To stretch the pulse in time we apply a chirp to it. This means that the different frequency components of the pulse arrive at different times.
The stretchers that we use in Vulcan are formed by a combination of gold gratings separated by an imaging system to apply the desired chirp to achieve the required stretch.
The different frequencies of the pulse will travel different path lengths in the stretcher so that the longer wavelength (red) components of the pulse travel a shorter distance through the system and arrive ahead of the shorter wavelength (blue) components.
There are two stretchers in operation, one for the west target area and one for the petawatt target area, using different gratings, optics and geometry.
The action of the stretcher in applying a chirp to the pulse has to be undone in the target areas to achieve a short pulse. This compression is achieved by applying the opposite chirp to that applied by the stretcher. In this way the imaging system in the stretcher has to generate image planes that have the equivalent distance between the planes and the gratings of the stretcher as the separation of the gratings in the compressor.