Laser Physics

Components of LASER

In order to understand the working principle of a laser, we should first know about the essential components of the laser. These are given below.

  1. Active medium
  2. Pumping
  3. Optical resonator

optical resonator
1) Active medium

The active medium is a collection of atoms or molecules, which can be excited into a population inversion situation, and can have electromagnetic radiation extracted out of it by stimulated emission . The active medium can be in any state of matter: solid, liquid, gas or plasma. The active medium determines the possible wavelengths that can be emitted from the laser.

Examples of Active medium:
  1. Atoms such as helium-neon (He-Ne) in the Helium-Neon laser emitting red light.
  2. Ruby which is Al2O3 with some of the aluminum (Al) atoms replaced by the chromium (Cr) ions. In this case, chromium (Cr) ions are the active centre or active media.

2) Pumping

It is an external source of energy which provides the necessary energy to the active medium to produce a state of population inversion, essential for lasing action. The most commonly used methods of pumping are:

  1. Optical pumping

    2. As the name suggests, in this method, light is used to supply energy to the laser medium. An external light source like xenon flash lamp is used to create the population inversion. This type of pumping is used in solid state lasers such as Ruby lasers & Nd:YAG lasers.

  2. Electric discharge

    3. The intense electric field accelerates the electrons to high speeds and they collide with neutral atoms in the gas. As a result, the atoms in the lower energy state gains sufficient energy from external electrons and jumps into the higher energy state. This method is mainly used in Argon and He-Ne lasers.

  3. Chemical reactions

    4. If an atom or a molecule is produced through some chemical reaction and remains in an excited state at the time of production, then it can be used for pumping. Lets consider the example, H2 + F2 →2HF. The hydrogen fluoride molecule is produced in an excited state when hydrogen and fluorine gas chemically combine. The number of produced excited atoms or molecules is greater than the number of normal state atoms or molecules. Thus, population inversion is achieved.

3) Optical resonator

In a laser, the active system or the gain medium is enclosed in an optical cavity usually made up of two parallel surfaces, one of which is perfectly reflecting reflector and the other surface is partially reflecting reflector. In this resonant cavity, the intensity of photons is raised tremendously through stimulated emission process.