Yes, the OTDR's light-receiving ratio is generally smaller than that of an optical power meter because it relies on backscattered light rather than direct optical power measurement.

Measurement Principles

An OTDR (Optical Time Domain Reflectometer) works by sending short laser pulses into a fiber and measuring the backscattered and reflected light that returns to the device. This backscattered signal is only a small fraction of the total light launched into the fiber, which inherently limits the amount of optical power the OTDR receives at any given moment . The OTDR then uses this weak signal to generate a distance-based trace, showing events like splices, connectors, bends, and breaks along the fiber . In contrast, an optical power meter (OPM) measures the absolute optical power directly at the fiber end. When paired with a calibrated light source, it captures nearly all the transmitted light at that point, providing a highly accurate and direct measurement of received power in dBm or mW . This direct measurement results in a much higher light-receiving ratio compared to the OTDR.

Practical Implications

• Sensitivity and Accuracy: Because OTDRs rely on backscatter, their measurements are less sensitive to small power changes and can have higher uncertainty in absolute loss measurements. Power meters, by capturing the full optical signal, provide more precise end-to-end loss readings .
• Use Cases: OTDRs are ideal for troubleshooting, locating faults, and characterizing fiber links over long distances, where knowing the position of events is critical. Power meters are better for certifying link loss, verifying power budgets, and routine maintenance, where absolute power measurement is required .
• Dynamic Range Considerations: OTDRs have a selectable dynamic range, typically 35–45 dB for singlemode fibers, which determines how far and sensitively they can detect backscattered light. This is still less than the full optical power captured by a power meter at the fiber end .

Summary

The OTDR receives only a small fraction of the launched light due to its reliance on backscatter, making its light-receiving ratio smaller than that of an optical power meter, which measures the full optical power directly. This difference explains why OTDRs are better suited for event location and fiber characterization, while power meters are preferred for accurate end-to-end power measurements and loss certification .