I am almost sure, that we require a LOT of energy to pump 8x116mm active area of Ruby rod. Let's look on tests data - it required to pump 1000J of flash lamp pulse to receive only 3.5J lasing output! Ruby has very high lasing threshold, so I am sure, that 100mw diode will not pump it over the threshold. Yes, if we are using laser diodes, emitting in maxumum absorption of laser rod, we could use low pump energies, like in DPSS Nd:YAG green lasers.
But Ruby is quite another material!
405nm is in the maximum absorption range of a ruby rod, that's not the problem.
The problem is that ruby absorbs the same wavelength it emits, so whatever parts of the ruby are not being pumped hard enough will be removing power from the beam. To top it off, most ruby rods have high doping levels, so you need to pump it very hard when compared to Nd:YAG lasers.
1000J in for 3.5J out probably means it was pumped with close to 1MW for 1ms, which only flashlamps can provide. However, most of the energy from the flashlamp is lost, so you can probably get away with a few KW of bluray for the same output. Of course, 3.5J output is a lot, so the 50-100W guess may very well be in the ballpark for a sensible output power. My guesstimate was that one would need 15-60W to get any reasonable output from it. The most rational option is to focus the pump beam tightly, so that a much smaller part of the crystal is being used, since the energy density determines how quickly you pass the lasing threshold.
Of course, this is why I recommend people play with ruby half ball lenses instead if they want to use bluray diodes for pumping: the power requirements are much smaller with a tiny piece of ruby that is lightly doped. If one has access to lapidary equipment, one can always use a diamond blade to cut a rod into thin slices and polish them, but most of us do not. The tedium of polishing the surface by hand is probably a show stopper for most, hence stock ruby half ball lenses that cost a mint.
Great rod for flashlamp pumping, though, or for anyone that has the equipment and finances to diode pump the whole rod as well as it deserves. A large mirror coated cylinder with lots of 3.7mmØ holes drilled in it to accomodate the cans of countless 6x diodes will certainly do a better job, but is also a whole lot of work and an insane amount of money if one doesn't actually need that kind of power.
To say nothing of what the missus might think of a decoratively glowing ruby rod over a bulky laser that does "nothing."
