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A Decreasing the power density of a laser

  1. Jun 3, 2017 #1
    Hello! I have an issue with the power density of a 488 nm continuous wave laser. I'm using this laser to excite my sample. The problem is that even at low power, the laser is causing the samples to form a burn deposit on the quartz sample holder. Between the laser source and the sample, there are a series of dialectric mirrors, pinholes, and a focussing lens. What are the ways in which power density can be decreased at the focal point?
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  3. Jun 3, 2017 #2


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  4. Jun 3, 2017 #3
    Thanks for the reply, but I can't afford that. Are there any other cheaper alternatives?
  5. Jun 4, 2017 #4


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    There are various cheap filters that can reduce the laser intensity. Adding another pinhole at the right place could work as well, simply by blocking a part of the laser light.

    You'll have to figure out if that disturbs the measurement.
  6. Jun 4, 2017 #5
    How about defocusing the lens a little?
  7. Jun 4, 2017 #6
    Thanks for the reply, I have tried that but my sample still decomposes.
  8. Jun 4, 2017 #7
    Thanks for the reply, the focussing lenses have knobs to adjust the focus in the x,y, and z direction. Do you think it matters which direction is defocus? Also, does it matter where in the focussing lens the laser beam hits? Thanks a lot.
  9. Jun 4, 2017 #8


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    good grief !! what is the power of the laser ??
  10. Jun 4, 2017 #9


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    A smaller pinhole, at a point where the laser beam is wider? A pinhole off-center?
    There must be some way to make the beam weaker if you block a part of it.

    I don't know the requirements on the beam parameters, they might change with an additional pinhole.
  11. Jun 4, 2017 #10
    Thanks for the question, the laser is set to 1 watt.
  12. Jun 4, 2017 #11
    Thanks for the reply.
  13. Jun 4, 2017 #12
    I don't know how your x, y and z directions are defined, but the relevant one is the optical axis along the laser beam. Let's call that the z-direction. I'm not sure what you mean by changing the focus in the x- and y-direction. Anyhow. the focusing lens is presumably bringing the laser beam to tight beam waist, thus concentrating its power in a small volume. If you move the focusing lens either toward or away from the laser, the beam waist at the sample will expand, thus lessening the power density.

    The laser beam should go through the center of the focusing lens, otherwise it will be displaced in the x- or y-direction.
  14. Jun 4, 2017 #13
    Thanks for the reply, our focussing lens are mounted in an assembly like this https://www.newport.com/p/77259. That is what I meant.
  15. Jun 5, 2017 #14
    This might be naive on my part, but what about a beam splitter. A simple splitter between the laser and the beginning of the optics would reduce the power by 50% and might not require any adjustment to the optics. And I think there are some splitters that allow for adjusting the split ratio.
    Last edited: Jun 5, 2017
  16. Jun 5, 2017 #15
    Thanks for the reply, I will look into this.
  17. Jun 7, 2017 #16
    If interference effects are not deadly to you just add a bunch of microscope slides (with air spaces) somewhere in collimated laser path at some angle (approx. 45 degree). Each reflection will attenuate about 10 % of light if the incidence is s-polarized light (simply just rotate the slide and check for biggest reflection). Avoid direct back reflection as it will make laser unstable! Its cheap and easy.
  18. Jun 7, 2017 #17


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    Is the laser too powerful to use neutral density filters? Or pop out some lenses from cheap sunglasses?
  19. Jun 16, 2017 #18
    Thanks for the reply, I'll look into this.
  20. Jun 16, 2017 #19
    Thanks for the advice.
  21. Jun 16, 2017 #20


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    1 W is a very powerful laser. Please be careful of stray reflections (off the pinhole diaphram or ND filter, for instance), and use proper lab safety procedures (no public access to room when laser is running, warning signs, laser-filtering safety goggles for you and other lab personnel, etc.). Also be careful of attenuating filters. If they are placed in a position where the beam is focused, they can crack due to heating from the absorption. It is common to defocus the beam, attenuate it, and then refocus.
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