The vertical axis of an OTDR trace represents optical loss or the strength of the backscattered/reflected signal, typically measured in decibels (dB).

Understanding the Vertical Axis

In an OTDR trace, the vertical axis shows the amount of light returned to the device from the fiber under test. This includes both Rayleigh backscatter from the fiber itself and Fresnel reflections from discontinuities such as splices, connectors, bends, or breaks . The values are usually expressed in decibels (dB), indicating the relative loss of signal: higher points on the vertical axis correspond to stronger returned signals, while lower points indicate greater loss or attenuation .

How It Relates to Fiber Analysis

• Splice and connector losses: Sudden drops in the vertical axis indicate points where light is lost due to splices or connectors .
• Fiber attenuation: A gradual downward slope along the vertical axis represents the inherent loss of light as it travels through the fiber .
• Fault detection: Sharp dips or spikes in the vertical axis can reveal breaks, bends, or other defects in the fiber .

Interaction with the Horizontal Axis

The horizontal axis represents distance along the fiber, usually in meters or kilometers. By combining the vertical axis (signal strength) with the horizontal axis (distance), technicians can pinpoint the location and severity of losses or faults in the fiber optic link . This graphical representation, called an OTDR trace, provides a “picture” of the fiber's condition, allowing for installation verification, maintenance, and troubleshooting .

Summary

The vertical axis of an OTDR is crucial for interpreting fiber performance. It quantifies the returned optical signal, helping technicians identify losses, reflections, and faults along the fiber. Understanding this axis, in conjunction with the horizontal distance axis, is essential for accurate fiber optic testing and network maintenance .