Flat Dispersion Profile Research Articles

Design and analysis of the C6H6-filled circular photonic crystal fiber with ultra-flat dispersion for broadband supercontinuum generation with peak power of 130 W and 250 W

This article introduces a new model of a circular silica-based photonic crystal fiber with a hollow core filled with C6H6. The difference in the air hole size and the distance between them in the first ring around the core has a profound effect on the dispersion, leading to ultra-flat dispersion with values as low as ±0.996 ps nm−1· km in wavelength range 0.74 µm. The high nonlinear coefficient of several 1000 W−1 · km−1 and the low confinement loss of a few tens of dB m−1 suggest proposing three fibers with dispersion and nonlinear properties suitable for broadband supercontinuum generation at low peak power. The influence of peak power on the broadening of the supercontinuum spectrum is also investigated. Fibers with a flat all-normal dispersion profile provide a smooth spectrum with bandwidths of 1.215 and 1.626 µm at 30 dB with a peak power of 250 W. A fiber with an anomalous dispersion regime generates a supercontinuous spectrum, broadening to 3.868 µm in the mid-infrared region (2.467 µm bandwidth at 30 dB) under laser pulse excitation with 130 W peak power. Our results provide further insights into the generation of broadband mid-infrared supercontinuum using liquid-core silica-fibers, which have great potential for applications in the fields of optical communications and optical sensing.

Supercontinuum Generation in Dispersion Engineered Highly Doped Silica Glass Waveguides

AbstractThe effect of a lower index oxide layer inclusion within a highly doped silica glass slot waveguide is investigated for optimized supercontinuum generation at telecom wavelengths. By controlling the thickness of the oxide slot, it is demonstrated that one can engineer the waveguide dispersion profile in order to obtain supercontinua with vastly different spectral broadening dynamics and bandwidths. Using this approach, a waveguide with a low and flat dispersion profile of less than 43 across a wavelength range spanning over 1000 nm is designed and fabricated. It is shown that, when pumped at the telecom C‐band, a supercontinuum that spans over 1.5 octaves can be generated from 817 to 2183 nm. The numerical simulations, whose parameters are derived from the measured waveguide dimension and material indices, exhibit good agreement with experimental measurements, where one can observe both a qualitative and quantitative match in the supercontinuum overall spectrum and specific features (e.g., soliton and dispersive wave locations). This study represents an important step forward in the control and manipulation of dispersive and nonlinear dynamics in highly doped silica glass waveguides, paving the way toward advanced on‐chip broadband light manipulation.

Star cluster progenitors are dynamically decoupled from their parent molecular clouds

ABSTRACT The formation of stellar clusters dictates the pace at which galaxies evolve, and solving the question of their formation will undoubtedly lead to a better understanding of the Universe as a whole. While it is well known that star clusters form within parsec-scale overdensities of interstellar molecular gas called clumps, it is, however, unclear whether these clumps represent the high-density tip of a continuous gaseous flow that gradually leads towards the formation of stars, or a transition within the gas physical properties. Here, we present a unique analysis of a sample of 27 infrared dark clouds embedded within 24 individual molecular clouds that combine a large set of observations, allowing us to compute the mass and velocity dispersion profiles of each, from the scale of tens of parsecs down to the scale of tenths of a parsec. These profiles reveal that the vast majority of the clouds, if not all, are consistent with being self-gravitating on all scales, and that the clumps, on parsec-scale, are often dynamically decoupled from their surrounding molecular clouds, exhibiting steeper density profiles (ρ∝r−2) and flat velocity dispersion profiles (σ∝r0), clearly departing from Larson’s relations. These findings suggest that the formation of star clusters correspond to a transition regime within the properties of the self-gravitating molecular gas. We propose that this transition regime is one that corresponds to the gravitational collapse of parsec-scale clumps within otherwise stable molecular clouds.

Ongoing hierarchical massive cluster assembly: The LISCA II structure in the Perseus complex

We report on the identification of a massive (∼105 M⊙) substructured stellar system in the Galactic Perseus complex likely undergoing hierarchical cluster assembly. This system comprises nine star clusters (including the well-known clusters NGC 654 and NGC 663) and an extended and low-density stellar halo. Gaia-DR3 and available spectroscopic data show that all its components are physically consistent in 6D phase-space (position, parallax, and 3D motion), and homogeneous in age (14–44 Myr) and chemical content (half-solar metallicity). In addition, the system’s global stellar density distribution is that of typical star clusters and shows clear evidence of mass segregation. We find that the hierarchical structure is mostly contracting toward the center with a speed of up to ≃4 − 5 km s−1, while the innermost regions expand at a lower rate (about ≃1 km s−1) and are dominated by random motions. Interestingly, this pattern is dominated by the kinematics of massive stars, while low-mass stars (M < 2 M⊙) are characterized by contraction across the entire cluster. Finally, the nine star clusters in the system are all characterized by a relatively flat velocity dispersion profile possibly resulting from ongoing interactions and tidal heating. We show that the observational results are generally consistent with those found in N-body simulations following the cluster violent relaxation phase, strongly suggesting that the system is a massive cluster in the early assembly stages. This is the second structure with these properties identified in our Galaxy and, following the nomenclature of our previous work, we named it LISCA II.

Dual Concentric-Ring-Core Fiber With Four Zero-Dispersion Wavelengths for Beyond Three-Octave OAM Supercontinuum Generation

Orbital angular momentum (OAM) beams, featuring with unique spatial field distributions, have been extensively investigated and applied into a wide range of fields. In most applications, OAM beams covering a certain frequency band are usually required, and supercontinuum (SC) generation is a feasible method to provide broadband OAM source. As a short pulse is incident into a specially designed optical fiber with flat dispersion over broad bandwidth, a broadband SC could be generated along propagation due to the nonlinear spectral broadening. In this work, we propose a dual concentric-ring-core fiber design to achieve near-zero and flat dispersion profile over 3007-nm wavelength range with four zero-dispersion wavelengths for the OAM mode. The precise position of the zero-dispersion wavelengths can be tailored by varying the structural variables and the germanium-doped concentration of the silica-based fiber. The performance of the spectral broadening in terms of flatness and bandwidth is investigated under different input pulse and propagation conditions. Simulation results depict that the OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3,1</sub> mode supercontinuum spectrum can achieve beyond three-octave spanning from 445 to 3942 nm at −40 dB level by pumping a 50-fs 600-kW Gaussian pulse at the central wavelength of 1900 nm into a 2-mm designed fiber. By further optimizing the proposed fiber structure for the OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1,1</sub> mode with <60 ps/(nm·km) dispersion variation over a 3210-nm bandwidth, the generated supercontinuum can cover nearly three octaves.

Mid-IR supercontinuum generation in dispersion flattened As38Se62 chalcogenide photonic crystal fiber

We have created a PCF with square shaped core using chalcogenide material As38Se62 as core and air holes in IR region, which yield very high optical non linearity of 1290 W−1Km−1 at 3.5 μm, further gives a flat dispersion profile. The fiber has been used for numerically simulate SCG using low power pump pulse width (TFWHM) at 50 fs duration at 3.5 μm. At the end of 10 mm fiber, SC broadening of about 2000 to 12000 nm can be achieved with pulses of 4200 W peak power.

Numerical study on flat dispersion in a single-layer Si3N4 coupled-waveguides with four zero-dispersion wavelengths for supercontinuum generation

We achieve a flat broadband dispersion profile in a dual-core silicon nitride (Si3N4) waveguide utilizing the mode coupling between the quasi-TE00 mode in a narrow waveguide and the quasi-TE10 mode in a wide waveguide. Mode coupling extends the anomalous dispersion region and flattens the entire dispersion, creating four zero dispersion wavelengths. The generated dispersion profile varies between −10 and 13 ps/(nm∙km) over an octave bandwidth from 1150 nm to 2425 nm. A two-octave supercontinuum spectrum from 750 nm to 3001 nm can be generated through the 5 mm long Si3N4 waveguide by launching a 50-fs 5-kW hyperbolic secant input pump pulse at anomalous dispersion region. Significant spectral broadening is also observed, by pumping in the normal dispersion region. The proposed Si3N4 waveguide has only a single-layer core structure and is compatible with the photonic Damascus process. Moreover, the obtained dispersion profile is insensitive to the gap distance between the narrow and wide waveguides, making it easier to fabricate than slot-assisted waveguide structures.

Mid-infrared ultra-broadband optical Kerr frequency comb based on a CdTe ring microresonator: a theoretical investigation.

Microresonator Kerr frequency combs are coherent light sources that emit broadband spectrum of evenly spaced narrow lines in an optical microresonator, which provide breakthroughs in many technological areas, such as spectroscopy, metrology, optical telecommunications, and molecular sensing. The development of mid-infrared (MIR) optical frequency comb (OFC) based on microresonators could pave the way for high performance spectroscopy in the MIR "molecular fingerprint" region. However, the generation of microresonator MIR OFC, especially towards the long-wavelength MIR (>10 µm) region, is prohibited by the transmission window of the commonly used Kerr optical media such as Si and Si3N4, and low nonlinearity at long wavelengths. Here, we seek the possibility to realize an ultra-broadband frequency comb operating in the long-wavelength MIR region based on a cadmium telluride (CdTe) ring microresonator. CdTe features a broad transmission range covering the wavelengths of 1∼25 µm, a flat dispersion profile, and an extraordinary third-order nonlinear refractive index (∼1.4 × 10-17 m2W-1 at 7 µm) which is 2-order greater than that of Si3N4, making it a promising platform to realize MIR Kerr frequency comb. Based on the above excellent optical properties, we design a CdTe/cadmium sulfide (CdS)/Si heterojunction microring resonator to generate an ultra-broadband MIR OFC. Through the numerical simulation, the geometric parameters (width, height, and radius) of the microresonator, polarization, wavelength of the pump, and quality factor are investigated and optimized. As a result, a MIR OFC covering 3.5∼18 µm is numerically demonstrated by using the pump wavelength of 7 µm and a pump power of 500 mW. This is the first simulation demonstration of Kerr OFC with the spectral range extending beyond 10 µm, to the best of our knowledge. This work provides new opportunities for the realization of ultrabroad microresonator frequency combs based on novel Kerr optical medium, which can find important applications ranging from calibration of astronomical spectrographs to high-fidelity molecular spectroscopy.

High-dispersion TIR-GRISMs with flattened angular dispersion profile

Modern grating imaging spectrographs typically show aberrations due to the nonlinear dependence of angular dispersion on wavelength. This error can be reduced by a combination of diffracting and refracting components (grating and prism). Here, we present a dispersive optical component, namely, total internal reflection (TIR) grating + prism, with a grating attached to one facet of a prism. The elements work within the limit of TIR and exhibit three advantages compared to standard diffraction gratings: first, high diffraction efficiency up to 98%; second, enormous angular dispersion up to 0.75 ∘ n m − 1 ; and third, a nearly constant angular dispersion profile. As an example, the fabrication of such a disperser designed for the short-wave infrared2 spectral range is demonstrated. We report on details of the manufacturing process, which relies on a dedicated combination of technologies, i.e., electron beam lithography, atomic layer deposition, and hydrophilic direct bonding.

Dispersion engineered mid-IR supercontinuum generation using multi-core silicon-rich nitride photonic integrated waveguide

The multi-core silicon-rich nitride photonic integrated waveguide is presented here to numerically model the spanning of supercontinuum generation in the mid-IR region beyond the 4 μm wavelength range by engineering the dispersion profile such that maximum phase-matching can be achieved. Tunable dispersive wave generation has been observed in a long mid-IR region up to 5.2 μm wavelength by the variation of gap spacing between the cores. This unique geometry enables a higher phase matching from 1.2 μm to 2.6 μm wavelength range resulting in the flat dispersion profile in the near-IR region by coupling the fundamental mode in narrow waveguide core to the higher-order mode in wide waveguide core that gives rise to anomalous GVD which plays an essential role in compensating chromatic dispersion in the mid-IR region.

The large-mode-area multicore fiber with low and flat normal dispersion profile at 1.5–2.3 μm

For the purpose of realizing the large-mode-area (LMA) fibers with low and flat normal group velocity dispersion (GVD) profiles, the approach based on the multicore fiber (MCF) consisting of identical cores is proposed. The contribution of the coupling between two adjacent cores to tailoring the GVD of the MCF can be analyzed by using the dispersive equation, which is evaluated by the coupled mode theory (CMT) and can describe the GVD relation between the MCF and single-core waveguide. Furthermore, the designed 19- and 37-core MCFs with the step-index waveguides exhibit extremely flat normal GVD profiles with values of close to 0 ps/nm·km in the wavelength range of 1500 – 2300 nm, while the effective mode area of the 37-core MCF can reach 440 μm2 at 1900 nm. For the case of the MCF based on the photonic crystal fiber (PCF) structure, the 7-core MCFs can offer the concave-shaped normal GVD profiles in the 1900-nm wavelength region, where the minimum GVD values are around −46 ps/nm·km, and the wavelengths of the dip center can shift from 1880 to 2110 nm by tuning the core spacing. The effective mode areas are increased beyond 170 μm2 at 1900 nm. The proposed LMA normal-dispersion MCFs have potential applications in high-power ultrashort-pulse fiber lasers and amplifiers.

Efficient few-cycle Yb-doped laser oscillator with Watt-level average power.

So far, the operation of ultrafast bulk laser oscillators based on Yb-doped gain materials and directly emitting few-cycle pulses have been restricted to low optical-to-optical efficiencies and average output powers of only a few milliwatt. This performance limitation can be attributed to the commonly-applied standard collinear pumping scheme in which the optical pump is transmitted through a dichroic mirror whose spectral transmission and dispersion properties severely perturb the oscillating pulse when its optical spectrum extends towards the pump wavelength. In this study, we report on a novel pumping scheme relying on cross polarization that overcomes this challenge. In our concept, the pump transmitting mirror is highly transmissive for the pump light in p-polarization, while it is highly reflective for the laser light in s-polarization over a broad wavelength range, even covering the pump wavelength and beyond. In contrast to a standard thin-film polarizer featuring similar polarization dependent properties, it provides a low and flat dispersion profile over a broad spectral range for the s-polarization. Implementing this pumping scheme in a soft-aperture Kerr-lens mode-locked bulk laser oscillator based on the gain material Yb:CALGO, we achieve clean 22-fs soliton pulses at 729 mW of average output power and an optical-to-optical efficiency of 25%. In a second configuration optimized for the highest average output power, we demonstrate a high optical-to-optical efficiency of 36.6%, which was obtained for 31-fs pulses at 1.63 W of average output power. In a third configuration we experimentally confirm the limiting effect of a dichroic mirror commonly used in the standard collinear pumping scheme. All the results presented here and obtained in the first and second configuration generate pulses with a center wavelength ranging from 1030 nm to 1056 nm, well within the spectral region of high gain cross sections of Yb:CALGO. While this initial demonstration was realized using a commercial diffraction-limited fiber laser as pump source, the pump geometry appears also well suited for pumping with laser diodes coupled into multimode fibers. This novel approach opens up new opportunities for compact and cost-efficient high-power few-cycle bulk laser oscillators based on Yb-doped gain materials and can be applied to any gain material with small quantum defect.

Highly birefringent all-normal dispersion silica fiber with flat dispersion profile in the 1200–2100 nm wavelength range

Highly birefringent all-normal dispersion silica fiber with flat dispersion profile in the 1200–2100 nm wavelength range

Single-mode, single-polarization and dispersion-flattened waveguides based on silicon carbide and diamond

Fundamental properties of an optical waveguide are mainly related to its guided modes and associated loss, dispersion, polarization and nonlinearity. Single-mode and single-polarization waveguiding has long been of interest, i.e., so-called truly single mode. We propose SiC- and diamond-based waveguides, which are truly single-mode and simultaneously dispersion-flattened. This is preferred for broadband nonlinear applications, such as octave-spanning supercontinuum and frequency comb generation. An ultra-low and flat dispersion profile with four zero-dispersion wavelengths (ZDWs) can be obtained over an octave-spanning bandwidth from 1.8 to 3.6 μm. Meanwhile, only TE00 mode is supported in the proposed waveguide over the low-dispersion bandwidth. Scattering loss of the waveguide is greatly reduced by 8.6 times, compared to a standard strip waveguide. Fabrication of the proposed waveguide is easier than previous dispersion-tailored waveguides, without dry etching SiC and diamond. It is expected that the proposed waveguides can find applications in nonlinear photonics, optomechanics and quantum optics.

Testing for Dark Matter in the Outskirts of Globular Clusters

Abstract The proper motions of stars in the outskirts of globular clusters are used to estimate cluster velocity dispersion profiles as far as possible within their tidal radii. We use individual color–magnitude diagrams to select high-probability cluster stars for 25 metal-poor globular clusters within 20 kpc of the Sun, 19 of which have substantial numbers of stars at large radii. Of the 19, 11 clusters have a falling velocity dispersion in the 3–6 half-mass radii range, 6 are flat, and 2 plausibly have a rising velocity dispersion. The profiles are all in the range expected from simulated clusters that started at high redshift in a zoom-in cosmological simulation. The 11 clusters with falling velocity dispersion profiles are consistent with no dark matter above the Galactic background. The six clusters with approximately flat velocity dispersion profiles could have local dark matter, but are ambiguous. The two clusters with rising velocity dispersion profiles are consistent with a remnant local dark matter halo, but need membership confirmation and detailed orbital modeling to further test these preliminary results.

Low pump power coherent supercontinuum generation in heavy metal oxide solid-core photonic crystal fibers infiltrated with carbon tetrachloride covering 930-2500 nm.

All-normal dispersion supercontinuum (ANDi SC) generation in a lead-bismuth-gallate glass solid-core photonic crystal fiber (PCF) with cladding air-holes infiltrated with carbon tetrachloride (CCl4) is experimentally investigated and numerically verified. The liquid infiltration results in additional degrees of freedom that are complimentary to conventional dispersion engineering techniques and that allow the design of soft-glass ANDi fibers with an exceptionally flat near-zero dispersion profile. The unique combination of high nonlinearity and low normal dispersion enables the generation of a coherent, low-noise SC covering 0.93-2.5 µm requiring only 12.5 kW of pump peak power delivered by a standard ultrafast erbium-fiber laser with 100 MHz pulse repetition rate (PRR). This is a much lower peak power level than has been previously required for the generation of ANDi SC with bandwidths exceeding one octave in silica- or soft-glass fibers. Our results show that liquid-composite fibers are a promising pathway for scaling the PRR of ANDi SC sources by making the concept accessible to pump lasers with hundreds of megahertz of gigahertz PRR that have limited peak power per pulse but are often required in applications such as high-speed nonlinear imaging, optical communications, or frequency metrology. Furthermore, due to the overlap of the SC with the major gain bands of many rare-earth fiber amplifiers, our source could serve as a coherent seed for low-noise ultrafast lasers operating in the short-wave infrared spectral region.

Mass–Velocity Dispersion Relation in MaNGA Brightest Cluster Galaxies

Abstract We investigate a kinematic scaling relation between the baryonic mass and the flat velocity dispersion, i.e., mass–velocity dispersion relation (MVDR), from the brightest cluster galaxies (BCGs) to the galaxy clusters. In our studies, the baryonic mass of BCGs is mainly estimated by photometry. The velocity dispersion profiles are explored with the integrated field unit by Mapping Nearby Galaxies at Apache Point Observatory (MaNGA). For the first time, we reveal two significant results with 54 MaNGA BCGs: (1) the flat velocity dispersion profiles; (2) a tight empirical relation on the BCG-cluster scale together with cluster samples, i.e., MVDR, log ( M bar / M ⊙ ) = 4.1 − 0.1 + 0.1 log ( σ los / km s − 1 ) + 1.6 − 0.3 + 0.3 , with a tiny lognormal intrinsic scatter of 10 − 1 + 2 % . This slope is identical to the acceleration relation in galaxy clusters, which is reminiscent of the spiral galaxies, albeit at a larger characteristic acceleration scale. The residuals of the MVDR represent a Gaussian distribution, displaying no correlations with four properties: baryonic mass, scale length, surface density, and redshift. Notably, the MVDR on the BCG-cluster scale provides a strict test, which disfavors the general prediction of the slope of three in the dark matter model.

Coherent ultrabroad orbital angular momentum supercontinuum generation in an AsSe2-As2S5 microstructured fiber with all-normal dispersion

We design and simulate a hollow AsSe2-As2S5 microstructured fiber surrounded by air holes with nearly-zero flattened all-normal dispersion profile. Simulation results show that the coherent first-order orbital angular momentum (OAM1,1) supercontinuum (SC) spectrum extending from 1.3 to 11 μm at −40 dB level, which can be achieved by pumping a chirp-free hyperbolic secant pulse with 100 fs pulse duration and 20 kW peak power at the wavelength of 6 μm into an 11 mm proposed fiber. Due to the flexible controllability for chromatic dispersion of this fiber, several coherent SC spectra generated by different OAM modes are analyzed.

Dispersion Optimization of Silicon Nitride Waveguides for Efficient Four-Wave Mixing

Silicon nitride waveguides have emerged as an excellent platform for photonic applications, including nonlinear optical signal processing, owing to their relatively high Kerr nonlinearity, negligible two photon absorption, and wide transparent bandwidth. In this paper, we propose an effective approach using 3D finite element method to optimize the dispersion characteristics of silicon nitride waveguides for four-wave mixing (FWM) applications. Numerical studies show that a flat and low dispersion profile can be achieved in a silicon nitride waveguide with the optimized dimensions. Near-zero dispersion of 1.16 ps/km/nm and 0.97 ps/km/nm at a wavelength of 1550 nm are obtained for plasma-enhanced chemical vapor deposition (PECVD) and low-pressure chemical vapor deposition (LPCVD) silicon nitride waveguides, respectively. The fabricated micro-ring resonator with the optimized dimensions exhibits near-zero dispersion of −0.04 to −0.1 ps/m/nm over a wavelength range of 130 nm which agrees with the numerical simulation results. FWM results show that near-zero phase mismatch and high conversion efficiencies larger than −12 dB using a low pump power of 0.5 W in a 13-cm long silicon nitride waveguide are achieved.

Wave dark matter and ultra-diffuse galaxies

ABSTRACT Dark matter (DM) as a Bose–Einstein condensate, such as the axionic scalar field particles of String Theory, can explain the coldness of DM on large scales. Pioneering simulations in this context predict a rich wave-like structure, with a ground state soliton core in every galaxy surrounded by a halo of excited states that interfere on the de Broglie scale. This de Broglie scale is largest for the low-mass galaxies as momentum is lower, providing a simple explanation for the wide cores of dwarf spheroidal galaxies. Here we extend these ‘wave dark matter’ (ψDM) predictions to the newly discovered class of ‘ultra-diffuse galaxies’ (UDG) that resemble dwarf spheroidal galaxies but with more extended stellar profiles. Currently, the best-studied example, ‘Dragon Fly 44’ (DF44), has a uniform velocity dispersion of ≃33 km s−1, extending to at least 3 kpc, that we show is reproduced by our ψDM simulations with a soliton radius of ≃0.5 kpc. In the ψDM context, we show that relatively flat dispersion profile of DF44 lies between massive galaxies with compact dense solitons, as may be present in the Milky Way on a scale of 100 pc and lower mass galaxies where the velocity dispersion declines centrally within a wide, low-density soliton, like Antlia II, of radius 3 kpc.