Ultra-flattened negative dispersion for residual dispersion compensation using soft glass equiangular spiral photonic crystal fiber

Abstract

We present a numerical investigation of an equiangular spiral photonic crystal fibre (ES-PCF) in soft glass for negative flattened dispersion and ultra-high birefringence. An accurate numerical approach based on finite element method is used for the simulation of the proposed structure. It is demonstrated that it is possible to obtain average negative dispersion of –526.99 ps/nm/km over 1.05–1.70 μm wavelength range with dispersion variation of 3.7 ps/nm/km. The proposed ES-PCF also offers high birefringence of 0.0226 at the excitation wavelength of 1.55 μm. The results here show that the idea of using the proposed fibre can be potential means of effectively directing for residual dispersion compensation, fibre sensor design, long distance data transmission system and so forth.     

Design of ultra-flattened nearly zero dispersion and ultrahigh non-linear slot microfibres with ultrahigh birefringence

We proposed a new simple design of microfibre employing an elliptical silica rod in the centre of fibre core region as a slot core for the purpose of controlling the chromatic dispersion properties of the microfibre and enhancing the performance of non-linearity and birefringence. The simulation results show that the proposed slot microfibre has ultra-flattened near-zero dispersion of 0.94 ps/(nm km) for quasi-TE mode over a 50-nm wavelength range, ultrahigh birefringence up to the order of 10^?1, and ultrahigh non-linear coefficients of 38.35 and 37.92 W^?1 m^?1 for the fundamental quasi-TE mode and quasi-TM mode at the wavelength of 1.55 μm. The outstanding advantage of this new design is that nearly zero ultraflattened dispersion, ultrahigh modal birefringence and ultrahigh non-linearity can be realized simultaneously simply using a slot fibre core. Benefiting from its excellent performance, the proposed slot microfibre will have great potential for all-optical signal processing applications.     

Highly birefringent large negative dispersion-flattened photonic crystal fibre for broadband residual dispersion compensation

A triangular lattice photonic crystal fibre is presented in this paper for residual dispersion compensation. The fibre exhibits a flattened negative dispersion of ?992.01 ± 6.93 ps/(nm-km) over S+C+L wavelength bands and ?995.83 ± 0.42 ps/(nm-km) over C-band. The birefringence is about 4.4 × 10^?2 at the excitation wavelength of 1550 nm which is also very high. Full vector finite element method (FEM) with a perfectly matched absorbing layer (PML) boundary condition is applied to numerically investigate the guiding properties of this PCF. The fibre operates at fundamental mode only. All these properties endorse this fibre as a suitable candidate for compensating residual dispersion and polarization maintaining applications.     

Analysis of birefringent and dispersive properties of photonic crystal fibers

Two types of high birefringence photonic crystal fiber (PCF) which import four or six big circular air holes near the elliptical-hole are proposed. Their birefringent and dispersive properties are analyzed by full-vector finite-element method (FEM). Numerical analysis demonstrates that importing the big circular hole near the center of elliptical-hole PCFs can achieve a high birefringence. When the ratio (d/Λ) of diameter to hole spacing is larger than 0.8, the proposed two types of PCF have a larger birefringence than that of sole elliptical air hole ones. When the ratio d/Λ is equal to 0.95, the birefringences of these two types PCF can be as high as 4.27×10(-3) and 5.09×10(-3) at the wavelength of 1.55 μm, respectively. Besides, PCF with the four big circular air holes has a large negative dispersion at the long wavelength in x-polarized mode, which indicates a potential in single-polarized mode dispersion compensation.     

Numerical investigation of birefringence and confinement loss formed by rectangular/elliptical/circular air holes photonic crystal fibers

In this paper, high birefringence and low confinement loss of rectangular air holes photonic crystal fibers (PCFs) are numerically investigated and compared with elliptical and circular patterns using the finite element method. The mode birefringence of the proposed PCFs with rectangular air holes at λ?=?1.55?µm reaches 8.1?×?10^?2 and the confinement loss is less than 5?×?10^?3?dB/km. Besides, a high birefringence up to 2.76?×?10^?2 is also achieved from the proposed circular air holes PCF, which is the highest value compared to conventional circular air holes PCF.     

Ultra-broadband,coherent mid-IR supercontinuum expanding from 1.5 to 12.2 μm in new design of AsSe2 photonic crystal fibre

We propose a new design of AsSe₂ photonic crystal fibre (PCF) with all-normal dispersion, nearly zero flat-top used to generate an ultra-broadband supercontinuum spanning from 1.5 to 12.2 μm. Simulated results show that, when we use only 1 mm of AsSe₂ PCF, a broadband mid-infrared supercontinuum with the specrum extent from 1.5 to 12.2 μm is obtained with a very low input energy of E = 1.3 nJ at the wavelength of 3.5 μm and pulse duration of 100 fs. We study the temporel and spectral impact of optical wave breaking in the development of the continuum. The influence of the fibre length, the input energy and the full width at half maximum is investigated. Compared to previous research works, we have obtained the broadest, coherent supercontinuum, which could be applicable in biomolecular sensing, cancer diagnostics, infrared spectroscopy and free space communication.     

Improving of high birefringence photonic crystal fiber with low confinement loss using small elliptical air holes in the core region

In this paper, a novel design double lattice photonic crystal fiber is proposed for achieving both high birefringence and low confinement loss. In this structure, circular air holes are arranged as octagonal lattice in the cladding and elliptical as rectangular lattice in the core region. Numerical results illustrate that the birefringence in such fibers is determined not only by the double lattice but also the changing of the shape and the arrangement of the air hole in the first inner rings of the cladding. The birefringence property and confinement loss are studied by employing the finite difference time domain method with transparent boundary condition. The numerical results demonstrate that the maximal birefringence and lowest confinement loss of our optimized structure PCF at the excitation wavelength of λ = 1550 nm can be achieved at 5.16 × 10^?2 and 0.003 dB/km, respectively.     

Design of an As2Se3-based photonic quasi-crystal fiber with highly nonlinear and dual zero-dispersion wavelengths

We propose an As₂Se₃-based highly nonlinear photonic quasi-crystal fiber with dual zero-dispersion wavelengths (ZDWs). Using a full-vector finite element method, the proposed fiber is optimized to obtain high nonlinear coefficient, low confinement loss and two zero-dispersion points by optimizing the structure parameters. Numerical results demonstrate that the proposed photonic quasi-crystal fiber (PQF) has dual ZDWs and the nonlinear coefficient up to 2600 W^?1 km^?1 within the wavelength range from 2 to 5.5 μm. Due to the introduction of the large air holes in the third ring of the proposed fiber, the ability of confining the fundamental mode field can be improved effectively and thus the low confinement loss can be obtained. The proposed PQF with high nonlinearity and dual ZDWs will have a number of potential applications in four-wave mixing, super-continuum generation, and higher-order dispersion effects.     

一种高双折射低限制性损耗光子晶体光纤设计

本文设计了一种适用于长距离光纤通信的新型光子晶体光纤。该光纤包层内椭圆形和圆形空气孔呈交错排列,纤芯两侧为两个小椭圆空气孔。利用有限元分析方法对所设计光纤的传输特性进行分析并对其结构进行了优化,确定了最佳结构。结果表明,波长为1550 nm时,此新型光子晶体光纤在最佳结构下可提供高达3.51×10^-2的高双折射和低至1.5×10^-9 dB/m的限制性损耗。与现存的引入椭圆形空气孔的光子晶体光纤相比,本文中的光子晶体光纤的双折射系数有较大提高,限制性损耗系数降低了5个数量级。另外,本文还详细研究了光子晶体光纤的色散随光子晶体光纤结构的变化以及其布里渊增益特性,并分析了其可制造性。基于其高双折射和低限制性损耗特性,此种光纤可应用于长距离光纤通信系统。     

Three octave spanning supercontinuum by red-shifted dispersive wave in photonic crystal fibers

This article presents a three-layer index guided lead silicate (SF57) photonic crystal fiber which simultaneously promises to yield large effective optical nonlinear coefficient and low anomalous dispersion that makes it suitable for supercontinuum (SC) generation. At an operating wavelength 1550 nm, the typical optimized value of anomalous dispersion and effective nonlinear coefficient turns out to be ~4 ps/km/nm and ~1078 W^?1km^?1, respectively. Through numerical simulation, it is realized that the designed fiber promises to exhibit three octave spanning SC from 900 to 7200 nm using 50 fs ‘sech’ optical pulses of 5 kW peak power. Due to the cross-phase modulation and four-wave mixing processes, a long range of red-shifted dispersive wave generated, which assists to achieve such large broadening. In addition, we have investigated the compatibility of SC generation with input pulse peak power increment and briefly discussed the impact of nonlinear processes on SC generation.     

Design of ultra-low and ultra-flattened dispersion single mode photonic crystal fiber by DE/EDA algorithm

This paper proposes a combination of differential evolution (DE) and estimation of distribution algorithm (EDA) to design photonic crystal fiber structures with desired properties over the C communication band. In order to determine the effective index of propagation of the mode and then, the other properties of structure, a finite difference frequency domain (FDFD) solver is applied. The results revealed that the proposed method is a powerful tool for solving this optimization problem. The optimized PCF exhibits a dispersion of 0.22 ps nm^?1 km^?1 at 1.55 µm wavelength with a variance of ±0.4 ps nm^?1 km^?1 over the C communication band and a nearly zero dispersion slope.     

Generation of stable and narrow spacing dual-wavelength ytterbium-doped fiber laser using a photonic crystal fiber

Abstract

We demonstrate the design and operation of novel narrow spacing and stable dual-wavelength fiber laser (DWFL). A 70-cm ytterbium-doped fiber has been chosen as the gain medium in a ring cavity arrangement. Our design includes a short length photonic crystal fiber, acting as a dual-wavelength stabilizer based on its birefringence coefficient and nonlinear behavior and tunable band pass filter (TBPF) to achieve narrow spacing spectrum lasing. Our laser output is considered to be highly stable, with power fluctuation less than 0.8 dB over a period of 15 min. The flexibility and tunability of TBPF, together with polarization controller enable the spacing tuning of the DWFL from 0.03 nm up to 0.07 nm for 1040 nm region, and 0.10 nm up to 0.40 nm for 1060 nm region. The tunable wavelength spacing shows the flexibility of the DWFL in addition to stable and reliable properties of fiber laser in 1-μm region.     

A potential candidate design for nanosecond-order delay based on high-group index four rows optimized air holes line-defect photonic crystal waveguide

In order to meet the demand of ultra-high optical delay within a limited transmission distance, a high-group index line-defect photonic crystal waveguide (PCW) is designed. By adjusting the parameters of holes adjacent to the defect, a high constant group index about 680 and the corresponding flat bandwidth approximately 0.4 nm are obtained. Group velocity dispersion in the flat-band slow light region is in the order of 10⁹ ps²/km, which is non-negligible. Thus, the pulse broadening caused by dispersion that limits the transmission distance of the pulse in PCW is evaluated through broaden factor (BF). Considering the limitation of BF, a highest time delay up to 70.9 ps for 24 μm delay line is obtained. Assuming return-to-zero modulation format, the above optimized delay line supports a bit rate of the signal up to 12.9 bit/s. The study provides an important approach for realizing large time delay with small size using line-defect PCW.     

Comprehensive characterization of PVDF-TrFE thin films for microelectromechanical system applications

This paper presents a comprehensive characterization of a polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) thin film with 75/25 molar ratio for piezoelectric MEMS applications. PVDF-TrFE film was deposited on a silicon substrate using spin coating, and electrodes were formed using sputtering. Dielectric constant and dielectric loss factor were measured at different frequencies. Frequency and temperature dependence of the ferroelectric response was examined to investigate required poling conditions and maximum operating temperature. The lower limit for the coercive field was measured as 55 V/μm at room temperature. Coercive field decreased with temperature with a slope of ?0.1 V/μm K, and ferroelectric to paraelectric transition occurred between 100? and 108?°C. Piezoelectric displacement measurements were performed using an atomic force microscope based method. Average value of the effective piezoelectric d₃₃ coefficient was measured as ?23.9 pm/V. No degradation was observed in this value after 2?×?10⁵ unipolar excitation cycles. On the other hand, significant fatigue was observed in the piezoelectric response due to polarization switching; 1.8?×?10⁵ cycles caused an average reduction of 33% in the effective d₃₃. Presented data corroborates with the previous studies in the literature and can be used in the design of PVDF-TrFE based MEMS devices utilizing its dielectric, ferroelectric, and piezoelectric properties.     

Tailoring of broadband dispersion-compensating photonic crystal fibre

This paper presents a broadband dispersion-compensating photonic crystal fibre (B-DCPCF) with a high compensation ratio of 30:1. We theoretically tailored the negative dispersion in a photonic crystal fibre (PCF) to nullify the positive dispersion in the transmission fibre over a bandwidth range of as wide as possible. The numeric results indicate that the effective dispersion within ±0.64 ps/nm/km over a bandwidth range of 226 nm (from 1338 to 1564 nm), cover the E + S + C wavelength bands. Finally, the confinement loss and the modal properties were examined to verify that the proposed B-DCPCF with extremely low confinement loss and should be operated in single mode throughout the operating band.     

Particle size distribution control and related properties improvements of tungsten powders by fluidized bed jet milling

Fine grinding process of different particle size tungsten powders was carried out by fluidized bed jet milling. The results showed that the jet milling treatment caused deagglomeration of tungsten powders, which led to particles sufficient dispersion and narrow particle size distribution. Grinding gas pressure of 0.70 Mpa made the particles achieve high speed which promoted the particles collision contributing to particle dispersion and shape modification. For tungsten powder with particle size of 3 μm FSSS, a higher packing density with 5.54 g/cm³ was obtained, compared with the 3.71 g/cm³ of the original powder. For tungsten powder with particle size of 1 μm FSSS, the big agglomerates disappeared and the particle size distribution become narrower, while small aggregates about 2–3 μm still exist after the jet milling process. For tungsten powder with particle size of 5 μm and 10 μm FSSS, different medium diameter particle size and narrow particle size distribution of monodisperse tungsten powders can be produced by the optimized jet milling parameters. More important, the effective dispersion, favorable shape modification and precise classification have been achieved in the simple process.     

Dispersion-compensating photonic crystal fiber with wavelength tunability based on a modified dual concentric core structure

We present a 5-layer air-hole dispersion-compensating photonic crystal fiber (PCF) with a modified dual concentric core structure, based on central rod doping. The finite element method (FEM) was used to investigate the structure numerically. If the structural parameters remain unchanged, a high degree of linear correlation between the central rod refractive index and the operating wavelength can be achieved in the wavelength range of 1.5457–1.5857 μm, which suggests that the operating wavelength can be determined by the refractive index of the centre rod. A negative dispersion coefficient between –5765.2 ps/km/nm and –6115.8 ps/km/nm was obtained by calculation and within the bandwidth of 108 nm (1.515–1.623 μm) around 1.55 μm, a dispersion coefficient of –3000 ps/km/nm can be ensured for compensation. In addition, this proposed PCF also has the advantage of low confinement loss, between 0.00011 and 0.00012 dB/m, and ease of fabrication with existing technology. The proposed PCF has good prospects in dispersion-compensating applications.     

A novel structure of photonic crystal fibre for dispersion compensation over broadband range

This paper studies a novel structure of photonic crystal fibre (PCF) for dispersion compensation at broadband wavelengths. The application of broadband is investigated using a design model based on combination of modal properties and dispersion compensation. The newly designed PCF with defect introduced is recorded over transmission spectrum range 146.7–256.98 THz, i.e., 1.16–2.04 µm. The modal characteristics and dispersion compensation of 2D PCF with circular air holes defect introduced are investigated and compared to those of conventional hexagonal 2D PCF. Changes in bandwidth behaviour are also observed by changing refractive index and geometric parameter of PCF.     

Design and modelling of dispersion-engineered rib waveguide for ultra broadband mid-infrared supercontinuum generation

We report a dispersion-engineered As₂Se₃ chalcogenide glass rib waveguide structure for ultra broadband mid-infrared supercontinuum generation across molecular ‘fingerprint region’. The proposed rib waveguide structure offers non-linear coefficient as high as 18,250 W^?1 km^?1 at 2.5 μm. Supercontinuum spectrum spanning 2–15 μm, which not only covers the both atmospheric transparent windows (3–5 μm and 8–13 μm) in the mid-infrared domain but also covers the important molecular ‘fingerprint domain’, is obtained using only 4 mm-long rib waveguide structure. To the best of our knowledge, such broadband mid-infrared supercontinuum spectrum in As₂Se₃-based chalcogenide waveguide geometry is reported for the first time. The proposed design of rib waveguide has potential for robust, integrated and low-cost supercontinuum sources in various applications including frequency comb generation, chemical sensing, food quality control and early cancer diagnostics.