I know we have a few people here that are interested in lasers, and I've put some thought into this (I blame Fernando!). I'm putting down some thoughts.
The two most useful I could see being used with a RepRap is a CO2 laser and a nitrogen laser. TEA lasers actually. Google will provide a more complete answer than I can as to what that is.
The CO2 version would be great for doing cutting, but the problem with CO2 is that for a simple "flowing gas" version, you need an exacting mix (about 10% CO2, 20% N2, and the remainder He), while for a sealed version you'd need some very exotic additives. It's also extremely powerful and hence dangerous. Even though 10.6um is considered eye safe, I don't think any 25W heat beam is really all that safe. Standard polycarbonate safety glasses will act as a block at least.
A Nitrogen TEA laser would actually be far more useful. The main advantage for RepRap is that it is a simple 337.1nm wavelength laser. This is UV-A, so it should work well for spot hardening Fernando's polymers.
While tricky, they work off of welding gas N2 or even plain air. Cooled air N2 (from one design online I've seen) can do 120pps at 0.5J per pulse (2mm beam?) and the useful average power is 180mW. Most normal ones do a few pulses per second. The instantaneous power is something like 50KW due to the pulse width being a few nanoseconds. I think power is reduced by about 75% if you can get the same design to work on air. Many of the parts can be easily acquired or made via RepRap. No glass at all is required, but a rear aligned mirror will boost output by a factor of 4.
Now, for issues with a Nitrogen TEA laser. The common capacitor material is a little hard to come by (extremely thin double sided FR4). The voltages involved are hazardous in the extrme. Normal power for something like this is taken from a wall plug, through an auto transformer, through a HV transformer, then up a voltage multiplier to get a few milliamps at 25KV. This would be difficult to really get working well off a 12V power supply and not poison the area with high frequency noise (but not impossible!). Neon sign transformers are also QUITE popular. The beam shape of this is square, not tubular. That may not be too bad, though. These designs commonly use spark gaps, so there's a lot of UV that gets tossed around.
Darwin itself is well suited for an off-machine laser system. The planar XY system could easily mount redirection mirrors and a focusing lens system.
Other laser systems could be useful, but they're not something I think your average user could assemble.
The two most useful I could see being used with a RepRap is a CO2 laser and a nitrogen laser. TEA lasers actually. Google will provide a more complete answer than I can as to what that is.
The CO2 version would be great for doing cutting, but the problem with CO2 is that for a simple "flowing gas" version, you need an exacting mix (about 10% CO2, 20% N2, and the remainder He), while for a sealed version you'd need some very exotic additives. It's also extremely powerful and hence dangerous. Even though 10.6um is considered eye safe, I don't think any 25W heat beam is really all that safe. Standard polycarbonate safety glasses will act as a block at least.
A Nitrogen TEA laser would actually be far more useful. The main advantage for RepRap is that it is a simple 337.1nm wavelength laser. This is UV-A, so it should work well for spot hardening Fernando's polymers.
While tricky, they work off of welding gas N2 or even plain air. Cooled air N2 (from one design online I've seen) can do 120pps at 0.5J per pulse (2mm beam?) and the useful average power is 180mW. Most normal ones do a few pulses per second. The instantaneous power is something like 50KW due to the pulse width being a few nanoseconds. I think power is reduced by about 75% if you can get the same design to work on air. Many of the parts can be easily acquired or made via RepRap. No glass at all is required, but a rear aligned mirror will boost output by a factor of 4.
Now, for issues with a Nitrogen TEA laser. The common capacitor material is a little hard to come by (extremely thin double sided FR4). The voltages involved are hazardous in the extrme. Normal power for something like this is taken from a wall plug, through an auto transformer, through a HV transformer, then up a voltage multiplier to get a few milliamps at 25KV. This would be difficult to really get working well off a 12V power supply and not poison the area with high frequency noise (but not impossible!). Neon sign transformers are also QUITE popular. The beam shape of this is square, not tubular. That may not be too bad, though. These designs commonly use spark gaps, so there's a lot of UV that gets tossed around.
Darwin itself is well suited for an off-machine laser system. The planar XY system could easily mount redirection mirrors and a focusing lens system.
Other laser systems could be useful, but they're not something I think your average user could assemble.