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How do lasers work?
by Nicholas Psaroudakis
In the 1960’s, T.H.Maiman ET. al., published a paper in Nature under the title ‘Stimulated Optical Radiation in Ruby’ and made an enormous contribution to technology, the laser. The word LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. A new field was born, photonics, which deals with the interaction at the quantum level, between photons and bulk matter.
The foundations for the laser were put quite many years before, by Albert Einstein, when he showed in 1917 that stimulated emission of radiation could occur. He was working in the cavity radiation problem, the same one that in the hands of Planck, gave birth to what we refer today as, quantum mechanics.
The laser involves the excitation of the atoms of a system from one energy state to another of higher energy. This can be achieved with radiation falling on the system, of energy equal to the energy difference of the two levels. The system absorbs the radiation and some of its atoms jump to a higher energy state. After that, each excited atom can undergo two processes. Those are spontaneous emission, and stimulated emission.
Spontaneous emission is what would happen to the excited atom, if someone waited a long enough time (in this case a mean time t ) and no radiation was present. Under those circumstances, the system would undergo a transition from the excited state to its ground state and the release of a photon of energy equal to the energy needed to excite the system would follow. Actually, no time passes for the emission of the photon. It is a process that happens instantaneously, as the atom jumps from the higher energy state to the lower.
Stimulated emission is what happens to the excited system, when radiation of energy equal to the amount needed to excite the atoms is present after the excitation of the atoms. This radiation drives the system down to its lower energy state giving birth to another photon. Now two photons exist where one photon existed before the excitation of the system. The two photons are identical. They bear the same energy, direction, phase and state of polarization. These photons can cause new ones to appear and the whole process is clearly an amplification process. For this to happen, a chain reaction is needed where the photons produced excite new atoms and those produced, undergo stimulated emission due to the presence of photons of the energy needed for the effect to take place.
Usually the system consists of a collection of atoms lets say of population N. Let N1 atoms be in the lower energy state and N2 atoms be in the excited state. For any system at thermal equilibrium, the Maxwell – Boltzmann distribution gives that the number of atoms in the higher energy state will be less than the number of atoms in the lower energy state. This holds for systems where the only factor that determines the levels of the populations is thermal agitation. If this system is exposed in radiation, the dominant process will be absorption. However, if the populations were inverted, under the presence of radiation, the dominant process would be stimulated emission, and as a result of it, laser light would be produced.
Population inversion can be achieved using a clever trick that involves three energy states; E1, E2 and E3. By means of radiation, the atoms are excited from their ground state E1, to an energy state E3. The atoms stay in this state, for a very small amount of time. This happens because this energy state is chosen so that it has a very small lifetime t . They then jump by means of spontaneous emission to an intervening energy state E2, referred to as metastable state, which has a very long lifetime t for spontaneous emission to occur. In the presence of radiation, the metastable state can become more populated than the energy state E1, so, population inversion has been achieved, and stimulated emission follows and laser light is being produced. This is what usually happens with lasers using crystalline material (ruby or others) as the lasing medium.
Other kinds of laser use mixtures of inert gases. One usual mixture is that consisting of 80% of helium and 20% of neon. This kind of laser involves four energy states. Those are a common ground state E0, a metastable state of helium E1, an energy state of neon E2 that is slightly lower than E1, and another state of neon E3, which is closer to the common ground state. An electrically induced discharge in the tube is accomplished. The helium atoms and ions jump to the state E1; they then collide with the neon atoms, pulling them in the energy state E2. The energy state E1 is metastable, so the atoms don’t frequently fall to the ground state by spontaneous emission. The excited neon atoms fall slowly to E3, by means of stimulated emission, and then they fall rapidly to the ground state by means of spontaneous emission. Lasing takes place in the region between energy states E2 and E3. This is achieved because of the frequent collisions between helium and neon atoms, and the rapid drop of the neon atoms from energy state E3, to the ground state; energy state E2 becomes more populated than energy state E3, and lasing occurs. At the beginning, most of the photons produced in the jump from E2 to E3, are not parallel with the axes of the tube and they are stopped by the walls. The photons that are parallel are reflected back and forth inside the tube, using two concave mirrors in the beginning and end of the tube, focusing at the centre of the tube. One of the mirrors is highly reflective for the photons produced with the stimulated emission process, and the other is a little bit leaky, so that a fraction of those photons can escape. The photons are reflected many times inside the tube, producing more photons parallel to the tube and lasing is achieved. The photons that manage to escape from the leaky mirror form the laser beam.
A method used to achieve greater population inversion than normal is Q-switching. It is a technique for obtaining short intense bursts of laser oscillation. The cavity where the population inversion is achieved is separated from the lasing medium and population inversion in the cavity is built up. This is accomplished using a shutter separating the cavity from the lasing medium.
The shutter can be a rotating mirror replacing one of the mirrors. As the mirror is rotating, the radiation losses are great. Just before the two mirrors become parallel, a triggering mechanism initiates the pumping of the laser. As the mirrors are not yet parallel, population inversion builds up with out lasing occurring. At the instant they become parallel, the laser pulse is produced. The repetition rate of laser firing is controlled by the triggering mechanism and not the angular velocity of the rotating mirror, as this would result in a lot more pulses per second, which would be prohibiting as the laser tube would become increasingly hot.
Another kind of Q-switching, is passive Q-switching. Here the separating material is a saturable absorber. At the beginning, the absorber is opaque, but as irradiance within the cavity increases, the saturable absorber can no longer absorb, and Q-switching occurs. The advantage of passive Q-switching is that it is very easy to implement, and it doesn’t produce low peak pulses that the rotating mirror does.
There are many kinds of lasers. Those can be distinguished in four groups. Those are doped insulator, semiconductor, gas and dye lasers. Doped insulator lasers, have active mediums that consist of crystalline or amorphous (glassy) host material, containing active ions. There is a wide range of ions that can serve this purpose, and a wide range of host materials. The advantages of crystals are that they are optical uniform and that they do not have defects that would act as scattering points.
They have low thermal expansion coefficients and high thermal conductivity, so they maintain the same length and refractive index when heated up at the process of lasing.
Another advantage is that it is easy to grow the doped crystals, with a quite freely level of doping, to serve the purpose for which it is needed. Glasses are even easier to fabricate and dope, and the uniformity is even greater. Doped insulator lasers are easier to maintain and are capable of generating high peak powers. Some typical examples are the ruby, which was the first successful laser, Nd:YAG, alexandrite, YLF and silicate glass lasers.
Semiconductor lasers use p-n junctions as the active medium. There is no need to use external mirrors for positive feedback as the semiconductor material ensures that the reflectance at the material/air interface is high enough.
Gas lasers are the most widely used. They range from low power lasers, to high power ones. Several gases can be used. Helium-neon low power lasers are used in teaching laboratories and carbon dioxide high power lasers are used in industry. In gas lasers, the transitions can be between the energy levels of atoms or ions, or between the vibrational-rotational levels of molecules. Gases used in those lasers are, helium-neon mixtures where the transitions occurring are atomic, noble gases like argon or krypton where the transitions are ionic and mixtures of carbon dioxide where the transitions are molecular. There have been developed gas lasers, emitting in the ultraviolet-visible region using gases like nitrogen and excimer. Excimer gases are the ones produced by associating molecules or atoms in the excited state with others in the ground state.
Liquid dye lasers are the last category of lasers. These have many advantages compared to crystal, semiconductor and gas lasers. There is no difficulty in creating liquids having optical homogeneity and they are not under potential damage if overheated. They have greater density of active atoms than gases and can be tuned over a significant range of wavelengths. The active medium in those is an organic dye dissolved in a solvent. One of the most successful dyes is rhodamine-6G with methanol as a solvent.
What makes lasers so attractive and useful is the properties they exhibit. Lasers are highly monochromatic. Compared to the light from single lines in a gas discharge tube, the sharpness of definition of laser light is about a thousand times greater. Laser light is highly coherent having coherence lengths of several kilometers or even hundreds of kilometers long, compared to tungsten filament lamp or gas discharge tube where the coherence length is typically less than one meter. Laser light is highly directional. A laser beam departs from strict parallelism only because of diffraction effects, determined by the wavelength and the diameter of the exit aperture. Light from other sources can be made approximately parallel by the use of lenses and mirrors, but the beam divergence is a lot higher. Laser light can be sharply focused. This property is again related to the parallelism of the laser beam. The size of the focused spot of a laser beam is only limited again by diffraction effects, and not the actual size of the source as someone would expect. Crystals of the size of the pinhead are used as active media for lasers that are built to drive optical fibres with light pulses for telecommunications. The largest lasers fill a building and are used for laser fusion research.
A big application of lasers, is holography. Holography is the science of producing images by wavefront reconstruction. With holography, it is possible to truly reconstruct three-dimensional images, magnified or reduced in size, in true color. In principle, the image obtained from the hologram should be indistinguishable from the original one. With holography, it is possible to store a large number of images on the same plate and then be able to reconstruct them. The first object is illuminated, producing a diffraction grating pattern on the plate. Then the plate is rotated and the second object is illuminated producing a second diffraction grating pattern on the plate. The two sets of patterns are not parallel to each other (the plate was rotated during exposure). For someone to reconstruct the first image he would have to illuminate the plate with the laser at the same angle of incidence as the angle at which the photosensitive plate was illuminated during the exposure. Under those circumstances, with the incident beam satisfying the Bragg condition with respect to the pattern corresponding to the first exposure, the first image will be reconstructed. The second image would not show up until the plate is rotated by an angle q , satisfying now the Bragg condition for the second exposure.
Another application of lasers is in optical lattices. Claude Cohen-Tannoudji and other researchers, used optical lattices, three-dimensional sets of atoms ordered by several laser beams, and managed to observe Bloch oscillations of cold atoms in a light potential in the time domain for the first time. Bloch oscillations, a purely quantum effect were studied theoretically in the 1930’s for the case of electrons in periodic crystals, but were not observed until recently in semiconductors, superlattices. The laser beam damps the atomic velocity so that the typical atomic temperature is in the micro-Kelvin range.
There are many applications of lasers. They are used in everyday life, from compact disks to optical communication networks and from entertainment to medical applications. It is possible with lasers to establish communication between synchronous satellites, eversince the problem of atmospheric absorption and distortion of the beams is not present.
When T.H.Maiman in 1960 demonstrated for the first time experimentally
lasers using ruby as the active medium, none had imagined the breakthrough
the lasers would bring.
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