Reducing the short pulses stretching caused by lenses
A short optical or x-ray pulse focused by a system of lenses is stretched in focus due to several reasons. In this paper the material effect is discussed, which occurs due to retardation of the pulse when propagating inside a lens. Two methods are proposed to minimize these effects. The first one is to insert the compensation element in front of the lens. The second method is to use an unusual “up-down” lens combination. These methods keep the focusing distance reasonably short.