Introduction to Fiber Optic Cables: Multimode and Single Mode
Fiber optic cables are crucial for modern communications infrastructure—from wireless backhaul and DAS deployments to data centers. These cables transmit digital information by encoding data as light pulses, where light on = “1” and light off = “0”, traveling down a glass core by internal reflection.
Light Transmission Principles
The core of a fiber acts like a conduit, guiding light with minimal loss. Since the signal travels via reflection, no power is needed once the light is launched—only lasers or LEDs at each end. The farther the light travels, the signal weakens and may eventually require regeneration.
Core Size and Its Impact
Multimode Fiber: Core diameters of 50 µm or 62.5 µm allow multiple light paths or “modes.”
Single Mode Fiber: A much smaller core (~8–10 µm) permits only a single propagation path.
Smaller cores result in more focused light paths, reducing dispersion and allowing for longer distance transmission.
Wavelengths in Play
Multimode typically uses 850 nm and 1300 nm wavelengths.
Single mode operates at 1310 nm and 1550 nm.
Different wavelengths affect reach and performance, much like different RF bands carry signals uniquely.
Multimode Fiber: OM1–OM5
ISO/IEC 11801 classifies multimode fiber using “OM” grades:
| Type | Core Ø | Wavelengths | Speed | Max Reach | Typical Use |
|---|---|---|---|---|---|
| OM1 | 62.5 µm | 850 nm | 1 Gbps | ≈300 m | Basic LAN/backbone |
| OM2 | 50 µm | 850 nm | 1 Gbps | ≈600 m | LANs |
| OM3 | 50 µm | 850 nm | 10 Gbps | ≈300 m; 40/100 Gbps ≈100 m | Data centers |
| OM4 | 50 µm | 850 nm | 10 Gbps | ≈550 m; 100 Gbps ≈150 m | High‑speed campus/backbone |
| OM5 | 50 µm | 850–953 nm | 40/100 Gbps | Enhanced reach | SWDM & future data centers |
Multimode fiber is cost-effective, easy to install, and ideal for short-to-medium distances. Advances like OM3, OM4, and OM5 help it support ever-higher data rates with improved modal dispersion.
Single Mode Fiber: OS1 & OS2
Single mode fiber supports a single light mode. The two main types are:
OS1: Tight‑buffered, designed for indoor use, supports ≤ 10 Gbps up to ~10 km.
OS2: Loose‑tube, outdoor-rated, capable of ≥ 100 Gbps over ~200 km.
Both utilize 9 µm cores, colored typically yellow, and use lasers at 1310/1550 nm.
Single mode fibers have very low loss (0.2–0.4 dB/km at 1550 nm), enabling long-haul transmission and support for WDM systems.
Comparing the Two: Multimode and Single Mode
| Feature | Multimode (MMF) | Single Mode (SMF) |
|---|---|---|
| Core Diameter | 50–62.5 µm | 8–10 µm |
| Light Source | LED/VCSEL | Laser (FP, DFB) |
| Reach | Hundreds of meters | Dozens to hundreds of kilometers |
| Bandwidth | Medium–high | Very high |
| Cost | Lower optics & cables | Higher upfront |
| Use Case | Offices, data centers | Telcos, backhaul, metro |
| Dispersion | Modal dispersion limits reach | Minimal modal dispersion |
Practical Guidance for Choosing Fiber
Short range, cost-driven installs: Use OM3/OM4 multimode fiber—easier to terminate, may require cheaper transceivers.
Long-distance or high-bandwidth links: Choose single mode, OS2 fiber—supports high-speed laser equipment and future-proofs networks.
Summary
Understanding Multimode and Single Mode fiber is essential when designing networks:
Multimode fiber (OM1–OM5) works well for distances from a few meters to ~550 m and uses LEDs or VCSEL lasers.
Single mode fiber (OS1/OS2) with its ~9‑µm core enables laser-based transmission over kilometers and integrates seamlessly with WDM systems.
Selecting the right fiber depends on distance, bandwidth, and budget—ensuring optimal performance and scalability, whether within buildings or across continents.
