Beam achromatic expanders

A bandwidth-limited pulse has a spectral width given by the Fourier transform of its time-domain profile. Consequently, a 10-fs FWHM Gaussian pulse centred at 800nm has a bandwidth of 94nm (1466 cm-1). When a short pulse travels through a dispersive medium, the component frequencies are separated in time.

Firstly, the centre of the pulse is delayed with respect to a pulse travelling in air. This is usually called the group delay, which is not a broadening effect. Secondly, normally-dispersive media like glass impose a positive frequency sweep or ‘chirp’ on the pulse, meaning that the blue components are delayed with respect to the red.

Consequently, if optics are used for very short pulses, especially for femto-second pulses, the optics have to be chosen very carefully. Sill Optics provides suitable beam achromatic expanders and f-theta objectives that take these issues into account in order to minimise the chirp on the pulse.