Fiber characterization
This domain tests characterization of fiber for high-capacity or long-haul systems, where insertion loss alone is not enough. The exam expects familiarity with chromatic dispersion (CD), polarization mode dispersion (PMD), optical return loss (ORL), spectral attenuation, and the units and limits used to judge a link.
What is measured
CD is pulse broadening versus wavelength, expressed in ps/(nm·km) for fiber and ps/nm for a link; link CD is coefficient × length. PMD is a statistical differential group delay, expressed in ps and related to a fiber PMD coefficient in ps/√km; link PMD scales with the square root of length. ORL is the ratio of reflected to incident power, expressed as a positive dB value in common link specifications. Spectral attenuation shows loss versus wavelength and is used to verify fiber and cable performance across a band.
Worked example
A 40 km single-mode route has CD coefficient 17 ps/(nm·km) at 1550 nm. Link CD = 17 × 40 = 680 ps/nm. For a 10G NRZ system with roughly 800 ps/nm chromatic-dispersion tolerance, the link passes with about 120 ps/nm margin. If the PMD coefficient is 0.1 ps/√km, link PMD is approximately 0.1 × √40 = 0.63 ps; this is well inside a typical 10G PMD limit of about 10 ps. If the route were upgraded to 40G, both margins would need re-evaluation against the new system limits.
Traps
- Using an OTDR or OLTS as if it measured CD or PMD.
- Confusing the fiber coefficient with the link value; length scaling differs for CD and PMD.
- Reporting PMD as a simple linear sum instead of a statistical √length relationship.
- Reversing the sign or meaning of ORL; follow the specification’s sign convention.
- Applying 10G limits to a 40G or 100G design without checking the new tolerance.