LASER | Definition | Its Working | 4 Important Applications



What is a laser?

LASER stands for “light amplification by stimulated emission of radiation“, is an instrument in which light is emitted by the process of optical amplification thru stimulated emission of electro magnetic radiation. The first laser has been invented and designed by Theodore Maiman in the year 1960, The design of this instrument was influenced by the theoretical works of  Charles Hard Townes and Arthur Leonard Schawlow and The light emitted by a laser is coherent in nature i.e. the phase-difference is also constant. This device is used for a wide variety of applications in the field of medicine, research, manufacturing, military, etc.

What is stimulated emission?

An electron present occupying a lower energy state absorbs some external energy present in the form of light (photons) or heat (phonons) in order to occupy a higher energy state and this e- transition from one state to another is possible only when the energies of the photon or phonon is equal to the energy-diff in-between these 2-states. Therefore, these electrons or atoms are capable of absorbing only a specific frequency of light for transitioning.

The electrons cannot stay in the higher excited state forever. They tend to return back to their ground state. These electrons are sometimes externally influenced to fall from a higher excited state to a lower excited state or ground state. The photon emitted after the high-low transition matches the externally supplied photon in terms of direction, phase, and wavelength. This process of releasing photons is referred to as stimulated emission and this forms the basis of laser-working.

For stimulated emission, the first requirement is to excite the electrons or the atoms with the help of a gain media, because in a normal medium the no of atoms in the lower energy state is greater than in the higher energy states at the thermal equilibrium condition hence, the rate of absorption exceeds the rate of stimulated emission in normal media.

Demonstration of stimulated emission, Image source: V1adis1av contribs), Stimulated EmissionCC BY-SA 4.0

What is a Maser?

MASER or “Microwave amplification by stimulated emission of radiation“, is a device in which coherent microwave electromagnetic emission is generated by amplification through stimulated emission mode. Maser has invented at Columbia University, in 1953 by the scientist James P. Gordon, Charles H. Townes, and  Herbert J. Zeiger. Masers have their applications in devices like atomic clocks, and radio telescopes. Masers can also produce electromagnetic radiations belonging in the range of radio and infrared.

A hydrogen radio frequency discharge in a hydrogen maser. Image source: Courtesy NASA/JPLCaltech
Hydrogen maser

What is the difference between a laser and a maser?

Laser vs Maser

This instrument produces coherent electromagnetic emission over a wide frequency range (mainly the visible, UV and IR frequency).Maser produces coherent electromagnetic emission having frequency in the microwave and radio freq range.
This instrument is used for a wide variety of applications in the field of medicine, research, manufacturing, military, etc.This instrument is mainly used for microwave communication and in several astronomical instruments.
This instrument commonly works by exciting atoms of Helium, Neon, Carbon dioxide, etc.This instrument commonly works by exciting atoms of Ammonia, Hydrogen, etc.

What is an amplifying medium?

In lasers, amplifying medium or optical gain medium is a material that amplifies the power of the light beam generated. The gain medium compensates for the power loss due to the resonator. The gain medium amplifies the light by taking up energy through the process of electrical pumping (or at times optical pumping). The gain medium can be of several types such as Nd:YAG (neodymium-doped yttrium aluminum garnet YAG lasers) medium, Yb: YAG (ytterbium-doped YAG) medium,  gallium arsenide, gallium nitride, or indium gallium arsenide semiconductor medium, ceramic gain medium, optical fiber medium, etc.

How does a laser work?

Generally, these instruments contain a gain or amplifying medium, a pumping mechanism, and a system for providing optical feedback. Lasers work on the principle of photoelectric absorption and stimulated emission. These instruments have a gain medium that can be a solid, liquid, or gaseous material. This medium receives the external energy and directs it towards the atoms or electrons to excite them to their higher energy states and this material can be adjusted in term of shape and sizing, concentration and purities.

Population inversion refers to the state where the no of particle present in an higher excited state has exceed the no of particle present in the lower-excited state. In this state, Stimulated photon emission rates will exceed the rate of energy absorbed by the electron. Therefore, the light beam emitted in the form of photons gets amplified.

An optical cavity is present inside the device. It is primarily a pair of mirrors (also called output couplers) present in each side of the gain medium to make the light beam bounce back and forth through the medium being amplified every time it strikes the mirror, and One of the two mirrors is partially transparent allowing some light to escape through it and if the mirrors present are curved then the light comes out in the form of a narrow beam and if the mirrors are flat then the light beam is spread out.

Component Descriptions. : 1. The Gainmedium. 2. The Pumping energy. 3. The High reflector. 4. O/P coupler. 5. the Light beam.
Image source. User:TatouteLaserCC BY-SA 3.0

What are the two modes of operation of a laser?

Coherent light beam can generated either in pulsed mode or in continuous mode.

Pulsed mode operation of a laser :

In pulsed mode, the optical power follows the pattern of a pulse and has a repetition rate based on a certain duration of time. The pulsed mode is used for generating high power pulses by lowering the rate of pulses. The process of ablation and drilling that required high power outputs often used pulsed mode at peak pulse power. Processes that require the application of nonlinear optical effects use the pulsed mode relying on the maximum pulse power or energy. Sometimes amplification in continuous mode cannot be achieved, so the pulsed mode is used.

Continuous mode operation of a laser:

In continuous mode, the output power remains constant over time. In this mode, the frequency variation is negligible and does not influence the lasing application. This mode requires a steady pump source so that the population inversion of the amplifying medium can be achieved. Pumping the lasers at high power levels continually may result in damaging the laser due to excessive heating. For this reason, the continuous mode has a limited power output level. This mode is used for experimental and medical purposes mainly.

What are the applications of lasers?

Applications of lasers

Military applications of laser

Several types of lasers like carbon dioxide lasers that operate and emit infrared light are used for several military applications. The earth’s atmosphere is comparatively more transparent to Infrared light rays. Due to this reason, such lasers prove to be efficient for military range-finding using methods such as LIDAR (light detection and ranging). The laser beam provides information about the distances of the observer and the target position.

Medical applications of laser

IR Lasers , Excimer Lasers used in the Medical field.

Industrial applications (cutting and welding) of laser

Lasers provide high power beams that can be effective for several industrial applications like welding process, etching process, peening and drill process, clad preparation, and laser based cutting for hard metal or glass-cutting process etc. Nowadays, these instruments are also used for surface cleaning that involves eradicating the impurities and contaminants off the surface of a material. The CO2 used for engraving on materials and this devices are also used in the selective manufacturing processes of SLS or Selective laser sintering.

Research applications of laser

The SILEX (Separation of isotope by laser excitation) procedure utilized to enrich uranium also involves IR laser, several other important applications such as fabrication of microfluidic devices also involves the use of these instruments as the common plastic poly (methyl methacrylate) is a good absorbent of IR waves.

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About Sanchari Chakraborty

I am an eager learner, currently invested in the field of Applied Optics and Photonics. I am also an active member of SPIE (International society for optics and photonics) and OSI(Optical Society of India). My articles are aimed towards bringing quality science research topics to light in a simple yet informative way. Science has been evolving since time immemorial. So, I try my bit to tap into the evolution and present it to the readers.

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