Advanced Fiber Types and Optical Transmission
Domain 2 tests whether you can select the right fiber for a route and predict transmission limits before installation. It covers ISO/IEC fiber categories, single-mode variants, and the impairments that set reach.
Fiber categories and variants
- Multimode categories: OM1 (62.5/125 µm), OM2 (50/125 µm), OM3 and OM4 (laser-optimized 50/125 µm), and OM5 (wideband 50/125 µm for short-wavelength division multiplexing). Higher OM grades improve effective modal bandwidth, not attenuation.
- Single-mode: OS1/OS2 per ISO/IEC; ITU-T G.652.D is the standard low-water-peak fiber for 1310/1550 nm. G.657.A1/A2/B2/B3 are bend-insensitive, with tighter bend tolerance but not automatically lower attenuation. G.655 is non-zero dispersion-shifted for DWDM; G.654.E is large-area ultra-low-loss for long-haul.
- Transmission impairments: attenuation (dB/km), chromatic dispersion (ps/(nm·km)), polarization mode dispersion (PMD, ps/√km), and nonlinear effects such as self-phase modulation and four-wave mixing. OSNR limits amplified WDM systems.
Worked example
A 10 Gbps NRZ link uses 80 km of G.652 at 1550 nm with chromatic dispersion coefficient 17 ps/(nm·km) and source spectral width 0.1 nm. Accumulated dispersion = 17 × 80 × 0.1 = 136 ps. The bit period at 10 Gbps is 100 ps, so the dispersion exceeds one bit period. Reasoning: a simple directly modulated laser and direct detection would suffer severe intersymbol interference; the design needs dispersion compensation, a narrower-spectrum source, coherent DSP, or a shorter span. This is why 1310 nm (near-zero dispersion in G.652) is attractive for shorter unamplified links despite higher attenuation.
Traps that cost marks
- Assuming OM4 always reaches farther than OM3 at every speed; reach tables depend on wavelength, lane count, and connector loss.
- Treating G.657 bend-insensitive fiber as lower-loss fiber than G.652.
- Using a single dispersion number without source spectral width; CD penalty scales with both.
- Scaling PMD linearly with length instead of with √length.
- Confusing CWDM channel spacing (20 nm) with DWDM grid spacing (50 or 100 GHz).