I am using a laser diode to count aerosol particles flowing through a gas stream. The refracted light is being funneled to a photo-multiplier via fiber optics. When there is no gas flowing (and therefore no particles) we are seeing some strange results from the output signal of the photomultiplier. After troubleshooting this problem for more than a year, I am left to believe that there is something going on with the laser diode that I am not able to capture. Is it possible that the light being emitted from the diode is shifting around and causing slight changes in the focal point? What I'm seeing, electrically, is an occasional burst of very small square waves coming from the PMT (photomultiplier). Sometimes it'll last for less than a minute, sometimes it'll last for more than an hour. It starts as occasional spikes (either positive or negative) which get wider and more frequent until the square wave develops and then fades out to more spikes inverse to what it started as. This is my first foray into working with lasers after 20 years with electronics. A more optically experienced hand would be very appreciated. I would also like to know if the Iop of a diode is posted at a given mA, is it advisable to use it at that current, or should it be operated somewhere between the posted operating current and the threshold current?