How Lasers Work - A Complete Guide

everyone has seen them and have probably teased many caps with them maybe some of you have had unwanted hair removed or maybe you have built one and popped some balloons with it bottom line lasers are ubiquitous not only in scientific research but also in industry just how do these little devices manage to put out that nice powerful collimated beam of light all this and more coming up that some may or may not know laser is actually an acronym it stands for light amplification by stimulated emission of radiation however nowadays it is so common that people don't bother to capitalize it and simply write laser a very brief history of the laser starts in 1971 Einstein introduced the concept of stimulated emission which will be explained shortly then in 1954 the first major was demonstrated by Charles Townes the M standing for microwave the ammonium a there was the first device based on Einstein's predictions and obtained the first amplification and generation of electromagnetic waves with a wavelength of about one centimeter which is in the microwave range this is recognized as the precursor to the laser it wasn't until 1960 when Theodore Maiman developed the first working laser at Hughes research lab may islands early lasers are used to powerful energy source to excite atoms in a synthetic Ruby to higher energy levels the development of the laser was the collaborative effort by scientists and engineers who were leaders in optics and photonics okay so why are lasers useful why are they ubiquitous the answer can be broken down to three unique properties the laser holds the first being lined with the purity of a laser referred to as the lined with can be quite narrow more so than any other light source in layman's terms this is a measure of what frequencies are contained in the emitted light the narrower is aligned with the closer the emitted light is to a single frequency single if you will thus a laser is said to be monochromatic in reality it does output a small range of frequencies the smaller this range the better the line width and quality of the laser in contrast an incandescent bulb has a very large line width and emits a broad spectrum which is why the emitted life is white white light is a superposition of all the colors in the visible spectrum having a narrow line width is useful because many scientific experiments want to analyze stuff with certain energies different wavelengths of light corresponds to different energies hence having a source with one energy is helpful the second is coherence the light emitted by a laser is coherent light this means it is all polarized in the same direction as well as being in phase the laser is said to output highly coherent monochromatic light an LED on the other hand is also monochromatic one color but it emits incoherent light an analogy with synchronization and harmony can be made imagine an orchestra playing if the orchestra is in sync and everyone is playing the parts correctly it will be pleasing to the ear the laser if some players are playing out of sync but still playing the parts correctly it won't sound as good the LED coherence is important because all the photons have their energies together and we can then focus them on a small spot over some distance lastly power lasers make it possible to deliver a high intense light to a small area of course militaries are particularly interested in this aspect of the laser as well as medical applications laser eye surgery for example now let's take a look at how a laser works the workings of a laser are quite complex as it requires an understanding of quantum mechanics there are some commonalities behind every laser the first part can be broken down to three key pieces stimulated absorption spontaneous emission and stimulated emission which is what the SE part of laser stands for let's take a look at the first concept stimulated absorption we will need a nucleus that is made up of protons and neutrons that has an overall positive charge and an electron the Trezza negative charge hey there little guy most textbooks show electrons existing in discrete energy states of a material but actually electrons exist in probability density clouds around the nucleus as they have wave-like behavior and the orbitals represent the average distance one is likely to find it let's use this average distance to define the orbital and ignore the probability distribution for simplicity mostly always electrons are found in the lowest energy State or ground state everything in nature wants to be in a low energy state as it is easier for it to exist at this level in other words it minimizes energy think of a ball on a hill and how easy it is for it to roll down it wants to roll down because the energy state is lower closer to the Earth's core than further away in this case potential energy however it is possible to excite electrons by some kind of external means just like we can exert a force on the ball that is rolled down and push it back up light can be this pushed to excite electrons if a photon of light which is one unit of mics comes across an electron in a low energy State it can sacrifice itself and push the electron to a higher energy state the photon is annihilated but the energy of it is now part of the excited electron it should be noted that each material has different levels of energy in other words if the ground state is one unit and the next energy level is five units then the photon of light must have exactly four units of energy to excite the electron to that energy level anything lower will not suffice and anything higher would not as well as there is nowhere for that extra energy to go unless a higher energy state exists if the incident photon is very high in energy the electron would be ionized to continue our analogy it would be like trying to push the ball up the hill with not enough force the ball would just roll back down too much force and it would roll down the other side go to another plateau will be launched into space an exact amount of energy is required to elevate it to a particular energy state again this process is called stimulated absorption as we are stimulating the electron and it absorbs the photons energy the next mechanism we will look at it's spontaneous emission we now have an excited electron what happens now well again this higher energy level is quite unstable and after a very very short time about 100 nano seconds of being there the electron will eventually fall for some perspective light travels about 29 meters in 100 nanoseconds when it falls back down it will release a photon with energy equal to the difference and energy levels the higher the fall the higher the energy of the photon will be should the energy value of the photon that is released be in the visible range we would perceive it as color you may be thinking if the electron reaches the higher energy level through the previously-mentioned stimulated absorption mechanism why exactly does it fall back down well referring back to the ball example imagine the ball on a hill but now with the top having zero friction and a sharp point the ball can remain there only if it is perfectly balanced but any tiny little force in either direction will cause it to start rolling the electron in this higher energy state is in a similar situation the forces that push it are small perturbations in vacuum energy this is a quantum mechanical effect space or vacuum is not as empty as we think things are popping into and out of existence constantly it is these vacuum events that perturb the electron this is also responsible to why things are ferromagnetic that's a different story though again this process is called slow Dainius emission as the process that the electron falls back down to the lower energy state is more or less spontaneous the last quantum process we will talk about and the most important for lasers is stimulated emission this occurs when a photon interacts with an electron that is already excited this photon can act as a type of perturbation and force the electron to fall back down to a lower energy state and emit a photon we then will have two photons photons actually like to be together so if one comes near a situation where another one could be present such as the electron falling back to a lower energy state the situation usually will play out the important part is that the emitted photon will be identical to the one that stimulated it meaning same frequency phase and polarization they will be coherent with each other so if we could somehow Avalanche this process we would have a laser after all that is basically what a laser is as if tilly an identical coherent photons being emitted in contrast if two electrons undergo spontaneous emission the emitted photons will unlikely be traveling in the same direction nor be in-phase but in order for electrons in the excited energy level to be able to undergo stimulated emission and not spontaneous emission enough time has to be available the life time of an electron in the excited level is just too short however some materials have so-called metastable States these are excited States with slightly lower energy than the excited States these states allow the electron to remain there for much longer lifetimes milliseconds instead of nanoseconds enough times that a passing photon can cause it to undergo stimulated emission of course an initial spontaneous emission from the metastable state to the ground state must occur in order to have the initial photons that can stimulate are their excited electrons in the metastable States to sum up if a ground state electron is hit with a photon it will absorb it and move from the ground state to the excited state the photon must have the energy equal to the difference between these levels this electron will then transition to the metastable state if one exists this transition does not emit a photon and is said to be a radiation less transition the energy difference is dissipated in other ways heat or phenoms now this electron if a photon stimulates it will emit a photon with equal energy phase and direction these are the ones that make up the laser beam it should be apparent that the photon which pumps the electron from the ground state to the excited state has a different energy than the photons that are being lazed this is because the energy difference between the ground state and the excited state is different than the difference between the metastable state and the ground state the pumping photons are always higher in energy than the photons being lazed we obviously want lots of electrons in this metastable state more so than the ground state in order for them to be in a situation where stimulated emission can occur something known as creating a population inversion is required if we only had a two levels we would reach a point of saturation where 50% of the electrons are excited and 50% are not the excited electrons simply spontaneously emit too fast essentially our medium becomes transparent to photons by introducing the metastable state we force the pumping photon to excite the ground state electrons that then transition to the metastable state so the photons that are emitted by the transition from the metastable state to the ground state are primarily used to stimulate other electrons in the metastable state enough time exists for this to happen yes some of these photons will excite ground state electrons directly into the metastable state but the pumping photons should take care of the majority and create a situation where there are more excited electrons in the metastable state than ground state electrons the population inversion by the way the above is describing a three level laser four level lasers exist and are more efficient again we want to create an avalanche effect where the spontaneously emitted photon that was created when an electron transitions from the metastable state to the ground state gets amplified through the means of stimulated emission we don't want just a single puny photon we want lots all working together it is not practical to create a laser that is extremely long so the solution is to put the laser medium in a cavity let's take a closer look at how a cavity will influence the light waves and how exactly this will create the amplification we desire since light is a wave it will be subject to constructive and destructive interference we want constructive interference in our cavity to take place in order to have a high intensity beam a laser cavity has a mirror on one side and a partial mirror on the other it is partial because we want some of the beam to escape that's the beam we see now when light waves are created through spontaneous emission they will initially travel in random directions but the ones traveling perpendicular to the mirrors will reflect back and forth let's take a look at one of these like waves it is first emitted via spontaneous emission and quickly becomes large in amplitude through stimulated emission it travels towards the mirror and is reflected back because we continue to stimulate atoms in the left and right directions we get two waves in the cavity again one moving to the left and one moving to the right waves will add their amplitudes when interfering with each other in this case we will get a standing wave meaning instead of a wave noticeably moving to the left or right the combined wave will appear to be going up and down rest assure this is just an illusion this is the effects of two waves hitting each other head-on and their left and right components cancel out but their up-and-down components add together so when the wave looks flat this is a moment when the two waves are destructively interfering with each other and of the maximum they are in a constructive interference point here are a few examples of some standing waves in a cavity that are resonating resonance is just a fancy word for having these waves being in a state where standing waves are being produced a mode being just what n you have N equals one is a mode N equals two is another one N equals three etc is there an equation that will tell us what modes can exist in the cavity sure there is the left part is the frequency that exists in the cavity n is the mode which is always an integer v is the velocity of the wave and L is the distance between the two sides of the cavity the velocity in our equation is the speed of light C which is 300,000 kilometers per second the L is just the distance between the mirrors like traveling from the left of the cavity will now interfere with light traveling from the right so again we have these possible modes where the lights can produce standing waves and beam resonance not all frequencies are able to exist in a cavity but a lot are also let's be clear that the standing waves produces are a collection of trillions and trillions of light waves all working together they are produced by stimulated emission and the cavity allows them to keep amplifying each other they are coherent with each other recall this was one of the big reasons why we care about lasers if we didn't have this synergy between light waves we will just have an ugly LED I bet you can't make your casts go crazy with a red LED well maybe but you get my point question what frequencies are allowed to exist in a red laser pointers cavity answer a cheap red laser pointer has a cavity length of about one millimeter and the speed of light is C 300 million meters per second plugging in these values to our equation we would get a difference between allowed frequencies of about 150 gigahertz now red light has a frequency of about 400 points 0-5 terahertz which corresponds to an N value of 2667 recall n must be an integer so if 400 point zero five terahertz is an allowed frequency then the next one would be when N equals 2668 which is a frequency of 400 point to terahertz we can plot all allowed frequencies as we know 150 gigahertz we'll separate them the plot will look like this here we have n equal to 2,000 667 and the corresponding frequency of 400 point 0 5 terahertz here is two thousand six hundred and sixty eight two thousand six hundred and 69 and so on these are the frequencies that are allowed to resonate in this laser cavity so if you wanted your laser to have a frequency of four hundred point one terahertz you would first have to change the cavity length for this to be allowed as it is not possible in this red lasers cavity about two thousand six hundred frequencies in the visible spectrum would be able to resonate in this red lasers cavity now there is slightly more to the story about these allowed frequency lines we have assumed the mirrors are perfect which is practically impossible the imperfectness of the mirrors and other slight variations add a thickness to the frequency lines the actual allowed frequencies in a laser cavity looks like this again this is due to imperfections the last piece of the puzzle is to mention the gain medium itself gain medium is just the material we are using for our laser different materials will have different energy levels hence photons of different energy will be released during stimulated emission for example different materials will need to be used to create the blue laser then that of a red laser since the energy levels in a material are discrete one would think that exactly one frequency would be emitted out of a laser but only if this is also a frequency allowed in our laser cavity we can superimpose these ideas on this graph we assume here that indeed the stimulated emitted photon is a frequency that is allowed in the cavity however there is much more to the story the frequencies being emitted out of the laser actually takes a shape like this this was briefly mentioned at the beginning of this video when discussing lines with what is going on here are complicated event such as the Doppler effect stark effect and other quantum behavior the takeaway is but the game medium does output a small range of frequencies and has this gain curve it is still extremely narrow and said to be monochromatic it's not but it's close enough to sum up certain frequencies are allowed to exist in a laser cavity there is some relaxation to these frequencies as the mirrors and such are not perfect the laser gain medium emits photons in a certain frequency as well but again there is some broadness to this as certain effects influence this we can superimpose these two frequency plots and get the following the frequencies under the gain curve that have enough intensity to overcome other cavity losses are the ones the laser emits there are plenty of laser active medium these days any frequency you wish to lace is pretty much possible here is a picture of different laser material and the frequency they output some are in the gas state some solid and it is even possible to use a liquid as a listing material this concludes this episode on the laser if you enjoyed the content and learned something please consider doing all that stuff every other video asks you to do you know what I am talking about [Music]

Channel: Scientized

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Keywords: Laser, light, quantum mechanics, stimulated emission, coherence, lasers, interference, photons, einstien, science, technology, linewidth, spontaneous emission, stimulated absorption, energy, frequency, gain, standing waves, resonator, light amplification, laser animation, how a laser works, how lasers work, how a laser pointer works, laser pointer, radiation, em radiation, electromagnetic radiation, electron, proton, energy levels, waves, phase, physics, solid state, condensed matter, maiman, maser

Id: _JOchLyNO_w

Length: 20min 45sec (1245 seconds)

Published: Sun Mar 26 2017