Lasers

INTRODUCTION
          Laser is a source of light that is known to emit light through a process of optical amplification. Laser is an acronym for "Light Amplification by Stimulated Emission of Radiation". Laser emits light waves whose frequencies and wavelengths are equal, this property is called as Coherence. Light emitted by Sun and other light sources is incoherent where as light emitted by a Laser is highly Coherent which makes them very special. The Coherent nature of laser allows it to emit light beams narrow over great distances, it allows to emit light of any single color which makes them very applicable for measuring devices and pointers such as Laser pointers. Lasers have many applications, some of them are Laser Surgery, Skin treatments, Cancer treatment, Holograms, Laser printers, barcode scanners, DNA sequencers, Fiber optic and optical communications, cutting and welding materials, measuring devices and for display lights.

HISTORY
          The theoretical concepts of a laser was first established by Albert Einstein in the year 1917. He derivated Max Planck's law of radiation based upon Einstein Coefficients for absorption, spontaneous emission and stimulated emission of electromagnetic radiation. After that in the year 1957, Charles Hard Townes and Arthur Leonard Schawlow began to study about infrared laser and developed ideas for it based upon the Visible light. However the first Laser was built in the year 1960 by T.H Maiman at Hughes Research Laboratories. The first functioning laser was built using a ruby crystal to produce red laser light at a wavelength of 694 Nanometers. The device was only capable of Pulsed operation and flashes of laser was produced. The drawbacks of this laser were its high cost and not continuous emission of light. Later in that year, the first gas laser, constructed by Ali Javan and William R. Bennett was capable of continuous operation and they were awarded by The Albert Einstein Award in the year 1993. Later on experiments were carried out and many sources of laser were known like Semiconductors, Liquid Nitrogen, etc. Lasers beams of different wavelength and colors were also constructed.

WORLD'S FIRST WORKING LASER

TERMINOLOGY

• STIMULATED EMISSION

          Since lasers are emitted through stimulated emission, you might be wondering that what is stimulated emission? As you know that electrons in an atom change their energy levels by emitting or absorbing energy. When electrons absorb energy they move to higher levels and when they emit energy they move to lower energy levels. This emission is called spontaneous emission. Stimulated Emission is the process by which the electron moves to lower energy levels when stimulated by external photons. The higher energy level electrons fall to lower energy levels and emit energy. The photon which stimulates the electron either destroys and new photon is produced after the emission or the photon absorbs the energy and leaves the source.

• POPULATION INVERSION

          If all the electrons at higher energy levels are stimulated and no electron is left then the laser stops functioning. To overcome this Einstein proposed a condition called as the "Population Inversion". When an electric current is passed through the source of laser the electrons absorb the energy and gets excited and move to higher energy levels. This is continued till a state is reached where electrons in higher energy levels are more than the lower energy levels. This state is called as the Population Inversion.

WORKING
         The working of a Laser is pretty simple. The Lasers require a material known as the gain medium which can amplify ( increase power ) light by stimulated emission. Light that passes through the gain medium is amplified. For the gain medium to amplify light it needs energy which is provided by the pumping process. This pumping operation is done by supplying electric current or by a light of different wavelength. When light is used for pumping operation, the process is called as Optical Pumping. Similarly it is called Electrical Pumping when electric current is used.

          In the process of optical pumping the basic source of light is a Flash Lamp or any other Laser. The optical pumping operation provides the required photons and stimulates the electron to emit energy, this energy is then absorbed by the photon and it emits out in the form of amplified light called as laser light. A partially transparent mirror is used at the end of the gain medium which allows the light to pass only in one direction. This helps in focusing the light at one point and the light does not spread.

          In Electrical pumping an electronic oscillator is used to provide an electric current to the gain medium. Two mirrors are attached to the gain medium, one is partially reflecting which is attached at the front end and another is fully reflecting at the other end. The electric current is used to create a state called population inversion in the medium and for pumping. The light emitted is amplified and falls on the partially reflecting mirror which allows the perpendicular beams to pass through it and the inclined beams bounces off on the other end and works as the stimulating photon.

          For different lasers different gain medium is used. Some examples are Ruby crystal, Helium-Neon Gas, Carbon dioxide, Hydrogen Fluoride, Semiconductors such as Silicon, Gallium arsenide  etc.. Laser light emitted by all these sources is very strong such that even a small quantity of pure laser can destroy anything in its path. Therefore sources used in lasers are mixed with impurities and the power of the laser can be adjusted by adjusting the amounts of impurities. Even the impure lasers are hazardous. They can permanently blind a person when directly focused on their eyes.

A 100MW LASER

USES
          There are many uses of Lasers in today's technology in many different fields such as Medical, Military, Research, Communication, Industries, Commercial products etc.

• Lasers are used in the field of Medicine in various ways. Eye Surgeries now require lasers for better treatment and improved results. Kidney Stone treatments have new methods which include lasers. They are also used in Skin treatments, Ophthalmoscopy, Cancer treatment by destroying Tumors and cancer cells. Lasers are more precise than traditional surgery methods and cause less damage, pain, bleeding and swelling. Surgeons must have specialized training as they can be harmful if misused. 

• Lasers have the ability to permanently blind a person when directly focused at their eyes. This makes them useful in making weapons that can incapacitate enemies. Lasers of even a fraction of watt in power can immediately cause vision loss making them non-lethal but incapacitative. Researchers are still experimenting lasers for checking their capacity to destroy an object in its path. They can be used in weapons for destroying the enemies and their weapons. 

• Lasers are used in optical fibres for free space communication and can also be used in space. They have advanced the ways of communication. They also allow services like internet in modern communications. 

• The first use of lasers in daily life was the barcode scanners in supermarkets. Later followed by devices such as Laser printers and scanners, Laserdisc player, Lasers for cutting and welding, Holograms, electronic circuits and measuring devices. Today lasers have become an important part of the modern technology. They have a vast use in nearly every field and are very efficient when compared to previous methods.