from a denser medium (like glass, n ≈ 1.46) into a less dense one (like air, n = 1.00), and the angle of incidence exceeds a critical angle, refraction becomes impossible. All light reflects back into the denser medium. No energy escapes.

The critical angle θ_c = arcsin(n₂/n₁). For glass-air, this is approximately 43°. Light entering the fiber within the acceptance cone bounces at angles exceeding 43° and propagates along the fiber essentially without loss at the interface.

Optical fibers exploit this by using two concentric layers of glass: a core with higher refractive index surrounded by a cladding with lower refractive index. Total internal reflection at the core-cladding interface traps light in the core.

Fiber Architecture: Single-Mode vs. Multi-Mode

Single-mode fiber uses an 8–10 µm core — so narrow that only one light mode (traveling straight down the axis) can propagate. This eliminates modal dispersion — the spreading of pulses caused by different modes arriving at different times. Single-mode fiber is the backbone of global telecommunications.

Multi-mode fiber's wider core allows easier coupling of light and uses lower-cost vertical-cavity surface-emitting lasers (VCSELs), but modal dispersion limits its reach. It dominates within data centers and buildings.

The Signal: Encoding Data as Light Pulses

Data is encoded by switching light sources on and off (on-off keying) or using more sophisticated modulation schemes. Modern coherent optical systems use quadrature amplitude modulation (QAM) — encoding information in both the amplitude and phase of light waves, just as advanced radio modulations encode information on radio waves.

• 10G PON: Basic passive optical networks delivering 10 Gbps to residential users
• 100G coherent: Used on long-haul terrestrial routes; 100 Gbps per wavelength using DP-QPSK modulation
• 400G ZR: Metro/regional routes using 16-QAM modulation, 400 Gbps per wavelength
• 800G and 1.2T: Emerging standards for next-generation backbone networks

Wavelength Division Multiplexing: Many Colors, One Fiber

Dense Wavelength Division Multiplexing (DWDM) is the technology that makes modern fiber capacity possible. Different wavelengths of infrared light (each a different "color") travel simultaneously in the same fiber without interfering. Each wavelength carries an independent data stream.

DWDM systems use wavelengths spaced 0.8 nm apart in the C-band (1,530–1,565 nm) and L-band (1,565–1,625 nm). Commercial DWDM systems deploy 80–160 wavelengths per fiber. With each wavelength carrying 400 Gbps, a single fiber pair carries 32–64 Tbps of total capacity — enough to serve tens of millions of broadband users simultaneously.

Optical Amplifiers: Sustaining Signal Across Oceans

Signal strength weakens as light travels through fiber due to scattering (Rayleigh scattering from microscopic density variations) and absorption. Modern single-mode fiber has attenuation of approximately 0.2 dB/km at 1,550 nm wavelength. Over 10,000 km of submarine cable, total attenuation would be 2,000 dB — the signal would vanish entirely without amplification.

Erbium-doped fiber amplifiers (EDFAs) solve this problem without converting light back to electricity. A short section of fiber doped with erbium ions is pumped with 980 nm laser light. Incoming signal photons stimulate the excited erbium ions to emit identical photons — pure optical amplification. EDFAs are spaced every 50–100 km along submarine cable routes, maintaining signal integrity across ocean floors with no active electronics at depth.

Manufacturing Ultra-Pure Fiber

Telecom-grade silica fiber must be among the purest glass objects ever made. Impurities at the parts-per-billion level cause excessive absorption. Fiber is made by chemical vapor deposition (CVD): silicon tetrachloride (SiCl₄) and germanium tetrachloride (GeCl₄) are oxidized to create ultra-pure silica and germanium oxide layers in a preform rod. The preform is then drawn into fiber at 2,000°C in a draw tower, stretching from a 100 mm diameter preform to 125 µm fiber at speeds of 1,500–2,500 meters per minute.

Submarine fiber cables add to the fiber bundle: aluminum foil water barrier, high-strength steel wires, copper power conductor (supplying 10,000 volts DC to power the EDFA repeaters), polyethylene jacket, and armoring near shore where ships' anchors and fishing equipment pose the primary failure risk. Most cable failures — over 70% — are caused by human activity in shallow water, not by spontaneous failure of the cable itself.