Complex sentence: When it exits the beamlines it is recombined in a similar set of gratings to produce a single very short pulse, but because the pulse now has very high power, the gratings have to be large (approx 1 m) and sit in a vacuum.
Split into simpler sentences: When it exits the beamlines it is recombined in a similar set of gratings to produce a single very short pulse. But because the pulse now has very high power, the gratings have to be large (approx 1 m) and sit in a vacuum.