Scott,

Did you have any trouble with temperature-sensitivity of the optical filter?   
When doing experiments at 633 nm (He-Ne red) I had to use a narrow band filter 
to keep that part of the ambient light out of the photomultiplier, even though 
the lab was lit with a mixture of blue and green fluorescent tubes to minimise 
the amount of red.

The first attempt did not work at all.  The photomultiplier was cooled with dry 
ice* and the optical filter pass band shifted too much.  I had to isolate the 
PMT and the filter with some optical fibre to keep the filter at room 
temperature.

73
John F5VLF
* a bit below the temperature you get in winter I think

On 8 Oct 2011, at 23:14CEST, Scott Tilley wrote:

> Hi Stefan
> 
> I've created a wide-band audio link before using a laser.  In fact, I used it 
> to bounce the audio I wanted to send off of a passive reflector.  Wideband 
> and excellent fidelity is possible, just ensure the object you are bouncing 
> off is very stable otherwise you'll get microphonics (useful if you want to 
> hear what's going on somewhere else :-)........  I strongly recommend the use 
> of a very narrow band interference type filter for the laser wavelength you 
> are using, mount this over the PIN diode detector...  This way the system is 
> usable day or night without noticeable degradation due to sunlight.
> 
> 73 Scott