Why do lasers have various colors?

- 2021-08-26-

The color of light is determined by the wavelength (or frequency) of the light. A certain wavelength corresponds to a certain color. The wavelength distribution of sunlight ranges from 0.76 microns to 0.4 microns, and the corresponding colors range from red to purple, and there are 7 colors in total, so sunlight is not monochromatic. A light source that emits a single color of light is called a monochromatic light source, and the light wave emitted by it has a single wavelength. For example, krypton lamps, helium lamps, neon lamps, hydrogen lamps, etc. are all monochromatic light sources, which only emit light of a certain color. Although the wavelength of the monochromatic light source is single, it still has a certain range of distribution. For example, the krypton lamp emits only red light, which has good monochromaticity and is known as the crown of monochromaticity. The wavelength distribution range is still 0.00001 nanometers. Therefore, the red light emitted by the krypton lamp still contains dozens of red light if carefully identified. . It can be seen that the narrower the wavelength distribution range of light radiation, the better the monochromaticity. The light output by the laser has a very narrow wavelength distribution range, so the color is extremely pure. Take the helium-neon laser that outputs red light as an example, the wavelength distribution range of its light can be as narrow as 2×10-9 nanometers, which is two ten-thousandths of the wavelength distribution range of the red light emitted by the krypton lamp. It can be seen that the monochromaticity of the laser far exceeds that of any monochromatic light source. In addition, the laser has other characteristics: good coherence. The frequency, vibration direction, and phase of the laser are highly consistent, so that when the laser light waves overlap in space, the light intensity distribution in the overlapping area will appear a stable strong and weak phase phenomenon. This phenomenon is called interference of light, so laser light is coherent light. The light emitted by ordinary light sources, whose frequency, vibration direction, and phase are inconsistent, is called incoherent light. The flash time can be extremely short. Due to technical reasons, the flash time of ordinary light sources can not be very short, and the flash time of a camera flash is about one thousandth of a second. The flash time of pulsed laser is very short, reaching 6 femtoseconds (1 femtosecond=10-15 seconds). Light sources with extremely short flash times have important uses in production, scientific research and military affairs.