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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. Multimode nonlinear dynamics in spatiotemporal mode-locked anomalous-dispersion lasers

   https://doi.org/10.1364/OL.471457  

   Wu, Yuhang; Christodoulides, Demetrios_N; Wise, Frank_W (August 2022, Optics Letters)

   Spatiotemporal mode-locking in a laser with anomalous dispersion is investigated. Mode-locked states with varying modal content can be observed, but we find it difficult to observe highly-multimode states. We describe the properties of these mode-locked states and compare them to the results of numerical simulations. Prospects for the generation of highly-multimode states and lasers based on multimode soliton formation are discussed. 

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3. Deep learning of ultrafast pulses with a multimode fiber

   https://doi.org/10.1063/5.0007037  

   Xiong, Wen; Redding, Brandon; Gertler, Shai; Bromberg, Yaron; Tagare, Hemant_D; Cao, Hui (September 2020, APL Photonics)

   Characterizing ultrashort optical pulses has always been a critical but difficult task, which has a broad range of applications. We propose and demonstrate a self-referenced method of characterizing ultrafast pulses with a multimode fiber. The linear and nonlinear speckle patterns formed at the distal end of a multimode fiber are used to recover the spectral amplitude and phase of an unknown pulse. We deploy a deep learning algorithm for phase recovery. The diversity of spatial and spectral modes in a multimode fiber removes any ambiguity in the sign of the recovered spectral phase. Our technique allows for single-shot pulse characterization in a simple experimental setup. This work reveals the potential of multimode fibers as a versatile and multi-functional platform for optical sensing. 

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4. Multimode-fiber-based single-shot full-field measurement of optical pulses

   https://doi.org/10.1364/OL.388616  

   Xiong, Wen; Gertler, Shai; Yilmaz, Hasan; Cao, Hui (April 2020, Optics Letters)

   Multimode fibers are explored widely for optical communication, spectroscopy, imaging, and sensing applications. Here we demonstrate a single-shot full-field temporal measurement technique based on a multimode fiber. The complex spatiotemporal speckle field is created by a reference pulse propagating through the fiber, and it interferes with a signal pulse. From the time-integrated interference pattern, both the amplitude and the phase of the signal are retrieved. The simplicity and high sensitivity of our scheme illustrate the potential of multimode fibers as versatile and multi-functional sensors. 

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5. A systematic analysis of parametric instabilities in nonlinear parabolic multimode fibers

   https://doi.org/10.1063/1.5044659  

   Lopez-Aviles, H_E; Wu, F_O; Sanjabi_Eznaveh, Z.; Eftekhar, M_A; Wise, F.; Amezcua_Correa, R.; Christodoulides, D_N (December 2018, APL Photonics)

   We provide a systematic analysis of geometric parametric instabilities in nonlinear graded-index multimode fibers. Our approach implicitly accounts for self-focusing effects and considers dispersion processes to all orders. It is shown that the resulting parametric problem takes the form of a Hill’s equation that can be systematically addressed using a Floquet approach. The theory developed indicates that the unstable spectral domains associated with such geometric parametric instabilities can be significantly altered as the power levels injected in a parabolic multimode fiber increase. These predictions are in excellent agreement with experimental data gathered from graded-index multimode structures. 

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