Highly polarization-dependent periodic coupling in mechanically induced long period grating over air-silica fibers

A very flexible and versatile tunable mechanical grating platform is introduced, with which highly polarization-dependent mode coupling is observed for three types of air-silica microstructured fibers: hollow core fiber, hexagonal-boundary holey fiber (HHF), and circular-boundary holey fiber. The resonances of gratings showed highly polarization-dependent broadband coupling compared with conventional single-mode fibers due to their unique beat-length dispersions between the core and the cladding modes, which could find applications in wideband polarization-dependent loss compensation. We further present significance of the spatial symmetry of HHF in distinct mode coupling for different rotation angles around the fiber axis.     

Tunable Ytterbium-Doped Fiber Laser Based on a Mechanically Induced Long Period Holey Fiber Grating

We describe a tunable double-clad Yb-doped fiber laser based on a long period fiber grating mechanically induced in a section of single mode holey fiber inserted into the laser cavity. The mechanically induced long period holey fiber grating acts as a wavelength-selective fiber filter whose central wavelength, linewidth, and strength can be tuned by changing the period, the length of the grating, and the applied pressure. The fiber laser gives a ∼12.6 nm tuning range, from ∼1079:4–1092nm, with slope efficiencies of 18.7–26.3% at this wavelength range, with respect to the launched pump power.     

长周期光纤光栅传感器温度和应变灵敏度分析

从耦合模理论出发,分析了长周期光纤光栅的温度和应变灵敏度.结果表明,长周期光纤光栅的温度和应变灵敏度不仅与光栅周期和纤芯参数有关,还强烈依赖于包层参数.通过选择适当的参数,可以制成对温度(或应变)不敏感的应变(或温度)长周期光纤光栅传感器,从而可以解决光纤光栅传感器的温度和应变交叉敏感问题.同时,利用基模与不同包层模耦合时温度和应变灵敏度的不同,可以用一根长周期光纤光栅传感器同时测量温度和应变.     

长周期光纤光栅模式与耦合的研究

长周期光纤光栅是在光纤纤芯中沿轴向形成折射率周期性调制的带阻型光纤器件。与光纤布拉格光栅不同，它表现为前向传播的纤芯基模与同向传播的各阶次包层模式在特定波长的耦合。研究了纤芯基模、包层模式（HE1t/EH1t）及其有效折射率随波长的变化，研究了纤芯基模与包层模式（HE1t/EH1t）的耦合系数。研究表明，纤芯基模与一阶低次包层模式的有效折射率随波长增大而减小，纤芯基模与HE1t产生的耦合系数远大于与EH1t的耦合系数，并且包层模式次数较低时，耦合系数随次数的升高而增加。这与以前的研究结果有所不同。     

Tilted fiber Bragg grating sensors

Optical fiber gratings have developed into a mature technology with a wide range of applications in various areas, including physical sensing for temperature, strain, acoustic waves and pressure. All of these applications rely on the perturbation of the period or refractive index of a grating inscribed in the fiber core as a transducing mechanism between a quantity to be measured and the optical spectral response of the fiber grating. This paper presents a relatively recent variant of the fiber grating concept, whereby a small tilt of the grating fringes causes coupling of the optical power from the core mode into a multitude of cladding modes, each with its own wavevector and mode field shape. The main consequence of doing so is that the differential response of the modes can then be used to multiply the sensing modalities available for a single fiber grating and also to increase the sensor resolution by taking advantage of the large amount of data available. In particular, the temperature cross‐sensitivity and power source fluctuation noise inherent in all fiber grating designs can be completely eliminated by referencing all the spectral measurements to the wavelength and power level of the core mode back‐reflection. The mode resonances have a quality factor of 10⁵, and they can be observed in reflection or transmission. A thorough review of experimental and theoretical results will show that tilted fiber Bragg gratings can be used for high resolution refractometry, surface plasmon resonance applications, and multiparameter physical sensing (strain, vibration, curvature, and temperature).     

Highly sensitive bend sensor with hybrid long-period and tilted fiber Bragg grating

We demonstrate a new type of fiber optic bend sensor with a hybrid structure made up of a long period grating (LPG) and a tilted fiber Bragg grating (TFBG). The sensing mechanism is based on the spectrum of power transfers between the core and cladding modes from a TFBG located downstream from a LPG. We show that the curvature of a beam can be determined by the reflected power difference between the core mode and the recoupled cladding modes. We further provide design rules for the LPG and TFBG to optimize and linearize the sensor response. In addition, the temperature cross-sensitivities of this configuration are also investigated for two different types of fiber.     

长周期光纤光栅耦合常数的研究

长周期光纤光栅表现为前向传播的纤芯基模与同向传播的各阶次包层模式在特定波长时的模式耦合。由纤芯基模、一阶包层模式的场分布推得其功率分布 ,进一步得到两种模式的归一化常量。推导了纤芯基模与一阶各次包层模式 (HE1l/EH1l)的耦合常数。结果表明 ,纤芯基模与HE1l包层模式产生的耦合常数远大于与EH1l包层模式的耦合常数。次数低时 ,耦合常数随包层模式次数的升高而增大     

光子晶体光纤布拉格光栅传输谱特性的分析

研究光子晶体光纤中光纤光栅的传输谱特性对于研制基于光子晶体光纤的光纤光栅器件有着重要的意义。结合耦合模理论和光束传输相关函数方法,对一种典型光子晶体光纤中的布拉格光栅(FBG)传输谱进行了理论分析。比较了光子晶体光纤中布拉格光栅与常规布拉格光栅的传输谱。数值分析了光纤截面结构变化对于光栅传输谱的影响,并给出这种影响的定性解释。计算结果显示,与常规光纤光栅相比,包层模共振引起的损耗峰与正反向纤芯模耦合引起的损耗峰可以相比拟,而包层模共振的间隔也比常规光纤中光纤光栅的包层模共振间隔要大。同时给出了晶体光纤截面上空气孔的占空比,空气孔的排布层数对于传输谱影响的规律。     

High-efficiency broadband fiber-optic mechanical intermodal converter

We demonstrate a highly efficient, broadband fiber-optic intermodal converter. The technique relies on a long period grating mechanically induced in a two-mode fiber. A compact, portable apparatus was designed and fabricated, where period-variable metallic corrugation is implemented to form periodic micro-bends along the fiber. The coupling strength between the interacting fiber modes and the grating period can be tuned continuously and individually using two control knobs in the apparatus. Experimental results show that the complete coupling between the LP₀₁ and LP₁₁ modes is achieved, which is confirmed by an observed over-coupling while increasing the grating strength. For the short fiber length of <1.9 cm (33 grating periods), large band-rejection of −32.5 dB was obtained at resonance. The band rejection efficiency over 98.6% have been achieved in the entire communication C-band. As the grating strength increased, two over-couplings were observed at resonance, which indicates the high efficiency of the device. Experimental results are confirmed by our numerical simulations.     

Single-mode single-polarization holey fiber using anisotropic fundamental space-filling mode

We present the single-mode single-polarization regime of a circular-hole holey fiber consisting of a core with large elliptical holes. The elliptical holes in the core, which produce large anisotropies, split the fundamental mode into two orthogonally polarized fundamental modes, often referred to as slow and fast modes. This fiber can guide only one polarization state of the fundamental mode when a fundamental space-filling mode index of the cladding region is designed to lie between these indices of the slow and fast modes of the core region. We demonstrate one design example of this fiber and show that the single-polarization regime can be achieved over a wide wavelength range.     

模式耦合理论在圆周对称长周期光纤光栅建模中的应用

长周期光纤光栅是不同于光纤Bragg光栅的一种光纤光栅器件 ,根据模式耦合理论 ,长周期光纤光栅表现为前向传播的纤芯导模和同向的各阶次包层模式之间的耦合。分析研究了长周期光纤光栅轴向的模场变化。忽略轴向的模式耦合以及包层模式之间的相互耦合作用 ,并认为折射率指数的调制只存在于纤芯中 ,建立了简化的长周期光纤光栅数学模型。对圆周对称轴向均匀型长周期光纤光栅谱特性进行了仿真 ,其结果与实验结果基本吻合 ,表明了简化的数学模型的合理性     

Tuning properties of long period gratings in photonic bandgap fibers

We investigate the thermal tuning properties of long period gratings (LPGs) in a fluid-filled photonic bandgap fiber (PBGF). The combination of strong, resonant waveguide dispersion, characteristic of all PBGF modes, and the large thermo-optic coefficients of fluids yields highly tunable grating resonances. We measure grating resonances in three transmission bands with large tuning coefficients of up to -1.58 nm/degrees C, which match numerical results. We derive an analytic model for the PBGF LPG tuning coefficient to show how it depends on both the shift of the transmission bands and the dispersion of the coupled modes.     

Fiber-optic vector vibroscope

A directional vibration sensor based on polarization-controlled cladding-to-core recoupling is demonstrated. A compact structure in which a short section of multi-mode fiber (MMF) stub containing a weakly tilted fiber Bragg grating (TFBG) is spliced to another single-mode fiber without any lateral offset. Multiple core modes of the MMF are coupled at the junction and appear as well defined resonances in reflection from the TFBG. Some of those resonances exhibit a strong polarization and bending dependence. Both the orientation and the amplitude of the vibrations can be determined unambiguously via dual-path power detection of the orthogonal-polarimetric lowest order LP(1n) modes. Meanwhile, the unwanted power fluctuations and temperature perturbations can be referenced out by monitoring the fundamental LP(01) mode resonance.     

长周期光纤光栅中的包层模谐振

本文给出了光纤中包层模的电场形式,指出包层膜的电场形式与弱导光纤中导模的电场形式不同,对光纤中基模与包层模电场沿光纤横截面分布的重叠情况的分析表明,长周期光纤光栅除了能将HE₁₁导模耦合到HE_1p包层模外,也可以将其耦合到EH_1p包层模。基于耦合模理论的计算表明,耦合到EH_1p和EH_1,p-1包层模的耦合波长相近,对于典型的单模光纤,当p>4时耦合到这两个模式的耦合系数是可比的。     

Highly efficient lens couplers for laser-diodes based on long period grating in special graded-index fiber

A theoretical scheme of a new device consisting of a lensed fiber and a long period grating (LPG) imprinted in a special single mode gradient-index fiber (SSMGIF) is presented. The SSMGIF consists of the single mode uniform core and the graded-index cladding. The performance of the proposed device is based on specific properties of SSMGIF. For high efficiency coupling between number of cladding modes and co-propagating core mode in this special type of the fiber we can use a number of weak LPGs with the same amplitudes of the refractive index modulation and with almost the same lengths. This scheme provides higher coupling efficiency, relaxed longitudinal alignment tolerance, and smaller device size, than an identical scheme with the LPG imprinted in a step-index (SI) fiber. Two different types of fiber lenses (hemispherical and hyperbolic) are presented for comparison.     

Holey optical fiber with circularly distributed holes analyzed by the radial effective-index method

We analyze a holey fiber that consists of a circular distribution of air holes by the radial effective-index method. By this method, we show that the holey fiber is a leaky structure and its extended single-mode operation is governed by the differential leakage loss between the first two modes of the fiber. The effects of the hole size and the hole separation on the leakage losses of the first two modes are calculated. The leakage loss of the fundamental mode of the fiber is found to be comparable to that of a conventional holey fiber that has a hexagonal distribution of air holes.     

不同包层直径的倾斜光纤光栅折射率传感特性

倾斜光纤光栅的透射谱中有纤芯模和大量的包层模,它们具有与布拉格光栅相同的温度特性.利用HF酸腐蚀的方法得到具有不同包层直径的倾斜光纤光栅,研究了其对外界折射率的传感特性.结果表明,外界环境折射率在1.333～1.4532之间变化时,同一直径倾斜光纤光栅的高阶包层模的敏感性要比低阶包层模强;随着包层直径的减小,包层模的敏感性增强,且在折射率比较高的环境中有更高的敏感性.因此,利用倾斜光纤光栅的温度特性不仅可以解决温度交叉敏感问题,而且通过小同的腐蚀程度能定制所需要的灵敏度,以实现对环境折射率的高灵敏度测量.该办法可应用于对生物和化学等高灵敏度传感领域的各种溶液进行实时监控.     

Coupling between counterpropagating cladding modes in fiber Bragg gratings

We present an experimental demonstration of energy transfer between counterpropagating cladding modes in a fiber Bragg grating (FBG). A strong FBG written in a standard photosensitive optical fiber is illuminated with a single cladding mode, and the power transferred between the forward propagating cladding mode and different backward propagating cladding modes is measured by using two auxiliary long period gratings. Resonances between cladding modes having 30 pm bandwidth and 8 dB rejection have been observed.     

Fabrication of long-period fiber gratings by twisting a standard single-mode fiber

A new method of fabrication of long-period fiber gratings by twisting of a standard single-mode fiber at high temperature is presented. The method relies on the fact that there always exists some core-cladding eccentricity in the optical fiber. Therefore, when the fiber is twisted, its core follows a helicoidal path inside the cladding. The transmission spectrum of the helicoidal long-period fiber grating that is produced contains several dips that correspond to resonances with the fiber cladding modes.     

Numerical Analysis of Highly Birefringent Photonic Crystal Fibers with Bragg Reflectors

We analyze theoretically the coupling properties of Bragg gratings written in highly birefringent photonic crystal fibers with doped core and show how they can be tuned by the parameters of a microstructured fiber. We apply the coupled mode theory combined with a fully vectorial mode solver based on the plane-wave method. The results indicate large differences in interaction with the grating for the two linearly polarized fundamental modes. We show fiber designs, which provide single-mode operation with high birefringence and at the same time a high coupling efficiency of the grating. Such features can be used in fiber sensors, fiber laser configurations or to introduce a polarization dependent feedback in a long external cavity system with a semiconductor laser.