Hello,
I will need to measure real output power of fake 200 mW green laser pointer I have ordered on eBay, so I have been searching for some simple and cheap approximate laser power meter.
My first idea was simple photodiode + ammeter. I am sure it would work (and it could be even pretty accurate and have fast responses), but the problem is that I can't calibrate it (I don't have any other laser sources with precisely known output power). So I have searched the net and found Laser Power Meters chapter in Sam's Laser FAQ. And there is one brilliant idea:
What do you think about it? It can be made really easily and it would be extremely cheap. It would have slow responses (so you can measure only average power, not peak power), but it can be relatively accurate (at least for my needs, I assume error about +- 5 mW). Do you have any experiences with this design?
I will need to measure real output power of fake 200 mW green laser pointer I have ordered on eBay, so I have been searching for some simple and cheap approximate laser power meter.
My first idea was simple photodiode + ammeter. I am sure it would work (and it could be even pretty accurate and have fast responses), but the problem is that I can't calibrate it (I don't have any other laser sources with precisely known output power). So I have searched the net and found Laser Power Meters chapter in Sam's Laser FAQ. And there is one brilliant idea:
Diode detectors are a pain to calibrate unless you have a light source of known energy at the same wavelength you're trying to measure. A method which resolves (mostly) the calibration problem is to use a small thermistor. Epoxy a 1/4 watt resistor to one side and coat the other surface with lamp black. Put thermal insulation around all of it except the smoked side. Apply about 1/4 watt of power to the resistor and let it come to equilibrium and measure the resistance of the thermistor. Then focus the beam of the laser on the smoked thermistor and reduce the power to the resistor to keep the thermistor resistance at the same value. The laser power should be equal to how much the resistor power was reduced. It's very cheap, fairly accurate, uses your DMM for the readings, and will measure CW or average power of small pulsed lasers.
What do you think about it? It can be made really easily and it would be extremely cheap. It would have slow responses (so you can measure only average power, not peak power), but it can be relatively accurate (at least for my needs, I assume error about +- 5 mW). Do you have any experiences with this design?