Re: polarized lasers?

[gastineau@badgerden.com (John E. Gastineau)]

If you look inside a typical HeNe laser you'll either see glued-on 
mirrors (for randomly polarized lasers) or external mirrors with 
brewster windows on the gas discharge tube (for polarized lasers). 

The glued-on mirror approach means that there is no preferred 
polarization mode, not that the output is unpolarized. As various 
lasing oscillation modes (think of the two mirrors supporting 
standing waves) come and go you see various polarizations. Especially 
if the laser tube has a large internal diameter you can see some with 
some intensity structure as well (the TEM modes). Why the time scale 
is on the order of seconds I don't know. I've observed just what you 
report. Usually several oscillation modes are active at one time, so 
the intensity of any particular polarization doesn't vary by a lot, 
but you can detect it easily by measuring the power transmitted 
through a linear polarizer. 

Lasers with brewster windows are strictly polarized, since there is 
essentially no loss at the window for the preferred polarization. 

The vast majority of research lasers are polarized due to brewster 
windows or other surfaces at brewster's angle (eg, a dye laser jet). 
This makes actually using the laser easier since you often have to 
bounce the beam off of several mirrors, (which have different 
reflectances for different polarization) and so the intensity will 
vary with time if the laser is not polarized. 


On 10 Apr 97 at 21:30, kyle forinash wrote:

> I was showing a polarizer filter in class today and a student asked
> if laser light was or could be polarized. I didn't know and tested
> it out on a NeHe laser and one of those solid state ponter pen
> lasers. Both showed some polarization.
>
> Interestingly the NeHe laser polarization ROTATES with about a 9s
> period (holding the filter still the intensity of the transmitted
> beam varied over time). Can anyone give me an explanation of what is
> going on, why polarized and why rotating?

--
John E. Gastineau                (304) 296-1966
900 B Ridgeway Ave.              gastineau@badgerden.com
Morgantown, WV 26505
www.badgerden.com/~gastineau