More fun with lasers!
I discovered (why didn't I realise this the other day?) that a 5mm LED fits snugly into the muzzle of this laser module, so I can take very accurate readings of the voltage across the LED now without the stupid brass tube getting in the way.
Hooking up my Fisher Price oscilloscope to the LED sensor gives me the following waveform...
I was driving this laser by providing a 5kHz 50% duty cycle PWM signal using a Parallax Propeller and using a 2N3904 transistor (with no base resistor!) in order to supply enough current to the laser module.
This scope trace is basically a direct reading of the light output which is then captured by the silicon die on the LED sensor. The scope probes are crap, the scope is crap so this isn't a good measurement really.
However the trace was very solid so I'm happy that I can use this setup to test for improvements to the modulation pattern of this laser. This trace would suggest that the rise time is quite favourable - I'd estimate about 25microseconds. The flat bit is maybe 65uS and the falling slope takes the remaining 110uS.
Complications:
1. I can't get the laser module out of it's metal housing - I suspect it's epoxied in there to deter foolish tinkerers like myself meddling with things they should not wot of.
2. There looks to be a voltage regulation circuit in there (unsurprisingly) that will be fighting against my attempts to drive the laser by applying PWM to the battery terminals as I have been doing so far. I've noticed that with increasing PWM frequencies, the resultant light output from the laser becomes smoothed out over the whole PWM amplitude so that ultimately at infinite PQM frequency, the laser would simply emit a steady beam at around 50% peak intensity (that's a naíve description of course, life's not that simple in practice).
Conjecture:
1. If I can get the casing off this laser then perhaps I can bypass the voltage regulator circuitry and drive the diode with a circuit of my own making that will allow me to run the PWM faster and with sharper rise and fall times.
If I try to use this setup on my UV laser plotter then the resultant images would look a bit blurred. In fact, due to that slow fall time it would probably look like a motion blur - as if the image was moving upwards! No good at all.
The plan is to get the rise and fall times to at least be equal - which would prevent any blurring in the final images. If I can make the whole waveform modulate a lot faster as well then that would be great because I could regulate the exposures more accurately and produce a sharper and more detailed image.
Now I have to find a way to get this case open.