The Difference Between OTDR Pulse Width (in meters) and Spatial Event Resolution

Converting an Optical Time-Domain Reflectometer (OTDR) pulse width from nanoseconds (ns) to the equivalent distance covered in meters (m) can be confusing to people getting into the trade. The key is to distinguish between the physical length of the light pulse within the fiber and the OTDR’s spatial event resolution.

A. Physical Pulse Length

To find the literal physical length that test pulses' light occupies inside the glass, at any single moment, divide the pulse width by the cable's velocity of propagation:

• Physical Length (m) = Pulse Width (ns) ÷ Propagation Delay (ns/m)

• Example: A 100 ns pulse width results in a light burst that stretches across 20 meters of fiber: 100 ns ÷ 5 ns/m = 20 m.

From a practical OTDR trace analysis standpoint, however, this academic calculation offers very limited value.

B. OTDR Spatial Event Resolution

In practical fiber testing, the more useful calculation is the OTDR’s spatial event resolution—the minimum separation needed for the instrument to distinguish between two adjacent events, such as splices, connectors, or patch cords. Because an OTDR measures the light’s round-trip travel, this resolution is half the physical pulse length: 

• Event Resolution (m) = Pulse Width (ns) ÷ [2 × Propagation Delay (ns/m)] 

• Example: A 100 ns pulse width yields an effective event resolution of 10 meters: 100 ns ÷ (2 × 5 ns/m) = 10 m.
• Any two events closer than 10 meters apart will merge together into a single event on the OTDR trace, so they will be indistinguishable.