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1. General theory and observation of Cherenkov radiation induced by multimode solitons

   https://doi.org/10.1038/s42005-021-00640-1  

   Eftekhar, M. A.; Lopez-Aviles, H.; Wise, F. W.; Amezcua-Correa, R.; Christodoulides, D. N. (December 2021, Communications Physics)

   null (Ed.)

   Abstract Advancements in computational capabilities along with the possibility of accessing high power levels have stimulated a reconsideration of multimode fibers. Multimode fibers are nowadays intensely pursued in terms of addressing longstanding issues related to information bandwidth and implementing new classes of high-power laser sources. In addition, the multifaceted nature of this platform, arising from the complexity associated with hundreds and thousands of interacting modes, has provided a fertile ground for observing novel physical effects. However, this same complexity has introduced a formidable challenge in understanding these newly emerging physical phenomena. Here, we provide a comprehensive theory capable of explaining the distinct Cherenkov radiation lines produced during multimode soliton fission events taking place in nonlinear multimode optical fibers. Our analysis reveals that this broadband dispersive wave emission is a direct byproduct of the nonlinear merging of the constituent modes comprising the resulting multimode soliton entities, and is possible in both the normal and anomalous dispersive regions. These theoretical predictions are experimentally and numerically corroborated in both parabolic and step-index multimode silica waveguides. Effects arising from different soliton modal compositions can also be accounted for, using this model. At a more fundamental level, our results are expected to further facilitate our understanding of the underlying physics associated with these complex “many-body” nonlinear processes. 

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2. Clots reveal anomalous elastic behavior of fiber networks

   https://doi.org/10.1126/sciadv.adh1265  

   Zakharov, Andrei; Awan, Myra; Gopinath, Arvind; Lee, Sang-Joon John; Ramasubramanian, Anand K; Dasbiswas, Kinjal (January 2024, Science Advances)

   The adaptive mechanical properties of soft and fibrous biological materials are relevant to their functionality. The emergence of the macroscopic response of these materials to external stress and intrinsic cell traction from local deformations of their structural components is not well understood. Here, we investigate the nonlinear elastic behavior of blood clots by combining microscopy, rheology, and an elastic network model that incorporates the stretching, bending, and buckling of constituent fibrin fibers. By inhibiting fibrin cross-linking in blood clots, we observe an anomalous softening regime in the macroscopic shear response as well as a reduction in platelet-induced clot contractility. Our model explains these observations from two independent macroscopic measurements in a unified manner, through a single mechanical parameter, the bending stiffness of individual fibers. Supported by experimental evidence, our mechanics-based model provides a framework for predicting and comprehending the nonlinear elastic behavior of blood clots and other active biopolymer networks in general. 

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3. Coherence properties of light in highly multimoded nonlinear parabolic fibers under optical equilibrium conditions

   https://doi.org/10.1364/OL.483282  

   Selim, Mahmoud_A; Wu, Fan_O; Pyrialakos, Georgios_G; Khajavikhan, Mercedeh; Christodoulides, Demetrios (February 2023, Optics Letters)

   We study the coherence characteristics of light propagating in nonlinear graded-index (GRIN) multimode fibers after attaining optical thermal equilibrium conditions. The role of optical temperature on the spatial mutual coherence function and the associated correlation area is systematically investigated. In this respect, we show that the coherence properties of the field at the output of a multimode nonlinear fiber can be controlled through its optical thermodynamic properties. 

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4. Review of a Decade of Research on Disordered Anderson Localizing Optical Fibers

   https://doi.org/10.3389/fphy.2021.736774  

   Mafi, Arash; Ballato, John (November 2021, Frontiers in Physics)

   Nearly a decade ago, transverse Anderson localization was observed for the first time in an optical fiber with a random transverse refractive index profile. This started the development of a whole new class of optical fibers that guide light, not in a conventional core-cladding setting based on total internal reflection, but utilizing Anderson localization, where light can guide at any location across the transverse profile of the fiber. These fibers have since been used successfully in high-quality endoscopic image transport. They also show interesting nonlinear and active (lasing) properties with promising applications. This review will cover a brief history of these fibers with personal accounts of the events that led to their development in our research groups. It will then follow with recent progress and future perspectives on science and applications of these fibers. 

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5. Effect of an input beam’s shape and curvature on the nonlinear effects in graded-index fibers

   https://doi.org/10.1364/JOSAB.379253  

   Ahsan, Amira_S; Agrawal, Govind_P (February 2020, Journal of the Optical Society of America B)

   We present a general framework capable of describing the nonlinear propagation of pulsed optical beams of arbitrary shapes and phase fronts inside a graded-index (GRIN) fiber. The main assumption made is that the spatial self-imaging features of the beam are not affected by the temporal evolution of optical pulses. A propagation kernel known from the work done in the 1970s is used to obtain a distance-dependent nonlinear coefficient that captures all spatial effects within an effective nonlinear Schrödinger equation. We consider three specific beam shapes (Gaussian, circular, and square) to study the impact of the shape, position, and curvature of optical beams on the complex spatiotemporal dynamics specific to GRIN fibers. In particular, we focus on the impact of an input beam’s shape on the modulation-instability sidebands and the generation of multiple dispersive waves when higher-order solitons form inside a GRIN fiber. The results of our numerical analysis indicate that for beam widths chosen to yield the same value of the effective mode area at the input end of the fiber, the nonlinear effects are pronounced considerably when a Gaussian beam is launched into the fiber. We also found that even though the self-imaging period is doubled when an off-centered Gaussian beam is launched into a GRIN fiber, it does not affect the nonlinear evolution because the effective beam area still maintains the same periodicity, as long as the shift in the beam’s center is not so large that it does not remain confined to the fiber’s core. 

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