September 2017 9
Before we get to the detector, let’s think about what
kind of modulation might be desirable. The first order
approach would be to switch the laser on and off with
each bit that you want to send. While that will work in
some circumstances, it may not be robust in a noisy
At the very least, we might want to add some forward
error correction, or FEC. We might also want to consider
using a carrier signal that would help us to distinguish our
signal from other background noise. In a case like this,
probably a Manchester coding scheme would work well.
Manchester coding is similar to phase modulation
in that there is essentially a 180˚
phase shift between a 1 and a 0.
The advantage of this scheme is that
a clock can be recovered from the
encoded signal regardless of the data
in the encoding. That clock frequency
can help us distinguish signal from
The principle is quite simple: A 1
is encoded as a 01 sequence, and a 0
is encoded as a 10 sequence. Figure 1
shows waveforms that illustrate how it
With this scheme, we have a viable
data stream. We can then add FEC-like
Reed-Solomon error correction coding
to attempt to fix up or at least detect
the inevitable errors in the bitstream.
There are many different Reed-Solomon polynomials to choose
from which trade off redundancy
for resilience. It might require some
experimentation to see what the
statistical properties of the errors are
in your environment, but some Reed-Solomon coding will almost certainly
be better than none, regardless of the
There is a good article on
Wikipedia about Reed-Solomon at
There are some interesting choices for detector
devices. The simplest would be a photodiode.
In general, silicon photodiodes have low noise
characteristics, but there are other materials that trade
off properties like speed, light spectral response,
sensitivity, dark current, etc.
To go to higher sensitivity, an avalanche
photodiode could be used. These are highly sensitive,
and ambient light filtering becomes critical so that the
device is not saturated.
These devices function a bit like a photomultiplier
tube (very interesting devices in themselves) in that one
photon is effectively multiplied when it strikes a target, and
releases more photons by the photoelectric effect that are
subsequently detected by a semiconductor junction.
These devices can be very fast. You will need a several
hundred volt power supply to bias them properly. I found a
company called Laser Components that has an entire line
of APDs that might work ( www.lasercomponents.com/us/
Don’t forget to use a dense IR filter and a focusing lens
to filter and concentrate the energy. NV
QUESTIONS and ANSWERS
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n FIGURE 1. Manchester encoding (Wikipedia, public domain).