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Old 07-17-2016, 08:59 PM #1
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Default LPM using a photodiode

Hi everyone!
First of all sorry for my bad english...
Recently i started to study photodiodes, and i thought: "what about building a laser power meter with a photodiode?"
I got a look to this photodiode:
PIN-10DPI SB
PIN-10DPI/SB | OSI Optoelectronics PIN-10DPI/SB Visible Light Si Photodiode, Through Hole Metal | OSI Optoelectronics
I know that a photodiode is provided with a chart wich explain you the response of the current in relation to the optical power, considering that this A/W ratio depends by the wavelenght we are measuring.
On photodiode's datasheet the A/W spectral response chart has got a very high resolution so its very easy to find the A/W ratio about the wavelenght i would like to measure. But some days ago somebody told me that the values wich i read on the chart are only "tipical values", wich may be very different from real values about the photodiode i got in my hands.
Does anybody know more about that?
Thanks for listening.


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Old 07-17-2016, 09:11 PM #2
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Default Re: LPM using a photodiode

Welcome. It would be nice if you could include your location so we know where you are. Photodiodes have been used for low power LPMs for a long time. You will need to calibrate any LPM you make yourself, so keep that in mind.
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Old 07-17-2016, 11:31 PM #3
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Default Re: LPM using a photodiode

There are a few professional LPMs that use photodetectors of various types. The Coherent Lasercheck is probably the most popular of them.

As long as you have a way to compensate for different wavelengths, I think you will quite successful.

Anybody attempting this might either want to use stable labbies or a proven thermal LPM to compare numbers with though.
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Old 07-18-2016, 01:30 AM #4
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Default Re: LPM using a photodiode

You could always use it under controlled test conditions to compare one laser to another or see if your laser has weakened over time.
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Old 07-18-2016, 02:40 AM #5
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Default Re: LPM using a photodiode

There are several problems with photodiodes used as laser power measuring devices.

One of them is wavelength calibration, which could be overcome to some degree using lookup tables etc.

The other is they can only handle very limited amounts of power, making them good for the 1 mW-ish type of laser, but useless for the 1W-ish type that would quickly burn out the sensor.
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Old 07-18-2016, 07:11 AM #6
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Default Re: LPM using a photodiode

How about defocus the laser being tested to make a 50mm wide spot on white paper from 200mm distance with the photodiode set at 250mm from the paper facing the backscatter and 75mm above the lasers centerline. Same each time.

No smoke in the air during the test and the same humidity.
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Old 07-18-2016, 08:07 AM #7
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Default Re: LPM using a photodiode

You still have the problem of non-linearity across the spectrum and that would be difficult to reproduce among differing laser powers. If you needed to use a photo sensor for higher powers it would be better to us neutral density filters or partially reflective mirrors. Still, that pesky wavelength problem.
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Old 07-18-2016, 02:09 PM #8
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Default Re: LPM using a photodiode

Thanks for answering to me
So, since it would be too much difficult for me to calibrate a homemade LPM, i was looking at this cheap LPM on Ebay:
SANWA LP1 Optical Laser Power Meter 40mW 400?1100nm optical sensor Leistungsmesr | eBay
It comes with a correction table for the wavelenght you are measuring.
Will a good quality ND100 density filter allow me to measure higher outputs?

P.S. A 2W Laserbee LPM is cheap for me, but shipping costs to Italy are a pain in the neck...
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Old 07-19-2016, 01:20 AM #9
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Default Re: LPM using a photodiode

You could use a LPM like that with a neutral density filter to increase it's power range. As long as you know the exact wavelength of your laser diode (which can be a problem for red ones especially) it should work.

I'm not sure how good the wavelength compensation is on the unit you linked though. Coherent has the 'lasercheck' that works with a photodiode and look-up table to display the result directly, but that's limited to handling only 10 mW.

I guess you could put a ND1000 filter in between to extend that to 10 watts, although at that point i'd start to worry about burning a hole through or even just bleaching out the filter.
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Old 07-19-2016, 03:12 AM #10
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Default Re: LPM using a photodiode

It would be putting a lot on a photo diode to measure powers at 10 W. I think you could use several filters if you wanted to push the envelope, though. Using an HR mirror might be the best option for every high powers. But, most photo diode LPMs are for lower powers.
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