An OTDR, or Optical Time Domain Reflectometer, is a diagnostic instrument used to analyze the integrity and performance of fiber optic cables.

It works by launching a series of high-speed optical pulses into the fiber and measuring the light that is scattered or reflected back due to imperfections, splices, connectors, or breaks. By calculating the time it takes for the reflected signals to return, the OTDR determines the distance to each event and maps the entire fiber link as a trace, which visually represents signal loss over distance.

This allows technicians to identify and locate faults, measure splice and connector losses, assess reflectance, and evaluate overall attenuation. OTDRs are indispensable for commissioning new fiber installations, troubleshooting existing links, and documenting network quality, especially in long-haul and access networks where physical inspection is impractical.

A PM/LS setup, which stands for Power Meter and Light Source, is a fundamental tool pair used to measure the optical power and insertion loss in fiber optic links. The light source emits a stable, calibrated signal at a specific wavelength—commonly 850 nm, 1310 nm, or 1550 nm—while the power meter detects the received signal strength at the far end of the fiber.

This setup enables accurate measurement of end-to-end loss, helping technicians verify that the link meets design specifications and that splices, connectors, and fibers are performing within acceptable limits. Unlike OTDRs, which provide a graphical view of the fiber’s internal structure, PM/LS tools offer direct, quantitative measurements of optical power and loss, making them ideal for certifying patch cords, short links, and verifying continuity.

Together, OTDR and PM/LS form a complementary testing strategy—one for mapping and fault localization, the other for precise loss verification.

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