More on Photodiodes

[Colin Kaminski]

I found this on usenet. The author is Jonathan Hardis (not our Jonathan).





A few effects that you may not have considered:

1) Photodiodes are usually protected from air by a thin quartz (SiO2) layer
or window, which can act as an etalon. The responsivity of silicon
photodiodes to laser light varies in an oscillatory manner by frequency.
To counteract this problem, photodiodes used with laser are often
constructed with "wedged windows" (e.g., protective layers with
parallel surfaces. If you want to search the literature on photodiodes
used with lasers, you can search on "wedge-window detectors".

2) Photodiodes inherently count photons that create electron-hole pairs.
They are not inherently power measuring devices. So, if you actually
care about power sensitivity vs. wavelength, even where the photodiode
has a "flat" response, you need throw in a 1/lambda factor. (A photon's
energy is inversely proportional to its wavelength). If you really care
about power and want a flat response in power, you need to use a thermal
detector, such as a thermistor behind a black absorver. These are
generally what are used for higher powers, anyway.

3) The responsivity of a photodiode depends on its temperature. If you
use a photodiode in a higher power laser, its temperature will rise
and its responsivity will change. This is sort of like a non-linear
power effect except that there would be a hysteresis to it also.
Good power meters based on photodiodes regulate the temperatures of the
photodiodes (e.g., with Peltier coolers).

4) There are no "rules of thumb" for the linearity of photodiodes in general,
each type of photodiode is different. You can test the linearity yourself
-- you don't have to guess.

Method 1: Shine two beams on the detector. (If you only have one laser,
use a beam splitter. Or a screen with two holes in it.) Measure the
detector's signal for each hole separately and both holes together.
Do they add up? If not, by how much do they differ?

Nethod 2: Shine two beams on the detector, one strong and one weak.
(You can create a weak beam by using a microscope slide as a beam
splitter, which gives you about about a 95/5 split.) Make the
weak beam turn on and off, e.g., by spinning a slotted disk in it.
Note the amplitude of the oscillatory part of the photodiode signal,
e.g., with an oscilloscope. The question is, does this oscillatory
signal vary in amplitude if the intensity of the strong beam changes?
Think about it -- it's a differential way of determining linearity.
Most crudely, you can block/unblock the strong beam. Better, you can
try the test with various neutral density filters, defocusings, etc.

Both of these methods are well documented in the technical literature.

- Jonathan