[Laser] Pointing lights at each other

[Clint Turner]

Hi all,

Having been involved in this for some time now, I thought that I might 
reiterate a few things about setting up optical paths.

First of all, take a look at the web page:

http://modulatedlight.org/optical_comms/using_laser_pointers.html

The most important things are practice and planning:  Aiming the things 
over a manageably-short range for practice is EXTREMELY helpful to get 
to know how things work.  For aiming over short distance for testing, I 
built this:

http://modulatedlight.org/optical_comms/PIMG0043.jpg

One the left side is a 555 oscillator using an LDR (CdS cell) to make a 
light-sensitive oscillator.  Using an  HT into an attenuator (30dB or 
so) the tone is transmitted to allow an audible indication of aiming.  
It is this sort of device that I also use for adjusting my lightbeam 
transceivers:  The other side (which as a dim LED modulated at about 
1kHz - a 500 kHz ceramic resonator divided by 512) is spaced the 
center-to-center distance of the two Fresnel lenses allowing focusing 
and peaking while setting paraxial alignment.

When using such a setup one can get the "feel" of how aiming works - and 
one realizes how very touchy pointing about anything - LED transmitters, 
lasers, etc. - really is!  As seen on the "using laser pointers" page 
(linked above) I mention how important it is that if using lasers, one 
MUST use a special devices to allow precise and REPEATABLE aiming.

With LEDs-based communications devices, the transmit beamwidth is a bit 
less tricky, but it is possible - with good-quality Fresnel lenses - to 
obtain beamwidths well under 0.1 degree, but you may not want to do 
that.  Since the beamwidth is likely to be limited by the ratio of the 
apparent size of the emitter and the aperture (with the focal length 
thrown in there) one will, using practical LEDs, end up with a 
larger-than-minimal beamwidth:  Using an emitter with a larger area will 
not increase the far-field flux, but just increase the beamwidth.  Aside 
from increasing the power consumption of the transmitter, this isn't 
necessarily a bad thing as it DOES make aiming easier.  Using Luxeon III 
and Phlatlight devices, the beamwidth is on the order of 0.25 degrees - 
more or less - which makes for a very manageable value.

Setting up the path is really a chicken-and-egg problem:  You don't know 
were to look/aim you receiver unless you can see the transmitter - and 
you can't aim the transmitter unless someone at the receiver can see 
it.  As noted on the page above, we do the legwork beforehand (figure 
out where to point, annotated pictures - real and virtual - to show the 
sites, etc.) but there's still the issue of just getting oriented in the 
dark.

The first thing we will often try is a cheap (usually <$15) 500,000 
candlepower spotlight.  At distances of only a few 10's of km this is 
easy to spot, although it can get lost in a sea of lights.  If there is 
only one light to go around, it is best used at the receive end so that 
those at the transmitter know where to aim - although if one is setting 
up for a full-duplex path, the one light would be best-used at the end 
with the least amount of "light clutter" so that it may be spotted.

Having spotted the light, the transmitter is aimed.  Using simplex radio 
communications (or cell/mobile phones - although those don't work nearly 
as well due to delay/compression!) the LED transmitter is then aimed.  
Since our devices are all coupled TX/RX devices (transceivers) aiming 
one (the RX) also aims the TX by default - or gets one very close to it 
- so the act of aiming the receiver also aims the other end's transmitter.

While our lasers are mounted using devices to allow fine adjustments, 
the LED transceivers are not:  We have found that flat tables suffice.  
Since our LED transceivers have built-in "elevation" adjustments, one 
simply rotates the entire transmitter on the flat table to set the 
azimuth.  For this I would recommend a slightly rough surface:  One of 
our parties (K7RJ) often uses a small, fold-up plastic table, but the 
polyethylene top allows the transceiver to slide too easily, dragged by 
cables or moved by the wind so he has taken to using another surface 
(piece of cardboard, square of carpet or the floor mat from the car!) to 
increase friction.  Ron also made a very small table seen here:

http://modulatedlight.org/optical_comms/DSCN6711.jpg

This sits very low to the ground and has adjustable legs of nesting 
aluminum tubing with stainless-steel hose clamps that allow it to be 
leveled:  The transceiver simply sits atop it and is rotated in its 
entirety to adjust azimuth.

Another method worth considering is the use of a heavy-duty tripod:  A 
surveyor's tripod works fine - if you have one - and a strong 
photographic tripod would also work.  I don't personally use those, 
however, as I prefer the stability of the table to hold all of the gear, 
plus I often pile batteries on the back side of the transceiver to 
further weigh it down - and I've usually tied up my tripods during these 
tests with other gear anyway!

By far the most useful tool in our arsenal is our "Audible S-Meter."  I 
can't really overstate how helpful this device is for setting up an 
optical path.  The device that I use is described here:

http://modulatedlight.org/optical_comms/optical_comm_audio_interface_device.html#audible_s_meter

This devices uses a 1 kHz tone from the transmitter and in the receiver, 
the amplitude of the received 1 kHz tone is converted to a tone of 
varying pitch.  As it turns out this is EXTREMELY sensitive and is able 
to "hear" a 1kHz tone that isn't readily apparent to the human ear - 
plus it responds instantly.  Having about a 40dB range and some 
intrinsic integration, extremely weak tones that are buried in the noise 
are detectable by a slight difference in the resulting tone pitch.  The 
use of a varying pitch is particularly important in that even 1dB of 
signal level change is discernible by the listener in the 40+dB range as 
a definite change in pitch - a difference that would far too small to 
detect had a change in amplitude (that is, just relying on the tone's 
loudness) for the human ear.  By having a variable pitch one can 
more-easily couple this tone into the radio link back to the "other" 
end, allowing those at the receive site to relay, in real time, the 
signal quality back to the person pointing the transmitter.  Typically 
this is done just by having the microphone of the radio (or telephone) 
anywhere near enough to the speaker conveying the S-meter tone to allow 
it to be picked up.  Unless one is severely tone deaf this method works 
very well.


Once we have set up our LED-based links we'll often use this to set up 
our Lasers:  Using the same 1 kHz tone and our already-set-up receivers, 
pointing the lasers is a relative cinch, usually taking only a few 
minutes and using our LED link coming back in the other direction to 
relay the S-meter tones back to us!

Here is a recording of one-such alignment session done over a 172 km path:

http://modulatedlight.com/optical_comms/optical_insipiration_20070903_laser_pointer1b.mp3

In this recording we hear the left channel containing the audio being 
transmitted back to me containing the S-meter tone that I'm using for 
alignment - this being conveyed by a one-way LED link back to me.  
(Since I was aiming the laser, my LED transmitter was turned off.)

In the right channel we hear what was actually being "heard" on the 
optical receiver.  We can hear the 1 kHz tone as it swoops past and, 
eventually, as I "dial in" pointing of the laser.  Although not heard in 
the recording, once I have finished alignment - or if I want to get the 
other end's attention - I'll turn the light on and off three times at 
which point they'll switch from the "S-meter" mode back to the "audio" 
mode - or at least start paying attention to their radio!

* * *

Now I'm sure that there are other methods that can be used, but these 
are among those that have worked for us.

Comments are welcome, of course.

73,

Clint
KA7OEI