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Transverse modes describe the energy distribution across the beam profile and determine the spatial distribution of the laser light in the beam and the nature of the laser focus. This description of the radiant power or energy in the beam's cross‐section is one way to know if the laser will focus on a clean circular pattern or form several patches of light distributed over a larger area. A “single mode,” or “fundamental mode” (noted as TEM₀₀), has a Gaussian (normal) distribution of power in the beam profile. This is desirable in many circumstances, since it can be focused to the smallest possible spot of any transverse mode. On the other hand, the fundamental mode can represent only a small fraction of the laser's output and may not be desirable where extremely high energy levels are required for a given task.

Most laser designers attempt to achieve a single transverse mode of operation when possible, since the beam of a single‐mode laser can be focused to the smallest theoretical spot for that wavelength. However, single‐mode operation is not always possible and the beam profile may be irregular in shape. This irregular profile is referred to as “multimode.”

The term “mode‐locking” describes a method that fixes the way the photons bounce back and forth in the resonant cavity. Technically, mode‐locking controls the number of longitudinal modes in a cavity. The net result is that a train (i.e. group) of evenly‐spaced pulses is produced, where each individual pulse has an extremely short duration; such pulses are sometimes referred to as “ultra‐short pulses.” One can visualize longitudinal modes by imagining several groups of photons separated in space along the longitudinal or long axis of the cavity. These groups of photons are racing separately at the speed of light (c) between the two mirrors which are separated by the cavity length (L). The transit time between mirrors is c/L seconds and the time for a complete round trip to the starting point is 2 c/L seconds. If only one bundle of photons move back and forth between mirrors, a short pulse of light will leave the cavity through the partially silvered mirror every 2 c/L seconds. For example, since light travels about 30 cm (1 foot) in one nanosecond, a one‐foot long cavity would have ultra‐short pulses being emitted every 2 ns. This allows control of the light in the laser so the light energy bunches into a very concentrated short packet, delivered in a time determined by the length of the laser cavity.