What does a variable optical attenuator do?
A variable optical attenuator (VOA) adjusts loss in an optical path. It can support receiver tests at multiple input powers, link-loss simulation and production testing. A fixed attenuator normally supplies one nominal loss value; a VOA is useful when power must change continuously or in steps. For either device, confirm connector interfaces, operating wavelengths, permissible input power and fiber type.
Keysight's receiver sensitivity example combines a calibrated optical signal, programmable attenuator and bit error ratio tester to observe BER at multiple input powers. This is an example test system; a VOA alone does not establish compliance with an interface standard.
Attenuation setting versus output power setting
| Check | Attenuation setting | Output power setting |
|---|---|---|
| Objective | Set optical loss | Set a target output power |
| Input power change | Output will generally change | A device with feedback and tracking can compensate within its operating range |
| Evidence | Wavelength, actual input and loss setting | Feedback measurement location, calibration and tracking range |
| At the receiver | Downstream cords and connections still add loss | Instrument output reading is not automatically the receiver input |
The VIAVI VOA data sheet distinguishes attenuation-only, external power-monitoring and integrated-monitoring configurations. Its specified tracking option adjusts attenuation when input power changes. Closed-loop power control is not present on every VOA: check the exact model, options and operating limits.
Establish power at the receiver reference plane
For a single controlled channel, when all readings use the same wavelength and average optical power definition, transmitted power minus actual VOA attenuation and downstream losses provides an initial receiver-power estimate. Record additional connections, splitters, adapters and any movement of the reference plane. The nominal attenuation setting alone is not the power at the device under test.
Using a compatible calibrated power meter, verify power at the device reference plane under the instrument and project procedure; record wavelength, connections, measurement range and uncertainty. Then connect the device using the approved setup. Requirements for BER, OMA, extinction ratio and signal waveform depend on the relevant interface test method; an average-power reading does not automatically replace them. Observe source, instrument and receiver maximum input limits and site laser-safety procedures.
Test and procurement checklist
- State whether the task is fixed link power control, a laboratory sweep or automated production testing; specify channels and control interfaces.
- Record fiber category, connector and polish, wavelength range, maximum input power, required attenuation range and minimum insertion loss.
- Check attenuation accuracy, repeatability and settling time, and whether output monitoring, power tracking and an external reference meter are required.
- Agree reference-plane calibration, test cord and connection loss, report fields, calibration evidence and acceptance conditions.
- Inspect and clean fiber end faces when needed. At each sweep point record actual power, wavelength, test duration and error statistics; use the project method to judge results.
Procurement FAQ
- Is a VOA setting in dB the same as receiver power in dBm? No. dB expresses attenuation as a ratio; dBm expresses power relative to one milliwatt.
- Does power control automatically compensate the receiver patch cord? Not necessarily. Check the feedback reference plane and how downstream loss is calibrated.
- Can a fixed attenuator replace a VOA? Consider it for a fixed loss when tolerance and interfaces fit; a multi-point sweep needs an adjustable method.
- Does one VOA support every fiber and wavelength? Do not assume this. Check the model's calibrated range, fiber and interfaces.
Related guidance
See receiver power windows and attenuator selection, dB, dBm and power reports and test reference cables. The test-equipment discussion is general education and does not claim ARRANEX supplies VOAs.
Sources
Reviewed 2026-11-07: Keysight receiver sensitivity characterization and VIAVI variable optical attenuator data sheet.
Sources checked: 2026-11-07