Calculation of the frequency response in step-index plastic optical fibers using the time-dependent power flow equation

The time-dependent power flow equation, which is reduced to its time-independent counterpart is employed to calculate frequency response and bandwidth in addition to mode coupling and mode-dependent attenuation in a step-index plastic optical fiber. The frequency response is specified as a function of distance from the input fiber end. This is compared to reported measurements. Mode-dependent attenuation and mode dispersion and coupling are known to be strong in plastic optical fibers, leading to major implications for their frequency response in data transmission systems.     

Frequency response in step-index plastic optical fibers obtained by numerical solution of the time-dependent power flow equation

The time-dependent power flow equation is employed to calculate frequency response and bandwidth in addition to mode coupling and mode-dependent attenuation in step-index plastic optical fibers. Frequency response is specified in the paper as a function of fiber length. Results are found to match reported measurements better than the existing analytical solution does. Mode-dependent attenuation and mode dispersion and coupling are known to be strong in plastic optical fibers, leading to major implications for their frequency response.     

Double random mirror Hi–Bi photonic crystal fiber Sagnac based multiwavelength fiber laser

In the present work, a multiwavelength Raman fiber laser based in a Sagnac configuration is presented. The Sagnac configuration is composed of a Hi–Bi photonic crystal fiber in the middle of two dispersion compensation fibers. The pump power induces two random mirrors to work in opposite directions in the Sagnac, leading to the amplified signal and the multiple scattering signals from the random mirror traveling through the Hi–Bi photonic crystal fiber in a counter-propagating way interfering in the output port.     

Soliton formation in hollow-core photonic bandgap fibers

The formation of solitons upon compression of linearly chirped pulses in hollow-core photonic bandgap fibers is investigated numerically. The dependence of soliton duration on the chirp and power of the input pulse and on the dispersion slope of the fiber is investigated, and the validity of an approximate scaling relation is tested. It is concluded that compression of input pulses of several ps duration and sub-MW peak power can lead to a formation of solitons with ∼100 fs duration and multi-megawatt peak powers. The dispersion slope of realistic hollow-core fibers appears to be the main obstacle for forming still shorter solitons.     

Two-octave spectral broadening of subnanojoule Cr:forsterite femtosecond laser pulses in tapered fibers

Spectral broadening of femtosecond Cr:forsterite laser pulses is enhanced due to the use of tapered fibers. Supercontinuum generation with unamplified subnanojoule femtosecond Cr:forsterite laser pulses is observed for the first time. With 40-fs 0.6-nJ pulses of 1.25-μm Cr:forsterite laser radiation coupled into a tapered fiber having a taper waist diameter of about 2 μm and a taper waist length of 90 mm, we observed the spectra spanning more than two octaves at the output of the fiber in the regime of anomalous group-velocity dispersion. This result opens the way for the creation of compact femtosecond Cr:forsterite laser plus tapered fiber systems for optical metrology and biomedical applications. Received: 23 October 2001 / Accepted: 16 January 2002 / Published online: 14 March 2002     

48.8 km Ultralong Erbium fiber laser in active mode-locking operation

By the use of installed fibers inside the city we demonstrated a 48.8 km ultralong Erbium-doped fiber laser in modelocking regime with repetition rate varying from 1–10 GHz. The shortest pulse duration of 42 ps at 2.5 GHz was obtained by optimization of intracavity dispersion.     

三包层WⅠ和WⅡ型单模光纤波导色散特性的研究

研究了三包层WⅠ和WⅡ型单模光纤的波导色散特性。结果发现在相同条件下，三包层WⅠ和WⅡ型单模光纤零色散点的调节范围比传统的双包层W型单模光纤明显增大。详细分析了几何参量P、Q和光学参量R1、R2对单模传输时的波导色散特性和低次模截止频率的影响。所得的研究结果为获得更为理想的色散补偿、色散平坦光纤及设计新型无源光器件提供了重要的依据。计算波导色散的方法可推广到多包层光纤。     

Anisotropic Phantoms for Quantitative Diffusion Tensor Imaging and Fiber-Tracking Validation

Different fiber materials (hemp, linen, viscose rayon, polyamide and dyneema twine) were tested for their suitability as fiber phantoms for diffusion tensor imaging (DTI) calibration on clinical magnetic resonance imaging systems with common diffusion-weighted echo planar imaging sequences. Additionally, the potential for fiber tracking validation of these fiber phantoms was investigated. For phantom manufacturing the fibers were wound up into a bundle of parallel fibers enwrapped by plastic ribbon. The most homogenously distributed fractional anisotropy (FA) values (0.63 ± 0.10) were determined in the dyneema and polyamide (0.3 ± 0.1) fiber phantom. FA values in the viscose, linen and hemp bundles were at high variations (about 0.2 ± 0.10). The dispersion of the direction of the principal eigenvector in the polyamide and dyneema phantom was less than 7°, for the other fiber phantoms it was over 30°. Thus, the presented results may indicate that polyamide- and dyneema-based fiber phantoms provide the opportunity for verification and validation of DTI sequences on clinical scanner. Additionally, they can be applicable for testing the accuracy of fiber tracking algorithms. A strong parallel alignment of the fibers with a constant compression grade of the fiber bundles could be achieved by machine-made production. This could also provide highly reproducible diffusion properties within the anisotropic fiber phantoms. Authors' address: Kamil A. Il'yasov, Physics Department, Kazan State University, Kremlevskaya ulitsa 18, Kazan 420008, Russian Federation     

基于光纤自相位调制多波长全光再生的色散管理优化

针对基于光纤自相位调制效应的多波长全光再生系统,分析了光纤平均色散对再生器品质因子改进量的影响,提出一种色散管理优化方法.由于波长间色散差异会导致再生系统性能的稳定性下降,通过选择色散补偿光纤和单模光纤的长度,使其合成的色散曲线与各波长分别达到最优再生性能时对应的色散曲线近似拟合,同时保证该色散曲线落在性能满意度高的色散区域,然后调整信号输入功率和滤波器的偏移量,可有效改善多波长再生性能的稳定性,实现再生器的色散管理优化.     

Design of hybrid photonic crystal fiber: Polarization and dispersion properties

A highly birefringent dispersion compensating hybrid photonic crystal fiber is presented. This fiber successfully compensates the chromatic dispersion of standard single mode fiber over E- to L-communication bands. Simulation results reveal that it is possible to obtain a large negative dispersion coefficient of about −1054.4 ps/(nm km) and a relative dispersion slope of 0.0036 nm⁻¹ at the 1550 nm wavelength. The proposed fiber simultaneously provides a high birefringence of order 3.45 × 10⁻² at the 1550 nm. Moreover, it is confirmed that the designed fiber successfully operates as a single mode in the entire band of interest. For practical conditions, the sensitivity of the fibers dispersion properties to a ±2% variation around the optimum values is carefully studied and the nonlinearity of the proposed fiber is also reported and discussed. Such fibers are essential for high speed transmission system as a dispersion compensator, sensing applications, fiber loop mirrors as well as maintaining single polarization, and many nonlinear applications such as four-wave mixing, etc.     

Modelling of linearly chirped fiber bragg gratings by the method of single expression

To model chirped fiber Bragg gratings (FBGs) for dispersion compensation in optical fibers a novel method of single expression (MSE) is used. The reformulation of Helmholtz’s equation in the MSE to the full set of first-order differential equations leads to dealing with the electric field amplitude, its derivative, power flow density and phase distributions in any aperiodic media. The phase derivative obtained numerically permits to compute the dispersion slope in the time delay of investigated chirped gratings. Reflective and time delay spectra of linearly chirped gratings of different lengths and chirp coefficients are computed. A self-similarity law for the gratings of the same strength but different lengths and chirp coefficients is revealed. The apodization of gratings is applied to reduce sidelobes in gratings’ reflection spectra and eliminate oscillations in the time delay characteristics.     

Simultaneous measurement of anomalous group-velocity dispersion and nonlinear coefficient in optical fibers using soliton-effect compression

We present a novel and simple method to measure both the value of the second-order dispersion coefficient and the nonlinear coefficient in optical fibers. This method is based on the higher-order soliton-effect pulse compression phenomenon and is valid for dispersion values greater than 0.5 ps/km/nm. A non-zero dispersion-shifted fiber, a standard single-mode fiber and a highly-dispersive highly-nonlinear fiber have been measured using this method.     

Transmission loss and dispersion in plastic terahertz photonic band-gap fibers

We present an investigation into the transmission loss and dispersion of terahertz waves in plastic photonic band-gap fibers having a cladding with a finite number of air hole rings. The leakage loss and absorption loss caused by background material are analyzed by a full-vectorial two-dimensional finite difference frequency domain method and the lowest power transmission loss of 6.126 dB/m at 1.75 THz is realized. Numerical results show that a larger diameter-to-pitch ratio is suitable for lower transmission loss and lower group-velocity dispersion in plastic terahertz photonic band-gap fibers. PACS 41.20.Jb; 41.20.Cv; 42.81.Dp     

Dispersion engineering of tellurite holey fiber with holes formed by two glasses for highly nonlinear applications

Tellurite nonlinear holey fiber is characterized by a high nonlinearity and a broad transparent window. However, these advantages are canceled by its unflattened dispersion in most practical applications. The unflattened dispersion is due to the fact that tellurite holey fiber with complex structure is difficult to fabricate. In this work we develop a dispersion flattened tellurite composite holey fiber (TCHF). The TCHF has only one ring of holes. The holes are formed by two tellurite glasses. The heavy deformation, which probably occurs for tellurite complex microstructured fiber during the fabrication process, is avoided by this simple structure. The fiber is made of two glasses with different refractive-indices, which improves the flexibility in dispersion engineering. By using this structure the dispersion is engineered to be the most flattened for the highly nonlinear soft glass fiber within 1.5–1.6 μm. Owing to the flattened dispersion and high nonlinearity, more than one octave supercontinuum generation is demonstrated by a femtosecond fiber laser.     

Tailoring the dispersion properties of photonic crystal fibers

Photonic crystal fibers (PCFs) have had a substantial impact on nonlinear fiber optics and shortpulsed fiber laser systems due to their novel dispersion properties. The large normal or anomalous waveguide dispersion available in such fibers opens up a number of new opportunities not accessible with standard fiber technology. In this contribution, the fundamentals of PCF dispersion are briefly reviewed along with earlier results. In addition, some of our recent work on dispersion tailoring to facilitate nonlinear processes, and dispersion control in lasers will be presented.     

Analysis of chromatic dispersion effect on coherent optical CPFSK system and performance improvement with the use of combinations of various optical fibers

The performance of a coherent optical continuous-phase frequency shift keying (CPFSK) system is evaluated theoretically including the combined effect of fiber chromatic dispersion, non-zero laser line width and receiver noises. The system limitations imposed by chromatic dispersion and laser phase noise are depicted. The system performance can be improved by shifting the zero-dispersion wavelength to the operating wavelength of the system and this shifting can be achieved efficiently by using a suitable combination of different types of optical fibers. Dispersion shifting characteristics are demonstrated using four kinds of fibers, namely: standard single-mode fiber (SMF), dispersion shifted fiber (DSF), dispersion compensated fiber (DCF) and large core fiber (LCF). The effect of dispersion shifting on the system performance is evaluated in terms of transmission distance and bit rate.     

Numerical Investigation on Performance of In-Building Plastic Optical Fiber Transmission Systems and Role of Digital Television Broadcasting

Abstract

This article reports a numerical investigation on the transmission performance of multilevel systems operating in building links encompassing step-index plastic optical fibers. For such an aim, a simplified model for the multimode fiber propagation is introduced. A sub-carrier multiplexing technique is also simulated to demonstrate the distribution of broadcasting television channels by adopting such fibers. The reported results show that a unique building network based on step-index plastic optical fibers is suitable to carry both Ethernet and broadcast TV signals in all rooms.     

光纤波导色散的比例性质研究

基于麦克斯韦方程组的比例性质,讨论了光纤波导色散的比例性质,并对普通光纤、Bragg光纤和光子晶体光纤的波导色散进行了理论研究。以Bragg光纤为例,提出了一种根据波导色散的比例性质灵活设计光纤色散特性的方法。     

Optimum Compensator Positioning to Reduce Four-Wave Mixing in Wavelength Division Multiplexing Optical Communication Systems Using Dispersion-Managed Fibers

Abstract

The impact of the compensator's location along the optical fiber on a variation of four-wave mixing power penalty is investigated theoretically and numerically for dispersion-managed fibers in wavelength division multiplexing systems. The power penalty is analyzed for fibers with different attenuation coefficients and lengths, and the optimum positions for the compensator along the fiber is discussed in cases of both dispersion compensation of each fiber section between two amplifiers and whole-compensation.     

色散补偿光子晶体光纤结构参数对其色散的影响

光子晶体光纤由于其灵活可调的色散特性用作色散补偿具有极大的应用潜力. 设计了一种色散补偿光子晶体光纤, 并运用频域有限差分法模拟了其色散特性,从理论上分析了其结构参数孔间距Λ和空气占空比d/Λ对该光子晶体光纤的色散系数的影响, 并且实际制备出了3种不同结构参数的光子晶体光纤. 通过对其色散曲线对比分析表明: 当光子晶体光纤孔间距在1 μm附近时, 其色散系数随着孔间距Λ和占空比d/Λ的增大而增加, 但对于孔间距Λ的变化比占空比d/Λ更为敏感, 并且随着孔间距Λ的增加,其对色散系数的影响能力逐渐减小. 设计并制备的光子晶体光纤在1550 nm处的色散系数为-241.5 ps·nm^-1·km^-1, 相对色散斜率为0.0018, 具有较好的色散补偿能力. 关键词： 色散 色散补偿 光子晶体光纤 结构参数