How to use a thermal sensor for a pulsed laser beam

Often times we are approached with a question about measuring low energy or power.  For example, what if the laser has a low pulse energy but its average power is well above the thermal sensor’s minimum threshold?  Can the power be measured?  What about the energy over some time?

When considering minimum thresholds, as opposed to damage thresholds, it all depends on what is being measured.  If one is measuring single-shot energy, it is important to remember that thermal sensors don’t recognize pulses more frequent than once in a few seconds.  Instead they treat the entire measurement, whether pulsed or cw, as one long pulse.  Imagine for example a 50Hz pulsed laser beam with 1μJ of energy per pulse.  If the beam is measured over one second, the thermal head sees one long 50μJ pulse.  So even though the laser is rated at 1μJ, which is below the minimum measurable energy of a 3A thermal sensor, the total energy of the train of pulses can be measured in this case without a problem.

What about the average power?  One millijoule over one second is one milliwatt.  The minimum measurable power is 60μW so there’s no problem measuring 1mW, right?  Not so fast.  Thermal sensors have a reaction time of about 3 seconds, so although all the measurements are well above the noise levels, we can’t measure power unless the laser is on for at least 5-10 seconds.  If that is not an option, one can alternatively measure the energy and calculate the average power by dividing the total energy by the amount of time the laser was left on.

Thermal power sensors have many useful applications.  Understanding the specs of your laser measurement sensor can help you use the sensor to its full capability.

Questions?  Comment below.