Background
In virtually all laser scanning microscopes from other manufacturers, the laser intensity is regulated via an AOTF (Acousto-Optic Tunable Filter), which is calibrated so that intensity increases linearly with the software setting. PicoQuant systems work differently, which can lead to unexpected behaviour if you are used to other systems.
Answer
In SymPhoTime and SEPIA II software, it is not the laser intensity that is regulated directly, but the current driving the laser diode. Every diode has a different intensity response, and the lasing threshold differs between individual diodes.
- If the driver is set too low, the current stays below the lasing threshold and the laser will not emit light.
- Slightly above the threshold, the narrowest pulse is typically obtained.
- Further above the threshold, the intensity response is steepest with increasing power setting, then gradually saturates towards 100%.
Because pulse shape changes with the driver/software settings, we recommend using optical attenuation to regulate the laser power rather than the software intensity setting. On PicoQuant systems this is available via the attenuation option on the Laser Combining Unit (e.g. Luminosa, or the standard Laser Combining Unit on a LSM upgrade kit), or via the attenuation screw in front of a directly-coupled laser head.
Note: PicoQuant laser couplers such as the Luminosa can regulate laser intensity optically (without changing the driving current). If your system uses such a laser combining unit, optical attenuation is already built into the workflow and you can use the coupler's intensity control instead of the driver setting.
Additional Info
- Recommended default: Set the driver intensity to 100% in the software or on the driver, and attenuate only optically. This is sufficient for most experiments unless the sample lifetime is extremely short.
- For fiber-coupled lasers, the intensity response can differ depending on whether the seed or amplification stage is being regulated (relevant for older vs. newer fiber laser designs).
- The lasing threshold differs between pulsed and CW mode, and can shift slightly at different repetition rates.