"Experts"? ;D That's a good one... Gazoo qualifies as an expert in my opinion... Benm does too... And Chimo...
;D :
Here's the deal...
To start things off... let's start with the capacitors' risk of killing the diodes by... discharging spontaneously? :-?
Umm... think about it... The capacitor is doing us a favor... It's filling up like a battery right before the diode anyway... it's in fact a PERFECT battery, or about as close to it as we can get in the real world! The output of a capacitor is exactly how a diode wants it... nice and clean with no spikes or sudden drop outs or start ups. A sudden start for a diode is the same as a slow-acting poison... they hate it... and sudden shut downs are just as bad... :
A slow up/down ramp, even if it's the order of pico-seconds, is ALWAYS better... the capacitor in the LM317 circuit does that and more...
The LM317 circuit needs a capacitance... it's good for it... Any regulator requires a capacitance to make sure the output is nice and filtered and as, in fact, DC.
The discharging that the capacitor does is quite healthy too, mind you... It will charge up to some specific amount and then start to discharge the rest into the diode as fast as it is getting it in... in the process of charging up, it'll sake the first jolt, then slowly start to give the diode more and more... until it reaches full potential. At which point the current is pretty much just being filtered by the cap and then sent off to the LD.
Why would you use a current limiting resistor BEFORE the LD and after the capacitor when you are already limiting the current going into the cap? Are you expecting the cap to be a black hole for energy? It's not going to suck the energy and store it indefinitely... it's going to have to come out one way or the other... and the whole reason we are using the LM317 is to limit the current... so it's already nice for the LD, we just need to take out the spikes and whatnots.
Why 47uF? It was handy... and not for no reason... I picked it out of a stack of over 1000 different sizes. It's what I personally believe is a good number that distinguishes between a 'too small' and a 'too big'. It's the right size and fits just right. Your opinion may be different... and I encourage that. I have seen circuits that use as low as a 0.1uF capacitance for the LD... as a matter of fact, do you know those big and fat lasers that pump out more than a watt of power? Or the even beefier ones that all the laser shows use nowadays? These probably do not top 1uF ever... If you start using more than 0.1uF your output to the diode will be too slow and the show will be ruined as a result of all the slow start and slow fade out... BUT, on the other hand, the input to the circuit is heavily filtered... The better the module is, and the more expensive it is, the heavier the filtering before the circuit... If you have tried turning them on... they usually need a minute or so before they kick on... or you have to start the power supply in advance and just keep it on... it's because the capacitors have that long of a ramp-up.
The IDEAL circuit for a laser diode is one that runs from a capacitor... and endless capacitor... I have seen circuits where the input is so regulated that it is actually running off of a series of capacitors that charge one after the other. The lasers that top-end companies use to calibrate with are ones just like that... The lasers used in some military and medical purposes are also just like that... the process uses a stacked capacitor charge-discharge circuit where by the first capacitor is charged from a V-source, and after some time constant the circuit flips and lets the cap charge another cap, and then that cap charges yet another... and that last one discharges into the laser diode... the result? A perfectly filtered and uniform DC source
Now... to cover the full spectrum, I agree with using a couple different kinds... the better idea I would say is probably to add one before the circuit (10uF or so) and then add another at the back of the LD... this should cover the full spectrum... or even better and take it one step further... add both at the beginning of the circuit and have the input to the regulator (output from batteries or any other source) filtered, and then add a smaller 1uF cap on the back of the LD...
It's an endless set of choices and with limitless configurations... take your pick with what you see is your best choice and what gives you your best results. I presented my idea as a helpful and guiding post for other to come
If you need any help, let me know
--DDL
has anyone tried the lm317 driver without the cap? :-? has anyone scoped this circuit to see how it acts on powerup, and power down? :-? It obviously works verry well with the cap, and very likely needs the cap there to prevent spikes, I was just wondering if anyone has actually done the tests with a scope? :-? HMMM, I wonder if this could be the reason a few blu rays have died with no explanation? :-? or maybe they were hit by that one cosmic ray that came all the way accross the universe to hit that one little diode a million light years from its origin? 
