空芯反谐振光纤,以其空芯导光所带来的低延迟、高容量、低色散、低非线性、高损伤阈值、低散射噪声和其结构特点所带来的易制备、纤芯尺寸大、模场能量与材料交叠低等独特优势,为光纤通信、光纤传感、高功率激光传输及太赫兹波传输等系统瓶颈问题的突破,提供一个新的解决途径。因此,空芯反谐振光纤已成为特种光纤领域备受关注的研究方向。本学位论文在国家自然科学基金项目“单偏振空芯微结构光纤及在高精度小型化谐振式光纤陀螺的应用研究”和北京交通大学研究生创新基金项目的共同资助下,面向近红外、中红外以及太赫兹波段应用所急需的高性能空芯光纤的发展需求,开展应用于近红外、中红外以及太赫兹波段的多种高性能空芯反谐振光纤的理论与实验研究工作。论文取得的主要创新性成果如下:1.推导出空芯反谐振光纤非谐振区域模式有效折射率的近似解析表达式。从光波导基础理论Helmholtz方程出发,以无限壁厚中空管作为替代模型,推导得出能够用于计算空芯反谐振光纤非谐振区域内不同模式有效折射率的近似解析表达式,丰富了空芯反谐振光纤理论,为新型空芯反谐振光纤的研究与发展奠定了坚实的理论基础。2.提出一种双层管包层结构的中红外单模空芯反谐振光纤。以全圆管包层结构降低制备难度,在2.5μm到3.3μm的超宽波长范围内,损耗低于4×10-3 d B/m,高阶模消光比高于1000,实现优异的低损耗单模传输特性。最高高阶模消光比在2.8μm波长处高达16600,在此波长处有效模场面积高达2314μm~2。与单层圆管结构相比,其高阶模消光比水平提升了2至3个数量级。研究成果为高功率中红外激光传输提供了优质传输媒介。3.利用折射率调制和几何尺寸控制相结合,提出一种单模单偏振中红外空芯反谐振光纤。引入两种尺寸包层管从而同时抑制LP11模式与LP21模式,保证光纤单模性能,并引入高折射率包层管获取折射率分布的不对称性,实现对y偏振态基模的抑制,实现单偏振传输。在3.0μm波长处,x偏振态基模损耗为0.048d B/m,偏振消光比与高阶模消光比分别高达5085和495。此外,通过结构尺寸调整,该结构光纤可以在2.0μm至4.0μm波长范围内进行拓展,实现在该波长范围内任何期望波长处的单偏振单模传输。4.提出一种差异壁厚单偏振单模空芯反谐振光纤。以双层包层结构设计保证单模传输,引入差异壁厚打破结构对称性,利用加厚包层管壁上的模式与特定偏振态基模的耦合来实现对某一偏振态的抑制。在1550 nm波长处,实现偏振消光比和高阶模消光比分别高达17662及393的极佳单偏振单模传输特性。其偏振消光比水平,为目前同类相关研究中的最高水平。5.提出一种在双层交替管单模空芯反谐振光纤中引入高折射率材料镀层实现单偏振传输的方案。引入镀硅包层管来打破双层交替管单模空芯反谐振光纤结构对称性,使y偏振态基模与镀硅管模式发生耦合,最终实现单偏振单模传输。在1550 nm波长处,偏振消光比与高阶模消光比分别高达4803和308,x偏振态基模损耗为0.11 d B/m,总单偏振单模带宽高达45 nm。并在不同结构的空芯反谐振光纤中利用这一方案实现单偏振传输,相比于差异壁厚包层管设计,单偏振传输带宽由数nm提升至数十nm。6.提出一种不对称包层结构的高双折射太赫兹空芯反谐振光纤及一种开槽壁包层结构的太赫兹空芯反谐振光纤。以不同尺寸包层管构建类椭圆不对称芯区,从而实现高几何双折射,在1.0-1.24 THz频率范围实现高于7×10-4的双折射,最高双折射8.7×10-4位于1.04 THz频率处。另外,以壁上开槽的设计在厚壁太赫兹空芯反谐振光纤中实现近似薄壁的传输特性,以调解3D打印技术最小壁厚限制与太赫兹空芯反谐振光纤中损耗和带宽性能对薄壁厚需求之间的矛盾,并详细研究开槽壁对传输特性的影响。3D打印所制备的开槽壁太赫兹空芯反谐振光纤在太赫兹时域光谱系统中的实验检测结果,证实了该设计的有效性。

We propose and experimentally demonstrate a multiwavelength erbium-doped fiber laser (EDFL) based on a compound filter. The compound filter is composed of polarization controllers (PCs), polarizers and a piece of home-made polarization-maintaining photonic crystal fiber (PM-PCF). Due to the high birefringence and few mode properties of the PM-PCF, Lyot filter and multimode interference filter can be realized simultaneously in the compact structure. Hence, the EDFL can achieve flexible lasing output. By adjusting PCs, the lasing can obtain single-, dual-, triple-and quad-wavelength output, respectively. The tuning range of single-wavelength lasing output can exceed 20 nm with side-mode suppression ratio (SMSR) above 40 dB. For dual-wavelength lasing output, the tuning range with fixed interval can reach 16 nm. Besides, the wavelength interval can be adjusted from 2.5 nm to 17.4 nm. The great interval adjustability is also observed in triple-wavelength output. The wavelength interval can be adjusted from 8.2 nm to 14.7 nm. Compact structure, flexible lasing output, and good stability make our proposed EDFL more convenient and practical to the applications such as sensing and wavelength division multiplexing.

A denoising method based on singular value decomposition (SVD) with particle swarm optimization (PSO) is proposed to improve the signal-to-noise ratio (SNR) of far-end disturbances in distributed sensor system based on phase-sensitive optical time-domain reflectometry ( $\Phi $ -OTDR). Also, an improved clustering algorithm is introduced to locate the position of disturbance. The effective sensing distance of the $\Phi $ -OTDR system is 25.05 km and four kinds of disturbance events, including watering, knocking, climbing and pressing, are applied on the sensing fiber respectively. A series of experiments of single-point far-end disturbances and five-point disturbances are carried out. Experimental results demonstrate that the SNR of far-end disturbance can be effectively improved to over 12dB, the processing time is less than 3 seconds, and the average location accuracy rate is more than 96%. Compared with the commonly used denoising methods, such as empirical mode decomposition (EMD), wavelet-1D and wavelet-2D, the SVD denoising with PSO method has better performance in SNR enhancement and real-time, which is beneficial for accurate positioning.
© 2001-2012 IEEE.

A novel hybrid hollow-core polarization-maintaining fiber is proposed by combining the photonic bandgap mechanism and anti-resonant effect. High birefringence can be achieved by introducing a pair of negative curvature anti-resonant layers into the air core of a 7-cell HC-PBGF to obtain two-fold rational symmetry. Within the wavelength range from 1.45 mu m to 1.65 mu m, the birefringence reaches up to the magnitude order of 10(-3) and high-order mode suppression ratio, which is defined as the ratio of the lowest loss of high-order modes to the highest loss of fundamental modes, is greater than 100. Hence, the proposed fiber can realize high birefringence and single mode simultaneously in the 200 nm wide wavelength range. Besides, an extended structure by changing the position of the anti-resonant layers also proves the advantages of hybrid structure in achieving high birefringence and single mode operation. The proposed hybrid structure owns great potential for polarization-sensitive applications and provides a new idea to design hollow-core polarization-maintaining fibers with high birefringence and single mode.

An antiresonant hollow core effective single-mode terahertz fiber is proposed and successfully fabricated with a photosensitive resin (SomosEvoLVe 128) by a 3D printer. The single mode characteristics of the fiber are found to be related to the area of the semielliptical tubes in the cladding. By optimizing the cladding structure parameters, a Higher Order Mode Extinction Ratio (HOMER) of above 6 can be achieved. A 30 cm long sample with optimized structural parameters is obtained by cascading two 15 cm long fiber samples fabricated by a 3D printer. The transmission loss and the electric field distribution are measured by a terahertz time domain spectroscopy (TDS) system. The electric field distribution measured at the output end of the fiber sample has the Gaussian profile, which accords with single-mode field distribution. Experimental results show that the average loss is 0.048 cm-1 within the frequency range from 0.2 to 1 THz, and the minimum loss is obtained as low as 0.009 cm-1 at the frequency of 0.82 THz.
© 2013 IEEE.

We propose a temporal Fresnel filtering based repetition rate multiplication (RRM) scheme suitable for multi-wavelength optical pulse trains. Residual chirp terms inside and outside the temporal Fresnel integral contribute to preserving of the multi-wavelength feature. Implementation of our scheme is very simple and compact, and only one linearly chirped Bragg fiber grating (LCBG) and one temporal grating (TG) are needed. Optical signals before and after the TG share the same LCBG by forward and backward passing to implement temporal original and inverse Fresnel transforms, respectively, avoiding possible mismatch between these two transforms and minimizing the influence of the third and higher order dispersion coefficients. Electrical programming of the TG contributes to the tunability of multiplication factor m. In numerical simulations, our scheme succeeds to preserve the multi-wavelength feature of an input pulse train, while the current temporal Fourier filtering and temporal Talbot effect based schemes fail, verifying the novelty of our scheme. Our work is especially useful for tuning the scanning resolution of steering angles of microwave beams, formed by photonic-assisted phased array antennas, in the application of five-generation (5G) wireless communications.
© 2020