[Laser] stable audio oscillator?

[Andrew T. Flowers, K0SM]

I'm not really sure whom to ask about this, but I'm guessing that 
someone has run into a similar problem at one time or another...

I'm building a laser communication system and I'm looking at different 
modulation schemes for the transmitter.  I'd like to try QRSS CW--that's 
CW in 0.5wpm or even less, using DSP to show the elements on the screen 
using spectran or other analysis program.  I thought about keying the 
laser on and off into a 12vdc fan to create a squarewave tone.  It works 
fine for normal speed CW, but the frequency drift is +/- 10Hz or so. 
 For QRSS to be effective, I need rock-solid stability, preferably less 
than 100 millihertz/min or better.  Any ideas here?  I'd like to keep it 
cheap.  Is there some version of the 555 timer do this with good 
components?  I was also thinking of getting some ovenized xtal thing in 
the low RF range and then dividing down.

Ideas??

Andy, K0SM