Because the laser is a luminous phenomenon based on the amplification of stimulated radiation, it has many distinct characteristics compared with the ordinary light source based on spontaneous emission.
1. Good monochromaticity
It is known that light of different colors has different wavelengths. For example, the wavelength of red light is 0.647 ~ 0.700μm.
The so-called monochromatic light is actually a section of radiation with a very small wavelength range.
The narrower the spectral line width (the smaller the wavelength range), the better the monochromaticity of light, and the easier it is to be modulated in optical communication.
Because the laser is the stimulated radiation generated after the inversion of particle number between specific energy levels, and through the frequency selection of the resonant cavity, the spectral line width of its output light is very small, that is, it has good monochromaticity.
For example, the krypton lamp with the best monochromaticity among ordinary light sources emits 0.6057μm light with a spectral line width of 4.7 × 10-7μm, while the width of 0.6328μm spectral line emitted by general He-Ne laser can reach 10-10μm .
Good spectral lines can reach 10-12μm, 100000 times higher than krypton lamp.
2. Good directionality
Usually, the divergence angle of light is used to describe its directivity.
The smaller the divergence angle is, the better the directivity is.
The best searchlight among ordinary light sources has a divergence angle of 0.1rad.
If it is irradiated on the moon 400000 km away from the earth, its spot diameter is tens of thousands of km.
In the laser, because the direction of the excited atom is the same as that of the external light, and the resonant cavity only allows the light propagating along the axis to be amplified, the output laser has good directionality, the divergence angle can reach 10-5rad, and the spot diameter is less than 2km when it is irradiated on the moon.
3. High brightness
Because the laser can emit light intermittently, so that its energy can accumulate to a certain extent and then burst out, it has high power, up to 1014W.
In addition, the good directivity of the laser makes its brightness (the radiation power emitted by the light source per unit area within the range of unit solid angle in a given direction) extremely high, hundreds of billions of times higher than the brightness of the sun.
Only the strong flash at the moment of the hydrogen bomb explosion can be compared with it.
4. Good coherence
The so-called coherence refers to the characteristic that two beams of light can interfere to form a stable light dark interference image.
Because the light emitted by stimulated radiation atoms is the same as foreign photons in frequency, phase and vibration direction, the laser has good coherence, which is close to the ideal and completely coherent electromagnetic wave.
It is widely used in optical communication, holography and precision measurement.