Laser is an acronym for light amplification by stimulated emission of radiation. In the last century many types of lasers have been used for many different applications from welding to surgery to military and even many uses in every day life by harnessing the principles of light and stimulated emission.
To understand how lasers work we must first understand the physics behind light waves. Light is emitted from a source and travels in straight lines and when it strikes an object is either absorbed, reflected, or refracted. Light behaves primarily like a wave and its this wave nature of light that allows lasers to work. Constructive interference is what amplifies light. Since light is a wave it has a frequency calculated by the equation:
f = c/
where f is the frequency c is the speed of light which is equal to 3.00 x 108 m/s and is the wave length of light. Also waves have crests, the high points of waves, and troughs, the low points. Constructive interference occurs when two waves of the same frequency meet at a crest or trough therefore combining to form a wave that has an amplitude equal to the sum of the individual amplitudes of the original waves. Stimulated emission is the process that the laser works on, which was first proposed by Albert Einstein in 1917. When a sufficient number of atoms, either gas solid or liquid, absorb energy so that they are in an excited state of higher energy stimulated emission can occur. Light of a specific wavelength can produce more light with the same phase and direction these light waves will be coherent. Stimulated emission amplifies the coherency of this radiation and gives the radiation a very narrow beam spread.
The combination of light amplification and stimulated emission creates a laser. Laser light is coherent because the atoms are stimulated to emit waves of light that are in phase creating constructive interference producing a powerful and intense laser light. The emitted light is monochromatic, meaning only one wavelength, and one directional. For a laser to work three components are needed. First a gain medium that can amplify light through it. Second an energy pump source to create a population inversion, (this is a condition when electrons in high energy levels are more numerous than electrons in lower energy levels), in the gain medium. Finally two mirrors that form a resonator cavity where small units of energy released from the atoms called photons move back and forth between the mirrors triggering more stimulated emissions. The energy of a photon is calculated using the equation:
where E is the energy of a photon, h is Planck’s constant (equal to 6.63 x 10-34) and v is the frequency. As the photons are moving back and forth between the mirrors an intense, directional, and monochromatic laser light seeps through one of the mirrors which is only partially silvered.
Lasers are generally based on the medium used, and are classified as solid state, gas, semiconductor, or liquid. Solid state lasers have a solid gain medium in which atoms are excited. The first ever laser was a solid state laser with a ruby core and pumped buy a xenon flash tube. This laser was made by Theodore Maiman in 1960. The most common solid state lasers are constructed out of ruby crystal, neodymium-doped or yttrium-aluminum and coated with a highly reflecting nonmetallic film. These lasers have the highest power output and are usually operated in a pulsed manner to generate bursts of light over a short period of time. These short bursts are useful for studying physical phenomena over a extremely short time interval. The frequency of these lasers fall in the infrared spectrum, but can be multiplied to ultraviolet by multiplying the laser with potassium dihydrogen phosphate and X-ray wavelengths by aiming the beams at yttrium.
Gas lasers have a medium of pure gas, a mixture of gases or metal vapor enclosed in a glass or quartz tube with one fully reflective mirror and one partially reflective mirror (see illustration above). Gas lasers are pumped by ultraviolet light, electric current, chemical reactions, or electron beams. Some gas lasers have wavelength with in the visible spectrum. A helium-neon laser has a wavelength of 632.8 nm, an Argon ion laser is measured at about 488.0 nm. These lasers have high frequency stability, color purity and a very low beam spread.
The most compact laser is the semiconductor laser. It consists of a junction between layers of material that have electrical conductivity between metals and insulators called semiconductors, usually constructed of gallium arsenide, the laser cavity is confined to this junction by two reflective plates. Semiconductor lasers are pumped by a direct electric current across the junction. These lasers can operate in a continuous wave mode with over fifty percent efficiency.
A liquid or dye laser has an inorganic dye as its medium contained in glass prisms. These lasers are pumped by intense flash lamps in a pulse mode or by a gas laser in a continuous wave mode. A dye laser is tunable,
that is that the frequency can be adjusted by rotating the dye prism inside the laser cavity.
The free electron laser is one using electrons unattached to the atoms and pumped to their lasing capacity by an array of magnets. First developed in 1977 they are now becoming an invaluable research tool. They are tunable like dye lasers and theoretically a small number could cover the entire spectrum from infrared to X-rays and could become capable of producing very high powered radiation that is currently to expensive to produce.
Even more vast than the different types of lasers is the different applications that lasers can be used for. A lasers use is only restricted by the cost to run it and imagination of the user. Lasers are widely used in industry scientific research and medicine, communication, in the military and in every day home uses. In industry lasers are used for their intense heat and precision. Its heat is used for the spot welding of two metals. A focused beam can easily heat, melt or even vaporize a material thus making it useful in any industry. It’s used for drilling and cutting gems like diamond by use of a laser drill or laser scriber, to construct and shape machine tools, precisely cut fabrics, synthesize new materials, and to attempt to induce controlled nuclear reactions. A powerful short laser pulse can be used for high speed photography with an exposure time of a few trillionths of a second. Also lasers are used to detect certain types of air pollution and cloud ceilings. High speed laser activated switches are being developed for use inside particle accelerators and these are only a few of the industrial applications.
The use of lasers in research and medicine over the years has been invaluable. Narrow beams of light can cut and cauterize certain tissues making it useful for many surgeries. The latest is laser eye surgery. This is a process in which a laser is used to cut a layer of tissue off the lens of the eye allowing light to reach the retina from a standard distance. Also laser photocoagulators are used to send laser beams through the eye for retina therapy. In research a lasers directional, monochromatic nature can be used to detect trace substances in lab samples, study the molecular structures of matter, and to selectively induce chemical reactions.
The signal strength of laser light can travel long distances with very little reduction in signal strength making lasers ideal for space communication. Laser light can be used for high density information recording allowing it to record and transmit audio and optical information to a radio or monitor. The laser has great importance to the future of communication. Modulated beams of the coherent laser light can transmit a much larger number of messages at a time than the current ordinary telephone systems.
The military is one of the most vast areas in which lasers play a key roll. Lasers are used in the military for guidance, defense and detection. Innovations such as a laser guidance unit is used to locate or “paint” a target and provide guidance information to a missile or attack unit. Inversely a laser jamming unit is used to direct energy to a hostile attacker to jam or confuse their radar, guidance, tracking, and other devices. A very commonly known detection system is the laser intrusion detector. This is a thin beam that can be placed in entrances so that when broken will alert an intrusion, this device is also used in many commercial and home based alarm systems. Research is currently being done to develop offensive military weapons a concept that has been in the works since the cold war. In March of 1983 President Ronald Regan first proposed the development of an antiballistic missile defense system to provide the United States total protection from nuclear attack. The goal was to intercept incoming missiles in midcourse. This would require advanced technologies some of which were only in the preliminary stage of research. Given the name “Star Wars” the plan required space and ground-based nuclear X-ray lasers, subatomic particle beams and computer-guided projectiles fired by electromagnetic rail guns. Also a network sensors and specialized mirrors for laser targeting would be required. Some of the systems were in development, but some, particularly the laser systems, were unattainable. The total cost of the system was almost three hundred times more than the departments annual budget.
Aside from the many uses of lasers that most people will never see, there are endless uses in our every day lives the most common today is the compact disks. A compact disk is a plastic disk with a thin metallic layer. A high precision laser beam is used to burn microscopic pits in the thin metal layer of the disk called a master disk. The pits are laid out in a pattern that can be read by compact disk player and thousands of disks can be copied off the master. A compact disk player is constructed with a low power laser and high precision lenses and mirrors. A server motor position an objective lense to a track on the disk the laser then directs a narrow beam of light on to the track along the track regions with pits scatter light in different directions. This sequence of light scattering represents sound. A photo detector picks up the light sequence and sends a signal to a microprocessor which then converts the light to sound. This same process can also be used for storing information, animation, and video. Lasers have also simplified the recording of three-dimensional images called holograms. Holograms are made by splitting a beam in to two. One beam will travel through a lense (beam spreader) and then
scattered by the object being holographed and be projected on to a film. The other will only travel through a separate lense and then on to the film the two beams interfere with each other when they reach the film because they have taken different paths and are no longer in phase with each other. The film records the interference pattern which is the hologram. Also because of its high frequency laser light can carry one thousand times the television channels than that of the channels today being carried by microwaves. Low-loss optical fibers have been developed to transmit laser light for communication in telephone and computer systems.
Lasers, though valuable in their many different uses, can be extremely dangerous and should be handled with care and common sense. A helium-neon laser though low in power and radiation can cause serious eye damage if exposed for just a few seconds.
With its endless applications and constant development there is no telling how far the technology can be developed. Wether it is used to produce image or sound, construct a tool, or destroy a star ship the laser will always be a key tool in modern day civilization.