Measure the Speed of Light
lens position so that the beam of light passes through the
holes. Then place the corner-reflector about five to 10 feet
from the bench. While, in theory, the corner-reflector can
be at any angle to the beam, it’s best if the laser light hits
it head on. This will reflect light back and through the lens
and beam splitter. A white card or piece of paper will show
where the beam is focusing into a point. This is precisely
where the optical receiver IC must be located. (Note that
you will probably see two closely-spaced points of light.
This is because the light reflects from both the front and
back surfaces of the beam splitter. With 1/8-inch-thick
plastic, the two points will be about 1/8-inch apart. Thinner
plastic brings the points closer together.)
Once the optical alignment is complete, connect the
high-speed oscilloscope to Pin 3 of the 74S86 (summed
output). (I used an oscilloscope with a 300 MHz bandwidth.) In Photo 2, you can see that I brought this out to a
test point. Connect the probe as close to the chip as possible and do not use a cable or a long length of wire.
Remember, we want to measure nanosecond signals. You
should use a 10X probe for better signal response. With
the corner-reflector removed, you should see the “clock”
signal, as shown in Figure 1. With the corner-reflector in
place, you should see the signal change to the “output
pulse” form (also shown in Figure 1). You may want to fine
tune the optical alignment with the oscilloscope. A good
alignment is indicated by a stable signal with the narrowest
pulse width (more on that later).
In order to adjust the delay circuits properly, we need
to reflect the beam right back from the face of the lens.
Unfortunately, it’s difficult to do this because the beam is
very narrow at that point and reflects back through the
holes in the lens and beam splitter without reflecting onto
the receiver IC. I “cheated” a little by
reflecting some of the beam just as it
came out of the laser directly onto
the IC. This distance “error” is about
six inches (or about 0.5 ns) and I felt
that it was insignificant.
With a direct signal into the
receiver circuit, adjust the capacitors
so that the output pulses narrow and
disappear. At this point, the delay
from the delay circuit exactly equals
the delays from the laser and receiver. (You may have to fiddle with the
delay-component values as previously mentioned. However, my receiver
worked well without any component
adjustments.) Now, we’re ready to
actually measure the speed of light.
shows that same pulse with the bandwidth limited to 20
MHz. As you can see, you will need a 100-MHz bandwidth
for reasonable results. The corner-reflector was placed 10,
15, and 20 feet from the optical bench. This means that
the actual travel distance was twice that (out and back).
Here’s what I measured:
42. 4 ns
Okay, so the results aren’t all that accurate, but — for
$20.00 worth of parts — it isn’t that bad. I suspect that
much of the problem lies with the receiver delay. I would
be very surprised if the light intensity didn’t have a significant effect on its delay (which is why I adjusted the
optics for the shortest pulse). This is evidence that the
intensity changes the delay. I didn’t measure this.
(However, you can do this by keeping the corner-reflector at a fixed distance and changing the laser intensity with an optical filter. This will provide “first order”
compensation values that can be used to correct the
There are a number of variations to the design that are
possible. As mentioned before, a mirror can be used
instead of the corner-reflector. In that case, the beam splitter and lens may not be required (although a lens may still
be helpful). You can measure the speed of light in water by
submerging the corner-reflector in a swimming pool. You
can always redesign the receiver using discrete components for better delay characteristics.
Use your imagination. NV
Photo 3 shows a typical output
pulse with a 300-MHz ‘scope. Photo 4
Circle #100 on the Reader Service Card.