Synonymous with rise and fall time, cycle time is the time it takes for a fiber optic light source to cycle through a rise to its peak and a fall to its trough level of signal intensity, or power. Digital fiber optic systems use Amplitude Modulation (AM), so the faster the light source can cycle, the higher the bit rate. (Note: Most fiber optic system are digital, although a few are analog in nature.) Light sources never completely turn off, as that would limit their speed, so there is always some amount of residual signal present. Diode lasers are the fastest light sources, followed by vertical cavity surface-emitting lasers (VCSELs) and light-emitting diodes (LEDs). See also AM, diode laser, LED, and VCSEL.