[Laser] Re: Our 100 + miles optical DX record.

[Chris L]

Well!

It seems I threw a cat among the pigeons!

However - a few facts:

(1) LEDs are not coherent sources. They do not exhibit synchronous plane 
parallel wave emission. They can be fairly monochromatic, but only a diode 
LASER emits coherent flux.

(2) The output of various types of lasers are coherent to different degrees. 
HeNe gas lasers are highly coherent, most diode lasers in the visible range 
are only poorly coherent. These generally emit in an oval pattern of around 
7 degrees in one direction and around 30 degrees the other way, parallel to 
the junction. Their output is almost invariably collimated by a small lens. 
Ideally, their collimation should include a cylindrical lens to make the 
beam circular, but this is rarely done in practice. Diode lasers are NOT 
inherently fully coherent and collimated.

(3) As far as eligibility for ARRL records goes, I think I speak for Dr 
Groth (VK7MJ) and myself in stating, like Rhett Butler, "Frankly, Miss 
Charlotte, I couldn't give a damn!" Dr Groth is a licensed ham, I am not, 
though I have a professional engineering background. Any system that 
requires no license is, to me, a boon - as it must be for all those of the 
increasing public gang using 2.4 GHz LAN freeband! All I want is a system 
that works, cheaply, effectively, broadband and through a dynamic and 
changing atmospheric medium. Let's take just one example of this 
practicality. To collimate a laser source with a large aperture by beam 
expansion - a necessary pre-requisite for DX - one needs optics with a 
surface accuracy of around 1/4 wavelength to avoid cancellations and 
irregularity of light flux from the coherent source through the transmitted 
beam cross section. With Luxeons, I can get a beam of high flux density and 
almost perfect homogenity some 16 inches in aperture with a fresnel lens 
costing US$28.60. Look up the cost of a 16 inch telescope objective accurate 
to 1/4 wave at 630 nM. When you've done with the smelling salts and the 
heart bypass, get back to me... (!) Let's just get practical here...

(4) Coherent sources do not remain coherent after passing through any 
reasonable distance of atmosphere. The atmosphere is moving, contains 
bubbles of high and low temperature as well as greater and less density. 
Coherent beams transmitted through the atmosphere remain relatively narrow 
band, but the wave fronts decohere rapidly with distance, producing severe 
decohering noise and scintillation. Look up the work of Dr A A Michelson on 
estimating stellar diameter by means of interference fringes, and you'll 
recognise the reasons for a major source of optical comms scintillation. In 
this context, coherence is NOT an advantage - quite the opposite.

(5) The usage of narrow band optical interference filters for receiving 
becomes especially difficult with lenses of high numerical aperture, as 
interference filters are only intended to work with the light perpendicular 
to the filter pellicle. An f1 lens focusses light to its prime focus over a 
90 degree angle, rendering small aperture int. filters difficult or 
impossible to use. A better approach is simply to raise your input signal 
level until ambient input is insignificant. Large optics and sources of high 
intensity like the Luxeon are one approach to this problem. The Luxeon can 
be linearly modulated to at least 10 MHz by my measurement. There, the 
resemblance to the non-coherent sources that WA4QAL denigrates diverges 
sharply. An arc cannot be modulated to that frequency. A fluorescent light 
cannot. A neon lamp cannot. A mercury vapour arc cannot - though I could get 
10 KHz out of one (refer my article in Australian ''Amateur Radio'' 
magazine, January 1979).

(6) No presently available commercial laser operating at visual frequencies 
gives all of its output at a single discrete frequency. Even the output of 
HeNe lasers (632.8 nM, from memory)  is spread over a noise spectrum of 
several GHz (refer for example the book: ''Laser Receiver''s by Monte Ross). 
To some extent, these are still emitters of modulated noise. They are still 
the 'spark transmitter' that WA4QAL finds so retrograde.

So long as the optical comms system gives 10 MHz bandwidth and reasonable 
s/n, do I give two hoots about being seen as retrogessive for not going to 
coherent light? Pass me the morse key, the spark coil, and the coherer, 
Guglielmo! I just wanna communicate without my wife giving me concussion 
from the rolling pin when she sees the depletion of the family bank account!

But in communicating 100 + miles with the Luxeons, full duplex - have we 
made our point?

More cats, more pigeons! All the best, fellas,

Chris Long.