AT&T Archives: Principles of the Optical Maser

this is a crystal of calcium tungstate it is being pulled from the melt as it grows this type of crystal prepared with a small amount of neodymium as an impurity was used in the first continuously operating solid-state optical mouse on the optical maser is a radically new device which represents a significant step forward in our ability to generate light in a controlled way an optical maser is an optical oscillator in many ways similar to a radio oscillator except that it gives out light instead of radio waves it can be used to generate a beam of light which is at the same time very powerful and highly coherent coherent means very roughly well organized we'll see what it means more precisely later first let's take a look at these five gas mazes which show the achievement of optical maser action in the noble gases they are helium neon argon Krypton and xenon although these gases fluoresce in kala all produced maser beams which are in the infrared this optical maser uses a ruby crystal instead of a gas it gives out an enormous leap powerful burst of maser light in the red which Lost's about a thousandth of a second to show you how powerful is the burst of Nezha light I have arranged to lens so as to focus the beam on a graphite block I shall count down from three and fire on zero three two one what you saw just then was a fright haast burst of hot gas leading the block from the point at which the focused maser beam hits the sample let's try it again now that you know what to look for three two one here is the graphite block after several flashes of the post Ruby Mesa I said at the beginning that an optical maser is an optical oscillator which is like a radio oscillator except that it gives out light instead of radio waves as I'm sure you know radio waves and light are the same in kind they are both electromagnetic waves they differ only in frequency and wavelength the frequency at which the optical maser oscillates is around ten to thirty thousand times higher than that of any previous generator the optical maser in fact generates light but it does so in a way very different from sources with which we have previously been familiar here are some familiar light sources all of them give white light or something reasonably close to it white light may be thought of as composed of all frequencies between the lowest which the eye can see at the red end of the spectrum to the highest at the blue with a modern prism spectrometer like this one white light is directed through a prism which splits the light into its component colors this spectrometer also enables us to see a single color will place a card here in order to display the spectrum a second card goes back here to displayed a single color which is passed to a slit between the cards as we darken the room the spectrum becomes quite visible on the front card when the front card is removed the single color red in this case is passed through a slit to the rear card when I move the spectrum across the slit I can select any color I please in this way I can use the spectrometer to pick out a restricted range of frequencies you will notice that I said just now not frequency but range of frequencies in fact at the moment the band of frequencies coming out of the spectrometer amounts to perhaps 20 trillion cycles per second or in radio language 20 million mega cycles and that's quite a frequency band I can narrow this band by narrowing the entrance and exit slits of the instrument maybe to 1 million mega cycles which is about the best I can do with this particular instrument but as you will see by the time I have narrowed the slits to get as near as I can to a single frequency there is hardly any light left and this is a chronic fact of life as long as we use white light sources plus some sort of filter which is what a spectrometer is the nearest we can approach to a single frequency is still a band which is very gross by radio standards and even this can only be done at the expense of having very little power left over here I have a laboratory sodium lamp when we shine light from this lamp through the spectrometer we find that it consists as near as this experiment can tell of just one band of frequencies in the yellow actually an instrument of higher resolving power will show that there is not one line but two each line being some tens of thousands of megacycles wide so we do better with this lamp than we do with a white light and a filter but for many purposes it still isn't good enough what we would like both the scientific and communications needs is this a source of light of as nearly as possible just one frequency which radiates that light in as nearly as possible just one direction this is what the optical measure does in order to explain how the optical measure works it will be well for us to look a little more closely at conventional light sources in order to see how they defer from radio transmitters here we have a diagram of a Hertzian dipole representing for example a simple half wave antenna connected to a radio oscillator you can see lines of force and the way in which they move in time as the dipole radiates energy but dipole is oscillating at a perfectly definite frequency because the wavelength of light is so much smaller than that of radio waves a half wave antenna tuned to light frequencies would be in conveniently small even if we knew how to excite it fortunately however the atoms in a hot body or in a gas discharge themselves behave like tiny hats and dipoles in such a source each dipole radiates independently of all the others and each at a different frequency I'm sorry that we haven't been able to include the moving lines of force in this picture but as you can imagine it would be quite a job and that's just the point the radiation coming out of this block will be a confused jumbo more or less uniform in intensity in all directions and with no particularly well-defined frequency what we have is annoys generator look now at what happens in a sodium lamp here too the atoms behave like little Hertzian dipoles but now they all radiate at much more nearly the same frequency but as you will see there is no particular correlation between the instants of time at which the light attached to each miniature Hertzian dipole flashes out we can describe this lack of correlation suppose we put a clock next to the dipoles then we pick a dipole and notice what time it flashes as the hand swings it indicates when the other dipoles flash as number five flashes number one is nearly ready to flash again the total time elapsed is one period of the oscillation now we can quote by what fraction of the period each of the other dipoles is late this quantity we call the phase which can be quoted in factions of period or in radians or in degrees the point about the sodium lamp is now that the phases are random there is no correlation of phase between one dipole and the next the expression we use to describe this situation is that there is no phase coherence in the atomic system I think you can now see that what we need in order to obtain a proper optical oscillator is precisely a phase coherence this is just what happens in an optical maser the trick of course is to set the phases right what we need is a system set up in such a way that as the light wave you're generating travels along the phases of the atomic Hertzian dipoles are Sur ranged that they always help the wave graph now let's see how it's done let me start with an acoustic analogy the analogy isn't quite exact an allergist seldom are but it will help us to get started here we have two tuning forks let's leave one aside for the moment and look at the vibrations of the other under this microscope through the microscope we see the top surface of one of the tines the filing marks on the tine are quite bold and will serve as a good reference to observe the amount of amplitude as I strike the tuning fork the tines start moving and sound is radiated the amplitude clearly starts at a maximum and then decays as the sound gets weaker one reason the amplitude decays is that the energy the tuning fork has while it's vibrating is being converted into sound the pork is all by itself at this stage and emits energy for no other reason than that it has energy to emit let me call this process spontaneous emission now we're ready for the next step the second book is brought up very close to the first I'm going to start the second fork into vibration and I want you to watch what happens through the microscope you can see the amplitude of the fork build up this means that energy is being taken up by the first fork from the radiated energy of the second this process I shall call absorption now let's see what happens when the second focus struck while the first is already vibrating you will have noticed but sometimes the amplitude builds up when the second fork is struck and sometimes it dies away rapidly instead this is because the instant at which the second fork is struck relative to the vibration of the first is entirely a matter of chance that is the phasers are uncorrelated in those cases where the amplitude builds up we again speak of absorption of energy by the first fork where the amplitude dies away fast instead clearly the second fork is compelling the first to radiate its energy faster than it would if left to itself this process we call stimulated emission now let's go back to the atomic Hertz in dipole now of course the sound field is replaced by the electromagnetic or light field exactly the same three processes can occur if the atom is not excited it can absorb energy from the light field if it is excited it can emit energy into the light field and the same distinction can be made between spontaneous and stimulated emission if the atom is all by itself it can emit spontaneously but it can also be stimulated to emit faster than that if there is light of the right frequency present but the analogy must not be pushed too far a tuning fork can be given any amount of energy we please and its phase is exactly measurable atoms on the other hand in stationary states can only be in one of a discrete number of well-defined energy levels and if you know definitely that an atom is in one of these energy levels you will not be able to find out anything about its phase so do not expect to find an easy analogy to the step in the acoustic experiment in which we changed from absorption to stimulated emission by changing the relative phases of the two forks unlike a tuning fork if an atom is definitely in the upper of two energy levels it can only emit and stimulated emission will occur regardless of the phase of the incoming light in the process of absorption an atom is pictured as moving upwards from a lower to a higher level of energy similarly in emission where the spontaneous or stimulated the antrum moves down as it loses energy to the radiation field the only difference between spontaneous and stimulated emission is that in stimulated emission we hasten the transition by forcing it with the same frequency as that which the atom is prepared to in it furthermore suppose we have just one atom and one Photon if the atom is in its lower stage there is a certain probability of an absorption occurring if in its upper state a certain probability of a stimulated emission now the point is that these two probabilities are the same thus if we have a large number of atoms and there are more of them in the lower than in the upper state the system will on balance absorb energy from the light wave if on the other hand there are more in the upper state then in the law the system will on give off energy to the light wave which in consequence will grow when there are more atoms in the upper than in the lower state we say that the system is in a state of negative temperature such a state can only exist if we have some way of driving atoms up into the upper state by some outside source this is not always possible but in some systems we can do just that the process of generating a negative temperature that is the process of selectively exciting more atoms into the upper state than there are in the lower we call pumping the fact that a light wave passing through a negative temperature medium will grow enables one to make an optical oscillator here's how it's done we have a collection of atoms driven in some way so as to be in a state of negative temperature this collection of atoms is placed between two parallel mirrors I can best explain what is expected to happen as a bootstrap operation suppose that you start a plane wave having the correct frequency moving between the mirrors as the wave moves forward it causes stimulated emission to occur and consequently it grows in amplitude when it hits the mirror it bounces back since no mirror is perfectly reflecting it will lose some amplitude in the process some of the energy lost stays in the mirror if the mirror is thin enough some gets through on the return trip the amplitude builds up again as more stimulated emission occurs if there is little or no gain from stimulated emission the wave will die away after several reflections the break-even point occurs when the stimulated emission gained just makes up for the losses at the ends if this breakeven point is exceeded the result will be that the wave should continue to build up forever or rather until it has exhausted our ability to contain enough atoms in the state from which stimulated emission occurs the sort of photon chain reaction rather like the neutron chain reaction which happens in an atomic bomb a few moments ago I described what happens as a bootstrap operation this was because I had to ask you to consider that a plane wave was present at the beginning in order to get the thing started actually of course so long as you're past the break-even point you don't need to introduce an external plane wave at all this is because sooner or later as a result of fluctuations in the field there will be something like a tiny plane wave present and as soon as such a blackshoe Asian occurs the plane wave will keep on growing to the exhaustion point so to make an optical maser you need three things first you need a cavity a means for keeping the light bouncing to and fro or in the language of electronics to introduce feedback a pair of flat parallel mirrors constitutes one type of catch a second you need a medium a collection of atoms capable of giving gained through stimulated emission at some frequency these atoms must possess some suitable pair of energy levels between which transitions can occur with the absorption or emission of light of the desired frequency third in order that stimulated emission shall dominate over absorption you need a pump in effect a means for driving the system into a state of negative temperature here is the helium neon optical maser the two mirrors are in the Box lag ends the median has as its active part the gas neon in fact you can see the tube glowing pink just as a neon sign does pumping is done by including helium along with neon in the maser tube and exciting the mixture by coupling to a radio frequency generator the generator excites the helium atoms and puts them into a long-lived excited state collisions with neon atoms then occur and a negative temperature it's that in the neon the signal given out by this Meza is in the infrared at a frequency about 30% lower than the lowest you can see the beam of this maser can be seen directly by using an image converter tube this tube has a tiny television screen on which the beam of the maser produces a green spot the beam is extremely parallel and has a divergence of less than a minute of Arc this is smaller than the I can resolve the reason for this of course is that only light traveling to and fro between the ends very close to the axis of the structure stays in the cavity long enough to join in the oscillations thus we see that a cavity with flat ends generates plane waves or something very close to it and these waves escape through the ends as a parallel beam here we have another experiment which demonstrates the purity of frequency of the signal given out by the helium neon Mesa what we are doing is beating together two different output frequencies from the mesa just as you can do with two radio frequencies sources the two oscillating frequencies fall on the detector which mixes them together and generates the difference or beat we Qin say the beat is displayed on this frequency analyzer here is another version of the helium neon Mesa which I can use to illustrate some other features of the device this one gives out a beam of visible light instead of infrared the principle difference between the two lies in the frequency which the end mirrors are made to reflect because the beam is visible you can in fact follow it right through the apparatus and out the ends notice that this time we get on the screen a pattern more complicated than just a simple spot from the pattern we can deduce in what mode of the cavity the Mesa has been excited now I want to talk about a different sort of optical maser this one uses a solid the medium here is a crystal of calcium tungstate neodymium such crystals are made in the way you saw at the start the cavity is just like the one used for the gas maser except that instead of having separate detached mirrors we make the ends of the crystal very flat and very parallel and cover them with a reflecting coating one big difference between this and the helium neon Mesa is the pump this is the pump for the neodymium maser it is a small but powerful mercury lamp which floods the crystal with white light and sets up the condition of negative temperature this elliptical mirror focuses the light from the lamp on the crystal the top is a flat mirror and fits over the housing when the apparatus is working as it is here the beam given art is in the infrared and an oscilloscope can be used to demonstrate the existence of oscillations the lower trace shows the pumping light and the upper trace the maser signal this is another solid state optical maser it gives out light that you can see the apparatus is rather different from the one you just saw the pumping lamp is here and its light is reflected off this parabolic mirror and focused on the crystal the crystal is Rubik and is positioned inside a door the light given out by this maser is red here is the maser beam and it is directed into our camera the one degree reference gives you some idea how slight is the divergence of the beam the ruby crystal itself is cut in the shape of a trumpet to make maximum use of the light which is focused on the end of the crystal the reason for this is that you need a much higher pumping intensity to make Ruby oscillate historically the first optical measure ever made generated pulses of nasal light rather than a continuous signal we are still very much interested in such devices because although the signal doesn't last very long it's much more powerful while it's on than anything we can get out of a continuous measure here is a pulsed Ruby laser which will show you what I mean instead of a continuously running lamp we have a flash tube shown here with the Ruby crystal it produces a flash lasting only a thousandth of a second but hundreds of times brighter pilots on then even the brightest continuous lamp the Ruby crystal is placed inside the spiral and the whole assembly looks like this when mounted in a housing the lamp is flashed by discharging a bank of condensers through it to look at the maser beam I have inserted a glass tube filled with smoke between the maser and a card remember the beam is only visible for about a thousandth of a second so I will count down from three three two one will go in a little closer and do it again three two one it's of course impossible to give you much idea of how bright the spot really is in the continuous Ruby optical maser the power in the beam was about 10 milli watts in that flash the peak beam power was about 10 kilowatts or around million times as much and there are tricks we can use to get the power a thousand times higher still which is to say 10 megawatts provided we are content with a flash duration of a few tenths of a millionth of a second these powers may sound impressive and they are with them scientists have been able for the first time to make two photons coalesce to give one or twice the frequency to make ultraviolet light out of red it isn't very efficient process to be only a part per million but with ordinary lighting sensitives you can't detect it at all this experiment is only one of a number which have been made possible by the high light intensity as generated by the Ruby optical maser at sufficiently high light intensities the optical properties of many materials are quite peculiar and we're going to have quite a time finding out why we have in the optical Mesa an entirely new scientific tool with which many new kinds of experiment will be possible on the one hand with continuous optical lasers we are now able to carry over all of the standard radio and microwave techniques such as the beating experiment which you saw through the optical frequency range on the other hand the fantastic light bars made possible with post optical may enable us to see new optical effects as in the doubling up experiment and each new scientific discovery will be examined for its technological implications Oh you you

Channel: AT&T Tech Channel

Views: 40,137

Keywords: AT&T Tech Channel, AT&T, Laser, AT&T Archives, how it works, science

Id: vuORqMb481k

Length: 29min 8sec (1748 seconds)

Published: Fri Jun 10 2011