Optical Fiber & Connectors

Cable

Light guided down hair-thin glass by total internal reflection — the backbone of the internet and the Optical Communication unit. Numerical aperture, attenuation in dB/km, and splice loss: the syllabus terms all live in this cable.

What it looks like

A typical cable — exact shape, colour and markings vary between manufacturers.

Types & variants

Single-mode (9 µm core — long haul, telecom)Multimode (50/62.5 µm core — short links, easier launch)Step-index vs graded-indexPlastic optical fiber (POF — toys, TOSLINK audio)Armoured/aerial/duct outdoor cablesPatch cords & pigtails (pre-connectorised)

Key specs

Core/cladding diameter

µm

9/125 single-mode, 50/125 or 62.5/125 multimode — the numbers on every patch cord. Mismatched cores lose light at joints.

Attenuation

dB/km

The headline exam figure: ~0.35 dB/km at 1310 nm, ~0.2 dB/km at 1550 nm (why long-haul uses 1550), 2–3 dB/km for multimode at 850 nm, ~150 dB/km for plastic.

Numerical aperture

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NA = √(n₁²−n₂²) — the acceptance cone for launching light; the first numerical every student solves.

Bandwidth / dispersion

MHz·km / ps/nm·km

Modal dispersion limits multimode distance; chromatic dispersion limits single-mode — the reason graded-index and DFB lasers exist.

Connector type

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SC (square snap), LC (small, ubiquitous in switches), FC (screw, lab), ST (bayonet, legacy); each in UPC (blue) or APC (green, angled — don't mix) polish.

Bend radius

Tight bends leak light (macro-bending loss) — visible red light escaping at a kink is a live demonstration of the spec.

Markings

Jacket printing: fiber count, type (G.652D single-mode, OM3/OM4 multimode), and metre marks. Jacket colours: yellow = single-mode, orange = OM1/2, aqua = OM3/4. Connector boot colours follow polish (blue UPC/green APC).

Standard values

Home broadband (FTTH): G.657 bend-tolerant single-mode drop cable with SC/APC connectors into the ONT. Data centres: LC-LC OM3/OM4 patch cords. Lab kits: 1 m POF links with 650 nm LEDs.

How to choose

1) Distance rules: >500 m or telecom = single-mode; in-building/rack = multimode (cheaper optics); hobby/audio = POF. 2) Match connector AND polish to the equipment (APC-to-UPC mating damages both). 3) Outdoor runs: armoured or aerial-rated cable, spliced to indoor pigtails in a joint box. 4) Count a power budget: source dBm − (fiber dB/km × km) − splice/connector losses must exceed receiver sensitivity — the standard design numerical.

Pinout & package

Light in, light out — but joints matter: fusion splices (~0.05 dB, machine-made), mechanical splices (~0.2 dB), and connectors (~0.3 dB each). Patch panels, splice trays and SFP transceiver ports make up the physical layer.

Example circuits

FTTH drop: OLT → splitter → SC/APC drop fiber → ONT at home
Lab: LED/laser source + fiber spool + optical power meter measuring attenuation in dB
TOSLINK optical audio between TV and soundbar over POF
OTDR trace locating a JCB-cut fiber to the metre

Common failures

Dirty connector end-faces (the #1 cause of loss — one dust speck covers the whole 9 µm core), tight bends and staples crushing drop cables, broken fiber at connector boots, water in joint closures, and mismatched APC/UPC matings gouging ferrules.

How to test

Visual fault locator (red laser pen) lights up breaks and tight bends through the jacket. Power meter + source measures end-to-end dB loss against the budget. OTDR maps every splice, bend and break with distance. Always clean ferrules (dry wipes/click pens) before mating — and never look into an active fiber.

Substitutes

Cat6/copper substitutes below ~90 m; wireless links substitute where trenching is impossible. Within fiber, single-mode does anything multimode can (with pricier optics); the reverse is false. Media converters bridge fiber to RJ45 copper.

Where to buy

Amazon Robu LCSC