[Laser] Re: laser speckle

[Karel Kulhavy]

> Years ago (~20), I had access to a really nice spectrophotometer, and used 
> it to make some measurements of LEDs and Lasers.  LEDs had quite a 
> broad emission spectrum, dozens to hundreds of nm wide, while LEDs
> had essentially a zero width spectrum (Well, it was so good it was beyond
> the measurement capabilities of the spectrophotometer, and that
> spectrophotometer was one of the best ones available.).  So, there's a world
> of difference between an LED and  a laser.  I suppose you could consider
> it similar to a spark gap transmitter and a quartz crystal controlled transmitter [1].
> 
> [1] I wonder if the comparison between a LED and a laser with a spark gap
> transmitter and a quartz crystal controlled transmitter has anything to do 
> with the ARRLs requirement on optical communications?
> 
> In any case, laser diodes have gotten almost as good as HeNes (if not better).
> The early semiconductor lasers had some real problems with frequency
> hopping and multi-modes.  However, the current generation has improved.
> I can remember when semiconductor lasers wouldn't support holography,
> nor could you see the laser speckle due to the laser jumping frequencies so
> rapidly.  However, now, semiconductor lasers are stable over long time periods.

Why does the laser jump from one frequency to another? Is it that the
Barkhausen criterion for oscillator is met for several frequencies and
it
just randomly hopes here and there as it wants?

What if the loop gain is say 10 for 650.00 and 3 for 650.01nm
and there are no other points where Barkhausen is met? What will the
laser do? Run most time on 650.00 and sometimes do a holiday on 650.01?

Or is it somehow guaranteed that it will run on the "stronger" one and
never make an excursion? Or is the whole just a plain old chaotic system
that does what it wants?

> So, I would have to answer that semiconductor lasers are stable, which would
> imply that their light output is very coherent.
> 
> As for how coherent modern semiconductor lasers are, I think that can be
> determined by viewing the laser speckle, which is due to optical interference.
> 
> Dave
> WA4QAL