The optical power meter reading is 0 dB
A 0 dB reading on an optical power meter indicates that the measured power is equal to the reference power set on the meter, often corresponding to 1 milliwatt if using dBm.
Absolute vs Relative Measurements
Optical power meters can display readings in dBm or dB. dBm is an absolute measurement of optical power referenced to 1 milliwatt, so 0 dBm equals exactly 1 milliwatt of optical power. Negative dBm values indicate power less than 1 milliwatt, while positive values indicate power greater than 1 milliwatt .
In contrast, dB (without the “m”) is a relative measurement, showing the difference in power between two points. A 0 dB reading in this mode means the measured power is equal to the reference level previously set on the meter. For example, if you set a reference using a short patchcord, any subsequent measurement of the same power will read 0 dB, and any loss or gain along a fiber link will appear as a positive or negative dB value relative to that reference .
Practical Interpretation
- 0 dB in relative mode: No loss or gain compared to the reference. The fiber or connection is transmitting the same power as the reference.
- 0 dBm in absolute mode: The optical power is 1 milliwatt. This is typical for some laser sources in LANs or telecom systems, though most fiber signals are below 1 milliwatt and often show negative dBm readings .
Key Points for Fiber Testing
- Always set a reference before measuring loss in dB. This ensures that 0 dB corresponds to the reference power.
- Use dBm to check absolute power levels of transmitters or receivers.
- Use dB to measure insertion loss or relative changes along a fiber link. The difference between input and output power gives the loss in dB .
- Typical fiber losses: multimode fibers may lose 3 dB/km at 850 nm, singlemode fibers around 0.25–0.4 dB/km, and connectors or splices add small additional losses .
In summary, a 0 dB reading indicates that the measured optical power matches the reference level, and understanding whether the meter is in absolute (dBm) or relative (dB) mode is essential for correct interpretation.
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