Sixty years ago, on May 16, 1960, the young American physicist Mayman successfully developed the first laser in human history, the ruby laser, which can generate light with a single frequency and a high concentration of direction. It is one of the four major inventions in the 20th century that can be as famous as atomic energy, semiconductors and computers, and has a profound impact on the development of human society. In order to commemorate this major historical event, UNESCO announced in 2018 that the birth day of the laser, that is, May 16 each year, will be designated as the "International Day of Light".
Laser is the light emitted by the excited particles in the atoms of certain substances. It is different from ordinary light. The light waves radiated by it have the same phase, frequency and direction of vibration. The English name of the laser is "Laser", which is the abbreviation of the first letter of each word in English Light Amplification by Stimulated Emission of Radiation. If it is explained in Chinese, it means "light amplification by stimulated emission", which actually reflects the laser Working principle.
Lasers are different from ordinary light sources, which are mainly reflected in three aspects: First, the laser has good directivity and highly concentrated energy. The light emitted by the ordinary light source is divergent, facing in all directions, while the divergence angle of the laser is extremely small. It is said that humans used lasers to illuminate the moon in the 1960s (the mirror on the moon was left by the United States when it landed on the moon) and return to the earth. As a result, the spot on the surface of the moon showed a radius of less than 2 kilometers. In addition, because the photons emitted by the laser can be concentrated in a very small space, the energy density is extremely high, so the strong laser can even generate a high temperature of hundreds of millions of degrees Celsius.
Secondly, the laser has good monochromaticity. The wavelength range of light emitted by ordinary light sources is very wide, and it is not true monochromatic light. The laser is different, because it is amplified by stimulated radiation, so all the photons it emits are exactly the same as the photons excited by the outside world, so the wavelength range of the laser is very narrow.
Finally, the laser has good coherence. The light-emitting mechanism of ordinary light sources is spontaneous emission. The spontaneous emission light generated by different atoms is different in frequency, polarization direction and propagation direction, and is disorderly; while laser light is different, its working mechanism is stimulated emission, so all photons and The photons that excite it from the outside world are the same regardless of frequency, polarization direction and propagation direction.
Therefore, the coherence of laser is very good. When used in light interference experiments, it is easy to observe interference fringes. For example, the test device that detected gravitational waves in 2016 used a Michelson interferometer, and the light source used was a laser.