OTDR Testing Basics: Measuring Fiber Links and Locating Faults

When a fiber link goes down, the OTDR (Optical Time-Domain Reflectometer) is the instrument that tells you exactly where and what the problem is. Unlike a simple power meter, which only confirms that light is or is not arriving, an OTDR sends pulses into the fiber and measures the backscattered light to build a trace of the whole link - showing every splice, connector, bend and break with its distance. This guide covers the physics in plain terms, the settings you must get right (wavelength, pulse width, averaging, refractive index), and how to interpret a trace to find faults fast.

How an OTDR Works

The OTDR launches a short light pulse into the fiber and measures the returned light over time. Two effects produce the return signal:

• Rayleigh backscattering: microscopic density variations in the glass scatter a tiny fraction of light back toward the source - this creates the continuous trace slope.
• Fresnel reflections: at discrete events (connectors, mechanical splices, the far end, a break), a strong reflection occurs - visible as spikes on the trace.

Since the speed of light in fiber is known, time converts to distance: distance = (c × time) / (2 × group index).

Key OTDR Settings

Reading a Trace: Events

• Connector: sharp loss step + reflectance spike (e.g., 0.3 dB loss, -40 dB reflectance).
• Splice: small loss step, no reflectance (fusion splices typically 0.02-0.1 dB).
• Macrobend: gradual loss with no reflectance - usually a tight bend, check with a visual fault locator.
• Break: large reflectance followed by the trace dropping to noise floor - the fault is at that distance.
• End of fiber: final reflection (or flat end) - the trace ends.

Distance accuracy matters for dispatch: a 100 m error can send a technician to the wrong manhole. Verify the RI setting before trusting distances.

Dead Zone and the First Event

Every OTDR has a dead zone after the launch: the first few meters are saturated and events close to the OTDR cannot be resolved. Use a launch fiber (pigtail, 100-500 m) to push the first event beyond the dead zone - this is mandatory for premises/jumper testing where the first connector is often the one failing.

Step-by-Step Fault Location Workflow

1. Connect the OTDR with a launch fiber; set wavelength, range, pulse width and RI.
2. Run the trace and identify the event list (distance, loss, reflectance).
3. Compare against the baseline trace from commissioning - differences are the faults.
4. Confirm a suspected break with a visual fault locator (VFL) on short links.
5. Re-test after repair and store the new baseline for future comparison.

Common Mistakes

• Wrong RI → distances off by several percent; on a 10 km link that is hundreds of meters.
• No launch fiber → the most common failure (the first connector) is hidden in the dead zone.
• Testing 1310 nm but the link carries 1550/1625 nm (DWDM) → loss and events differ.

Related: WDM: expanding fiber capacity and optical module fault diagnosis.

原文链接：https://community.fs.com/article/otdr-basics.html