If we're going to do anything useful with our test setup, it's necessary to know the power of our laser.
We used four methods to measure the laser power. Each of the methods has its 'issues' but they agree closely enough to give us some confidence in the results.
Method
Laser Power
Light Dependent Resistor compared with bright sunlight
0.1 mw
Photodiode compared with bright sunlight
0.15 mw
Photodiode based on specifications
0.27 mw
Camera based on specifications
0.1 mw
Light Dependent Resistor (LDR)
We compare the response of a LDR to the laser and to the bright sun.
Why the sun?
The sun is close to being a reliable standard illuminator.
The sun and a low power laser have about the same irradience, approximately one milliwatt per square millimeter.
The illumination of the bright sun can be calculated for any time of day, for any day of the year, and for any latitude. link
The resistance of the LDR varies roughly inversely with the power of the incident light.
The Setup
The Light Dependent Resistor (LDR) was mounted behind a 7/64 diameter aperture. When the laser was shone through the aperture, the hole was slightly fringed with red light. The intent was that light from the sun and light from the laser would illuminate the same area on the surface of the LDR.
Results
Resistance due to laser light: 148.2 ohms
Resistance due to sunlight: 20.1 ohms
Sunlight power per square mm: 0.75 mw
laser power = 0.75 x 20.1 / 148.2 = 0.1 mw
Photodiode compared with bright sunlight
The current of the photodiode is roughly proportional to the number of photons striking it. The sensitive area is a 1 mm square.
Results
Current due to laser light: 0.081 ma
Current due to sunlight: 0.393 ma
laser power = 0.75 x 0.081 / 0.393 = 0.15 mw
Comment
The photodiode has a huge variation in sensitivity across the spectrum of solar radiation. It is highly biased toward the red end of the visible spectrum and near infrared. This calculation above ignores that fact.
Photodiode using specifications
The datasheet gives a figure for quantum yield (0.9 at 850 nm) plus a graph of relative sensitivity vs. wavelength.
For 640 nm (the wavelength of the laser) the electrons generated per photon is: 0.9 x 75% = 0.675.
Calculations
The calculations for this section and the next are based on:
1 - Lumenera
2 - Strolls with my Dog
Strolls with my Dog has done excellent work uncovering the technical details of the Raspberry Pi High Quality Camera.
The following is from a jupyter notebook. It's mostly python3 code but also includes the results of print statements.
#laser wavelength
lmbda = 640e-9
#diode spectral sensitivity per datasheet
ss = 0.75
#measured current (ma)
i = 0.094e-3
#coulomb
cou = 6.24e18
#electrons per second
eps = i * cou
#conversion @ 850 nm electrons per photon
n = 0.9
#photons produced by laser beam
pp = eps / (n * ss)
#energy per photon e = h v = h c / lmbda
h = 6.63e-34
c = 3e8
ep = h * c / lmbda
#energy (watts) = photons per second x energy per photon
pwr = pp * ep
print("Laser Power = ", round(pwr*1000,2), "mw")
Laser Power = 0.27 mw
Using the Camera to Calibrate the Laser
That's sufficiently complicated that it merits its own page. link