Ultrashort light pulses in confocal imaging systems

Abstract

Tight focusing of ultrashort light pulses is very sensitive because of small amounts of chromatic aberration present in the optical system. This has to be considered if imaging with high spatial and temporal resolution is required. Introducing the concept of time-dependent amplitude point spread functions of lenses illuminated by a pulse of given amplitude spectrum, we can describe the imaging process using Fourier-optical treatment. We show that in a confocal syntem (e.g., a laser scanning microscope) the temporal resolution is determined by the pulse width of the incident pulse if a suitable annular objective pupil and chirped pulses are used, even if the objective shows chromatic aberration. The spatial resolution in discussed in terms of the time-averaged point spread function (PSF) as well as in terms of the optical transfer function (OTF). The OTF shows clearly the influence of chromatic and spherical aberration and defocusing on the imaging process. We analyze this with the aim of obtaining high spatial and temporal resolution simultaneously. Applications of such a confocal system are suggested.