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Removing speckle in Nd:YAG laser

  1. Nov 1, 2011 #1
    I am trying to use an Nd:YAG laser for pulsed laser melting My laser is an EKSPLA NL313, which has a 5 ns pulse. I need to produce a decent size spot (~1 cm2) with a fluence of about 1 J/cm2 for the laser melting, which is no problem with this laser. However, the spot needs to be very uniform in intensity. The YAG laser has very good spatial and temporal coherence, which means that the spot has a lot of speckle. I have looked at several solutions, but I am having trouble getting anything to work in practice.

    I have tried using an engineered diffuser, and this works to a certain extent, but there are still unacceptable variations in the intensity. I have tried using a beam homogenizer, but I still end up with fringes in the spot, almost like the beam was just folded back onto itself. This may be due to the fact that the spot is circular, while the homogenizer is rectangular. I have also tried using a pinhole to eliminate spatial harmonics, which also kind of works, but not well enough. I still have some fringes. Basically, I have tried several solutions, but none of them is getting me where I need to be. I have looked at using custom microlens arrays, but the high cost and lead time (6 months) makes that impossible.

    Does anyone know of a way I can homogenize the beam? Is there a combination of elements I could use to do this?
     
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  3. Nov 1, 2011 #2

    sophiecentaur

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    Wouldn't you achieve what you want by using what is effectively a 'mode stirrer' (as in a microwave oven)? All you need is to have some element in the light path that will move the diffraction pattern around so that the high / low power spots are evened out over time. A vibrating or rotating optical wedge would do this, I think.
     
  4. Nov 1, 2011 #3
    There are rotating diffusers available, and those are a great solution for CW lasers. Unfortunately, this solution does not work for short pulses. As you said, the intensity spots are evened out over time. With a 5 ns pulse, there is not sufficient time for the diffuser to rotate enough to even out the intensity. It's just too short of a time scale. I just started working with the pulsed laser, and I am finding that it is somewhat more complicated than the CW case due to the short time scale and high intensity. Thanks for your reply.
     
  5. Nov 1, 2011 #4

    sophiecentaur

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    Wow. 5ns isn't long to do anything much with a pulse, is it?
    You mention a pinhole. Is there not some location on the beam that you could place an appropriate diameter of aperture that could make the speckles coarser so that they overlap? Or would that spread the spot too much?
    I really don't know much at all about the special problems of laser light - I'm just applying what I know about optics and antennae in general. Perhaps I should shaddup. haha
     
  6. Nov 1, 2011 #5
    Not at all, I appreciate your replies. The pinhole suggestion may be something to look at. Actually, the pinhole that I have now involves a somewhat complicated setup. I focus the light down to about 1mm to go through the pinhole. The laser is so powerful that it actually causes the air to break down when it is focused. So, I have the pinhole housed in a pipe that is kept under vacuum, with windows on either side to admit light.

    Nanosecond scale pulses are useful for a variety of applications. In this case, I am using the pulses to melt and recrystallize silicon in order to produce layers that are supersaturated with transition metal impurities. There are a multitude of other uses as well. Generally, any time-resolved optical spectroscopy that is on a nano- to micro- second time scale needs a ns time scale excitation source.
     
  7. Nov 1, 2011 #6

    Andy Resnick

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    Whoa- that's a beast of a laser. What's the spec on the beam uniformity?
     
  8. Nov 1, 2011 #7
    Focal spots from gigantic Nd:YAG lasers are smoothed with phase plates placed in the beam near the lens - this is basically a more-or-less random phase screen that broadens the focal spot while smoothing it. Spectral dispersion is then used to very rapidly change the speckle pattern on very small scales. I suspect this is more complicated and expensive than you want, though.

    What sets your requirement on beam uniformity on various spatial scales? Melting requires heating, which takes time and allows for diffusion of heat that will smooth out non-uniformities on scales finer than some characteristic distance set by the timescale of the experiment and the speed of heat diffusion. I ask because it's possible to spend an enormous amount of time and money getting rid of non-uniformities in focused laser spots, and it would be a waste of said time and money if you later discover that the effort wasn't worthwhile.
     
  9. Nov 6, 2011 #8

    Claude Bile

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    It sounds like you have tried the standard solutions. As they have not worked, you need to get imaginative. Harnessing nonlinear interactions to smooth the intensity profile of a beam may be an avenue to chase. To be honest, it is a difficult problem and one where a simple solution may not yet exist! I look forward to a publication if you eventually solve this issue. Good luck.

    Claude.
     
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