[Laser] Information about a QEX article on Laser Transciever

[C. Turner]

While I haven't built the project as a whole, over the years, I've built 
the equivalent of the various pieces over the years and can make a few 
comments on things.

Transmitter:

Current-modulating a laser diode isn't too hard to do, but is a real 
pain - especially if that diode module already comes with its own 
regulator board that wants to do its own thing:  Sometimes this board 
can resist modulation - which is its job, really - making good quality 
modulation difficult to obtain.  Also, many diodes aren't particularly 
forgiving of mishandling and can easily be blown up if the current 
somehow gets just a bit too high.

If you get a cheap laser pointer or something else with a laser in it (I 
picked up a bunch of laser levels for $3 each a few years ago) then it 
will have a small circuit board that does the job of regulating the 
current.  While you can get "raw" laser diode modules, these are usually 
much more expensive, and if you follow the advice at Sam Goldwasser's 
site (Google it!) you'll be better off using a laser diode with a board, 
anyway.

Being that these laser modules can be difficult to amplitude (e.g. 
current) modulate, the obvious workaround is to pulse-width modulate 
them.  I've done this a number of times (I even have an 8-pin PIC that 
can do audio AGC, tone generation etc. along with  PWM'ing) and it is a 
no-brainer, as one simply uses a transistor to turn the laser module 
on/off, powered by a voltage source regulated to the same voltage as the 
batteries from which the module was originally run.

I've heard it said that some laser modules include capacitors that 
prevent modulation, but none of the cheap laser pointer/modules that 
I've ever used have had this problem, and I have on/off modulated a 
couple at up to several hundred kHz, although my PWM systems have 
typically operated around 20 kHz.

***

Receiver:

The receiver described in the article is a typical TIA (Transimpedance 
Amplifier) type.  Having built a number of these, I can see from the 
design that it will be comparatively deaf.  I'm sure that this is 
largely intentional, as it was intended to be usable when it was still 
fairly light outside.  Having experience with several of these types of 
circuits, I can tell that the receiver is likely to be 30-60dB 
less-sensitive that it could be:  It is fairly easy to build an 
audio-frequency optical detector that can produce copyable speech from a 
distant light source that is too dim to see with the naked eye - if you 
use a reasonably large lens.

 From some reason, he also specifies the use of a large-area 
photodiode.  While this can make aiming easier, it can make it much more 
difficult to obtain good ultimate sensitivity while at the same time 
achieving good bandwidth, although the relatively heavy feedback in the 
TIA quashes most of the deleterious effects of that high capacitance.

Perhaps the biggest drawback of using the large photodiode is its 
price:  A BPW34 can be had for less than $1 in small quantities and is 
an excellent performer and has a much smaller surface area while the 
diode specified is, ahem, more expensive than that!

For systems with moderate sensitivity, the TIA topology is just fine and 
one would use a feedback diode in the 1-10Meg range for audio purposes.  
It is, however, extremely important that one locates the front end of 
the TIA very close to the photodiode to absolutely minimize lead 
length:  I'd call 2" of wire connecting the photodiode to the circuit to 
be too much - and don't use shielded cable, either as that adds too much 
capacitance!  If one plans to use this sort of circuit during daylight 
hours - or even with "bright" twilight, don't be surprised if they are 
totally swamped:  "High ambient light" conditions require somewhat 
different strategies ranging from the use of a neutral density filter 
(or a baffle with small holes in it) to AC-coupling of the front end.

If you are fairly close to the transmitter and its light is fairly 
bright, you can also use the fact that the BPW34's plastic case is clear 
as an advantage:  If a "peephole" is arranged so that the backside of 
the photodiode can be viewed, you can actually see the BPW34's case 
light up as the light from the far end hits it.

***

Lenses:

As for the mechanics of the entire system, what is shown is a reasonable 
starting point.  Needless to say, the larger the lens of the receiver, 
the better off you are - and for a number of reasons that should be 
obvious.  The lens - which would be a "positive" type such as a Plano or 
Double Convex itself isn't particularly critical as long as it is of 
reasonably good quality - and about anything you'd get from Edmund or 
Surplus Shed would be adequate:  The cheap acrylic lenses  - or even a 
magnifying glass lens - will usually focus a distant point-source of 
light down to a spot smaller than the active area of the photodiode, and 
that's all you really need it to do!

Also, do not overlook the cheap, rigid page-size Fresnel lenses that 
often show up at stationary/office supply stores these days for under 
$10 - or under 1/3 of that if you get them online.  The main 
consideration of these cheap page-size lenses is that they have a 
reasonable focal length:  I've noted that they seem to come in two 
distinct ranges - in the 10-14" range, and others in the 24" or so 
range:  The latter lenses would make for a very large, awkward box while 
the former are much more-reasonable to use, so check them out in the 
store before you buy them!  Even the flexible vinyl ones work OK, but 
you have to keep them flat:  A pair of cheap picture frames - using only 
the glass from one of them - work nicely to sandwich and protect the lens.

***

Aiming!

Finally, one of the things that is overlooked by many constructors 
(including us, originally) is that it is extremely difficult to aim the 
stupid things:  Forget about trying to adjust a camera tripod to aim the 
laser!  It is simply not possible to use a standard camera tripod Az/El 
adjustments to aim a laser like this - unless you enjoy standing around 
for hours in the dark, pulling out clumps of hair!  Oddly enough, we 
have found that cheap tripods without fluid heads are easier to align 
than those with as the viscous grease tends to be a liability when 
trying to make precise adjustments.

For our laser experiments, the laser pointer itself was mounted on a 
reasonably stout tripod - but with a vernier pointing device:  The 
tripod was set, using "sighting sticks" (that is, a couple of handy tree 
branches, sticks in the ground - or just some volunteers) that are 
in-line with the far end to provide a visual reference as to where, 
approximately, the beam is pointed.  Once the tripod is locked-down, the 
vernier device is used to provide fine and repeatable adjustments to 
azimuth and elevation.

Such a device is shown in Figure 3 here:  
http://modulatedlight.com/optical_comms/contest_2008.html

I've also successfully mounted a laser pointer to the top of my 8" 
Celestron telescope, using it's 1/4-20 mount and the scope's own vernier 
adjustments.

If one is using a standard tripod, it's imperative that the thing 
attached to the tripod have a fairly tightly-contained center mass - and 
this rules out using large boxes that extend outward in any direction, 
as these things tend to become rather wobbly and act as sails in even 
very light breezes.  What this probably means is that this rules out 
using a single box with both the laser pointer and receiver and putting 
it on a tripod.  Since the receiver's beamwidth is going to be far wider 
than that of the laser, you can probably get a way with putting it on 
its own, good-quality tripod - if tripods are your preference, that is.

As far as aiming goes there is, in my opinon, only one way to accomplish 
this easily:  An audio feedback scheme.

In the simplest form, this would involve the transmitter being modulated 
with a tone and the receive end feeding back the audio from the receiver 
via a radio (NOT a cell phone!) link.  With this method, even the 
briefest "pip" of tone can be heard as one scans back and forth and with 
a little bit of practice, one can get the "feel" of how close the beam 
is to the target based on the instantaneous audio feedback.  The 
"instantaneous" part is quite important - and it rules out the use of a 
cell phone as that tends not only to introduce a bit of delay, but the 
digital nature of a cell phone can obfuscate the nature of what is being 
conveyed via audio!

We use a radio link - typically a simplex frequency - to relay the audio 
back.  Making sure that one's transmitting doesn't jam the optical 
receiver or transmitter, we typically go in 30 second sessions to allow 
some chance of actually communicating with the other end if so-needed!

For our system, the optical transmitter being aligned is modulated with 
a 1 kHz tone and the receive end uses a bandpass filter coupled with a 
logarithmic amp and voltage-to-frequency converter to convey the 
strength of the received 1 kHz tone as a changing pitch:  This works 
extremely well, having over 40dB of dynamic range and since the strength 
is conveyed as pitch rather than amplitude, it is intuitively obvious 
and far easier to align and peak than simply listening for the received 
tone's amplitude on a radio link that would have, on a good day, only 
20dB or so of useful dynamic range.  This get around the fact that the 
human ear is really terrible at trying to judge absolute amplitude - 
especially if one is trying to hear a wildly-varying microphone-coupled 
signal over a transmitter that may or may not be driven into clipping!  
Using this system, we were able to align a pair of laser pointers over a 
107+ mile path in a matter of a few minutes - see the link above.

***

These probably answer the questions below to some extent.  For more 
info, you can visit the modulatedlight.org web site, or email me.

73,

Clint
KA7OEI

> I was wondering if anyone has information about this article from the
> November/December 2001 issue of QEX:
> "A Laser Transceiver for the ARRL 10-GHz-and-Up Contest"
>
> 1) Wondering if anyone has built it and how it performed?
> 2) Looking at the parts and trying to find them, I have noticed some
>    have become 'unobtainium" and what might be suggested as replace-
>    ments?
> 3) Is it worth it to obtain the boards from Far Circuits and use?
>
> I am thinking of this as an addition to the W8PGW VHF+ efforts during
> contests.
>
> James W8ISS
>