Near-zero Flattened Dispersion Research Articles

Mid-infrared supercontinuum generation in As2S3-circular photonic crystal fibers pumped by 4.5 µm and 6 µm femtosecond lasers

A study on As2S3 chalcogenide photonic crystal fiber (PCF) and its potential in supercontinuum generation (SCG) applications is presented. The designed fibers exhibit near-zero flattened chromatic dispersion, facilitating coherent and broad SCG utilizing femtosecond lasers at 4.5 and 6 µm wavelengths. A continuous spectrum spanning from 1.5 to 8 µm is achieved when the initial fiber is stimulated with an input power of 20 kW. With a pulse width of 100 fs and input power of 6 kW, the second fiber provides soliton-induced SCG with 2 to 15 µm spectral bandwidth. Furthermore, the integration of these large core diameter PCFs with high-power laser pulses guarantees the preservation of optical fiber integrity without damage. Consequently, these fibers hold promise for delivering SC spectra characterized by high power density, catering to a diverse range of practical applications including optical communications, spectroscopy, sensing, metrology, and calibration.

Highly birefringent photonic crystal fibers with near-zero flattened dispersion for polarization-maintaining terahertz transmission

In this work, we present a novel hexagonal lattice porous-core photonic crystal fiber (PCF) exhibiting high birefringence, low loss, and near-zero flattened dispersion in the terahertz regime. The core design incorporates asymmetric rectangular and triangular air holes to achieve enhanced birefringence performance. The background material, TOPAS, is carefully selected to minimize dispersion and transmission losses. Furthermore, by employing the finite element method, the guiding properties of the proposed PCF are comprehensively investigated. Our simulation results reveal a near-zero flattened dispersion value of ±0.2ps⋅THz−1⋅cm−1 within a broad frequency range of 0.8−1.6THz, accompanied by an impressive birefringence value of 0.072. Additionally, the PCF demonstrates outstanding characteristics in terms of confinement loss, effective material loss, effective mode area, core power fraction, and bending loss. Considering its exceptional attributes, the proposed PCF holds significant promise for polarization-maintaining applications and terahertz communication systems.

Design of large core AS2S3 chalcogenide PCF pumped at all-normal and anomalous dispersion regimes for supercontinuum generation

We design and simulate a near-zero flat dispersion As2S3 photonic crystal fiber (PCF) with a circular shape to generate a broad and smooth supercontinuum (SC). A near-zero flattened dispersion in the all-normal dispersion PCF is maintained in the 4–6.2[Formula: see text]m wavelength range. A 6[Formula: see text]m hyperbolic pulse is injected into this fiber, producing a flat spectrum from 2.45 to 8.4[Formula: see text]m at a peak power of 5[Formula: see text]kW. The generated SC in anomalous dispersion PCF can cover multi-octaves by a pump laser with a 6500[Formula: see text]nm wavelength, 10[Formula: see text]kW of peak power, and a 90[Formula: see text]fs duration in 10[Formula: see text]cm of the proposed fiber structure. These results are achieved by optimizing the fiber structure through the core size. In general, the proposed structures could be used for broadband smooth low-cost SC generation.

A new type of supercontinuum generation in hexagonal lattice C6H6-core PCF with broadband and low-power pump

Most of the spectral bandwidths of previous publications are still limited by high input powers making them economically less than ideal. By using a benzene core (C6H6) photonic crystal fiber (PCF) as a new supercontinuum (SC) light source, it is possible to achieve a very large spectral broadening with hundreds of times lower peak power. Due to the change in the diameter of air holes in the first ring near the core, near-zero flattened dispersion, high nonlinearity and small attenuation can be achieved for spectral broadening. The structural geometries of two C6H6-PCFs are optimized to generate wide SC at low input energy. The SC spectrum produced in 1[Formula: see text]cm long of all dispersion fiber extends from a part of visible light to the near-infrared range at 1.3[Formula: see text][Formula: see text]m wavelength and a small pulse energy of 18[Formula: see text]pJ (or 450[Formula: see text]W of electrical input). The second PCF shows wide soliton-induced SC from 0.8 to 4.2[Formula: see text][Formula: see text]m with 71[Formula: see text]pJ pulse energy (or input power approximately 790[Formula: see text]W) at 1.5[Formula: see text][Formula: see text]m wavelength within a fiber of 12[Formula: see text]cm. The proposed structures have the potential to become a new class of microstructured optical fibers for low-cost, broad-spectrum SC generation.

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.

ENZ material filled microstructured optical fiber with high birefringence and near-zero flattened dispersion at terahertz frequency

为了实现太赫兹波的保偏波导传输，设计了一种含有纤芯缺陷孔和椭圆形包层空气孔的高双折射微结构光纤。通过在包层空气孔中选择性地填充太赫兹近零介电常量（epsilon-near-zero, ENZ）材料，引入了几何结构和材料分布的双重不对称性，破坏了2个偏振基模的简并以获得高双折射特性。应用有限元方法研究了光纤的双折射、损耗和色散等传输特性随结构参数的变化规律。在0.5 THz~2 THz的宽频段范围内获得了大于0.01的高双折射。<i>x</i>和<i>y</i>偏振基模的损耗在0.8 THz附近具有最小值，分别为0.903 dB·cm⁻¹和0.851 dB·cm⁻¹。纤芯缺陷孔可以有效调节色散特性，<i>y</i>偏振基模在1 THz~1.8 THz范围内具有 (0±0.054) ps·THz⁻¹·cm⁻¹近零平坦色散特性。光纤的传输特性对ENZ材料的折射率变化不敏感。研究结论为研制太赫兹保偏光纤提供了理论参考。

Design of normal dispersion high nonlinear silica fiber and generation of flat optical frequency comb

&lt;sec&gt;The scheme of generating optical frequency comb mainly includes mode-locked laser, electro-optic modulation comb, nonlinear Kerr micro-resonator comb, and nonlinear supercontinuum comb. For the nonlinear supercontinuum comb scheme, the silica-based high nonlinear fiber with near-zero flattened normal dispersion is required. However the fiber dispersion varies along the fiber due to the fabrication inaccuracy. Furthermore, nonlinear supercontinuum comb generation based on the nonlinear fiber has not been systematically studied. In this paper, an optimal design of four-clad flat normal dispersion high nonlinear silica fiber with a triangular core refractive index distribution for the flat optical frequency comb generation is carried out. The effects of the fiber cladding width and refractive index on the fiber dispersion characteristics and cut-off wavelength are studied through using the finite element method mode solver. The optimally designed fiber can obtain relatively flat near-zero normal dispersion in a wavelength range of 1400–1700 nm, the dispersion range is –3–0 &lt;inline-formula&gt;&lt;tex-math id="M5"&gt;\begin{document}$ \rm{p}\rm{s}/(\rm{k}\rm{m}\cdot \rm{n}\rm{m}) $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221115_M5.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221115_M5.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, and the dispersion slope is close to 0 at nearly 1550 nm. The effective mode field area of the nonlinear silica fiber is about 11&lt;inline-formula&gt;&lt;tex-math id="M6"&gt;\begin{document}$ {\text{μm}}^{2} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221115_M6.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221115_M6.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, and the nonlinear coefficient can reach 12.8&lt;inline-formula&gt;&lt;tex-math id="M7"&gt;\begin{document}$ {\rm{W}}^{-1}{\cdot \rm{k}\rm{m}}^{-1} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221115_M7.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221115_M7.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;.&lt;/sec&gt;&lt;sec&gt;Based on the electro-optic modulation pulse pumping the flat normal dispersion high nonlinear silica fiber, the flat optical frequency comb generation is systematically simulated with the generalized nonlinear Schrödinger equation. The time-frequency evolutions of a hyperbolic secant pulse, a Gaussian pulse and a super Gaussian pulse are simulated by using the X-Frog technology. The time-frequency spectrograms connect the time domain and the frequency domain of the pulse, clearly showing the change of pulse chirp during the propagation. The effects of various parameters on the optical frequency comb are studied, such as the fiber length, second-order dispersion, third-order dispersion, pulse peak power, pulse half width, pulse initial chirp, and pulse shape. An optical frequency comb with 3-dB flatness and about 40-nm bandwidth can be achieved based on hyperbolic secant pulse or Gaussian pulse pumping. Compared with the hyperbolic secant pulse and Gaussian pulse, the super Gaussian pulse can produce a flatter optical frequency comb. An optical frequency comb with 2-dB flatness and about 92-nm bandwidth can be achieved based on the super Gaussian pulse pumping. Therefore, based on the proposed high nonlinear fiber with normal dispersion , it is possible to realize an optical frequency comb with a repetition rate above 10 GHz, power flatness within 3 dB, and spectral bandwidth of about 40–90 nm. The simulation results are beneficial to promoting the localization of normal dispersion high nonlinear silica fiber and its application in flat optical frequency comb.&lt;/sec&gt;

Rectangular Porous-Core Photonic-Crystal Fiber With Ultra-Low Flattened Dispersion and High Birefringence for Terahertz Transmission

We propose a novel porous-core photonic crystal fiber (PCF) consisting of an asymmetric rectangular air-holes in core and six-ring hexagonal lattice circular air-holes in the cladding for achieving low loss polarization terahertz transmission in wide frequency range. By assuming TOPAS as the host material, finite element mothed (FEM) is used to investigate its properties. The near-zero flattened dispersion of -0.01±0.02 ps/THz/cm is achieved over a frequency range of 1.0-2.0 THz, as well as a high birefringence of 7.1×10-2 which can be useful for polarization maintaining applications. Also, critical parameters such as modal field distribution, effective modal loss, confinement loss, effective mode area, are discussed in detail. Further, fabrication possibilities are discussed briefly by comparing recent work on similar waveguide structures.

Highly Birefringent, Low-Loss, and Near-Zero Flat Dispersion ENZ Based THz Photonic Crystal Fibers

In this paper, we present and compare two experimentally feasible photonic crystal fiber (PCF) designs ( $\textit{Type-I}$ and $\textit{Type-II}$ ) which ensure near-zero flattened dispersion with ultra-high phase and group birefringence at THz frequencies. Both structures are based on a subset of a triangular array of circular air-holes, which define the cladding of the PCF and a central elliptical air-hole which breaks the symmetry of the structure, thus introducing high levels of birefringence. Additionally, we investigate the possibility of further enhancing the birefringence properties of $\textit{Type-II}$ structure by selectively filling the air-holes with Potassium Chloride (KCl) as strong Epsilon-Near-Zero (ENZ) material. Our investigation reveals that significant enhancement of birefringence can be achieved than its original counterpart with birefringence to be as high as 0.0627 at 6.2 THz and near-zero flat dispersion of $-$ 0.54 $\pm$ 0.04 ps/THz/cm over the frequency range of 6.2 $-$ 6.3 THz.

Low loss and highly birefringent photonic crystal fibre for terahertz applications

A highly birefringent photonic crystal fibre, with horizontal arrangement of three elliptical air holes in the core and hexagonal arrangement of circular air holes in the cladding, has been proposed for transmission of terahertz waves. Guiding properties of the fibre have been rigorously investigated using finite element method with perfectly matched layer boundary conditions. It has been demonstrated that high birefringence of 0.08 can be achieved with low effective material loss of 0.03 cm−1 at 1.3 THz. Moreover, it has been shown that about 42 % of useful power flows through the elliptical core air holes. Near-zero flattened dispersion of 1.18 ± 0.01 ps/THz/cm is obtained for the x-polarisation mode of the fibre. Other important optical properties such as confinement, bending and total losses have also been investigated; further strengthening potential applicability of the proposed fibre for next-generation terahertz research.

Numerical investigation on coherent mid-infrared supercontinuum generation in chalcogenide PCFs with near-zero flattened all-normal dispersion profiles**Project supported by the National Natural Science Foundation of China (Grant Nos. 61421002 and 61435003).

We design a novel all-normal flat near-zero dispersion chalcogenide photonic crystal fiber (PCF) for generating mid-infrared (MIR) supercontinuum (SC). The proposed PCF with a core made of As2Se3 glass and uniform air holes in the cladding is selectively filled with As2S5 glass. By carefully engineering the PCF with an all-normal flat near-zero dispersion profile, the anomalous-dispersion soliton effect is reduced, thus enabling broadband highly coherent SC to be generated. We also investigate the influence of the pulse parameters on the SC generation. Broadband SC covering with perfect coherence is achieved by pumping the proposed 3-cm-long PCF with 3- 100-fs pulses. The results provide a potential all-fiber realization of the broadband coherent MIR-SC.

Near-zero dispersion flattened, low-loss porous-core waveguide design for terahertz signal transmission

We demonstrate a photonic crystal fiber with near-zero flattened dispersion, ultralower effective material loss (EML), and negligible confinement loss for a broad spectrum range. The use of cyclic olefin copolymer Topas with improved core confinement significantly reduces the loss characteristics and the use of higher air filling fraction results in flat dispersion characteristics. The properties such as dispersion, EML, confinement loss, modal effective area, and single-mode operation of the fiber have been investigated using the full-vector finite element method with the perfectly matched layer absorbing boundary conditions. The practical implementation of the proposed fiber is achievable with existing fabrication techniques as only circular-shaped air holes have been used to design the waveguide. Thus, it is expected that the proposed terahertz waveguide can potentially be used for flexible and efficient transmission of terahertz waves.

Dispersion properties of a photonic quasi-crystal fiber with double cladding air holes

We propose a novel photonic quasi-crystal fiber (PQF) with a near-zero flattened dispersion in the 1450–1650nm optical telecommunication window. A 10-fold quasi-crystal lattice of holes is used to constitute the double clads. Using a full-vectorial finite element method, we analyze the relationship between the dispersion properties of the optical fiber and the structure parameters of cross section. According to the numerical simulation results, in 1450–1650nm optical telecommunication window, the dispersion of the PQF can be kept within (−0.25±0.31)ps/(nmkm) and there are dual zero dispersion wavelengths, while the confinement loss is limited to less than 0.04dB/km. Meanwhile, in this work it is demonstrated numerically that little variation in the fabricated affects moderately the figure of merits such as dispersion or confinement loss. Our structure and optical properties predictions are expected to provide valuable reference and basic data to aid in relative experiments and fabrication.

Numerical Modelling of Fundamental Characteristics of ZBLAN Photonic Crystal Fiber for Communication in 2-3μm Mid-Infrared Region

We numerically studied ZrF 4 -BaF 2 -LaF 3 -AIF 3 -NaF (ZBLAN) photonic crystal fiber (PCF) for potential implementation in optical communication within a 2to 3-μm midinfrared wavelength region. We focused on solid-core uniform air-hole-size hexagonal lattice ZBLAN PCF. The fundamental characteristics such as normalized frequency, confinement loss, chromatic dispersion, effective mode area, and nonlinearity were simulated through a full-vectorial finite-element method with perfectly matched layer boundary condition. Two different structural design ZBLAN PCFs with nonuniform airhole size were designed with zero dispersion wavelength shifted to 2.5 μm. In addition, a near-zero flattened chromatic dispersion ZBLAN PCF within a 2to 3.5-μm wavelength region was achieved. Furthermore, the sensitivity of the dispersion properties to a ±2% variation in the optimum parameters is studied for fabrication tolerance.

Two octaves spanning supercontinuum in highly nonlinear As2Se3nanophotonic crystal fiber for midinfrared applications

A new design of all-normal and near-zero flattened dispersion based on chalcogenide nanophotonic crystal fiber (PCF) has been proposed to generate smooth and ultra-broadband supercontinuum (SC) in the midinfrared (IR) region. With the optimized geometric parameters, the As2Se3 nano-PCF has been found to be suitable for two-octave supercontinuum generation (SCG). We designed a nano-PCF having a flat top dispersion curve with a maximum value of −2.3 [ps/(nm km)] and a large nonlinear coefficient equal to 7250 (W km)−1 around the wavelength of 5.24 μm. By numerical simulations, we predict the generation of a very broadband SC in the mid-IR region extending from 2 to 10 μm in only 2-mm fiber lengths by using a femtosecond laser having a full-width at half-maximum of 50 fs and a relatively low energy of E=80 pJ. The generated SC demonstrates perfect coherence property over the entire bandwidth. SC generation extended into the mid-IR spectral region has potential usefulness in a variety of applications requiring a broad and mid-IR spectrum, such as WDM sources, fiber sensing, IR spectroscopy, fiber laser, and optical tomography coherence.

Design of high duty ratio nonlinear photonic crystal fiber with near-zero flattened dispersion after 1.205 μm

The paper presents a high nonlinear photonic crystal fiber (HN-PCF) with highly duty ratio for time-stretch analog-to-digital conversion (TSADC). The simulation results show that a nonlinear of 42.26W−1km−1 and the flattened dispersion of less than 1.0ps/(nmkm) are obtained in more than 495nm waveband (1205–1700nm). Owing to its high nonlinear coefficient and flattened dispersion, the high nonlinear PCF is expected to be suitable for supercontinuum (SC) generation. The numerical simulation results demonstrate that the proposed high duty ratio HN-PCF can generate wideband SC.

A highly nonlinear photonic quasi-crystal fiber with low confinement loss and flattened dispersion

We proposed a novel photonic quasi-crystal fiber with near-zero flattened dispersion, highly nonlinear coefficient, and low confinement loss by using the dual concentric core structure. By optimizing the structure parameter, the proposed photonic quasi-crystal fiber can achieve a nonlinear coefficient larger than 33W−1km−1 and near-zero flatten dispersion of 0±3.4ps/nm/km with a near-zero dispersion slope of 8.5×10−3ps/nm2/km at the wavelength of 1550nm. Near-zero flattened dispersion and low confinement loss in the ultralow order of 10−7dB/m are simultaneously obtained in the wavelength range from 1373 to 1627nm. Furthermore, two zero dispersion wavelengths can be achieved in a wide wavelength ranger from 1373 to 1725nm. From the point of view of practical fabrication, the influence of deviation of each air hole diameter within 3% of imperfections on dispersion, nonlinearity, and is discussed to verify the robustness of our design.

Design of highly nonlinear octagonal photonic crystal fiber with near-zero flattened dispersion at 1.31μm waveband

This paper presents a highly nonlinear octagonal photonic crystal fiber (HN-OPCF) with a Ge-doped core for dental optical coherence tomography (OCT) applications. The simulation results show that a nonlinear coefficient of 56.3W−1 km−1 and the flattened dispersion of less than −1:0 ps/(nm·km) are obtained in 75 nm waveband (1265–1340 nm). Owing to its high nonlinear coefficient and flattened dispersion, the HN-OPCF is expected to be suitable for supercontinuum generation, which is very important in OCT applications. The numerical simulation results demonstrate that the proposed HN-OPCF can generate wideband supercontinuum.

Polarization-maintaining photonic crystal fibers with near-zero flattened dispersion in 1.06 μm waveband for medical applications

In this paper we present the design of a modified hexagonal photonic crystal fiber (PCF) having high birefringence and a near-zero flattened dispersion. Using the finite-difference method (FDM), it is shown that the proposed multiple Gedoped core hexagonal PCF exhibits a high birefringence of order 10−3 and a nearly zero flattened dispersion in the optical coherence tomography (OCT) waveband. In addition, the proposed PCF has a confinement loss of less than 10−8 dB/m at 1.06 μm. PCFs with such properties are considered suitable for both endoscopic OCT and other experimental setups employing 1.06 μm lasers.

Design of terahertz photonic crystal fibers by finite difference frequency domain method

Terahertz photonic crystal fibers are analyzed and designed using a full-vectorial 2D finite difference frequency domain method with nonsplit-field anisotropic perfectly matched layers absorbing boundary conditions. By analyzing the effective mode areas of fundamental mode and second-order mode, a phase diagram which describes the regions of single-mode and multimode operation as well as the non-confined fundamental mode regime is obtained in the terahertz region. The single-mode terahertz photonic crystal fibers with near-zero flattened dispersion are designed based on this method by optimizing the structure parameters.