The word ‘LASER’ is the acronym of ‘Light Amplification by Stimulated Emission of Radiation’. Let’s explain this strange concept more.

 

We begin with the electromagnetic waves. Electromagnetic waves, from a spectroscopic point of view, are classified into two different spectral ranges: ultraviolet (UV) waves with the wavelengths between 10 nanometer to 400 nanometer, visible light waves with the wavelengths in the range 400 nanometer (ultraviolet) to 800 nanometer (red light), infrared (IR) waves with the longer wavelengths between 1 micron to 100 micron. The UV and IR waves are invisible and we can see only the light waves in the visible part of the spectrum.  So, how does the laser amplify light?

Generally speaking, the light in the world originates from the relaxation of electrons from a higher energy level to a lower energy level. This process can be done in two different ways that are called “spontaneous emission” and “stimulated emission”. The spontaneous emission of electrons between atomic energy levels occurs in different wavelengths with no correlation between each component. As a result, the natural light in the environment and the light of the lamps in our house create lighting and don’t have the laser properties. The amazing properties of the laser come from the process of stimulated emission. In the stimulated emission, initially, the electron is in a high energy level and then goes to a level with a lower energy due to the interaction with a photon. Hence, this light which is produced by another light is called “the stimulated emitted light”. It travels in the same direction, with the same wavelength and the same phase of the first light. So, the laser light is much stronger that the light of the lamps and can travels in very long distances. The material in which the electrons are displaced between the energy levels and the emission is then occurred is called the “gain medium”. There are different lasers classified based on their gain materials, including solid-state lasers such as Nd-YAG lasers, liquid lasers such as dye lasers and gas lasers such as CO2 lasers. The process in which the electrons jump to a higher energy levels and then relax to a lower energy level and generate light is called “the pump process” or the “gain pumping”. This process is done by electrical discharge in CO2 lasers, flash lamp pumps in some medical lasers for hair removal or diode pumps by special light emitting diodes (LEDs) in engraving lasers and vanadate (Nd:YVO4) cutting lasers.

Now the question is that how we can amplify this stimulated emission?

The amplification of the stimulated emission light is done by a laser resonator, which is usually composed of two mirrors at the beginning and the end of the cavity. The gain material is placed between these two mirrors. Theses mirrors cause the light to stay more in the cavity and pass the gain material for several times. Hence it can amplify to a desirable amount, very quickly.

Now that we are familiar with the laser and its different parts, it is the time to see its special features that distinguish it from conventional light. It has four features due to the stimulated emission process:

1.     Coherency: it means that the laser light maintains its form, phase and wavelength at a special time duration (time coherency) and for a special distance (length coherency). As a result, the laser light features can be fixed and one can deliver special information to several million kilometers by optical fibers without any modification in its form and contents.

2.     Monochromaticity: it means that the laser light contains only one or a few wavelengths. One of the important applications corresponding to this feature is in the medical sciences. Since every material absorbs a special wavelength and emits another special wavelength, the surgeon can burn the cancerous cells by laser without any damage to its adjacent healthy cells.

3.     Low divergence: this feature makes the laser light to travel several kilometers in a straight line without a much reduction in its intensity.

4.     Beam collimation: this characteristic causes the intensity of the laser light to be very high and the diameter of its spot to be very small in the order of a few microns. Hence, incisions in dental and gingival surgery with small tears and no bleeding are possible.

 

Today, pulsed lasers are also realized with the emitted light in a pulsed form and in very small time durations on the order of nanoseconds, femtoseconds and picoseconds. With the aid of these lasers, the very difficult and painful eye surgeries can be accomplished more easily and the patients can continue their daily life without the need to rest for even an hour after the operation.