How does a LASER work?

  

       A LASER is a type of device in which the light is produced in a narrow beam. The reason for this narrow beam is, that the light waves in the laser are parallel to each other, whereas in normal light the light waves are not parallel to each other. So all the light waves in a laser travel in one particular direction. Also, the laser is very coherent and monochromatic (one color) in nature.

Different light sources and their nature of light

      The abbreviation of LASER is Light Amplification by Stimulated Emission of Radiation, which means that the light is produced by the increase in intensity; through the process of stimulated emission. As the light produced is in a narrow coherent manner, it won’t get distorted as much, over longer distances. So it is used for measuring longer distances, such as measuring the distance between the Earth and the Moon. There are generally different types of lasers, which produce light in the form of UV light to infrared light. The way the lasers produce this light depends upon the gain medium used. The gain medium decides the wavelength of the light, which further decides the color and power of the laser light to be emitted.  The lesser the wavelength; the stronger the laser light becomes and vice versa. The operation of the laser is an amazing process. Let’s dive into it.

         The basic construction of a laser contains a Gain medium; two mirrors out of which one is fully reflective (100%) and the other is partially reflective (98%); an external source which may be a light or an electrical source. The external source makes the gain medium active. The Gain medium is the internal source, which may be Solid, Liquid or gas.

Construction of Solid-State Laser

    The laser is made, by placing the gain medium at the center. Then the mirror with fully reflective is placed on one side of the medium and the mirror with partially reflective is placed on another side of the medium. When the external source is switched ON, (Let’s take the light source for this example), the photons (light) from the source are first inserted into the medium. The process of inserting photons into the medium is called optical pumping. At first, whatever the gain medium may be used, it will be all made up of atoms. So when a light particle is inserted into the gain medium, the electrons in the atom absorb the light energy and jump into a higher conduction band, making it to be excited. Conduction band means, the shells or the orbiting pattern of the electron that surrounds an atom. Let’s consider three conduction bands here, namely E0 (ground state) and E1 (metastable state), and E2 (higher state). If an atom has an electron in a higher energy state (excited condition) the atom is said to be an excited atom. But the electron in the higher conduction band cannot retain in that state for a longer period. So the excited electron quickly jumps back to the lower conduction band (E0), by releasing the same amount of energy that it had absorbed at first. This process is called spontaneous emission. The operation of the spontaneous emission can be observed in the below figure.

Spontaneous emission

    But to make a laser beam, there must be a stimulated emission rather than a spontaneous emission. So to make the laser beam, the photon inserted into the Gain medium must be more powerful, which should make the electron in the Gain medium to jump into the conduction band E2 rather than E1. As soon as the electron jumps into level E2 the atom becomes excited. Soon after a certain time, the electron jumps into level E1 by emitting a less amount of energy which is liberated as heat. Because in the E2 state, the excited electron will be there for less time; thus jumping off from E2 to E1 state, does not release any energy. But still, the atom is in the excited state; because the excited electron is in the E1 state. In this stage, absorbing another photon from the external source does not create a stimulated emission as there are two different energy states in the atom to create stimulated emission. Here stimulated emission means, creating an extra photon in addition to spontaneous emission. Thus creating two photons at the same time by pumping in a single photon that has the energy same as the energy difference between two conduction bands.

Stimulated Emission

    So after a certain amount of time, the electron in level E1 comes to E0 by emitting a photon with higher energy through spontaneous emission. Now, this photon has the energy to cause the stimulated emission. Hence this high-energy photon (which has the same energy difference between two conduction bands) collides with another newly excited atom that has its excited electron in an E1 state. So by this collision, the new atom gives off two new photons which are in same in amplitude, frequency, and direction. Thus creating a stimulated emission. So these ejected photons collide with other excited atoms and the chain reaction continues, thus increasing the stimulated emission. The ejected photon which hits the excited atom only in one particular direction can cause stimulated emission.

Stimulated Emission

       So by causing a stimulated emission, the number of atoms in the excited state becomes greater than the number of atoms in the ground state (non-excited state). This situation is called population inversion. As more photons are produced in the gain medium, it all travels in one particular direction and hits the mirror on the side of the gain medium. If the photons hit the fully reflective mirror, the photons are reflected back and pass through the gain medium and hit the partially reflective mirror. When it hits the partially reflective mirror, 2% of that light is passed through the mirror and making that 2% of light as a useful loss. So only the light with the exact wavelength is made to pass through the mirror. And the remaining 98% of light is reflected back by the partially reflective mirror. So the remaining light goes again and hits the fully reflective mirror through the gain medium. As the light passes through the gain medium every time it gets amplified. So the intensity of the laser light gets higher.  That’s why it is called Light Amplification by Stimulated Emission of Radiation.

Working of LASER