The light obtained through stimulated emission travels in all four directions. Its wavelength is uniform, but its phase is not. Guiding this light into a tube with mirrors at either end (one of which is a translucent half mirror), where it reflects back and forth a number of times, causes only light with uniform phase and direction to intensify and remain.

This results in a beam of "coherent light" that has a fully uniform wavelength and phase, and easily interferes with another beam of coherent light. When the intensity of coherent light rises above a certain level, it penetrates the half mirror to reach the outside. This is known as a laser beam. A variety of materials are used for lasers. When a ruby is used, yellowish-green and blue light rise to the fifth floor (excited state) and then emit red light with a wavelength of 694 nanometers when returning from the fourth floor to the first floor (ground state), thereby producing a red laser beam. Semiconductor lasers use a diode combining p-type and n-type silicon.