[Laser] Lenses, Fresnels, LEDs and detectors (Was: Another TX+RX system coming to life)

[C. Turner]

Hello Paolo,
> Nothing hi-tech, but hopefully a starting point for some fun.
>
> My TX currently uses a 10mm clear case red LED driven at 10mA average, 488 Hz 
> MCW carrier.
>
> The RX is a OPT202 with additional 3M3 series resistor to reduce the device 
> bandwidth, soon to be followed by a bandpass active filter.
>
> I have achieved a copy of the signal through ceiling reflection; feeding the 
> OPT202 output to the MIC input of a laptop running Spectran (but it was 
> audible too). No focusing lens was used. Total distance: about 6m ("Personal 
> Best", HI).
>
> A few of questions.
>
> If I add a second LED in series rather than dissipating heat in the 
> controlling resistor, I double the output power, don't I? Provided the current 
> flowing is unchanged.
>   

There will be twice the optical output power, but it may not be useful 
in any way - more on this in a moment.
> Using several LEDs rather than one will not affect the radiation pattern, as 
> occurs with Yagi antennas at RF, right? (I'm pretty sure about this one.)
>
>   
This partly depends on how you wish to produce the pattern - whether 
using the LED's lens by itself, or have additional optics in front of them.
> Why do many people seem to use Fresnel lenses? Can't I use a el cheapo 
> magnifying lens from the china shop around the corner? It's just a weight matter?
>   
In microwave use, the larger the antennas in the system (e.g. dishes) 
the more gain will be available.

This is also true of an optical system:  Clearly, the larger the receive 
antenna (e.g. lens) the more "gain" it will have.

This is also true for transmitting - but the analogy may not be 
immediately apparent as to why:  For any given collimation system, the 
smaller the emitter is as compared to the diameter of the collimating 
lens, the lower the divergence will be - and this is analogous to a 
larger dish having a smaller beamwidth = higher gain.  This, of course, 
assumes "perfect" optics.

For going just a few hundred feet, you are probably just fine using 
lenses only a few cm across such as those from an inexpensive magnifying 
glass:  The divergence will likely be quite poor, as will (likely) be 
the "illumination efficiency" for the transmitter..

For minimizing divergence, one will want to use a larger lens - and in 
terms of cost, weight, and simplicity, you really can't beat Fresnel lenses.

If you are using lenses, however, the use of additional LEDs isn't 
really of much help.  Because the amount of divergence is based on the 
size of the emitter, putting another LED at the focus will simply 
enlarge that distant spot, increasing the divergence - but this also 
means that the observer won't see any extra brightness - only that the 
"spot" will be wider than before.  This may not be intuitively obvious, 
but it is a mistake that is often made.

Again, if you choose to avoid using external lenses - relying on the 
LED's own pre-focused lens - then multiple LEDs would be of help, but 
remember that a single LED behind even a small lens will probably be far 
more effective than a bunch of LEDs without a lens.
> What are your DX experiences /without/ a lens in front of the detector? 
> Probably with a laser rather than a LED...
>
>   
Using a lens in front of the detector is arguably more important than 
using a lens with the emitter:  Most LEDs already have built-in optics 
to project a "spot" rather than radiate isotropically.  A detector, on 
the other hand, has a more-or-less hemispheric response making it 
susceptible radiation from about any angle - not a good thing in terms 
of using it in an urban environment.  Also, a typical detector has an 
area of only a few square mm.  Let's take the cheap, common BPW34 with 
its 7mm^2 area.  If you compare this with even a small magnifying lens 
of, say, 5cm diameter, just from the aperture alone you can expect a 
gain of over 20dB - and this doesn't take into account the S/N increase 
that one might expect from a reduction of off-axis interfering sources.  
In this way, the improvement gained by using a lens with a detector may 
end up being more than due to the gain of the added lens alone!

Honestly, I really haven't tried for DX using a detector *without* a 
lens as the use of a lens is such an easy way to *dramatically* improve 
performance of a system - and it does so "noiselessly," and without 
requiring any extra power.  If one wanted to keep it as simple as 
possible and avoid lenses as much as possible, one would, once again, be 
better off omitting the emitter's lens rather than the detector's lens, 
for the reasons mentioned above.
> I will soon need a LOS wireless voice intercom that should work over a 
> distance of about 300m. Would this work with a LED as TX in a urban 
> environment? Fewer accurate beaming requirements and no risk to beam a laser 
> into someone's eyes. Any circuit to recommend?
>
>   
No ordinary LED at 10 mA is going to pose a risk to anyone.

As for circuits to use:  For short-range, the OTA (Operational 
Transconductance Amplifier) circuit - such as that used in the OPT202 - 
is adequate, but not particularly good.  If you are trying to reduce 
bandwidth, however, you'd be better off doing so by using a capacitor 
across the amplifier rather than a series resistor, as any resistor in 
the circuit will contribute noise and reduce sensitivity.  Another way 
to reduce bandwidth is to simply use a higher-value feedback resistor, 
if possible, as this will further increase gain and sensitivity - at the 
expense of bandwidth.  The obvious hazard of using large-value feedback 
resistors is that the circuit becomes increasingly susceptible to being 
saturated by extraneous light as the amplifier will more-easily smash 
into a rail - but this sort of problem can be mitigated somewhat with 
the addition of blocking capacitors.

There are also other circuits, such as K3PGP's:  This circuit is not 
well-suited for operation in the presence of other light sources as it 
is easily driven to a rail, but modification (addition of resistors and 
blocking capacitors) such as those suggested by K3PGP can also mitigate 
this - at the expense of sensitivity, of course.

If this circuit needs to work in the daylight, there are a lot of other 
considerations as well, but that's beyond the scope of this email.

***

You have probably seen it, but one fairly simple system of urban (data) 
communications over limited distance is the Ronja system:  Information 
can be easily found with a web search.


> Cloud bounce. I read about K3PGP's experiments and others: were they carried 
> in rural areas or are there some chances in big cities as well?
>
>
>   
A number of groups have done cloud bounce.  Using banks of high-power 
LEDs, some hams in Tasmania have managed >50km with 1-way voice via 
cloudbounce - and this was done in a rural area to avoid problems with 
light pollution.

***

As for Fresnel lenses, I have had some experience with these and their use.

- Avoid overhead projector Fresnel lenses:  They do NOT lend themselves 
to being used in any sort of optical communications system as they do 
not focus/collimate worth a darn.  If you have some of these, they still 
can be used to melt/burn things on the sidewalk...

- Vinyl page magnifier lenses - typically of almost the same size as "A" 
or "A1" sheets of paper and infinitely superior to overhead projector 
Fresnels in terms of optical communications.  These are cheap and 
readily available - and surprisingly good, but they require a mounting 
frame.  Sandwiching them between two panes of clear glass and mounting 
them in a modified picture frame (masking the edges as necessary to 
avoid stray light) works well.  I have used a pair of these (they cost 
me $2 each, surplus) for full-duplex voice communications over a 
distance of over 170km.  Their problem is that they suffer from a bit 
more scattering than higher-quality acrylic lenses - mostly a problem in 
the presence of other, off-axis light sources.  Properly mounted, these 
are capable of focusing most of the received energy down to a spot of 
about a millimeter diameter or so - depending on the ultimate quality of 
the lens, however.

- High-quality acrylic Fresnel lenses.  These are occasionally found on 
the surplus market, often sold to be placed in front of a monitor to 
increase the apparent screen size - a popular thing with video gamers.  
These lenses are surprisingly good and the ones that I have tested will 
focus a star (I used Vega) down to a spot smaller than a millimeter:  
For the lenses that I have tested, the "blur circle" was between 
1/1500th and 1/2500th of the focal length - with the shorter focal 
length lenses providing the better numbers.  I have used such lenses to 
successfully complete Voice and Morse communications over a distance of 
more than 278km.  It is worth mentioning that millimeter sizes are below 
those of typical emitters and many photodetectors.

73,

Clint
KA7OEI