Fiber Optic Testing: Instruments and Acceptance
Instruments
Know the tools and what each measures. The optical loss test set (OLTS: light source plus power meter) measures end-to-end insertion loss and is the primary acceptance test. The OTDR measures loss and distance to events, reflectance, and ORL, and locates breaks and macrobends. The visual fault locator (VFL) finds near-end breaks and bad connectors. Inspection scopes and cleaning tools are mandatory before mating connectors. Understand reference methods (one-jumper, two-jumper, three-jumper) because they change measured loss by including or excluding connectors.
Test parameters and standards
Candidates must know wavelengths (850/1300 nm multimode; 1310/1550 nm single-mode), launch conditions (mandrel wrap or mode filter for multimode), and pass/fail limits from standards or the design budget. OTDR settings: pulse width, range, averaging, and index of refraction; incorrect IOR shifts distance readings. Bidirectional OTDR testing is required for accurate splice loss because of gainers.
Traps and worked example
Traps: accepting a one-way OTDR splice value; forgetting to clean and inspect; using the wrong reference method and comparing to the wrong limit; measuring multimode without a launch conditioner; and confusing dB with dBm (dBm is absolute power). Worked example: a link shows 2.1 dB loss on the OLTS; the budget allows 2.5 dB, so it passes, but a 1550 nm OTDR shows a reflective event at 40 m with -45 dB reflectance, indicating a dirty connector; clean and retest rather than accepting a marginal pass.