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 29/10/2016

Tutorials Spatial Light Modulators

Setting up a DMD: Diffraction effects

I recently acquired a Digital Micromirror Device (DMD) and when I started setting up the experiment, I faced a problem I did not anticipate which is closely related to blazed gratings. Due to the fact that the surface of a DMD is not flat, diffraction orders are shifted compared to the optical axis. This shift depends on the pixel pitch, the wavelength, and the incident angle. A close look at this diffraction phenomenon is important to configure an experimental setup properly. It is even relevant to consider this effect before choosing the appropriate DMD model to buy.

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Created by sebastien.popoff on 17/10/2016

Tutorials Spatial Light Modulators

Control a Vialux DMD with Python

Vialux provides Texas Instrument DMD (Digital MicroMirror Devices) chips with an electronic board to send and display image sequences at high speed (up to 30kHz). While they provide a C++ dll, Labview, and Matlab codes, I did not find any tool for Python. I share here a simple module that wraps the C++ functions for Python. It allows using in a simple manner the basic functions while providing the advanced features of the ALP API.

DOI

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