0.2 Fiber Optic Cables

Fiber optic cables transmit data as pulses of light through a glass or plastic core. Fiber cables offer immunity to EMI, support longer distances than copper, and serve as the standard choice for backbone links, inter-building runs, and environments with severe electrical interference.

How Fiber Works

Light travels through the core (high refractive-index glass). Total internal reflection off the cladding (lower refractive-index glass) keeps the light inside the core. A coating (acrylate buffer) surrounds the cladding, and a jacket forms the outer layer.

Cross-section of a fiber strand:
  ┌─────────────────────────────┐
  │  Jacket (outer protection)  │
  │  ┌───────────────────────┐  │
  │  │  Cladding (125 µm)    │  │
  │  │  ┌─────────────────┐  │  │
  │  │  │  Core (8–62.5µm)│  │  │
  │  │  └─────────────────┘  │  │
  │  └───────────────────────┘  │
  └─────────────────────────────┘

Single-Mode vs Multimode

The core diameter determines whether the fiber operates as single-mode or multimode.

Single-Mode Fiber (SMF)

• Core diameter: 8–10 µm (narrow enough for only 1 light mode to propagate)
• Cladding: 125 µm
• Zero modal dispersion — light travels in a single path
• Supports distances up to 80 km (and beyond with amplifiers)
• Requires a laser light source (higher-cost transceivers)
• Standard: OS1 (tight-buffered, indoor) and OS2 (loose-tube, outdoor/long-haul)

Multimode Fiber (MMF)

• Core diameter: 50 µm or 62.5 µm
• Multiple light modes travel simultaneously at slightly different angles
• Modal dispersion limits distance and bandwidth
• Uses lower-cost LED or VCSEL light sources
• Suitable for campus and intra-building backbone (up to ~550 m at 10G)

Multimode OM Grades

Grade	Core	Bandwidth (850 nm)	Max Distance at 10G	Max Distance at 100G
OM1	62.5 µm	200 MHz·km	33 m	Not supported
OM2	50 µm	500 MHz·km	82 m	Not supported
OM3	50 µm	2000 MHz·km	300 m	100 m
OM4	50 µm	4700 MHz·km	400 m	150 m
OM5	50 µm	28000 MHz·km	400 m	400 m (SWDM4)

Note

OM1 and OM2 are legacy grades. Use OM3 minimum for new installations. OM4 suits 10G+ links. OM5 supports short-wavelength division multiplexing (SWDM) for 40G/100G over a single fiber pair.

Fiber Connector Types

Connector	Form Factor	Common Use
LC	Small form	SFP transceivers, data centers, most modern installs
SC	Square push-pull	Older installs, some industrial switches
ST	Bayonet twist-lock	Legacy; still found in older OT sites
FC	Threaded	Test equipment, high-vibration environments
MPO/MTP	Multi-fiber (12 or 24)	High-density data centers, 40G/100G
E2000	Spring-loaded shutter	Telecom, some industrial

Note

LC dominates as the connector for SFP-based switches. When patching fiber in an industrial cabinet, use duplex LC (2 fibers in 1 clip) to eliminate accidental TX and RX swapping.

Polish Types

The end-face of a fiber connector receives polishing to minimize back-reflection:

Polish	Return Loss	Use Case
PC (Physical Contact)	≥ 40 dB	Multimode, general use
UPC (Ultra PC)	≥ 50 dB	Single-mode, data networks
APC (Angled PC, 8°)	≥ 60 dB	Long-haul, CATV, high-sensitivity

APC connectors are green. UPC connectors are blue. Never mate APC to UPC — mating APC to UPC damages both connectors and causes high insertion loss.

Optical Loss Budget

Every fiber link has a loss budget — the maximum attenuation the transceiver tolerates. Verify that the link loss stays within budget.

Sources of loss:

• Fiber attenuation: ~0.35 dB/km (SMF at 1310 nm), ~0.2 dB/km (SMF at 1550 nm), ~3.5 dB/km (MMF at 850 nm)
• Connectors: ~0.3–0.5 dB per mated pair
• Splices: ~0.1 dB (fusion), ~0.5 dB (mechanical)
• Bends: maintain the minimum bend radius (10× cable diameter)

Example calculation — 500 m OM4 link at 10G:

Fiber loss:    0.5 km × 3.5 dB/km = 1.75 dB
4 connectors:  4 × 0.3 dB         = 1.20 dB
2 splices:     2 × 0.1 dB         = 0.20 dB
─────────────────────────────────────────────
Total:                               3.15 dB

SFP+ 10GBase-SR budget: typically 7.3 dB → PASS ✓

Note

Add a 3 dB safety margin to the calculated loss. The margin accounts for connector aging, future splices, and measurement uncertainty.

Wavelengths and Windows

Window	Wavelength	Fiber Type	Typical Use
1st	850 nm	MMF	Short-range (SX, SR)
2nd	1310 nm	SMF	Medium-range (LX, LR)
3rd	1550 nm	SMF	Long-range (ZX, ER, ZR)
CWDM	1270–1610 nm	SMF	Wavelength multiplexing

BiDi (Bidirectional) Fiber

Standard fiber links use 2 strands — 1 for TX, 1 for RX. BiDi transceivers use a single strand with 2 different wavelengths (e.g., 1310 nm TX / 1550 nm RX). Deploy BiDi transceivers in matched pairs — the TX wavelength of 1 transceiver matches the RX wavelength of the other.

Fiber in Industrial Environments

• Armored fiber (steel or Kevlar reinforcement) for direct burial or areas with rodent exposure
• Tight-buffered cables for indoor runs. Loose-tube cables for outdoor and long-haul runs.
• Industrial-grade patch panels with IP-rated enclosures for factory floors
• Fiber serves as the preferred medium for substation automation (IEC 61850) due to galvanic isolation — zero ground loops between HV equipment and control systems
• In explosive atmospheres, fiber is inherently safe (zero electrical spark exposure)
• Respect the minimum bend radius during installation — kinking fiber causes permanent loss increase

Cleaning Fiber Connectors

Dirty connectors are the #1 cause of fiber link issues. Follow these steps:

1. Inspect with a fiber scope before mating.
2. Clean with a one-click cleaner or lint-free IPA wipe.
3. Keep bare fingers away from the end-face.
4. Cap unused connectors with dust caps.