Spatial and temporal properties of an ultrashort pulse of light are naturally scrambled upon propagation in thick scattering media. Significant progresses have been realized over the last decade to manipulate light propagation in scattering media, mostly using monochromatic light. However, applications that require a broadband ultrashort pulse of light remain limited, as the pulse gets temporally broadened because of scattering effects. A monochromatic optical transmission matrix does not allow temporal control of broadband light. Although measuring multiple transmission matrices with spectral resolution allows fine temporal control, it requires lengthy measurements, as well as stability of the medium.
In this work, we show that a single linear operator that we named Broadband Transmission Matrix, can be straightforwardly measured for a broadband pulse with a co-propagating reference. We exploit this operator for focusing purposes, and we analyze its phase conjugation properties. While the operator naturally allows for spatial focusing, unexpectedly, the focus duration is on average shorter than the natural temporal broadening due to the medium. More precisely, we observe a two-fold temporal recompression at the focus that we fully explain theoretically. We also explore the spectral content at the focus, and demonstrate a narrowing of the spectrum.
These results are particularly relevant for non-linear imaging techniques in biological tissues, at depth where an ultrashort excitation pulse is broadened.
Mickael Mounaix, Hilton B. de Aguiar, and Sylvain Gigan, "Temporal recompression of an ultrashort pulse of light with a broadband transmission matrix (Conference Presentation)," Proc. SPIE 10502, Adaptive Optics and Wavefront Control for Biological Systems IV, 105020S (Presented at SPIE BiOS: January 28, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2290893.5751631623001.
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