text: 271,39,1419953782721495060,956539964795301889,Jacopo Bertolotti,"New paper on @arXiv! <LINK> This work was done by a former PhD student in my group (Alba Paniagua-Diaz, currently working with @pablo_artal), but while it was a chapter in her thesis, we never found the time to put it into a paper shape. Until now. 🧵 1/ This work marks the point where Alba moved from ""I am doing what my supervisor tells me to do"" to ""I have these ideas I want to try out"", which is probably the most important moment in a PhD 🙂 So, what is it about? 2/ ""Wavefront shaping"": by controlling the input wavefront, you can control the output even when the beam is completely scrambled by multiple scattering. How good is your control over the beam determines how good is your control over the output <LINK> 3/ In this paper we look at the problem from the opposite direction: what if our input beam is already completely scrambled (i.e. it looks like a speckle pattern)? Can we use wavefront shaping to recover a ""good"" beam? 4/ If you had perfect control over the beam, and you could change its amplitude and phase however you wanted, the answer would be a trivial ""yes"". But you don't have perfect control over the beam. 5/ Apart from technical limitations, you just can't take a point in the speckle pattern where there is no intensity, and multiply it by infinity to get a finite one. It just doesn't work. So the question becomes: how well can you do it ""in practice""? 6/ But first: who cares? We care because there are many cases where we would like to refocus the light coming out of a multimode fibre (e.g. because we have a high-power fibre laser), and the output of a multimode fibre is at best a speckle pattern. 7/ So in this paper we study how well we can take the output of a multimode fibre and form a nice Gaussian focus with it, with particular attention to the various possible limiting factors. It is a very ""practical"" paper, which hopefully will be useful to people 🙂 8/8 @sylvaingigan @arxiv @pablo_artal Let's see what the referees have to say 😉 (One big problem with very delayed papers, is that if referees start insisting in new measurements, things get tricky)",https://arxiv.org/abs/2107.10601,"A perfectly collimated beam can be spread out by multiple scattering, creating a speckle pattern and increasing the etendue of the system. Standard optical systems conserve etendue, and thus are unable to reverse the process by transforming a speckle pattern into a collimated beam or, equivalently, into a sharp focus. Wavefront shaping is a technique that is able to manipulate the amplitude and/or phase of a light beam, thus controlling its propagation through such media. Wavefront shaping can thus break the conservation of etendue and, in principle, reduce it. In this work we study how much of the energy contained in a fully developed speckle pattern can be converted into a high quality (low M2) beam, and discuss the advantages and limitations of this approach, with special attention given to the inherent variability in the quality of the output due to the multiple scattering. ","Wavefront shaping to improve beam quality: converting a speckle pattern