Can you pass 400mJ laser light through a fiber? What is the energy limit for fibre optics? For a dual-pulsed nd:yag laser firing at 10Hz with energy roughly 2x 400mJ, and pulse width ~5-10ns. My gut says that this is far too high to pass through fiber optics, but I have no experience in this matter.
 A: It depends.  The first challenge is damage at the surface.  Is your fiber single-mode or multi-mode?  What is the mode-field diameter?  If it is single-mode then the mode diameter will probably ~10um, or so, and the fluence will be very large.  If you are using multi-mode fiber the mode could be quite large, or not, depending on the fibre.
The second challenge, if you can get the light into the fibre without surface damage, will be nonlinearities as the light propagates.  Brillion scattering, Raman scattering, etc will act to "modify" your pulses in some way.  The longer the fiber the worse these effects get.  So, this challenge depends upon the length, and the mode-field diameter.
A: Back of envelope - .4 J in 10 ns is 40 MW peak power. Put that into a 10 µm fiber, and the peak flux is $5\cdot 10^{17} W/m^2$. Nonlinearity sets in at field strengths above 10$^8$ V/m. Relationship between power and field is $$E = \sqrt{\frac{P}{\epsilon_0 c}} = 1.4\cdot 10^{10}V/m$$ - so your power density is two orders of magnitude above where nonlinearity sets in. It's likely that you will have trouble transmitting the power - in the nonlinear regime, fibers can get pretty weird (although exactly how they respond depends on the type of fiber).
If you used a bigger fiber (multi mode) it might work. Be careful to avoid any surface where absorption could take place. Clean, clean, clean. But if you are using this kind of laser you already know that.
