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When light propagates through opaque material, the spatial information it holds becomes scrambled, but not necessarily lost. Two classes of techniques have emerged to recover this information: methods relying on optical memory effects, and transmission matrix (TM) approaches. Here we develop a general framework describing the nature of memory effects in structures of arbitrary geometry. We show how this framework, when combined with wavefront shaping driven by feedback from a guide-star, enables estimation of the TM of any such system. This highlights that guide-star assisted imaging is possible regardless of the type of memory effect a scatterer exhibits. We apply this concept to multimode fibres (MMFs) and identify a `quasi-radial memory effect. This allows the TM of an MMF to be approximated from only one end - an important step for micro-endoscopy. Our work broadens the applications of memory effects to a range of novel imaging and optical communication scenarios.
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Time-of-flight (ToF) 3D imaging has a wealth of applications, from industrial inspection to movement tracking and gesture recognition. Depth information is recovered by measuring the round-trip flight time of laser pulses, which usually requires projection and collection optics with diameters of several centimetres. In this work we shrink this requirement by two orders of magnitude, and demonstrate near video-rate 3D imaging through multimode optical fibres (MMFs) - the width of a strand of human hair. Unlike conventional imaging systems, MMFs exhibit exceptionally complex light transport resembling that of a highly scattering medium. To overcome this complication, we implement high-speed aberration correction using wavefront shaping synchronised with a pulsed laser source, enabling random-access scanning of the scene at a rate of $sim$23,000 points per second. Using non-ballistic light we image moving objects several metres beyond the end of a $sim$40 cm long MMF of 50$mu$m core diameter, with millimetric depth resolution, at frame-rates of $sim$5Hz. Our work extends far-field depth resolving capabilities to ultra-thin micro-endoscopes, and will have a broad range of applications to clinical and remote inspection scenarios.
Beam self-cleaning (BSC) in graded-index (GRIN) multimode fibres (MMFs) has been recently reported by different research groups. Driven by the interplay between Kerr effect and beam self-imaging, BSC counteracts random mode coupling, and forces laser beams to recover a quasi-single mode profile at the output of GRIN fibres. Here we show that the associated self-induced spatiotemporal reshaping allows for improving the performances of nonlinear fluorescence microscopy and endoscopy using multimode optical fibres. We experimentally demonstrate that the beam brightness increase, induced by self-cleaning, enables two and three-photon imaging of biological samples with high spatial resolution. Temporal pulse shortening accompanying spatial beam clean-up enhances the output peak power, hence the efficiency of nonlinear imaging. We also show that spatiotemporal supercontinuum generation is well-suited for large-band nonlinear fluorescence imaging in visible and infrared domains. We substantiated our findings by multiphoton fluorescence imaging in both microscopy and endoscopy configurations.
The input numerical aperture (NA) of multimode fiber (MMF) can be effectively increased by placing turbid media at the input end of the MMF. This provides the potential for high-resolution imaging through the MMF. While the input NA is increased, the number of propagation modes in the MMF and hence the output NA remains the same. This makes the image reconstruction process underdetermined and may limit the quality of the image reconstruction. In this paper, we aim to improve the signal to noise ratio (SNR) of the image reconstruction in imaging through MMF. We notice that turbid media placed in the input of the MMF transforms the incoming waves into a better format for information transmission and information extraction. We call this transformation as holistic random (HR) encoding of turbid media. By exploiting the HR encoding, we make a considerable improvement on the SNR of the image reconstruction. For efficient utilization of the HR encoding, we employ sparse representation (SR), a relatively new signal reconstruction framework when it is provided with a HR encoded signal. This study shows for the first time to our knowledge the benefit of utilizing the HR encoding of turbid media for recovery in the optically underdetermined systems where the output NA of it is smaller than the input NA for imaging through MMF.
Multimode fibers (MMF) are used in many applications from telecomunications to minimally invasive micro-endoscopic imaging. However, the numerous modes and their coupling make light-beam control and imaging a delicate task. To circumvent this difficulty, recent methods exploit priors about the transmission of the system, such as the so-called optical memory effect. Here, we quantitatively characterize a chromato-axial memory effect in step-index MMF, characterized through its slope $delta z/delta lambda$ and its spectral and axial widths. We propose a theoretical model and numerical simulations in good agreement with experimental observations.
Multimode fibres (MMFs) are attracting interest for complex spatiotemporal dynamics, and for ultrafast fibre sources, imaging and telecommunications. This new interest is based on three key properties: their high spatiotemporal complexity (information capacity), the important role of disorder, and complex intermodal interactions. To date, phenomena in MMFs have been studied only in limiting cases where one or more of these properties can be neglected. Here we study MMFs in a regime in which all these elements are integral. We observe a spatial beam-cleaning process preceding spatiotemporal modulation instability. We show that the origin of these processes is a universal unstable attractor in graded-index MMFs. Both the self-organization of the attractor, as well as its instability, are caused by intermodal interactions characterized by cooperating disorder, nonlinearity and dissipation. The demonstration of a disorder-enhanced nonlinear process in MMF has important implications for telecommunications, and the multifaceted complexity of the dynamics showcases MM waveguides as ideal laboratories for many topics and applications in complexity science.
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