Laser working principle and types of lasers

The properties of laser light are different from ordinary light sources like LEDs, incandescent or fluorescent bulbs. Laser light is monochromatic, coherent and highly directional. The basic principles of a laser were first proposed by physicist Charles H. Townes and Arthur L. Schawlow. Some common types of lasers include solid state lasers, gas lasers, liquid lasers, dye lasers, homojunction and heterojunction semiconductor lasers and so on. They are used for specific applications as per their working principles and unique characteristics.

What is Laser?

• Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. From the term, it appears that it is a device which amplifies light radiation which is obtained by stimulated emission process.
• Laser light is monochromatic, bright, unidirectional and coherent. Monochromaticity means luminous waves emitted come out with same wavelength and energy. Brilliancy means light beam emitted is extremely intense and angularly well centered. Coherency means all the photons emitted vibrate in phase agreement both in space and time. Unidirectional means all the photons travel in uni direction.
• Light from laser has very low divergence. Hence it can travel over great distances or can be focused to a very small spot with brightness greater than the sun.
• Laser principle can be understood from Bohr's model.
• Light is made up of particles called "photons". Each photon has energy (E) expressed as follows.
➨E = h* v, where h is planck's constant and v is frequency of light.
➨λ * v = c
➨E = h*c/λ

laser working principle

Let us understand working principle of Laser.
• The figure-1 depicts three processes viz. absorption, spontaneous emission and stimulated emission.
• Absorption: For an atom to absorb light, the energy of single photon must equal, almost exactly, the energy difference between the two states. Hence wavelength of the photon must be λ = h*c/ΔE, where, ΔE = Em-En.
• Spontaneous emission: When electron from its excited energy state decays to lower level, it gives off photon of radiation. This is known as spontaneous emission.
• Stimulated emission: In this process, photon is emitted at exactly the same wavelength, exactly same direction and exactly the same phase as the passing photon. For stimulated emission to dominate, there must be more atoms in excited states than in ground state. Such a configuration of atoms is called a population inversion.

The lasers are superior to LEDs and widely used as dedicated light sources in various high performance applications. Lasers overcome all the limitations of LEDs such as large spectral broadening, limited Bandwidth, lower intensity due to chromatic nature of output radiations and so on.

Applications of Laser sources

Based on types of laser, it is used in various applications as described below.
• Storing information in CDs and DVDs
• High speed transmission of information over fiber optic cable (i.e. communication)
• Metallurgical and manufacturing uses such as for metal cutting, drilling, welding etc. with very high power
• Distance monitoring and measurement
• Holography i.e. playback of hologram
• Laser radar and laser simulators for military applications
• Medical tools with very low power used for surgery and other medical treatments

Different types of lasers

Heterojunction semiconductor laser

Based on structure and principle of operation, Laser types are categoried as follows.
➨By active media there are various types which include semiconductor laser, solid state laser, Gas laser, liquid laser or dye laser etc.
➨By mode of operation, there are two types viz. CW laser and pulsed laser. CW (Continuous Wave) laser produces beam of constant amplitude. In normal pulsed laser, the excitation mechanism is pulsed and laser is produced for short time while pumping energy is great enough to keep the active medium above the gain threshold.
➨By pumping and laser levels, there are two types viz. 3-level laser and 4-level laser.
➨Laser can also be classified based on other parameters such as Gain of the laser medium, power delivered by laser, Efficiency or Applications of usage.

Let us understand basics of these different types of lasers.

Homojunction and heterojunction semiconductor laser

Semiconductor lasers are compact in size as they are made using semiconductor materials on nanometer scale accuracy. It is similar to transistor and has operation like LED, but the output beam has characteristics of laser light. The material most often used in semiconductor laser is GaAs (Gallium Arsenide). Hence it is known as gallium arsenide laser. It is also called as injection laser.
Examples of semiconductor lasers are Homojunction laser, double heterojunction laser (as shown in figure-2), Quantum well laser, distributed feedback laser, tunable laser, surface emitting laser etc.

Homojunction and heterojunction semiconductor lasers are types of semiconductor lasers that differ in their design of junctions and materials used. In homojunction laser, active region and surrounding layers have same composition material where as in heterojunction laser they are made of different semiconductor materials with different bandgaps. Homojunction lasers are relatively simple in design and fabrication. Homojunction lasers have lower performance compared to heterojunction lasers in terms of power output, efficiency and temperature stability.

Solid State lasers

They are high power lasers and used for industrial applications such as welding, drilling, cutting, molding etc. These applications require very high power with peak value in kilowatts to megawatts. solid state lasers use high density solid media as active laser materials.
Examples of solid state laser types are ruby laser, Nd:YAG laser etc.

Gas lasers

They are widely available for all power (mwatts to Mwatts) and wavelength (UV, IR) requirements. It uses low density gaseous materials as active media. Electrical pumping (continuous, RF or pulsed) is used. Gas lasers can be made from neutral atoms (He-Ne, metal vapor etc.), ions (e.g. Ar+) or molecules (e.g. CO2).
Examples of gas laser types are argon laser, CO2 laser, He-Ne laser etc.

Liquid lasers

Liquid lasers use liquid as active medium. In dye laser, liquid material is called dye (e.g. rhodamine B, sodium fluoresein, rhodamie 6G) is used as an actibe medium, which causes to produce laser light. These lasers produce output whose wavelengths are in visible, ultra-violet and near infrared spectrum. It is used as research tool in medical applications.

Dye lasers

Dye lasers are commonly used in scientific research and spectroscopy experiments to cover a wide range of wavelengths.

Conclusion : The importance of lasers lies in their ability to produce coherent and intense light beams with specific properties such as directionality, wavelength and efficiency. Due to these characteristics lasers are indispensable as precise and powerful light sources for diverse purposes. They are used in wide range of applications viz. communications, manufacturing, medicine, research, defense and so on. As technology continues to evolve, the importance of lasers is likely to grow.

Advantages and disadvantages of laser types

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