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Practical implementation of focused UV laser

  1. Oct 16, 2011 #1
    As a quick heads up, I'm buried over my head in an imager project, and the portion that has given me the most headache is the UV source.

    Essentially, I need the source to make nice little 13 micron dots in the 405 nm range. I've been assured by a pair of vendors that this is an easy matter of purchasing a very expensive laser module, launching it into a fiber, and using a precision fiber-lens assembly to focus it.

    All very nice except I'm going bankrupt purchasing the 3 micron positioning equipment, having parts machined, and bringing together the raster scan computer / position control.

    So, my question is this:
    Can I cheat, purchase a cheap Chinese laser from Ebay, place a 12 micron spatial filter between the laser and target, and focus the beam such that its periphery is restricted by the filter prior to reaching the focal point. Then after reaching the focal point allow it to expand to about 13 microns at the target. Thus, I think I'm imaging the pinhole onto the target.

    Does this sound viable or am I getting myself into a world of difficulty?

    Thanks in advance,

  2. jcsd
  3. Oct 16, 2011 #2
    I don't follow exactly what you mean to do, but the usual way to focus a laser beam to a small spot is to use a microscope objective designed for an infinite image distance and run it backwards. This is what you would probably use anyway to focus the beam into an optical fiber. If the resulting spot isn't good enough, you put a spatial filter at the focus location, let the beam expand, then focus it again with a second lens. You effectively image the spatial filter onto your target, and the spatial filter cleans up the wings of the far-field pattern by throwing away light, so this presumes you have a bright-enough laser to be able to afford the loss.
  4. Oct 16, 2011 #3
    Thanks Jeff,

    This is fairly consistent with what I've been told. Essentially, the method I've been presented with utilizes the nature of a single mode fiber to establish a spatial filtering and then a coated lens is placed a distance from the fiber based upon a focus distance you specify. I've heard it referred to as a focuser (as opposed to a collimator?)

    I'm not overly repulsed by using this apparatus as it's not overly expensive, but the laser + fiber coupling to drive the thing quickly approaches the $2000 range. That seems expensive when I'm only attempting to make a 13 micron dot.

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