Tuesday, October 23, 2012

Endoscopy - focusing and depolarization

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. 

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. 

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. 

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.    

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