Contrary to popular belief it is very difficult to force a laser to be truly monochromatic, that is to have a single wavelength.
The laser is generally capable of operating on a number of different wavelengths or frequencies and tends to jump between them. If these wavelengths can be locked together in phase then they will beat together and result in a short pulse envelope.
This technique is called modelocking and it is modelocked lasers that provide the seed pulses for Vulcan’s short pulse capability. The more wavelengths that we can lock together then generally the shorter the pulse we can generate. These lasers also produce a train of pulses with a repetition rate ~100 MHz.
The seed source for the Petawatt target area is a commercial Ti:Sapphire laser. The seed source for Target Area West is a commercial Nd:glass laser, modelocked using a saturable absorber.
