An ultrashort pulse laser is a laser that emits ultrashort pulses of light, generally of the order of femtoseconds to ten picoseconds. They are also known as ultrafast lasers owing to the speed at which pulses "turn on" and "off"—not to be confused with the speed at which light propagates, which is determined by the properties of the medium (and has an upper limit), particularly its index of refraction, and can vary as a function of field intensity (i.e. self-phase modulation) and wavelength (chromatic dispersion).