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

[Karel Kulhavy]

> (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.

I am currently slowly working on a laser optical head for Ronja. As soon
as I have it done, we will all know, how big impact this decoherence
will have on optical comms in comparison with today's Ronja's standard
LEDs ;-)

> 
> (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 

Then laws start to be significant. Infrared Ronja already has problems
with eye safety - when you look with a telescope from proximity into
this, you get out of the permitted dial :(

I think brute forcing this will lead just to an unusability for
practical purpose due to safety reasons (which are according to my
opinion definitely not artificial obstacles, but reasonable protection
of public health).

> 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 

How did you perform your measurement? Have you any oscilloscope
screenshots?

> 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!

Today spark transmitters are not used because the wide spectrum. But you
have the whole light for you - it's been reserved for you by your No. 1
supplier of everyday optical noise - Sun! Therefore you can spark there
as much as you want and noone may complain ;-)

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

No, I would like to have 10Mbps full duplex here with <=10^-9 BER :)

Anyway the advantage of lasers is that you can squeeze 100MHz easily and
1GHz when careful probably too out of them. You are not going to do this
with LED's :(

CL<