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We present an analytical treatment of ultra-short pulses propagating in an optical fiber in the strong nonlinearity regime, in which the interaction between self-phase modulation (SPM) and group-velocity dispersion (GVD) substantially broadens the input spectrum. Supported by excellent agreement with the simulation results, these analytical solutions provide a convenient and reasonable accurate estimation of the peak position of the outermost spectral lobes as well as the full width at half maximum of the broadened spectrum. We show that our unified solutions are valid for either Gaussian pulse or hyperbolic secant pulse propagating inside an optical fiber with positive or negative GVD. Our findings shed light on the optimization of SPM-enabled spectral broadening in various applications.

The production of bimetallic nanoparticles with ultrasmall sizes is the constant pursuit in chemistry and materials science because of their promising applications in catalysis, electronics and sensing. Here we report ambient-temperature preparation of bimetallic NPs with tunable size and composition using microfluidic-controlled co-reduction of two metal precursors on silicon surface. Instead of free diffusion of metal ions in bulk system, microfluidic flow could well control the local ions concentration, thus leading to homogenous and controllable reduction rate among different nucleation sites. By controlling precursor concentration, flow rate and reaction time, we rationally design a series of bimetallic NPs including Ag-Cu, Ag-Pd, Cu-Pt, Cu-Pd and Pt-Pd NPs with ultrasmall sizes (∼ 3.0 nm), tight size distributions (relative standard deviation (RSD) < 21%), clean surface, and homogenous elemental compositions among particles (standard deviation (SD) of weight ratios < 3.5%). This approach provides a facile, green and scalable method toward the synthesis of diverse bimetallic NPs with excellent activity.

We propose the generation of wavelength-tunable femtosecond pulses with µJ energy based on spectral broadening in hollow-core fibers (HCFs) filled with noble gas. A proper combination of gas type, gas pressure, and the HCF core radius ensures that the broadened spectrum consists of isolated spectral lobes; the selection of the outermost spectral lobes produces nearly transform-limited pulses. We performed a detailed numerical investigation of this method based on an antiresonant HCF with a 20 µm core radius filled with 17.3 bar Xe gas. By using 1.03 µm, 200 fs pulses with up to 20 µJ energy as the excitation pulse, this method can produce 100 fs pulses tuned from 0.74 to 1.25 µm with up to 5 µJ pulse energy. Further energy scaling is limited by the onset of ionization, which reduces the wavelength tuning range and causes a significant spectral blue shift. Increasing the input pulse energy beyond 45 µJ creates a strong spectral lobe peaking in the wavelength range of 0.92–0.95 µm with >10µJ energy. These results represent a two orders of magnitude improvement in the pulse energy compared to current tunable femtosecond sources based on nonlinear wavelength conversion in solid-core optical fibers.

SPM-enabled spectral selection (SESS) constitutes a powerful fiber-optic technique to generate wavelength broadly tunable femtosecond pulses. In the current demonstration, the maximum tuning range is 400 nm and the energy conversion efficiency from the pump source to the outmost spectral lobes is ∼25%. In this submission, we apply the particle swarm optimization method to the generalized nonlinear Schrödinger equation to identify the optimal parameters that maximize both the tuning range and the conversion efficiency. We show that SESS in an optical fiber with the optimized dispersion can deliver SESS pulses tunable in one octave wavelength range and the conversion efficiency can be as high as 80%. We further show the feasibility of experimental implementation based on specially designed fibers or on-chip waveguides.

Pre-chirp managed amplification (PCMA) allows the generation of optical pulses with a duration well below 100 fs. However, the pulse peak power is limited to 30 times improvement in both pulse energy and peak power compared with current Yb-fiber PCMA systems.

OSA continuum 3(7), 1988-1998 (2020). doi:10.1364/OSAC.392981, We incorporate two techniques into pre-chirp managed amplification (PCMA) to achieve high-energy ultrashort pulses with the duration well below 100 fs. Numerical simulations confirmed by our experimental results demonstrate that seeding PCMA with circularly polarized pulses instead of linearly polarized pulses can increase the amplified pulse energy by 1.5 times. We also employ high-dispersion chirped mirrors to compress the amplified pulses with the throughput efficiency as high as 98%. These two techniques allow us to demonstrate an Yb-fiber PCMA system that emits 50-MHz, 47-fs pulses with 101.2-W average power., Published by OSA, Washington, DC

Post-pulse-compression is demanded to produce energetic few-cycle pulses. We propose pre-chirp-managed adiabatic soliton compression (ASC) in gas-filled pressure-gradient hollow-core fibers to suppress the detrimental pedestals and therefore significantly improve the compressed pulse quality. We show that two-stage ASC can compress 125 μJ, 130 fs pulses at 2 μm to a nearly two-cycle pulse 15 fs in duration. Our analytical analysis suggests that ASC is in favor of compressing pulses centered at a longer wavelength. As an example, a 280 μJ, 220 fs Gaussian pulse at 4 μm is compressed to 60 fs with minimal pedestals. We expect that the resulting high-quality, energetic few-cycle pulses will find important applications in high-field science.

Self-phase-modulation-enabled spectral selection (SESS) allows generation of widely tunable femtosecond pulses. We propose pre-chirp managed SESS, in which the excitation pulse is properly pre-chirped to control the fiber-optic nonlinear spectral broadening and hence generate separated spectral lobes—a feature demanded by SESS. Besides offering improved efficiency compared with conventional energy-tuned SESS, this new method is able to reshape an input pulse of unwanted shape into a bell-shaped pulse that is desired for implementing SESS. The excellent energy scalability of pre-chirp managed SESS can produce megawatt-level femtosecond pulses widely tunable in the wavelength range of 0.86–1.3 µm. Such a powerful source is well suited to drive multiphoton microscopy to achieve deep-tissue imaging.