Optical power measurement
This domain tests absolute and relative optical power measurement with a power meter and stabilized source. The exam expects correct detector selection, wavelength setting, reference handling, and recognition of the errors that make a link look better or worse than it is.
Meter fundamentals
A power meter displays average optical power in dBm or µW. Detector type must match the wavelength: silicon for 850/1300 nm multimode, indium gallium arsenide (InGaAs) for 1310/1550 nm single-mode, and germanium for broader coverage. Set the calibration wavelength to the source wavelength; a 1310 nm source read with a 1550 nm calibration is wrong even if the display looks stable. Power meters are typically accurate to about ±0.2 dB, so do not report differences smaller than the meter uncertainty without qualification.
Reference and measurement
For loss work, connect the source directly to the meter through the reference cord and set the display to 0.00 dB, then insert the link under test. The difference is insertion loss. For absolute transmitter output, read dBm at the source; for receiver input, read dBm at the receiver. Difference between two dBm readings is dB, which is valid for loss when the same wavelength and detector are used.
Worked example
A 1310 nm source is specified at −10.0 dBm. With a one-jumper reference the meter reads 0.00 dB; after the link it reads −13.2 dB. The link loss is 13.2 dB. If the same meter is accidentally left at the 1550 nm calibration, the number is not a valid 1310 nm loss result. Recheck wavelength, connector cleanliness, and reference before judging the link against its budget.
Traps
- Using a silicon detector at 1550 nm or an InGaAs meter without checking calibration.
- Overloading the detector with too much power and compressing the reading.
- Leaving a dirty connector in the reference path; the link can then appear better than it is.
- Confusing dBm (absolute power) with dB (relative difference).
- Reporting sub-0.1 dB differences from a meter rated at ±0.2 dB.