The FIFA 2018 finale left millions of people in utter bewilderment when two underdog teams France and Croatia set the pace in the stadium. Who would have thought that these two teams would make it through! But they made it!
The laser is one such discovery in the modern discoveries of science which was earlier considered to be a ‘toy’ and today this toy finds application in many fields like medicine, forensics, industries, communications, etc.
Fundamentals of laser
A laser is an acronym of ‘Light Amplification by the Stimulated Emission of Radiation’. It is a device that creates a narrow and low-divergent beam of coherent light with a phase that varies randomly with time and position.
The principle of laser is based on an understanding of the energy transition phenomena in the atoms of its active medium which includes spontaneous emission, stimulated emission, stimulated absorption, population inversion, and amplification.
By quantum mechanics, the lower energy level is more stable than higher energy levels, so electrons tend to occupy the lower level. Those electrons in higher energy levels decay into lower levels, with the emission of electromagnetic radiation. This process is called spontaneous emission. The radiation emitted is equal to the energy difference between the two levels.
Supposing the atoms of the active medium are initially at an energy level (say E2). If external electromagnetic waves with frequency (say n0) that is near the transition frequency between E2 and E1 (ground energy level) is incident on the medium, then there is a set possibility that the incident waves will force the atoms to undergo a transition E2 to E1. Every E2-E1 transition gives out an electromagnetic wave in the form of a photon. We call this stimulated emission since the process is caused by an external excitation. The emitted photon is in phase with the incident photon, has the same wavelength as it and travels in the same direction as the incident photon.
If the atom is initially in the ground level E1, the atom will remain at this level until it got excited. When an electromagnetic wave of frequency n0 is incident on the material, there is again a finite probability that the atom will absorb the incident energy and jump to energy level E2. This process is called stimulated absorption.
Population Inversion and Amplification
Generally, the population of the lower energy levels is larger than that of the higher levels. The processes of stimulated radiation/absorption and spontaneous emission are going on at the same time, yet even if we ignore the decay factors, stimulated absorption still dominates over-stimulated radiation. This means that the incident electromagnetic wave cannot be amplified in this case.
Amplification of incident waves is only possible when the population of the upper level is greater than that of the lower level. This is called Population Inversion. This is a mechanism by which we can add more atoms to the metastable level and hold them there long enough for them to store energy, thereby allowing the production of great numbers of stimulated photons.
If population inversion exists, the incident signal will be amplified. This means that the signal will increase exponentially when there is a population inversion. This continues until the population inversion reaches a certain point, then the signal saturates and reaches a steady state. And that is how the narrow beam of light is ready to display its charm.
What makes laser special?
Laser light has 4 unique characteristics that differentiate it from ordinary light. Laser beams are coherent, monochromatic, have high intensity and are directional.
All the photons emitted in laser have the same energy, frequency or wavelength and are in phase making it coherent and monochromatic.
Ordinary light travels in a random direction whereas, in laser, all photons travel in the same direction. Therefore, the laser emits light only in one direction. This is called the directionality of laser light. The width of a laser beam is extremely narrow. Hence, a laser beam can travel to long distances without spreading.
We know that the intensity of a wave is the energy per unit time flowing through a unit normal area. In an ordinary light source, the light spreads out uniformly in all directions. In laser, the light spreads in a small region of space and in a small wavelength range. Hence, lasers light has greater intensity when compared to 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, and semiconductor laser.
The solid-state laser, gas laser and liquid laser uses solid, liquid and gas as a laser medium. 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. 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.
Applications of lasers
Lasers, due to their unique properties are employed in a wide range of sectors.
Laser communications systems can be easily employed because they are inexpensive and small. They have low power and they do not require any radio interference studies, Data exchange is relatively easy to combine with accurate range metering which is essential in many applications. Lasers are able to see through the dense foliage, and they can allow for space communication from distances measured in millions of miles.
In health-related topics, lasers had great impact. By using lasers scientists and doctors are able to point out cancer cells to destroy them and sometimes they do not need to cut the patient’s body in order to apply their surgery in cases that cutting may create other diseases and in some parts may not even be possible.
High power lasers are remarkable tools that can be used for a wide spectrum of material processing applications in nuclear reactors. The use of optical fibers for laser beam delivery adds a new dimension to their use in rather inaccessible areas and highly radioactive environments.
There are guns that by using laser beams it will fully show the place of the bullet in the target’s object. And more heavy machinery may use the laser as a weapon to create a devastatingly strong armor. Laser communications and laser sensing are important in mortar defense and other crucial aerospace and defense applications.
The high stability and intensity of laser enable us to get the internal view of objects better than X-ray. This technique is called holography and is widely employed in the fields of medicine and engineering.
As early critics called lasers as toys, a laser is actually used as a toy too. We all have used mini lasers in our childhood to tease our younger siblings, our pet cats, and the annoying neighbor. But lasers are used massively for entertainment purposes too.
Discos, pubs, and clubs get their groovy environment with the fascinating beams of lasers. Lasers add amazement to musical fountains and rain dance sections in amusement parks. Lasers are also used in CDs, DVDs, etc.
Apart from these mainstream applications, scientists have found many tasks and uses of lasers. Lasers are devised to cut, drill, weld, read, write, solve crimes, etc. Over and over again the laser has proved to be an extremely practical tool.
The future growth in lasers
During the last few decades, laser technology has advanced unceasingly. New types of lasers as ultra-short pulsed lasers in the femtosecond regime entered medical applications in ophthalmology. Diode lasers became more powerful and smaller with a broader range in wavelengths. In the future, new sources will also be used in medicine, fiber lasers, LEDs, etc. Plastic foils as surface emitters could become important as irradiation source in Photo-Dynamic radio Treatment (PDT). But not only advancement in light sources unbolted new fields in medical laser applications, application development with optimized applicators and tool holders broadened the scale of applications for the same laser, e.g. in dentistry.
Microsurgery is still a challenge where Nano-surgery in cells already in employment. Also, new technology in medical diagnostics enters the scene. Optical coherence tomography with high resolution opens the view into the skin or new sophisticated fluorescence microscopy techniques image metabolism of cells. Online diagnosis in combination with laser treatment will open the market and stimulate further development.
Considering the ‘not-so-warm-welcome’ the laser received when it was first discovered, the milestones it has achieved are fascinating and the ever-increasing ocean of technology has offered a clear surface for the laser to flow. This makes the future of lasers more exciting and promising.