Introduction LASER or laser (Light Amplification by Stimulated Emission of Radiation) is a mechanism for emitting electromagnetic radiation, often visible light, via the process of stimulated emission. Electromagnetism
Introduction The main use of laser as a light amplifier: is as an optical oscillator or transducer for converting energy into highly collimated beam of optical radiation ( light).
Introduction Characteristics of Lasers: Laser light has three unique characteristics, that make it different than "ordinary" light. It is: Monochromatic means that it consists of one single color or wavelength. Even through some lasers can generate more than one wavelength, the light is extremely pure and consists of a very narrow spectral range. Directional means that the beam is well collimated (very parallel) and travels over long distances with very little spread. Coherent means that all the individual waves of light are moving precisely together through time and
Laser Physics We need to begin with the idea of atomic energy levels. In chemistry you will probably remember doing Shell diagrams of atoms. The electrons placed on circles around the atom, two in the first layer, eight in the second, etc. thus for Oxygen with eight electrons we drew:
Laser Physics Now, we shall consider an idealized atom with two electron energy levels and one electron. The electron may be in either of the two energy levels, thus:
Laser Physics We may make the analogy of a person who is only allowed to stand on a chair or on the ground, two defined levels. If the electron is in the higher level it may fall down into the lower level. In its doing this it must give up an amount of energy equal to the energy difference between the two levels. This is the law of conservation of energy being applied. This energy is given up in the form of light.
Laser Physics Light is also quantized. It may be represented as groups of photons. Each photon carries one quantum of light energy. The amount of energy in a quantum depends upon the wavelength ( colaour) of the light.
Laser Physics So we see that a short wavelength such as blue light at 470 nm has a high energy, and red light at 670 nm has a low energy per photon. The important point is that the wavelength of light is linked to the energy of a photon in a defined way. Thus our electron in the idealised atom which has given out a photon of defined energy emits light of a defined wavelength or colour. This is seen in street lights. They contain Sodium atoms which take electrical energy to move their electrons into higher levels, these electrons then fall back down to the initial, lower, state giving light at 589 nm the characteristic orange of street lights.
Laser Physics This process is known as SPONTANEOUS EMISSION The atom emits light spontaneously, without external influences. If however the atom is not isolated, other effects may occur. Photons of the same energy as the energy of the upper level may use their energy to move an electron from the lower level to the upper one. This is known as ABSORBTION, as the photon is destroyed in the process.
Laser Physics If a photon of the correct energy passes an atom with its electron in the upper level, then it may cause the electron to fall to the lower level. This STIMULATED EMISSION is very different from spontaneous emission. In the spontaneous process the photon may travel in any direction and be emitted at any time, Stimulated emission, however, causes the emitted photon to travel in the identical direction to the passing photon and at the same time.
Laser Physics Now we finally get to lasers. L.A.S.E.R. is an acronym for Light Amplification by the Stimulated Emission of Radiation. Which is why I have had to explain all about Stimulated emission. The three processes above all happen if we have a group of N atoms some of which (N2) have their electrons in the upper level and some (N1) with their electrons in the lower level. In a laser we want stimulated emission to be the biggest effect ( as indicated by the acronym ). Let us then look at the rate at which each process occurs:
Laser Physics Spontaneous emission.This is only dependent on the electron being in the upper level. A certain proportion ( call it a ) of the upper level electrons will emit in a given time, so:Spontaneous rate = a N2 Absorbtion.This depends upon the electron being in the lower level and there being a photon present. Using the number of photons as n and the proportion of possible interactions which occur as b :Absorbtion rate = b N1 n Stimulated emission.This is the same as for absorbtion, but the electron must start in the upper level. :Stimulated emission rate = b N2 n
Laser Physics We require that the last of these expressions is the largest. Now a and b are constants which depend upon the particular atom used and are thus not under our control. So for stimulated emission to be greater than spontaneous emission, we require n to be large - we need many photons in the laser. For stimulated emission to be greater than absorbtion we require N2 to be greater than N1 - more atoms have their electrons in the upper level than have their electrons in the lower level.
Laser Physics This is known as an Inversion as it is normal for electrons to be in their lowest energy level. This may be readily seen from the fact that in the absence of external influences, ie. no photons n=0, the only process which can occur is spontaneous emission which will allow any electron which began in the upper level to fall to the lower level, but not vice-versa.
Laser Physics The fundamental difficulty in producing a laser is creating this necessary inversion in the populations of the two levels. It must be stressed here that we are talking about many atoms each with a single electron which may be in one of two levels. We have not got many electrons in one atom which would be restricted in their movements between levels by the rules governing how many electrons may occupy a single level of a particular atom.
Laser Physics Now assuming that we have an inversion, N2 greater than N1 , then we can get the SE part of laSEr. Now how do we amplify light using this? consider a single photon entering a region with the atoms in. This photon will pass by an atom with its electron in the upper level and cause it to emit a second photon travelling in the same direction, by the process of stimulated emission. There are now two photons, each of which can cause stimulated emission in two more atoms to give four photons , and so on.
Laser PhysicsThus we have amplification, which is also knownas gain. The region containing the atoms isknown as the gain medium. The final stage in alaser is to get this first photon to amplify. This isdone by placing the gain medium between twomirrors. This forms what is known as a laser
Laser Physics Initially there is no light in the cavity. The only possible process for the atoms to undergo is therefore spontaneous emission, and this duly occurs. As stated earlier, this may travel in any direction out of the gain medium, and most is lost from the cavity. However out of the millions of photons emitted by the millions of atoms in any real medium, there is bound to be at least one which travels directly to one of the mirrors and is reflected back to the gain medium.
Laser Physics This is now our first photon. As it passes through the gain medium, it causes stimulated emission as described above and by the end of the gain medium there are, say, ten photons. Now the important part is that these are all travelling int the SAME DIRECTION as the first photon, so will be reflected back to the gain region by the other mirror. These ten photons now each cause stimulated emission, and when they get out of the medium to the first mirror again there are one hundred which are reflected back to the gain medium again and are amplified to 1000 etc...
Laser Physics Thus we very rapidly get very many photons travelling back and forward in the cavity. Obviously in this idealised case where no photons are lost from the steadily amplified beam, the photon number just goes on increasing. In any real laser some photons are lost, for many various reasons. One of these is quite deliberate. One of the mirrors is made to reflect only part of the light, and to allow the rest through.
Laser Physics This is then the output beam of the laser and the leaky mirror is referred to as the output coupler. A steady state may then be reached where the gain exactly replaces the photons lost from the cavity by the output coupler. There is then a constant number of photons in the cavity at any time. For example a laser with a gain of 1.12 ( much more realistic than the gain of ten used earlier as the illustration ) and an output coupler which reflects just 80% of the light we have:
Laser Physics The output beam thus has photons which are travelling in a fixed direction and also have a fixed wavelength (colour) defined by the energy levels of the electrons in the atoms of the gain medium.
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