Types of Lasers with examples and their working principles
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. Lasers, an acronym for Light Amplification by Stimulated Emission of Radiation, have revolutionized technology and science. From precise surgical tools to cutting edge communication systems, lasers are integral to various applications. Some common types of lasers include homojunction and heterojunction semiconductor lasers, solid-state, gas, liquid and dye lasers. 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
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
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.
Liquid Laser Example: A widely used liquid laser example is the neodymium doped glass laser, which employs a liquid solvent as the medium for light amplification.
Dye lasers
Dye lasers are a unique type of laser that utilize organic dye solutions as the lasing medium, typically dissolved in a liquid solvent. Their most distinguishing feature is their tunability, allowing them to emit light across a wide range of wavelengths by simply adjusting the dye or the laser cavity configuration. This makes dye lasers highly versatile for applications requiring precise wavelength control, such as spectroscopy, medical diagnostics, and laser induced fluorescence. The working principle involves the excitation of dye molecules by an external light source, often another laser, which then emits light as they return to their ground state. Despite their advantages, dye lasers require careful handling due to the toxicity and degradation of dye materials over time.
Dye Laser Example: Rhodamine 6G is a popular example of a dye laser, known for its tunable emission in the visible spectrum, often used in spectroscopy applications.
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. Understanding the types of lasers and their working principles highlights their versatility across industries. Whether it's the precise beam of a solid state laser or the tunable spectrum of dye lasers, each type has unique advantages tailored to specific applications.