Generation Of Ultrashort Pulses Research Articles

Second-harmonic generation of ultrashort pulses in refractive index linearly modulating quantum dot structure

While other works use segments of nonlinear crystal to compensate for walk-off, this work utilizes a quantum dot (QD) nanostructure for such compensation. Such nanocrystal structures allow two benefits: the high nonlinear susceptibility in these nanocrystals and the high surface density of QDs in the structure. Good results are obtained for pre-chirping and modulation parameters. The results are computed at 10 and 100 kV/cm applied electric fields. Compared to ordinary nonlinear crystals, the pulse width, intensity, and efficiency of the second harmonic (SH) pulse are all higher in QD nanocrystals, even with a low electric field applied. Higher applied fields give higher results.

Transverse mode switchable mode-locked laser with narrow bandwidth

Transverse mode switchable ultrashort optical pulses with narrow bandwidths can create potential for exploring what we believe are new physical effects. We demonstrate the generation of transverse mode switchable ultrashort pulses with narrow bandwidths in an all-fiber mode-locked laser by exploring a mode-selective photonic lantern (MSPL). The laser cavity serves not only as a ring resonator but also as an intrinsic spectral filter. For mode-locking with the LP01, LP11a, and LP11b modes, the bandwidths are 3.0 nm, 86.7 pm and 101.7 pm, respectively. The narrowband pulses with higher-order modes are generated by an intrinsic spectral filter due to the spectral-domain intermodal interference. Mode-locked pulses with a signal-to-noise ratio better than 60 dB for LP01, LP11a, and LP11b modes are independently generated, i.e., transverse mode switchable by changing the input port of the MSPL. The mode-locked wavelength can be tuned for the LP11a mode and LP11b mode by adjusting the state of polarization. Furthermore, our experimental results also show that, the slope efficiency of LP11a and LP11b modes can be improved, by the use of LP11 mode pump scheme. We anticipate that, narrowband pulses with complex mode profiles can be generated by simultaneously phase-locked transverse and longitudinal modes.

Generation of soliton molecules in a 1.5-μm ultrafast fiber laser based on Zinc 2,5-dihydroxyterephthalate saturable absorber

Two-dimensional (2D) Zinc 2,5-dihydroxyterephthalate (Zn-MOF-74) is a kind of metal–organic framework (MOF), which has been intensively studied in gas adsorption, gas storage, and catalysis. However, the potential applications of Zn-MOF-74 in the fields of optics, especially in the areas of nonlinear optical response and ultrafast optics need further exploration. Here, the nonlinear optical properties of Zn-MOF-74 are systematically investigated. In this work, we demonstrate that it gradually becomes saturated with the increase of the intensity of the incident pulse, which makes Zn-MOF-74 an effective saturable absorber in a mode-locked Er-doped fiber laser for ultrashort pulse generation. Our experiments yield stable single solitons as well as soliton molecules with tunable characteristics. When the pump power is 126 mW, conventional solitons with an output power of 2.35 mW and a pulse width of 0.93 ps are obtained. The generated pulses characterize a central wavelength of 1554.84 nm and a 3-dB bandwidth of 3.07 nm. By adjusting the pump power and polarization appropriately, the center wavelength of the soliton molecules is tunable from 1532.76 nm to 1554.24 nm. The experimental observations further proves that Zn-MOF-74 has great potential in ultrafast optics.

Crystal growth, polarized spectral properties, and 2.3 µm laser performance of Tm:LuVO4 crystal

We report on the Czochralski crystal growth, polarized optical spectroscopy, and the first continuous-wave laser operation of 1.5 at.% Tm:LuVO4 crystal on the 3H4 → 3H5 transition. The polarized absorption and stimulated-emission properties of Tm3+ ions in LuVO4 were revised and the crystal-field splitting of the Tm3+ multiplets was determined by low-temperature (12 K) spectroscopy. The maximum stimulated-emission cross-section for the 3H4 → 3H5 transition is 2.48 × 10−20 cm2 at 2363 nm for π-polarization corresponding to an emission bandwidth of 28 nm. Evidence of phonon-assisted emissions of Tm3+ ions above 2 µm is presented. The broadband emission properties of the Tm:LuVO4 crystal make it promising for ultrashort pulse generation. Additionally, pumped by a 796 nm fiber-coupled laser diode, the Tm:LuVO4 laser generated a Watt-level output power at 2279-2295 nm with a slope efficiency of 9.2% and linearly polarized emission (π-polarization).

Frequency Сonversion of Femtosecond Ti:sapphire Laser Pulse to the Long-wave Mid-IR Range with BaGa2GeSe6 Crystal

Generation of ultrashort mid-IR pulses spanning from 8.5 to 10.5 μm with high energy (up to 4.5 μJ) was experimentally demonstrated through difference frequency generation in BaGa2GeSe6 crystal pumped by 100-fs 0.95-μm Ti:sapphire laser pulses. Optical damage threshold and two-photon absorption coefficient were determined for this pump pulses. Frequency conversion efficiency reached 0.24% at 1.85 mJ pump pulse energy and was decreased at higher one. Estimations indicate that application of 15 mm in diameter wide-aperture BaGa2GeSe6 sample will allow one to increase pump pulse energy up to ~10 mJ, and to increase the mid-IR pulse energy up to 24 μJ at the same efficiency.

Exploring the Dynamics of Dark and Singular Solitons in Optical Fibers Using Extended Rational Sinh–Cosh and Sine–Cosine Methods

Exploring the Dynamics of Dark and Singular Solitons in Optical Fibers Using Extended Rational Sinh–Cosh and Sine–Cosine Methods

Analytical solution to second-harmonic generation of ultrashort laser pulse

We present an analytical method to evaluate the second-harmonic generation (SHG) process of ultrashort laser pulses in a nonlinear medium within the small-signal regime. Building upon the broadband nonlinear coupled wave theory, we derive an analytical solution that allows one to investigate the fundamental physics and detailed dynamics of the nonlinear interaction between ultrashort laser pulses. We find that SHG for ultrashort laser pulses in a nonlinear crystal arises from a series of self-SHG and self sum-frequency generation processes involving different frequency components. We have applied the analytical method to investigate specific pulse profiles with Gaussian and sech functions and found good agreement between analytical theory and numerical simulations, validating the accuracy of the analytical approach. The analytical method enables one to explore the evolution of the second-harmonic wave under various pulse profiles, pulse widths, and crystal thicknesses, and offers a more efficient and insightful approach to study the nonlinear optical dynamics of ultrashort laser pulses.

Ultrafast nonlinear optical response of layered violet phosphorus for femtosecond noise-like pulse generation

As a new member of two-dimensional (2D) phosphorene, 2D layered violet phosphorus (VP) has unique optoelectronic properties and good environmental stability, showing its huge advantages in optoelectronic applications. In this paper, the ultrafast nonlinear optical (NLO) properties of layered VP nanosheets at 1 µm band were explored, which exhibit an obvious saturable absorption response with a modulation depth of ∼1.97%. Meanwhile, the fast and slow carrier lifetimes of VP nanosheets at 1µm band were also determined as 295.9 fs and 2.36 ps, respectively, which are much shorter than that of most reported 2D materials. The excellent saturable absorption response combined with ultrashort carrier lifetimes indicate the prospect of layered VP nanosheets as a fast saturable absorber (SA) for ultrafast laser modulation. Then we demonstrated a Yb-doped fiber laser based on the VP-deposited taper-shaped fiber (TSF) SA, which delivers stable Q-switched mode-locked (QSML) pulses, dual-wavelength mode-locked pulses and 404-fs noise-like pulses. This work fully demonstrates the great potential of 2D VP materials for 1 µm ultrashort laser pulse generation.

Structural, spectroscopi c and laser properties of transparent Tm:YScO3 ceramic based on gas-phase synthesized nanoparticles

Owing to the combination of good thermo-optical properties and inhomogeneously broadened fluorescence spectra suitable for ultrashort laser pulse generation, rare-earth doped solid solutions of sesquioxides (Y,Lu,Sc)2O3 have attracted considerable interest in recent years. In this study, compositionally disordered Tm3+-doped YScO3 transparent ceramic was fabricated using solid-state vacuum sintering of nanopowder produced using laser ablation of solid target in air flow. The as-synthesized powder consisted of weakly aggregated spherical particles having 20 nm diameter and a mostly monoclinic (space group C2/m) crystal structure, whose symmetry was transformed into cubic (space group Ia-3) by calcination at 1050 °C. From high resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray analysis (EDX), we observed a homogeneous distribution of components both throughout individual nanoparticles and 100-nm-thick ceramic foil resulting in high structural and optical quality of the obtained samples. The sintered ceramic featured an optical transmission of 77% at 600 nm and above 81% near 2 μm, an average grain size of 24.2 μm and a content of scattering centers of 26.1 ppm. The absorption spectra at several temperatures, from 93 K to 293 K, were measured and the corresponding absorption coefficients were obtained by Lambert-Beer formula in the range from 600 nm to 900 nm and from 1400 nm to 2100 nm. In particular, the room temperature absorption cross section at 794 nm and 1634 nm were determined to be σabs = 4.25 × 10−21 cm2 and σabs = 5.01 × 10−21 cm2 respectively. Concerning the emission cross section it was calculated by Fuchtbauer-Ladenburg equation starting from the fluorescence spectrum acquired at the laser work temperature: at 1951 nm and 2106 nm corresponding to the main two emission peaks belonging the curve, it was found σem = 9.2 × 10−21 cm2 and σem = 4.9 × 10−21 cm2, respectively. Finally, it was tested the laser behavior of the ceramic: pumped at 793 nm in quasi-CW mode it delivered 1.27 W of output power with a slope efficiency of 10.33% at 2081 nm. The tunability curve spans from 1942 nm to 2110 nm (i.e. 168 nm).

Cr2AlB2/PbS heterostructure as saturable absorber for ultrashort pulse generation

Rapid progress has been realized in the nonlinear optical device applications of 2D materials. 2D materials are attractive due to their characteristics optical nonlinearities and distinctive light-matter interactions, which assure them promising nonlinear optical devices for ultrashort pulses generating. Cr2AlB2, a member of the new series of 2D materials known as MBenes, has been predicted to have potential saturable absorption properties. In this study, the saturable absorption properties of Cr2AlB2 and Cr2AlB2/PbS vdW heterostructure SAs are investigated. In comparison with Cr2AlB2, the Cr2AlB2/PbS vdW heterostructure exhibits better saturable absorption. The enhancements of nonlinear optical properties can be attributed to the carrier migration between heterostructure. The simulation results based on the Ginzburg-Landu equation are employed to predict the laser performances of Cr2AlB2 and Cr2AlB2/PbS heterostructure SA. By incorporating SAs in the cavity, highly stable conventional solitons were observed. Both simulation and experimental results demonstrate that Cr2AlB2/PbS vdW heterostructure has superior pulse performance compared to the Cr2AlB2 SA. These results demonstrate that Cr2AlB2/PbS vdW heterostructure processes better saturable absorption properties and serves as a promising SA in ultrashort pulse generation.

Intrinsic free carrier absorption limited THz generation from Bi2Te3 and Bi2Se3 topological insulators

We investigate the influence of intrinsic free charge carriers on the ultrashort THz pulse generation efficiency of three-dimensional topological insulators (3dTIs). Wavelength dependence of the optical penetration depth of the femtosecond excitation pulses is exploited to vary the excitation volume within Bi2Te3 and Bi2Se3 crystals and accordingly, the free carrier population, to contribute to THz attenuation. The standard free carrier absorption (FCA) formalism is inadequate to explain wavelength-dependent enhancement in the THz emission as observed in the experiments. Within a modified framework, the THz attenuation by FCA is accounted for accurately, which consistently explains the experimental results for samples having different carrier density and mobility. We conclude that the THz generation efficiency of 3dTIs can be enhanced by engineering samples with high carrier mobility and low intrinsic carrier density and by using excitation wavelengths of minimal penetration depth.

General framework for ultrafast nonlinear photonics: unifying single and multi-envelope treatments [Invited

Numerical modeling of ultrashort pulse propagation is important for designing and understanding the underlying dynamical processes in devices that take advantage of highly nonlinear interactions in dispersion-engineered optical waveguides. Once the spectral bandwidth reaches an octave or more, multiple types of nonlinear polarization terms can drive individual optical frequencies. This issue is particularly prominent in χ(2) devices where all harmonics of the input pulse are generated and there can be extensive spectral overlap between them. Single-envelope approaches to pulse propagation have been developed to address these complexities; this has led to a significant mismatch between the strategies used to analyze moderate-bandwidth devices (usually involving multi-envelope models) and those used to analyze octave-spanning devices (usually involving models with one envelope per waveguide mode). Here we unify the different strategies by developing a common framework, applicable to any optical bandwidth, that allows for a side-by-side comparison between single- and multi-envelope models. We include both χ(2) and χ(3) interactions in these models, with emphasis on χ(2) interactions. We show a detailed example based on recent supercontinuum generation experiments in a thin-film LiNbO3 on sapphire quasi-phase-matching waveguide. Our simulations of this device show good agreement between single- and multi-envelope models in terms of the frequency comb properties of the electric field, even for multi-octave-spanning spectra. Building on this finding, we explore how the multi-envelope approach can be used to develop reduced models that help build physical insights about new ultrafast photonics devices enabled by modern dispersion-engineered waveguides, and discuss practical considerations for the choice of such models. More broadly, we give guidelines on the pros and cons of the different modeling strategies in the context of device design, numerical efficiency, and accuracy of the simulations.

Sub-60-fs mode-locked Tm,Ho:CaYLuAlO4 laser at 2.05 µm

Tm,Ho:CaYLuAlO4 (Tm,Ho:CALYLO) crystal has wide emission spectra both for π-polarization and σ-polarization, showing significant potential for the generation of ultrashort pulses. Here, a widely tunable and passively mode-locked laser operation based on Tm,Ho:CALYLO crystal under two polarizations was demonstrated for what we believe to be the first time ever. For π-polarization, a maximum output power of 1.52 W and a tuning range of 255.3 nm were achieved in the continuous wave (CW) regime. In the mode-locked regime, a pulse duration of 68 fs and an average output power of 228 mW were achieved upon GaSb-based semiconductor saturable absorber mirror (SESAM). As for σ-polarization, a broader tuning range of 267.1 nm was realized, leading to the shorter pulse duration of 58 fs at 79.7 MHz repetition rate.

Stable Ultrashort Pulse Generation Using a Coiled and Tapered Few-Mode-Fiber Polarization Controller as a Saturable Absorber

Stable Ultrashort Pulse Generation Using a Coiled and Tapered Few-Mode-Fiber Polarization Controller as a Saturable Absorber

Intermodal effective phase matching for second harmonic generation of ultrashort pulses in high index contrast optical fibers

Second harmonic generation (SHG) in glass optical fibers calls for creating a second order susceptibility in the fiber glass and for achieving phase matching between the pump and the second harmonic signal. The latter is very challenging when using ultrashort pulses, given that the group velocities of the pump and the second harmonic should also be matched. We have shown in previous work that it is possible to achieve simultaneous modal phase matching (MPM) and group velocity matching (GVM) when the pump and the second harmonic are propagating in the LP01 and LP02 modes, respectively, in high GeO2-content double-clad optical fibers. However, simultaneous MPM and GVM can only be obtained in optical fibers with dedicated designs and within very tight geometrical tolerances. In this paper, we show that instead of considering the matching of phase and group velocities separately, we can consider a more general or “effective” phase matching approach, in which we consider all the dispersion terms up to the second order in the expressions of the propagation constants of the pump and second harmonic signals. This allows introducing the pulse duration as a controllable parameter that helps to enforce the said effective phase matching in fibers with designs that deviate by as much as 10% from the target, while providing for temporal walk-off lengths in excess of several centimeters. The impact of this finding goes beyond SHG only and can be applied to other ultrashort laser pulse-based nonlinear optical processes in fibers and waveguides.

FURION: modeling of FEL pulses propagation in dispersive soft X-ray beamline systems.

Modern X-ray free-electron lasers (XFELs) can generate pulses with durations ranging from femtoseconds to attoseconds. The numerical evaluation of ultra-short XFEL pulses through beamline systems is a critical process of beamline system design. However, the bandwidth of such ultra-short XFEL pulses is often non-negligible, and the propagation cannot be simply approximated using the central wavelength, especially in dispersive beamline systems. We developed a numerical model which is called Fourier optics based Ultrashort x-Ray pulse propagatION tool (FURION). This model can not only be used to simulate dispersive beamline systems but also to evaluate non-dispersive beamline systems. The FURION model utilizes Fresnel integral and angular spectrum integral to perform ultra-short XFEL pulse propagation in free space. We also present the method for XFEL pulse propagation through different types of dispersive gratings, which are commonly used in soft X-ray beamline systems. By using FURION, a start-to-end simulation of the FEL-1 beamline system at Shenzhen superconducting soft X-ray free electron laser (S3FEL) is carried out. This model can also be used to evaluate gratings-based spectrometers, beam splitters, pulse compressors, and pulse stretchers. This work provides valuable insights into the start-to-end simulation of X-ray beamline systems.

Anomalous spectral shift of localized surface plasmon resonance.

We report the first observation of spectral blue shift of plasmon resonance of synthesized silver nanoparticles (AgNPs) due to a negative optical nonlinearity of a local ambient medium, i.e., indigo carmine (IC) solution at around 420 nm wavelength. The blue shift occurred at a larger concentration of AgNPs or at a larger concentration of IC solution, being in obvious contrast to spectral red shift which was widely witnessed in plasmon spectral shift in a linear regime. Plasmon-enhanced local fields could excite the third-order optical nonlinearity for blue shift even under continuous (non-pulsed) light illumination. We also found that the plasmon-excited nonlinearity could allow for differential nonlinear response of the IC solution to be even greater than its differential linear response, though appearing to be somewhat inconsistent with what was generally known in light-matter interaction. The demonstrated properties of such anomalous shift of plasmon spectral peaks and its accompanying properties indicated that plasmon technologies could be exploited not only in linear but also in nonlinear aspects for critical optimization in plasmon-energy harvesting systems such as in surface enhanced spectroscopy/microscopy, biomedical imaging/sensing, laser frequency conversion, ultrashort pulse generation, and all-optical switching.

Numerous optical soliton solutions of the Triki–Biswas model arising in optical fiber

A model condition for the propagation of ultrashort pulses in optical fiber frameworks could be established through the extensive generalization of the nonlinear Schrödinger equation introduced by Triki and Biswas. This study uses two significant and effective trustworthy methods to examine the Triki–Biswas (TB) equation. Sardar Subequation Methods (SSM) and the generalized Kudryashov (KUD) method generate solutions for trigonometric, hyperbolic, exponential, and rational functions. These approaches are specifically designed for handling solitary wave patterns, dark-singular-mixed solitons, combined dark-bright solitons, singletons, bright solitons, exponential and rational functions, as well as periodic solitons. All of these solution classes contribute to the physical dynamics of outcomes. These findings represent an innovative extension of the soliton domain within the TB model and are being reported for the first time in our investigation. In addition, Mathematica simulations are used to display the 3D and 2D graphs to explain the identified solutions’ physical dynamics.

Propagation of electromagnetic pulses with nonzero area in dissipative media

The propagation of ultrashort electromagnetic pulses with a nonzero electric area in dielectric and conducting media is studied in the frame of a unidirectional propagation model. General solutions for the electric pulse area are obtained for different types of media with a linear response to the external field. It is shown that the evolution of the electric area of the pulse is dramatically different for conducting and non-conducting media. In the case of dielectrics, where the current induced by an external field arises from the polarization of bound electrons, the electric pulse area is an invariant of pulse propagation in spite of the dissipation process. For media with free charge carriers (plasma or semiconductors), the electric pulse area decreases with time due to Joule heating of the media by the static component of the field.

Characterization of the nonlinear optical properties of Zn and Ag nanoparticles using light at 1550 nm

Nonlinear optical properties of metallic and oxide metallic nanoparticles have been extensively studied as saturable absorbers in fiber-optic lasers in the infrared region for the generation of short and/or ultrashort pulses. In this paper, we report an experimental study of the nonlinear optical response of silver and zinc nanoparticles for continuous-wave illumination at 1550 nm. Nonlinear absorption was investigated using the power-dependent transmission and Z-scan techniques. The results obtained from the first technique showed that the zinc and silver particles behave like saturable absorbers, the modulation depths obtained were 15 % and 32 %, respectively. The Z-scan technique showed the presence of nonlinear refraction and absorption in both samples, the open aperture configuration confirmed the behavior of the materials as saturable absorbers. Additionally, the nonlocality of the nonlinear response was analyzed using a numerical model.