For a single-mode laser diode the emission spectrum consists of a single spectral line. For a multimode laser diode it consists of several individual spectral lines that differ in frequency by the free spectral range (FSR) of the device, given by FSR=c/2L with c being the speed of light and L the cavity length. The term “mode” is used to describe how the power of a laser beam is distributed. There are longitudinal and transverse modes.