Background
In virtually all laser scanning microscopes (LSMs) and some confocal systems, the laser power is typically controlled using an acousto-optical tunable filter (AOTF) or an acousto-optical modulator (AOM). These devices modulate the laser power by deflecting a fraction of the beam using sound waves generated by an applied radio frequency (RF) signal.
The output power of the laser is not strictly linear with the control voltage or percentage set in the software. Instead, the response follows the diffraction efficiency of the acousto-optical device as a function of the RF amplitude. This is inherently nonlinear: at low drive levels, efficiency rises steeply; at higher levels, it saturates or may even decrease.
Answer
In SymPhoTime and SEPIA II software, the laser power control is calibrated and normalized to approximate a linear response over a defined range. However, at very low power settings (typically below ~10–20% of the maximum), the laser output can deviate significantly from linearity or fall below the device’s minimum threshold to lase.
This means:
Non-linear response at low power settings: The relationship between the set power and actual output is highly nonlinear near the minimum operating point.
Laser not turning on below a threshold: Some laser models have a minimum power threshold below which they will not operate (lase). Setting the laser to a very low percentage may simply not be sufficient to drive the laser above lasing threshold.
Additional info
This behavior is documented in the SEPIA II help system for each laser module.
If you need precise very-low-power measurements, consider using neutral density filters instead of relying on software power control at low settings.
If your laser does not turn on at all, check whether the power setting is above the minimum threshold for your specific laser model.