Modification of cavity ring-up spectroscopy

Cavity ring-up spectroscopy (CRUS) can be used to sense ultrafast phenomena with a timescale of a nanosecond and it has been experimentally demonstrated in whispering-gallery-mode (WGM) microresonators. Here, we present a modification of CRUS in which the intensity of the monochromatic laser used to excite a WGM microresonator is changed from one nonzero value to another nonzero value within a very short time. The modification doubles the sampling rate of CRUS at almost no cost, which is important in ultrafast sensing. We first give a theory of the modified CRUS and then show its experimental observation. The obtained results make the developing CRUS more complete.