Improved single mode property in elliptical air hole photonic crystal fiber

The single mode property of a holey, index-guided two-dimensional photonic crystal fiber (PCF) is investigated using the alternate direction implicit method. The modal analysis of a three-ring elliptical air hole photonic crystal fiber is performed and compared with the equivalent three-ring circular air hole photonic crystal fiber keeping the air-fill fraction the same for both cases. The fiber is investigated for single mode operation. The effect of the rotation of the ellipse axes on these characteristics is also considered. It is observed that a PCF with elliptical air holes exhibits a better single mode property over a wider wavelength range in the optical domain. Also when the diameter of the air hole is properly catered for, they can show an endlessly single mode property.     

Linear and nonlinear optical properties of hollow core photonic crystal fiber

We review the optical guidance properties of hollow-core photonic crystal fibers. We follow a historical perspective to introduce the two major optical guidance mechanisms that were identified as operating in these fibers: photonic bandgap guidance and inhibited coupling guidance. We then review the modal properties of these fibers and assess the transmission loss mechanisms in photonic bandgap guiding hollow-core photonic crystal fiber. We dedicate a section to a review of the technical basics of hollow-core photonic crystal fiber fabrication and photonic microcell assembly. We review some of the early results on the use of hollow-core photonic crystal fiber for laser guiding micro-sized particles, as well as the generation of stimulated Raman scattering, electromagnetically induced transparency and laser frequency stabilization when the fiber core is filled with a gas-phase material. We conclude this review with a non-exhaustive list of prospects where hollow-core photonic crystal fiber could play a central role.     

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.     

正方形空气孔光子晶体光纤特性分析

为了同时获得高双折射和色散平坦特性的光子晶体光纤,本文提出了一种包层以椭圆空气孔为纤芯,四周环绕正方形空气孔的光子晶体光纤结构。基于不同纤芯椭圆率、不同纤芯填充材料,对所提光子晶体光纤结构的双折射、色散、非线性等性能进行了讨论。结果表明,在波长1.55 μm处,当纤芯椭圆率不同,填充材料相同时,最大双折射值为0.37,最大非线性系数值277.76 W-1·km-1;当纤芯填充材料不同,椭圆率相同时,最大双折射值为0.34,最大非线性系数值为307 W-1·km-1。在波段1.26 μm~1.8 μm范围,色散呈现出近零色散平坦特性,变化范围不超过±12.5 ps/(nm·km),带宽0.6 μm。     

Low chromatic dispersion and high birefringence investigated in elliptical air hole photonic crystal fiber

An index-guided, two-dimensional photonic crystal fiber (PCF) with triangular lattice and solid air core was designed and investigated using the effective index method. A modal analysis of three-ring elliptical air hole photonic crystal fiber is presented: first the innermost ring was made elliptical and then the cells of the inner two rings were made elliptical and finally all three rings contained elliptical cells. The above configuration was then compared with three-ring circular air hole photonic crystal fiber keeping the area of air holes and simulation parameters of the wafer the same to obtain the fundamental space-filling mode (FSM). The analysis was performed using full vector FDTD for TE and TM polarizations. The complex effective refractive index generated was then used to calculate the chromatic dispersion and birefringence properties of the PCF.     

Ultra-bandwidth polarization splitter based on soft glass dual-core photonic crystal fiber

A novel ultra-bandwidth polarization splitter based on soft glass dual-core photonic crystal fiber (DC-PCF) is designed in this paper, which is analyzed through the finite element method (FEM). The coupling characteristics of the designed DC-PCF can be enhanced by a high refractive index As₂S₃ core. Numerical results show the ultra-bandwidths of the x- and y-polarization modes can reach to 86 nm and 60 nm as the extinction ratios better than −20 dB and −30 dB at the vicinity of the wavelength of 1.31 μm. The length of the designed soft glass DC-PCF is 52.29 mm and the extinction ratios of the x- and y-polarization modes are −85.57 dB and −56.81 dB at the wavelength of 1.31 μm, respectively. In addition, the designed splitter has a tolerance of ±10 nm in its all structure parameters, which make the design not sensitive to the perturbation during the fabrication process.     

Advanced design and optimization of single mode photonic crystal fibers

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 properties of PCFs such as dispersion, dispersion slope and loss, an artificial intelligence method, the Nero-Fuzzy system, is applied. In addition, a special cost function which simultaneously includes the confinement loss, dispersion and its slope is used in the proposed design approach. The results revealed that the proposed method is a powerful tool for solving this optimization problem. The optimized PCF exhibits an ultra low confinement loss and low dispersion at 1.55 µm wavelength with a nearly zero dispersion slope over the C communication band.     

Ultrabroad supercontinuum generation in tellurite equiangular spiral photonic crystal fiber

Simulations are presented of a very broad and flat supercontinuum (SC) in both the normal and anomalous group velocity dispersion regimes of the same equiangular spiral photonic crystal fiber at low pumping powers. For a pump wavelength at 1557?nm and average pump power of 11.2?mW, we obtained a bandwidth >3?μm (970?nm–4100?nm) at 40 dB below the peak spectral power with fiber dispersion ～2.1?ps/km nm at 1557?nm. In the same fiber, at pump wavelength 1930?nm and average pump power of 12?mW the SC bandwidth was more than two octaves (1300?nm–3700?nm) and dispersion was ～1.3?ps/km nm at 1930?nm. This demonstrates the potential use of the fiber for multi-wavelength pumping with commercially available sources at fairly low power.     

Multi-wavelength Q-switched Erbium-doped fiber laser with photonic crystal fiber and multi-walled carbon nanotubes

A simple multi-wavelength passively Q-switched Erbium-doped fiber laser (EDFL) is demonstrated using low-cost multi-walled carbon nanotubes (MWCNTs)-based saturable absorber, which is prepared using polyvinyl alcohol as a host polymer. The multi-wavelength operation is achieved based on non-linear polarization rotation effect by incorporating 50?m long photonic crystal fiber in the ring cavity. The EDFL produces a stable multi-wavelength comb spectrum for more than 14 lines with a fixed spacing of 0.48?nm. The laser also demonstrates a stable pulse train with the repetition rate increasing from 14.9 to 25.4?kHz as the pump power increases from the threshold power of 69.0?mW to the maximum pump power of 133.8?mW. The minimum pulse width of 4.4?μs was obtained at the maximum pump power of 133.8?mW while the highest energy of 0.74 nJ was obtained at the pump power of 69.0?mW.     

Tunable ultrashort electro-optical power divider using coupled photonic crystal waveguides

Photonic crystals (PCs) have many potential applications because of their ability to control lightwave propagation. We have investigated a tunable ultrashort electro-optical power divider in two-dimensional PC structures. The power divider, composed of a dielectric cylinder in air, is studied by solving Maxwell's equations using the plane wave expansion method and finite-difference time-domain method. The power-splitting mechanism is analogous to that of conventional directional couplers, utilizing coupling between guided modes supported by line defect waveguides. To increase the coupling coefficient of the PC coupler, the radius of the rods between two waveguides is reduced. The switching mechanism is a change in the conductance in the coupling region between the waveguides and hence modulating the coupling coefficient, and eventually switching is achieved. Such a mechanism of wavelength multiplexing should open up a new application for designing components in photonic integrated circuits.     

A kind of single-polarization single-mode photonic crystal fiber

A kind of single-polarization and single-mode totally internal reflection photonic crystal fiber (SPSM TIR-PCF) is proposed in this paper. It is a PCF structure with elliptical air holes in the cladding and four large holes in the first ring. A full-vector plane wave expansion method is employed to analyze this PCF structure. The numerical results show that this PCF structure can realize an ultra-broad SPSM bandwidth of 540?nm with a confinement loss less than 0.1?dB?km^?1, the broadest bandwidth to the best of our knowledge. Moreover, the structure that we proposed can realize a high nonlinear coefficient.     

Surface plasmon resonance biosensor based on large size square-lattice photonic crystal fiber

A surface plasmon resonance biosensor based on large size square-lattice photonic crystal fiber has been designed and simulated by finite element method. The square-lattice airholes are first coated with a calcium fluoride layer to provide mode confinement, then a nanoscale gold layer is deposited to excite the plasmon mode, and finally, the sample is infiltrated into the holes. The numerical results reveal that the resonance properties are easily affected by many parameters. The refractive index resolution of corresponding sensor can reach 4.3 × 10^?6 RIU when the optimum parameters are set as the radius of curvature of the airhole r = 2 μm, the thickness of the core struts c = 200 nm, the auxiliary dielectric layer s = 1 μm, and the gold film d = 40 nm. In addition, the effective area and nonlinear coefficient are calculated.     

Polarization-maintaining photonic crystal fiber based quarter waveplate for temperature stability improvement of fiber optic current sensor

Fiber optic quarter waveplate (QWP) is widely used in all kinds of optical fiber systems such as fiber sensing systems. Specifically, in fiber optic current sensor (FOCS) system, which is now applied in power grids to measure current and monitor operation, fiber optic QWP is a key device and has serious impact on temperature stability and accuracy of the system. We fabricated a QWP using polarization-maintaining photonic crystal fiber (PM PCF), and studied its impact on the temperature characteristic of a home-made FOCS system. We also made a QWP with conventional polarization-maintaining fiber (PMF) for comparison purposes. The measured system error of the FOCS prototype using a QWP made of PM PCF between ?40°C and 70°C was 8.45‰, which is five times smaller than with a QWP made of conventional PMF.     

Generation of a train of ultrashort pulses using periodic waves in tapered photonic crystal fibres

We consider a light wave propagation in tapered photonic crystal fibres (PCFs) wherein the wave propagation is described by the variable coefficient nonlinear Schrödinger equation. We solve it directly by means of the theta function identities and Hirota bilinear method in order to obtain the exact periodic waves of sn, cn and dn types. These chirped period waves demand exponential variations in both dispersion and nonlinearity. Besides, we analytically demonstrate the generation of a train of ultrashort pulses using the periodic waves by exploiting the exponentially varying optical properties of the tapered PCFs. As a special case, we discuss the chirped solitary pulses under long wave limit of these periodic waves. In addition, we derive these types of periodic waves using the self-similar analysis and compare the results.     

Highly birefringent photonic crystal fibers with near-zero dispersion at 1550 nm wavelength

This paper presents highly birefringent photonic crystal fibers with simultaneously near-zero dispersion and low confinement losses. The finite difference time domain method with anisotropic perfectly matched layer boundaries is used as the simulation software. According to simulation, it is shown that photonic crystal fibers with hybrid cladding and artificial defects along one of the orthogonal axes sufficiently results in a very high birefringence of the order 10^?2 which is two orders of magnitude higher than that of the conventional polarization maintaining fibers. Such a fiber also assumes both near-zero dispersion and low confinement losses at the 1550 nm wavelength. Optical fibers with novel properties such as high birefringence, near-zero dispersion, and low confinement losses may have applications in optical sensing applications.     

Semiconductor optical amplifier-based multi-wavelength ring laser utilizing photonic crystal fiber

A multi-wavelength laser source is demonstrated with a semiconductor optical amplifier (SOA) as a gain medium. A multi-wavelength comb with equal spacing is achieved due to Fabry–Pérot modes of the SOA which oscillates in the ring cavity. A 100 m long photonics crystal fiber (PCF) is inserted in the ring cavity to provide a nonlinear gain by four-wave mixing (FWM) so that the output comb spectrum can be greatly broadened and flattened. The stability of the ring laser is also increased due to the efficient FWM phenomenon occurring in the PCF. The SOA-based laser can generate 35 lasing lines with equal spacing of 0.28 nm and extinction ratios of more than 30 dB at room temperature. The number of channels of the multi-wavelength laser can be controlled flexibly by changing the ratio of the coupler used in the ring cavity configuration as well as controlling the polarization state of the oscillating laser.     

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。     

Nonlinear analysis of a photonic crystal laser

An approximate nonlinear analysis of light generation in two-dimensional square- and triangular-lattice photonic crystal lasers including gain saturation effects is presented for the TE modes. This model extends earlier studies which took into account only TM modes. Our approach is based on coupled mode theory. With the help of an energy theorem and a threshold field approximation an approximate formula relating the small signal gain required to obtain a given output power to the structure parameters has been obtained. It has been used to calculate laser characteristics revealing an optimum coupling strength for which laser structure achieves maximum power efficiency.