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Thread: Need some advice from experienced laser operators

  1. #1
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    Default Need some advice from experienced laser operators

    Greetings,

    I obtained a blue laser about a year ago from someone who doesn't know anything about it, and I'm just getting around to trying it out. This is my first laser and I'm being very careful because I've read about how dangerous lasers are (in particular blue ones). Not wanting to risk eye injury, I purchased some safety glasses that have a very high OD of 7+ in the blue region. Here is a link to the glasses I purchased:

    https://www.thorlabs.com/thorproduct.cfm?partnumber=LG3

    The lenses of the glasses have the following information laser-printed at the top of the lenses:

    180-532nm OD 7+ 48%VLT
    180-315 D LB7 + IR LB4 >315-532 DIRM LB6 NOIR CE

    The laser is a 3W 450nm focusable laser from China. It runs off of a 12VDC power supply and it has an input that is labeled "TTL".

    The glasses arrived yesterday and I proceeded to turn on the laser for the first time. I set up a frequency generator to drive it with a 5 kHz PWM signal at a low duty cycle. I found the laser turns on at about 0.3% duty cycle. I briefly set it to a high duty cycle and it easily burned a piece of paper. It works, so nothing to complain about there. But being a newbie, I don't have the experience to know what I should be able to see through those safety glasses. I am thinking that I shouldn't be able to see anything in the blue spectrum. Blue colors on my computer monitor are completely blocked out by the glasses. But when I observe the laser on a piece of white paper at the lowest intensity and very poorly focused I can see it through the glasses. Is this normal with glasses that have a relatively high VLT? At low intensity and poorly focused, I viewed it on the paper without the glasses and it is indeed blue. Is it a feature of some lasers to output some light in other spectrum ranges so that you can see the laser while wearing safety glasses? Or do you think this laser is defective?

    The light isn't uncomfortable to view through the glasses, and there isn't any persistence when I close my eyes. I expect to see some light when it burns things, but should I be able to see it when the power is very low?

    I am attaching a picture of the laser and the power supply. If anyone has any information about it, please let me know.

    Thank you very much in advance,

    Greg
    Attached Thumbnails Attached Thumbnails 20170128b.jpg  


  2. #2
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    I don't have the most technical experience in this area so I can't offer too much information, but I can say this; the wavelength of your laser may vary, but it shouldn't vary significantly. Essentially, a 450nm laser may vary +/- 5nm or so depending on drive current and temperature. If it is a DPSS laser it's possible for there to be a large amount of IR energy as well, but more than likely it is a diode module so IR should be negligible I think.

    Anyway, an OD of 7 indicates a transmission level of .00001%. It will not block ALL blue light but it will block most of it. The glasses appear to block all of the blue light from your monitor because the light sources within it are significantly less powerful than your laser as well as the fact that they are not coherent. The amount of blue light from you monitor that passes through the glasses is so small that your eyes cannot detect it. The laser on the other hand is so much more powerful as well as being coherent that the amount of blue light that passes through is much more perceptible. 3 watts of blue passing through your glasses should appear as approximately 300nW (nano watts). To be honest, I don't know what 300nW should look like. Does what you see through the glasses appear bright, or is it simply perceptible? I actually do not own a pair of safety glasses yet so I don't have any personal experience with them I just know how the math is supposed to work.

    It could also have something to do with the TTL driver. A .3% duty cycle doesn't actually mean that the laser is only putting out .3% of its power, it just means that it's putting out 100% of its power .3% of the time. This is also assuming that the driver itself is capable of such a short duty cycle. A .3% duty cycle at 5000 kHz correlates to a 600nS pulse width. I could be wrong but that seems like an awfully small pulse width for what is probably a relatively cheap laser driver.

    Hopefully someone with more knowledge can chime in with some specifics. I'm just speculating since I don't actually have any experience here.
    Last edited by ZeroPoint; 01-30-2017 at 00:11.
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  3. #3
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    ... look here, at post #6: http://www.photonlexicon.com/forums/...-for-Christmas

    You can see a blue laserdiode cutting wood with roughly 3Watts of power without shielding or through a "laser-window" with a similar blocking ability to your glasses.

    A little amount of blue light can pass, but most of the visible "orange" light is either from the created plasma (which is emitting "white" light), or "secondary emission" from fluorescency or such -- not sure, how the fluorescency is with 445nm, but a noticeable effect with 405nm ...

    Viktor

    *** EDIT ***

    added the images, so no need to scroll thorugh the other thread

    - without shielding:

    Click image for larger version. 

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    - ... with:

    Click image for larger version. 

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    Last edited by VDX; 01-30-2017 at 07:47.

  4. #4
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    Thank you ZeroPoint for answering my questions.

    Quote Originally Posted by ZeroPoint View Post
    I don't have the most technical experience in this area so I can't offer too much information, but I can say this; the wavelength of your laser may vary, but it shouldn't vary significantly. Essentially, a 450nm laser may vary +/- 5nm or so depending on drive current and temperature. If it is a DPSS laser it's possible for there to be a large amount of IR energy as well, but more than likely it is a diode module so IR should be negligible I think.

    Anyway, an OD of 7 indicates a transmission level of .00001%. It will not block ALL blue light but it will block most of it.
    I agree, and that is what is perplexing to me.

    The glasses appear to block all of the blue light from your monitor because the light sources within it are significantly less powerful than your laser as well as the fact that they are not coherent. The amount of blue light from you monitor that passes through the glasses is so small that your eyes cannot detect it. The laser on the other hand is so much more powerful as well as being coherent that the amount of blue light that passes through is much more perceptible. 3 watts of blue passing through your glasses should appear as approximately 300nW (nano watts). To be honest, I don't know what 300nW should look like. Does what you see through the glasses appear bright, or is it simply perceptible? I actually do not own a pair of safety glasses yet so I don't have any personal experience with them I just know how the math is supposed to work.
    I was thinking that the laser at lowest intensity was comparable to the computer screen. But it is much brighter than I thought. The best way to answer your question about the brightness is to take some pictures with my cell phone.

    Here are pictures of it at the lowest power level. The first image is direct, and the second image is as viewed through the glasses.

    Click image for larger version. 

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    You can see that it is quite bright through the glasses.

    I noticed something interesting. I happened to look at a power supply that was sitting nearby and the colors of the LEDs were hardly affected by the glasses. Here are two images. The first is direct and the second through the glasses. You can tell the difference because there is a blue LED nearby that is casting some light. You can't see the blue in the image taken through the glasses.

    Click image for larger version. 

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    That surprised me because everything in the room is so orange in color with the glasses on. The red in particular was such a vivid red with and without the glasses.

    It could also have something to do with the TTL driver. A .3% duty cycle doesn't actually mean that the laser is only putting out .3% of its power, it just means that it's putting out 100% of its power .3% of the time. This is also assuming that the driver itself is capable of such a short duty cycle. A .3% duty cycle at 5000 kHz correlates to a 600nS pulse width. I could be wrong but that seems like an awfully small pulse width for what is probably a relatively cheap laser driver.
    That is a good point. Perhaps I should find a phototransistor and hook it up to my oscilloscope. I should be able to see it pulse if it isn't a continuous wave.

    I focused the laser on a piece of paper at the lowest intensity and let it sit there for several minutes. There wasn't any indication that it burned the paper. I think I should try this test again to make sure I did it right. I can also try some other duty cycles. If it was pulsing at high power for a short amount of time at 5 kHz, wouldn't that be enough to cause a little burning? I'll run some more tests when I have some time.

    Thanks again for your help.

    Greg

  5. #5
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    ... for burning paper you'll have to focus the beam to a spot smaller than 0,2mm in diameter and rise the power - you can try with the black back of a CD-case or a piece of black adhesive, when it starts to melt and/or fume ...

    Viktor

  6. #6
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    Quote Originally Posted by VDX View Post
    ... look here, at post #6: http://www.photonlexicon.com/forums/...-for-Christmas

    You can see a blue laserdiode cutting wood with roughly 3Watts of power without shielding or through a "laser-window" with a similar blocking ability to your glasses.

    A little amount of blue light can pass, but most of the visible "orange" light is either from the created plasma (which is emitting "white" light), or "secondary emission" from fluorescency or such -- not sure, how the fluorescency is with 445nm, but a noticeable effect with 405nm ...

    Viktor
    Thank you for finding that post and the pictures Viktor. Any idea what the OD of that shielding is? Perhaps I should ask the person that posted that picture .

    In my case the light is not coming from the created plasma, because at low power I can see the light and it isn't burning the paper. But the "secondary emission" you mentioned sounds like a strong possibility. I will need to research this. It looks like I get a fair number of results searching for "blue laser fluorescence" and "blue laser-induced fluorescence". If this is a common phenomena, that means I didn't need to purchase this:

    https://www.thorlabs.com/thorproduct...artnumber=VRC1

    The card definitely works. It makes the card glow for a period of time after the laser has been shining on it. But maybe I didn't need it...

    Thank you for your help.

    Greg

  7. #7
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    Quote Originally Posted by VDX View Post
    ... for burning paper you'll have to focus the beam to a spot smaller than 0,2mm in diameter and rise the power - you can try with the black back of a CD-case or a piece of black adhesive, when it starts to melt and/or fume ...
    When I did that test for several minutes, it was focused. If I had turned the power up it would have burned the paper instantly.

    Greg

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    Hi Greg,

    ... you'll need a specific energy-density level to "carbonize" white paper - if it wents brown, then it will burn through at once for much higher absorption rates than with the original white paper.

    With a black plastic sheet you'll see melting effects with much lower energies, so could be better for testing.

    And without shielding goggles damages in the eye will start with even lower energies at "direct hits" (e.g. reflected beam on a shiny metal surface)

    Viktor

  9. #9
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    Another test you could try which would eliminate the florescent theory is to briefly place the glasses in the beam path to see how much light passes through. Still do this at low power and don't hold them there long so as not to damage them... and make sure you're not wearing them when you do it lol. If light still makes it through then you know it is not caused by florescence or a wavelength shift from the beam striking a particular surface. If no light passes through then you can assume that what you saw in your other test was light resulting from the beams interaction with a surface.
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  10. #10
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    Quote Originally Posted by VDX View Post
    ... you'll need a specific energy-density level to "carbonize" white paper - if it wents brown, then it will burn through at once for much higher absorption rates than with the original white paper.
    Good point Viktor. I will test it out with a black plastic CD case and report what happens at low power. I am sure it will melt it at high power.

    Thanks,

    Greg

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