# Wavefront shaping techniques in complex media

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- Category: Multimode fibers
- Published on Saturday, 29 December 2018 16:35

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## [tutorial] Numerical Estimation of Multimode Fiber Modes and Propagation Constants:

## Part 2: Bent Fibers

We saw in the first part of the tutorial that the profiles and the propagation constants of the propagation modes of a straight multimode fiber can easily be avulated for an arbitrary index profile by inverting a large but sparse matrix. Under some approximations [1], a portion of fiber with a fixed radius of curvature satisfies a similar problem that can be solved with the same numerical tools, as we illustrate with the PyMMF Python module [2]. Moreover, when the modes are known for the straight fiber, the modes for a fixed radius can be approximate by inverting a square matrix of size the number of propagating modes [1]. It allows fast computation of the modes for different radii of curvature.

- Details
- Category: Multimode fibers
- Published on Thursday, 13 December 2018 10:29

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## [tutorial] 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 presence of bending. I released a beta version of the Python module pyMMF based on such 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 spare matrix representing the equation on the discretized space.

- Details
- Category: Spatial Lights Modulators (SLMs)
- Published on Friday, 21 September 2018 09:18

## How to calibrate linearly aligned nematic liquid crystal based SLMs [tutorial]

We published previously 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 treating 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 the phase fluctuations.

Written by Paul Balondrade and Sébastien M. Popoff

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- Category: Postdoc offers
- Published on Friday, 14 September 2018 09:53

## Postdoctoral Research Associate/Fellow

## Spatiotemporal properties of light in multimode optical fibre

### School of Information Technology and Electrical Engineering (ITEE), The University of Queensland, Brisbane, Australia.

We seek applications from suitably qualified candidates who will focus on the development of optical systems for the measurement and manipulation of the spatial, spectral and temporal properties of light in multimode optical fibre. The position is funded under an Australian Research Council (ARC), Discovery Project in collaboration with the University of Southampton and industrial partners Nokia (Bell Labs, US) and Finisar Australia.

Examples of previous work in this area available here : https://www.youtube.com/user/joelacarpenter

Duration: 2-3 years

#### More information: here

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- Category: Events
- Published on Tuesday, 10 April 2018 08:17

## OIP'21 : Optics for information processing in the 21th century

### May 23th - May 26th 2018

### Villa Finaly

### Florence, Italy

**Hosted by:** Sylvain Gigan (LKB, Sorbonne Université Paris, Paris), Rafael Piestun (University of Colorado, Boulder, USA)

## Link: here

- Details
- Category: Events
- Published on Monday, 26 February 2018 11:02