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3d stand up - Electronic components - Featured - HIGH VOLTAGE - History - KNOWASIAK originals - News - Recommendations - Uncategorized - September 14, 2018

Types Of Lasers In The World And Their Applications.

What is a laser?

LASER stands for Light Amplification by Stimulated Emission of Radiation. A laser is a device which produces highly directional light. It emits light through a process called stimulated emission of radiation which increases the intensity of light.

A laser is different from conventional light sources in four ways: coherence, directionality, monochromacity, and high intensity.

The light waves of ordinary light sources have many wavelengths. Hence, the photons emitted by ordinary light sources are out of phase. Thus, ordinary light is incoherent.

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On the other hand, 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.

The light waves from laser contain only one wavelength or color so it is known as monochromatic light.

The laser beam is very narrow and can be concentrated on a very small area. This makes laser light highly directional.

The laser light spreads in a small region of space. Hence, all the energy is concentrated on a narrow region.Therefore, laser light has greater intensity than the ordinary light.

Types of lasers

Lasers are classified into 4 types based on the type of laser medium used:

  • Solid-state laser
  • Gas laser
  • Liquid laser
  • Semiconductor laser

Solid-state laser

A solid-state laser is a laser that uses solid as a laser medium. In these lasers, glass or crystalline materials are used.

Ions are introduced as impurities into host material which can be a glass or crystalline. The process of adding impurities to the substance is called doping. Rare earth elements such as cerium (Ce), erbium (Eu), terbium (Tb) etc are most commonly used as dopants.

Materials such as sapphire (Al2O3), neodymium-doped yttrium aluminum garnet (Nd:YAG), Neodymium-doped glass (Nd:glass) and ytterbium-doped glass are used as host materials for laser medium. Out of these, neodymium-doped yttrium aluminum garnet (Nd:YAG) is most commonly used.

The first solid-state laser was a ruby laser. It is still used in some applications. In this laser, a ruby crystal is used as a laser medium.

In solid-state lasers, light energy is used as pumping source. Light sources such as flashtube, flash lamps, arc lamps, or laser diodes are used to achieve pumping.

Semiconductor lasers do not belong to this category because these lasers are usually electrically pumped and involve different physical processes.

Gas laser

A gas laser is a laser in which an electric current is discharged through a gas inside the laser medium to produce laser light. In gas lasers, the laser medium is in the gaseous state.

Gas lasers are used in applications that require laser light with very high beam quality and long coherence lengths.

In gas laser, the laser medium or gain medium is made up of the mixture of gases. This mixture is packed up into a glass tube. The glass tube filled with the mixture of gases acts as an active medium or laser medium.

A gas laser is the first laser that works on the principle of converting electrical energy into light energy. It produces a laser light beam in the infrared region of the spectrum at 1.15 µm.

Gas lasers are of different types: they are, Helium (He) – Neon (Ne) lasers, argon ion lasers, carbon dioxide lasers (COlasers), carbon monoxide lasers (CO lasers), excimer lasers, nitrogen lasers, hydrogen lasers, etc. The type of gas used to construct the laser medium can determine the lasers wavelength or efficiency.

 Liquid laser

A liquid laser is a laser that uses the liquid as laser medium. In liquid lasers, light supplies energy to the laser medium.

A dye laser is an example of the liquid laser. A dye laser is a laser that uses an organic dye (liquid solution) as the laser medium.

A dye laser is made up of an organic dye mixed with a solvent. These lasers generate laser light from the excited energy states of organic dyes dissolved in liquid solvents. It produces laser light beam in the near ultraviolet (UV) to the near infrared (IR) region of the spectrum.

Semiconductor laser

Semiconductor lasers play an important role in our everyday life. These lasers are very cheap, compact size and consume low power. Semiconductor lasers are also known as laser diodes.

Semiconductor lasers are different from solid-state lasers. In solid-state lasers, light energy is used as the pump source whereas, in semiconductor lasers, electrical energy is used as the pump source.

In semiconductor lasers, a p-n junction of a semiconductor diode forms the active medium or laser medium. The optical gain is produced within the semiconductor material.


Ruby Laser

Ruby laser definition

A ruby laser is a solid-state laser that uses the synthetic ruby crystal as its laser medium. Ruby laser is the first successful laser developed by Maiman in 1960.

Ruby laser is one of the few solid-state lasers that produce visible light. It emits deep red light of wavelength 694.3 nm.

Construction of ruby laser

A ruby laser consists of three important elements: laser medium, the pump source, and the optical resonator.

Laser medium or gain medium in ruby laser

In a ruby laser, a single crystal of ruby (Al2O: Cr3+) in the form of cylinder acts as a laser medium or active medium. The laser medium (ruby) in the ruby laser is made of the host of sapphire (Al2O3) which is doped with small amounts of chromium ions (Cr3+). The ruby has good thermal properties.

Pump source or energy source in ruby laser

The pump source is the element of a ruby laser system that provides energy to the laser medium. In a ruby laser, population inversion is required to achieve laser emission. Population inversion is the process of achieving the greater population of higher energy state than the lower energy state. In order to achieve population inversion, we need to supply energy to the laser medium (ruby).

In a ruby laser, we use flashtube as the energy source or pump source. The flashtube supplies energy to the laser medium (ruby). When lower energy state electrons in the laser medium gain sufficient energy from the flashtube, they jump into the higher energy state or excited state.

Optical resonator

The ends of the cylindrical ruby rod are flat and parallel. The cylindrical ruby rod is placed between two mirrors. The optical coating is applied to both the mirrors. The process of depositing thin layers of metals on glass substrates to make mirror surfaces is called silvering. Each mirror is coated or silvered differently.

At one end of the rod, the mirror is fully silvered whereas, at another end, the mirror is partially silvered.

The fully silvered mirror will completely reflect the light whereas the partially silvered mirror will reflect most part of the light but allows a small portion of light through it to produce output laser light.

Working of ruby laser

The ruby laser is a three level solid-state laser. In a ruby laser, optical pumping technique is used to supply energy to the laser medium. Optical pumping is a technique in which light is used as energy source to raise electrons from lower energy level to the higher energy level.

Consider a ruby laser medium consisting of three energy levels E1, E2, Ewith N number of electrons.

We assume that the energy levels will be E1 < E2 < E3. The energy level E1 is known as ground state or lower energy state, the energy level E2 is known as metastable state, and the energy level E3 is known as pump state.

Let us assume that initially most of the electrons are in the lower energy state (E1) and only a tiny number of electrons are in the excited states (E2 and E3)

When light energy is supplied to the laser medium (ruby), the electrons in the lower energy state or ground state (E1) gains enough energy and jumps into the pump state (E3).

The lifetime of pump state E3 is very small (10-8 sec) so the electrons in the pump state do not stay for long period. After a short period, they fall into the metastable state Eby releasing radiationless energy. The lifetime of metastable state E2 is 10-3 sec which is much greater than the lifetime of pump state E3. Therefore, the electrons reach E2 much faster than they leave E2. This results in an increase in the number of electrons in the metastable state E2 and hence population inversion is achieved.

After some period, the electrons in the metastable state E2 falls into the lower energy state E1 by releasing energy in the form of photons. This is called spontaneous emission of radiation.

When the emitted photon interacts with the electron in the metastable state, it forcefully makes that electron fall into the  ground state E1. As a result, two photons are emitted. This is called stimulated emission of radiation.

When these emitted photons again interacted with the metastable state electrons, then 4 photons are produced. Because of this continuous interaction with the electrons, millions of photons are produced.

In an active medium (ruby), a process called spontaneous emission produces light. The light produced within the laser medium will bounce back and forth between the two mirrors. This stimulates other electrons to fall into the ground state by releasing light energy. This is called stimulated emission. Likewise, millions of electrons are stimulated to emit light. Thus, the light gain is achieved.

The amplified light escapes through the partially reflecting mirror to produce laser light.



Nd:YAG laser

Nd:YAG laser definition

Neodymium-doped Yttrium Aluminum Garnet (Nd: YAG) laser is a solid state laser in which Nd: YAG is used as a laser medium.

These lasers have many different applications in the medical and scientific field for processes such as Lasik surgery and laser spectroscopy.

Nd: YAG laser is a four-level laser system, which means that the four energy levels are involved in laser action. These lasers operate in both pulsed and continuous mode.

Nd: YAG laser generates laser light commonly in the near-infrared region of the spectrum at 1064 nanometers (nm). It also emits laser light at several different wavelengths including 1440 nm, 1320 nm, 1120 nm, and 940 nm.

Nd: YAG laser construction

Nd:YAG laser consists of three important elements: an energy source, active medium, and optical resonator.

Energy source

The energy source or pump source supplies energy to the active medium to achieve population inversion. In Nd: YAG laser, light energy sources such as flashtube or laser diodes are used as energy source to supply energy to the active medium.

In the past, flashtubes are mostly used as pump source because of its low cost. However, nowadays, laser diodes are preferred over flashtubes because of its high efficiency and low cost.

Active medium

The active medium or laser medium of the Nd:YAG laser is made up of a synthetic crystalline material (Yttrium Aluminum Garnet (YAG)) doped with a chemical element (neodymium (Nd)). The lower energy state electrons of the neodymium ions are excited to the higher energy state to provide lasing action in the active medium.

Optical resonator

The Nd:YAG crystal is placed between two mirrors. These two mirrors are optically coated or silvered.

Each mirror is silvered or coated differently. One mirror is fully silvered whereas, another mirror is partially silvered. The mirror, which is fully silvered, will completely reflect the light and is known as fully reflecting mirror.

On the other hand, the mirror which is partially silvered will reflect most part of the light but allows a small portion of light through it to produce the laser beam. This mirror is known as a partially reflecting mirror.

Working of Nd:YAG laser

Nd: YAG laser is a four-level laser system, which means that the four energy levels are involved in laser action. The light energy sources such as flashtubes or laser diodes are used to supply energy to the active medium.

In Nd:YAG laser, the lower energy state electrons in the neodymium ions are excited to the higher energy state to achieve population inversion.

Consider a Nd:YAG crystal active medium consisting of four energy levels E1, E2, E3, and E4 with N number of electrons. The number of electrons in the energy states E1, E2, E3, and Ewill be N1, N2, N3, and N4.

Let us assume that the energy levels will be E< E<E<E4. The energy level E1 is known as ground state, E2 is the next higher energy state or excited state, E3 is the metastable state or excited state and E4 is the pump state or excited state. Let us assume that initially, the population will be N1 > N2 > N3 > N4.

When flashtube or laser diode supplies light energy to the active medium (Nd:YAG crystal), the lower energy state (E1) electrons in the neodymium ions gains enough energy and moves to the pump state or higher energy state E4.

The lifetime of pump state or higher energy state E4 is very small (230 microseconds (µs)) so the electrons in the energy state E4 do not stay for long period. After a short period, the electrons will fall into the next lower energy state or metastable state E3 by releasing non-radiation energy (releasing energy without emitting photons).

The lifetime of metastable state E3 is high as compared to the lifetime of pump state E4. Therefore, the electrons reach E3 much faster than they leave E3. This results in an increase in the number of electrons in the metastable E3and hence population inversion is achieved.

After some period, the electrons in the metastable state E3 will fall into the next lower energy state E2 by releasing photons or light. The emission of photons in this manner is called spontaneous emission.

The lifetime of energy state E2 is very small just like the energy state E4. Therefore, after a short period, the electrons in the energy state E2 will fall back to the ground state E1 by releasing radiationless energy.

When photon emitted due to spontaneous emission is interacted with the other metastable state electron, it stimulates that electron and makes it fall into the lower energy state by releasing the photon. As a result, two photons are released. The emission of photons in this manner is called stimulated emission of radiation.

When these two photons again interacted with the metastable state electrons, four photons are released. Likewise, millions of photons are emitted. Thus, optical gain is achieved.

Spontaneous emission is a natural process but stimulated emission is not a natural process. To achieve stimulated emission, we need to supply external photons or light to the active medium.

The Nd:YAG active medium generates photons or light due to spontaneous emission. The light or photons generated in the active medium will bounce back and forth between the two mirrors. This stimulates other electrons to fall into the lower energy state by releasing photons or light. Likewise, millions of electrons are stimulated to emit photons.

The light generated within the active medium is reflected many times between the mirrors before it escapes through the partially reflecting mirror.

Advantages of Nd:YAG laser

  • Low power consumption
  • Nd:YAG laser offers high gain.
  • Nd:YAG laser has good thermal properties.
  • Nd:YAG laser has good mechanical properties.
  • The efficiency of Nd:YAG laser is very high as compared to the ruby laser.

Applications of Nd:YAG laser

Military

Nd:YAG lasers are used in laser designators and laser rangefinders. A laser designator is a laser light source, which is used to target objects for attacking. A laser rangefinder is a rangefinder, which uses a laser light to determine the distance to an object.

Medicine

Nd: YAG lasers are used to correct posterior capsular opacification (a condition that may occur after a cataract surgery).

Nd:YAG lasers are used to remove skin cancers.

Manufacturing

Nd:YAG lasers are used for etching or marking a variety of plastics and metals.

Nd:YAG lasers are used for cutting and welding steel.



Helium-Neon laser

Helium-Neon laser definition

Helium-Neon laser is a type of gas laser in which a mixture of helium and neon gas is used as a gain medium. Helium-Neon laser is also known as He-Ne laser.

What is a gas laser?

A gas laser is a type of laser in which a mixture of gas is used as the active medium or laser medium. Gas lasers are the most widely used lasers.

Gas lasers range from the low power helium-neon lasers to the very high power carbon dioxide lasers. The helium-neon lasers are most commonly used in college laboratories whereas the carbon dioxide lasers are used in industrial applications.

The main advantage of gas lasers (eg: He-Ne lasers) over solid state lasers is that they are less prone to damage by overheating so they can be run continuously.

What is helium-neon laser?

At room temperature, a ruby laser will only emit short bursts of laser light, each laser pulse occurring after a flash of the pumping light. It would be better to have a laser that emits light continuously. Such a laser is called a continuous wave (CW) laser.

The helium-neon laser was the first continuous wave (CW) laser ever constructed. It was built in 1961 by Ali Javan, Bennett, and Herriott at Bell Telephone Laboratories.

Helium-neon lasers are the most widely used gas lasers. These lasers have many industrial and scientific uses and are often used in laboratory demonstrations of optics.

In He-Ne lasers, the optical pumping method is not used instead an electrical pumping method is used. The excitation of electrons in the He-Ne gas active medium is achieved by passing an electric current through the gas.

The helium-neon laser operates at a wavelength of 632.8 nanometers (nm), in the red portion of the visible spectrum.

Helium-neon laser construction

The helium-neon laser consists of three essential components:

  • Pump source (high voltage power supply)
  • Gain medium (laser glass tube or discharge glass tube)
  • Resonating cavity

High voltage power supply or pump source

In order to produce the laser beam, it is essential to achieve population inversion. Population inversion is the process of achieving more electrons in the higher energy state as compared to the lower energy state.

In general, the lower energy state has more electrons than the higher energy state. However, after achieving population inversion, more electrons will remain in the higher energy state than the lower energy state.

In order to achieve population inversion, we need to supply energy to the gain medium or active medium. Different types of energy sources are used to supply energy to the gain medium.

In ruby lasers and Nd:YAG lasers, the light energy sources such as flashtubes or laser diodes are used as the pump source. However, in helium-neon lasers, light energy is not used as the pump source. In helium-neon lasers, a high voltage DC power supply is used as the pump source. A high voltage DC supplies electric current through the gas mixture of helium and neon.

Gain medium (discharge glass tube or glass envelope)

The gain medium of a helium-neon laser is made up of the mixture of helium and neon gas contained in a glass tube at low pressure. The partial pressure of helium is 1 mbar whereas that of neon is 0.1 mbar.

The gas mixture is mostly comprised of helium gas. Therefore, in order to achieve population inversion, we need to excite primarily the lower energy state electrons of the helium atoms.

In He-Ne laser, neon atoms are the active centers and have energy levels suitable for laser transitions while helium atoms help in exciting neon atoms.

Electrodes (anode and cathode) are provided in the glass tube to send the electric current through the gas mixture. These electrodes are connected to a DC power supply.

Resonating cavity

The glass tube (containing a mixture of helium and neon gas) is placed between two parallel mirrors. These two mirrors are silvered or optically coated.

Each mirror is silvered differently. The left side mirror is partially silvered and is known as output coupler whereas the right side mirror is fully silvered and is known as the high reflector or fully reflecting mirror.

The fully silvered mirror will completely reflect the light whereas the partially silvered mirror will reflect most part of the light but allows some part of the light to produce the laser beam.

Working of helium-neon laser

In order to achieve population inversion, we need to supply energy to the gain medium. In helium-neon lasers, we use high voltage DC as the pump source. A high voltage DC produces energetic electrons that travel through the gas mixture.

The gas mixture in helium-neon laser is mostly comprised of helium atoms. Therefore, helium atoms observe most of the energy supplied by the high voltage DC.

When the power is switched on, a high voltage of about 10 kV is applied across the gas mixture. This power is enough to excite the electrons in the gas mixture. The electrons produced in the process of discharge are accelerated between the electrodes (cathode and anode) through the gas mixture.

In the process of flowing through the gas, the energetic electrons transfer some of their energy to the helium atoms in the gas. As a result, the lower energy state electrons of the helium atoms gain enough energy and jumps into the excited states or metastable states. Let us assume that these metastable states are F3 and F5.

The metastable state electrons of the helium atoms cannot return to ground state by spontaneous emission. However, they can return to ground state by transferring their energy to the lower energy state electrons of the neon atoms.

The energy levels of some of the excited states of the neon atoms are identical to the energy levels of metastable states of the helium atoms. Let us assume that these identical energy states are F3 = E3 and F5 = E5. E3 and E5 are excited states or metastable states of neon atoms.

Unlike the solid, a gas can move or flow between the electrodes. Hence, when the excited electrons of the helium atoms collide with the lower energy state electrons of the neon atoms, they transfer their energy to the neon atoms. As a result, the lower energy state electrons of the neon atoms gain enough energy from the helium atoms and jumps into the higher energy states or metastable states (E3 and E5) whereas the excited electrons of the helium atoms will fall into the ground state. Thus, helium atoms help neon atoms in achieving population inversion.

Likewise, millions of ground state electrons of neon atoms are excited to the metastable states. The metastable states have the longer lifetime. Therefore, a large number of electrons will remain in the metastable states and hence population inversion is achieved.

After some period, the metastable states electrons (E3 and E5) of the neon atoms will spontaneously fall into the next lower energy states (E2 and E4) by releasing photons or red light. This is called spontaneous emission.

The neon excited electrons continue on to the ground state through radiative and nonradiative transitions. It is important for the continuous wave (CW) operation.

The light or photons emitted from the neon atoms will moves back and forth between two mirrors until it stimulates other excited electrons of the neon atoms and causes them to emit light. Thus, optical gain is achieved. This process of photon emission is called stimulated emission of radiation.

The light or photons emitted due to stimulated emission will escape through the partially reflecting mirror or output coupler to produce laser light.

Advantages of helium-neon laser

  • Helium-neon laser emits laser light in the visible portion of the spectrum.
  • High stability
  • Low cost
  • Operates without damage at higher temperatures

Disadvantages of helium-neon laser

  • Low efficiency
  • Low gain
  • Helium-neon lasers are limited to low power tasks

Applications of helium-neon lasers

  • Helium-neon lasers are used in industries.
  • Helium-neon lasers are used in scientific instruments.
  • Helium-neon lasers are used in the college laboratories.

Applications of Lasers

Laser is an optical device that generates intense beam of coherent monochromatic light by stimulated emission of radiation.

Laser light is different from an ordinary light. It has various unique properties such as coherence, monochromacity, directionality, and high intensity. Because of these unique properties, lasers are used in various applications.

The most significant applications of lasers include:

  • Lasers in medicine
  • Lasers in communications
  • Lasers in industries
  • Lasers in science and technology
  • Lasers in military

Lasers in Medicine

  1. Lasers are used for bloodless surgery.
  2. Lasers are used to destroy kidney stones.
  3. Lasers are used in cancer diagnosis and therapy.
  4. Lasers are used for eye lens curvature corrections.
  5. Lasers are used in fiber-optic endoscope to detect ulcers in the intestines.
  6. The liver and lung diseases could be treated by using lasers.
  7. Lasers are used to study the internal structure of microorganisms and cells.
  8. Lasers are used to produce chemical reactions.
  9. Lasers are used to create plasma.
  10. Lasers are used to remove tumors successfully.
  11. Lasers are used to remove the caries or decayed portion of the teeth.
  12. Lasers are used in cosmetic treatments such as acne treatment, cellulite and hair removal.

Lasers in Communications

  1. Laser light is used in optical fiber communications to send information over large distances with low loss.
  2. Laser light is used in underwater communication networks.
  3. Lasers are used in space communication, radars and satellites.

Lasers in Industries

  1. Lasers are used to cut glass and quartz.
  2. Lasers are used in electronic industries for trimming the components of Integrated Circuits (ICs).
  3. Lasers are used for heat treatment in the automotive industry.
  4. Laser light is used to collect the information about the prefixed prices of various products in shops and business establishments from the bar code printed on the product.
  5. Ultraviolet lasers are used in the semiconductor industries for photolithography. Photolithography is the method used for manufacturing printed circuit board (PCB) and microprocessor by using ultraviolet light.
  6. Lasers are used to drill aerosol nozzles and control orifices within the required precision.

Lasers in Science and Technology

  1. A laser helps in studying the Brownian motion of particles.
  2. With the help of a helium-neon laser, it was proved that the velocity of light is same in all directions.
  3. With the help of a laser, it is possible to count the number of atoms in a substance.
  4. Lasers are used in computers to retrieve stored information from a Compact Disc (CD).
  5. Lasers are used to store large amount of information or data in CD-ROM.
  6. Lasers are used to measure the pollutant gases and other contaminants of the atmosphere.
  7. Lasers helps in determining the rate of rotation of the earth accurately.
  8. Lasers are used in computer printers.
  9. Lasers are used for producing three-dimensional pictures in space without the use of lens.
  10. Lasers are used for detecting earthquakes and underwater nuclear blasts.
  11. A gallium arsenide diode laser can be used to setup an invisible fence to protect an area.

Lasers in Military

  1. Laser range finders are used to determine the distance to an object.
  2. The ring laser gyroscope is used for sensing and measuring very small angle of rotation of the moving objects.
  3. Lasers can be used as a secretive illuminators for reconnaissance during night with high precision.
  4. Lasers are used to dispose the energy of a warhead by damaging the missile.
  5. Laser light is used in LIDAR’s to accurately measure the distance to an object.

Transverse and Longitudinal Waves

Transverse waves

If the particles of the medium vibrate in a direction perpendicular to the direction of propagation of the wave, it is called a transverse wave.

In transverse waves, the particle movement is perpendicular to the direction of wave propagation.

Light and other types of electromagnetic radiation are examples of transverse waves. Some other examples of transverse waves include a ripple on a pond and a wave in a string.

The particles do not move along the wave, they simply move up and down relative to the wave propagation.

Example 1:

The circular ripples produced on the surface of the water expand and propagate through water.

As the ripples move horizontally across the surface of water, the water particles vibrate up and down. Thus, the water waves (ripples) propagate horizontally, the particles of the medium (water) vibrate perpendicular to the direction of wave propagation.

Example 2:

Take a string of certain length, with one end attached to a fixed support. Hold the other free end of the string in hand, stretch and vibrate it in a perpendicular direction to the length of the string. A wave pattern is observed in the string as shown in the figure.

The uppermost point of the wave is known as crest (C) and the lowest point in the wave is known as trough (T).

As the wave propagates from left to right, the particles of the string vibrate up and down, thus forming a transverse wave in the string.

Longitudinal waves

If the particles of the medium vibrate in a direction parallel to the direction of propagation of the wave, it is called a longitudinal wave.

In longitudinal waves, the particle movement is parallel to the direction of wave propagation.

Longitudinal waves can travel through solids, liquids, and gases, as the medium requires only elasticity of volume for its propagation.

The longitudinal waves travel through a medium in the form of compressions and rarefactions. The region of high pressure is called compression and the region of low pressure is called rarefaction.

Sound waves and waves in a stretched spring are some examples of longitudinal waves.

Some waves are not purely transverse or longitudinal. For example, the seismic (earthquake) waves produced in the interior of earth travel both in the form of longitudinal and transverse waves.

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