A fiber optic cable is a strand or bundle of very thin, transparent fibers (usually glass, sometimes plastic) that transmit data as pulses of light instead of electrical signals. Each fiber has a high-index core surrounded by a lower-index cladding, coated by protective buffers and an outer jacket for strength and environmental protection.

It works by guiding light through the core via total internal reflection: because the core’s refractive index is higher than the cladding’s, light entering at acceptable angles reflects along the core, even around bends, with minimal loss. Light is generated by LEDs or laser diodes at specific wavelengths (commonly 850, 1310, or 1550 nm). At the far end, photodiodes convert the light back into electrical signals.

There are two main types:

• Multimode fiber (larger core, e.g., 50/62.5 µm): carries multiple light modes, suitable for shorter distances (up to a few kilometers) at high speeds.
• Single-mode fiber (small core, ~9 µm): carries a single mode, enabling very long distances (tens to hundreds of kilometers) and highest bandwidth.

Advantages include extremely high data rates, long reach, low attenuation, immunity to electromagnetic interference, electrical isolation, and difficulty to tap without detection. Limitations include sensitivity to tight bends (macrobend/microbend losses), more precise installation/splicing, and higher transceiver cost. Applications span internet backbones, data centers, FTTH, undersea cables, medical imaging (endoscopy), industrial sensing, and military/aerospace systems.

Single-mode (SMF)

• Core/cladding: ~8–10 µm core, 125 µm cladding; carries a single light mode.
• Wavelengths: 1310 nm and 1550 nm (also 1625 nm).
• Light sources: Lasers (DFB, CWDM/DWDM).
• Dispersion: Minimal modal dispersion; chromatic and polarization-mode dispersion dominate at long haul.
• Bandwidth/distance: Virtually unlimited bandwidth; tens to thousands of km with amplifiers.
• Types: OS1 (tight-buffer, indoor; up to ~10 km), OS2 (loose-tube, outdoor/low-water-peak; long-haul/metro).
• Use cases: Telecom, metro/WAN, long campus runs, 5G fronthaul/backhaul.
• Cost: Fiber inexpensive; transceivers typically higher cost per port.

Multimode (MMF)

• Core/cladding: 50 or 62.5 µm core, 125 µm cladding; many modes.
• Wavelengths: 850 nm (primary), 1300 nm (legacy).
• Light sources: VCSELs or LEDs.
• Dispersion: Significant modal dispersion; limits reach at high speeds.
• Bandwidth/distance (typical with 850 nm VCSELs):
  • OM1 (62.5/125): legacy; up to ~33 m at 10G.
  • OM2 (50/125): legacy; up to ~82 m at 10G.
  • OM3 (50/125, 2000 MHz·km): up to 300 m at 10G, 70–100 m at 40/100G (SR4).
  • OM4 (50/125, 4700 MHz·km): up to 400 m at 10G, 150 m at 40/100G.
  • OM5 (wideband MMF): similar to OM4 but supports SWDM across 850–953 nm; 100/150 m gains with SWDM.
• Use cases: Data centers, short campus links, patching—cost-effective for short reach.
• Cost: Fiber and optics cheaper for short distances; higher count needed at very high speeds (parallel optics).

Common notes

• Connectors and cladding diameter (125 µm) are standardized.
• MMF is more tolerant of alignment; SMF offers superior reach and future-proofing.

• Bandwidth and speed: Fiber offers vastly higher bandwidth and supports multi‑gigabit to terabit rates. Copper (twisted pair) tops out practically at 1–10 Gbps over typical runs, with higher speeds requiring short, high‑grade cabling.
• Distance and attenuation: Fiber transmits over kilometers with low loss (tens to hundreds of km with amplification). Copper Ethernet is limited to ~100 m per segment without repeaters.
• Interference and noise: Fiber is immune to electromagnetic interference, crosstalk, and radio frequency noise; copper is susceptible and may require shielding.
• Security: Fiber is difficult to tap without detection; copper can leak signals and is easier to intercept.
• Latency: Propagation in fiber is slightly slower than in copper (glass vs. electrical), but overall latency is typically lower due to fewer repeaters and higher throughput.
• Reliability and environment: Fiber is not affected by lightning or ground loops and is ideal for industrial/noisy environments. Copper can conduct surges and needs proper grounding.
• Size, weight, and density: Fiber cables are lighter, thinner, and allow higher port densities than copper.
• Scalability: Fiber supports wavelength‑division multiplexing to multiply capacity on a single strand; copper lacks an equivalent.
• Power delivery: Copper can deliver PoE/PoE+; fiber cannot carry power (needs separate power or hybrid cable).
• Installation and handling: Copper is more forgiving, cheaper terminations, and easier for short runs. Fiber requires careful handling, cleaning, bend‑radius management, and specialized tools.
• Cost: Fiber transceivers and installation can be higher upfront, but for high bandwidth/long distance, total cost per bit per meter favors fiber. Copper is cost‑effective for short, low‑to‑moderate speed links.
• Use cases: Fiber for backbones, data centers, long‑haul, and EMI‑heavy sites. Copper for desktop runs, short patching, and PoE devices.

• Access (to homes/businesses, passive optical networks):

  • GPON: 2.5 Gbps down / 1.25 Gbps up (shared across splits 1:32–1:128).
  • XG-PON: 10 Gbps down / 2.5 Gbps up.
  • XGS-PON: 10 Gbps symmetric.
  • 10G-EPON: 10 Gbps symmetric.
  • 25G PON: 25 Gbps symmetric (emerging deployments).
  • 50G PON: 50 Gbps (asym/sym options; standard finalized, pilots underway).
  • NG-PON2 (TWDM-PON): up to ~40 Gbps aggregate via multiple 10G wavelengths.

• Enterprise/datacenter:

  • Multimode (OM3/OM4/OM5): 10/25/40/100 Gbps up to a few hundred meters; 200/400 G short-reach with parallel fibers.
  • Single-mode: 10/25/50/100/200/400/800 Gbps links; 1.6 Tbps emerging. Reach from 2–10 km (LR) to 40 km (ER/ZR), 80 km (ZR+), 120+ km with amplification.

• Metro/core/backbone (coherent DWDM on single-mode):

  • Per-wavelength: 100/200/400/600/800 Gbps; 1.2–1.6 Tbps trials.
  • Channels per fiber: typically 80–120 in C-band; 160–200 with C+L.
  • Aggregate per fiber: ~8–96 Tbps common; 100+ Tbps with C+L and high-order modulation.
  • Reach: 500–3,000+ km per span; transoceanic with repeaters.

• Subsea/ultra-long-haul:

  • Per fiber pair: tens to 100+ Tbps using many wavelengths and space-division multiplexing (many fiber pairs per cable).

• Physical bandwidth of silica fiber:

  • Usable optical bandwidth per band: C-band ~4.4 THz; C+L ~9 THz; broader with S-band.
  • Ultimate data rate limited by signal-to-noise, nonlinearities, and distance (Shannon limit), not by the glass itself.

• Practical user speeds today:

  • Consumer FTTH: 1–10 Gbps typical; symmetrical 10G increasingly available.
  • Near-term upgrades: 25G–50G to premises and multi-100G to nodes/backhaul.

It depends on fiber type, bitrate, wavelength, launch power, receiver sensitivity, dispersion, and whether “repeaters” excludes any in‑line optical amplifiers.

• Multimode (OM1–OM5), no in‑line devices:

  • 1G: ~300–550 m (OM2/OM3), up to ~2 km with legacy 1000BASE‑LX on 62.5 µm.
  • 10G: ~70–400 m (OM2–OM4); OM5 similar for SWDM.
  • 40/100G SR: ~100–150 m (OM4/OM5).

• Single‑mode, no in‑line devices (transceiver power budget only):

  • “IR/LM” optics (LR/LR4): ~10 km.
  • “ER/ER4”: ~30–40 km.
  • “ZR/ZR+”: ~70–100 km (some up to ~120 km with low‑loss fiber and good margins).
  • Coherent pluggables (no in‑line amps): ~80–200 km depending on baud rate/FEC.

• Power‑budget view (typical 1550 nm fiber at ~0.20 dB/km):

  • 25–30 dB budget → ~125–150 km theoretical; after margins, splices, aging, chromatic dispersion and nonlinear penalties → ~80–120 km practical.

• Unrepeatered long‑haul (no in‑line repeaters/regenerators, but using high launch power and end‑station EDFAs/Raman): commercially ~300–500+ km; demonstrated records ~600–700 km on ultra‑low‑loss fiber with optimized spans. These are still called “unrepeatered” because there are no in‑line devices, only terminal amplification.

Rule of thumb:

• Multimode: ≤2 km.
• Standard single‑mode enterprise/metro: 10–40 km.
• High‑budget/ZR-class: 70–120 km.
• Coherent without in‑line amps: up to ~200 km.
• Unrepeatered long‑haul with terminal amplification: ~300–500+ km.

Advantages:

• Extremely high bandwidth and data rates; supports long-distance, high-capacity links.
• Very low attenuation; fewer repeaters over long spans, stable performance.
• Immune to electromagnetic interference and crosstalk; excellent signal integrity.
• Enhanced security; difficult to tap without detection and no RF emission.
• Lightweight, small diameter; higher cable density in ducts and racks.
• Electrically nonconductive (dielectric); safe in explosive or high-voltage environments.
• Scalable via WDM (dense/coarse) to multiply capacity over a single pair.
• Corrosion resistant; suitable for harsh or underwater deployments.
• Future-proof path for upgrades by changing optics rather than cable.

Disadvantages:

• Higher upfront costs for transceivers, splicing, test gear, and installers.
• Fragile compared to copper; sensitive to bend radius, pulling tension, micro/macro-bending losses.
• Installation and repair require skilled labor, precise terminations, and cleanliness; contamination at connectors degrades performance.
• Cannot carry power; no PoE—requires separate power or media converters.
• Optical-electrical conversion at endpoints adds complexity and potential points of failure.
• Dispersion and nonlinear effects at high speeds/powers require careful design (e.g., dispersion compensation).
• Field troubleshooting can be harder; needs tools like OTDR and inspection scopes.
• Temperature and mechanical stress can impact performance; protective routing and enclosures needed.
• For very short, low-speed, low-cost runs, copper can be cheaper and simpler.

• Planning and pathway prep: survey route; choose cable type (SM/MM), fiber count, OFNR/OFP; verify bend radius and pull tension limits; install conduit/innerduct or aerial hardware; pull or blow cable with lubricant, swivels, and breakaway links; leave service loops; label and document.
• Cable prep: secure to patch panel/closure, remove jacket and strength members, install glands/strain relief; maintain min bend radius.
• Termination (connectors):
  • Options: epoxy/polish (UPC/APC), anaerobic, pre-polished mechanical, or fusion splice-on connectors.
  • Steps: strip buffer/coating, clean with lint-free wipes and IPA, precision cleave, attach connector per system, cure if epoxy, polish to spec (UPC ~ -50 dB, APC ~ -60 dB reflectance), inspect endface (IEC 61300-3-35), test loss; protect and cable-manage.
• Splicing:
  • Fusion splicing (preferred): strip, clean, 8–10 mm cleave, align (core/clad), arc fuse, apply heat-shrink splice protector, store in splice tray; typical loss ≤0.05–0.1 dB.
  • Mechanical splicing: index-matching gel alignment; faster but higher loss/reflectance; use for temporary or mixed environments.
• Enclosures: use rated closures for outside plant; seal, desiccant if needed, organize trays, label fibers.
• Testing and inspection:
  • Endface: “inspect-clean-inspect” with 200–400x scope; meet IEC zones.
  • Insertion loss/length: OLTS at proper wavelengths (MM: 850/1300 nm; SM: 1310/1550 nm), reference method (1/2/3-jumper); verify polarity.
  • Reflectance and event location: OTDR (SM: 1310/1550/1625 nm; MM: 850/1300 nm); check splices, connectors, macro-bends.
  • Visual fault locator for near-end breaks and polarity.
  • Acceptance: compare to link budget/TIA limits; document traces, losses, labels, as-builts; remedy high-loss or reflectance events and retest.