First of all, to find out the laser mode has transverse mode and longitudinal mode, why to distinguish? Because the nature of the two is different! 1, longitudinal mode: longitudinal mode is the form of stable standing wave formed in the laser resonator! There will be many standing wave forms in the same resonator, because there are a lot of phase matching conditions in the cavity, so long as this condition is satisfied, a stable standing wave can be formed (the amplitude condition is ignored here. We assume that the gain spectrum in the active region of the laser is very wide, and a standing wave form corresponds to a longitudinal mode! Understand the longitudinal mode, the subject can be linked to the laser wavelength, you can simply understand that a wavelength corresponds to a laser longitudinal mode. Transverse mode: the transverse mode refers to the distribution of the field of the laser (including electric and magnetic fields) on the cross section perpendicular to the direction of the laser propagation. The simple understanding is what the field looks like on this surface! For example, the fundamental mode, its distribution on the cross section is a spot, high order mode in the cross section has multiple spot! Note that when you see a spot on the cross section, the fundamental mode, it doesnundefinedt have to be a single longitudinal mode! 3, a single mode and a multimode: Iundefinedll default here that the subject is talking about the longitudinal mode, because what we usually mean by the single mode multimode refers to the longitudinal mode. And the transverse module says the base module and the higher order module! If the wavelength of the laser is associated with the longitudinal mode as I said earlier, the difference between a single mode and a multimode is obvious! Single mode is the laser output only one wavelength of light, multi-mode, the laser output light contains multiple wavelengths!