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Thread: Poor greenie beam profiles

  1. #1
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    Default Poor greenie beam profiles

    :x Is there anyone that knows where to get a true 75 or 100mW green laser that has a divergence of less than a spotlight? I have a couple of greenies here but the beams are just horrid at 15 meters. Somewhere in the range of 2Mrad or better. Too pissed to even calculate. Only one in the house is a measured 53mW and is doing a nice 1.3mrad the best I can calculate. There seems to be a flood of green.........no usable ones around. I hope someone can prove me wrong about this as it is disturbing.
    You are the only one that can make your dreams come true....and the only one that can stop them...A.M. Dietrich

  2. #2
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    Default Beam divergence comparison - photos...

    What do you consider "terrible beam quality"?

    I've got a Lasever 100 mw DPSS green laser that has a ~ 1.5 mm beam (measured a few feet away from the head). I knew that it had good beam quality, but I didn't know how it compared to any of my other lasers. I was also curious as to how large the spot would be at a great distance. (I've never had a throw longer than about 30 feet.)

    Anyway, I just took my Lasever unit out to the backyard and set it up along with one of my Spectra-Physics single-line (488nm) Argons and a 17" Melles Griot HeNe. Surprisingly, the beam from the DPSS was *MUCH* smaller than either the Argon or the HeNe at 105 ft... (32 meters) Have a look:



    Take a look at the album I've set up in the gallery for a closer view of the three beam spots. (Oh, and a big thanks to my son for acting as a backstop to display the beams on!)

    I have to admit that I was quite surprised to discover that my Lasever DPSS unit had the best beam quality of the three. I expected the HeNe to be the best, with the Argon a close second. But I guess Lasever has their act together after all...

    So, if you're looking for a decent green DPSS, I guess I'd have to recommend the Lasever unit - though I'm sure that there are other models out there that are even better...

    Adam

  3. #3

    Default Re: Poor greenie beam profiles

    Quote Originally Posted by MechEng3
    :x Is there anyone that knows where to get a true 75 or 100mW green laser that has a divergence of less than a spotlight? I have a couple of greenies here but the beams are just horrid at 15 meters. Somewhere in the range of 2Mrad or better. Too pissed to even calculate. Only one in the house is a measured 53mW and is doing a nice 1.3mrad the best I can calculate. There seems to be a flood of green.........no usable ones around. I hope someone can prove me wrong about this as it is disturbing.
    The flood you stated above is the problem really, and its really only a rough relation if any under 300mW in the 532nM wavelength. There are tons of variables past that that really make a mess like diode stripe size, optical cavity design, and the walkoff of whatever crystals you are using.

    Theres this nasty thing going around on ebay where people quote "half angle" in divergence... 1.5mrad half angle is really 3mrad.

    It has a lot more to do with manus using cheap optics to lower the price. Some lasers here have a divergence thats .5mrad with a 2mm source beam.

    For reference: all optics being equal within one specific wavelength, the larger the aperture beam the lower the divergence that can be attained.

    For future reference: a radian is a measurement of size *over* a given distance, for mRad specifically its +1mm in diameter per meter of throw distance + aperture beam size.

    Given all this: you are right, "decent quality greens" really are hard to find. I'll ask around.

  4. #4
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    Default

    Spec;

    I thought you had a line on the Lasever units.?. I didn't buy mine from you (only because I didn't know you 2 years ago when I bought it!) but I've seen the power supply for your Lasever DPSS blue unit, and it's identical to the one for my 100 mw Lasever DPSS green.

    I though you were an importer / distributor for them?

    Adam

  5. #5

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    to call me a "distributor" is pushing it...

    I give them money, and they take their sweet time sending me units.

  6. #6
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    Default

    Hello,
    I think you may be mistaking divergence for bad beam quality. Most green DPSS lasers have short cavities and do have more divergence than say a 10 mW He which is under 1 mR. You may always use external optics to correct this trading a larger beam diameter for lass beam spread. Laser collimators with say a factor of 2-3X mayl help. Even the Coherent 315M green laser has ~2mR divergence.
    Phil Bergeron( AKA 142laser)

  7. #7
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    Default Divergence in a Lasever DPSS 100 mw unit

    Quote Originally Posted by 142laser
    I think you may be mistaking divergence for bad beam quality.
    I don't think so. MechEng3's original post specifically mentioned divergence. He said he wanted one with a divergence that was "less than a spotlight". Granted, he was undoubtedly exaggerating, but he certainly was talking about poor divergence of his laser.

    My picture posted above was taken at a distance of 105 feet from the 3 laser heads used in the experiment. Note also that each of the three beams was less than 2 mm in diameter at the output aperature of the respective head. (I didn't try to measure them exactly; I merely confirmed that none of the three were significantly larger/smaller than the others.

    So it's not as if the Lasever DPSS unit has a large collimator stuck on the business end that expanded the beam to 10mm or something right at the start. No - it simply has lower divergence than either the Argon or the HeNe.

    As for overall beam quality, they all show a pretty good power distribution and TEM00 operation, though the Argon seems to be the closest to a gaussian profile.
    Most green DPSS lasers have short cavities and do have more divergence than say a 10 mW He which is under 1 mR.
    I know, which is why I was so surprised at the outcome of the experiment.

    Adam

  8. #8
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    I still say a larger divergence is not the same as bad beam quality and is more of a design issue. Look at the Coherent 315M with it's 0.33 mm diameter beam and 2 mR divergence. The M square of the 315M approaches 1.1. A 3X collimator gives a 1.0mm diameter beam with 0.66 mR divergence...abpou perfect I would say. Bad beam quality lasers have spots that look like a football with diffraction rings at a large distance or worse...like a shotgun blast of high order multi-modes! Now THAT would be hard to correct!
    Phil Bergeron( AKA 142laser)

  9. #9
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    Default Divergence vs overall beam quality

    Hey Phil;

    Yeah, I agree that multi-mode beams, football-shaped beams, and other beam abberations like you mentioned are all difficult to correct for. (Not impossible, but probably more work than anyone here is willing to go through!)

    However, that wasn't the problem that MechEng3 was describing. He said he had bad divergence. I think it's fair to lump divergence in with the other beam quality issues you mentioned above.

    Also, if you look at the beam profiles in the picture above, you'll see that all three of them are TEM00 with nice round spots. So, strict beam "quality" (in terms of mode and shape) isn't really a factor here.

    Thus the only remaining quality of the beam left to compare is divergence. And in the case above, the Lasever DPSS unit soundly trumps the other two lasers.

    I'm not saying that the Lasever unit is a superior laser in terms of stability; just that it has significantly lower divergence than the other two lasers, and thus would probably be a good fit for MechEng3's application.

    Remember that in MechEng3's application, we're talking about a laser used for display purposes, so longitudinal mode stability is irrelevent. (Now, if we were talking holography, I agree that the Coherent 315 would be the hands-down winner.)

    Beam quality (in terms of shape and transverse mode) is certainly important to a display laser. (Especially the football shaped beam you mentioned.)

    But divergence is VERY important, as is the initial beam diameter. (Especially if you have a long throw...) Trying to get three lasers lined up so that the near and far field spots are both the same size AND in the same place is a lot of work.

    In fact, I'd go so far as to say that as long as the spot is ROUND, divergence matters more than the transverse mode. (I've seen the results of a TEM01 beam being mixed with other lasers for a whitelight projector, and the results looked surprisingly good.)

    Another point that I think is worth mentioning is the fact that the Coherent 315 is a LOT more expensive than either the Lasever unit or any of the other Chinese DPSS units. But then again, you get what you pay for, and in the case of the Coherent there are a lot more things you can do with that laser besides painting pretty pictures on the wall!

    Adam

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