Lasers - An Introduction (1970)

[Music] until modern times most of the light encountered by man has been of a certain kind the kind emitted from substances that are extremely hot light given off by the sun has basically the same characteristics as light given off by other heated substances the color of such light varies according to how hot the source becomes but the light always shows a continuous range of wavelengths when passed through a prism such light produces a continuous spectrum but not all objects produce continuous spectra this rock when exposed to ultraviolet radiation emits a discontinuous spectrum similar spectra are produced by all elements when properly excited when the element copper is placed in a flame a characteristic green color is given off the element strontium gives off a characteristic red color the strontium spectrum is limited to light of a few wavelengths here we see a discontinuous spectrum produced by a gas argon gas energized by an electric discharge neon gas can be energized in the same manner although the dominant color is red other wavelengths can be seen in the neon spectrum in an ordinary neon tube atoms give off light haphazardly in various directions at various times a more precisely regulated light known as coherent light is given off by the device known as a laser a laser is based upon the behavior of individual atoms if we pump energy into an atom we raise its energy level if we pump in more energy we excite it to a still higher level each time changing the structure of the atom when the energy of an atom is raised the atom is in an unstable state if left alone it will return to its ground state giving off the excess energy as light in a laser such light waves stimulate other energized atoms of the same kind to give off their light waves in step with the first a crude mechanical example of what happens can be demonstrated with pendulums we'll set one of them swinging eventually all of the pendulums having the same length as the first begin swinging in step similarly with atoms of different kinds in an excited state when one of them emits a light wave it stimulates the other atoms of the same kind to release their energy in step this is coherent light the process has come to be called light amplification by the stimulated emission of radiation which is where the word laser comes from the intensity of laser light is built up by sending it back and forth down a tube having mirrors at both ends each time it passes down the tube it picks up more energy from the atoms the atoms must be continually re-energized from an outside source eventually the wave reaches a maximum intensity some of this energy passes through a partial mirror at one end while some of the energy is reflected back down the tube to keep the lasing process going here we see one end of a helium neon laser with one of the mirrors being screwed into place before the laser can give off amplified coherent light mirrors must be inserted at both ends and adjusted so that they're exactly parallel the light must be kept going back and forth within the tube without losing any out the sides when alignment is exact the light begins to build in intensity until lazing occurs now it's emitting a beam of coherent parallel radiation through the partial mirror graph paper placed a few feet from the end of the tube shows that the diameter of the beam fills less than four squares the diameter is the same when the paper is placed much farther from the laser the almost perfectly parallel beam is a distinctive feature of a laser another type of laser uses argon gas the amplified light is greenish blue characteristic of argon argon lasers can be made more powerful than helium neon lasers the intensity of this form of light is another distinctive feature of a laser unlike the helium neon laser argon lasers can function on more than one wavelength passing the beam through a prism shows that it is composed of several different wavelengths it's possible to adjust an argon laser so that it functions on many wavelengths simultaneously the laser can also be tuned to lays on only one color at a time by using a small prism at the end of the tube instead of a reflector the prism is rotated almost imperceptibly selecting light of a particular wavelength green blue blue green a still more powerful laser uses carbon dioxide gas the plasma glow in the tube results from an electric discharge that energizes atoms of gas so that lasing can occur the actual laser beam is infrared below the visible spectrum we can't see it but we can detect it the beam can be focused just like any other light beam directing the energy to a tiny spot enough concentration of energy to melt a razor blade increasing the length of a laser increases the number of atoms that can be energized thus increasing its power [Applause] not all lasers are made of gas-filled tubes some are made of solid crystalline materials one such laser is composed of itrium aluminum garnet high intensity lights are directed at the crystal to pump the atoms to a higher energy level sealing the unit in a reflective casing maximizes the transfer of energy inside one of the best known laser crystals is the ruby a compound of aluminum oxide containing chromium atoms a xenon flash lamp is used to energize the atoms the ruby laser is also covered with a reflective shield to increase the efficiency of light transfer inside ruby lasers are pulsed they give off their energy in short bursts lasers are used industrially for precision welding and drilling doctors are using razors to perform surgery within the human eye the light can be focused within a transparent structure without damaging the surface as we can demonstrate using clear plastic another use of lasers is to make holograms photographs that can produce images of an object in three dimensions no lens is required all that's needed is a frame to hold photographic film on one side of the model and a laser on the other the light from the laser is split into two beams one beam strikes the model and is reflected the other beam goes directly to the film where the two beams meet they interfere with one another forming a hologram when the film has been developed it can be inserted into a frame and used to reproduce a view of the model as complete as the original now the model is gone only the hologram remains as a laser beam is passed through the hologram it appears as if the model is once more in place and can be seen from different viewpoints as we move across the film such photographs have tremendous potential for storing and comparing information about three-dimensional objects another use of lasers is in tunnel construction one problem in digging such a tunnel is in making it follow a straight line with a laser beam for linear guidance tunnels as long as two miles have been dug with a total deviation of less than one inch the unequaled straightness of a laser beam has wide use for the alignment of tunnels bridges and dams [Music] probably the most promising use of lasers is in communication experimental systems are using lasers to transmit messages because of the high frequency of light waves as compared to radio waves a single laser beam at least in theory can carry as many messages as are now being carried by all the radio television and telephone systems of the world combined today lasers are being used as tools of communications science medicine and industry but this is just a beginning the future will bring additional uses as well as deeper explorations of the scientific principles underlying the laser you