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I Determining Appropriate Focal Length

  1. Apr 27, 2017 #1
    So I'm working on a project where we're deciding a new lens to use for our laser diode. I need to determine the appropriate focal length based on the equation: 1/ƒ = 1/μ + 1/σ where ƒ is our focal length, μ is the distance from the lens to image and σ is distance from lens to object. For the purposes of our project we've determined we can assume that our object is infinitely far away and thus we have 1/ƒ = 1/μ or our focal length should be the same as the distance from our lens to our image.

    Now I just need some help finding that distance. I was looking on the schematic diagram for our laser diode and wanted to determine that distance. I imagined the distance from the "image" to the lens for a laser (as opposed to a camera) would be from the point of emission of the laser to the lens. I've attached a picture of this schematic, if anyone can help me find the distance σ I would truly appreciate it.

    ** The little distance indicator that I drew in pencil on the diagram is where I believed the distance σ would be. But there's no way to determine this distance from the info given.
  2. jcsd
  3. Apr 27, 2017 #2
    Sorry but I coulnd't understand the phisical situation.

    You have a laser a lens and a plane where the image will be formed, right? Where should the object be?
  4. Apr 27, 2017 #3
    Yes we have laser light coming out of a laser diode (what I interpret to be the "image" -- please correct me if I am wrong) which then passes through and gets refracted by a lens and which then travels through a crystal and through a beamsplitter into photo-detectors. Now we can assume the object that the laser is hitting is far enough away such that we can neglect the term associated with it. This then means the equation 1/ƒ = 1/σ tells us the appropriate focal length will be the length from the "image" (again, where the laser is emitted, I believe) to the lens.

    I need to find that distance.
  5. Apr 28, 2017 #4


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    From the LASER spec sheet, look up the beam diameter and the beam divergence angle. From these, calculate where a point would have to be to match the listed values. Note that the beam from a bare diode laser is elliptical, not circular; i.e. there is alot of astigmatism. Some, not all, prepackaged diode lasers have a built-in correction lens to yield a somewhat circular beam.
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