How laser light is different from ordinary light
Friday, September 10, 2021
The sun, stars and fire are natural sources of ordinary light. We recognize the light received by the human eye as white, whether it is the light of a lamp or the sun. However, we do not notice the spectrum of colors that make white, except through a prism by which light can be decomposed into different components.
Let’s find out about the differences between ordinary light and laser light.
Ordinary light
Ordinary light sources mainly emit polychromatic light and their emission is spatial. The source of ordinary light emits many rays in all directions, randomly. Ordinary light consists of a mixture of rays with different wavelengths, where each wavelength corresponds to one color that your eyes register.
Then why is the laser light different from ordinary light?
Because laser light is:
- Monochromatic - It contains only one specific wavelength and hence one color.
- Coherent - The motion of all photons is coordinated.
- Directional - The laser beam is very narrow, concentrated and therefore, it is a high intensity source.
Laser (Light Amplification by Stimulated Emission of Radiation) is an artificial source of light radiation that emits a coherent beam of photons, as the source is stable in frequency, wavelength, and power. Unlike the light emitted by common sources, such as light bulbs, laser light is mostly monochromatic, i.e. only one specific wavelength (color).
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When we talk about highly monochromatic lasers, we consider that the beam linewidth is very narrow. You can use these lasers especially for applications such as laser spectroscopy or coupling with optical fibers in fiber communications.
Take a look at other types of light sources: incoherent light sources (light bulbs, LED, stars). These sources produce radiation by spontaneous emission in all directions, with a spread of wavelengths and no interrelationships among individual photons. Spontaneous emission is a random process. On the other hand, lasers use stimulated emission, producing photons with identical properties (all photons move at the same wavelength and direction).
You can be surprised by the fact that measured linewidths of real solid-state lasers, such as diode-pumped YAG lasers, are <1 kHz. This is not the bottom line. In fact, linewidths less than 1Hz are also achievable by suppression of external noise influence. Narrow-linewidth lasers are used in holography, frequency metrology and light detection and ranging (LIDAR).
The laser beam is highly coherent, which means that the electromagnetic waves are in the same phase with each other and propagate in the same direction. You can get a laser beam of high intensity and directivity by superposition of electromagnetic waves that are in phase. Such a highly directional beam can be focused on a very sharp point, which is not possible with ordinary light.
However, real laser light is not purely directional because some propagation effects can distort the laser beam, especially if it is interacting with nonlinear media. When you propagate laser beam it gets broadened with distance. In order to achieve an adequate beam size and directivity you need to additionally precisely control, measure and direct laser beam with lenses and mirrors.
Laser light properties are used all around us
Specific properties of artificial light obtained by lasers are advantageous for transmitting data for hundreds of miles, up to terabits per second, by a convenient, single wavelength coherent source. Nowadays, you can transmit data with laser beams through free-space optics technology over moderate distances with many Gbit/s data rates.
The laser has also become an important research instrument and has found its application in many fields, such as the correction of vision, the sharpening of the astronomical image from space, testing the DNA molecule and in obtaining pure energy by laser fusion of atoms.
Since the discovery of the laser, more than 30 Nobel Prizes have been awarded in the field of natural sciences for scientific discoveries directly related to lasers. In 2018, Donna Strickland became the second woman to receive a Nobel Prize in physics, which she was awarded for her work on ultra-short and high-intensity laser pulses. Measuring the laser's properties with precision and accuracy is key to such research, which is why Gentec-EO develops and manufactures high-accuracy laser power meters and laser energy meters.
