LASER stands for Light Amplification by Stimulated Emission of Radiation. In 1960, the first successful laser was demonstrated by Theodore Maiman using a ruby crystal. The light produced by the laser exhibits special characteristics such as high directionality, coherence, monochromaticity, and high intensity.

• Coherence: The light waves of laser light have only one wavelength. Hence, all the photons emitted by laser light are in phase. Thus, laser light is coherent.
• Directionality: The laser beam is very narrow and can be concentrated in a very small area. This makes laser light highly directional.
• Monochromaticity: The light waves from laser contain only one wavelength or color so it is known as monochromatic light.
• High intensity: The laser light spreads in a small region of space. Therefore, the entire energy is focused within a narrow region. As a result, the intensity of laser light is higher compared to ordinary light.

The basic principle behind the lasing action is stimulated emission which was predicted by Einstein in 1917. In stimulated emission, an incident photon with an appropriate frequency stimulates an atom in the excited state to make a transition to the lower energy state by emitting radiation, which results in the amplification of the incident beam. Lasers are used for various applications such as holography, optical communications, spectroscopy, etc.

Absorption, Spontaneous Emission, and Stimulated Emission

Atoms are characterized by discrete energy states. They can interact with electromagnetic radiation in three different ways. 

Figure 1: Stimulated Absorption

The first one is absorption. It is a process in which electromagnetic radiation of an appropriate frequency can pump the atom to its excited state. i.e; When a photon of energy hν is incident on an atom in the lower level, it gets excited to the upper level. This phenomenon is called absorption. The incident photon should have energy equal to or greater than the energy difference between the upper level and lower level. The process of stimulated absorption is shown in figure 1.

Figure 2: Spontaneous Emission

The second way is spontaneous emission. Atoms in the upper level have a very short lifetime, therefore, they make a spontaneous transition to the lower energy level by emitting a photon. Even without the presence of radiation, this process can still take place. Figure 2 represents the process of spontaneous emission.

Figure 3: Stimulated Emission

The third one is stimulated emission. When an external photon triggers an atom or molecule in the higher energy level, it relaxes to the lower energy level by emitting radiation at the same frequency as that of the external photon. This is the basic principle behind laser. The representation of stimulated emission is shown in figure 3.

Basic Structure of Laser

The basic structure of a laser is shown in figure 4. There are three main components for any laser: active medium, pumping source, and optical resonator. The active medium consists of atoms, molecules, or ions that are capable of amplifying the light waves. 

Figure 4: Schematic Representation of a Laser

The amplification inside a laser occurs when a condition of population inversion can be fulfilled. This condition implies that the number of atoms or molecules in the higher energy level should be greater than the number of atoms or molecules in the lower energy level. The pumping mechanism helps the laser attain population inversion. Different types of pumping can be provided such as optical pumping, electrical pumping, thermal pumping, etc. The active medium is kept inside the optical resonators that act as an oscillator which feeds back the output energy of the oscillator. The optical resonators can be a pair of mirrors that are kept facing each other. One of them is highly reflecting and the other one is a partially reflecting mirror. The laser output comes out of the partially reflecting mirror after optical amplification. The output of the laser is a coherent beam of light that is very intense and tightly focused. The wavelength of the laser depends on the properties of the gain medium and the optical cavity.

Pumping in Lasers

Pumping in lasers is the process of providing energy to the laser medium, typically in the form of light or electricity, to stimulate the emission of coherent and monochromatic light. This energy input excites the atoms or molecules within the laser medium to a higher energy state, creating a population inversion, where more atoms or molecules exist in the higher energy state than the lower one. When these atoms return to their lower energy state, they emit photons in a synchronized manner, resulting in an intense and focused laser output beam. 

Three-level and Four-level Laser System

Lasers usually work with three or four energy levels. In a three-level laser, initially, an atom is excited to a high-energy state, but this state has a short lifetime. It then gets de-excited to a slightly lower-energy state called a metastable state. This metastable state is important because it has a long lifetime. This helps in creating a special condition called population inversion. When this happens, the atoms can be stimulated to release laser light and return to their normal state. One example of a three-level laser is the ruby laser, invented by Theodore Maiman.

In a three-level laser, it works best when the ground state doesn't have too many atoms or molecules in it. When atoms give emit light, they tend to collect in this ground state. Here, they can stop the laser from working by absorbing the laser light. So, three-level lasers usually make short bursts of laser light.

A four-level laser is a solution for this. It adds another state between the metastable state and the ground state. This extra state helps to prevent too many atoms or molecules from accumulating in the ground state. As a result, four-level lasers can produce a steady and continuous beam of laser light that can last for a long time, often for days.

Types of Lasers

Some of the different types of lasers are given below:

• Solid State Lasers: These lasers use solid materials (crystals or glasses) as the active medium. Examples include ruby lasers, neodymium-doped YAG (Nd:YAG) lasers, and erbium-doped fiber lasers.
• Gas Lasers: These lasers use different gases as the active medium. Examples include helium-neon lasers, argon lasers, and krypton lasers.
• Semiconductor Diode Lasers: These lasers emit visible or infrared light when an electric current passes through them. They are commonly used in various applications.
• Fiber-Optic Lasers: These lasers utilize optical fibers to amplify and guide laser light. They are commonly used in telecommunications and data transmission.
• Dye Lasers: Dye lasers use liquid containing organic dye molecules to emit light across a range of wavelengths, making them tunable and versatile.
• Chemical Lasers: These lasers rely on chemical reactions to produce excited molecules for stimulated emission. They can generate high-powered laser beams.
• Free-Electron Lasers: Free-electron lasers emit light when electrons pass through a changing magnetic field, covering a wide range of wavelengths

Applications of Laser

Lasers are used in various medical procedures, including eye surgeries, dermatology, dentistry, and cancer treatment. They are used to precisely cut or vaporize tissues, remove unwanted hair, and even destroy cancer cells.

Lasers are used for industrial applications in various manufacturing processes such as cutting, welding, and drilling, where high precision is required. They are used in material processing, printing, and 3D printing.

They are used in many scientific fields, including physics, chemistry, and biology. They are used to measure distances, study the properties of materials, and even manipulate individual atoms and molecules.

Lasers are used in fiber optic communication systems to transmit large amounts of data over long distances at high speeds.

In military and defense applications, lasers are used for range-finding, targeting, and even for disabling enemy vehicles or weapons.

They are used in various entertainment applications such as light shows, laser tag, and laser pointers.

Lasers are used for environmental monitoring and control, such as measuring air pollution and detecting greenhouse gases.

Click here to learn more about mode locking in lasers.

Click here to learn more about lasers.