Hey folks..
Multimode fibers are great for endoscopy type scanning
applications. But a beam focused at a multimode fiber generates a scrambled
random speckle pattern at the output, with the fiber acting like a turbid
medium. Digital phase conjugation can be used to suppress this speckle.
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Ioannis
Papadopoulos spoke on this topic at Frontiers in Optics this year. They
interfere the speckle pattern with a plane reference beam and image the
resultant off-axis hologram at a detector. They calculate the phase of the
speckled field from the hologram and project it on a phase-SLM. The reference
beam is now reflected back from the SLM generating a conjugate field which is
projected at the fiber tip. Because of
this pre-aberration, the light output at the other end of the multimode fiber
now is sharply focused. This principle can be used to compute required phases
at the SLM and digitally, axially scan the focused spot.
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Julian Fade spoke about sensing the
depolarization of a material through fiber endoscopes. It’s difficult to
control polarization for light propagating through a birefringent material such
as fiber. But polarimetric orthogonality is not affected by propagation through
a fiber. So, the authors use a dual frequency, dual polarization probe beam and
propose measuring the depolarization strength of a material from the way it
breaks the orthogonality between the two orthogonal polarization states
incident on it.
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They showed measurements for a single pixel (fiber), but if the
fiber is scanned over a field of view they could obtain images which would be
very useful application of a fiber endoscope.
