Category: Electronics Workbench

The Gentec ED-200 optical joulemeter from the Box o’ Optical Stuff is so thoroughly obsolete that no datasheet exists for it anywhere online:

The best I could come up with, after many dead ends, is a 2001 capture from gentec-eo.com at archive.org with the barest hint of specifications:

The Max Energy Density spec suggests longer pulses are allowed to deposit more energy, probably because more time gives thermal diffusion an opportunity to spread the heat across the target; at CO₂ laser wavelengths that may not apply.

With the platform lowered as far as it goes, the ED-200 is 130 mm below the laser nozzle where the beam diameter is about 6 mm for an area of 0.3 cm². Ignoring the ideal Gaussian beam profile by smearing 60 W uniformly across the circle gives a power density of 200 W/cm², which means the laser pulse must be less than 0.5 W·s / 200 W = 2.5 ms to stay inside the power density limit.

I sincerely hope Gentec overbuilt and underspecified their detector.

Also, there’s a useful overview document from Genetc-eo.com, wherein it is written:

The Voltage Response
The result is a voltage pulse that rises quickly with the response time of the device to a level proportional to the laser energy (Figure 2). It then decays exponentially over a longer period of time that is a function of the pyroelectric device and load impedance. Figure 2 also shows that there is a longer recovery time to return to the initial state of the detector. This is a function of thermal phenomena and is not affected by the load impedance as are the rise and decay times. The integrated pulse energy over this period is proportional to the peak voltage.

Pulse Width Versus Rise Time

Usually the applied laser pulse must be shorter than the rise time of the detector for all of its energy to be represented by the peak voltage. Pulse energy received after the detector voltage has peaked will not be fully integrated into that value. For very long pulses, the peak voltage will actually represent peak power rather than pulse energy.

Gentec Energy Detectors, page 2

Figure 2 shows the overall waveform:

Which looks a lot like this 10 ms pulse at 50% duty cycle:

The pulse was 10 ms long, much longer than the 1.5 ms ED-200 risetime spec, but the overall shape looks right. Dividing the 3.3 V peak by the detector’s 10.78 J/V calibration value (11 J/V works for me) says the pulse delivered 300 mJ = 300 mW·s. Dividing 300 mJ by 10 ms gives 30 W, a beam power astonishingly close to the expected value.

The OMTech laser has a nominal 60 W output, although the tube life drops dramatically with regular use over 70% = 40 W. Power does not scale linearly with the laser tube current displayed on the power supply milliammeter, with the maximum value presumably preset to the tube’s 20 mA limit producing 60 W. The 20 kHz PWM duty-cycle chopping applied by the controller should linearly scale the average power downward from there.

It looks like the ED-200 might deliver reasonable results for millisecond-scale pulses at low PWM duty cycles, but it was obviously intended for much milder lasers.

On the other paw, it’s fully depreciated …

The Box o’ Optical Stuff disgorged an ancient Gentec ED-200 Joulemeter:

It’s an optical pyrometer producing, sayeth the dataplate, an output of 10.78 V per joule of energy applied to its matte black absorber. Whether it’s accurate or not, I have no way of knowing, but aiming the business end toward the sun and waving my fingers over it produced a varying voltage, so there was hope.

It has a 1/4-20 socket on one side and my spare magnetic mount expects a 3/8 inch rod, so I drilled a suitable hole in a suitable aluminum rod and cut the head off a suitable bolt:

A dab of Loctite intended to secure bushings completed the assembly:

I later replaced the nut with a finger-friendly nylon wingnut.

Which allows a measurement setup along these lines:

The white disk atop the sensor is a homebrewed target to indicate the active sensor area and its center point:

The 1 mm graticule lines give a jogging suggestion to hit the center, assuming you (well, I) manage to hit anywhere on the target at the first shot. The beam is supposed to fill most of the central region, which is obviously not going to happen here, and it must not be focused to a pinpoint. The previous owner (or his minions) put a few scars on the surface and I expect to make similar mistakes.

Early results look encouraging:

The SVG image as a GitHub Gist:

I added the two mounting ears in anticipation of putting the joulemeter in the beamline between the mirrors to measure their loss.