Laser Power meter and Photometer for Holography
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 !