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We're often asked about details of measuring power of very low power pulsed beams.
To measure average power of beams down in the nW and mW range, we normally use a photodiode-based
sensors.
Depending on the spectral region, it might be the silicon-based PD300 or the germanium-based
PD300-IR for example.
One of the characteristics of photodiode-based sensors is that they have a very fast physical
response time, typically on the order of nanoseconds.
The processing and calculations required to get an actual measurement might take a bit
longer, but what this means is that when measuring a pulsed beam, the detector signal can generally
actually keep up with the pulses.
This is unlike the situation for example with a thermal sensor, which normally has a time
constant, on the order of about a second.
We can see over here a pulsing source, and the reading that's obtained using a thermal
sensor and a photodiode-based sensor, in this case the PD300.
With the thermal sensor the pulses are physically averaged out by the sensor itself, but as
we can see, the photodiode-based PD300 is actually keeping up with the pulses.
Can we, then, use a photodiode-based sensor to measure the average power of such pulsed
beams?
Normally the average power of these beams might be too low to use a thermal sensor.
In general, yes we can use photodiode based sensors for such applications, but two physical
issues need to be kept in mind:
One: If the pulse frequency is close to the typically 15 Hz sampling rate of the instrument
-- let's say ten or tens of Hz -- we often get a "beat frequency," which means essentially
that the reading is going to jump around and getting a measurement is going to be difficult.
Similarly, when we're working at very low pulse repetition rates we also can have a
reading that jumps around and is difficult to properly record.
The solution to this is to use the "averaging" function available on Ophir meters.
What this does is it displays not the moment-by-moment real time reading, but rather the running
average over a time period that the user can select.
Two: It's possible for a pulsed beam to have average power within the specified average
power range of the sensor and yet be made of pulses whose pulse energy is high enough
to cause momentary saturation of the detector.
This could result in a reading that's incorrect, and yet no over-range warning will indicate
to the user that there is anything wrong, because the average power is, after all, within
specification.
It's important then, to be sure that the pulse energy and not only the average power is within
the specified range
This parameter -- maximum pulse energy -- is actually included in all specifications of
the PD300 family of photodiode-based sensors, for just this reason.