Special properties of laser

- 2021-09-01-

Laser has many special properties, and people use these properties to create laser applications with special value in many aspects.

1. Directional light

Ordinary light sources emit light in all directions. To allow ordinary light to propagate in one direction, it is necessary to install a certain condensing device for the light source. For example, the headlights and searchlights of automobiles are equipped with condensing mirrors.

The laser light emitted by the laser is naturally emitted in one direction, and the divergence of the beam is extremely small, only about 0.001 radian, which is close to parallel. In 1962, human beings used laser light to illuminate the moon for the first time. The distance between the earth and the moon is about 380,000 kilometers, but the laser spot on the surface of the moon is less than two kilometers. If the condensing effect is very good, the seemingly parallel searchlight beams shoot towards the moon, and according to its spot diameter, it will cover the entire moon.

2. Very high brightness

Before the invention of the laser, the high-pressure pulsed xenon lamp had the highest brightness among the artificial light sources, which was comparable to the brightness of the sun, while the laser brightness of the ruby laser could exceed tens of billions of times that of the xenon lamp.

The brightness of the laser is extremely high, it spreads extremely far, and can illuminate objects at a long distance.
The light beam emitted by the ruby laser can produce 0.02 units of illuminance on the moon, the color is bright red, and the light spot is clearly visible. If the most powerful searchlight is used to illuminate the moon, the resulting illuminance is only about one part of a trillion, which cannot be detected by the human eye. The main reason for the extremely high brightness of the laser is directional light emission.

A large number of photons are emitted in a very small space, and the energy density of the laser will naturally be high.

3. Extremely pure color

The color of light is determined by the wavelength (or frequency) of the light. A certain wavelength of light corresponds to a certain color. The wavelength of sunlight is between 0.76 microns and 0.4 microns, and the corresponding colors range from red to purple, and there are 7 colors in total, so sunlight is not monochromatic.

The krypton lamp, helium lamp, neon lamp, hydrogen lamp, etc. we see in daily life are all monochromatic light sources, which only emit light of a certain color. Although the color of a monochromatic light source is single, it still has a certain range of distribution. For example, although the krypton lamp only emits red light, if you carefully identify it, it contains dozens of red light.

Basic knowledge of physics tells us that the narrower the wavelength distribution range of light, 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 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.

Good coherence
Physics tells us that the conditions for the interference of two beams of light are: the same frequency, the same direction of vibration, and a constant phase difference.

The frequency, vibration direction, and phase of the light emitted by the laser are highly consistent; when the two lasers overlap in space, the light intensity distribution in the overlapping area will have a stable strong and weak interference phenomenon. So laser is coherent light. The light emitted by ordinary light sources is incoherent because its frequency, vibration direction, and phase are inconsistent.

5. The flash time can be extremely short

The flash time of ordinary light sources cannot be very short, and the flash time of the flash used for photography is about one thousandth of a second. The flash time of a pulsed laser is very short, up to 6 femtoseconds (1 femtosecond = second).

6. Laser frequency

Lasers emitted by different lasers have different powers, and their frequencies are between infrared and ultraviolet.