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Single and multimode optical fibers

  1. Feb 20, 2014 #1
    Hi everyone,

    Sorry this is probably a really dull question but i'm intrigued nonetheless. I use a microscope that has multiple visible light lasers for excitation. The incident light is fed to the microscope through optical fibers. There is a single mode and a multimode fiber. I have been told that the single mode fiber is for use in Total internal reflection (TIRF) microscopy and the multimode is for widefield.

    A little background to the instrument, the incident light exits the fiber on the microscope side and passes through a phase transmission grating, splitting the light into zero and +1, -1 orders. These orders are then allowed to interfere producing a 3D grid pattern which is projected onto the sample through an objective lens. Before the laser light enters the multimode fiber it is "scrambled" by a motorised rotating holographic diffuser built into the laser rack to remove spatial coherence. So my questions are:
    -For imaging and specifically TIRF/widefield why is it necessary to use separate single and multimode fibers? My understanding of modes is that multimode allows multiple ray paths down the fiber (is this an over-simplification)
    -What does a rotating holographic diffuser do to the light before it enters the multimode fiber? I know to remove spatial coherence but why would you do that?

    Many thanks!

  2. jcsd
  3. Feb 20, 2014 #2
  4. Feb 20, 2014 #3

    Andy Resnick

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    Can you provide the make/model of the scope and illumination module?

    Broadly speaking, for TIRF you need to illuminate at a high incidence angle, meaning the entrance pupil of the condenser (or objective, if this is an epi-illumination method) must be completely filled, or even slightly overfilled to provide more uniform illumination. Multi-mode fiber, being larger in diameter, generally has a small 'cone angle' of light exiting the fiber, so the light exiting the multi-mode fiber may not completely fill the pupil. Another possible response is that a single-mode fiber provides better spatial control of illumination, for example better control of the incident angle of (TIR) illumination (this is why I asked for the make/model- TIRF can be implemented in a variety of ways).

    The use of a structured illumination element is slightly confusing- is that *always* in place?

    As for question 2, spatial coherence (speckle) makes imaging extremely difficult. A rotating diffuser destroys the spatial coherence, removing speckle, and provides a uniform illumination field.
  5. Feb 20, 2014 #4
    Hi andy,

    Yes its a structured illumination microscope (Nikon N-SIM):
    http://www.nikoninstruments.com/en_GB/Products/Microscope-Systems/Inverted-Microscopes/N-SIM-Super-Resolution [Broken]

    Yes its an inverted episcopic illumination method. The TIRF angle is set by the phase grating in the SIM illuminator module. The gratings are changed depending on if you're doing 2D/3D imaging or 2D TIRF imaging. There is no way to change the angle of incidence. I should also add that the gratings for TIRF are single wavelength, so only one wavelength down the fiber at any one time which may partly explain why a rotating diffuser is not required, but is required for multiple wavelengths as used in the multimode fiber.

    Last edited by a moderator: May 6, 2017
  6. Feb 20, 2014 #5
    Hi andy,
    I was reading about speckle patterns and the explanation seems to go against the nature of coherent light:
    "The speckle effect is a result of the interference of many waves of the same frequency, having different phases and amplitudes, which add together to give a resultant wave whose amplitude, and therefore intensity, varies randomly"

    Coherent light has the same frequency but i thought it would also generally have the same phase-i thought that was one of the fundamental effects of stimulated emission.. Any interference should be constructive. Or at least it has a constant phase relationship.
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