Mode recoupling in a novel Bragg grating pair

A new type of mode recoupling in a Bragg grating pair, in which one of the gratings is written in the fiber cladding, is proposed to overcome limitations of concatenated long-period and fiber Bragg gratings reported earlier [Opt. Lett. 27, 1214 (2002)]. Its reflection spectrum is similar to that of the previously reported concatenated grating structure; however, the reflectivity can be much larger than the previous limitation of 50%. Furthermore, avoidance of the loss induced by a long-period grating makes such a Bragg grating pair more attractive for practical applications.     

Theoretical analysis of novel fiber grating pair

A novel fiber grating pair that consists of a conventional long-period fiber grating and a fiber Bragg cladding grating (FBCG) is proposed. The FBCG is a new type of fiber grating in which refractive index modulation is formed in the cladding. Through the coupled-mode theory, we accurately calculate the coupling coefficients between modes supported in the fibers. And some other mode coupling features in the fiber cladding gratings are analyzed in detail. The calculation of the modes involved in this paper is based on a model of three-layer step-index fiber geometry. Then, we have investigated the sensitivity characteristics for variation of the modulation strengths of the fiber Bragg cladding gratings’ resonance peaks and the long-period cladding gratings’ (LPCGs) dual resonant peaks. Finally, the modulation strength sensitivity of the grating pair’s three resonant peaks is demonstrated, and the results indicate that these grating pairs may find potential applications in optical fiber sensing.     

Measurement of refractive-index variation with temperature by use of long-period fiber gratings

The thermo-optic coefficient of the core material of a fiber is analyzed by use of a pair of long-period fiber gratings. First the effective index difference between the core and the cladding modes is measured from the peaks of the interference fringe generated by the grating pair. The order of the cladding mode is decided by the cutoff wavelength and the numerical aperture of the fiber. The material index of the fiber core is obtained in terms of wavelength. At each wavelength the index is chosen to minimize the difference between the measured and the calculated spectra of the grating pair. Finally the thermo-optic coefficient of the fiber core is calculated by repetition of the measurement at different temperatures. With a germanosilicate-core fiber and a boron codoped germanosilicate-core fiber, the thermo-optic coefficients were 1.1x10(-5)/( degrees )C and 0.75x10(-5)/( degrees )C, respectively.     

Tapered Mach–Zehnder interferometer based on two mechanically induced long-period fiber gratings as refractive index sensor

A Mach–Zehnder interferometer formed in single mode fiber is implemented. The interferometer is built by two mechanically-induced long-period gratings. In addition, a fiber taper in the middle section is inserted. The spectral properties of the whole system are analyzed. Visibility of the interference fringes up to 0.80 (the higher ever reported using mechanically-induced long-period gratings) with fringe spacing in the 4.1 to 0.86 nm range are experimentally demonstrated. The proposed device allows reducing the fiber diameter of the section between gratings with a minimal effect in the interference fringe spacing. The sensitivity of the interferometer to external refractive index changes was also studied. It is experimentally shown that, due to the nature of the cladding mode excited, it is necessary to taper the fiber to improve the system sensitivity to external refractive index. Fiber tapers with different diameter, inserted between the long-period gratings pair were fabricated and tested for measuring external refractive index changes. A maximum resolution of 2.3×10^?4 RIU in a refractive index range from 1.36 to 1.402 is achieved.     

Propagation of ultrashort pulses in a nonlinear long-period fiber grating

By solving a pair of normalized nonlinear coupled-mode equations, we analyze in detail the propagation of ultrashort optical pulses in a nonlinear long-period fiber grating and investigate the conditions for the grating to function as an optical limiter or a saturable absorber. We show that the function of the grating depends on whether the nonlinear effect counteracts or enhances the detuning effect, and the nonlinear effect can be weakened significantly by the group-delay difference between the core mode and the coupled cladding mode. We present simulation results to illustrate these effects and discuss the physical conditions required for an effective operation of a nonlinear long-period grating.     

Analysis of a widely tunable long-period grating by use of an ultrathin cladding layer and higher-order cladding mode coupling

A widely tunable long-period grating in single-mode fiber is analyzed by use of an ultrathin cladding layer and higher-order cladding mode coupling. The numerical simulation shows that a 225-nm tuning range in the newly designed ultrathin long-period grating (cladding thickness, 35 microm) with third-order cladding mode coupling can be obtained. The analyzed tuning range is seven times wider than those of the other known long-period gratings. We believe that the proposed highly sensitive long-period grating will be widely used as a gain-flattening filter for ultrawideband optical amplifiers and fast tunable filters in dynamic optical communication systems.     

Fiber Modal Index Measurements Based on Fiber Gratings

A novel method based on fiber gratings for measuring the effective indices of fiber modes is proposed. The effective index difference between the core mode and a cladding mode was obtained by analyzing the interference fringe of a pair of cascaded long-period fiber gratings. In order to extract the core mode index from the measured index difference, an index matching oil immersion method is proposed. By analyzing the interaction between the cladding mode and the oil applied on the cladding surface, the mode order and the effective index of the involved cladding mode might be calculated. Experimental results about the interference fringe shifts induced by the oil index and the oil-applied length are also presented.     

Fiber Modal Index Measurements Based on Fiber Gratings

A novel method based on fiber gratings for measuring the effective indices of fiber modes is proposed. The effective index difference between the core mode and a cladding mode was obtained by analyzing the interference fringe of a pair of cascaded long-period fiber gratings. In order to extract the core mode index from the measured index difference, an index matching oil immersion method is proposed. By analyzing the interaction between the cladding mode and the oil applied on the cladding surface, the mode order and the effective index of the involved cladding mode might be calculated. Experimental results about the interference fringe shifts induced by the oil index and the oil-applied length are also presented.     

Optimization of a long-period grating-based Mach-Zehnder interferometer for temperature measurement

An approach to optimize the design of the long-period grating pair as a temperature sensor device is presented, implemented by using a long-period grating (LPG) pair with a small separation (of around 2 mm) and scaling down their physical length by a factor greater than 2. The technique allows the interferometer formed not only to measure temperature variations over distances as small as the overall length of the grating pair (18 mm) but also to reduce the cladding losses between the LPGs forming the pair. This approach enhances the sharpness of the interference fringes (IFs) and the pits (Pts) in the transmission spectrum and, as a result, a high resolution sensor is obtained. The LPG pair is fabricated in the appropriate photosensitive single mode/core fibres, without being restricted to the use of dual core or other special fibres, thus exploiting the sensitivities of various fibres and reducing the overall system cost. In this work, the effectiveness of this technique is demonstrated by fabricating a small-scale LPG pair in a boron-germanium co-doped single mode fibre, with particular attention being paid to the higher order cladding modes. The sensitivity of the device thus created is 0.31 nm/°C with a root-mean-square (rms) deviation of 0.28 nm in the wavelength measurement, which corresponds to a temperature variation of approximately 0.9 °C. This was achieved while using a relatively low-resolution (0.6 nm) Optical Spectrum Analyzer to detect the wavelength changes of the device and was further improved to 0.7 °C when using an OSA with a resolution of 0.1 nm.     

表面镀层长周期光纤光栅双峰谐振及其透射谱研究

采用严格的耦合模理论,通过求解表面镀层长周期光纤光栅的特征方程,给出了双峰谐振波长的确定方法,以及它与光栅周期和模式序数之间的关系。结果表明,对应于较高次的包层模式,存在双峰谐振现象,且包层模式序数越高,与芯模产生谐振耦合所需的光栅周期越小。进一步讨论了双峰谐振波长的间距随薄膜参量与光栅参量变化的关系,描绘了这些参量对透射谱衰减谐振峰的影响,理论分析结果与X.W.Shu的无镀层长周期光纤光栅实验结果一致。这些研究为建立高灵敏的双峰谐振薄膜传感器提供了结构优化的理论支持。     

Cladding index modulated fiber grating

A new type of fiber grating with only cladding index modulation is presented. Characteristics of both cladding index modulated short-period fiber grating (FBG) and long-period fiber grating (LPFG) are analyzed. The calculation of the modes involved in this paper is based on a model of three-layer step-index fiber geometry. Transmission of a mode guided by the core through a cladding index modulated grating when evanescent field coupling occurs is analyzed with couple-mode theory. Evanescent field coupling causes a power flowing from the core to the cladding, so the attenuation of the new grating is analyzed as well. Lower attenuation, flexible spectral characteristics are demonstrated in comparison with traditional fiber core index modulated grating.     

长周期光纤光栅耦合模理论分析

长周期光纤光栅作为透射型光无源器件,以其在通信和传感领域具有广阔应用前景而引起人们的关注。要得到满足性能要求的光纤光栅,对其透射谱的分析是基础。通过耦合模方程确定了基模和一阶各次包层模之间的耦合常数。在设定的光纤光栅参数条件下,通过Matlab进行模拟得到了最终需要的长周期光纤光栅透射谱,并由此得出：长周期光纤光栅谐振波长出现的位置主要由光栅的周期决定,损耗大小可以通过调节光栅长度和折射率来实现。     

Deposition of overlays by electrostatic self-assembly in long-period fiber gratings

It was proved that the deposition of an overlay material onto a long-period fiber grating causes important shifts in the wavelengths of the typical attenuation bands that are caused by coupling between cladding and core modes [Opt. Lett. 27, 682 (2002)]. A theoretical model for analyzing a multilayer cylindrical waveguide is presented that permits the phenomenon to be understood and predicted. An overlay of higher refractive index than the cladding starts to guide a mode if a certain thickness value is exceeded. This causes large shifts in the resonance wavelength induced by the grating. One important application of this phenomenon to sensors is enhancement of the sensitivity of a long-period fiber grating to ambient conditions. Theoretical results are corroborated with experimental ones obtained by electrostatic self-assembly.     

Cladding-mode-assisted recouplings in concatenated long-period and fiber Bragg gratings

We investigate new types of mode recouplings in a concatenated grating structure comprising a long-period grating and a fiber Bragg grating. It is demonstrated that the light coupled out to the cladding mode by one of these gratings can be recoupled back to the guided mode by the other grating. Theoretical analysis based on the coupled-mode theory is presented, together with experimental results.     

CO2激光脉冲边缘写入的长周期光栅折射率特性研究

报道了一种用高频CO2激光脉冲在普通通信光纤包层边缘单侧写入的新型长周期光纤光栅。研究发现,这种长周期光纤光栅的折射率变化主要发生在光纤包层区域,而纤芯的折射率变化较小;同时该光栅的附加损耗低于0.5 dB。进一步折射率特性实验研究表明,由于其特殊的折变结构,这种光栅具有较高的外界环境灵敏度,当外界折射率在1.41~1.45范围内变化时,其谐振波长漂移量高达15.52 nm,比实验测得的用传统方法写入的长周期光纤光栅谐振波长漂移量高出近3倍,这种光栅结构在光纤传感中将具有重要的应用。     

Fiber laser with combined feedback of core and cladding modes assisted by an intracavity long-period grating

We present a fiber laser made in a single piece of conventional doped-core fiber that operates by combined feedback of the fundamental core mode LP((0,1)) and the high-order cladding mode LP((0,10)). The laser is an all-fiber structure that uses two fiber Bragg gratings and a long-period grating to select the modes circulating in the cavity; the laser emits at the coupling wavelength between the core mode LP((0,1)) and the counterpropagating cladding mode LP((0,10)) in the Bragg gratings. This work demonstrates the feasibility of high-order mode fiber lasers assisted by long-period gratings.     

Simultaneous measurement of refractive-index and temperature with high sensitivity by combined use of long-period grating and defect cavity in photonic crystal fibers

A dual-parameter sensor for measuring refractive-index and temperature is designed based on a magnetic fluids infiltrated photonic crystal fiber. By inserting a defective cavity in the cladding layer and inscribing a long-period grating on the fiber core, the proposed sensing mechanism can be achieved directly. The mode coupling and the confinement loss of cladding modes are investigated, and a dual-parameter matrix method is utilized to solve the cross-sensitivity problem. In addition, the relationship between resonant wavelength and external magnetic-field has been studied, which can be used for measuring the magnetic field.     

级联长周期光栅光谱特性

利用传输矩阵法分析了级联长周期光栅的光谱特性,讨论了级联处光纤的长度、位置以及包层模损耗系数对级联长周期光栅光谱的影响,并对级联长周期光栅和相移长周期光栅的光谱进行了比较。结果表明两者光谱存级联光纤长度较小或级联位置靠近光栅两端时具有较强的一致性,而在级联处光纤较长并且级联位置在中间时,两者表现出截然不同的光谱特性;在不考虑其他损耗的情况下,如果只改变级联处光纤长度,级联长周期光栅总量守恒;此外,当级联长周期光栅在级联处光纤包层模损耗系数较大时,级联长周期光栅的光谱等效于两个长周期光栅光谱的非相干叠加,从而为长周期光栅增益均衡器的优化设计和制作提供了一个简便有效的方法。     

长周期光纤光栅温度稳定性分析及其改善

利用模式耦合理论推导出长周期光纤光栅(LPG)温度特性的一般关系式;通过测试周期为400～600μm的长周期光纤光栅的温度特性,确定了芯内导模与被耦合的不同包层模间的热光系数差,并结合长周期光栅温度特性关系式总结出长周期光栅温度灵敏度与光栅周期和耦合包层模阶次的对应关系;在理论与实验的基础上,提出了改善长周期光栅温度稳定性的方法。     

三包层长周期光纤光栅谐振波长与薄膜光学参数的关系

通过求解严格的耦合模理论建立的三包层结构长周期光纤光栅特征方程,研究了三包层长周期光纤光栅谐振波长与第二包层(薄膜)的折射率和厚度之间的关系。结果发现,随着膜厚及折射率的增大,谐振波长偏移的变化分成三个区域,这与Nicholas D R的实验结果相符。利用HE/EH模的判据数,对三个区域的模特性进行了分析,给出了区域划分的衡量标准。给出了在不同薄膜参数时的长周期光纤光栅透射谱,发现一阶低次HE模式的耦合强度要远大于一阶低次EH模式。