Seeing through chaos in multimode fibres

Abstract

In a similar fashion to diffusers or other highly scattering media, multimode fibres deliver coherent light signals in the form of apparently random speckled patterns. In contrast to other optically random environments, multimode fibres feature remarkably faithful cylindrical symmetry. Our experimental studies challenge the commonly held notion that classifies multimode fibres as unpredictable optical systems. Instead, we demonstrate that commercially available multimode fibres are capable of performing as extremely precise optical components. We show that, with a sufficiently accurate theoretical model, light propagation within straight or even significantly deformed segments of multimode fibres may be predicted up to distances in excess of hundreds of millimetres. Harnessing this newly discovered predictability in imaging, we demonstrate the unparalleled power of multimode fibre-based endoscopes, which offer exceptional performance both in terms of resolution and instrument footprint. These results thus pave the way for numerous exciting applications, including high-quality imaging deep inside motile organisms.T he theoretical description of light transport processes within ideal multimode fibres (MMFs) has been developed for over half a century 1-4 . This elaborate theoretical model is, however, frequently considered inadequate to describe real-life MMFs, which are manufactured by drawing melted silica preforms. Such fibres are commonly seen as unreliable, and the inherent randomization of light propagating through them is typically attributed to undetectable deviations from the ideal fibre structure. It is a commonly held belief that this additional chaos is unpredictable and that its influence grows with the length of the fibre. Despite this, light transport through MMFs remains deterministic.The prospect of deterministic light propagation within MMFs has only recently been used through methods of digital holography and by adopting the concept of empirical measurement of the transformation matrix (TM) 5-11 . This technique, developed in studies of light propagation through highly turbid media 12-17 , has opened a new window of opportunity for MMFs to become extremely narrow and minimally invasive endoscopes, allowing sub-micrometre resolution imaging in deep regions of sensitive tissues 9,18 .However attractive, this technology suffers from several major limitations, the most critical being the lack of flexible operation. Any bending or looping of the fibre results in changes to its TM, rendering the imaging heavily impaired. All current methods exploiting MMFs for imaging require open optical access to the distal end of the fibre during the time-consuming measurement of the TM. Furthermore, this characterization must be repeated upfront for every intended configuration (deformation) and any axial distance of the focal plane behind the fibre before the system can be used for imaging 7,19 . The necessity to determine the TM empirically is therefore a major bottleneck of the technology, and it would be immense...

Single multimode fiber endoscope

“…A second creative solution uses a single mode fiber and a holographic film in conjunction with the MMF to create a virtual beacon source at the distal tip of the fiber [4] enabling digital phase conjugation compensation of fiber bending. Furthermore, recent advances in theoretical models [10] have shown the possibility of predicting the bending angle based on the analysis of the transmission matrix of the fiber, and its potential for image correction.…”

Towards an ultra-thin medical endoscope: multimode fibre as a wide-field image transferring medium

“…One potential candidate for forming these image conduits is multimode fibre [2][3][4][5][6][7][8] . A typical multimode fibre can potentially transmit an order of magnitude more information than a fibre bundle of the same diameter due to the large number of modes it supports 7 .…”

“…Hence, if incorporated into a rigid needle, a multimode fibre could act as an image conduit if a prior calibration allows the distortion to be corrected. One approach to this is to use a spatial light modulator (SLM) to generate a series of predetermined complex light fields at the proximal tip of the fibre which result in a scanned, focused point of light at the distal end [2][3][4][5][6] . If all light returning from each focused point is collected, an image can then be assembled.…”

Particle-like wave packets in complex scattering systems

“…As we shall see, particle-like scattering states give rise, in the time domain, to wave packets that remain focused in time and space throughout their trajectory within the medium. This crucial feature makes these states uniquely suited for many applications in a variety of fields, ranging from high intensity focused ultrasound [18,19] or underwater acoustics [20,21] to endoscopic microscopy [22][23][24], fibre optics [25][26][27][28][29] or telecommunications [30][31][32]. * alexandre.aubry@espci.fr…”