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Old 01-30-2011, 04:59 PM #1
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Default Neutral Density Filter

I was excited to hear that one of my physics professors had a laser power meter sitting around one of the labs. I was less excited once I found out that it was more of an analytical laser power meter which only metered up to 20 mW. Here is a link to the meter:

Power Meters Digital Photometer IF PM - Industrial Fiber Optics, Inc.

My professor mentioned that I could possibility use a neutral density filter to meter higher power lasers with, Here is a link to a filter from DX:

DealExtreme: $7.46 Premium ND8 Camera Lens Filter (72mm)

Does anyone have any experience with doing this? I would like to have a pretty accurate reading and im not sure if this would provide inaccurate readings.

Also I am wondering if the banana connector could be hooked up to a voltmeter to measure higher power lasers. Would this potentially damage the protector head?

Thanks!


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Old 01-30-2011, 06:03 PM #2
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Default Re: Neutral Density Filter

Beyond a few tens of mw the detector will saturate, so ND filters will be your best option. You only have to calibrate you OD filter, but that's not that hard. Use glass OD filters if possible, OD filters like that absorb the light, so it shouldn't melt. The cheapest are usually plastic.

But you also have to know the sensitivity of the detector for the wavelength you use, that's again a calibration.
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Old 01-30-2011, 07:08 PM #3
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Default Re: Neutral Density Filter

There are some nice wider range meters in the Laser Pro section that might interest you. They're comparably priced, but better value in my opinion.

Last edited by Bionic-Badger; 01-30-2011 at 07:12 PM.
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Old 01-30-2011, 07:42 PM #4
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Default Re: Neutral Density Filter

You could also use a diffraction grating or any other number of transparent materials. Measure a very stable low power laser. Next, measure one of the diffraction orders and you have a ratio. The X% reflection from a pane of glass will work, too. A ND filter might not be able to take a lot of power before it melts/bleaches/deforms.
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Old 01-31-2011, 09:36 AM #5
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Default Re: Neutral Density Filter

I depends on how far you want to go with power. I see the meter has a maximum of 20 mW, so using a ND8 filter, that would increase the maximum to 160 mW, 140 of which are absorbed by the filter. This should be fine for a good quality glass filter.

Note that the ND number is a fraction and the OD number is logarithmic, such that OD3 = ND1000. When used in photography, ND filters are usually multiples of 2, like 8, 16 or 32.

You could stack 2 ND8 filters to make a ND64, but this will be less accurate since each filter-to-air transition introduces some reflection, and those should be compensated for.
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Old 02-01-2011, 11:33 PM #6
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Default Re: Neutral Density Filter

Those filter to air transitions are significant, but the attenuation is far from calibrated, you you'll have to determine the attenuation yourself anyway, and with a good measurement you measure all the losses at once.

The reflection from a glass surface at normal incidence would work in theory, but that would send the beam back to the laser. And the index of the glass would have to be known to calculate the reflectance. Under an angle the polarisation comes into play which makes it more complicated.

Quality ND filters like from Thorlabs can take up to a watt, they're a bit expensive but probably worth it.
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Old 02-02-2011, 01:34 AM #7
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Default Re: Neutral Density Filter

It also depends on what you want with it. Sometimes accuracy is not that important, like when you just want to get relative measurements on any improvements you make (swichting lenses or so).


If you do want accuracy using one or more ND filters, you would probably need a laser of known power to calibrate things. This can also be a low powered laser that you can measure directly and through the filter setup with the same meter.
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Old 02-02-2011, 08:42 AM #8
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Default Re: Neutral Density Filter

It's more important to have the meter calibrated for your wavelength. If you laser has a too high power, use one ND filter to attenuate it and then measure the attenuation of another ND filter. Then switch the filters around and you have two known ND filters.
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