Page 165 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Control of Ambient Light

158 Chapter Seven

It is important that the apertures is sharp, the idea being to present only a
small area which scatters light. These can be fashioned very effectively from
thin metal foils. Don’t just try to use a drill press with the foil on a soft support.
Holes can be drilled cleanly by first clamping the foil tightly between two thick
scrap plates, ideally of a similar material. Use a sharp drill bit, lubricate with
oil-emulsion or kerosene, and run the drill-press at the correct linear-velocity
for the bit. Feed-rate is also important, and a few attempts to get it right will
be needed. If it is right, edge damage can be less than the foil thickness. This
even works well with the thinnest polymer foils such as Saran Wrap, which are
impossible to drill directly. For the thinnest metal foils (<25mm) photolithogra-
phy and one- or two-sided etching is a good alternative approach.

7.6 Coherent Detection and Localization
The difficulty of making sensitive optical measurements in a confined space
occurs frequently. Let’s imagine we want to detect the light scattered from a
small dust particle drifting around the room (Fig. 7.17). We use a collimated
6
laser beam, but if the scattering efficiency is only 1 in 10 , we get just 10nW
back from our 10mW laser. The majority of the light passes by to illuminate
the apparatus walls, contributing to scatter which could be far more intense
than the signal from the particle. The whole experimental environment is illu-
minated by an interfering source. We could of course modulate the laser light
and synchronously detect, and this would be effective in separating it from room
light. However, all the disturbing wall-scatter is equally modulated, and will
not be filtered out by the lock-in process. This is an opportunity for coherent
detection.
Figure 7.17b shows an option. The detector signal is made up of the weak
particle-scatter and a reference beam obtained by splitting a fraction of a
percent from the main beam. We will assume that the coherence length of
the laser source is much longer than the difference in path-lengths beam-
splitter/particle/detector and beam-splitter/detector. Then the two signals will
“interfere,” meaning that we must calculate the detected intensity by first
summing the light amplitudes, not powers. For the two complex amplitudes we
write:
r
A = Ae jf1 (7.6)
r
B = Be jf2 (7.7)

where A, B are the real field amplitudes and f 1, f 2 are the phases of the two
waves. The total detected amplitude is then:
v v v
+
E = A B (7.8)
r r
and the intensity I is proportional to E · E *, where the * denotes the complex
conjugate. We obtain:

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