[Laser] Coherent Source - back on my soap box

[Charles Pooley]

I have been a long time lurker, posting only a couple times, but do want to add 2c here.

As an advocate of laser diode data comm (see http://www.microlaunchers.com/7816/L3/laser/laser-link.html )
The diode laser is not meant to be coherent (they are only to on order of a cm or so), but the better collimation and narrower optical spectrum.

The former allows the transmit optics to be very small; the latter use of interference filters to reduce background light.

Atmospheric effects are an aspect to experiment with by using active tracking of beam wander and if needed, multiple receive optics spaced a few meters apart.

The receiver detector can be PIN or avalanche diodes before going on the more expensive photon counter diodes.

Tim's mention of multiple LED colors being available is available for lasers.  They come in 50 or so wavelengths in near IR and red.
Also you can "tune" them by controlling the temperature with a peltier cooler.  

Interference filters are tunable by tilting from normal to the optical axis.

If anyone is interested in the space comm situation described in Microlaunchers, or in any aspect, I would like to hear.

Charles Pooley                  KD6HKU, Microlaunchers




________________________________
From: Tim Toast <toasty256 at yahoo.com>
To: laser at mailman.qth.net
Sent: Mon, November 9, 2009 6:46:12 AM
Subject: Re: [Laser] Coherent Source - back on my soap box

I dont claim to understand the underlying math completely, 
but the degree of coherence of a source is directly related 
to the source's spectral bandwidth mathematically. In the 
simplest form the formula is the reciprocal of bandwidth. 
(coherence = 1/bandwidth) Other formulas include the 
refractive index of the medium (air), center frequency and C. 

Technically, requiring coherence without specifying a number, 
means that you cannot use a source with an infinite bandwidth 
or zero coherence length. So technically any real light source 
qualifies as "coherent" because none have infinite bandwidth.

At the other extreme, a hypothetical laser producing a single 
frequency which does not drift (ever) would have perfect 
temporal coherence and infinite coherence length. 

These two imaginary extremes are the only two that can be 
described fully with "coherent and not". In the spirit of the 
rule though, i would say they imply a small bandwidth and a 
resulting degree of coherence from a real light source. 

Besides LEDs having usually less than 50nm bandwidth, i 
thought a 50nm spacing would divide up the visible spectrum 
nicely into 6 bands (Blue, Cyan, Green, Yellow, Orange and 
Red) easily estimated by eye, and with a few UV and IR bands 
beyond each end. 







      
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