What characterizes a 'true laser'?

A true laser is characterized by its ability to produce a single coherent wavelength of light. This coherence means that the light waves are in phase and have a fixed relationship in time and space, which is a fundamental quality of laser light. The singularity of the wavelength allows for precise applications in various fields, including medicine, communications, and industry, because it can be focused more tightly than other types of light sources.

The coherence, along with monochromaticity (the emission of a single wavelength), enables lasers to achieve high intensity and allows for the use of interference and diffraction in various applications. These attributes distinguish lasers from other light sources, such as LEDs, which emit light over a range of wavelengths and lack coherence. Additionally, the ability to produce this coherent light requires an energy input to excite the atoms or molecules in the gain medium, which is necessary for the laser process of stimulated emission.