High-temperature stability of long-period fiber gratings producedusing an electric arc

We report, for the first time to our knowledge, the excellent optical performance of long-period gratings written in three standard fibers and in two nonstandard fibers, namely, S-doped and N-doped fibers, using the flexible and low-cost electric arc technique. The fundamental mechanism for their formation is proposed. The grating thermal behavior and the effect of fiber preannealing prior to writing have been investigated. We have demonstrated a good stability of these gratings at high temperatures     

Step-changed long-period fiber gratings

The spectral characteristics of a new type of long-period grating (LPG) structure, step-changed LPG, are investigated theoretically and experimentally. It is shown that the spectral profile of such gratings can be tailored by adjusting the step parameters. A good agreement between simulated results and measured spectra is obtained. This type of LPG has potential applications in gain flattening of erbium-doped fiber amplifiers     

通信中长周期光纤光栅温度敏感性的补偿方法

为减小环境温度变化对通信中长周期光纤光栅(LPFG)器件谐振波长的影响,文章设计了一种补偿谐振波长漂移的方案,即在光栅表面涂覆一层随温度升高折射率变大的薄膜.计算结果表明,该方案是可行的,实现了谐振波长的零漂移,在通信领域具有广阔的应用前景.     

Analysis of phase-shifted long-period fiber gratings

Phase-shifted long-period fiber gratings (LPFG's) are analyzed by the coupled-mode theory together with the fundamental-matrix method. The effects of introducing multiple phase shifts at various locations along a LPFG are highlighted. The results compare well with published experimental data     

Sensitivity characteristics of long-period fiber gratings

We present a detailed investigation into the sensitivity of long-period fiber gratings (LPFGs) as a function of temperature, strain, and surrounding refractive index, with particular attention to the higher order cladding modes and the possibilities for ultrasensitive sensors. From a general theoretical analysis, we identify a general sensitivity factor which offers new physical insight into LPFG behavior and represents a useful design aid in conjunction with a set of measurand-specific sensitivity factors. Our analysis reveals the existence of turning points in the mode dispersion characteristics at which ultrasensitive operation may be obtained. In an extensive set of coordinated experiments, we verify the theoretical predictions with close agreement and provide demonstrations of the device behavior close to the turning points. Alternative sensor schemes for temperature, strain, and refractive index based, respectively, on measurement of the dual resonance characteristic of the modes and on the transmission characteristics close to the turning points, utilizing higher order modes of the LPFG, are presented. For two variables at least, we record the highest LPFG sensitivities yet reported     

Gain-flattening filter using long-period fiber gratings

We propose a novel structure for a gain-flattening filter, in which a conventional long-period fiber grating (LPFG) and a phase-shifted LPFG are written closely. The transmission characteristics are investigated in the case of a closely arranged configuration. For the fabrication of a phase-shifted LPFG, a UV-trimming method to control the amount of the phase shift is examined theoretically and experimentally. Finally, a 67-mm-long LPFG gain-flattening filter designed for an erbium-doped fiber amplifier is fabricated and the measured spectrum is shown     

Analysis of two parallel long-period fiber gratings

A wavelength-selective coupler constructed with two parallel identical uniform long-period fiber gratings (LPFGs) is analyzed by the coupled-mode theory. The effects of introducing a longitudinal offset distance between the two gratings on the transmission characteristics of the coupler are discussed and the conditions for achieving a 100% coupling efficiency are highlighted. It is found that 100% coupling can be achieved with a practical grating length by using a proper grating design and a suitable index-matching surrounding medium. The theoretical results agree well with the experimental data.     

Electrically controllable long-period liquid crystal fiber gratings

An electrically controllable fiber-optic filter for broad-band rejection has been demonstrated by periodically poling a liquid-crystalline core in a hollow-core fiber by means of an external long-period-combed electrode. The periodically poled liquid-crystalline core in a period of 483 μm couples the fundamental core mode to the leaky cladding modes. A maximum loss dip of approximately 15-nm bandwidth and 6 dB band rejection has been obtained for a nonpolarized light with a combed 250-V external voltage modulation. The experimental result matches well with a numerical expectation, which has been analyzed by the discretized coupled-mode theory for an anisotropic perturbation     

Nonlinear pulse switching using long-period fiber gratings

We show that the intensity required to achieve all-optical switching in long-period fiber gratings can be reduced by orders of magnitude through the use of uniform phase-shifting regions between gratings. Predicted switching intensities are of the order 100 MW/cm² compared to 10 GW/cm² for normal device configuration     

Design of long-period fiber gratings with fast-varying parameters

A transfer matrix of an arbitrary period in a long-period fiber grating (LPFG) is derived. Cascade of such matrixes can easily be used to calculate the overall performance of LPFG, with fast variation in parameters (which is common practice in LPFGs) such as period length, strength, etc. Results show that the difference in transmittivity between our work and traditional coupled-mode theory becomes obvious, when abrupt change such as π phase-shift occurs. This method can be used in the mask design aiming at overall performance optimization through period-by-period pattern adjustment     

Coated long-period fiber gratings as high-sensitivity optochemical sensors

In this paper, the numerical and the experimental analyses of coated long-period fiber gratings (LPFGs) as a high-sensitivity optochemical sensor are presented. The proposed structure relies on LPFGs coated with nanoscale high refractive index chemical-sensitive overlays. The deposition of overlays with refractive index higher than the cladding one leads to a modification of the cladding-mode distribution. If the overlay features are properly chosen, a strong field enhancement within the overlay occurs, leading to an excellent sensitivity of the cladding-mode distribution to the coating properties. The effects of overlay thickness and cladding-mode order on sensor performances have been numerically and experimentally investigated. In order to provide a high-sensitivity and species-specific optochemical sensor, this mechanism has been proved with nanoscale overlays of syndiotactic polystyrene (sPS) in the nanoporous crystalline /spl delta/ form. The sensitive material has been chosen in light of its selectivity and high sorption properties towards chlorinated and aromatic compounds. Sensor probes were prepared by using dip-coating technique and an adequate procedure to obtain the /spl delta/-form sPS. Experimental demonstration of the sensor capability to perform subparts-per-million detection of chloroform in water at room temperature is also reported.     

长周期光纤光栅的相移和色散特性

推导了长周期光纤光栅的相移和色散与透过系数之间的关系,指出了它们之间存在着内在的联系。对于一典型的长周期光纤光栅,给出了相移和色散系数曲线并结合理论推导对其特性进行了分析。结果表明,相对于Ｂｒａｇｇ光纤光栅,长周期光纤光栅只具有很小的,几乎可以忽略的色散（约为Ｂｒａｇｇ光纤光栅色散的１×１０－５）。进一步的分析指出,这是由长周期光纤光栅不同于Ｂｒａｇｇ光纤光栅的相位匹配条件决定的。     

A quasi-mode interpretation of radiation modes in long-period fiber gratings

We have found an efficient approximation based on a quasi-mode method to analyze radiation-mode coupling in long-period fiber grating (LPFG) devices, and which was deduced by consideration of the best confinement condition of radiation-mode spectra inside an optical fiber. A single-mode representation via the quasi-mode method enables one to solve the radiation-mode-coupling problems analytically, which is the advantage of the method. The numerical accuracy is found to quite acceptable in comparison with the conventional method of the Green-function representation of radiation-modes via numerical computation of the integral. Utilizing the proposed method, we present several numerical examples for radiation-mode coupling problems both in uniform and nonuniform LPFGs.     

Reproducibility of splicer-based long-period fiber gratings for gain equalization

We fabricated long-period fiber gratings(LPFGs) using electric arc discharges. We observed that the fiber becomes slightly tapered due to longitudinal tension during the arc:this effect depends on the arc current and time length. We experimentally investigated how these characteristics can influence grating's performances,especially in view of employing the LPFG as gain equalizer for an erbium-doped optical amplifier. As expected,we found that the spectral response of the grating depends on the period Λ,the intensity of the perturbation,the grating length and the type of mode-coupling induced. Since this last parameter cannot be estimated directly from the transmission spectra,we propose a method to determine the mode-coupling occurring in the device and to assess the index modulation induced by the electric arcs. This method combines both experimental and simulated data,and can be used to characterize LPFGs made-up by any method.     

Transmission spectra of coated phase shifted long-period fiber gratings

The transmission spectrum of the coated phase-shifted long-pcriod fiber gratings （LPFCrs） with single and multiple phase shifts is analyzed by the coupled-mode theory and the transfer matrix method, and the influences of the film parameters on the spectral characteristics are also studied. It is shown that these parameters will affect the LPFG filtering characteristics. The loss peak of Wansmission spectrum decreases with the increase of film thickness, and the peak position shifts with the film refractive index. Compared with the non-coated phase-shifted LPFG, the coated one has the similar desirable filtering characteristics, and it has a flexible ability to adjust the transmission properties.     

倾斜长周期光纤光栅薄膜传感器特性研究

为设计高灵敏光纤光栅薄膜传感器,基于耦合模理论研究了表面镀制高折射率敏感薄膜的倾斜长周期光纤光栅（TLPFG）耦合特性和薄膜折射率传感特性。TLPFG能激发高阶（角向序数l1）包层模耦合,高阶包层模耦合系数随倾角增大而增大,80时,高阶包层模耦合系数远小于1阶包层模耦合系数。研究表明,光栅倾角变化不影响包层模耦合谐振波长,但影响光谱透过率。进一步研究了TLPFG在80时1阶5次包层模耦合的光谱透过率对薄膜折射率变化的响应,分析了光栅结构参数（L、、）和薄膜参数（h3,n3）对薄膜折射率响应灵敏度的影响,结果表明,响应灵敏度相比于非倾斜LPFG可提高一个数量级,对薄膜折射率的分辨率可达10-9。     

Nanodeposition of materials with complex refractive index in long-period fiber gratings

An overlay of higher refractive index than the cladding is deposited on a long-period fiber grating (LPFG). This causes large attenuation-band shifts in the transmission spectrum, which permits the improvement of the sensitivity of the device to ambient and overlay refractive-index changes. To obtain maximum sensitivity for specific overlay and ambient refractive indexes, an optimum overlay thickness (OOT) must be selected. For complex overlay refractive indexes, there is an additional phenomenon of vanishing of the attenuation bands. This occurs for specific overlay-thickness values. The problem is analyzed with a numerical method based on linearly polarized (LP)-mode approximation and coupled-mode theory. Experimental results are contrasted with theoretical ones.     

Nonlinear property analysis of long-period fiber gratings usingdiscretized coupled-mode theory

We present generalized mathematical expressions for coupled-mode equations for nonlinear pulse propagation in fiber gratings using discretized coupled-mode theory and quantitatively analyze the nonlinear properties of long-period fiber gratings, considering multimode coupling between the core mode and several cladding modes. The calculations yield nonlinear responses for the case of long-period fiber gratings, including pulse shaping and all-optical switching in the self- and cross-phase modulation regimes. In addition, we briefly discuss the group delay properties of long-period fiber gratings and present several numerical examples of nonlinear pulse compression, which is related to strong dispersion and soliton-like behavior in fiber gratings     

基于光子晶体光纤长周期光栅的传感器研究

详细研究了光子晶体光纤长周期光栅(LPG-PCF)的应力和温度特性,利用成功制作的温度不敏感PCF-LPG,实现了两种结构简单光纤传感器.在温度不敏感应力传感器中,PCF-LPG作为温度不敏感的应力传感头,当以波长为1553nm的分布反馈(DFB)激光器为入射光源、用分辨率为0.001dB的光功率计测量时,PCF-LP...     

在光纤拼接处刻写长周期光纤光栅及其传感应用

在"单模光纤-细芯光纤"拼接处刻写了 Type Ⅱ 型长周期光纤光栅,构成一种高温光纤传感器.该传感器实际由2 段不同的长周期光纤光栅级联构成,在第二个光栅形成了 Mach-Zehnder 干涉,因此,传感器透射谱由长周期光纤光栅与Mach-Zehnder 干涉仪共同作用形成.利用传感器的透射谱与环境温度的线性变化关系...