Enhanced Adsorption of Methyl Orange from Aqueous Phase Using Chitosan-Palmer Amaranth Biochar Composite Microspheres.

To develop valuable applications for the invasive weed Palmer amaranth, we utilized it as a novel biochar source and explored its potential for methyl orange adsorption through the synthesis of chitosan-encapsulated Palmer amaranth biochar composite microspheres. Firstly, the prepared microspheres were characterized by scanning electron microscopy and Fourier transform infrared spectroscopy and were demonstrated to have a surface area of 19.6 m2/g, a total pore volume of 0.0664 cm3/g and an average pore diameter of 10.6 nm. Then, the influences of pH, dosage and salt type and concentration on the adsorption efficiency were systematically investigated alongside the adsorption kinetics, isotherms, and thermodynamics. The results reveal that the highest adsorption capacity of methyl orange was obtained at pH 4.0. The adsorption process was well fitted by a pseudo-second-order kinetic model and the Langmuir isotherm model, and was spontaneous and endothermic. Through the Langmuir model, the maximal adsorption capacities of methyl orange were calculated as 495.0, 537.1 and 554.3 mg/g at 25.0, 35.0 and 45.0 °C, respectively. Subsequently, the adsorption mechanisms were elucidated by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy investigations. It is indicated that electrostatic interactions, hydrogen bonding, π-π interactions and hydrophobic interactions between methyl orange and the composite microspheres were pivotal for the adsorption process. Finally, the regeneration studies demonstrated that after five adsorption-desorption cycles, the microspheres still maintained 93.6% of their initial adsorption capacity for methyl orange. This work not only presents a promising method for mitigating methyl orange pollution but also offers a sustainable approach to managing Palmer amaranth invasion.

Slow saturable absorption for optimal operation in a soliton comb laser.

In this article, we study how the choice of parameters of a slow saturable absorber (SSA) affects the stable operation of a soliton fiber comb laser. We show that a shorter recovery time for the SSA does not always lead to shorter modelocked pulses. Instead, increasing the cavity gain plays a critical role in generating stable modelocked pulses with higher energy and shorter durations. We find that more stable, shorter, and more energetic output pulses can be achieved with lower saturation energies of the SSA and/or higher anomalous dispersion within the cavity.

Design of a dual-channel modelocked fiber laser that avoids multi-pulsing.

Multi-channel modelocked lasers and their design have attracted much attention. Here, we use the Swift-Hohenberg equation to study dual-channel simultaneous modelocking (DSML) in a fiber laser. When a quartic filter is added to the laser cavity, the stable dual-channel simultaneous modelocking can be obtained for a given filter bandwidth when frequency separation, ωs, is less than a calculated threshold, ωth. When ωs>ωth, a multipulsing instability occurs. We use a linear stability analysis to determine the limit that the multi-pulsing instability imposes on DSML, and we propose a cavity design that avoids the multi-pulsing instability.

Hybrid Graphene-Silicon Based Polarization-Insensitive Electro-Absorption Modulator with High-Modulation Efficiency and Ultra-Broad Bandwidth.

Polarization-insensitive modulation, i.e., overcoming the limit of conventional modulators operating under only a single-polarization state, is desirable for high-capacity on-chip optical interconnects. Here, we propose a hybrid graphene-silicon-based polarization-insensitive electro-absorption modulator (EAM) with high-modulation efficiency and ultra-broad bandwidth. The hybrid graphene-silicon waveguide is formed by leveraging multi-deposited and multi-transferred methods to enable light interaction with graphene layers in its intense field distribution region instead of the commonly used weak cladding region, thus resulting in enhanced light⁻graphene interaction. By optimizing the dimensions of all hybrid graphene-silicon waveguide layers, polarization-insensitive modulation is achieved with a modulation efficiency (ME) of ~1.11 dB/µm for both polarizations (ME discrepancy < 0.006 dB/µm), which outperforms that of previous reports. Based on this excellent modulation performance, we designed a hybrid graphene-silicon-based EAM with a length of only 20 µm. The modulation depth (MD) and insertion loss obtained were higher than 22 dB and lower than 0.23 dB at 1.55 µm, respectively, for both polarizations. Meanwhile, its allowable bandwidth can exceed 300 nm by keeping MD more than 20 dB and MD discrepancy less than 2 dB, simultaneously, and its electrical properties were also analyzed. Therefore, the proposed device can be applied in on-chip optical interconnects.

Averaging-free vector Brillouin optical time domain analyzer assisted by reference probe lightwave.

We propose and experimentally demonstrate a vector Brillouin optical time domain analyzer (BOTDA) system, which enables both distributed Brillouin gain spectrum (BGS) and Brillouin phase-shift spectrum (BPS) measurements without trace averaging. The proposed scheme generates and launches a reference light into the fiber under test (FUT), together with the Stokes probe light to indicate the phase noise and distortion in the probe. The commercial integrated coherent receiver at the detection end, using a local oscillator (LO) generated with single sideband (SSB) modulation, on the one hand, improves the signal-to-noise ratio (SNR) to avoid the need of trace averaging for the signal acquisition. On the other hand, by processing and analyzing the receiver outputs at specific intermediate frequencies (IF), the amplitude and phase signals carried by the Stokes probe and the reference light can be resolved at once. In this way, both Brillouin gain and Brillouin phase-shift signals can be obtained simultaneously. By scanning the the Stokes probe light frequency and recording the Brillouin response at each scanning frequency, both distributions of Brillouin gain spectrum and phase-shift spectrum have been acquired. The configuration is approved by experiments carried out over an 18.2 km FUT with the spatial resolution of 2 m.

Experimental generation of discrete ultraviolet wavelength by cascaded intermodal four-wave mixing in a multimode photonic crystal fiber.

In this Letter, we demonstrate experimentally for the first time, to the best of our knowledge, discrete ultraviolet (UV) wavelength generation by cascaded intermodal FWM when femtosecond pump pulses at 800 nm are launched into the deeply normal dispersion region of the fundamental guided mode of a multimode photonic crystal fiber (MPCF). For pump pulses at average input powers of Pav=450, 550, and 650 mW, the first anti-Stokes waves are generated at the visible wavelength of 538.1 nm through intermodal phase matching between the fundamental and second-order guided mode of the MPCF. The first anti-Stokes waves generated then serve as the secondary pump for the next intermodal FWM process. The second anti-Stokes waves in the form of the third-order guided mode are generated at the UV wavelength of 375.8 nm. The maximum output power is above 10 mW for Pav=650 mW. We also confirm that the influences of fiber bending and intermodal walk-offs on the cascaded intermodal FWM-based frequency conversion process are negligible.

Deep-ultraviolet second-harmonic generation by combined degenerate four-wave mixing and surface nonlinearity polarization in photonic crystal fiber.

Deep-ultraviolet (UV) second-harmonics (SHs) have important applications in basic physics and applied sciences. However, it still remains challenging to generate deep-UV SHs especially in optical fibers. Here, for the first time, we experimentally demonstrate the deep-UV SH generations (SHGs) by combined degenerate four-wave mixing (FWM) and surface nonlinearity polarization in an in-house designed and fabricated air-silica photonic crystal fiber (PCF). When femtosecond pump pulses with average input power P av of 650 mW and center wavelength λ p of 810, 820, 830, and 840 nm are coupled into the normal dispersion region close to the zero-dispersion wavelength of the fundamental mode of the PCF, the anti-Stokes waves induced by degenerate FWM process are tunable from 669 to 612 nm. Then, they serve as the secondary pump, and deep-UV SHs are generated within the wavelength range of 334.5 to 306 nm as a result of surface nonlinearity polarization at the core-cladding interface of the PCF. The physical mechanism of the SHGs is confirmed by studying the dependences of the output power P SH of the SHs on the PCF length and time. Finally, we also establish a theoretical model to analyze the SHGs.

Comprehensive analysis of passive generation of parabolic similaritons in tapered hydrogenated amorphous silicon photonic wires.

Parabolic pulses have important applications in both basic and applied sciences, such as high power optical amplification, optical communications, all-optical signal processing, etc. The generation of parabolic similaritons in tapered hydrogenated amorphous silicon photonic wires at telecom (λ ~ 1550 nm) and mid-IR (λ ≥ 2100 nm) wavelengths is demonstrated and analyzed. The self-similar theory of parabolic pulse generation in passive waveguides with increasing nonlinearity is presented. A generalized nonlinear Schrödinger equation is used to describe the coupled dynamics of optical field in the tapered hydrogenated amorphous silicon photonic wires with either decreasing dispersion or increasing nonlinearity. The impacts of length dependent higher-order effects, linear and nonlinear losses including two-photon absorption, and photon-generated free carriers, on the pulse evolutions are characterized. Numerical simulations show that initial Gaussian pulses will evolve into the parabolic pulses in the waveguide taper designed.

Spectrally-isolated violet to blue wavelength generation by cascaded degenerate four-wave mixing in a photonic crystal fiber.

Generation of spectrally-isolated wavelengths in the violet to blue region based on cascaded degenerate four-wave mixing (FWM) is experimentally demonstrated for the first time in a tailor-made photonic crystal fiber, which has two adjacent zero dispersion wavelengths (ZDWs) at 696 and 852 nm in the fundamental mode. The influences of the wavelength λp and the input average power Pav of the femtosecond pump pulses on the phase-matched frequency conversion process are studied. When femtosecond pump pulses at λp of 880, 870, and 860 nm and Pav of 500 mW are coupled into the normal dispersion region close to the second ZDW, the first anti-Stokes waves generated near the first ZDW act as a secondary pump for the next FWM process. The conversion efficiency ηas2 of the second anti-Stokes waves, which are generated at the violet to blue wavelengths of 430, 456, and 472 nm, are 4.8, 6.48, and 9.66%, for λp equalling 880, 870, and 860 nm, respectively.

On-chip integratable all-optical quantizer using strong cross-phase modulation in a silicon-organic hybrid slot waveguide.

High performance all-optical quantizer based on silicon waveguide is believed to have significant applications in photonic integratable optical communication links, optical interconnection networks, and real-time signal processing systems. In this paper, we propose an integratable all-optical quantizer for on-chip and low power consumption all-optical analog-to-digital converters. The quantization is realized by the strong cross-phase modulation and interference in a silicon-organic hybrid (SOH) slot waveguide based Mach-Zehnder interferometer. By carefully designing the dimension of the SOH waveguide, large nonlinear coefficients up to 16,000 and 18,069 W(-1)/m for the pump and probe signals can be obtained respectively, along with a low pulse walk-off parameter of 66.7 fs/mm, and all-normal dispersion in the wavelength regime considered. Simulation results show that the phase shift of the probe signal can reach 8π at a low pump pulse peak power of 206 mW and propagation length of 5 mm such that a 4-bit all-optical quantizer can be realized. The corresponding signal-to-noise ratio is 23.42 dB and effective number of bit is 3.89-bit.

CMOS-compatible 2-bit optical spectral quantization scheme using a silicon-nanocrystal-based horizontal slot waveguide.

All-optical analog-to-digital converters based on the third-order nonlinear effects in silicon waveguide are a promising candidate to overcome the limitation of electronic devices and are suitable for photonic integration. In this paper, a 2-bit optical spectral quantization scheme for on-chip all-optical analog-to-digital conversion is proposed. The proposed scheme is realized by filtering the broadened and split spectrum induced by the self-phase modulation effect in a silicon horizontal slot waveguide filled with silicon-nanocrystal. Nonlinear coefficient as high as 8708 W(-1)/m is obtained because of the tight mode confinement of the horizontal slot waveguide and the high nonlinear refractive index of the silicon-nanocrystal, which provides the enhanced nonlinear interaction and accordingly low power threshold. The results show that a required input peak power level less than 0.4 W can be achieved, along with the 1.98-bit effective-number-of-bit and Gray code output. The proposed scheme can find important applications in on-chip all-optical digital signal processing systems.

Resolution-enhanced all-optical analog-to-digital converter employing cascade optical quantization operation.

In this paper, a cascade optical quantization scheme is proposed to realize all-optical analog-to-digital converter with efficiently enhanced quantization resolution and achievable high analog bandwidth of larger than 20 GHz. Employing the cascade structure of an unbalanced Mach-zehnder modulator and a specially designed optical directional coupler, we predict the enhancement of number-of-bits can be up to 1.59-bit. Simulation results show that a 25 GHz RF signal is efficiently digitalized with the signal-to-noise ratio of 33.58 dB and effective-number-of-bits of 5.28-bit.

Efficient and broadband parametric wavelength conversion in a vertically etched silicon grating without dispersion engineering.

An efficient and broadband parametric wavelength converter is proposed in the silicon-on-insulator (SOI) waveguide without dispersion engineering. The vertical grating is utilized to achieve the quasi-phase-matching (QPM) of four-wave mixing (FWM). By alternating the phase-mismatch between two values with opposite signs, the parametric attenuation is suppressed. The conversion efficiency at the designated signal wavelength is significantly improved, and the 3-dB conversion bandwidth is also extended effectively. It is demonstrated that the conversion bandwidth is insensitive to both the propagation length and the grating width, which alleviates the tradeoff between the conversion bandwidth and the peak conversion efficiency. For a continuous-wave (CW) pump at 1550 nm, a conversion bandwidth of 331 nm and a peak efficiency of -12.8 dB can be realized in a 1.5-cm-long grating with serious phase-mismatch.