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1. Weak beam self-cleaning of femtosecond pulses in the anomalous dispersion regime

   https://doi.org/10.1364/OL.430926  

   Wu, Yuhang; Pourbeyram, Hamed; Christodoulides, Demetrios_N; Wise, Frank_W (July 2021, Optics Letters)

   Kerr beam cleaning in graded-index multimode fiber has been investigated in experiments with sub-nanosecond pulses and in experiments with femtosecond pulses at wavelengths where the dispersion is normal. We report a theoretical and experimental study of this effect with femtosecond pulses and anomalous dispersion. In this regime, only weak beam cleaning is observed experimentally, along with strong temporal evolution of the pulse. Numerical simulations exhibit the qualitative trends of the experiments. 

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2. 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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3. Millijoule few-cycle pulses from staged compression for strong and high field science

   https://doi.org/10.1364/OE.417404  

   Stanfield, M.; Beier, N_F; Hakimi, S.; Allison, H.; Farinella, D.; Hussein, A_E; Tajima, T.; Dollar, F. (March 2021, Optics Express)

   Intense few-cycle laser pulses have a breadth of applications in high energy density science, including particle acceleration and x-ray generation. Multi-amplifier laser system pulses have durations of tens of femtoseconds or longer. To achieve high intensities at the single-cycle limit, a robust and efficient post-compression scheme is required. We demonstrate a staged compression technique using self-phase modulation in thin dielectric media, in which few-cycle pulses can be produced. The few-cycle pulse is then used to generate extreme ultravoilet light via high harmonic generation at strong field intensities and to generate MeV electron beams via laser solid interactions at relativistic intensities. 

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4. Spatial Beam Self-Cleaning in Second-Harmonic Generation

   https://doi.org/10.1038/s41598-020-64080-7  

   Krupa, K.; Fona, R.; Tonello, A.; Labruyère, A.; Shalaby, B. M.; Wabnitz, S.; Baronio, F.; Aceves, A. B.; Millot, G.; Couderc, V. (December 2020, Scientific Reports)

   null (Ed.)

   Abstract We experimentally demonstrate the spatial self-cleaning of a highly multimode optical beam, in the process of second-harmonic generation in a quadratic nonlinear potassium titanyl phosphate crystal. As the beam energy grows larger, the output beam from the crystal evolves from a highly speckled intensity pattern into a single, bell-shaped spot, sitting on a low energy background. We demonstrate that quadratic beam cleanup is accompanied by significant self-focusing of the fundamental beam, for both positive and negative signs of the linear phase mismatch close to the phase-matching condition. 

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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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