Near-zero Dispersion Research Articles

Low peak power broadband supercontinuum spectra generated in a square lattice toluene-core photonic crystal fiber with different air hole diameters

This study shows that it is possible to achieve a very large supercontinuum spectral broadening with a peak power hundreds of times lower than in previous publications. We can achieve by using a square lattice toluene-core photonic crystal fiber. With the heterogeneity in the diameter of air holes, our new design allows simultaneous optimization of characteristic quantities to ensure flat near-zero dispersion, small effective mode area, and low attenuation for efficient spectral broadening. Supercontinuum generation (SC) of proposed PCFs has been numerically simulated and studied under different pump source parameters. The first fiber with a lattice constant of 1.0 μm and filling factor of 0.65 operates in all-normal dispersion. It generates broadband of 1.045 μm by a pump pulse with a central wavelength of 1.3 μm, 40 fs duration, and energy of 18 pJ (corresponding peak power of 0.45 kW). Besides, two octave-spanning of the SC spectrum in the wavelength range of 0.775–4.3 μm are formed in the second fiber with an input pulse whose optical properties are 1.55 μm wavelength, 100 fs duration, and energy of 55 pJ (0.45 kW of the peak power). The lattice constant of second fiber is 2.0 μm and the filling factor is 0.3. They would be good candidates for all-fiber SC sources operating with low-power pump lasers. This would be a as cost-effective alternatives to glass core fibers.

Optimization of dispersion in hollow-core photonic crystal fibers filled with toluene

In this work, we thoroughly investigated the dispersion in SiO2-based photonic crystal fibers with a C7H8-infiltrated hollow core. By cleverly modifying the air hole diameters and lattice constants in the structural design, we achieved ultra-flat near-zero dispersion as small as 0.462 ps·nm–1·km–1 and diverse dispersion properties of PCFs, very beneficial for supercontinuum generation. Based on the simulation results, we propose three optimal structures with small and flat dispersion capable of generating a broad and smooth supercontinuum spectrum. The results of our study can be advantageous for fabricating fibers in low-cost all-fiber laser systems.

Supercontinuum generation in C6H5NO2-core photonic crystal fibers with various air-hole size

In this paper, we demonstrated the ability of a hexagonal photonic crystal fiber (PCF) with a hollow core infiltrated with nitrobenzene (C6H5NO2) to generate a broad SC spectrum at low peak powers. Due to the non-uniformity of the air hole diameters, our new design allows for simultaneous optimization of features, resulting in near-flat, near-zero dispersion, a small effective mode area, and low attenuation for efficient spectral broadening. We selected two optimal structures from the simulation results to analyze the nonlinear properties and supercontinuum generation. The first fiber, #HF1, with a lattice constant of 1.0[Formula: see text][Formula: see text]m and a filling factor of 0.45, operates in all-normal dispersion and produces spectral SC ranging from 0.81[Formula: see text][Formula: see text]m to 1.919[Formula: see text][Formula: see text]m with a pump wavelength of 1.56[Formula: see text][Formula: see text]m, a pulse duration of 90[Formula: see text]fs, and peak power of 0.133[Formula: see text]kW propagated in a 1 cm fiber length. The #HF2 fiber (lattice constant of 2.0[Formula: see text][Formula: see text]m, filling factor of 0.3) has an extended SC spectrum from 0.792[Formula: see text][Formula: see text]m to 3.994[Formula: see text][Formula: see text]m, a pump wavelength of 1.55[Formula: see text][Formula: see text]m, a pulse width of 110[Formula: see text]fs, a peak power of 0.273[Formula: see text]kW propagated in a 15[Formula: see text]cm fiber length. The proposed fiber may be a new-generation optical fiber suitable for low-peak power all-fiber optical systems to replace glass-core glass fiber.

Multi-megawatt pulses at 50 MHz from a single-pump Mamyshev oscillator gain-managed amplifier laser.

We have developed a compact all-PM-fiber ytterbium-doped Mamyshev oscillator-amplifier laser system generating compressed pulses of 102 nJ and 37 fs, thus having over 2 MW of peak power, at a repetition rate of 52 MHz. The pump power from a single diode is shared between a linear cavity oscillator and a gain-managed nonlinear amplifier. The oscillator is self-started by pump-modulation and a linearly polarized single-pulse operation is achieved without filter tuning. The cavity filters are near-zero dispersion fiber Bragg gratings with a Gaussian spectral response. To our knowledge, this simple and efficient source has the highest repetition rate and average power among all-fiber multi-megawatt femtosecond pulsed laser sources and its architecture holds potential for generating higher pulse energies.

A low-loss polarization-maintaining terahertz fiber

A novel low-loss THz polarization-maintaining fiber is analyzed numerically. The proposed fiber consists of two small thin dielectric tubes nested in a large dielectric tube. Numerical simulations performed using the full-vector finite element method show that the difference between the effective refractive index of LPx01 and the adjacent modes is up to 10−2 orders of magnitude in the frequency range from 0.32 to 0.7 THz. At the same time, the proposed fiber has low loss characteristics, and the total loss of LPx01 is only 0.17 dB/cm at 0.35 THz. In addition, near-zero flat dispersion of 0.3 ± 0.36 ps/THz/cm can be achieved by adjusting the structural parameters of the fiber.

Dispersion Engineering of Waveguide Microresonators by the Design of Atomic Layer Deposition

In this work, we demonstrate dispersion engineering of silicon nitride waveguide resonators with atomic layer deposition (ALD). We conducted theoretical and experimental analyses on the waveguide dispersion with air cladding, hafnium oxide (HfO2) cladding, and aluminum oxide (Al2O3) cladding. By employing ALD HfO2 as the cladding layer, the dispersion of waveguide can be tuned to a finer degree in the normal regime at a wavelength of 1550 nm. On the other hand, using ALD Al2O3 cladding provides the waveguide dispersion that spans regimes in normal, near-zero, and anomalous dispersion.

Continuous fabrication of polarization maintaining fibers via an annealing improved infinity additive manufacturing technique for THz communications.

We report the design and fabrication of a polarization-maintaining fiber for applications in fiber-assisted THz communications. The fiber features a subwavelength square core suspended in the middle of a hexagonal over-cladding tube by four bridges. The fiber is designed to have low transmission losses, high birefringence, high flexibility, and near-zero dispersion at the carrier frequency of 128 GHz. An infinity 3D printing technique is used to continuously fabricate a 5 m-long polypropylene fiber of ∼6.8 mm diameter. The fiber transmission losses are furthermore reduced by as high as ∼4.4 dB/m via post-fabrication annealing. Cutback measurements using 3 m-long annealed fibers show ∼6.5-11 dB/m and ∼6.9-13.5 dB/m losses (by power) over a 110-150 GHz window for the two orthogonally polarized modes. Signal transmission with bit error rates of ∼10-11-10-5 is achieved at 128 GHz for 1-6 Gbps data rates using a 1.6 m-long fiber link. The average polarization crosstalk values of ∼14.5 dB and ∼12.7 dB are demonstrated for the two orthogonal polarizations in fiber lengths of 1.6-2 m, which confirms the polarization-maintaining property of the fiber at ∼1-2 meter lengths. Finally, THz imaging of the fiber near-field is performed and shows strong modal confinement of the two orthogonal modes in the suspended-core region well inside of the hexagonal over-cladding. We believe that this work shows a strong potential of the infinity 3D printing technique augmented with post-fabrication annealing to continuously produce high-performance fibers of complex geometries for demanding THz communications applications.

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.

Multi-octave supercontinuum generation of optical vortices in an As2S3 photonic crystal fibers with all-normal dispersion

We designed an all-normal flat near-zero dispersion As2S3 photonic crystal fiber (PCF) with orderly arranged air hole rings for generating an orbital angular momentum (OAM) supercontinuum (SC). We optimized the characters of chromatic dispersion and confinement loss compared with previous studies by carefully engineering the PCF. Simulation results show that, by launching a 100 fs 250 kW chirp-free hyperbolic secant pulse at 7 μm into a 5-mm designed PCF, a 9471-nm SC of OAM1,1 forms from 757 to 10228 nm at a −40 dB level, which covers a 3.7-octave bandwidth. As the peak power increases, the higher-order OAM with a flatter chromatic dispersion profile in the normal dispersion region has a wider bandwidth. They are all highly coherent in the whole spectral range.

Predicting the evolution of the mid-infrared supercontinuum in the near-zero dispersion region under weak triggering using LSTM neural networks

To enable active control of rogue waves after pumping a pulse into an optical fiber, adding a weak trigger is necessary. However, this process typically involves numerous numerical simulations based on the nonlinear Schrodinger equation, which can be computationally intensive. Using neural networks to predict complex nonlinear propagation in optical fibers avoids the need for numerical solutions during research. We show a neural network using long short-term memory to predict the temporal and spectral evolutions of the mid-infrared supercontinuum generation in the near-zero dispersion region under weak triggering, simply by determining the input pulse strength in the time and frequency domains.

Novel optical soliton molecules formed in a fiber laser with near-zero net cavity dispersion

Soliton molecules (SMs) are stable bound states between solitons. SMs in fiber lasers are intensively investigated and embody analogies with matter molecules. Recent experimental studies on SMs formed by bright solitons, including soliton-pair, soliton-triplet or even soliton-quartet molecules, are intensive. However, study on soliton-binding states between bright and dark solitons is limited. In this work, the formation of such novel SMs in a fiber laser with near-zero group velocity dispersion (ZGVD) is reported. Physically, these SMs are formed because of the incoherent cross-phase modulation of light and constitute a new form of SMs that are conceptually analog to the multi-atom molecules in chemistry. Our research results could assist the understanding of the dynamics of large SM complexes. These findings may also motivate potential applications in large-capacity transmission and all-optical information storage.

Theoretical and experimental investigations of dispersion-managed, polarization-maintaining 1-GHz mode-locked fiber lasers.

High-repetition-rate (up to GHz) femtosecond mode-locked lasers have attracted significant attention in many applications, such as broadband spectroscopy, high-speed optical sampling, and so on. In this paper, the characteristics of dispersion-managed, polarization-maintaining (PM) 1-GHz mode-locked fiber lasers were investigated both experimentally and numerically. Three compact and robust 1-GHz fiber lasers operating at anomalous, normal, and near-zero dispersion regimes were demonstrated, respectively. The net dispersion of the linear cavity is adjusted by changing types of PM erbium-doped fibers (EDFs) and semiconductor saturable absorber mirrors (SESAMs) in the cavity. Moreover, the long-term stability of the three mode-locked fiber lasers is proved without external control. In order to better understand the mode-locking dynamics of lasers, a numerical model was constructed for analysis of the 1-GHz fiber laser. Pulse evolution simulations have been carried out for soliton, dissipative-soliton, and stretched-pulse mode-locking regimes under different net dispersion conditions. Experimental results are basically in agreement with the numerical simulations.

Sideband-free dispersion-managed Yb-doped mode-locked fiber laser with Gires–Tournois interferometer mirrors

High-order dispersion introduced by Gires–Tournois interferometer mirrors usually causes spectral sidebands in the near-zero dispersion region of mode-locked fiber lasers. Here, we demonstrate a sideband-free Yb-doped mode-locked fiber laser with dispersion-compensating Gires–Tournois interferometer mirrors. Both the simulation and the experiment demonstrate that the wavelength and energy of the sidebands can be tuned by changing the transmission coefficient of the output mirror, the pump power, and the ratio of the net cavity dispersion to the net third-order dispersion in the cavity. By optimizing these three parameters, the laser can generate a sideband-free, Gaussian-shaped spectrum with a 13.56-nm bandwidth at -0.0232 ps2 net cavity dispersion, which corresponds to a 153-fs pulse duration.

Nonlinear properties of circular solid-core photonic crystal fiber with air-hole diameter difference and spacing in the cladding

We emphasize the ability to control the nonlinear properties of silica-based circular solid-core photonic crystal fibers (PCFs) with a new design. In this fiber, the diameter of the air hole in the rings is different, and the lattice constant is ununiform in the cladding. The simulation results show that a near-zero ultra-flattened chromatic dispersion over a wide wavelength range and low attenuation in these PCFs is achieved. Two structures with the lattice constant, Ʌ, of 0.7 and 0.9 µm and filling factor, d1/Ʌ, of 0.45 in the first ring were selected and investigated in detail. These structures are capable of generating broad-spectrum supercontinuum.

Study on Optical Features of Circular Photonic Crystal Fibers with Various Air-hole Size

: In this workwe proposed a newly designed photonic crystal fiber (PCF) with a circular lattice, the difference between the air-hole diameters of the first ring and the other rings makes it possible to improve the nonlinear properties of fibers. We investigated the effect of varying the filling factor (d1/Ʌ) and lattice constant (Ʌ) on the nonlinear characteristics of photonic crystal fibers in the 0.5 - 2 µm wavelength range. The advantages of these photonic crystal fibers are the flat and near-zero dispersion, low attenuation, and high nonlinear coefficient. From simulation results, we have selected three optimal structures (Ʌ = 1.0 μm; d1/Ʌ = 0.4, Ʌ = 0.8 μm; d1/Ʌ = 0.6, and Ʌ = 0.8 μm; d1/Ʌ = 0.65) to analyze the nonlinear characteristics at the pump wavelengths. The proposed fibers are valuable for supercontinuum generation.&#x0D; Keywords:

Supercontinuum generation in ultra-flattened near-zero dispersion PCF with C7H8 infiltration

Supercontinuum generation in ultra-flattened near-zero dispersion PCF with C7H8 infiltration

Simulation for generation of several kinds of solitons in anomalous dispersion erbium-doped fiber lasers

We numerically demonstrated seven types of solitons in an anomalous dispersion erbium-doped fiber laser, including conventional soliton (CS), dissipative soliton (DS), dissipative soliton resonance (DSR), two- and triple-soliton bound states, and period doubling and period tripling solitons. We found that DSR is achieved when the net cavity dispersion is large. CS with a narrow pulse width is obtained in the near-zero dispersion region, and period doubling and period tripling solitons are achieved around CS by properly adjusting the polarization state. The large net cavity dispersion and high gain saturation energy contribute to the generation of DS. Furthermore, further increasing the gain saturation energy under specific dispersion condition enables DS to transform into two- and triple-soliton bound states, and bound states have smooth spectral profiles and steep edges similar to DS. Furthermore, the influence of the net cavity dispersion and gain saturation energy on pulse type is diagrammed intuitively.

Exploring High-Performance Slow Light Grating Waveguides By Means of Deep Learning

The geometric parameters of periodic units, which influence the refractive/reflective behaviors of grating waveguides, are usually designed using intuition combined with meticulous trial-and-error, making the design extraordinarily time-consuming and computationally intensive. In this work, we proposed an effective method based on deep learning to meet this challenge. Concretely, we employed fishbone slow-light waveguides as the object to be optimized, whose unit cell has multiple degrees of freedom, and trained a deep neural network (DNN) to predict their geometric parameters with the given discrete group index curves as the input. Our design is oriented for high group index and near-zero group velocity dispersion (GVD). To verify this functionality, we performed the controllable inverse design of GVD while keeping the group index at the same level (about 24), which made the bandwidth increase by 53.8% at the same time, and further explored near-zero GVD structures with a group index of 30.76/41.30/51.90. Our results lay the groundwork for the rapid design of complex slow-light grating waveguides.

Investigation of dissipative solitons in an Er-doped fiber laser through machine-learning online optimization based on the Gaussian process

We have applied a machine-learning online optimization method based on the Gaussian process to the numerical modeling of dissipative solitons in an Er-doped and nonlinear polarization rotation mode-locked fiber laser. Three types of dissipative solitons are achieved both experimentally and numerically in the negative, near-zero, and positive net cavity dispersion regions. Corresponding optimum cavity parameters in simulation can be determined quickly and precisely via optimization. The optimization goal is the high similarity between the experimental results and the simulation results, which is calculated by the sum of Fréchet distance of the normalized spectral waveforms and autocorrelation traces. In numerical analyses, the characteristics of the output pulse in different dispersion conditions, the pulse dynamics inside the laser cavity, and the initial process of mode-locking are also investigated. Our results demonstrate the effectiveness and universality of machine-learning online optimization based on the Gaussian process method in the optimization process of fiber laser modeling, which can further provide more insights and extend large-scale potential applications of machine-learning algorithms in fiber lasers.

Topological photonic crystal fiber with near-zero flattened dispersion

Robustness against structural disorder or defects is a fascinating feature of topological photonic systems, triggering the extensive investigations in modern photonics. Recently, the marriage of topological photonics and fiber optics resulted in a new research direction of photonics with promising applications. In this work, we propose a new type of topological photonic crystal fiber (PCF) with guiding topological corner mode, which has near-zero flattened dispersion. Due to the design flexibility of the fiber cross-section and topological robustness of waveguiding modes, the topological PCF exhibits near-zero flattened dispersion immune to structural disorder and defects at telecommunication wavelength for proper structural parameters. In the dispersion-flattened topological PCFs, symmetric supercontinuum (SC) and frequency comb based on four-wave mixing (FWM) spanning from 1300 nm to 1800 nm can be generated for short fiber lengths in the range of 1–1.5 m. The presented results will broaden the applications of topological PCFs for nonlinear fiber optics due to the structural defects-immunity.