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We're often asked: How do I accurately measure the energy of very low energy pulses? To measure
the energy per pulse in a repetitively pulsed beam, we normally use a pyroelectric sensor.
In some applications, you may find yourself measuring energy of very low energy pulses,
whose energy is near the lower limit of the specified measureable energy range of the
given sensor, but still within spec. And yet you may find yourself a running into problems
of jumpy readings with energy values that might not really make sense.
Very likely the problem is false triggering caused by an acoustic vibration in the environment
of the measurement. If the pulse frequency, as shown on the meter or the instrument, is
itself jumping around then acoustic vibration is almost certainly the problem.
Pyroelectric sensors are sensitive to acoustic vibrations and in fact they detect vibrations
using the same physical mechanism through which they detect laser pulses. On the more
sensitive scales of sensitive devices such as the PE-9 or PE10-C, they could actually
be very sensitive to acoustic vibrations. We can actually see here if we set such a
sensor - the PE10-C in this case - to a low energy scale and clap our hands once just
above the sensor surface we will, in fact, get a reading.
There are two solutions to this problem. One is too damp out the effect of the acoustic
vibrations, by preventing them from traveling through the solid material of the base and
mounting rod into the sensor. Remember that acoustic noise travels primarily through a
solid medium rather than through the air. Ophir offers a shock absorbing mounting post
for just this application, which damps out the vibrations traveling through the table
and trying to make their way into the sensor. A second solution is to make use of the energy
threshold. If you have one of the newer Pyro-C sensors, such as the PE10-C that we have here,
then you can set the minimum user-adjustable energy threshold to a level that's above the
energy of the noise, but still below the level of the energy pulses that we're trying to
measure. This user adjustable threshold allows you to set the threshold to a point that will
keep the noise out, but still allow the pulses in.
It's worth mentioning that Ophir offers a number of energy sensors that are sensitive
enough to go down to such levels of very low energy - down to nanojoules or even picojoules.
The PD10, for example, can measure all the way down to two nanojoules, while the PD10-pJ
can go down as low as ten picojoules, for the visible and near-infrared regions of the
spectrum. And there's an equivalent PD10-pJ-IR for longer wavelengths.