How a Laser Works

How a Laser Works


How a Laser Works EngineerGuy Series #4 For years lasers have been a hallmark of science fiction yet much of our technology today depends on them Range finding devices, optical communications, and of course, bar code scanners. The unique characteristics of laser light make all these things possible. However, its use by eye surgeons to reattach retinas highlights them all. Injury can cause the eye’s retina to peel away from the tissue supporting it. Without rapid treatment the entire retina can become detached causing blindness. Surgeons use green laser light of nearly a single wavelength because that color passes through the eye’s lens and vitreous humor without being strongly absorbed, and thus without causing damage. The laser beam then strikes the retina where the tissue strongly absorbs that light, using the high intensity light to weld the detached retina back into place. The beam’s narrowness allows the surgeon to affect only the area of the retina that needs to be repaired areas as small as 30 microns. How a laser creates light with these three characteristics is a tour de force of engineering. Let me show you. Any glow-in-the-dark toy can illustrate the basic principles of creating laser light. This glows because the zinc-copper based compound coating the inside can absorb energy from a light source and then later radiate it as light. The light provides energy to electrons in the coating promoting them to higher energy levels. Once the light is off, these electrons slowly lose their added energy and return to their lower energy ground states. The energy lost is given off as light. A closely related phenomena lies at the heart of a laser. Let me tell you about the engineering in the very first laser based on ruby. Here I have a tiny piece of ruby and some red colored glass beads. When I shine the blue light on the glass beads nothing much happens but shine it on the ruby and it glows red. Unlike the glow-in-the-dark ball the light appears immediately and when I shut off the blue light it disappears. In 1960, Ted Maiman demonstrated the first laser by taking a cylinder of ruby and surrounding it with a xenon arc flash lamp used in aerial photography. An intense burst of light from the lamp initiates lasing. To see how it works let’s look at what happens with a weaker lamp. A flash would promote a few electrons from the ground state to an excited state. They’d lose a bit of energy fall to a lower energy state without emitting light and then drop from there to the ground state giving off a burst of light. The light produced would be incoherent light a spectrum of colors and intensities just as my small laser made the ruby sphere glow. To create a laser takes an extremely powerful lamp. In the ruby laser repeated flashes called pumping make something amazing happens. They supply so much energy that a population inversion occurs Here more electrons in the energy level just above the ground state than in the ground state. Electrons from a population inversion returning to the ground state release light that starts an avalanche called stimulated emission. The photon produced when an electron decays induces other excited electrons to simultaneously decay and release nearly identical photons. That creates coherent light meaning that the crests and troughs of every light wave in the beam match up. Now, at this point we have coherent light but not yet the other two properties of laser light. To get a narrow beam with all the light rays parallel and of a nearly single wavelength requires an addition to the ruby rod. Maiman silvered the ends to reflect the light within the ruby cylinder. He made the two ends of the rod astonishingly parallel to each other From top to bottom the distance between these two mirrors differs by no more than 200 nanometers. Inside this resonant cavity, two things happen. First, any light rays that don’t line up with the axis eventually just exit out the side of the cylinder. And the light parallel to the axis becomes intensified and narrowed in wavelength. The mirrored ends create a standing wave which means only light of particular wavelengths can exist inside the cavity. By choosing the rod length correctly we can get the nearly single wavelength of light characteristic of a laser. A small hole in one of the mirrors or a partially silvered mirror allows the light to escape creating the familiar beam. Now, since the first Ruby laser was created lasers have become easy and cheap to manufacture. For instance this laser pointer uses a semiconductor diode to produce light. Although many innovations and improvements have occurred since 1960 the essential principles are the same. I’m Bill Hammack, the engineer guy. This video is based on a chapter in the book Eight Amazing Engineering Stories. The chapter features more information about this subject. Learn more about the book at the address below.

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About the Author: Oren Garnes

100 Comments

  1. Noooo… when i was a child.. I'm pointing my laser toy direct in my eyes๐Ÿ˜ญ now… i cant see well in far

  2. I envy anyone who understood this explanation, cause I didn't understand anything. great voice, great presentation, but unclear explanations

  3. While watching this video, I pretended to understand every word but when it ended, I realised I was back to square 1, or rather square zero.

  4. itโ€™s like one in the morning why is this in my recommended and why am i watching it

    itโ€™s like six years old ffs

  5. I have always taken time to explain how things actually work to my kids when they ask and having discovered your video's I am proud to say they were absolutely glued to the screen. Youtube should have more educational material like this.

    To bill directly you sir make the learning fun your selection of props examples and references are excellent. You have taken some very complex subjects and made them interesting and fairly easy to follow Thank you.

  6. understand the basic words of science then research your words then do your own research. study to show your self approved.

  7. Boy have LED lasers become impressive, and dangerous !ย  .005 watts is the legal limit, but China is underrating, or not rating at all, lasers making up to 20 watts!ย  that's , 20,000 MW or 4000 X the legal limit…. As you might expect, in this pathetic nation of laundrysoap eaters and synthetic heroin users, lots of permanent eye injuries and blindness have resulted from clinical morons (IQ 69 or lower) , who have no respect for anything….The most dangerous of all, are BLUE lasers !ย  which for some reason, project a rectangleย  beam that can temporarily blind from hundreds of feet or yards away.It is illegal to shine a laser into the air !!!!ย  if you hit a plane and are noticed , they will come!On the flip side,ย  well , use your imagination … it's a silent cheap effective self defense mechanism in a life or death situation…

  8. This is interesting I was born with my retinas detached so it's neat to know it's a specific wavelength that let me see

  9. How did Ted Maiman know to experiment with a ruby cylinder ( which i imagine he had get manufactured specifically ) inside the coils of a xenon arc flash bulb ?

  10. So, regarding that thing where the bulb wrapped around the ruby being pulsed to create an amplified wave. Is the frequency at which the bulb is pulsed essentially the light-resonant frequency? It seems very similar in concept to resonant frequency, that being pulses of energy input timed specifically to amplify the crest of the energy wave. But I've never heard of light resonant frequency. Is that a thing? Is there a relationship between these two phenomena?

    I don't see how you could ever end up with a beam of light that is normal to the face of the mirrors on the end of the rod if light enters the rod from the side and doesn't bend. How can light parallel to the axis make it's way into the space between the two mirrors without penetrating one of them?

  11. Any guy who is not in engineering may struggle with these since it takes 1 semester and 1 complete chapter about laser in Applied Physics subject to fully understand. Still the video was very accurate and simplified and all the things said was true and accurate.

    Explain : for those who dont get anything : the electrons of few materials like GaAs are made to excite such a way that a large no. Of electrons are made to jump on higher energy state (pumping & Population inversion) and then when they loose energy they go to metastable state (intermediate energy level), these are hit by light of certain energy and the electrons thus get back to ground level by producing stimulated emissions (2n waves) which is reflected back (end to end mirrors) to create a narrow straight beam of light (standing waves) which are directed out. Note that if more electrons are on lower energy level than higher than spontaneous emissions occur which is useless and random thats why we use heavy pumping to achieve population inversion (more elec on higher energy level). Its really difficult to explain without diagrams and easier words as there's lot and i tried to skip many parts here to make it more readable/comprehensive.

  12. Is the red color reason of glass beads as same as ruby bead? I mean do they have same TM or RE atom doping? If not, it is unfair to compare them with each other, and their shapes are also totally unfair to compare. I want to kown why the ruby so bright.

  13. I'm pretty sure that the government is spying me now, bought a small laser pointer 3 in 1 literally yesterday, this appeared on my recommended videos today

  14. Seems to me that total internal reflection would keep most of the light from exiting through the sides of the ruby rod. I would imagine that most off-axis light is lost at the silvered ends either through absorbance or transmission.

  15. I'm sorry but that hollywood music makes it so hard to watch. In that aluminium can video, you guys never had that so it was easy to watch and learn. Your content doesn't seem to be great for consumption anyway so I don't know whats the point.

  16. If any of you have taken a basic chemistry class you should be able to understand this video… I passed high school with the bare minimum science credits and I still understood.

  17. Hey, lets give a little love to Charles H. Townes who invented the MASER…the microwave precursor to the LASER.

  18. I built a helium neon laser in 7th grade….I still have it it's .5mw…I won the science fair for 7th through 12th grade overall. I was inspired by the 1985 movie Real Genius.

  19. pretty sure i get this.
    if i get some lipstick and wrap spaghetti around it, i can complete my stormtrooper outfit.

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