Design of large mode area multicore photonic crystal fiber with a flat-top mode

A large mode area multicore photonic crystal fiber with a flattened fundamental mode is proposed in this paper. Another two kinds of single core fibers are also presented as comparison. The modal characteristics such as effective mode area and confinement loss are investigated using the finite element method. Numerical results show that a combination of flattened fundamental mode, large mode area and ultralow loss is obtained in the multicore fiber by introducing higher refractive index in the core region.     

全固态八边形大模场光子晶体光纤的设计

提出一种新型的全固态八边形大模场低损耗的掺镱石英光子晶体光纤,利用多极法对光纤的结构和特性进行了模拟.这种结构的光子晶体光纤空气孔由掺有少量氧化硼的石英棒代替,简化了制备过程,提高了光纤的热损伤阈值.在波长为1064 μm处,光纤的模场面积可达2000 μm²,还可实现单模传输,而且其弯曲损耗很小,当弯曲半径为5 cm时弯曲损耗小于05 dB/m.这种光纤对光纤激光器和光纤放大器的发展有重要意义. 关键词： 光子晶体光纤 模场面积 弯曲损耗 限制损耗     

Bending insensitive large mode area photonic crystal fiber

A novel kind of large mode area photonic crystal fibers (PCFs) are proposed in this paper. In order to achieve large effective mode area, a novel technique is applied to seven missing air hole PCF structures. The modal characteristics of PCF structures such as effective mode area, confinement loss, chromatic dispersion properties with doped cores, are investigated by employing the full vectorial finite element method (FEM). Simulation results demonstrate that effective mode area and confinement losses of fundamental mode simultaneously improved by applying our novel technique to proposed structures. The effects of bending on confinement losses of the proposed PCFs have been thoroughly investigated. Additionally, confinement losses of first higher order modes are presented and the possibility of stripping them in a simple way is discussed.     

高功率光纤激光器全内反射型大模场光子晶体光纤设计

在高功率密度下产生的非线性效应和材料损伤等问题限制了光纤激光器输出功率的进一步提高。利用大模场光纤降低光纤能量密度,提高非线性阈值是一种最为直接和有效的手段。以空气孔尺寸为光波长量级的全内反射型光子晶体光纤为对象,采用等效折射率模型分析了光子晶体光纤的单模特性,利用有限元法分析了结构参数对光子晶体光纤的模场面积和色散等光束质量参数的影响。设计了一种工作波长为0.40~1.55 m,模场面积为112.74~258.87 m2,且在1.27 m附近可补偿色散的大模场光子晶体光纤。该研究可为高功率光纤激光器大模场光纤的进一步参数优化设计及元件加工提供重要参考。     

Characteristics of a Q-switched multicore photonic crystal fiber laser with a very large mode field area

We model and characterize the behavior of a Q-switched fiber laser. The fiber is a doped multicore photonic crystal fiber having six cores in a ring-type geometry. The fiber laser is Q-switched using an intracavity acousto-optic modulator. Using a mode filtering technique in the far field, a mode very close to the fundamental in-phase supermode is obtained with a mode field area of 4200 microm(2) and a divergence of 9 mrad. Pulses with energies of up to 2.2 mJ and durations of 26 ns (limited by end facet damage) at a repetition rate of 10 kHz are obtained.     

Dispersion-ultra-flattened square-lattice photonic crystal fiber with small effective mode area and low confinement loss

Proposed in this paper is a simple square-lattice photonic crystal fiber (PCF) with nearly zero flattened dispersion (NZFD) over a wide wavelength span. We make a trade-off between the coupling efficiency and the effective mode area in order to obtain relatively low confinement loss and high nonlinearity. Via full-vector finite element method with hybrid edge/node elements, over 1137–1710 nm, the dispersion coefficient is 0.3 ± 0.3 ps/(km nm), the confinement loss is relatively low, in level of 10⁻⁷–10⁻⁴ dB/km and the effective mode area remains 5.88–6.59 μm².     

High energy femtosecond supercontinuum light generation in large mode area photonic crystal fiber

A large mode area photonic crystal fiber (LMA PCF) with an effective area of 180 μm² is used to generate a high energy, micro-joule range, flat, octave spanning supercontinuum (SC) extending from ~ 600 nm to ~ 1720 nm. A train of femtosecond pulses from a widely-tunable parametric amplifier pumped by a Ti:Sapphire regenerative amplifier system are coupled into a 20 cm length of LMA PCF generating a SC of 1.4 μJ energy. We present an experimental study of the high energy SC as a function of the input power and the pumping wavelength. The spectrum obtained at a pump wavelength of 1260 nm presents spectral flatness variation less than 12 dB over more than 1.1 octave bandwidth. The physical processes behind the SC formation are described in the normal and the anomalous dispersion regions. Our experimental results are successfully compared with the numerical solution of the nonlinear Schrödinger equation.     

Highly dispersive large mode area photonic bandgap fiber

An all-silica photonic bandgap fiber composed of a low-index core surrounded by alternating high- and low-index rings allows us to achieve a large mode area (500 microm(2)) and large chromatic dispersion. Sharp resonances from the even Bragg mode to odd ring modes theoretically lead to 20,000 ps/(nm km) chromatic dispersion when large bends are applied. By nature, sharp resonances are sensitive to inhomogeneities along the fiber length. Under experimental conditions, the resonances are broadened and the dispersion coefficient is decreased to 1000 ps/(nm km). However, to the best of our knowledge, this is the largest dispersion coefficient reported using a large mode area fiber.     

新型单模大模场红外硫系玻璃光子晶体光纤设计研究

大模场光子晶体光纤在高功率激光传输、光纤放大器、光纤激光器中的广泛应用, 使其受到研究者的广泛关注.硫系玻璃在红外波段(1–20μm)具有优良透过性能, 且具有折射率高(2.0–3.5)、声子能量低(小于350 cm^-1)、 组分可调等特性, 成为制备红外光纤的理想材料. 本文设计一种基于Ge₂₀Sb₁₅Se₆₅硫系玻璃基质的新型单模传输、低损耗、超大模场面积光子晶体光纤结构, 经理论验证其在λ =10.6 μm处基模限制损耗远低于0.1 dB/m, 高阶限制模损耗大于2 dB/m, 模场面积约为13333 μm². 关键词： 硫系玻璃 大模场面积 红外光子晶体光纤 结构设计     

Squeezed hexagonal highly birefringent photonic crystal fiber with low effective modal area

A modified hexagonal index guiding photonic crystal fiber made of pure silica with high birefringence and a low effective modal area is proposed, and properties, including birefringence and effective modal area, are numerically analyzed using the multipole method. Numerical results show that high birefringence of 1.362 × 10⁻² and a low effective modal area of 3.435 μm² are achieved at 1.55 μm, simultaneously. Moreover, impacts of hole spacing and hole size on birefringence and effective modal area are also investigated in detail.     

Highly birefringent photonic crystal fibers with flattened dispersion and low effective mode area

A highly birefringent index-guiding photonic crystal fibers with flattened dispersion and low effective area is proposed by introducing elliptical air holes in the cladding and small holes both in the core area and in the cladding. With the plane wave expansion (PWE) method, the birefringent, dispersion and effective area of the fundamental modes in such photonic crystal fibers are analyzed in detail. The simulation result shows that high birefringence with a magnitude of the order of 10⁻³, flattened chromatic dispersion from 1100 nm to 1800 nm and low effective area (which mean high nonlinearity) are obtained. Furthermore, the influences on the birefringence and dispersion by geometrical parameters have also been discussed and a modest number of design parameters are given.     

Optimization of long-period grating inscribed in large mode area photonic crystal fiber for design of bandstop filter

By inscribing long period grating (LPG) in a photonic crystal fiber (PCF), optical filters for communication wavelengths can be fabricated. In this paper, by utilizing coupled-mode theory and enhanced improved vectorial effective index method, a design of bandstop filter based on optimal LPG parameters is presented where the optimized full-width half-maximum (FWHM), the number (N) and total length of gratings of LPGs (L) are determined. As a result, by the determination of optimized LPG parameters with an optimum length of L_opt=6.5 mm and optimum number of grating N_opt=27 inscribed in the PCF, a bandstop optical filter with FWHM=1 nm is designed.     

Supercontinuum generation and nonlinear pulse propagation in photonic crystal fiber: influence of the frequency-dependent effective mode area

Numerical simulations are used to study the effect of the frequency dependence of the effective mode area in photonic crystal fiber on supercontinuum generation. We quantify how the frequency dependence of the effective area influences the propagation characteristics through a modified optical shock term and identify the major consequence as a reduction in the extreme long-wavelength edge of the supercontinuum spectrum. Our results show that, for the parameter regimes used in many previous supercontinuum generation experiments using near-infrared femtosecond pump sources around 800 nm, this effect would be expected to be negligible. However, for pumps in the 1000–1500 nm range, the inclusion of this effect would be expected to be crucial for accurate comparison of simulations with experiment.     

一种与标准单模光纤高适配的低弯曲损耗光子晶体光纤

设计并研制出一种与普通单模光纤高适配的低弯曲损耗光子晶体光纤. 结构采用光纤预制棒制作工艺上易于实现的掺锗芯六孔结构. 应用间接测量方法, 对其模式、弯曲及色散特性进行了系统的评估. 在波长1550 nm处研制光纤的模场面积为79.26 μm², 色散为21.7 ps·km^-1·nm^-1, 模场面积和色散特性与标准单模光纤具有高的适配性. 在光纤弯曲半径为5 mm时, 在波长1550 nm处的弯曲损耗为0.0365 dB/圈, 小于G.657B的弯曲损耗0.5 dB/圈. 研究成果为光纤到户用低弯曲损耗光纤的实用化奠定了良好的基础. 关键词： 光子晶体光纤 低弯曲损耗 光纤到户 高适配性     

Preparation and characterization of a large mode area liquid-filled photonic crystal fiber: transition from isolated to coupled spatial modes

We describe in detail the manufacturing procedures for selectively closing holes in photonics crystal fibers and their infiltration with different liquids. We apply our method to create a large mode area liquid-filled photonic crystal fiber which consists of 19 liquid strands. By changing the mixing ratio between toluene and ethanol and by varying the temperature, we show continuous tuning from isolated to coupled behavior of the spatial mode profile. This demonstrates the versatility of selectively closed liquid-filled photonic crystal fibers for future photonic devices. Filling with nonlinear liquids, gases, metals, liquid crystals, low melting compound glasses, or quantum dots is possible, and spatial as well as temporal engineering of linear and nonlinear optical properties will become feasible, which should allow the observation of spatiotemporal solitons.     

Design of single polarization single mode dispersion compensating photonic crystal fiber

In this paper, we present a photonic crystal fiber based on hexagonal structure for improved negative dispersion as well as high birefringence in the telecom wavelength bands. It is demonstrated that it is possible to obtain negative dispersion coefficient of −712 ps/(nm km) and relative dispersion slope (RDS) perfectly match to that of single mode fiber (SMF) of about 0.0036 nm⁻¹ at the operating wavelength 1550 nm. The proposed fiber exhibits high birefringence of the order 2.11 × 10⁻² with nonlinear coefficient about 57.57 W⁻¹ km⁻¹ at 1550 nm. Moreover, it is confirmed that the designed fiber successfully operates as a single mode in the entire band of interest.     

Phase locking and supermode selection in multicore photonic crystal fiber lasers with a large doped area

We report on the laser properties of multicore photonic crystal fiber lasers. A stable phase locking of six- and seven-core structures through evanescent coupling is observed. Effective supermode selection is obtained by using both diffraction losses and the Talbot effect. A pure in-phase supermode is obtained (1.1 times diffraction limited). The laser operating in this mode has a slope efficiency of 70% with up to 44 W of output power. The modal area of the in-phase supermode multicore fiber is 1150 microm2, which makes it, to our knowledge, the single-mode fiber laser with the largest mode field area. In-phase laser action is stable when the fiber is bent.     

Comparative study of large-solid-core photonic crystal fibers: Dispersion and effective mode area

In this study, large-solid-core photonic crystal fibers with fixed air-hole diameter d = 0.84 μm and with fixed pitch length Λ = 4.2 μm are investigated for different d/Λ ratios. The dispersions and the effective mode-areas are obtained and compared for both the structures. It is seen that the dispersion management is easier by using the fixed d structures, but for working around the same zero dispersion points in a large interval of d/Λ the fixed Λ structures are more available. The A_eff values larger than 100 μm² are obtained with d/Λ smaller than 0.2 for both the two structures. A_eff increases rapidly with decreasing d/Λ to 0.1 and then reaches to A_eff value of 500 μm² at the d/Λ = 0.1 for the fixed d structures. The single-mode regime for the two structures is also discussed.     

Experimental verification of improved effective index method for endlessly single mode photonic crystal fiber

We report the experimental verification of Improved Effective Index Method (IEIM) for Endlessly Single Mode Photonic Crystal Fibers (ESM PCFs) from far field intensity measurements. In this paper all the key parameters i.e. V-Number, effective refractive index of the cladding, radius of the core and numerical aperture have been obtained from the experimentally measured far-field intensity pattern of ESM-PCFs. It is shown that the transmission characteristics of ESM-PCFs match with the simulation result obtained using IEIM and also with the core diameter of PCF obtained from scanning electron microscope (SEM) within the experimental limit.