[Laser] Optical comms at 100 + miles in Australia

[Chris L]

Thanks for your replies, Karel, Kerry et al...

As you may have gathered from the posting of the Southern Tasmania group's 
website, our experiments in optical comms have been going on since about 
1969 (in VK7MJ's case) and 1970 (in my case). Major articles were published 
on this by both of us in the Australian ''Amateur Radio"magazine, January 
1979 and Dec 1986-Jan 1987. It's not any case of trying to "re-invent 
Ronja'' but more a desire to adapt our ideas to locally available 
components, optical and electronic. Australia is not exactly the centre of 
the world's optical and electronic component supply, so we've had to scratch 
about a lot for some of the rarer components, or adapt locally available 
trash to suit.

With their enormous aperture, inherent freedom from spherical aberration, 
and light weight, acrylic fresnels have proven so infinitely superior to 
glass lenses for optical comms that we have all but abandoned glass lenses. 
The fresnels also have the advantage of being square, allowing them to be 
placed in adjacent (transmit and receive with a baffle between the two) 
plywood box mountings wasting the minimum amount of optically active front 
space. A disadvantage is that nearly all commercially available fresnels are 
designed to work with their grooved faces pointing at the longer conjugate, 
ie at infinity or exposed to the outside world. For this reason, we place 
both of our fresnels behind a single 2mm thick glass cover (picture frame 
glass will do) to eliminate dust and grime from the fresnels'grooved 
surface.

Just to prove a point, we did our 104 mile comms with the cheapest and 
nastiest fresnels that we could find - sold in opticians and in 
sight-impairment shops as '"flexible magnifying sheets", only 19 cm by 25 
cm. You will see these in operation on our Mount Barrow station photos - 
refer the two operating trasnceivers in the following picture - and that's 
me with the pom-pom beanie in the background....

http://members.optushome.com.au/~jason.reilly/DSCF0182.JPG

The two receivers are seen there, spaced laterally about 2 metres as 
described in my previous postings. You'll note that one transceiver has four 
lenses. This was originally used as a test bed to compare high-output 
GaAlAsP 660 nM LED's against even higher output GaAlAs 840 nM LEDs on the 43 
km path between myself and VK3KAU in 1991. The results were surprising - the 
IR LEDs worked better, but only by a factor of 3 dB or so, and they were 
damnably infuriating to focus and line up ços you couldn't see the buggers!

When we adapted the old optical unit for the red 1 watt Luxeons, the lower 
two lenses were devoted to the red modulated transmitting Luxeons, each 
running with about 200 mA standing current. You'll see the glow of the red 
scattered light out of the lower two lenses in the pictures. One of the 
upper two lenses had the receiver (BPW34 photodiode). The remaining lens of 
the four had one of the new 3 watt white Luxeons at its PF, keyed on and off 
by a 555 IC at about 2 Hz with 1 Amp standing current as a 'here I 
am'"alignment light. By bonding the edges of the four Luxeons with epoxy to 
a suitably cut black cardboard matte, which in turn was bonded to the single 
cover glass with epoxy, all lenses were exactly in the same plane, and were 
in the requisite flat plane. The box is baffled to block light behind any 
one of the fresnels getting into any others, and can still run full duplex 
without feedback. The light proofing is also aided by using either 
'"Fuller's Caulk in Colours (Black)"or black drain sealant silicone all 
around the edges of the lens and also liberally applied at the front edge of 
each of the internal baffles.

The focal length of each 19 cm by 25 cm fresnel is about 22 cm, and the 
disperal of the system is sufficiently great to permit the usage of a simple 
sliding box mounting, wooden wedge etc - I'd estimate the dispersal to be 
about 1/2 degree. You'll also note that I have a two-lens unit on a 
photographic tripod. This has minimised size by the usage of a truncated 
plywood pyramidsupport  structure behind the two fresnels, rather than a 
rectangular box. This is a lot more practical than trying to line up a 
collimated laser where even seismic disturbances are a worry!

Mike and I are seriously beginning to wonder whether laser communication 
through the atmosphere is worthy of pursuit - it certainly would be if we 
lived in a vacuum. However, the turbulent atmosphere will render a coherent 
beam progressively less coherentand less parallel with distance, as the wave 
fronts break up and/or are delayed to differing degrees by pockets of 
hot/cold dense/less dense air traversing the beam. De-cohering noise, 
spurious beam steering and scintillation occur much less with non-coherent 
sources such as the Luxeon - at least in our experience. Another problem 
with diode lasers is that even if you can modulate them linearly, most 
cannot be modulated fully to zero without dropping into sub-lasing mode. The 
red Luxeons are intense, fully modulatable, so robust that the only way of 
'popping'them seems to be to fuse their internal wires (!) inherently VERY 
linear and just large enough in source area to be well collimated by better 
fresnels - but don't bother trying fresnels liberated from old overhead 
projectors. They're usually twinned fresnels intended for close-field 
conjugates, not a practical proposition for collimating at infinity.

Incidentally, don't be trapped into thinking that the new 3 watt and 5 watt 
white/blue/green Luxeons are any good for optical comms. They are not so 
linear, and their source area is some four or more times the size of the 
chip in the red 1 watt Lambertian Luxeon - the red types have a much greater 
intensity. Also, avoid the 'batwing' Luxeons as they don't optically couple 
well into a fresnel collimator. The "Lambertian" (high dome) type provides 
far better far-field flux.

A very handy source of fresnels is the 3dLens company in Taiwan. They make a 
39.5 cm square (about 15.5 inches square) fesnel with 33 cm focal length, 
thickness 2 mm, and offer it on Net for $28.60 American dollars. That would 
give you terrific collection area for receiving, and a transmit beam spread 
sufficient to overcome many of the smaller turbulence bubbles encountered 
over 5 km or so - sufficient to make amplitude modulation quite practical. 
By contrast, most Amaerican fresnel suppliers such as Edmund charge a SMALL 
FORTUNE for fresnels - and when I bought a 19" by 25" fresnel from them in 
1984, I was amused to find "MADE IN JAPAN" stamped on the lens!!

So chuck out those glass lenses and go fresnel - so many advantages!

The remaining problem, of course, is to reduce the fresnel's field of view 
so that your transceiver doesn't burst into flames when sunlight gets '"down 
the chute""! A series of parallel tubes or a box structure in front of the 
lens would probably do the trick in limiting the FOV - but we've been doing 
most of our DX tests at night so that's not such a worry to us.

Pleased to find such an interested reaction to our work!

Thank you!

Chris Long (Melbourne - also for Mike Groth VK7MJ Hobart).