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B Combing Lasers To Create One Mega Laser

  1. Jun 15, 2016 #1
    Hello, I am pretty new to physics but have a large amount of experience with Calculus and other sciences like chemistry. The theory I have come up with is that if I combine 6-8 5w laser beams together in a convex lens that can be moved forward or backward to change the focal point distance, I could harness their power together to create one 30-40w laser beam. So, my question is, would it work better to use different powered beams together, or one single power with multiple beams. Is there a tool that I could put at the focal point that would make the single powerful beam a "unit" and stay as one beam as opposed to separating into their original beams after exiting the lens, and also, would it in fact combine into a 40w beam or just be one big 5w beam.

    Thanks, Cayden
     
  2. jcsd
  3. Jun 15, 2016 #2
    This very thing is used in nuclear fusion reactors, but without the lens. Hundreds of high powered lasers are aimed at a small amount of deuterium-tritium. This is called Inertial Confinement Fusion.

    Here is a link: https://lasers.llnl.gov/science/icf
     
  4. Jun 15, 2016 #3

    davenn

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    not really what the OP was asking and I didn't see any reference to "100's of lasers"

    this large system uses 20 lasers
    https://lasers.llnl.gov/science/icf/how-icf-works

    The OP wants to combine the beams of several lasers to produce a single more powerful beam
    ( think Starwars Death Star and its planet destroyer combined beams)

    to the OP, not sure, so others who work with lasers may chime in


    Dave
     
  5. Jun 15, 2016 #4

    Andy Resnick

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    It's easy to combine sources- you don't need lasers, regular lightbulbs will do. The catch is that the *intensities* will combine, not the *amplitudes*. NIF does indeed use this principle, there are multiple (pulsed) sources that are separately amplified and then, by carefully timing everything, all converge onto the hohlraum at the same time.

    https://lasers.llnl.gov/about/how-nif-works

    This is not easy. The reason why NIF uses multiple sources is that the optical amplifiers can't handle Petawatt powers (not much can handle that kind of power).

    The fancy word for all this (combining searchlights, NIF lasers, etc) is 'incoherent addition'. If you want to coherently add multiple sources, you need to figure out how to control each source so that the ensemble behaves as though ti was a single source:

    http://www.sciencedirect.com/science/article/pii/S0030401803012628

    Which is more difficult. For example, your tool that could go at the focal point is an optical amplifier:

    https://kgwg.nims.re.kr/kagra5th/kagra5th/KSuzuki.pdf
     
  6. Jun 15, 2016 #5

    Doc Al

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    The National Ignition Facility at LLNL uses 192 beams, not just 20. (The 20 beam system was long ago.)

    The Omega laser at Univ of Rochester uses 60 beams. (I actually visited that one a few times.)
     
  7. Jun 15, 2016 #6
    As I understand the OP, he wants to combine the separate beams into a single beam. Not just to add the powers delivered to a target by multiple beams but to get a single beam that propagates, at least for some distance, as a "parallel" beam.

    I don't know if you can do that. I mean, you could use a semitransparent mirror to combine two beams that propagate at 90 degrees initially but you loose power from each beam at the mirror. Maybe something like this. But they are not really overlaping. It would depend on what do you want to do with that strong beam.
    https://sites.google.com/site/argslasers/guides/how-to-combine-lasers
     
  8. Jun 15, 2016 #7

    Doc Al

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    Yep, we went off on a tangent. :sorry:
     
  9. Jun 15, 2016 #8

    davenn

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    yes exactly, which is what I restated some posts ago :smile:
     
  10. Jun 16, 2016 #9

    Andy Resnick

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    Right- that's 'coherent addition'. It's not simple (I posted links earlier) but can be done. Here's a decent summary that discusses methods and applications:

    http://conservancy.umn.edu/bitstream/handle/11299/56714/Khajavikhan_umn_0130E_10719.pdf?sequence=1

    That said, the 'death star' image of multiple beams that converge and at the common focus (in vacuum) change propagation direction is firmly sci-fi.
     
  11. Nov 15, 2016 #10
    andy resnick,
    i just joined up to find out more of the practical applications of lasers, and how to drive them,and combine beams of 2 or more laser diode out puts.Im a electronics technician,at least i studied this 20 odd years ago,and now i design audio eqipment for musical instrument use,my interest in lasers is probably initially a childhood fixation with star wars????,but i can use a hot laser for etching pcb's,front control panels, on a diy flat bed cnc engraver,made from a scanner.

    My thoughts were that laser diodes,as they are diodes,could be brightness controlled as a normal visible led,using pwm methods ,which can get more brightness out by the control of the average input power,which in short pulse peaks gives higher than rated output.

    I read lasers are current controlled devices,which would lead me to a fold back control to limit peak current and maybe a pwm after this to control frequency of opperation.Im speculating here,as i am not sure of the usual method for controling lasers.

    As for beam combination,i have on order(from china) around 20 5mW red lasers ,the frequency colour in nm is unknown to me at this point,but do i need to use"graiting refflector mirrors"? and guide mirrors to combine beams together.

    I could obtain a driver pcb and just copy it in surface mount components,as i have used them for switching controllers,and have the capacity to design and produce pcb's of this style on hand.

    If you could give me some info to forward this project i would be very gratefull,as i could eventually use dvd burner diodes to get good power output,failing this the chinese lasers will make a good laser security fence alarm system.

    johnamptech.
     
  12. Nov 16, 2016 #11
    A critical concept has been missing from this thread. It is implicit in some of the comments regarding coherence, but not stated plainly. The central question of combining laser beams is a property called beam quality. For you optics geeks this is the same as etendue and is directly related to the diffraction limit. In lasers it is often called brightness.

    What is the desirable property that makes a laser different than a lightbulb? It isn't the amount of optical power at the exit aperture. You can make insanely bright lights. Your bedside lamp puts out way more light than any laser you are likely to own. What is special about a laser is how bright you can make a spot of light at some distance away from the aperture. That is to say how well you can collimate the beam to project optical power at a distance, or equivalently how tightly you can focus a beam to make a high intensity spot.

    Consider a single high quality (diffraction limited) laser beam. For a given aperture radius w the minimum half angle of the beam divergence is

    theta = wavelength / (pi w)

    For a given sized output aperture size the spot size at some distance, and therefore the intensity has a limiting value: the diffraction limit.

    You can arrange several beams to be side by side (spatial beam combining). Let's say you arrange N beams to cover an aperture with an area N times larger than the individual lasers and so sqrt(N) larger in effective diameter. The array has a larger net aperture. However the far field spot size is still the same as what each individual beam can deliver. Don't get me wrong. That is still N times brighter than a single laser. However if those lasers are slaved to a master oscillator so that they are all coherent, the beam divergence will be diffraction limited for the larger aperture. The beam spread will be sqrt(N) smaller and the intensity of the spot in the far field, or at the focus of a lens will be another N times brighter. So for projecting power, coherent beam combining is the bomb, and that's the approach on most directed energy weapons.
     
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