[Laser] Atmospheric effects on optical DX, 50 miles +

[Chris L]

Dear laser people,

Further to my earlier posting, we had some interesting research results from 
the Australian DX contact over 104 miles on 19 February.

At my location on Mount Barrow in Northern Tasmania, we had two complete 
optical transceivers, all using fresnel lenses 19 cm by 25 cm. These two 
receivers were set about 2.5 metres apart, each working into an independent 
amplifier and loudspeaker - as such, constituting a primitive form of 
spatial diversity receiving system.

The fast flutter fade characteristic of long optical links was present on 
this simple amplitude modulated system, but even with receivers spaced two 
metres apart the fading was not co-incident at each receiver. In fact, 
standing and listening between the two speakers one could copy the speech on 
the modulated optical signal far better than by listening to either one of 
the receivers alone. It sounded like the signal was see-sawing back and 
forth from one receiver to the other.

All of this raises the question of the size of the 'bubbles' of atmospheric 
density causing the fast flutter fade. These bubbles move across the beam 
path with the prevailing wind. Small lenses up to, say, five inches in 
diameter seem to maximise the fading depth. Go up to 12 inch fresnels and 
the fading depth decreases, and the cyclic fading rate decreases. When I 
tried my large 18" by 36" fresnels, the effect minimised and fading cycles 
reduced further in frequency. A description of this effect was covered in 
some detail in a paper published in the Bell System Technical Journal, Vol. 
62, No 3, March 1983, pps 607 to 699. The same system, using an array of 
four fresnels feeding a matrix of four photomultipliers was patented by AT&T 
at the time (American Patent 4491982). The patent mostly related to the 
steering of a transmitted laser beam to hit the aforementioned fresnel array 
by an electronic feedback system derived from the relative illumination on 
the four adjacent receiving fresnels. This system was demonstrated over a 23 
mile path at the time.

All of this sends a simple message: if you want DX, you should MAXIMISE your 
beam area, not only to avoid diffraction and beam spread, but to minimise 
the fading effects in the intervening atmosphere.

In the BSTJ article, the researchers found that a fixed laser beam path was 
subject to diurnal variations in its vertical displacement owing to the 
vertical thermal contour of the intervening air. Striae of air above the 
ground bent the beam upwards or downwards, particularly at sunrise and 
sunset when the temperture of the ground and the air above it were changing 
relative to each other. It must be admitted that their beam dispersal was 
miniscule, as they expanded their laser beam through a 12 inch telescope 
mirror collimator, so that the whole beam was only about 3 metres in width 
after travelling some 35 or 40 km!

Using my largest fresnels, and holding my eye at the prime focus while 
pointing the optical system at a distant point light source, one can 
actually see the ripples of light intensity in the intervening air moving 
across the cross-sectional area of the receiving beam. The ripple intensity 
decreases as the source size is increased - emphasising the need for 
maximising the transmitting aperture of a modulated light system. Mostly, 
the ripples seem to occur at spacing between 5 and 20 cm, though of course 
the temperature and the state of the atmosphere has a great bearing on this 
- it's not a fixed size of density discontinuity by any means.

Therefore I'm intrigued to ask whether anyone else has found two spaced 
receivers to be superior in effect to one large one. Any experimental 
results here?

Also, a major problem with our optical DX has been the loss of lens 'gain' 
through dewing late at night. It always pays to keep the lens or its cover 
glass a fraction of a degree in temperature above ambient to avoid 
condensation. As I use a series class-A modulator on my 1-watt Luxeons, we 
dissipate quite a bit of power as heat from the series modulating transistor 
(type MJ15003) and the related current limiting resistor. Surely there must 
be some way of channeling this heat energy over the lens, perhaps with a 
small muffin fan, to reduce the tendancy to the formation of dew?

In a permanent link that I had in 1991 between VK3KAU and my home QTH, a 
distance of 43 km, the dewing of my lenses proved to be a major nuiscance. 
Perhaps even some form of hygroscopic oil film on the lens covers might 
help, in at least softening the surface tension to provide a uniform fluid 
coat? Any ideas here, people?

Lastly, has anyone tried using a Peltier to cool the photodiode? Most 
Peltiers currently on the surplus componenr market are quite large (2.5 cm 
square or larger) which is great for car fridges but rather too big for 
conveniently cooling a tiny photodiode. The big Peltiers require somewhere 
around 4 to 8 amps - one might end up throwing four times the power into the 
Peltier as into the whole of the rest of the optical transciever - and then 
there is the problem of condensation: hardly compatible with high-impedance 
photodiode operation!

Anyway I submit all of this for discussion and would value the opinion of 
anyone with practical experience of 'photophony'!

All the best to everyone,

Chris Long (Melbourne, Australia).