Incoming Inspection: Measuring Energy, Pulse Width and Divergence

Most of the arguments we have about a returned unit come down to how it was measured. These are the three measurements that matter, and the traps in each.

Energy

Use a pyroelectric sensor, not a thermopile. A thermopile averages over seconds and will happily report a number for a 5 ns pulse that has nothing to do with its energy.

Check the sensor's damage threshold before you fire a 10 mJ pulse into it, and check its aperture is bigger than the beam. If your meter is slow, measure average power and divide by repetition rate, but only if the rate is stable and you know the meter is linear at that wavelength.

Pulse width

InGaAs photodiode, 1 GHz or better, into a scope with at least that much bandwidth. A 5 ns pulse measured on a 200 MHz scope comes out wider than it is, and people then report the source as out of spec.

Take the width at half maximum, and record the detector and scope you used. Without that, the number isn't comparable with anything.

Divergence

Measure the beam diameter at two distances, well apart, and use the difference. For a full angle, divide by the separation; for a half angle, divide by twice it. Say which one you're reporting, because suppliers differ.

Watch the detector: a CCD or slit scan saturates on a 10 mJ pulse. Attenuate first, and check the attenuation is uniform across the beam.

Trigger-to-light delay

Photodiode on the output, trigger on the scope's other channel. That gives you delay and jitter in one shot, and both matter if the host gates a receiver.

Write it down

Record wavelength, energy, pulse width, divergence, rate, baseplate temperature, and the instruments used. When a unit comes back two years later, that sheet is the only way to know whether anything changed.

Ask for the test conditions on a model

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