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Light Amplification by Stimulated Emission of Radiation is more commonly known by the acronym LASER. It is now commonly used in everyday life in a variety of fields such as medicine, scientific research and the military. Its sales are now amounting to over £5billion worldwide and are still increasing. One of the technologies most well known for using lasers is the Universal Product Codes - or barcodes as they are more commonly known. The first thing that was bought using a laser scanned barcode was a packet of Wrigley's chewing gun back in 1974. 1, 7
For something that has been used in so many different technologies, the laser is one of the easiest concepts to understand in the physics world as it acts in a similar way to the atomic nucleus which has been known of since the early 1900s.
The laser emits light of a single wavelength, and this is determined by the input energy. This single wavelength means that it produces light of a specific colour. It is coherent, of a specific brightness and is only produced in one direction. The fact that it is coherent means that it does not lose any energy by spreading out in different paths. 6
A laser has two essential components, a gain medium and a resonant optical cavity.
The gain medium is where the excitation of electrons occurs. When energy is inputted into an atom, the electrons in the laser can move from their original energy state, to a higher state which is further away from the nucleus. Once it is in the higher energy state, it cannot stay there forever, and will eventually return to the ground state. The ground state is the lowest possible state in the atom. Whilst returning to the ground state, it releases energy in the form of a photon, which is a particle of light. 3
To produce an even greater emission, a process called population inversion is used. This is where there are more electrons in a higher energy state than in a lower energy state. After the laser has been excited, the electrons will then relax and will all fall back to the ground state together producing photons as previously mentioned. The wavelength of light produced depends on the difference in the energy between the excited state and the ground state. This dependence is shown by the equation:
E = hc/Û¬λ (1)
Where E is the difference in energy between the two energy levels, h is planks constant (6.63x10-34), c is the speed of light (3x108) and λ is the wavelength of light produced. A larger energy difference produces light of a smaller wavelength. Lasers which produce a specific colour of light work by ensuring that the atoms are stimulated to a certain energy state, by pumping in a particular energy to start off with. 4
This process is needed as there are normally a lot more ground state electrons than there are excited electrons so it would take a lot of energy, such as a very intense beam of light, to increase the number of excited electrons to a number which is comparable to the number of ground state atoms. If this process did not occur, then the rate at which energy was absorbed would by far outweigh the rate at which energy is produced due to the number of electrons in each state, and therefore would not be a very productive way of producing photons. 2, 3
The most common way of ensuring this occurs is by using a helium-neon laser, where a mixture of helium and neon gas is sealed in a glass enclosure which contains two electrodes. When a high voltage is applied, an electric discharge occurs. Collisions between the ionised atoms and electrons which are carrying the electrical discharge excite more atoms to energy states two or three levels higher than the ground state. 2, 3
As well as this excitation, energy can be produced in another process called stimulated emission which occurs in the optical cavity. Mirrors are placed at both ends of the linear tunnel of the cavity, which causes the electrons to bounce backwards and forwards. This produces a resonance-type effect where it induces each atom to emit another photon with the same phase, frequency and polarisation as the incident photon. This results in the photons continuously colliding with each other. The incident photon is unchanged during this process. This method causes a magnetic effect in which energy levels are transferred from one electron to another, this intensifies the light further. This is a cascade-type effect and is called light amplification.
The light that is produced is coherent as all the photons emitted are in the same phase. A laser used stimulated emission to produce a coherent beam consisting of such photons and the cavity becomes and amplifying medium. 2
One of the mirrors is 'half-silvered' which means it has a small slit in the centre of the mirror so that the beam can escape the cavity. Lenses are also placed inside the cavity to concentrate the beam further still. 5
There are other processes which can be used to produce the same effect. Examples of this are in semi-conductors and in chemical lasers. In a semi-conductor, a p-n junction is used, where an electric field drives electrons and 'holes' across a band gap. In the chemical laser, a chemical reaction is the thing that produces the population inversion, rather than an electrical discharge in the helium-neon laser. 2 These are just two examples, but in recent times a lot of research has gone into the laser and how it works. There are now many different types of processes to produce this effect.
The laser is not actually that old, although it has been developed and used regularly for the past few decades, the first working laser was only produced in 1960 but Theodore H.Maiman. The laser consisted of a ruby crystal irradiated by a xenon flash lamp. It produced a coherent light beam of red light. The light was emitted in short pulses, and it was of great spectral clarity with an interval between each burst. The light was so intense that at short ranges it could burn holes through strong materials such as steel. The beam could be modulated to carry signals such as music or television in a similar way to radio waves as it was coherent. 9
The laser was not the first of its kind, before its production there was a previous invention called the MASER, which stands for Microwave Amplification by Stimulated Emission of Radiation. This was a similar concept to the laser except it was a device that amplified the radiation from the lower frequency microwave part of the electromagnetic spectrum rather than visible light. The laser was an extension of the maser which was first achieved in 1954 by Charles Townes. 9
Soon after the breakthrough, more laser types were made.
In 1961, the gas laser was developed; this was the helium-neon laser which was described before. This laser produced a beam of light of even better spectral quality than the ruby laser and it also worked continuously rather than in a series of pulses. Originally the laser only produced light in the infra-red range of the spectrum and it was not until later that physicists were able to use the same concept to produce visible light.
In 1962, an injection laser was produced by various scientist research groups at around the same time. This used gallium arsenide which is a semi-conductor element. The laser light was produced by passing a large current through it.
Since 1962, these three types of lasers have been developed further and have been used for various applications.
Immediately after the laser had been developed, scientists were keen to use in medical uses as they could see it had a lot of potential for various parts of the industry, but the ruby crystal laser was not suitable as it was not the right colour or intensity for the work. So, in the mid 1960s, an argon laser was developed that was able to be used for surgery. Since then it has been used in many procedures such as laser eye surgery, removal of birth marks on the skin, extremely precise surgeries and also in procedures which require no anaesthetics. 10
The laser can also be used in some dental surgery practices, as they can be used to reduce the body's sensitivity to hot and cold, shape gums and can be a pain free replacement to some of the dental drill treatments such as fixing problems with dentures.
As well as the applications that lasers are already being used in, they could also become handy in other aspects too, such as the current energy crisis.
Lasers can produce intense amounts of energy and a laser at the Rutherford Appleton Laboratory near Oxford is no exception The Vulcan laser can produce petawatts (x1015) of energy - which is 10,000 times more powerful than the national grid - in 1 picosecond. This immense amount of energy could hold the key for nuclear fusion. 8
Nuclear fusion is the combining of tritium and deuterium to produce helium and a neutron. This reaction would result in a loss of mass, which would produce an immense amount of energy of about 17.6MeV. This has been predicted by using the equation:
E = mc2 (2)
Where E is the energy produced, m is the mass loss and c is the speed of light.
Nuclear fusion occurs naturally in the Sun, but this is due to the gravitational pressure in the core, and therefore on Earth which has a naturally lower atmospheric pressure, a larger amount of energy is required as an input to produce the same effect.
The energy produced by the laser could provide the high temperatures required for this to occur, and if this does happen, then the energy crisis could well be solved.
Since the first production of the laser in 1960, it has quickly become one of the top inventions of the 20th century, being used in many different areas of science and making an easier, better way of life for people. The number of resulting inventions that have been produced from this single idea has been astonishing in the short period of 50 years. If the advances in the research and technology of this invention continue at the current rate, it could lead to even more advanced ideas, including energy production and even better medical techniques. The possibilities seem to be endless and it is a very exciting time to be I the world of Physics with the advances that are occurring everyday around this simple concept.