High peak power operation and harmonic generation of a single-polarization, Yb-doped photonic crystal fiber amplifier

We report on the use of a single-polarization, 41 μm core-diameter, intrinsically single-mode photonic crystal fiber (PCF) to obtain high peak power (up to 800 kW), 1 ns-duration pulses in a 100:1 linearly polarized, intrinsically single-mode (M²  1.2) output. By transmitting the PCF output through nonlinear crystals, we also obtained efficient second, third, and fourth harmonic generation resulting in peak power >400 kW in the visible (green, 531 nm) and 200 kW in the UV (265.5 nm). To our knowledge these results represent the highest peak power obtained in a linearly polarized output from a fiber and the highest peak power in the visible and UV obtained through harmonic generation of the direct fiber output.     

双折射光子晶体光纤传输特性分析

采用时域有限差分法对光子晶体光纤导模的传输特性进行数值分析,通过该法可得到任意横向结构光子晶体光纤的色散特性和双折射特性。为提高精度,在计算中应用了各向异性完全匹配层作为吸收边界条件。光子晶体光纤的传输特性完全由其横向结构决定。用时域有限差分法对一类对称结构和两类非对称结构光子晶体光纤进行了数值分析,计算结果表明经合理设计的非对称结构光子晶体光纤中可存在较高的双折射(其双折射可达0．07)。表明时域有限差分法可有效应用于分析和设计具有特定色散和偏振特性的光子晶体光纤。     

Room temperature multiwavelength erbium-doped fiber ring laser using a highly nonlinear photonic crystal fiber

A multiwavelength laser source is demonstrated with a high power erbium-doped fiber amplifier as the gain medium. A highly nonlinear photonic crystal fiber (PCF) is inserted in the ring cavity to provide nonlinear gain by four-wave mixing. A Sagnac loop is incorporated in the ring cavity serving as a comb-like multichannel filter. The comparison between fiber ring laser without PCF and with PCF shows that the highly nonlinear PCF can generate a larger number of excited wavelengths and help stabilize the output power.     

温度对液晶填充光子晶体光纤传输特性的影响

利用液晶的折射率是温度和波长函数的特性,在光子晶体光纤(PCF)芯区的空气柱中填充向列相液晶,通过改变温度来改变液晶的折射率,构成了一种温度凋制光子晶体光纤.用阶跃有效折射率模型研究了温度对这种光子晶体光纤在不同光波长时传输特性的影响,并进行了数值计算.结果表明液晶填充使光子晶体光纤的色散减小,由于折射率对温度和波长变化敏感,改变温度可以使光纤在长波长区域出现单模传输,在短波长时不会出现单模传输,即使包层相对孔径很小也不会出现无截止单模传输.温度升高使光纤的色散值增大,零色散波长向短波长方向移动.这些特性对温度调制光子晶体光纤器件的设计和应用具有一定的参考意义.     

Multi-wavelength fiber laser based on nonlinear polarization rotation in semiconductor optical amplifier and photonic crystal fiber

A nonlinear polarisation rotation (NPR) based multi-wavelength laser source is demonstrated using a semiconductor optical amplifier (SOA) and photonic crystal fiber (PCF). The SOA acts as not only the gain medium but also the phase retarder for multi-wavelength oscillation in conjunction with an isolator. The incorporation of PCF improves the NPR effect and thus assists in the multiwavelength generation. The wavelength spacing reduces from 4.82 to 2.76 nm as the PCF length increases from 50 to 100 m. With a 100 m long PCF, the proposed laser produces at least 42 lasing wavelengths with optical signal to noise ratio of more than 10 dB.     

Build-up of supercontinuum in heated and unheated photonic crystal fibers using a chirped femtosecond laser

The build-up of supercontinuum in a photonic crystal fiber (PCF) has been investigated experimentally as a function of pump power using chirped 100-fs pulses from a Ti:sapphire laser. As compared with the PCF at room temperature, a new blue-shifted spectral component is observed in the initial steps of supercontinuum (SC) generation when the central part of PCF is heated to 120 °C by a hot plate. In addition, the slope efficiency of SG is slightly improved with the slightly extension of supercontinuum spectrum in blue edge at high pump powers. The change in dispersion property as well as the effective cascading of nonlinear photonic crystal fibers for heated PCF would be the main attributions.     

Optical Coherence Tomography Implemented by Photonic Crystal Fiber

We report a photonic crystal fiber (PCF)-based optical coherence tomography (OCT) system, in which conventional single-mode fiber for the transmission line and the fiber coupler for the beam splitter/combiner are replaced with PCF and PCF coupler. The PCF coupler fabricated using the fused biconical tapered (FBT) method showed a nearly flat coupling ratio over a broad spectral bandwidth of 400 nm, which provided an axial resolution of 3-μm for OCT imaging. With a white-light source, the 8-μm thick air gap between two stacked cover glasses was measured, and with a conventional superluminecent diode (SLD) source, the in vitro images of rat eye and Misgurnus mizolepis skin were successfully obtained. The PCF and PCF coupler might enable a white-light as the source for the cost effective and high-resolution OCT system.     

Tunable photonic crystal fiber coupler based on a side-polishing technique

A tunable photonic crystal fiber (PCF) coupler, which couples part of the optical power in one PCF with that in another PCF, has been made by side polishing. We fabricated the PCF coupler by mating two side-polished PCFs. We achieved evanescent field coupling between the core modes of the two PCFs by using side polishing to bring the cores close to each other. By adjusting the mating angle between the two side-polished PCFs we obtained as much as 90% tunability in the coupling ratio. The spectrum of the coupling ratio was almost flat, with small ripples, over a 400-nm wavelength range.     

Analysis of high birefringence photonic crystal fiber with octagonal and squarely lattice

Aiming at the requirement of high birefringence, a new kind of photonic crystal fiber (PCF) with octagonal and squarely lattice is proposed. In this structure, squarely lattices are added in the inner layer to obtain high birefringence. Birefringence and dispersion as a function of wavelength and size of PCF are analyzed by using Finite Element Method (FEM). Simulation results show that this kind of PCF exhibits high birefringence with a magnitude of 10^?3, and one zero dispersion point is obtained simultaneously. In addition, the characteristics of PCF can be tuned by changing the size of fiber.     

一种新型高双折射光子晶体光纤

提出了一种新的高双折射光子晶体光纤结构.应用全矢量频域有限差分方法所做的数值分析表明：该结构光纤基模的两个正交偏振态不再简并，其模式呈现很强的线偏振特性,并且模式双折射与结构参数设置有密切关系.通过选择合适的结构参数，可以使之达到10^-2量级，比传统的D型和熊猫型保偏光纤高出2个数量级.合理设计光纤包层的几何结构，可以取得理想的色散效果.这种结构的光子晶体光纤可用于制作具有适当色散特性或偏振特性的保偏光纤及相关光纤器件. 关键词： 光子晶体光纤 模式双折射 偏振特性 频域有限差分法     

Gas sensing properties of index-guided PCF with air-core

The gas sensing properties of index-guided photonic crystal fiber (PCF) with air-core are investigated via full-vector finite element method. The relationships between the gas sensing properties of index-guided PCF with air-core and the fiber parameters, including the fiber length and the operating wavelength, have been discussed. The simulations show that the gas sensing sensitivity of the fiber increases significantly as the diameter of the air holes increases, and it decreases with the holes pitch. Compared with the traditional PCF, there is a great improvement in the sensing properties of our design. The results are helpful for designing high performance PCF for gases or liquids sensing.     

Improved high birefringence photonic crystal fibres with dispersion flattened and single mode operation

A kind of improved high birefringence photonic crystal fibre (PCF) is proposed in this paper. The characteristics of birefringence, dispersion and leakage loss are studied by the multipole method. Numerical results show that the improved PCF possesses the properties of a flat dispersion and single mode operation. Moreover, with the operating wavelength λ = 1.55μm, the modal birefringence increases greatly in comparison with that of the original PCF, and the leakage loss is about 10⁴ times smaller than that of the original PCF because the modification gives rise to the strong confinement of guided modes. It is expected that the improved PCF can be used as high birefringence and dispersion flattened fibres.     

基于纤芯折射率增强的高双折射光子晶体光纤

通过增加光纤纤芯区域折射率实现了一种高双折射光子晶体光纤.采用全矢量有限元和平面波展开方法,系统地研究了这种高双折射光子晶体光纤在不同的高折射率区域参数(比如区域形状、折射率)情况下的光纤特性.模拟结果表明,光子晶体光纤的双折射可以在优化的参数条件下获得很大提高,光子晶体光纤的非线性系数(连同双折射)也可以同时得到提高.     

光子晶体光纤拉制中工艺参数的控制

分析了光子晶体光纤拉制中各工艺参数之间的相互影响,建立了工艺参数与最终光纤结构之间的对应关系.在温度和送料速度的协调控制下,通过调节气压参数可有效控制气孔结构.实验拉制出孔径孔距比分别为0.45和0.8的单模以及高占空比光子晶体光纤.在制备非均匀孔径光子晶体光纤时,仅靠调控工艺参数往往难以拉制出理想结构,本文以一种单偏振单模PCF结构为例,对预制棒结构进行了优化设计.计算表明可由此拉制出满足要求的光子晶体光纤. 关键词： 光子晶体光纤 工艺参数 气压控制 气孔结构     

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     

Solid-core photonic crystal fiber as a Raman spectroscopy platform with a silica core as an internal reference

We show that solid-core photonic crystal fiber (PCF) is a promising platform for evanescent-field Raman spectroscopy of low-volume analytes. The Raman peak ratio of a silica core as a background to acetonitrile solution as analyte contained in the air holes maintains a constant value despite varying laser power and fiber length in a set of measurements. The Raman signal from the silica core can be used to eliminate the need to account for the coupling losses. These results demonstrate the feasibility of quantitative measurements using PCF as a Raman platform with silica as an internal reference. In addition, integrated Raman intensity increases with the length of the PCF due to long path length of light.     

高双折射光子晶体光纤偏振模色散测量

对一种高双折射光子晶体光纤的偏振模色散进行了测量.实验用26 m长光子晶体光纤使皮秒光脉冲的两个正交偏振模产生了108 ps时延.运用脉冲时延法和固定分析仪法对这种高双折射光子晶体光纤的偏振模色散进行了实验测量，测量得到其偏振模色散参量可达4154 ps/km，对应的模式双折射度为1.25×10－3.这种新型的高双折射光纤可用于补偿光纤通信系统中的偏振模色散.     

Variable photonic crystal fiber optical attenuator combining air hole reduction induced radiation and bending loss

Recently, an optical attenuator has been important in fiber optic communication systems, because a transmission power in fiber has become higher due to a channel increment in wavelength division multiplexing transmission. A photonic crystal fiber (PCF) optical attenuator is fabricated by air hole diameter reduction in a part of PCF in which radiations are caused in the air hole diameter reduced part of PCF. A PCF optical attenuator has a high power resistance feature due to its radiation-induced operation of optical attenuation. In this paper, we proposed a variable PCF optical attenuator in which a bend was applied to the air hole diameter reduced part in PCF optical attenuator that was fabricated by CO₂ laser irradiation. In PCF optical attenuator fabrication, the attenuation was adjusted by the reduced air hole diameter with laser irradiation time control. It was demonstrated that 10.6–13.5 dB of variable attenuation was obtained at 1550 nm-wavelength with 0°–90° bending angle applied to the air hole diameter reduced part in PCF optical attenuator.     

用于液体传感的高双折射新型光子晶体光纤结构

提出一种用于液体传感的光子晶体光纤的结构和设计方法。将六边形光子晶体光纤的纤芯区域设计成2个椭圆空气孔,分别填充水和乙醇,进而比较两种光子晶体光纤结构的传播特性。采用全矢量有限元法对所设计光子晶体光纤结构的双折射、相对灵敏度、限制损耗、非线性系数进行数值分析,对光纤各项参数的优化及纤芯椭圆结构参数进行调整,结果表明:当椭圆率为0.6时,纤芯填充乙醇的光子晶体光纤PCF2在波长1.55 nm处相对敏感度可达到72.506 7%,同时限制损耗可以降至10?8级。所设计的模型可用于传感和生物传感研究及应用。     

Modeling photonic crystal fiber for efficient soliton pulse propagation at 850 nm

We numerically investigate the dynamics of soliton propagation at 850 nm in chloroform filled liquid core photonic crystal fiber (LCPCF) by using both finite element method (FEM) and split step Fourier method (SSFM). We propose a novel chloroform filled PCF structure that operates as a single mode at 850 nm featuring an enhanced dispersion and nonlinearity for efficient soliton propagation with low input pulse energy and low loss over small distances. We adopt the projection operator method (POM) to derive the pulse parameter equations which clearly describes the impact of fourth order dispersion on the pulse propagation in the proposed PCF. To analyse the quality of the pulse, we perform the stability analysis of pulse propagation numerically and compare our results of the newly designed chloroform filled PCF with that of standard silica PCF. From the stability analysis, we infer that the soliton pulse propagation in modified chloroform filled PCF is highly stable against the perturbation.