Generation Of Ultrashort Pulses Research Articles

Effect of gold-nanoparticle size on microfiber saturable absorber for mode-locked erbium-doped fiber lasers

The optical performance of passively mode-locked erbium-doped fiber laser incorporating gold-nanoparticles (Au-NPs) of different sizes as saturable absorbers (SAs) was compared. A composite of Au-NPs with polydimethylsiloxane was spin coated on the surface of a microfiber waveguide. The fabricated Au-NP-based SA was deployed in a ring cavity erbium-doped fiber laser and its lasing performance was evaluated by comparing SA characteristics as well as pulse qualities among the different Au-NP sizes. The optimum pulse performance was realized when the SA was fabricated with 20 and 40 nm Au-NP size. For the former, time-bandwidth product of 0.34 was demonstrated which was the closest to its bandwidth-limited pulse. As for the latter, the fabricated SA exhibited 4.0% modulation depth and an average pulse duration of 886.7 fs. The research work has demonstrated the functionality of Au-NPs as a saturable absorption material for ultrashort pulses generation. In addition, the influence of nanomaterial size towards the characteristics of saturable absorbers, ergo the quality of laser pulse, was verified. This finding is also valuable in customizing ultrashort pulse output that can effectively suit system performance need.

A novel scheme for ultrashort terahertz pulse generation over a gapless wide spectral range: Raman-resonance-enhanced four-wave mixing

Ultrashort energetic terahertz (THz) pulses have created an exciting new area of research on light interactions with matter. For material studies in small laboratories, widely tunable femtosecond THz pulses with peak field strength close to MV cm−1 are desired. Currently, they can be largely acquired by optical rectification and difference frequency generation in crystals without inversion symmetry. We describe in this paper a novel scheme of THz pulse generation with no frequency tuning gap based on Raman-resonance-enhanced four-wave mixing in centrosymmetric media, particularly diamond. We show that we could generate highly stable, few-cycle pulses with near-Gaussian spatial and temporal profiles and carrier frequency tunable from 5 to >20 THz. They had a stable and controllable carrier-envelop phase and carried ~15 nJ energy per pulse at 10 THz (with a peak field strength of ~1 MV cm−1 at focus) from a 0.5-mm-thick diamond. The measured THz pulse characteristics agreed well with theoretical predictions. Other merits of the scheme are discussed, including the possibility of improving the THz output energy to a much higher level.

Low-Noise Femtosecond SESAM Modelocked Diode-Pumped Cr:ZnS Oscillator

Cr-doped ZnS and ZnSe are excellent gain mediums for high power and broadband ultrashort pulse generation in the 2 – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3~\mu \text{m}$ </tex-math></inline-formula> wavelength range. SESAM modelocked Cr:ZnS oscillators have the advantage of reliable, self-starting passive modelocking. We present a diode-pumped, SESAM modelocked Cr:ZnS oscillator delivering ultrashort pulses of 189 fs at 550 mW average output power with a repetition rate of 435 MHz with low relative intensity noise (RIN) and timing jitter. We measured an integrated RIN of 0.05% within a frequency span of [10 Hz, 5 MHz] dominated by the 1560-nm pump diode, and a very low integrated timing jitter of 10.9 fs [2 kHz, 10 MHz]. This type of laser source benefits not only from very low noise but also from reduced complexity and cost due to direct diode pumping, which is suitable for many applications such as spectroscopy, ranging, and frequency conversion.

Spectral properties and high-efficiency broadband laser operation of Tm:CaY0.9Gd0.1AlO4 crystal

We report on spectral properties and a high-efficiency broadband laser operation of Tm:CaY0.9Gd0.1AlO4 (Tm:CYGA) crystal. The polarized absorption and emission spectra, gain cross sections and fluorescence lifetime are systematically studied, presenting a broad absorption and emission spectra and thus a smooth gain band. Furthermore, under an absorbed pump power of 4.35 W, a maximum continuous-wave (CW) laser output power of 1.43 W at 1967.8 nm is obtained, corresponding to a slope efficiency of 42.2 %. The beam quality factors (M2) in X-axis and Y-axis are measured to be 1.011 and 1.027, respectively. A tunable Tm:CYGA laser with a tuning range of 284.7 nm from 1837.5 to 2122.2 nm is achieved employing a birefringent filter (BF). The results indicate that the Tm:CYGA crystal is an excellent laser medium for generating high-efficiency broadband CW laser, indicating its potential for ultrashort pulse generation in the 2-μm spectral range.

Bright and dark modulated optical solitons for a (2+1)-dimensional optical Schrödinger system with third-order dispersion and nonlinearity

This work studies the dynamics of an extended nonlinear (2+1)-dimensional evolution Schrödinger equation having third-order nonlinearity and dispersion. We employ distinct efficient schemes to furnish analytical bright and dark soliton modulated solutions that can describe the propagation of ultra-short pulses in nonlinear dispersive mediums. We investigate the model parameters that admit appropriate constraint conditions on a variety of solutions. Further, we obtain other solitonic solutions such as exponential solutions, singular, periodic, and rational solutions. The work shows the relevant contribution of third-order dispersion and nonlinearity to the derived solutions of the examined Schrödinger system.

Self-starting coherent mode locking in a two-section laser with identical gain and absorber media

Coherent mode-locking (CML) represents a method for ultrashort pulse generation in lasers based on the self-induced transparency (SIT) phenomena, which allows us to overcome the principal limitations on the pulse duration imposed by the active medium's gain bandwidth in standard passively mode-locked lasers. So far, this regime was only studied in lasers with transition dipole moments in the absorber medium being twice larger than in the gain medium. This fact ensures that the $2\ensuremath{\pi}$-SIT-soliton in the absorber also forms the stable $\ensuremath{\pi}$-soliton in the gain but can be barely realized in experiments. In this paper we demonstrate theoretically that the self-starting stable coherent mode-locking regime can also arise if the same medium is used both in the gain and absorber laser sections, i.e., the transition dipole moments are equal. These results are both obtained analytically using the area theorem and confirmed by the numerical solution of the Maxwell-Bloch equations. Our findings can enable new opportunities in the area of ultrafast lasers.

Exceeding the Boundaries of the Paraxial Spatiotemporal Nonlinear Optics and Filamentation for Ultrashort Laser Pulses.

An impressive phenomenon of observed plasma instability and conical emission under the propagation of ultrashort laser pulses in the air is reported. The discussed novel findings demonstrating nonlinear effects are incapable to be explained in the standard spatiotemporal paraxial optics. Three main mechanisms are investigated. The first one is related to the nonlinear nonparaxial mechanisms for waveguiding of femtosecond pulses, and the second one considers the mechanism of single filament formation at weak ionization. The third mechanism demonstrates a new physical effect leading to collision ionization with intensities in the range of 1010-1011 W/cm2. Furthermore, a new ionization regime of instability is suggested at intensities below the critical thresholds for multiphoton and tunnel ionization. The experimental results and findings are supported by theoretical analyses and numerical simulations.

"Stealth Scripts": Ultrashort Pulse Laser Luminescent Microscale Encoding of Bulk Diamonds via Ultrafast Multi-Scale Atomistic Structural Transformations.

The ultrashort-laser photoexcitation and structural modification of buried atomistic optical impurity centers in crystalline diamonds are the key enabling processes in the fabrication of ultrasensitive robust spectroscopic probes of electrical, magnetic, stress, temperature fields, and single-photon nanophotonic devices, as well as in "stealth" luminescent nano/microscale encoding in natural diamonds for their commercial tracing. Despite recent remarkable advances in ultrashort-laser predetermined generation of primitive optical centers in diamonds even on the single-center level, the underlying multi-scale basic processes, rather similar to other semiconductors and dielectrics, are almost uncovered due to the multitude of the involved multi-scale ultrafast and spatially inhomogeneous optical, electronic, thermal, and structural elementary events. We enlighten non-linear wavelength-, polarization-, intensity-, pulsewidth-, and focusing-dependent photoexcitation and energy deposition mechanisms in diamonds, coupled to the propagation of ultrashort laser pulses and ultrafast off-focus energy transport by electron-hole plasma, transient plasma- and hot-phonon-induced stress generation and the resulting variety of diverse structural atomistic modifications in the diamond lattice. Our findings pave the way for new forthcoming groundbreaking experiments and comprehensive enlightening two-temperature and/or atomistic modeling both in diamonds and other semiconductor/dielectric materials, as well as innovative technological breakthroughs in the field of single-photon source fabrication and "stealth" luminescent nano/microencoding in bulk diamonds for their commercial tracing.

Numerical Simulation of the Nonlinear Propagation of Ultrashort Pulses in Fiber Amplifiers Using Time-Frequency Representations

To predict the behavior of high power pulsed lasers, fine numerical simulations of nonlinear propagation of short chirped pulses in fiber amplifiers are needed. In particular, the gain used in the nonlinear Schrödinger equation (NLSE) should take into account the simultaneous dependence of the gain with frequency (i.e. gain dispersion) and time (i.e. gain saturation). We propose here a method using a time-frequency representation (TFR) of the pulse to compute the gain that can be used in the NLSE. Basic TFRs can be computed by estimating the group delay or the instantaneous frequency of the pulse but the low numerical complexity of these methods introduce some limitations in term of cases that can be properly simulated. We show that the more time-consuming computation of the reassigned spectrogram (RS) of the pulse allows to obtain a gain more suitable for general cases than can be encountered in high energy ultrafast lasers.

Demonstration of multiple dissipative solitons with nickel-based metal organic framework saturable absorber

In this work, we have demonstrated the generation of dissipative soliton from single to multi-pulsing phenomenon. The generation of ultrashort pulses was achieved with nickel-based metal organic framework (Ni-MOF) as saturable absorber in a ring cavity erbium-doped fiber laser. The SA was fabricated by spin-coating Ni-MOF/polydimethylsiloxane composite on a tapered fiber substrate, which had an insertion loss of 2.82 dB, a modulation depth of 4.61 %, and saturation intensity of 30.12 MW/cm2. A self-started DS mode-locked pulse was observed at 1559.89 nm central wavelength with 3-dB bandwidth of 19.72 nm at threshold pump power of 33.5 mW. Its pulse width of 7.89 ps was recorded at 7.94 MHz repetition rate. The nature of pulse splitting was observed up to three pulses which were separated with 2.62 ns time spacing with the increment of pump power until 134.2 mW. The generated multiple dissipative soliton pulses realized in the laser cavity through Ni-MOF saturable absorber were stable and invariant for 1-hour stability measurement.

SiO2 Passivated Graphene Saturable Absorber Mirrors for Ultrashort Pulse Generation.

Owing to its broadband absorption, ultrafast recovery time, and excellent saturable absorption feature, graphene has been recognized as one of the best candidates as a high-performance saturable absorber (SA). However, the low absorption efficiency and reduced modulation depth severely limit the application of graphene-based SA in ultrafast fiber lasers. In this paper, a single-layer graphene saturable absorber mirror (SG-SAM) was coated by a quarter-wave SiO2 passivated layer, and a significantly enhanced modulation depth and reduced saturation intensity were obtained simultaneously compared to the SG-SAM without the SiO2 coating layer. In addition, long-term operational stability was found in the device due to the excellent isolation and protection of the graphene absorption layer from the external environment by the SiO2 layer. The high performance of the SAM was further confirmed by the construction of a ring-cavity EDF laser generating mode-locked pulses with a central wavelength of 1563.7 nm, a repetition rate of 34.17 MHz, and a pulse width of 830 fs.

Ultrashort spin–orbit torque generated by femtosecond laser pulses

To realize the very objective of spintronics, namely the development of ultra-high frequency and energy-efficient electronic devices, an ultrafast and scalable approach to switch magnetic bits is required. Magnetization switching with spin currents generated by the spin–orbit interaction at ferromagnetic/non-magnetic interfaces is one of such scalable approaches, where the ultimate switching speed is limited by the Larmor precession frequency. Understanding the magnetization precession dynamics induced by spin–orbit torques (SOTs) is therefore of great importance. Here we demonstrate generation of ultrashort SOT pulses that excite Larmor precession at an epitaxial Fe/GaAs interface by converting femtosecond laser pulses into high-amplitude current pulses in an electrically biased p-i-n photodiode. We control the polarity, amplitude, and duration of the current pulses and, most importantly, also their propagation direction with respect to the crystal orientation. The SOT origin of the excited Larmor precession was revealed by a detailed analysis of the precession phase and amplitude at different experimental conditions.

Explicit solutions from Darboux transformation for the two-component nonlocal Hirota and Maxwell–Bloch system

Based on the two-component Hirota and Maxwell–Bloch (H-MB) system, which describes the propagation of ultra-short pulses in erbium-doped nonlinear optical fibers, we derive two kinds of two-component nonlocal H-MB (NH-MB) systems by choosing suitable symmetries, namely [Formula: see text]-symmetric two-component NH-MB system and reverse space-time two-component NH-MB system. Then the explicit solutions of these two systems are obtained by the Darboux transformation.

Optical soliton solutions of (1 + 1)- and (2 + 1)-dimensional generalized Sasa–Satsuma equations using new Kudryashov method

In this paper, we aim to derive new soliton solutions of (1+1)- and (2+1)-dimensional generalized Sasa–Satsuma equations via the new Kudryashov method. In optical fiber transmission systems, the Sasa–Satsuma equation describes the effects of third-order dispersion, self-steepening and stimulated Raman scattering in the propagation of ultrafast pulses. The considered equations are encountered in various physical applications such as ultra-short and femto-second pulse propagation in optical fibers and dynamics of deep water waves. So, investigation of the novel solutions of the equations is one of the important topics. We have successfully extracted some soliton solutions for the considered equation. The various graphs of the obtained solutions have been depicted in the figures by selecting appropriate parameters. The singular and bright soliton solutions have been revealed in the figures. All acquired solutions have been confirmed to satisfy the considered equations. The results show that the approach may be used to find exact solutions to various nonlinear evolution equations. The new solutions and the paper results may enrich the understanding of the wave propagation in the optical fibers and may shed light on new studies.

Physics-Informed Neural Network for Optical Fiber Parameter Estimation From the Nonlinear Schrödinger Equation

For any system that follows rigorous mathematical and physical theories like fiber-optic system, system parameter estimation is crucial for system detection and monitoring. In this paper, a physics-informed neural network (PINN)-based method is proposed for optical fiber parameter estimation by solving the inverse problem of the nonlinear Schrödinger equation (NLSE), i.e., inferring the unknown parameters of NLSE. The proposed method considers both the prior knowledge of physical law and the characteristics of observation data from only the transmitted and received waveforms. In PINN, the NLSE and observation data are set as the loss function, and the typical fiber parameters (attenuation, dispersion, and nonlinear coefficient) are optimized iteratively until they satisfy the inferred NLSE by learning the limited observation data. Three scenarios are validated, including single Gaussian pulse propagation, ultrashort pulse propagation in highly nonlinear fiber, and optical signal transmission in standard single-mode fiber. The launch power of the signal is embedded in PINN as a parametric feature, which makes PINN learn the signal transmissions under different powers simultaneously. The weights of mean square error terms for NLSE and observation data in the loss function are properly adjusted to balance learning difficulties. Moreover, the statistical results of 128 different signals and model performance under different signal-to-noise ratios are analyzed. As an extension, fiber length estimation is also studied, and the error is 0.375% when the launch power is 0 dBm. Our work verifiably shows that the proposed approach performs well and can be further extended to multiple applications in fiber optics.

Optical Control of High-Harmonic Generation at the Atomic Thickness.

High-harmonic generation (HHG), an extreme nonlinear optical phenomenon beyond the perturbation regime, is of great significance for various potential applications, such as high-energy ultrashort pulse generation with outstanding spatiotemporal coherence. However, efficient active control of HHG is still challenging due to the weak light-matter interaction displayed by currently known materials. Here, we demonstrate optically controlled HHG in monolayer semiconductors via the engineering of interband polarization. We find that HHG can be efficiently controlled in the excitonic spectral region with modulation depths up to 95% and ultrafast response speeds of several picoseconds. Quantitative time-domain theory of the nonlinear optical susceptibilities in monolayer semiconductors further corroborates these experimental observations. Our demonstration not only offers an in-depth understanding of HHG but also provides an effective approach toward active optical devices for strong-field physics and extreme nonlinear optics.

The way to generating sub-femtosecond pulse trains with high-order stimulated Raman scatterings

The generation of ultrashort pulses is the key to exploring the dynamic behaviors of matter on ever-shorter timescales. Sub-femtosecond pulses can be used to explore atomic-scale electron motion which can reveal the nature of changes in chemical reactions and the function of biological systems. Recent developments have shown that sub-2-fs pulse trains can be produced in an experiment through molecular modulation. In this paper, we experimentally demonstrate the potential of yielding sub-femtosecond pulse trains with eight or more high-order stimulated Raman scattering components; and report the current bottleneck of generating sub-femtosecond pulse trains via molecular modulation.

Measurement of dispersion and index of refraction of 1-decanol with spectrally resolved white light interferometry.

The high density, high nonlinearity, and stability of liquids make them an attractive medium for spectral broadening and supercontinuum generation in ultrafast experiments. To understand ultrashort pulse propagation in these media, their indices of refraction and dispersions must be characterized. We employ a Mach-Zehnder interferometer to generate a series of interferograms, which we refer to as a spectrogram, to develop a new method of using spectrally resolved white light interferometry to determine the refractive indices of materials. We determine the indices of refraction of BK7, sapphire, ethanol, and 1-decanol at 24°C across the visible and near infrared. To our knowledge, this is the first reported dispersion and index of refraction measurement of 1-decanol.

High coherent supercontinuum generation in nitrobenzene liquid-core photonic crystal fiber with elliptical air-hole inner ring

A nitrobenzene liquid-core photonic crystal fiber (NLC-PCF) with eight elliptical air-holes in the innermost ring of the cladding is proposed in this work, and this fiber structure can contribute to flexibly adjusting dispersion and constraining the fiber core. The appreciable dispersion is realized by adjusting the structural parameters of NLC-PCF, which is characterized by three zero-dispersion wavelengths (ZDWs), flat dispersion with the fluctuation of fewer than 40 ps nm−1 km−1, and high nonlinearity as high as 5500 W−1 km−1. The propagation of femtosecond pulse and supercontinuum generation (SCG) in NLC-PCF is studied numerically when the pump wavelength is located in the normal and abnormal dispersion region near different ZDW through solving the generalized nonlinear Schrödinger equation (GNLSE) by the split-step Fourier method. The numerical results show that a highly coherent supercontinuum (SC) spanning from 1.3 to 2.8 µm is obtained when the pump pulse with the center wavelength of 1810 nm, the peak power of 1000 W, and pulse width of 50 fs propagated in the 5 cm long NLC-PCF. This research can find applications in the fields of novel liquid-core PCF design and ultrashort pulse propagation.

Generation of High Power Ultrashort Pulses in Tapered Yb-Doped PCF Through Self-Similar Compression

We present a numerical approach for analysing simultaneous amplification and compression of a chirped hyperbolic secant soliton using a tapered Yb-doped photonic crystal fiber (PCF) based self-similar compressor. Theoretical studies reveal the conservation of input soliton during the amplification process due to self-similar property, which in turn helps for high quality compression of amplified soliton. In order to satisfy the self-similar condition, we note that the Yb-doped fiber should exhibit a decreasing group velocity dispersion (GVD). Such a constraint is met using a tapered PCF in our work, which is modeled using a fully vectorial effective index method (FVEIM). Influence of spatial and spectral dependent gain, saturation energy, amplified spontaneous emission (ASE), dispersion and nonlinearity on pulse propagation are analyzed using the combined solution of dynamic rate equations (DRE) and the generalized nonlinear Schrödinger equation (GNLSE). Numerical outcomes upon solving the aforementioned equations show that a 1 ps pulse is compressed down to a pulsewidth of 55.4 fs with a peak power of 1.03 kW in a total fiber length of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$L=2.5$ </tex-math></inline-formula> m and taper ratio of 2.34. Overall quality of the compression scheme is evidenced by a relatively high compression factor (F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>c</i></sub> ) = 18.12, and low percentage of pedestal energy ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$PE$ </tex-math></inline-formula> ) = 10.95 %