Created by sebastien.popoff on 13/12/2018

Tutorials Multimode fibers

Numerical Estimation of Multimode Fiber Modes and Propagation Constants: 

Part 1: Straight Fibers

 

Under the weakly guided approximation, analytical solutions for the mode profiles of step-index (SI) and graded-index (GRIN) multimode fibers (MMF) can be found [1]. It also gives a semi-analytical solution for the dispersion relation in SI MMFs, and, by adding stronger approximations, an analytical solution for the parabolic profile GRIN MMFs [2] (note that those approximations do fail for lower order modes). An arbitrary index profile requires numerical simulations to estimate the mode profiles and the corresponding propagation constants of the modes. I present in this tutorial how to numerically estimate the scalar solution for the profiles and propagation constants of guides modes in multimode circular waveguide with arbitrary index profile and in the presence of bending. I released a beta version of the Python module pyMMF based on such an approach [3]. It relies on expressing the transverse Helmholtz equation as an eigenvalue problem. Solutions are found by finding the eigenvectors of a large but sparse matrix representing the equation on the discretized space.

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Created by sebastien.popoff on 21/09/2018

Tutorials Spatial Light Modulators

How to calibrate linearly aligned nematic liquid crystal based SLMs

 

I previously posted a tutorial presenting a technique to calibrate spatial light modulators (SLMs). The approach was based on measuring the interference between two paths that have been reflected off two different regions of the SLM. This technique is always valid but requires aligning a mask, using a lens, and capturing and processing images of interference patterns. Nowadays, most phase-only SLMs based on liquid crystals use linearly aligned nematic crystals. Unlike twisted nematic liquid crystals, they allow phase-only modulation on one polarization while not affecting the orthogonal polarization. This feature can be used to simplify the calibration setup to characterize the SLM with a common path interferometer, not requiring a precise alignment [1]. Furthermore, it only needs a photo-detector, compared to a digital camera in the previously presented approach. This is convenient to measure the inevitable phase fluctuations of an SLM, usually around a 100 to 400 Hz frequency. In this document, we briefly describe the principle of the characterization scheme as presented in [2] and show typical results of the calibration and phase fluctuations.

Written by Paul Balondrade and Sébastien M. Popoff

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Created by sebastien.popoff on 21/05/2017

Talks Wavefront shaping

Wavefront Shaping in Complex Media:
From the Compensation to the Exploitation of Randomness

Sebastien M. Popoff

CLEO 2017 - San Jose, CA (USA), May 16 2017

Abstract: In the past ten years, many techniques were developed to control light propagation in complex transmission media using spatial light modulators. It involved applications in numerous fields including biomedical imaging and therapy, fiber endoscopy, cryptography, optical micromanipulation, optical spectroscopy, telecommunications and random lasers and also served as a tool for fundamental studies of light propagation in complex environments.

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Created by sebastien.popoff on 26/12/2016

Talks Wavefront shaping

Controlling Light in Complex Media

Sebastien M. Popoff

LOM Master Seminar

Friday, November 26 2016

Abstract: Seminar talk about the control of light in scattering media for focusing and imaging applications.

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