Lasers versus LEDs:
experiment with any laser, here are a
few things that you should know:
- The use of a high-power laser (Class 3B and
Class 4 - those above 5 milliwatts) is
restricted in many countries in all but controlled
environments. It is up to YOU
to determine and comply with the regulations in your
area. Lower-power (Class 3R/3A, Class 2 or
even Class 1) laser pointers may be regulated
(or even banned)
in some jurisdictions.
- Lasers of any power level can be
hazardous! Even if they are of too-low
power to be capable of direct physical harm, flashes
from lasers can be distracting to drivers and pilots
if used in an irresponsible manner!
- The use of other than red lasers is not
recommended for these sorts of experiments if best
performance is the goal. Because the eye
far more sensitive to the green than the red
wavelengths, a green laser is more likely to be a
distraction. Additionally, some green (and
blue/violet) lasers are modulated -
intentionally or not - and have additional circuitry
- either of which can make modulating them
difficult. Many of these lasers are of the
"pumped" type (e.g.
DPSS) using a
crystal to transform the wavelength of the light and
as such, the temperature range over which they will
operate efficiently is quite limited. Finally,
note that silicon detectors are much less-sensitive
to shorter (green/blue) than longer (red)
wavelengths which means that your receiver (and your
links!) will simply not work as well!
- NIR (near-InfraRed) and IR lasers may be
implemented provided that their wavelength is
compatible with the detectors and optics, but can be
understandably hazardous difficult to use since
their beams may be invisible.
- Again, it is up to YOU to determine the
legality of the use of a laser in your locale
and to make sure that it is used in a safe and
Readers of this, the modulatedlight.org
web site, should be well-aware that it it our contention that
for some applications, non-coherent light is preferred over
- The atmosphere de-coheres light. Differences
in atmospheric density (caused by changes in temperature,
humidity - among other things) disrupts the phase-coherent
nature of laser light by the time it has traveled a few
kilometers through atmosphere, its coherence has already
been lost. Because coherence is rapidly lost anyway,
there would seem to be little advantage to starting out with
it in the first place.
- Coherence can exacerbate scintillation. In
the distance that the laser light travels while still being
somewhat coherent, the atmospheric variations - the same
ones that cause loss of coherence - result in constructive
and destructive effects on the light which causes random
fluctuations in brightness referred to as scintillation
and these variations can disrupt information being conveyed
on the light. See the Comparison
Light web page for a demonstration of the
effects of atmospheric propagation on these types of
- It is difficult to collimate coherent light to a large
diameter. When dealing with coherent light, it
is necessary to use very accurate "diffraction-limited"
optics - those that are accurate to 1/4 wavelength or better
- to minimize scattering and loss by those lenses. As
the diameter (and size) of these components increase, so
does the weight and cost - not to mention the practicality
of their use. It is desirable to use large-diameter
beams to minimize scintillation and as distances increase,
so does the preferred beam diameter.
Despite these (and other)
challenges, laser pointers are
attractive in that they are fun, cheap, readily available,
reasonably safe if low-power devices are used, that they fairly
easy to modulate using PWM techniques, and of course, lasers
A note about the techniques
and equipment described on this page:
For the purposes of this web page we are describing only the
hobbyist/experimental use of lasers to convey voice or
low-speed digital information. Other aspects of laser
experimentation such as holography, range-finding and
atmospheric profiling (to
name but a few) aren't covered.
The goals described here (e.g. long-distance laser-to-laser
communications at audio-frequency bandwidths) can also be
achieved through the use of lab-quality lasers, precision
optics, specialized detectors, and/or precise aiming devices
such as special-purpose tripods, detector mounts, telescopes
or survey equipment. If you own or have access to such
equipment, by all means - feel free to use it!
Note, however, that this page is
specifically directed toward those who wish to perform these
sorts of experiments using materials and equipment that would
likely to be available to a hobbyist with a
limited budget. Considerable efforts have
been made to describe simple and effective techniques and
high-performance equipment that could reasonably be replicated
with the patience and skill to do so.
Whatever you do, be safe!
Examples of laser-pointer communications systems:
Low-power, inexpensive red laser
are ubiquitous these days which make them
nearly ideal devices with which one can experiment while their
low power level makes them fairly safe to use. Even the
cheapest pointers have built-in lenses that produce reasonably
beams - albeit with source diameters of only a few millimeters -
that are capable of being seen over quite a distance with the
naked eye - over 100km under good conditions!
For voice operations, most inexpensive laser pointers are very
easy to amplitude-modulate
techniques and an example of a basic laser-based PWM system can
be seen in Figure 1
. This circuit, designed by
Ron, K7RJ, was intended to be as simple as possible
to demonstrate the use of
such techniques to modulate voice onto a laser pointer using
readily-available components - and it is this very same laser
pointer that can be seen in Figures 4a
below. Also on the schematic is a very basic
photodiode-based optical receiver, but because the intent of the
project was that of demonstration and to test the modulator
effort was made to
maximize sensitivity any more than necessary to achieve a very
short-range (up to 100 meters or so)
Figure 1: An
ultra-simple PWM-based AM laser communications system
designed by Ron, K7RJ.
Figure 1a - Top Left: The schematic of the
laser communications system. The receive circuit was
designed solely for short-range (across-the-room)
demonstration and absolutely no attempt was made to
optimize its sensitivity.
Figure 1b - Top Right: The
controller/modulator (on the table) and the laser pointer
module (on the tripod.) The two are separate units,
connected by a cable so that there are no adjustments on
the laser itself that could disturb the precise pointing.
Figure 1c - Bottom Left: Inside the laser
pointer module. A cheap laser pointer was "gutted"
and mounted in a small plastic project box with only the
laser, Zener diode and a few other components mounted with
it. Below the laser is a white piece of plastic
tapped with 1/4-20 threads to allow it to be screwed to a
standard tripod mount for testing.
Figure 1d - Bottom Right: Inside the
controller/modulator box. Extra board space was left
for the later construction of the tone generator that is
used to aid in the pointing of the laser.
Not shown in any of these pictures is the "receiver"
Click on an image for a larger version.
A somewhat more-complicated PWM circuit is that described in the
Simpler Pulse-Width Modulators for LEDs and whatnot".
This modulator includes the ability to generate various test
tones which are very helpful when it comes to setting up any
sort of optical communications system - whether they are LED or
If you don't have the desire to build your own system from
scratch there are a number of kits available, including the Ramsey
LBC6K Laser Communicator kit
. (Note: Ramsey Electronics is no longer in the kit business, but some kits may still be found.)
kit consisted of a pulse-width modulated laser pointer and a
simple optical receiver consisting of a phototransistor and
audio amplifier. The "transmit" performance has been
reported to be "adequate" for a laser pointer, although it's
been recommended that a switch-selectable "manual" gain control
be added to its circuit to supplement the
built-in "audio AGC
Again, the "receive" portions of the Ramsey kit and that of the
circuit shown in Figure 1
aren't really suitable for
distances longer than several hundred meters - and for several
- The use of a phototransistor. While cheap and
easy to use, phototransistors (as was used in the Ramsey kit)
the best choice when it comes to good receiver performance -
although the sensitivity of the Ramsey kit overall was better
than the much-simpler circuit depicted in Figure 1.
Not only are phototransistors relatively slow, but under
very low-light conditions their own noise contribution tends
to mask the weak photon-induced signals. In a simple
demonstration circuit where highest performance is not the
goal their inherent self-amplification make them reasonable
choices when simplicity
is of greater importance.
- The lack of low-noise, high-gain amplification.
The circuit in Figure 1a wasn't really designed for
either low noise or high sensitivity and it simply cannot
make full use of the weakest signals that might be coming
from the detector. Similarly, the circuit that was used in the
Ramsey LBC6K - while significantly better than that circuit
in Figure 1a - was not optimized for the best
- No additional optics are used.
Phototransistor or photodiodes by themselves have very small
photo-active areas and because of this they can only
intercept relatively few of the photons from the
laser. While a brief reference is made in the Ramsey
manual to add a lens, the basic kit does not include them
and the instructions give no guidance as to their selection
or use. Lenses are the best way to noiselessly
add considerable receiver gain over the "bare"
phototransistor and they have the advantage of limiting the
field-of-view of the receiver to prevent off-axis light
sources from degrading receiver performance: Even the addition of a small
"magnifying glass" lens can make a tremendous
If you wish to further-improve your receive capability, I'm
afraid that you'll probably have to build the gear yourself!
Doing so can be fairly easy and inexpensive, but it requires a
bit of patience and care. A few examples of systems that
can offer excellent receive performance can be found at these
- "A Highly-Sensitive
Optical Receiver Optimized for Speech Bandwidth"
- This describes a field-proven circuit - having been
replicated by many others - that offers excellent
sensitivity through 2-3 kHz - a bandwidth suitable for voice
and low-speed digital communications and has been
successfully used for subcarrier (20-30 kHz) operations from
the "flat" output as well.
Wiring and mounting a laser module
In addition to hand-held laser pointers, suitable
low-power laser modules may be found in tools such as levels
and often in give-away promotional items. While a
pen-shaped laser pointer may be easier to modify and
re-mount, it should be practical to (carefully!) extract and
re-mount the laser modules from these other devices as well.
Optical Enclosure - cheap version" - This
page describes an optical system constructed from
"foam-core" paperboard and using inexpensive "page
lenses. Despite its being cheap and
lightweight, it has been proven in the field to be fairly
rugged and capable of good performance, having been used to
receive optical signals over a distance greater than
172km (107 miles.) This page includes
links to yet higher-performance Fresnel-based lens
Note that all laser pointers consist
of more than just a switch with a connection to a
battery: There will be a simple circuit to limit laser
current - usually on a small circuit board attached to the
body of the laser module in some way. With the cheaper
laser pointers this circuit may consist of one or two
transistors with a few other passive components but some of
the very cheapest pointers use just a resistor for limiting
current! Whatever form this circuit takes it's a good
idea to document its connections and its original power
requirements to preserve and use it later on.
Minimally-modified laser pointer showing the power
connections made using a wooden dowel. This dowel
replaces the AAA-size batteries used to originally run the
laser, providing external power connections. The
laser pointer is glued to the black plastic box that
contains the voltage regulator for the laser and to this
box is attached an aluminum plate into which threads have
been tapped for the camera mount. A piece of foil
tape was used to hold the button in the "on" position.
Click on the image for a larger version.
If one is using a cheap laser pointer there are several ways
to mount it. In Figure 1, the front portion of
a laser pointer was removed from the rest of its body,
carefully noting where the original battery connections
went. In most (if not all) cases, a cheap, red laser
pointer has the positive side of the battery
connected to the case - and the unit shown in Figure 1
was no exception! Because of this, it is recommended
that the laser module be mounted in a plastic
case so that it may be electrically isolated from the
negative "ground" connection of other circuits.
Another example of a laser pointer being mounted is that
shown in Figure 2 (to the right). In
this case, I couldn't easily see how to remove the laser
module from the pointer's body without some
possibility of destroying it so I simply decided to
use it as-is and fashioned a "fake battery" to make the
necessary power connections. I found a wooden dowel
that was about the same diameter as the AAA-type cells
originally used to power the laser pointer and, with a saw,
cut a groove along its length and into that groove was laid
a wire that I soldered to a small screw at the end of the
dowel to make the negative power connection.
Around the end of the dowel opposite
the screw I wrapped some copper foil tape to make a snug fit
when screwed into the original threads at the end of the
barrel of the laser pointer and to this foil a piece of wire
was soldered for the positive power connection. The
dowel assembly was then put into the laser pointer
simulating the pair of AAA cells with the screw making
contact with the spring inside the laser and taped into
place. Finally, the laser's "on" button was simply
taped down and the laser pointer itself was attached (using
thermoset glue) to a small plastic box that contained the
simple electronics to regulate the voltage applied to the
laser as well as electrically isolating the positive pointer
body from everything else. This same laser pointer
module appears in Figure 2, 3 and in Figures
Because laser pointers typically run from a pair of
Alkaline cells or a lithium coin cell, their nominal voltage
is around 3.0 volts - although this can vary a bit.
The circuit shown in Figure 1a (above) can be used
to drive a laser pointer, as can the circuit shown in Figure 2b
on the "Simple
PWM Circuit" page.
- It is not recommended that you use a
raw "laser diode module" of the sort often found in
electronics parts catalogs unless you know exactly
what you are doing!
These are often more
expensive (in the $15-$100 range) and often do not
have the necessary current-regulation circuitry. If
you have one of these, I would recommend that you set it
aside and use a cheap laser pointer instead!
Important notes about modulation of
Do not attempt
to modulate a laser diode by varying the voltage!
Laser diodes, like plain, ordinary diodes, have
voltage/current curves that can be extremely steep and vary
with temperature and even seemingly-identical devices from
the same manufacturer can have significantly different
operating characteristics. Like other semiconductor
diodes, a laser diode will not seem to draw current at very
low voltage until they hit the voltage at which start to
conduct, at which point the amount of current that will flow
will go up more-or-less exponentially with respect to
voltage: The difference between a laser being "off"
and being destroyed by too much current may be only a few
10's of millivolts and it is for this reason that all
laser diodes have some sort of current regulation scheme
incorporated within their operating circuitry.
As mentioned above, most "cheap" red laser pointers have
very rudimentary current regulation circuits - some of them
being as simple as just a single resistor. In these
cheaper laser pointers, there are few components (such as
capacitors) contained in the regulation circuitry that will
significantly affect the ability of the laser diode from
being turned on/off quickly as needed for PWM, FM or
high-speed data - even into the megahertz region.
Ironically, some of the more expensive laser modules do
contain more-sophisticated circuits used to regulate and
protect the laser and it is often the case that these cannot
be so easily modulated owing to the inability of the circuit
to respond to being turned on/off rapidly. Attempts to
so-modulate such a laser may, at best, not work very well
and at worst, confound the circuits' operations and expose
the fragile laser diode to higher-than-intended currents and
damage or destroy it. Many "non-red" lasers (e.g. yellow, green, blue,
blue/violet) - as well as higher-power devices of any
color - fall into this category.
In other words, Cheaper
may be better! It is recommended that you
start out with the cheapest red laser pointer that you can
find (and buy several!)
and that way, if you accidentally destroy it, you won't be
out much money!
In addition to PWM, it is common to find schemes that
modulate the laser current directly. While this method
of modulating a laser is possible, it has several practical
difficulties - mostly relating to the problem of not
knowing exactly how much you can modulate the diode.
- If the diode current goes too low, it stops lasing.
too-little current, lasing stops and the laser operates more
like a standard LED.
- If it goes too high, it will also stop lasing - permanently!
Even an extremely brief pulse of excess current can destroy
a laser diode instantly! Any system that modulates a
laser by varying the current should have a "hard" limit to
set the maximum amount of laser current from, say, voice
peaks, "clicks" caused by powering up/down the gear or
transients from connecting it to another signal source such
a s a portable player or computer.
What's worse is that the "low" and "high" extremes vary widely
from diode-to-diode (even those with the same part number)
well as over temperature - particularly with the extremely cheap
laser pointers. Not knowing the full range over which the
current can be safely controlled makes it more difficult to
"100% modulate" the diode and this can reduce its effectiveness
for communications! It is also worth mentioning that the
relationship of light output to laser current isn't a linear one
over the entire operating range which means that some distortion
of the audio/signal will inevitably result - but unless your
application requires high linearity and very high quality audio,
this shouldn't be much of a problem.
In short, the use of PWM sidesteps most of these problems as the
laser is never exposed to excess current as it is simply
switched on and off to "simulate" modulation of the beam's
In other words, the safest way to modulate a laser diode is via PWM!
For practical information about the inner-workings of
lasers, laser pointers and laser safety, see Sam's
What about FM?
At this point it should be noted that thus far we have discussed
in depth only
schemes in which the information being conveyed via laser with amplitude
of a "baseband" signal rather than some sort of discrete, modulated
carrier that is, itself being modulated that is what carries the
intelligence. Over the years a number of other schemes have been
described in various articles, many of which utilize FM subcarriers
to convey voice and/or data. (Note:
Even the FM carrier is represented via amplitude modulation since
we are using radiometric detection rather than attemptiing to
"modulate" the optical signal by varying the wavelength of the light
The use of FM
has its merits:
- Noise rejection. The primary advantage of
frequency modulation is that its detection scheme inherently
rejects noise - as long as the received signal is
sufficiently stronger than the noise sources that
are inevitably present. Because the information is
conveyed as a varying frequency rather than a change in
amplitude, the detector can "limit"
the received signal - that is, convert it to a
constant-amplitude signal in the process of detection and
demodulation. With sufficiently-strong signals the
received audio will be free of noise from various sources as
well as free of the amplitude variations that result from
scintillation. In other words, an FM-based system can
sound really good - but only if
signals are strong enough.
- High carrier frequency. The modulated carrier
frequency of an FM-based system is typically above the
hearing range, placing it well above the frequency band in
which "hum" from city lights (and the harmonics) is
heard. Since there is little energy from these
potentially-interfering sources in the passband of a
properly-designed receiver operating at these "ultrasonic"
frequencies, further rejection of potentially-interfering
noise sources is afforded.
The use of FM does have a few disadvantages:
- Complexity. A disadvantage of frequency
modulation is that the receive system is significantly more
complex. To detect it, you must first build an AM
optical receiver and then feed the signal from it to
an FM demodulator of some sort to recover any audio.
If a computer is available, one may use a DSP program such
as DL4YHF's Spectrum Lab ("Google"
it!) to both modulate and demodulate signals - Contact me using the
information at the bottom of the page if you need more
- Reduced system sensitivity. The biggest "hit"
comes from the fact that out of necessity, relatively high
frequencies - those significantly above the speech range -
are used. Because of the nature of detectors such as
phototransistors and photodiodes it is extremely difficult
to achieve both good ultimate sensitivity and
high frequency response, as one must be traded for the
other. As it turns out - unless you were to use
more-exotic detectors (such as photomultipliers or avalanche
photodiodes) - you may lose 20-40dB of detector sensitivity
at the necessarily-high frequencies required for FM
subcarriers in comparison with a simple "amplitude
modulated" system that uses pulse-width (or current
modulation) and an "AM" type detector.
In other words, if you don't mind the added circuit complexity
and want very high-quality, noise-free communications - and you
don't mind the sacrifice of a significant amount of achievable
range to do so - an FM system may be appropriate. You
should be aware, however, that the scintillation experienced on
a laser-pointer communications system over a span of 10-20km can
easily exceed 40dB under normal conditions - a depth that is
likely to introduce noise into all but the most-robust FM-based
Several FM-based systems may be found in published sources as
well as elsewhere on the web - see the link to Max Carter's page below
and one was described in the CQ Magazine "Math's Notes" columns
in February and March, 2010. Since I have not experimented with a
wide variety of these circuits, I don't have a particular
recommendation of one over the other although Max Carter's circuits
appear to be well thought-out and generally representative of the one
of the better ways to accomplish the goal.
subcarrier schemes - SSB:
Over the past several years, the U.K. optical experimenters
have successfully use SSB (Single SideBand)
subcarriers to sidestep the problem with interference from the
hum of mains-powered city lights and their harmonics.
Converting from 80 meter (3.58 MHz)
to/from about 25 kHz, SSB
provides both narrower bandwidth, more voice transmission with a
given amount of power and lower signal thresholds than those
available from the FM schemes mentioned above. With such a
system significant weak-signal performance may be had when
compared to an FM-based system and line-of-sight ranges in the
10's of kilometers can be easily attained!
Unlike FM, the use of SSB requires a linear
modulator which is very easy to manage with LEDs but as noted
above, this is a bit of a challenge with laser diodes. The
recommended way to do this would be to simply use a PWM scheme
with the switching frequency being significantly higher (say, 3x
than the SSB "carrier" frequency using a circuit like
that depicted in Figure 1,
perhaps using a faster op amp than the 741 depicted to handle
the subcarrier frequency: For a 25 kHz subcarrier
frequency, a PWM frequency in the 80-120 kHz range would be
It is worth noting that with SSB, it is necessary to transmit
only one half of the RF cycle so the voltage on pin 3 of the 741
may be made variable (using a potentiometer in the 10k-100k
and adjusting it
so that the LED just
illuminated when there is no audio.
As with FM, one may use a computer running DSP software to
modulate/demodulate SSB: Contact me using the information at the bottom of the
page if you need more information.
If you are starting out, I'd STRONGLY
recommend that you begin with a simple AM
modulator/demodulator scheme and get that working before
using a subcarrier scheme of some sort - especially since
you'll likely be able to use the same transmit/receive
gear anyway with little modification!
How to set up a laser-pointer communications
system over very short distances
Before you go out into the "field" it is strongly
recommended that you set up a laser communications system over a
very short distance - say, across a yard or field that spans a
distance of no more than a few hundred meters. When you
plan such a test, the area should be selected that the beam
cannot find its way onto a roadway or across a nearby airport
either as the beam traverses to the distant end or as it goes
past the distant end as the distraction caused by even a very
low-power and otherwise "harmless" laser can still be dangerous!
Remember: There may be a road or airport beyond
your test range into which your laser beam can spill!
At these short distances it is possible for the person pointing
the laser to see the distant end and the "spot" produced by the
laser hitting the target. It should go without saying that
being able to see the spot produced by your transmitter greatly
simplifies the aiming process - and it also goes a long way
toward getting the "feel" for how your equipment will
work. It will also reinforce the realization that many
people who go out into the field to attempt laser-pointer
communications underestimate the practical difficulties
Even at such short distances it is highly recommended that you
have assistants helping you along with a 2-way communications
system if you don't want your voice to become hoarse from
yelling. If both parties are radio amateurs, simplex radio
communication is a natural, otherwise inexpensive FRS
to communicate back-and-forth. Finally, one could also use
telephones to communicate if you don't mind
burning up your airtime minutes and running down your batteries!
The use of cell phones do have a distinctive disadvantage
Because they are digital, they have a rather obvious end-to-end
delay that becomes increasingly apparent when you are trying to
do "real-time" pointing. Hearing the sound of your beam
going past the receive end's detector by listening to its
speaker via the telephone will be slightly delayed (up to
several hundred milliseconds)
and this delay can make aiming
awkward. Also, being digital, a tremendous amount of
"lossy" audio compression causes those brief tones and
background noises (such as those emanating from your optical
to "confuse" the audio compression, often resulting in
what you are hearing over the telephone sounding very
different from what you
would have heard directly from the receiver! (If you
have ever heard what "music-on-hold" sounds like via your
phone, you have already heard how badly the digital
compression can mangle common, everyday sounds!)
While applicable to only fairly short distances, it is strongly
recommended that one surrounds the target with reflective tape
and/or inexpensive bicycle/yard reflectors. Because of the
" construction of these many of these reflective
devices, they will readily light up when your laser hits them
making it much
easier to find
the distant target in the dark.
Even at such short distances it becomes very apparent how
"touchy" the aiming of a laser pointer really is and one of the
first things that is discovered is how useless
a typical photographic tripod can be as a means of aiming a
- There are certain types of tripods (such as those used for
motion picture production or survey equipment) that may be
suitable for these purposes, but these are likely to be
specialized, heavy and expensive devices and not
the sort of things that the average person is likely to have
The intent here is to describe a system that can be
assembled using components that are inexpensive and readily
available and/or constructed at home.
Why aren't standard tripods very good for aiming lasers?
- A standard photographic tripod is not a
precision pointing device. When pointing a
camera, you simply aim up/down, left/right as needed,
looking through the viewfinder. Almost all tripods
have some degree of "backlash" - that is, the tendency for
the tripod to move backwards slightly once you take your
hand off it. Since this amount of backlash is usually
less than a degree or two, and since there is usually no
reason to try to aim a camera with such precision, this
isn't really a problem for the photographer. When
trying to point a laser, even a fraction of a
degree of backlash is too much! Ironically, the cheap
tripods that don't have features like a "fluid head"
are slightly better in this respect as the viscous fluid is
one of the aspects of a tripod that can greatly contribute
to backlash! (General "flimsiness" contributes to
backlash as well, but this is generally quite manageable
on a reasonably well-built - but inexpensive - tripod.)
- There are no means by which minute, repeatable
adjustments may be made. When you use a tripod,
one simply moves it back and forth or up and down to point
the camera and instantly seeing the results through the
viewfinder. When doing so, one has little sense of
exactly how much one is moving it,
particularly if you can't easily see where the laser is
being pointed in the dark! For laser work, having a
sense of how far, exactly, the pointing has been moved is
important if you are trying to scan back and forth several
degrees while making minute adjustments to the
elevation. Not only is it difficult to know exactly
how far to the left and right you have moved each time, it
is arguably more difficult to adjust the elevation
(up and down) by a known amount with a tripod as all you can
do is loosen the elevation's lock screw, make a guess on how
much you have moved it, and re-tighten it.
one use for aiming the laser if a
tripod isn't suitable? We'll cover that shortly.
The above problems are difficult enough to deal with when you
are attempting to set up over very short distances and are able
to see what you are aiming at, but you don't need increase the
distance very much before you can't see your spot reflecting off
the far end and have to rely exclusively on feedback from the
distant end in your aiming!
How to aim your laser pointer with precision
For longer distances over which you cannot see the terminus of
your own beam, you will require some sort of feedback from the
other end to assist in aiming the laser and at the very least,
this can come from observers who are reporting what they are
seeing. If you are really serious about this, it is
possible to use an "electronic" aiming aid as will be discussed
This topic of precisely pointing the laser could be the subject
of several web pages by itself, but in the interest of brevity,
we'll cover only two methods:
- Using a telescope/mount, and
- Using a home-built device mounted to a suitable
While there are many other possible methods of
precisely pointing a laser such as using a theodolite or
transit - especially one that may, itself, contain a laser
that could be modulated - we are concentrating only on those
methods that are likely to be accessible to the average
experimenter and can be done with little cost.
Use a telescope mount:
Many "inexpensive" telescopes (i.e. those that can be had as
new for $300 or less)
have 2-axis mounts - either "Az/El
(left/right and up/down)
or a so-called "Equatorial
" - the latter often incorporating a "star
drive" motor (which we wouldn't be using in our
to track the apparent motion of celestial objects
as the Earth rotates. (It
is worth noting that typical Dobsonian telescopes have neither
Az or El adjustment screws and rely instead on friction pads
to keep the telescope stable and as with a typical tripod,
it can be difficult to make precise, repeatable up/down and
Figure 3: The laser
pointer module shown in Figure 2 (above) attached to the
camera mount of an 8" reflector telescope. The
Equatorial mount of the telescope provides a stable and
adjustable platform for pointing the laser.
Click on the image for a larger version.
Many of these same astronomical telescopes, such as the one
shown in Figure 3
, also have a 1/4-20 screw mount
intended for attaching a camera and one could also use this same
mount to attach a suitably-packaged laser pointer. If you
own such a telescope - but it does not
already have an
accessory/camera mount on it - it may be possible to add one,
possibly by using straps, hose clamps or stretch bands to attach
Such telescopes could be considered "Laser Ready" if they have a
knob or gear that will adjust each "axis" independently and in a
repeatable manner - that is, one can "scan" the distant end,
making systematic azimuthal sweeps while making incremental
adjustments to the elevation. If the distant end spots the
beam as it flashes past it is then a simple matter of repeating
the motion that caused that flash, backing up and re-tweaking
the axes to optimize pointing. To be sure, an Equatorial
mount telescope doesn't provide true Az/El adjustments, but both
axes are still
easily and precisely adjustable in a
Using a telescope/mount has another obvious advantage: It
a telescope as well as a wider-angle finder scope!
If one is careful, it is possible to align the laser pointer in
parallel with the telescope and using a visual cue from the
distant end (such as a spotlight, car headlights - or even the
other end's laser)
to provide approximate pointing of the laser
reducing the uncertainty of the aiming of the laser to get you
One disadvantage of a telescope - even one on a sturdy mount -
is that it can sometimes "bounce" as the wind hits its fairly
large surface area. Such movement - even if slight - can
cause the laser's beam to move rapidly on/off point at the far
end, disrupting communication.
Another disadvantage of a suitable telescope/mount is that fewer
people own these than, say, a reasonably-sturdy photographic
tripod. Even though a suitable telescope/tripod can be had
for only a few hundred dollars new or used, it is understandable
that many people would not wish to make such an
investment! Finally, an 8" telescope and its required
accessories takes up quite a bit of space in a vehicle!
Using a tripod:
What if you don't
such a telescope?
As mentioned before, standard photographic or video tripods by
themselves aren't particularly useful in the precise pointing of
a laser pointer. They can,
be used as a stable platform for a device that may be used for
aligning a laser.
In our earliest experiments we attempted to use standard tripods
by themselves as mounts for laser pointers - but with mixed
success. Over the course of several evenings, many hours
were spent in frustration trying to point our lasers at each
other, often getting only a few tantalizing flashes from the far
end. The problem was that reporting of the flashes by the
observer at the distant end was necessarily delayed by the
comparatively slow reaction time of the viewer, with the report
being made after
seeing a flash.
Upon having a report of the distant end seeing the flash, the
person pointing the laser pointer (using a tripod)
repeat the maneuver that resulted in that flash but with the
laser's narrow beam, doing so was, at best, hit and miss and
attempts at making very small changes in pointing often resulted
in overshoot or backlash with the end result being that the
laser was still off-point. Of particular difficulty was
the adjustment of the elevation of the tripod: It was
extremely difficult to move the laser up and down without also
affecting the azimuth at least slightly. If, by chance we
able to see the beam, there was the inevitable
temptation to "tweak" it slightly to achieve the same brightness
observed in previous, brief flashes but between the flexure of
the tripod, the viscosity of the fluid head, and the effects of
static friction of the parts of the tripod such minute
adjustments often failed, causing the beam to be lost entirely!
After a bit of this nonsense I simply resorted to using my 8"
Celestron reflector telescope's camera mount for the laser
pointer. While it worked very well, it wasn't particularly
convenient to haul around and set up this rather large, fragile
and expensive device and I really
couldn't expect that
everyone who wanted to participate in such activities also have
to get a suitable telescope just to point a laser!
After some discussion with Ron, K7RJ about the construction of a
device that could be attached to a standard tripod he decided to
build something that could provide the precision and
repeatability needed to successfully aim a laser pointer.
The results of his work may be seen in Figures 4a-f
device that we affectionately (and erroneously)
as the "Vernier Pointy-thingie."
Which types of tripods are usable with this pointing
device? The very light-weight tripods intended for small
point-and-shoot cameras aren't generally suitable as they are
typically too flimsy. Very short "table-top" type tripods
will work - provided that they can be placed on a very solid
surface such as the ground or a stone or concrete wall, but
placing a tripod on a vehicle is not recommended as they tend to
move or settle as gear (and people)
If someone leans against the vehicle - or if there is even
slight wind - the vehicle can also move, knocking the laser
Somewhat "heavier" tripods such as those intended to hold a
camcorder or a full-size SLR-type or medium-format camera are
generally suitable. In other words: If the "new"
cost of the tripod is at least $70-$100, there is a good chance
that it will be "good enough." The tripod shown in
figures 4c-4f is an inexpensive ($35-$50) "video"
tripod that has been used several times for laser
communications and has been found to perform quite well.
Figure 4: Examples of the
"Vernier Pointy-thingie" devices as built by Ron, K7RJ.
The "Vernier Pointy-thingie":
Figure 4a - Top Left: Front view with the
Laser Pointer (in the black box) mounted to it.
Figure 4b - Top Right: Rear view of the
pointing device showing the "hinges".
Figure 4c - Center Left: Another of the
"Vernier Thingies" after being slightly re-worked by
Ron. For this later version, finer (metric) threads
were used and a knob installed to more-easily allow
Figure 4d - Center Right: A side-view of the
device, mounted atop a tripod. This shows the
installation of a metric "T-nut" at the base of the
Figure 4e - Bottom Left: This shows KA7OEI's
laser pointer module being held place by a short elastic
cord. Note the multiple holes above and below the
laser pointer module: This allows the optimal
arrangement of the small "eye hooks" to which the elastic
cord (or rubber band) could be attached.
Figure 4f - Bottom Right: Yet another view of
the device, removed from the tripod. Here we see the
"bottom" view, with the black circle (at the left edge)
marking where the 1/4-20 tripod threads have been tapped
into the plastic base. As can be seen from these
picture, the original elastic bands have been replaced
with metal springs and elastic cords.
Click on a picture for a larger version.
This device was so-called because we didn't know what else to
call it at the time it was conceived: Even though there's
involved, the name is meant to imply a degree of precision and
repeatability with respect to the device's operation. We'll
refer to it simply as the "Pointing device" elsewhere on this
As can be seen from the pictures in Figure 4
device attaches to a standard photographic tripod and allows
adjustments to both the azimuth
and elevation of the laser pointer.
Ron threw the first version of this pointing device together in
the late evening/early morning before a planned outing and it
was constructed largely from scraps of high-density polyethylene
plastic obtained from the surplus bin of a local
distributor - but this material could have been cut from, say,
an inexpensive kitchen cutting board. HDPE has an
advantage when it comes to moving parts in that it is very
slippery and has reasonably low dynamic and static friction.
With specific goals in mind, the design of this device was very
- The ability to provide fine adjustment. It
should be possible to make even minute adjustments
to the azimuth or elevation.
- The ability to make repeatable adjustments.
is, perhaps, one of the most important aspects of this type
of device. If you do something that results in the
other end briefly seeing the laser, you want to be able to repeat that motion during
your efforts to aim it!
- Adaptable mounting of the laser pointer. As
can be seen from the pictures, our two laser pointer modules
are mounted in very different ways. If we
rebuild/improve our laser pointers using different packages,
we want to be able to re-use the same mount in the future.
- Stability. Once we make an adjustment, we
expect it to stay!
These devices were quickly put together using materials that
happened to be on-hand and there is little doubt that they may
be improved upon but since they do
work, it would
be reasonable to use them as a starting point for further
The assembly consists of three main parts:
- The base plate. This is the piece that
attaches to the tripod using standard 1/4"-20 threads.
Into it, a threaded rod is installed that pushes the rest of
the assembly up to raise the elevation. In practice,
one would first "raise" ("pre-set") the elevation a few
degrees and from there, be able to move it up and
down when scanning.
- The elevation plate. Attached to the base is
the "elevation plate" and it is this piece that is pushed up
by the screw in the base plate to move it up and down to
adjust the elevation of the laser - and it also carries the
azimuth plate. It is in the elevation plate that the
azimuth-adjust screw is mounted.
Now, a bit about a few of its components.
- The azimuth plate. Being attached to the
elevation plate, this portion - which includes the laser -
goes up and down. The azimuth screw pushes this plate
away from the elevation plate to provide a degree of
side-to-side movement. As with the elevation, one
would "pre-set" the azimuth outwards a few degrees to allow
both left and right motion.
Take a look at Figures 4a
and notice the
two blocks on the base plate: These two blocks, attached
to the bottom piece, prevent side-to-side motion of the rest of
When designing this device, one of the goals was to maintain
orthogonality and independence of adjustments - that is, as much
as practical the adjustment of the elevation was to affect only
the elevation, and
likewise for the azimuth. The hinges, which, themselves,
also have flex or side-play, would not be enough on their own to
prevent side-motion as the elevation was raised up and down -
particularly since the weight of the upper piece (which includes
wasn't symmetrical about the axis.
Taking another look at Figure 4a
that there are similar "guide blocks" above and below the piece
to which the laser is mounted. These prevent the laser's
pointing from sagging as the azimuth is adjusted "outward" or
even over time as the plastic slowly deforms under the forces
exerted on it - particularly as the laser is moved "outward" and
away from the main block, increasing the leverage. Because
these guide blocks are made from polyethylene, there is little
friction, but that also means that it is not possible to glue
them together. For this reason, all of the pieces
comprising the pointing device are screwed together.
In looking at Figures 4b
you can see that
the hinges are constructed using thinner pieces of flexible
plastic - also polyethylene - taken from a food container.
Why use pieces of plastic instead of real metal hinges?
Partly, this was done because the plastic was cheap and on-hand
at the moment of construction and suitable metal hinges
weren't. In retrospect, it could be argued that the
plastic hinges - especially in conjunction with the guides -
have little "sideplay" which helps to keep the adjustments both
smooth and repeatable. When the second unit was
constructed, it was simply a duplicate of the first, taking into
account improvements made to the prototype after having been
used in the field. It is also worth noting that small,
inexpensive metal hinges are generally quite "sloppy" - that is,
they tend to move around on their pin and would likely require
modification in order to be useful: These problems are
pretty much avoided using the plastic "hinges" although some
thin, flat sheets of "spring" steel would likely work nicely as
To some degree the weight of the assembly and the "memory" of
the plastic hinges will assist in downward travel, but this is
not guaranteed so additional force is exerted using return
"springs." For the azimuthal adjustments - where there is
the lack of "gravity assist" and there is friction against the
guide blocks - more force is needed to assure that the
adjustment will return to "zero" as the threaded rods are
retracted, so even more "springs" are used.
As can be seen from Figures 1a
bands were originally used as "return" springs. While
these are cheap and readily available, one must remember to keep
plenty of spares on hand as they tend to lose elasticity and
break as they age - especially if they are going to be used
outside in the cold!
Rather than have to try to remember to bring a wad of rubber
bands with me, I simply replaced the elastics with metal springs
from an assortment that I'd bought from Harbor Freight,
relocating the screws to which they were attached (using the
extra holes that Ron had thoughtfully provided)
necessary to get the proper amount of tension. As can be
seen from pictures Figure 4a
and Figure 4c
azimuth adjustment has two
return "springs" arranged symmetrically
about the adjustment screw as more return tension is required
since gravity is not assisting us along that axis!
Originally, 1/4-20 "carriage" (or "coach")
bolts were used with
set in the plastic as the "base" thread, using the large head of
the bolt as a knob. After use in the field, several things
became readily apparent:
- The 20 TPI threads on the bolts were too coarse for
"fine" adjustment. It took only a minute
adjustment to move the laser too much and a fair amount of
force was required to turn them.
- The heads of the carriage bolts didn't make very good
The heads of the carriage bolts have a rather thin edge,
which makes them more difficult to grip. A tighter
grip increases the likelihood that doing so will
accidentally disturb the laser's pointing during
adjustment. Also, the heads have no obvious markings
or flutes by which the amount of adjustment (in fractions of
a turn) can be judged.
- More care had to be taken to re-shape the ends of the
bolts so that the motion of the adjustment was
more-consistent. More on this below.
To solve the first problem Ron went to the hardware store to
look for finer-threaded rod and while looking, he found a
similar-diameter piece of metric rod (6mm or so)
and a matching
"T-nut" with much finer thread - and he also purchased a pair of
knobs for adjustment. The much-finer pitch of this metric
rod - plus the addition of the relatively large adjustment knobs
- made precise adjustments much easier.
Of added benefit was that the finer thread provided a "tighter"
fit between the rod and the T-nut, considerably reducing
mechanical "slop" that had been observed with the 1/4-20
hardware. While many T-nuts are intended to be hammered
into wood and held in place with their spikes, the
"slipperyness" of polyethylene plastic doesn't hold them
securely in place so the T-nuts used were of the type held into
place with small screws as can be seen in Figure 5d
Yet another advantage of the finer pitch was that less
rotational force was needed to turn the screw to make
adjustments - especially with the knob - which also made it
less-likely that doing so would disturb the pointing overall.
One of the problems that had been noted on the first version was
that the ends of the bolts that pushed against the plastic
blocks weren't particularly flat. What this meant was
that, in the case of the elevation, as the rod was turned in one
direction the elevation would actually go up and
the elevation block rode on the uneven end of the bolt. To
solve this problem, Ron carefully
ground the ends
of the threaded rods to symmetrical, blunt points.
The laser module is mounted to the side of the azimuth plate
using a number of small screw-in eye hooks, held in place with
rubber bands or a stretch cord. If you look closely at Figure
you'll note that there is a grid pattern of small holes
drilled into the plate and these allow the strategic placement
of eye hooks
to accommodate the different sizes and shapes of laser modules
that Ron and I have and by pre-drilling an array of such holes,
"field adjustments" can be performed to best-accommodate the
As can be seen in Figure 4f
, a hole (the one marked with the black
hexagon to identify it in poor light)
was drilled and
tapped with 1/4-20 threads to allow it to be fastened to a
standard photographic tripod and even though these threads are
tapped in plastic they have proven to be more than strong enough
to allow repeated use. If the ability of the plastic to
"hold" threads turns out to be a problem we will install some
inserts (such as "Helicoils" (tm))
additional strength and support.
In addition to replacing the rubber bands with springs I
replaced the rubber bands originally used to mount the laser
module with a small elastic stretch cord (a.k.a. a "bungee")
hold the laser pointer module to the side of the pointing
device, rearranging the eye hooks as necessary to best-fit the
shape of my laser module.
Although not immediately obvious from the pictures, careful
scrutiny of Figures 4e
will reveal that
a piece of self-adhesive felt was attached to the surface of
azimuth plate "under" the laser module to provide additional
friction to prevent the laser diode module from moving around on
the slippery plastic surface. In lieu of felt, a piece of
self-adhesive rubber mat (often used for non-skid surfaces)
could have been used.
Exactly how the "Vernier Pointy-thingie" is used will
be covered in the next section.
How to set up a laser-pointer communications system
- Longer distances
Once you get past the distance at which you can easily see the
laser's "spot" at the distant end, you are essentially flying
blind, relying exclusively on what is being reported by
observers at the far end.
To reiterate safety:
- Do not do any such tests where the beam -
either between the laser and the receiver, or in the
distance beyond the receiver - will directly
cross a road or busy air corridor!
- Even though a low-power beam may be physically harmless,
it can still be distracting! From a practical
standpoint, once you get farther than a few kilometers from
a typical laser pointer its level of distraction will be
very minimal owing to its low energy density and the
practical likelihood that the duration of any exposure will
be very brief as the observer crosses through the
beam. Even so, always err on the side
Based on past experience we have determined that the
following method does not work very well:
- Move the laser back and forth until the distant end
reports seeing a flash.
- Try to re-create the motion that resulted in the
distant end seeing the flash.
- Go back to step 1.
While it is
possible to use the above method to point a
laser, unless luck intervenes one can spend (literally!) hours
trying to aim it! Having spent hours standing in the dark
and cold, talking on the radio saying things like "Brief
flash... Dim flash... Bright flash" or, more often
than not, "Nothing!" we can attest to the awkwardness and
seeming futility of the above method. On more than one
occasion we simply ran out of time, got too cold, and/or simply
lost patience and gave up - usually after having been tantalized
by the occasional, brief flashes of the laser from the far end!
Using the "Vernier Pointy-thingie":
Having taken care of the first problem by being able to
accurately and repeatedly point the laser with the aid of a
telescope mount or a device like the "Vernier Pointy-thingie",
there is still
the problem of guiding the pointing of
the laser to the distant end. With the addition of the
pointing device (or a telescope mount)
we have a means of
repeatedly pointing the laser and being able to adjust it in
very small increments - which is precisely what is necessary for
The procedure for doing this is approximately thus:
- Pre-set the Azimuth and Elevation of the pointing
device to slightly offset both axis. Simply put,
one just adjusts the elevation up and the azimuth outwards
end by a few turns. Doing so allows you to go up and
down as well as back and forth from the starting
- Azimuth scan. Using the tripod itself, start
sweeping back and forth, adjusting the elevation on the
tripod a bit at a time until the observer at the distant end
starts to see flashes - even if only occasionally. At
this point one has a very "rough" idea of where the laser
should be pointed and the tripod's azimuth and elevation are
"locked down." Locking the tripod will often cause the
azimuth and/or elevation to shift slightly, but it should
still be within the adjustment range of the pointing device.
- Scan with the pointing device. Locking down
the tripod to the approximate position where the distant end
has first started seeing flashes, use the pointing device
scan the azimuth back and forth, adjusting the elevation
slightly each time. Simply by noting how much one has
turned the various knobs it is possible to go back and
repeat the same steps over again, keeping track of what one
has already done if the other end start to see flashes - or
stops seeing them!
Because a tripod is used as the base for the pointing device, it
is important that the tripod be of reasonable quality and
that it be on stable, solid ground (e.g. not
mud or a springy mat of vegetation)
shifting: Many tripods have a center hook or connecting
point in the center or on the legs from which a weight (such as the batteries used to power equipment)
hung - but make sure that what you hang doesn't swing in the
wind, flexing the tripod and affect pointing!
Knowing where to look/point:
Up to this point we have not mentioned two additional, very
- Knowing where to point the laser.
- Knowing where to look for the laser.
Validating the path
A useful tool is Google
(tm) in that it can provide a simulated
view along the path. While one can determine the viability
of a proposed path with some
certainty using Google
Earth, you must
still do an actual in-field
verification to find out if that the path really
does exist as the accuracy of Google Earth can only be
relied upon to a certain degree
: It does a poor
job of determining if trees or nearby buildings will be a
problem and its accuracy is simply not adequate to determine if
"marginal" paths (e.g. those that just barely clear
hills and ridges)
For an example of "simulated" visual paths, look at the "Revisiting the
107 mile path" page - and at Figures 2a and 2b on that page
Using Google Earth, one can produce not only maps showing the
projected path, but also produce "simulated" views from each
end: It is strongly
recommended that one annotates
such a picture with labels, arrows and circles to identify
distinguishing landmarks - including where, exactly, the distant
end is supposed to be among the clutter! In addition to
Google Earth, another useful tool is RadioMobile: This
program is specifically designed for radio paths, but can be
used to determine optical paths as well - but it requires far
more preparation and experience to use and has quite a steep
It is also highly
recommended that a daytime visit to
the two sites be arranged and that you just look, using
binoculars and telescopes, to see if the end-to-end path
exists! If the distance isn't too great (no more than a
few kilometers) the path can be verified by shining mirrors at
each other and/or waving large flags or tarpaulins. Doing
this does two important things:
- It verifies, for absolute certainty, that a line-of-sight
path exists from end-to-end.
- It provides a future visual reference point - that is, you
will know where to look!
It is strongly
recommended that pictures be taken
on such an outing using various levels of camera zoom. As
with the Google pictures, these, too should be annotated (with
arrows, circles, labels, etc.)
to show where, exactly, one
should be looking! During your site visit, you should also
add notes and arrows to the Google picture that you printed to
further-help in identifying elements of the landscape.
For an example of a composite picture containing both real-world
photographs and simulated computer views, see the View of Swasey Peak
For the October 3, 2007
optical communications outing
an annotated version
of the August 18 picture - along with the computer-generated
view - were very helpful in assuring that we knew what we were
looking at, providing visual cues based on other landmarks.
Identifying landmarks in the dark
Although it is no surprise that the entire landscape tends to
change when it gets dark, many people fail to realize how
disorienting this really is and in many cases, a familiar
vista becomes inscrutable as the sun goes down and well-known
visual references tend to disappear and others show up!
Usually, roads, radio towers and large buildings can provide
visual references for use at night - provided that you can
figure out what and where they are so one useful trick is to spend
some time, around sunset, making notes and taking pictures (including
as the daytime objects disappear and are
gradually replaced by the nighttime references.
If you are in a rural area with no obvious landmarks that are
visible at night, you must be more creative so unless you
are very familiar with the area, it is best that you arrive before
dark to prepare for the loss of recognizable landmarks.
few suggestions include:
- Train a telescope on the far end. This can be
used as a reference, if nothing else. If you park the
telescope on the far end while there is still light and then
leave it there as it gets dark, you can be assured of being
able to look in the right place.
- Provide markers of your own. A series of
sticks, rocks or other object inline with the far end can
give you a general idea as to where you should be
looking or point your laser. Inexpensive "glow
sticks" or "throwies"
(simple LED/battery devices) can also be laid out in
a line to provide an azimuthal reference. Remember,
the farther-out you go (as in tens - or even hundreds of
meters!) the more accurate the visual reference. Make
sure you pick up and take any devices that you used for
marking with you when you are done!
- Positions of stars. If you are an astronomy
buff you can, knowing the precise time and date, determine
which stars can be used to indicate the azimuth of the other
end of the path.
Map and compass
One should not forget the old standby: A map and
compass! A GPS receiver can also provide many of the
details that a map would - namely bearing and distance - and a
good quality compass (or by "walking" with a GPS receiver in a
straight line for some distance)
can provide, within a few
degrees, the bearing to the "other" site.
It is recommended, however, that one also obtains the bearing
for a few other (known)
landmarks as well so that you can compare the predicted and
calculated compass bearings to them - a procedure that provides
a "sanity check" in case you somehow get the magnetic
declination wrong or if there's a minor local magnetic anomaly
that can skew compass bearings. Having a nearby "known"
reference can also allow you to do approximate aiming if one
knows the angular difference
between it and the distant target.
Providing your own visual cues for the distant end
As mentioned before, car headlights or hand-held spotlights can
also provide useful visual references, the latter being more
convenient as it is not attached to a car and can easily be
pointed in any direction! With the naked eye, a "500,000
Candlepower" portable spotlight - a device that may obtained
inexpensively at many auto-parts stores and plugs into the
cigarette lighter of a vehicle - can be easily
spotted amongst other city lights at a distance of at least 10
km with the naked eye and far more than this (over 100km under
if the light isn't amongst a sea of others!
It is important that both
ends be able to spot each other in this way. Not only does
the transmit end need to know where to point the laser, but
those at the "receiving" end need to know exactly where
to look! While a bright flash of a laser as it sweeps by
can be an attention-getter, it is far better if all eyes are
looking in the direction from which the flash will come instead
of simultaneously trying to look for a flash and
figure out where, in the darkness, it might appear - especially
when trying to spot weaker, off-axis flashes!
If you have managed to set up a small telescope that is already
trained on the transmit end, even weaker, "off-axis" flashes too
dim to be visible to the naked eye may be seen, possibly cluing
those at the transmit end to the fact that they might be getting
It should be mentioned that xenon strobes/flash lamps are
surprisingly ineffective when it comes to providing a visual
reference for the far end. The problem is that much of the
light energy of a strobe is in the blue-green spectrum that is
more-easily absorbed by the atmosphere. Also, the flash is
very brief and occurs only intermittently, so unless it is very
bright it is not easily spotted unless the observer happens to
be looking in the right direction at the right instant. If
you are setting up a receiver it may be possible to "hear" the
click of the strobe, taking care to avoid confusing its sound
with that of the strobes from passing aircraft.
If you have a strobe and choose to use it please be aware that
it may attract "unwanted" attention if someone thinks that its
flashes are from a party in distress: Again, a portable
spotlight is more effective and cheaper!
Aiming the laser:
Unless you have "married" your laser pointer to a telescope
mount such that the two are precisely in parallel to each other
(taking into account parallax, of course!) you'll note that it
is very difficult to actually tell where the laser is
pointed! Unlike in the movies and on TV, you
will probably not be able to see the beam
emerging from a low-power red laser pointer!
Unless the air is very dusty (which would also mean that your
maximum distance would be limited) it takes a Class 3B or
higher-power red laser to produce an obviously-visible beam
through clean, clear air: If you are using a high-power
laser outdoors you may be breaking the law unless you have
managed to get the appropriate permission/variance from the
relevant regulatory agency!
Figure 5: "Lining
Rods" used in Heliography to determine where the
mirror-reflected sunlight was being pointed.
Fortunately, we can learn from some of the techniques used by Heliograph
operators over a century ago where they, too, had to figure out
where, exactly, the sunlight reflected from their mirror was
being directed - and track the sun at the same time!
For more information about the Heliograph, refer to "The Heliograph"
- a reproduction of a portion of the 1899 work "The Sun
Telegraph" by Col. King.
In particular, refer to a figure from the article reproduced to
the right in Figure 5
in which we see two bent rods
pushed into the ground with objects ("bullets")
thread in their "crooks." If we line these two "bullets"
up with the distant end we have, in essence, a sight line that
can be used to aim our light source. The small size of
these "bullets" blocked an insignificant amount of the light
reflected from the mirror (6-10cm or larger)
that was typically
Practically speaking we wouldn't be using exactly
procedure with a laser pointer as the size of the "bullet" would
completely block the small-diameter laser-pointer beam
itself! What we can
do is adapt this technique,
often improvising on what we have on hand in the field to get
"close" to the target.
While some heliograph mirrors have holes in the middle of them
to allow sighting of the rays to be done from the center of the
reflective surface, effectively eliminating parallax
with a laser one must be satisfied to sight near
the body of the device - but not exactly
along the axis
of the beam - a difference that introduces such errors.
When doing such aiming it is necessary that one sights along a
line as close to the laser as possible to minimize this error
and because of the narrowness of the laser's beam, even a slight
amount of parallax can cause a significant amount of error in
A few "alternative" techniques loosely based the technique
depicted in Figure 5
- A thread (and weight) hung from tripod, or multiple
threads strung between two posts in a manner resembling a
harp. At some distance in front of your laser,
place a tripod with a piece of white thread in which the
midpoint of this thread (perhaps marked in some way) would
be lined up between your laser pointer and the visual cue
from the distant end. Taking into account the
inevitable parallax between your eye and your laser pointer,
the thread will light up (when the laser hits it) and
provide an approximate reference as to where the laser is
actually pointed. The use of thread is suggested as it
will block relatively little of the beam and the method of
stringing between two posts eliminates any movement that
might be caused by the weight swinging in the wind.
The farther this sighting device is placed in front of your
laser, the less error there will be due to parallax.
Setting up two of these devices in line during daylight can
further refine the aiming and reduce errors.
- The "stick in the ground" technique. This is
a variation on the "thread and tripod" arrangement - in case
you don't have either an extra tripod or thread! For
this technique one simply finds a stick (one that you
have brought with you for this purpose - or one that you
have found laying about on site) and plants it in the
ground some distance in front of the laser and uses it as a
visual reference. This stick would be placed slightly
off to the side, "almost" in line with the distant end - but
not directly inline as it would block the laser's
beam. With the stick slightly off to the side
you can get a good approximation of the elevation of the
laser as well as a rough estimation of the azimuth,
providing a starting point for your "scanning"
technique. For this technique it is useful to mark the
stick in some way using tape, string, or perhaps a feature
of the stick (say, a knot, fork or branch) to provide a
visual reference for the elevation setting.
- Scatter dirt/dust in the beam. This will
temporarily illuminate the path of the laser and provide a
visual reference as to where it is pointed. The
farther away the dust is scattered from the laser, the
more-accurate this will be as this will reduce the degree of
parallax between your eye and the laser.
- The "wave something in the beam" method. This
is a variation of the "dirt in the beam" method, in which an
assistant waves a hand, tree branch, a stick, or a window
screen in a frame back and forth through the beam, providing
a visual reference when it is illuminated by the beam.
Again, this should be done at some distance in front of the
laser to minimize parallax.
- Tree branches. It is often the case that
there are trees near the path and these can be used as a
general reference. Sometimes, you aren't sure how high
your beam really pointing, so by swinging sideways to a
nearby tree one can often gauge the beam's elevation and
visually compare it to that of the distant end, getting an
idea as to where the laser is being pointed.
- Weeds/grass on the ground. As with a tree,
one can often point down to the ground to get an idea as to
the azimuth of the laser pointer.
Over the years we have used variations of all
above techniques and while they do all work, the first method -
which implies some prior planning and forethought - is probably
"Rough aiming" with a tripod:
Another "rough aiming" procedure mentioned above is to take
advantage of the fact that it is
possible on most
tripods to do a back-and-forth pan with reasonable
accuracy. By loosening the locking screw just enough
to allow one to pan the tripod back and forth, the elevation can
be adjusted (preferably with the pointing device)
incrementally. The object of this exercise is not
to accurately point the laser, but to (hopefully)
approximately where the distant end starts to see flashes as the
beam sweeps past.
Once the distant end does
start to see flashes, the
tripod is adjusted as close as practical to that bearing and the
azimuth and elevation locks are tightened. Again, note
that with most tripods simply tightening the locking screws will
often have a slight effect on both axes, causing pointing to be
slightly offset when doing so - but this small difference should
be well within the adjustment range of the pointing
device. It is recommended that before doing this
procedure, however, that one points the laser at a stationary
object and then loosens/tightens the tripod's lock screws to
observe how their adjustment shifts the beam's pointing:
In this way one will have an idea as to where and how much one
needs to correct for these changes by using the pointing device.
Remember: The purpose is simply to get "close"
to pointing in the right direction and be within the
adjustment range of the pointing device!
"Talking in" the other end
How we do
Over the past several years, we have, through trial and
error, refined our "laser pointing" techniques.
Some of these experiences are detailed in the "First Optical
QSO" and "More Optical Testing"
pages. Even with the elaborate planning of the 1963 Operation
Red Line they underestimated the
difficulties involved in pointing the laser!
While we use the methods outlined on this page, we have
developed a few "shortcuts" to setting up a laser
Because our recent experimentation has largely been with
the use of high-power LEDs instead of lasers, we have
done most our laser experiments in conjunction with
those same tests. Having already set up our
receivers for use with the LED link means that we can
use them to help us align our lasers.
In order to set up the LED-based optical gear, we have
already done the same preparation as described,
- We have already verified that we have a
- For the longer-distance paths, we'd prepared
annotated pictures - some simulated - showing where
we should be pointing.
- Using map and compass, we further identify our
landmarks and the proper bearing once we arrive on
- We typically arrive with remaining daylight so we
can correlate the daytime landmarks with those that
disappear and new ones that appear once it gets
One advantage of the LED-based gear over lasers is that
the beamwidth is greater. What this means is that
it is more-likely that we can simply pan our optical
transmitters back and forth (while incrementally
changing elevation) and be spotted at the "receive" end.
- We have a way to communicate with each
other. We use amateur radio as a means of
communication since some of the areas that we have
been have no phone coverage at all!
The LED-based gear, since it produces more total
light than a laser (to overcome the greater beam
divergence) also produces a visible beam in the darkness
due to Rayleigh
scattering (among other things) which also
aids in our ability to determine where the beam is being
Once the transmitter's beam has been spotted at the
receive site, a tone is modulated onto it and used to
point the receiver and peak the signal. A
particularly useful device has been the "Audible Signal
Meter" system that we use (described
here) that detects the tone being
transmitted and converts its loudness (which is in
proportion to how much light is being detected)
into a tone of varying pitch. To "peak" the
receiver, one simply adjusts for the highest pitch of
tone - a far more accurate method than trying to judge
how "loud" something is. With this system, a tone
that is too weak to be audible to the human ear can be
detected which also means that even a very weak,
off-axis signal is more likely to be detected and be
The final step is to relay, via radio, that same tone of
varying pitch back to the transmitter site so that they,
too, can re-peak the transmitter simply by adjusting for
the highest-pitched tone as well. At this point we
now have set up a 2-way LED-based communications system,
complete with receivers that have already been pointed
When we set up our laser experiments - which always
occur after we have set up the LED-based link -
we follow a similar procedure in that the laser is
modulated with the tone and we relay the Audible Signal
Meter's variable-pitch tone back to the laser
transmitter site - either via radio or bye one of the
LED-based systems that we have already set up.
With this method even the briefest "flash" of the laser
as seen at the receive end will instantly be relayed as
a "hit" on the pitch of the tone, giving the person
adjusting the laser immediate feedback and the "feel" as
to the proper laser pointing. In this way, we can
quickly and easily "dial in" our lasers!
For an audio recording demonstrating the detection
and peaking of a laser at a distance of over 172 km
using the audible signal meter, listen
to the recording at this link.
How well have we done using the techniques described
using just cheap, standard laser pointers? We
of over 23km with little difficulty
and have also established a 2-way laser pointer-to-laser
pointer link over a distance greater than 172km as described on this page.
Before you start sweeping back and forth with the pointing
device, make sure that you have:
- Done your best to "rough" aim the laser. Make
sure that you know about
where you should be pointing.
- "Pre-set" the pointing device. Make sure that
the pointing device is offset from the stop in both axes so
that you adjust in both positive and negative directions
from your starting point.
- Verified that the azimuth and elevation locking on the
tripod screws have been tightened. You don't
want either of the tripod's adjustments to drift/slip as you
- Turned the laser on! Not only should you make
sure there's light coming out of your laser, but you should
also check - with your local receiver - to verify that it is
being modulated in the way you think it should be (e.g. tone or music.)
With the above techniques it is possible to not only get the
laser "pretty close" to pointing in the right direction, but
also - with the aid of the pointing device (or your
telescope-mounted laser pointer)
- be able to move the laser
back and forth and up and down with the finesse required to
tweak it in.
At this point we'll assume that the only means that one has to
align the laser is to have observers at the "receive" end that
are looking for the beam. It is worth mentioning that when
doing this, the observer should be standing quite close to the
receiver's location because even a cheap laser pointer may have
a "width" of only a few 10's of meters at a distance of several
kilometers: If you are standing far away from the
may be able to see the laser, but
the receiver may be outside the beam!
Using the aforementioned "rough pointing" techniques as a
starting point and I prefer to begin scanning back and forth
using the azimuth, making a sweep from one extreme to the other
and back again
, thereby completing two sweeps
across the same azimuth
before adjusting the
elevation. At this point the advantage of using a device
capable of precise and repeatable movements becomes
apparent: As you proceed with your scan, keep track of how
much the elevation knob is adjusted so that you may can to go
back to your starting point.
If, as suggested, you have "pre-set" your elevation slightly, if
the beam has not yet been spotted you should return to the
original elevation and start going in the other direction.
For example, if you first started sweeping, moving the elevation
up 1/4th of a turn each time and the other side never saw
anything, you would return to the original elevation and then
re-start your scanning, going down in elevation 1/4th of a turn
at a time. When returning to the original elevation
position, it is best to overlap slightly - say, starting just above
the original position - just to be on the safe side in case
there was some confusion in the number of turns made in the
Depending on the pitch of the
threads and the "fine-ness" of your mechanism, 1/4th of a
turn may (or may not!) be a suitably fine increment of
adjustment. It is by having tested and becoming
familiar with your gear through previous experimentation
that you'll get a "feel" as to how much you'll need to
If you have planned well (and are lucky)
the receive end
will begin to report seeing brief flashes from your laser:
At that point you would go back and repeat the motion that
resulted in the other end seeing the flash to carefully "dial
in" the adjustments - first using one axis and then the other -
until maximum brightness is obtained.
If the other end doesn't
see any flashes, verify again that
your laser really
is turned on (or that the battery
and then re-do the "rough aiming" techniques
described above, always remembering to take into account the
inevitable parallax between your laser and where you are able to
sight along it.
It should go without saying that the above techniques require
that both ends of the path be in constant communication with
each other. Again, this is preferably done via radio,
although a mobile/cell phone can work remembering that not only
there is a slight delay when using a cell phone, but that you'll
probably be burning up a lot
of air time and battery
power while you are doing it!
It has been occasionally stated that the farther
apart the transmit and receive sites are, the more-difficult
it is to aim the laser as pointing becomes "touchier" - a fact
attributed to the narrowness of the laser's beam becoming
increasingly problematic as the distance increases. This
is, in fact, a fallacy as the laser's beam is the same number of
degrees wide no matter how
far away the receiver is!
What does increase the challenge with aiming
the laser over an increasingly-greater distance is the fact
that the beam becomes dimmer and that the weaker, off-axis
light is increasingly more-difficult to spot! Once you
are in the "main beam" however, the "angular size" is the
same, regardless of the distance.
Setting up the receiver:
If you have gotten to the point of being able to see the laser
from the far end, you can now set up the receiver.
At this point it is worth mentioning two design aspects of the
laser transmitter that will come in extremely
- A tone generator. If your laser modulator can
produce a distinctive audio tone, it is much easier to
properly point the receiver and peak the signal.
Remember: The laser light itself won't make any noise
at all (aside from maybe "hiss" or a "rumble" or a
"click" as it flashes past) and putting a tone on it
is extremely useful. Barring this, sending
recognizable, loud music across the beam using a portable
audio player will also work!
- Remote controls. Do NOT put
any of the controls on the laser pointer module
itself! If you manage to get the laser pointed
properly, you will already be painfully aware as to how
touchy it is and the last thing you want to do is to
accidentally knock it off-point by having to turn a knob or
flip a switch on the laser module! It is for this
reason that the laser module should be connected, with a
cable, to its control box: The wires should be wrapped
around or taped to the tripod so that they do not move in
the wind or be flexed by moving the controls and the control
box itself should be sitting on a nearby table, allowing you
to make changes to the settings of the laser without
having to go too near the tripod! (It
is best to maintain a "safe" distance from the tripod
during operation to prevent accidentally bumping it or
kicking one of the legs and knocking it off-point.)
With the laser sending out a tone (or music)
it is a pretty easy
matter to adjust the pointing of the receiver so that one gets
the best (usually loudest)
signal from the distant end.
Once the receiver is set up it is also possible to further-tweak
the pointing of the laser itself (if you dare!)
to see if any
additional improvement can be obtained.
Once a signal is being received from the far end, it is easier
to fine-tune the alignment of the laser as one can simply relay
- via radio or telephone - the audio that is being
received: If, for example, the laser briefly sweeps past
the receiver, a brief "hit" of tone will be noted, providing a
cue for the person pointing the laser as to where it is
pointed. It should go without saying that having an
audible "instantaneous" cue from the receive itself (as opposed
to the delayed reaction of someone saying "I saw a flash!")
far easier to work with, as this rapid response allows for much
quicker adjustment than with having a person provide (delayed)
reports! Once set up, the pointing device and tripod
system described above has proven to be capable of holding the
beam steady for the duration of the experiments with little or
no obvious drift.
Comments about receiver sensitivity:
- A "reasonably" sensitive receiver should be able to
provide readable voice from any laser signal
that is bright enough to be seen with the naked eye.
An exceptionally-sensitive receiver will be able to provide
copyable speech from a signal that is just below the visible threshold of the naked, dark-adapted eye.
- Typical "kit" receivers (such as that provided with the
Ramsey LBC6K Communicator) or a simple receiver like that
depicted in Figure 1 will not work reliably
over distances of even a kilometer unless modifications are
made, the least of which being the addition of as large a
lens as practical in front of the detector!
Audio recordings of actual laser-pointer
As noted, we have, on several occasions,
completed laser-pointer communications over distances
exceeding 100km and below are segments of a
recordings made on several occasions over a distance of
greater than 172km. Notes about the audio
recordings may be found below.
For this clip, a standard laser pointer
- mounted to an 8" reflector telescope (but not
using the telescope's optics) - was used. The pointer
was modulated with a 1 kHz alignment tone and, using
feedback from the audible S-meter from Inspiration Point,
after a minute or so of sweeping, I heard a "hit" as the
Laser pointer flashed past the far end's receiver.
After a bit more gentle tweaking, I was able to dial the
telescope's adjustments to peak the signal at the far end.
Recording from September 3, 2007 - For more info, see
the 107 optical mile path" web page
pointer (mp3, 2:20, 1.07 Meg) Stereo
audio file recorded at Inspiration Point
- The LEFT channel contains local
audio transmitted from Inspiration Point.
- The RIGHT channel contains the audio received
via the Laser pointer over the 107 mile path.
- 0:00-0:29: Sighting-in of the Laser pointer
clamped to the telescope. In the LEFT
channel, one can hear the audible S-meter while the RIGHT
channel contains the 1 kHz "alignment" tone being
received, having been transmitted via Laser, being used to
"key" the audible S-meter. In the first few seconds,
one can hear the Laser "swoop" past the receiver and then
get "dialed in" to peak the signal. The "wobble" of
the S-meter's tone is due to the scintillation of the
- 0:29-0:58: Music clip. Note that
the use of short duration (<30 second or 10%) music
clips is considered to be acceptable fair use
under current interpretations of U.S. Copyright
law. (Music: "Children"
[Dream Version] from the album "Dreamland" by Robert
- 0:58-2:20: Voice commentary about the
communications. (There's a bit of acoustic
feedback at the beginning due to my microphone gain
initially being too high.)
As can be heard, scintillation is rather severe, yet the
intelligibility is still reasonably good - mostly owing to the
redundant nature of human speech and the fact that the
scintillatory periods were, on average, far shorter than
syllables: This is an example of the ear and brain doing a
good job of "filling in" the gaps.
Recording from August 20, 2008 - For more info, see the "Microwave and Optical QSO
for the ARRL 2008 '10 Gig and up' contest" page:
- Laser Pointer
reception from Nebo, audio file - 1:04, MP3, 980kB
Note that the use of short duration (<30 second or
10%) music clips is considered to be acceptable fair use
under current interpretations of U.S. Copyright law.
(Music: Theme song of the movie Dark
Star by John Carpenter)
- For both ends, the already-aligned optical receivers for
the LED QSO were used.
- This is a "2-channel Mono" recording from the receiver
at Inspiration point only. Unfortunately, the audio
recorder on my end ran out of memory and stopped prior to
this portion of the evening's experiments.
- The occasional "squeak" that is heard is from a
long-range FAA RADAR, its RF getting into the optical
receiver's front end.
At the beginning of this file can be heard a brief segment of
the 1 kHz "alignment" tone, immediately followed by an
exchange: Note that Ron's voice can be heard in the
because of the open microphone on the
optical transmitter at the Nebo end picking up and retransmitting
receive audio from the local speaker - which means that his
voice went both ways
over the 172km+
Quite apparent in this audio clip is a sort of "rumbling hiss"
caused by the scintillation of the laser's light:
Measurements indicate that there is at least 40dB of
scintillation present on the audio, but the redundant nature of
human speech and the brevity of the most severe of these "dips"
in amplitude still allowed good intelligibility, albeit with
rather poor audio quality.
Interestingly, the scintillation experienced on this 172+km path
than what we had observed on a much shorter
path on several occasions. This is attributed to
the fact that the shorter path crossed the Salt Lake valley
skimming the top of a thermal inversion layer while the longer
path passed through the air volume at much higher elevations,
above such layers (>2600 meters ASL)
and with its
comparatively rarefied air. Coupled with that, on that
particular evening seeing conditions were somewhat degraded by
airborne smoke particles: We have observed, on several
occasions that, despite reducing signal levels overall, mild
degradation due to such particles seems act as a mild diffuser
to more-quickly "de-cohere" a laser's emissions and as well as
seeming to minimize the appearance of "local coherence"
- both being factors that
can affect scintillation.
A few comments on the above paragraph:
- We have observed on
several occasions that scintillation seems to be
less-severe than expected when mild/moderate atmospheric
particulates are present - a result that we believe to be
a result of, at least in part, by the presence of those
a discussion of methods used to partially de-cohere a
laser using diffusion media, refer to the works of Olga
Korotkova as linked from the Modulated
DX page. It is our suspicion that an
atmospheric volume that contains a moderate amount of
obscuring dust particles - but not so many that path-loss
attenuation is increased to the the point of making
communications impossible - act as a sort of mild diffuser
to more-quickly break up coherent wave fronts. Such
particles may also play a part in the prevention of "local
coherence" on light sources of small angular diameter as
perceived from the receive site. It should be stated
that we have yet to attempt any rigorous analysis or
conduct further studies to prove or disprove these
assertions and that it is, at this point, just a
- For a discussion of
"local coherence" and its relationship with aperture
diameters and scintillation, see article "The
Sizes of Stars" by Calvert.
It is very
important that you prepare beforehand if you
plan to set up a laser link in the field! If you are new
to this, you must first become adept at setting up the very
short-range links and in doing this you will not only become
accustomed to how "touchy" setup can be, but you will begin to
learn the quirks and capabilities of your own gear, making
improvements and modifications as necessary - and avoiding
Once you have mastered short distances, gradually move to
greater distances. This will not only further-hone your
skills but it will also more-clearly spell out the various
limits of your gear as you continue to increase distances.
Again, newcomers to this rather esoteric activity tend to
greatly underestimate some of the difficulties that they will
encounter as well as overestimating the abilities of their
gear! By repeated experimentation, practice and
modifications, you will not only gain experience but you should
quickly become adept at setting up the gear and maximizing its
If you don't
succeed in your first attempts, don't give
up: We have found that our greatest improvements in our
gear and techniques have resulted from things not
working as we had hoped or going as planned!
Remember: If we can do it, so can you!
This page is not intended to be the sole
guideline for laser operation and should not be considered
to be a definitive source of technical, legal, or safety
advice. It would be irresponsible for anyone reading
this page to conduct experiments without doing further
research to determine the suitability of the methods or
techniques described. Neither the author or the host
of this web page can take responsibility for the actions of
others, particularly if those actions are conducted in an
irresponsible manner - lawful or unlawful - and/or lead to
distraction and/or injury and/or result in physical and/or
property damage. A reader should not construe
discussions or references on this page to be any sort of
legal advice as such topics are beyond the scope of this
A few relevant links:
These are links that generally cover the topic of lasers:
It is up to you to use lasers in a safe, responsible
manner and avoid injury - either directly or indirectly -
keeping in mind that even if a laser does not have the
potential to cause direct physical harm, it can still pose a
hazard due to its potential to be distracting to the
operator of a vehicle such as a car or aircraft.
When conducting experiments such as those described above,
make sure that the laser's beam doesn't inadvertently enter
an area in which it could pose a hazard or cause a
distraction. One such example might include a scenario
in which, over a short test range, the laser beam crossed a
roadway and caused a distraction to drivers - either in
front of or behind the "receive" end.
It is not possible for this page to cover all eventualities
that might arise from the use of a laser. It is also
not possible to be able to determine the legality of
conducting such tests in your area. It is solely up to
you, the reader - and others who might be involved in your
tests or experiments - to assure that such activities are
done in a safe, legal manner!
Safety page. This page contains general
information as to laser safety, as well has having links to
other pages on related topics.
FAQ. This is a practical hands-on
reference to all sorts of lasers, how they operate, how they
can be used by an experimenter, and practical aspects of
Safety page gives some practical examples and
references related many aspects of laser safety and
potential legal aspects of which users should be aware.
A few more designs of laser pointer transmit/receive
These links describe various circuits and techniques used to
modulate a laser and detect its emissions - using both AM and
- Operation Red
Line. This page gives details of the
historic 1963 laser efforts, occurring mere months after the
development of the visible-light Helium-Neon laser. Don't
miss the Photo
Gallery page which has pictures of the
equipment and of the event itself.
laser page. This page - in both English
and German - details experiments done with long-distance
laser communications - including that involving the
transmission of video.
mailing list at qth.net - This is a mailing
list that, while mostly geared toward Laser-based
communications, also covers other non-Laser aspects of
optical communications as well. This link given
points to the mailing list archive. You may
subscribe to the list and receive individual emails or
daily digests. Subscribing is required if
you wish to participate.
Laser Pointer audio modulator. This
describes an FM-based system centered on approximately 75
kHz and is one of the better-designed, higher-performance
FM-based systems that I have seen on the web! Unlike
most pages that describe laser-pointer communications
systems, Max impresses on the reader the need for additional
optics to improve performance and actually shows how one
would use a lens at the receive end to (greatly!) improve
range. Additional links on related topics such as how
to mount the laser diode, photos from testing and other
things are sprinkled throughout the page. Go to
the bottom of Max's page under "related links" to find
other articles on related topics as well as to find more
info on how to build the circuits.
laser page. Experiments and equipment by
Michael using lasers, photomultipliers and lots of other
laser transceiver. Another simple PWM
laser transmitter and receiver. It is very similar in
operation and performance to the K7RJ and Ramsey devices
Laser pointer transmitter. A typical
"current"-type laser modulator. This describes a way
to modulate and detect a laser with the minimum of
parts. The described detector is suitable only for
very short-range testing, however.
Below are a few more links that relate in some way to lasers and
laser communications. They are listed in no particular
Please note that some of this information is quite dated and
does not reflect the current state of the art, nor does all of
the advice contained in these link correlate with our own
experiences and the advice given above. These links are
included because the do contain some useful information - both
historical and technical.
Return to the KA7OEI Optical
communications Index page.
If you have questions or comments concerning the
contents of this page, or are interested in this circuit, feel
free to contact me using the information at this URL.
pointer, Lightbeam communications, light beam,
lightbeam, laser beam, modulated light, optical
communications, through-the-air optical
communications, FSO communications, Free-Space
Optical communications, LED communications, laser
communications, LED, laser, laser voice, laser voice
transmitter, laser voice communicator, laser
communicator, laser transmitter, laser voice sender,
laser pointer transmitter, laser pointer
transceiver, laser pointer communicator, laser
pointer communications, laser pointer voice
communicator, laser pointer voice communications,
light-emitting diode, lens, fresnel, fresnel lens,
photodiode, photomultiplier, PMT, phototransistor,
laser tube, laser diode, high power LED, luxeon,
cree, phlatlight, lumileds, modulator, detector
This page and contents
copyright 2009-2016. Last update: 20160920