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Old 03-28-2008, 04:02 PM #1
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Default Idea: photodiode power meter?

Ok so here's what my brain just made up just now:
Most dvd cans have the third pin for the PD which is used to send some kind of feedback to the APC circuit to regulate the power the diode is getting.(right? :-/)
So I thought , wouldn't it be possible to hook this photodiode up to your circuit and add a tiny and sensitive digital voltmeter that you could tune with the help of the pot so that it displays the laser's power, right on your unit This would be totally cool if it actually worked, but it's just a theory.I didn't go deep into any research yet because I want to see if someone has a good strong reason for why this wouldn't work.So what do you think? :P


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Old 03-28-2008, 04:12 PM #2
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Default Re: Idea: photodiode power meter?

Yes it would work but each laser diode you use would be slightly different so you would have to calibrate each laser output with the PD gain. Generally the PD sits behind the LD and receives a bit of light. The stronger the light the greater the current in the PD and via a negative feedback loop, the input current to the LD is decreased eventually stabilizing the output of the LD. The drawback to PD diodes is they are sensitive to reflected light so any optics used must be properly AR coated and you must be careful not to have a reflection return to the emitter from an outside source (i.e. mirror on the lab table, or a power meter too close to the aperture, etc.)
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Old 03-28-2008, 04:19 PM #3
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Default Re: Idea: photodiode power meter?

Photodiodes are not very linear. In a regulator circuit, this doesn't matter because only a single response is needed (where the total gain of the close loop is 1. For measuring, a microcontroller might be needed to convert PD response to a linear value using a PD-dependent response graph (every PD is unique). This makes it close to infeasible, unless you're totally laser-geek, have too much free time and have the required skills. 8-) (the latter two sort of contradict each other though there might be exceptions)
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Old 03-28-2008, 05:11 PM #4
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Default Re: Idea: photodiode power meter?

furthermore, open can DVD writer diodes dont have a built-in photodiode
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Old 03-28-2008, 09:51 PM #5
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Default Re: Idea: photodiode power meter?

I knew someone would find somthing to ruin my dream, but what's left of it is good enough for me ;D
I don't have the skills but I can sure learn...I'll get into researching photodiodes in laser diode cans.And look for the smalles voltmeter or ampmeter.I would just build this in a big box with a 16x diode just for the heck of it It has a sort of cool factor to it you gotta admit :
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Old 03-28-2008, 10:29 PM #6
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Default Re: Idea: photodiode power meter?

You could do the same thing with a apc setup inside a regulated green. *a portion of the beam is &quot;split off&quot; to a photosensor and from that sensor is regulated. *Obviously it's setup to make sure it's never going over 5mw. *soooo do the same thing with any laser you do and bounce a portion off to the side to do exactly that!

Would be cool as hell to see built into the side of a kryton. Ohh my laser is running at ...###
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Old 03-28-2008, 11:19 PM #7
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Default Re: Idea: photodiode power meter?

If could use the internal PD, why not use an external Photodiode (IR receptor), at least will not be necessary the calibration.

More power, more light right?
So, using an o-amp to amplify the signal ... bla bla bla... what u think ?
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Old 03-29-2008, 01:21 AM #8
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Default Re: Idea: photodiode power meter?

You could use an external one and you can calibrate it with the help of you laser power meter so it shows the correct power I guess.But I was thinking you could do the same with the internal PD.It would save space.

Btw, I found this http://news.thomasnet.com/fullstory/13364 .I guess it could be turned into an ampmeter quite simply if you needed to.But I'm not sure if it will show mV :-/ But anyway, that thing is really tiny.It could fit on the side of a kryton barrel with a little work.So anyway, I bet the right one is out there.
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Old 03-29-2008, 06:06 AM #9
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Default Re: Idea: photodiode power meter?

Quote:
Originally Posted by Switch
You could use an external one and you can calibrate it with the help of you laser power meter so it shows the correct power I guess.But I was thinking you could do the same with the internal PD.It would save space.

Btw, I found this http://news.thomasnet.com/fullstory/13364 .I guess it could be turned into an ampmeter quite simply if you needed to.But I'm not sure if it will show mV :-/ But anyway, that thing is really tiny.It could fit on the side of a kryton barrel with a little work.So anyway, I bet the right one is out there.
don't worry, there is a very handy law which relates voltage to current across a resistor.
i think we will be talking about output currents of about 100uA (from the photodiode), so a 1K resistor across the photodiode will give 1mv per 1uA which would be suitable for the voltmeter you mentioned.

it would probably be a good idea to have a 2k multiturn pot to calibrate it.

as mentioned it won't be linear. those crappy ones on ebay would probably be better - anyone know how linear those are?
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Old 03-29-2008, 06:59 AM #10
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Default Re: Idea: photodiode power meter?

photodiodes, when operated in photovoltaic mode, are by far not perfect current sources. the PD current specified in LD datasheets is normally the short-circuit current. In any case, the open-circuit voltage across a Si photodiode wont exceed the bandgap voltage /0.55-0.6V/. Also, a huge drop in incident light intensity will not result in equivalent drop in voltage percentage-wise. The simplest straightforward thing to do, to begin with, is hook a tiny electromechanical microampere meter to the photodiode and adjust its sensitivity by means of a shunt resistor across its terminals.
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Old 03-29-2008, 12:06 PM #11
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Default Re: Idea: photodiode power meter?

or hook a mutlimeter in 200uA mode to it. basically they suck.
so what kind of sensors are in those ebay ones. and where do you get them, apart from ebay?
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