What are OTDR Gainer Events?

A gain, or “gainer,” event on an OTDR trace appears as a positive bump that resembles “negative loss", if such concept could exist. Since passive fiber cannot amplify light, this behavior can be confusing or counterintuitive, particularly for less experienced users. Although many articles may describe gainers as “false” readings, they are actual events that, when properly interpreted, provide useful diagnostic information.

Gainer events are moderately common in modern fiber optic testing, particularly when using traditional single-direction OTDR testing. Occasionally, an OTDR trace may show a gain event, while not a true increase in optical power, a gainer appears on a trace as an upward spike at a splice or connector point, often caused by connecting fiber sections with different backscatter coefficients, such as splicing G.652 (standard) and G.657 (bend-insensitive) fibers.

In practical terms, fiber optic gainer events appears when two fibers with different backscatter coefficients are spliced or connected, and the OTDR test pulses transition from a fiber with lower backscatter to one with higher backscatter. This situation is typically associated with Mode Field Diameter (MFD) mismatches, for example when standard SMF-28 is joined to bend-insensitive fiber, or when 50 µm and 62.5 µm multimode fibers (MMF) are interconnected.

Does splicing of disimilar fibers happen often in modern fiber networks?

That depends on the age of the fiber plant, service provider, fiber operator or data center. Commonly cited example are fiber-to-the-home (FTTH) and fiber-to-the-premises (FTTP) networks, where standard bend-sensitive fibers are typically deployed in the outside plant because the routes are well engineered and this is the most cost-effective option, while higher-cost bend-insensitive fibers are often reserved for indoor segments to accommodate the unpredictability of less controlled installation conditions.

Main Causes of Gainers in OTDR Traces

• Dissimilar Fibers: When fiber A has a lower backscatter coefficient than fiber B, the OTDR sees a sudden increase in backscattered light at the junction, interpreting it as a "gain" rather than a loss.

• MFD Mismatch: Splicing different types of single-mode fiber, specifically standard fiber to bend-insensitive fiber, causes significant backscatter differences.

• Multimode Mismatch: Mismatching fiber core sizes, particularly connecting 50µm to 62.5µm fiber, creates a large gainer, according to.

• Reflective Events: A highly reflective splice or connection can sometimes cause a temporary increase in the trace, misconstrued as a gainer.

Practical Implications