Yes, I already switched back to batteries..but no difference in the result. I read somewhere to better not use alkaline batteries. How important is this? ..since I can't seem to find any non-alkaline D batteries where I live..
So far, this forum and the people on it have been very helpful
It is critically important to have a STABLE laser diode so that mode hops are eliminated. This requires (a) a very stable power supply, and (b) a very stable output power. Alkaline cells have terrible voltage-vs-discharge curves. They start dropping in voltage for every microamp you take. Rechargeable NiMH cells have very stable voltage-discharge curves. They will be very steady around 1.2v for a long time until it gets close to empty. E.g. a 2000 mA-hr cell will go for at least 50-75 hours at a discharge rate of 20 mA. Voltage will start around 1.25v and will drop very gradually to 1.20v during that period. You can find AA rechargeables with 2000-2500 mAH easy on the internet, should be just as easy at your local electronics supplier.
That said, in your original case you were already getting very good results with the mains power supply. Your diode is already at 90%.
Stable output is maintained when you have stable power supply, stable operating temperature, and minimum backreflections. When your laser beam hits the plate and the objects, a small portion of laser light reflects back towards the diode's aperture. It doesn't take much reflection for the diode to get confused and produce fluctuations in output power. If your plate/object setup is fairly close to the diode, a good amount of reflection will occur. If the beam is pointed straight on to the plate, a LOT of reflection will feed back into the diode. You need to angle the plate and the laser diode so that reflections are steered away from the diode's aperture. It also helps to have a deep and dark background behind your setup, so that the diode doesn't see itself.
If you need to test the diode for stability, you will have to construct a Michelson interferometer. Watch the fringes over time and maybe you'll see if the diode is good or not.