Laser Power meter and Photometer for Holography

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Here is a simple circuit I developed for detemining the exposure times for holography, as well as light ratios, optical densities and simple laser power measurements (up to a few mW). It allows for moderately accurate measurements even without any adjustments (by replacing R7 by a fixed 1K resistor; R1, R2 and R7 should then be 1% or better).  It is a current-to-voltage converter with low input impedance and reverse bias, designed to be very linear up to several volts output; also, diode current is limited such as to protect the diode by strong illumination. Zero offset due to dark current turned out to be very small, ca 0.2mV.

Setting R2=100K yields an output voltage of U=100mV per uA diode current, while R2=1K yields U=1mV output per uA.. The former is intended for illumination intensitiy measurements, while the latter can be used for power measurements up to several mW.

By using a photo diode with given predetermined sensitivity and wavelength response, one can obtain an absolute accuracy of better than perhaps 10%  (I verified this with an accurate thermistor detector).  From data sheets one infers that typical diodes have a sensitivity of say 0.5mA/mW at 900nm, and  less than that at shorter wavelengths; usually they have a sensitive area of only 1-3 square-millimeters, which yields a quite low output.

After some experimentation, I converged to the PIN-10AP from UDT, which I had obtained conveniently from here; its data sheet is here. This diode has the dual advantage that is has a) a large surface area of 1cm^2, which helps to even out small scale intensity fluctuations and also allows to accurately aim a (weak) beam onto the sensitive area; b)  a calibrated spectral response curve that allows to match various wavelengths with good accuracy (2%).

Concretely, with R2=1K this gives for power measurements up to ca 3 mW (where the whole beam must hit the sensitive area):

488nm:   .055 mA/mW     --->  amplifier output  U =  55   mV/mW
514nm:   .162 mA/mW     --->  amplifier output  U =  162 mV/mW
532nm:   .238 mA/mW     --->  amplifier output  U =  238 mV/mW
630nm:   .073 mA/mW     --->  amplifier output  U =  73   mV/mW

On the other hand, with these data one can directly compute the exposure time for a given holographic film and wavelength (for this, set R2=100K for hundred-fold sensitivity), see my web interface.

Note added: I measured the sensitivity of the PIN-10AP sensor by comparison with a Thorlabs S20MM power meter, and found that the above values are matched remarkably well, ie, to within 3-5% (I checked only for 514nm and 532nm). Pretty good for an uncalibrated photo sensor !

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