Since our previous
test several things had been happened:
Top: Relative spot sizes and shapes of the LED (left) and
laser (right.) The color and saturation of the two spots has been
exaggerated to make them easier to see.
Center: The spectra of
Bottom: The spectra of
the LED. Note that there was some "thickening" of both spectral
lines owing to saturation of the camera's imager.
image for a larger version.
- I discovered that I had badly misfocused my optical ("lightbeam")
receiver: Subsequent refocusing yielded a 10-15dB improvement
in receiver sensitivity, along with better rejection of off-axis
signals. This testing was done with a simple "optical beacon" (a
500kHz resonator on a 4060 oscillator/counter chip with a normal LED
connected to the divide-by-512 output) placed about 525 feet (160
- I also checked the focus of the "cheap enclosure"
it to be with a fraction of a dB of being optimal.
- The "cheap enclosure"
modified and ruggedized. A prior complaint was that
the bottom of this enclosure (which is constructed of "foam core"
poster board) was not flat and rocked back-and-forth. I shimmed
the bottom of the enclosure and added some scraps of thin plywood to
provide a firm base.
- In order to facilitate aiming, Ron, K7RJ, constructed a simple
"elevation plate" - a simple contraption made from two hinged pieces of
wood with a bolt/nut/washer to adjust the angle between the two plates
and, thus, the elevation. This "elevation plate" may be seen
in the bottom picture of figure 5.
- In order to keep a continuous record of scintillation, we were
transmitting a 4 kHz "pilot" carrier set 12dB below peak
modulation. Because this tone is rather piercing and can be very
annoying, I constructed some 4 kHz notch filters to remove this tone
prior to being sent to the speaker amplifier: The signal being
sent to the audio recorder did not
have this notch filter in its signal path.
- In addition to testing modifications and refinements to the
existing equipment, I wanted to see for myself the difference between
how coherent and non-coherent light propagated, given equal optics in
Each of us when to the same locations as the
last time -
that is, I was on the west side of the Salt Lake valley
while Gordon and Ron were in the northeast corner of the valley, near
the Salt Lake City cemetery - a distance of 14.82 miles (23.85
km.) On this evening the temperature was about 55 degrees F (13
C) at the start and there was only a very slight breeze.
Interestingly, despite calmer air than last time, all of us noticed
that it seemed that the city lights "flickered" more than they had done
during the previous tests. I also noted that re-focusing of the
paid off: The pointing of the received signal seemed "sharper"
than before along with noticeably better off-axis rejection of other
Once a communications link had been established using the Luxeon and
Fresnel lens system, I set up the equipment for the evening's main
The experiment - Coherent versus
The objective of this evening's experiment was to quantify the
difference in scintillation between coherent light as emitted by a
laser diode and noncoherent light as emitted by a standard LED:
It has been suggested that nonuniformities (e.g. "cells" of air
with different temperature, pressure, and humidity) in the air volume
between two sites would result in greater apparent scintillation when
using coherent light sources as compared to noncoherent light sources.
While a similar experiment was done during the last test, it wasn't
fair comparison: While the light from the LED was emitted by a
fairly large aperture (a 250mm by 318mm Fresnel Lens) the cross-section
of the beam from the Laser pointer was only a few millimeters in
diameter. Because the amount of scintillation is strongly related
to the aperture of both the emitting aperture and detecting aperture,
by this fact alone the Laser's signal would have experienced more
This time, we wished to avoid this disparity so we decided to do two
- Use exactly the same optics for
both the coherent (Laser) and noncoherent (LED) transmissions.
- Use exactly the same optics for
reception of both sets of signals.
Because of the distance (about 15 miles) it was necessary to use good
optics to collimate the beam, so I decided to interface to an 8 inch
(20.3cm) reflector telescope (a Celestron C8) that I have. The
of this telescope, taking into account blockage by the secondary
mirror, is approximately 50.27 square inches (approx. 289 square cm) -
a respectable size.
Several months ago, I happened to note that a local hardware store had
Laser levels on a clearance sale for $3 each, so I bought several of
them, knowing that the $3 price tag was a good price for the Laser
alone. Attacking the level with a variety of tools (which
destroyed the level, as it turns out) yielded a self-contained Laser
module that ran directly from 3 volts.
As is, the Laser module could not usefully illuminate the telescope's
optics owing to its very low divergence, so I unscrewed the lens from
the laser module in
order to defocus the beam
to a divergence suitable for interfacing with the telescope's
While this significantly "de-focused" the beam, it was still far too
narrow to be useful: To
remedy this, I removed the plastic lens from the Laser module, turned
it around, and re-installed it, thereby increasing
the amount of divergence, allowing me to more-efficiently illuminate
The optical outputs of typical Laser diodes have a property that
complicates efficient beam collimation: They produce a narrow,
- a property illustrated in Figure 1
the optics, it is difficult to "capture" most of
the beam (something that would require the beam to be made more
rectangular) and properly illuminate the optics of the telescope, so
the only other option is to only take the very "center" of the beam,
discarding the majority of the optical energy: If the beam is
collimated too strongly before being applied to the telescope, the
collimated beam would fully not fill the aperture and would be cut off
on the sides.
For the noncoherent light source, I found a large (10mm) LED in my
parts bin - a Radio Shack
P/N 276-086. This LED is rated for an optical output of 5000 mcd
milliamps with a beamwidth of 30 degrees at a wavelength of 660 nm -
very close to the same wavelength of the Laser as can be seen in Figure
When testing, I could tell that the LED's built-in lens did not
efficiently illuminate the telescope's optics (that is, the LED's beam
was so wide that much light was "wasted" off to the sides) but I was
when comparing the optical output of the LED with that of the Laser,
that sufficient light was available to the telescope, so I did not add
any extra optics.
and Laser modules to the telescope:
Top: The back end of the Laser module. Center: The front end of the
module installed in the telescope's eyepiece mount.
image for a larger version.
The telescope has a standard 1-1/4 inch eyepiece mount and star
diagonal, so it made sense to use it to hold the Laser and LED
modules. Rummaging around my scrap metal, I found some aluminum
that was exactly the right size to slide into the compression-mount
eyepiece holder. To mount the LED and Laser, I
some old Thermalloy (tm) TO-5 transistor aluminum heatsinks which fit
inside some gray ABS pipe that I had laying around.
The Laser module and LEDs were first glued to the TO-5 heat sink, and
this, the heat sink (with the LED or Laser attached) was then glued
inside a piece of ABS pipe. To mount the ABS section inside the
tubing - and to provide a means of precise optical alignment - I
drilled and tapped three 8-32 screws: These setscrews can be seen
in the pictures of Figure 2
the LED/Laser, the screws were loosely tightened and
then, using a thin-blade screwdriver, I was able to reach in and adjust
the orientation of the emitter until it was properly aligned with the
telescope's optics. After this, the set screws were tightened,
alignment re-checked, and then the screws and ABS piece was glued into
When installed (see the bottom image
of Figure 2)
both the Laser and LED completely illuminated the entire aperture of
the telescope. Further testing also revealed that the beam could
be easily focused by the telescope to provide best possible collimation.
The only practical way to linearly modulate the Laser is to use PWM
(Pulse Width Modulation) techniques, so it was natural that I use my PWM
for this task. Because the Laser module
already included the circuitry necessary to be operated directly from a
3 volt battery, I simply constructed a voltage regulator using an NPN
transistor and two red LEDs to provide a regulated 3 volt source.
Because the Optical Modulator uses a current sink as its output stage,
it was necessary to provide a loading resistor to provide enough load
for the circuit to work, so the source voltage (from, the modulator)
was paralleled with a 20
ohm resistor: With proper adjustment of the drive current, the
Laser module was properly driven with the PWM waveform.
Interfacing the LED was much easier: A 150 ohm series resistor
was used to limit the current to the LED to a peak value of about 60
milliamps while a parallel power resistor of about 17 ohms was
used. While the 60 milliamp current exceeds the maximum rating of
LED, the average current would be half this owing to the nature of the
Aiming the telescope:
In our previous outing, it was noted that it was extremely difficult to
the Laser pointer while it was mounted on a tripod: Without
adjustments, the slightest touch would knock the Laser completely
off-point and without a sighting device, it was difficult to get even
The telescope has the obvious advantage in that it's a telescope!
With the 3-watt Luxeon in operation from the far end, I had a ready
upon which I could train the telescope and with it, I could not only
see the very bright red LED, but I could also see Ron's and Gordon's
flashlights as they were standing near the optical transceiver.
I then removed the eyepiece and substituted the Laser module, it took
only very minor adjustments (of both pointing and focusing) to peak the
signal, using my own received test tone being heard via the optical
link from the far end.
It was noted that, with the telescope, the apparent beamwidth of the
Laser was comparable to that of the LED. This telescope has some
very fine adjustments for declination and it took about the same amount
of adjustment to "pass through" the Laser's beamwidth as compared to
the LED's beamwidth, with the total beamwidth being much less than one
signal quality of the Laser versus the LED:
According to Ron and Gordon, the perceived luminous intensity of the
Laser was about the same as that of the 3 Watt Luxeon with the Fresnel
lens, but it was easy to see that it was "flickering" far more than the
LED had been. While the LED in the telescope was visibly dimmer
than the Laser, it
was clearly less "flickery" than the Laser: A quick listen to the
test tone revealed that there
was, in fact, very significant scintillation of the Laser's signal as
the test tone sounded
raspy, as the following clips demonstrate:
Important note: Due to the MP3 audio compression,
the scintillation are lost, but the audio files "sound" pretty much
identical to the uncompressed PCM files to which they were originally
All of the audio clips were transmitted using the 8 inch reflector
telescope and received using a 7"x10" (17.8 cm x 25.4 cm) Fresnel lens
- that is, an area of approximately 70 sq. in, or 452 sq. cm.
Unfortunately, the audio clip of the test tone from the Laser
experienced some clipping (overdriving) when the recording was made -
but clipping occurred only on higher audio peaks. In both clips,
some "clicking" can also be heard in the background (especially on the
LED clip) and this was from the strobes of aircraft flying slightly
above the optical signal path.
A more "real world" comparison may be heard in the clip below.
This clip directly compares the "sound" of the Laser's signal with that
of the LED. In this clip, the Laser is tested first, and then the
LED is immediately installed to assure that both tests were done with
the same atmospheric conditions.
This clip has been edited to reduce its length and improve
continuity: At the end of the clip, K7RJ's comments (transmitted
via LED from the end of the path where the bulk of the recording was
were mixed in from a recording made at my end as received via the
lightbeam link. During the entire
transmission (except during the 1kHz tone segment) a 4kHz "pilot" tone
was transmitted along with the normal audio, but this has been notched
out (except for the brief test segments) to make it more
"listenable" as the 4 kHz tone is really
than the removal of the pilot tone, no other
filtering or amplitude adjustment was done, hence the presence of the
120 Hz tone (and harmonics) from the city lights. The rhythmic
clicking of the strobes of passing aircraft can also be heard in
parts of this recording.
Top: Waveforms of a portion of the 4 kHz test signal
transmitted by the Laser.
Top-middle: Close-up of a portion of the scintillation
of the 4 kHz Laser-transmitted tone. This portion has been
"zoomed in" in both the vertical (amplitude) and horizontal (time) axes.
Bottom-middle: Waveform of a portion of the 4 kHz test
signal transmitted by LED.
Bottom: Close-up of a portion of the scintillation of
the 4 kHz LED-transmitted tone. Note that the rate-of-change of
amplitude is much slower than with the Laser as evidenced by noting the
time scale along the bottom of the images.
Click on an image for a larger version.
The audio clip below consists of the following segments:
- Transmitted via Laser:
- Brief music clip (0:13-0:25)
- Voice announcement of switching from the Laser to the LED (0:13-0:35)
- Transmitted via LED:
- Voice announcement (0:46-0:52)
- Brief music clip (1:01-1:10)
- Brief comments on audio quality (1:10-1:24)
A more in-depth analysis of the scintillation:
audio comparison (MP3 audio file, 1:24,
987kB) Note that the use of short duration (<30 second or
considered to be acceptable fair use under
current interpretations of
U.S. Copyright law. (Music: "Children" [Dream
Version] from the album "Dreamland" by Robert Miles)
The waveforms in Figure 3
offer close-up graphical analysis of
4 kHz tones from the original uncompressed PCM recordings show the true
nature of the scintillation. These waveforms have been
bandpass-filtered to from 1 kHz to about 6 kHz to remove 120 Hz energy
from the city lights and the segments from which the analysis was taken
have been amplitude-normalized to 16 bits full-scale.
As can be seen from the top image of Figure 3
significant scintillation that occurs at a very rapid rate with a
close-up being seen in in the top-middle image. Note that the
top-middle image has been "zoomed in" in terms of time to show the
rapid rate-of-change and amplitude to view the depth of the
scintillation: The reference of this image is, like the others,
based on a full-scale 16 bit sample. Analysis of the
original audio file reveals several things:
- While the "primary" period of scintillation is approximately 10
milliseconds (100Hz) but there is evidence that there are harmonics of
this rate to at least 2.5 milliseconds (400 Hz) - but the limited
temporal resolution of the test tone makes it difficult to resolve
these faster rates.
- Other strong scintillatory periods evident in the audio sample
occur at approximate subharmonics of the "primary" scintillatory rate,
such as 75 and 150 milliseconds.
- The rate-of-change of amplitude during the scintillation is quite
rapid: Amplitude changes of over 30 dB can occur in just 20
- The overall depth of scintillation was noted to be over 40dB,
with frequent excursions to this lower amplitude. It was noted
that this depth measurement was noise-limited owing to the finite
signal-noise ratio of the recorded signal.
The bottom-middle image of Figure 3
shows a typical example of
scintillation from the LED. Analysis of the original audio file
shows several things:
Observations with a 1kHz tone:
- The 10 millisecond "primary" scintillatory period observed in the
Laser signal is pretty much nonexistent while the 20 millisecond
subharmonic is just noticeable.
- 150 and 300 millisecond periods seems to be dominant, with strong
evidence of other periods in the 500 and 1000 millisecond period.
- The rate-of-change of amplitude is far slower: Changes of
more than 10 dB did not usually occur over a shorter period than about
- The overall depth of scintillation was noted to be about 25 dB
peak, but was more typically in the 15-18dB area.
It should be mentioned that on the previous
the measurement of the scintillation using the
Luxeon LED and Fresnel Lens was done using a 440 Hz tone. It was
noted at the time that the temporal resolution of this tone was too low
(e.g. too few cycles) to capture some of the short
term properties of the scintillation - particularly in the case of the
Laser's scintillation - and this resulted in somewhat lower measured
amounts of scintillation that were likely present.
In analyzing the 1 kHz tones from this test, it is more difficult to
realize the apparent depth of the scintillation: On the Laser,
depths of well over 30 dB are readily apparent, but due to clipping on
original recording, the true depth of some of the faster fades cannot
be accurately measured. On the LED, the noted scintillation depth
more in line with that observed using the 4 kHz tone (about 22dB peak)
because the changes were slower and were more easily captured at the
Observed scintillation using the Luxeon and the Fresnel Lens:
A final test was done using the Luxeon and Fresnel lens
combination for transmitting and the same Fresnel as before for
receiving. For this test, the LED current was reduced so that
it was visually similar in brightness to that of the LED in the
telescope. The Fresnel lens has a larger aperture than the 8 inch
reflector telescope - about 117 sq. in. (759 sq. cm) - a bit more
twice the area, so it was expected that lower scintillation would
result. A typical sample of the scintillation on the 4 kHz tone
may be seen in Figure 4.
Measuring using the 4 kHz tone, the peak scintillation was in the
17-20dB area and the rate of change of amplitude was comparable to that
observed when using the telescope and LED for transmitting. A
this may be heard in the following recording:
Typical scintillation of a 4 kHz tone transmitted from a 107sq in
(690 sq cm) Fresnel lens.
Click on the image for a larger version.
- Transmitted via LED with the Fresnel lens:
- Voice announcement (0:00-0:02)
- 1kHz tone (0:02-0:06)
- Brief music clip (0:10-0:14)
- Brief voice comment (0:14-0:16)
- Luxeon with Fresnel lens
(MP3 audio file, 0:16, 185kB) Note that the use of
duration (<30 second or 10%) music
considered to be acceptable fair use under
current interpretations of
U.S. Copyright law. (Music: X-Files theme by Mark Snow, DJ
As in the previous clip, the 4 kHz tone present through much of the
recording has been notched out - except during the specific 4 kHz test
signal. Also present is the 120 Hz hum from urban lighting.
Comparisons with the previous test:
As mentioned before, during the previous
we used some ordinary Laser
pointers and did comparisons with those and the Luxeon-Fresnel
combination, and as expected, the effects of scintillation were quite
severe. Unfortunately, for this
test I did not
have a standard
Laser pointer that could be modulated, and the Laser module installed
the telescope could not be used by itself because it no longer produced
a collimated beam.
While it would have been nice to do a side-by-side comparison, it is
likely that the air conditions during these two tests were quite
similar, so a rough comparison between the "small aperture Laser" and
"large aperture Laser" would still be valid.
As can be heard from these recordings:
Laser pointer - This audio clip is the sound of the unmodulated
Laser signal: The fluttery "hissing" noise (reminiscent of a flag
flapping in a strong wind) of the scintillation is clearly
audible. (MP3 audio file, 0:17 seconds, 346 kB)
Laser pointer - The distortion of the voice is due
to the extreme modulation by the scintillation. (MP3 audio
seconds, 34 kB)
There is a distinct "fluttery" noise in the background solely due to
the scintillation while the magnitude of the signal emitted by the
large aperture Laser transmitter exhibited this effect to a much lower
degree. Analysis of these recordings show that the rate-of-change
of amplitude of the light from the Laser pointer is higher than that of
the larger aperture telescope - a property that significantly distorted
Because Ron and Gordon reported receiving badly
distorted audio, I checked the audio quality of the signal that I was
transmitting using a local receiver and found it to be clean.
Also, because no 4 kHz tone was transmitted at the time of this test,
comparisons pertaining to the "dA/dT" (rate of change of amplitude over
time) of the Laser pointer's signal could not be made.
Is should be noted that in the case of the Laser pointer, its light
needs to travel only fraction of the distance of the total path (1 or 2
kilometers) before its
normal divergence causes the beam size to exceed that of the beam size
emitted by the telescope,
so much of the added distortion of the Laser pointer's signal is from
through only that first portion of the optical path.
As expected, the coherent light experienced more scintillation than the
noncoherent light and that the rate-of-change of amplitude of the
coherent light was much faster (and of greater depth) than the
noncoherent light. In this test, both light sources were
collimated approximately equally and from the same size of
aperture. While much of the Laser's scintillation was due to the
air turbulence, it is very likely that the severity of the
scintillation was increased due to wavefront cancellation occurring due
to the differing velocities in the cells of air through which the light
passed. Comment: No attempts were made to "dodge" any
nulls in atmospheric transmissivity that might have been occurring at
operating wavelength of the Laser.
It has occurred to us that the light being emitted by the telescope is
probably not phase-coherent across the entire aperture - and this
doubt, increase the likelihood of scintillation. It should be
noted that achieving such phase coherence across such a large area
would require extremely precise optics - something that the average
experimenter would not
likely be able to find or
afford. What is
significant is that, using just an ordinary high-brightness LED
operated at a few 10's of milliamps it was possible to get results that
were superior to those obtained when using the Laser.
It also occurred to us that not every experimenter has a large
reflector telescope that can be dragged around - and if they did, the
bulk, weight, and fragility of this piece of equipment would greatly
limit exactly where
it might be taken. While the
telescope has extremely good optics, it has also been shown that very
good results can be obtained by using an inexpensive plastic Fresnel
lens. While this lens is not as efficient a collimator as the
telescope, one can overcome the Fresnel's inefficiencies by brute
force, using an extremely high-output LED such as the Luxeon III - both
techniques that quite practical to the experimentor.
Using a normal LED with minimal optics:
Just prior to the test, Ron connected a standard red LED - mounted in a
small flashlight reflector - to his PWM Laser modulator just to see if
it could be detected, or if I could see it with the telescope:
Because I could see their individual LED flashlights, it was hoped that
LED would be powerful enough to be detectable: It wasn't.
This was surprising, actually, as I expected that something
would be detectable by ear. This test was thrown together at the
very last minute and it probably failed owing to the fact that the
reflector chosen (a cheap flashlight) was not well-suited for focusing
the LED that Ron chose. If the LED and reflector had been
better-matched, I have no doubt that something
would have been
According to the initial analysis of the audio file
using narrowband techniques, it is possible that a signal may
have been detected - but it was more 90dB below the peak audio level
received from the Luxeon and Fresnel lens.
Using the Luxeon emitter with its secondary lens:
Another experiment was one in which I removed the Luxeon emitter (with
its secondary lens) from the Fresnel lens enclosure resulting in an
effective beamwidth of around 40 degrees or so. Ron reported
that with binoculars, the 3 watt Luxeon LED (with its secondary lens)
visible and he could
hear the "peaks" of the tone via speaker. Further analysis of the
indicate that removal of the LED from the Fresnel reduced their
received optical power by 55-60dB on average with scintillatory peaks
at higher levels.
Here is a sample of the signal - but
turn your volume down for the first part :
- Luxeon with secondary
lens, but no Fresnel (MP3 audio file, 18 seconds, 217kB)
few seconds of this file are with the LED installed in the
Fresnel mount, but after 6 seconds, what tone is heard is from the
Luexeon and secondary lens, only.
Yelling at our gear:
Top: The method used to modulate the telescope.
Middle: Waveform of the Kennecott Strobe. Three
pulses (negative-going spikes) are evident.
Bottom: The "cheap" enclosure strapped down to Ron's
adjust" contraption - photo, by Ron, K7RJ.
Click on either image for a larger version
After we did all of the experimentation with the LEDs and Laser, we
were wondering what else to do before we tore down our gear. Ron,
out of curiosity, decided to shine a portable 1 Million Candlepower
spotlight at me: As expected, I could easily hear the thermal
roar as he did so - but I noticed something else: A "bong" has he
was handling the spotlight - apparent microphonics of the filament, as
the following audio file demonstrates:
Or, for an shorter version:
- Modulating tungsten (MP3
1:46, 1252kB) In this stereo file, I am heard
(via optical link at Gordon and Ron's location) on the left
channel while the right channel carries the audio from
my optical receiver. It is best to use headphones when
listening to this file. Minor editing and filtering as been done
on this file to reduce its length and to make it more listenable.
- Ron screaming at a
spotlight (MP3 audio file, 0:13, 213kB)
of this audio file is what was received while the second
part has noise reduction and pre-emphasis applied in an attempt to
improve intelligibility. (It didn't help...)
Not to be outdone, I decided to try a similar thing, so I installed the
LED and began shouting at various parts of the telescope's
anatomy - see the top image in Figure 5
. After a
few experimental shouts, we determined that the
star diagonal seemed to be the most sensitive part, so I screamed at
- Clint screaming
at a telescope (MP3 audio file, 0:17, 269kB)
the previous file, the first part is unprocessed while the
second part uses noise reduction to help with intelligibility. (Again,
Shortly after this (and with a hoarse throat from the screaming) we
decided that it was getting late (it was past midnight) so we packed
our gear up and headed home.
Pointing the receiver elsewhere:
From where Ron and Gordon set up, they could see the very tall (1215
the Kennecott smelter
to the west of
their location. Because of its height, it is well-marked with a
set of strobe lights to provide warning to aircraft.
Being curious, they pointed the optical receiver at the smokestack and
from the sound of it, they could hear what sounded like multiple
flashes. Later examination of the audio file bore this out, as
seen in the middle picture of Figure 5.
- Kennecott Strobe
(MP3 audio file, 0:35, 482kB) The "clicks" of
strobe are very apparent - as is the harmonic-rich 120 Hz hum from
As can be seen from the waveform, there are at least three
flashes. It is also apparent that the combination of optical
receiver and digital recorder inverts the polarity of the received
audio, so the initial impulses from the flashes are negative
with a little bit of overshoot on the return. Also apparent in
the image is the presence of 120Hz energy (from city lights) and
As mentioned before, we often heard the "clicks" of aircrafts' strobes
as they flew by: Even though these were off-axis, their strobes
were intense enough to be easily detectable. In addition to the
strobing of aircraft, I could also hear a nearly constant "tick" in the
background - but I never spotted the source of this noise.
More may be added to this page soon - check back again.
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laser beam, modulated light, optical communications, through-the-air
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Copyright, KA7OEI 2007-2011.
page last updated on 20110304