Yes, there is a reason why you wouldn't want to hook up a diode to a voltage source through some resistors. Resistors don't regulate the current, but just limit it. And it still depends on the source voltage. It would take A LOT of tweaking to determine the correct resistors, which would limit the current enough, that it would still be below what it is capable of.
For laser diodes you need current regulation, and then you need to know what current the diode needs. If you hook it up to a voltage source, you have no idea what the current will be. With a current regulated driver, you can set the current to whatever you want, and it will stay there, regardless of what voltage you put into the driver.
Also, when diodes heat up, they lose resistance, so more current will flow through them, which can kill them, even if you limited the current with resistors enough for the diode to be safe when cold. I lost many diodes by hooking up to a power supply, even tho my PSU can regulate the current.
You should have just measured the current before you took it appart. You could still solder it back to the driver, and try it with batteries first, while measuring the current. If it is not drawing any current, the diode is probably dead.
But are you sure you want to power up an invisible laser diode which could have as much as 200mW or maybe even more? You don't even want to look at the (invisible) reflection of that!
And even if you verify it is still lasing, you will probably not be able to realign it with the crystals. Once you separate the diode from the crystals, the fun is almost always over. I managed to get only half the original power, after many, MANY hours of careful and patient aligning with special tools i had to make for it, so that i could adjust the position and rotation of the crystals with the laser pointed away from me.
If you want a green laser that works, go with DealExtreme, like Razako recommended. They are good for lower power greens, and very cheap at that. It won't be $12, but it will work. And they have free shipping.
Trying to repair cheap green lasers is never worth it. Just a new pump diode of the same power would cost more. And it probably wouldn't be in the same shape so it wouldn't fit. And if it would fit, there is no guarantee it would even make the crystals put out some green. If you just take a working diode out of a working laser, and put it back in into the same position, there is no guarantee it would still work. The tiniest difference in alignment can reduce the output, sometimes even to zero. The only reason for trying to repair one would be if you wanted to learn how they work, and to learn if you have the patience..
If you never built a red laser before, go with that first, and just buy a new green. And you can't hook up red diodes to a voltage source either! Look up drivers in the Experimenst & Modifications section.
